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Post by JohnH on Nov 4, 2010 4:49:08 GMT -5
I just checked into a couple of my boxes. On my booster, currently the battery is flat, and it sounds dreadful running on it, but the blue LED is still glowing quite nicely with just 1.2mA through it
EDIT - with a new battery, its nice and bright with 3mA. Im running it with a 2.2k series resistor off the 9V battery.
J
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Post by ozboomer on Nov 4, 2010 5:06:03 GMT -5
I just checked into a couple of my boxes. On my booster, currently the battery is flat, and it sounds dreadful running on it, but the blue LED is still glowing quite nicely with just 1.2mA through it Ohhhh brother... So, that also answers another question - the LED is NOT going to be any use to indicate charge on the battery, now is it!? ...but did you measure that current on the BlueJuice unit? Looks like you have a 2.2k resistor in series with your LED which (nominally) runs at ~4mA... but would I be so cheeky as to use an 6.8k for example? ...or is that just tempting fate? Edit: You answered my question whilst we're both on-line...Dang.Experiment, experiment... I know ...and, just to confirm, are the LEDs you're referring to these ones?
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Post by JohnH on Nov 4, 2010 5:35:23 GMT -5
Im not sure! - there are a bunch of diferent ones, they dont have a chrome body though. They are clear and i think not the most expensive blue ones
BTW, alkaline batteries fade away gradually. Lithiums hold more, but die suddenly. I havea neat trick with a stereo jack plug, and two leads on it to the barrel and ring only. Plugged into the input of a stomp box, I can read the battery voltage across the leads, with a reading that is about 1/2V low since it is reading through the circuit. Fine for a quick check though.
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Post by ozboomer on Nov 6, 2010 1:38:05 GMT -5
I've given the unit (almost) a week of playing and such.. and I bit the bullet this arvo and added the power switch. I ended-up not bothering about the LED but I used a toggle switch so it was obvious the unit is on... and as I mainly use this unit from a desktop when recording, that'll suit me best.. Here's a rather boring/nodescript photo (whee): The thing I DID notice, though... was how the unit had a HUGE effect when plugged into my little Frontman 15R amp. ANOTHER chalk'n'cheese moment... 'coz I particularly noted how different SimpleMod-d sounds running through the booster/buffer box and into the amp... and it makes me re-think about the combinations I want to include/exclude in the next guitar wiring mod(!) Anyway, just thought I'd post these thoughts/notes... for completeness John
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Post by ozboomer on Nov 18, 2010 20:17:17 GMT -5
After building the buffer/booster and using it for a while, I found a couple of things that annoyed me... The first thing has already been resolved and described, being where I fitted a power switch, rather than using the cable/plug connection to power the unit off/on. The other thing that I didn't like was that there was a pretty nasty *Thump!* going on whenever the power was switched on/off. It was a relatively simple thing to fix, by simply applying a capacitor across the power supply... but after consulting our "learned colleagues", I found there was a particular way to do it. This then led to the following revised schematic: With this arrangement, there is a short, slow decay of the sound when the unit is turned off and a similar 'fade-in' of sound when it's turned on; a much less concerning and possibly damaging behaviour. So, that's the buffer/booster unit for now... until I re-work it to be a "real" foot pedal sometime in the future. HTH.. John
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Post by ozboomer on Dec 22, 2010 19:55:10 GMT -5
As a further 'sleeper development' of this buffer/booster, I'm currently scoping-out some options... one of which is to make the unit guitar-mounted. Now, I understand it will reduce the 'utility' of the electronics, when it could all be placed in a stompbox and therefore, would be available for use with ANY guitar... but it's still something I'm considering. The question is: How would a preamp such as this work with a 3x 3V Lithium button battery -style of power supply? Button batteries can be easily snuck into a corner of any of the cavities in a Strat (particularly where there is a 'swimming pool rout'... or in the back cavity...)... but this leads to the next question.. It's fairly simple to buy a single button battery holder locally... but I'd have to go to Mouser Electronics, for example, to get a double button battery holder... but I don't think anyone makes anything for 3x batteries...? Now, I could do something drastic, like cut out a hole in the back of the guitar to hold a 9V battery (as I don't want to 'block' the tremolo, which I understand some people do).. but I don't want to do that. ..and I'm not sure that 6V would be enough to power the preamp and some tone controls? Any thoughts!? ...and on a related matter... I'm thinking of making a change to one of my guitars where the wires connecting the tone capacitors to the pots, etc are extra long and lead into the rear cavity of the (Strat) guitar. Normally, the caps. are soldered more-or-less directly to the pot shell and pot lug. To have the caps in the rear cavity would mean the wires would need to be, maybe 6"-12" (150-300mm) long. Would there be too many losses/extra capacitance if I used plain wire for those connections? Would it be better if I used coax or some sort of shielded wires? I understand that I could just run the leads from under the pickguard and have the caps floating around the top surface of the guitar... but I'm vain enough that I don't WANT that look, thank you If I pull the wires through to the back cavity, I can simply take off the rear spring cavity cover and change the tone caps WITHOUT having to re-string or pull the guitar completeley apart, which makes life easier... ...and I don't want to go the Varitone route, thanks Would appreciate any and all thoughts, thanks... Regards John
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Post by newey on Dec 22, 2010 21:23:26 GMT -5
Oz- JohnH had something a while ago where he just taped 3 button cells together and soldered the connections, IIRC. I don't recall what specific project it was, however, so John will need to come in off his verandah . . . As far as the caps strung out to the trem spring cavity, I don't know why not, doesn't seem like a few extra inches of wire would make much difference. To use a shielded cable might be preferable but some drilling may be needed, as I don't think the small hole for the string ground can accommodate it. You could use a small piece of breadboard, the kind with the clips, for a quick-change cap setup, so no soldering required. But if it's just about searching for the right cap value, it's easier to do your testing outside the guitar, then wire in the value you feel works the best.
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Post by JohnH on Dec 23, 2010 0:05:41 GMT -5
Lithium button cells are quite neat, at 3V each, and they hold a lot of power in a small volume. They come in different diameters, take a long time to decay and also hold their voltage well untill depleted
You can get a holder to form 2 or 3 of them into a 6V or 9V stack. I used two in a simple JFET buffer and it worked fine, with the holder glued to the back of a pot. In a circuit with gain however, I think 9V is a good minimum.
I have also wired things into the rear cavity of a strat. If you have a lot of wires, some form of screened multicore is good, then theres no extra buzz induced, and you only need one hole from front to rear. i have used 4-core screened cable for LP wiring, and my piezo circuit uses most of the wired from an old printer cable, for battery connections and links to pots on the front..
If you are using this for tone caps, then the extra cable capacitance should not be an issue, particularly if you put the caps on the ground-side of the tone pots.
cheers John
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Post by ozboomer on Jan 12, 2011 20:01:56 GMT -5
More developments continue to be made with the little "clean boost" box... but I wanted to see if anyone had some thoughts on the following, please... The idea is that I want to add some tone control to the basic booster but I want to make it "switchable"; that is, although I might add the booster at the head of the signal chain from the guitar, I also might want to do some initial tone processing (or not) within the booster itself. This leads to a few options for how I might achieve this end, viz:- ...plus a few others, I'm guessing. I'd like to see what your thoughts might be on this... and if I was going to put some sort of "Level" control on the unit, would I do it on the 1st or the 2nd booster circuit (where applicable)? ...and why would I put it (preferably) on one and not the other? Appreciate earlier suggestions and any forthcoming thoughts about this tone control option, folks... John
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Post by ashcatlt on Jan 12, 2011 22:24:04 GMT -5
Just to check my understanding:
You've got a single booster in there now, and are thinking about adding a (big muff) tone stack and possibly another boost section?
It's my understanding that the BMP tone stack wants buffers surrounding since its action is sensitive to impedance on both sides. The booster does a fine job of buffering, though you might need to change some values somewhere to get it to work the same. Option C is out, though.
I'd be inclined to put the tone control before the booster. Being a "lossy" passive filter, it will buy at least a little bit of headroom through the booster, allowing a bit more boost before distortion. If it's already pretty clean I guess that's not completely necessary.
But the BMP follows the tone stack with an amp stage which both acts as a buffer and boosts the signal a bit to make up for the losses. You could just tack on a similar amp and call it part of the tone stack. There might be a way to switch a single resistor into the booster circuit to "auto-compensate" instead.
On the subject of headroom: Once you've run out of clean headroom in one booster stage, you're not going to get any more clean gain from a second unless that one has a higher supply voltage. At that point, we wonder why we didn't just use a higher supply for the first stage. But you don't have a gain control on the box you've got now (right), and if you're in an area where it's not close to distorting, you might have room in the second stage...
I think the reason for choosing placement of the Level control is related to this discussion.
I'm assuming you're talking about a "Gain" knob - which changes the gain of one of the boost stages - as opposed to a "Volume" knob - a passive voltage divider which attenuates the output after it's been gained up.
I think proper gain staging says we should get as much as possible/necessary as early as possible/practical. There will be at least some noise added to the signal at every stage along the way, and we'd like to amplify as little of that noise as possible.
But then we get to the question: Do you want to be able to overdrive one or both of these stages? If both, then the gain knob must go on the first booster. If only one, then you can put it on either. Put it on Boost1 and set it to max out before distorting that stage, and set static gain of Boost2 high enough that it will. Or put it on Boost2 and make sure that the static gain of Boost1 plus the max gain of Boost2 will push it into overdrive.
At this point, you might end up wanting a "Volume" at the end of it all, it's going to be REALLY LOUD!
If we're not specifically looking to overdrive, I don't think it makes any difference aside from the whole noise thing I talked about above.
Note - The definitions above of "Gain" and "Volume" I think are generally the way you see them used, but it's really my own distinction.
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Post by ozboomer on Jan 13, 2011 4:01:17 GMT -5
Just to check my understanding:
You've got a single booster in there now, and are thinking about adding a (big muff) tone stack and possibly another boost section? Yup, that's it... ...but before getting into all the detail, let me just elaborate somewhat... The booster is primarily used to clean-up the tone of my guitar (by reducing the loading issues) and to increase the volume level of the guitar a bit; this is also why I simplified JohnH's original buffer and booster designs so that the booster only had a "nominal" gain of maybe 8dB, mainly being determined by the bias voltage requirement for the JFET. Now, the booster is working very well but I thought I'd like to include some tone controls in the booster unit (and! be able to bypass those controls if required). After researching on-line and talking to a few people, I opted for simplicity and went for a Big Muff tone stack. Now, I realize it's going to have significant losses but, as was mentioned on the above-referenced page, I thought the booster would have "enough" in it to compensate for the losses... but probably no... hence, I would need an extra boost stage (see "Option A" above). Just to see how things worked, I built-up a breadboard circuit and dropped it into the signal chain thus: guitar -> booster -> Big Muff tone stack -> amp... and it sounded Ok to me... but then, I only came to that conclusion by running it into an amp and not into any recordings, so I don't really know how noisy or distorted the arrangement might be. Anyway, that's why I thought "Option C" might work... but again, probably no. The other options are just me thinking out loud, really... So, this is the starting point I'm coming from.. At any rate, many thanks for the posting, ash; I think I'm probably going to have to do something about a 2nd booster stage, so tha's Ok.. But then we get to the question: Do you want to be able to overdrive one or both of these stages? Actually, it's probably not a major requirement, really. I'd probably like a slight distortion to get some sustain and the extra harmonic content but again, maybe that's a "feature" I can bring in later on by changing some components. If both, then the gain knob must go on the first booster. If only one, then you can put it on either. Put it on Boost1 and set it to max out before distorting that stage, and set static gain of Boost2 high enough that it will. Or put it on Boost2 and make sure that the static gain of Boost1 plus the max gain of Boost2 will push it into overdrive. I'm going to have to think about those options a bit, I think... I can see an "offshoot" of SimpleMod for booster boxes on the horizon Still keen to read anyone and everyone's thoughts... Regards John
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Post by JohnH on Jan 13, 2011 16:23:47 GMT -5
Hi John - I had a look through your A to D options, and I'm not sure I quite see the one that I would do. But as you note, you need two boost stages. One of them is the stage that you already have, to give you the gain that you choose (call that boostA). The other is one precisely tweaked to match the loss due to the tone control (call that BoostB. Ideally you should be able to set the tone control at the mid position and then flick it on and off and hear no difference in tone or volume. BoostB is after the tone stack, to preserve headroom So the chain would be: input - BoostA - switch - tone stack - BoostB - switch - output the two switch stages are two halves of a dpdt, to loop past the tone stack and BoostB I'm sure you've fiddled with the Big Muff values on the Tone Stack Calculator. As installed, the Big Muff stack has a mid dip, but if you wish, you can adjust the values to get a flat response. I had a quick play: Zsrc should be set to the output impedance of the booster before it, and is about equal to the resistor between drain and +. R4 is the gate to ground resistor of BoostB. As shown, this would indicate the need for 11db of gain to compensate for the tone stack. Probably the basic booster makes a bit less gain than that. You can turn it up if put a largish cap and variable resistor in series, across the source resistor of BoostB. Pick these values so the resistor can be from 0 to about say 5x the source resistor, and the cap is large enough to have an impedance of at less than say 10% of the source resistor, at a frequency of 60Hz (ie, it will be linear all the way down to a dropped tuning on guitar) What do you think? cheers John
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Post by ozboomer on Jan 13, 2011 20:33:09 GMT -5
Hidy-Ho, JohnH.. ...One of them ... to give you the gain that you choose (call that boostA). The other is one precisely tweaked to match the loss due to the tone control (call that BoostB. Ideally you should be able to set the tone control at the mid position and then flick it on and off and hear no difference in tone or volume. BoostB is after the tone stack, to preserve headroom Golden Fact No. 1 ... and so we have the reason for the 2nd booster AND we know what we need it to do (for the current application); slowly the picture becomes clearer...! I'm sure you've fiddled with the Big Muff values on the Tone Stack Calculator. You bet I have... and I've been tempted to even install a boost+BMtone into one of the guitars... but that's another project... [...example flat response design...] One of the breadboard designs I built-up had that response in the calculator (and by fiddling R3 I was able to get it DEAD flat) and it actually sounded like a very smooth tone control (the LPF and HPF sections both had the same values, ie R1=R2 and C1=C2). O'course, I forgot all about Zsrc, didn't I!? *mutter*... Golden Fact No. 2 ... and R13 ~ Zsrc (and is 10k in the original JohnH design and my version). Golden Fact No. 3 ... In your example, you have R4 = 1.5M but in the original design (and is in my version), we have R4 = 2.2M... but it doesn't change the component values very much, viz:- Right. So, going back to the original design, where we are re-introducing "C6" (with a series variable resistor - a pot)... That's Ok conceptually... but I'm going to have to look-up about impedance, reactance, and all those goodies again to work out what's needed there... 'coz I don't really want to blow-up the couple of JFETs I have, so I can't just put on any old values and see what happens...(!) I think we have progressed a LONG way with these last couple of postings... Many thanks to ash and John! More later ( after I've attempted my impedance 'homework' ) John
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Post by ozboomer on Jan 14, 2011 6:01:58 GMT -5
Whilst I'm working away on some different layouts for the booster circuits, I had a question... Regarding the "original circuit", viz:- The 330nF capacitor is (physically) a pretty big thing, compared to the other components, let alone the 10000nF (10uF). Now, in some cases, these values are just not procurable in a polycap, or if they are, they might cost A$10 instead of A$0.25. So, what's the alternative? I can get an electrolytic version (A$0.35) for some of the larger values but then there's reliability issues, with leaking and such. Are tantalums any good for these audio applications (A$0.95)? ...and what are these polypropylene/metallised things (A$16.95)? I can't get any Multi-Layer Ceramics (A$1.11 from Mouser) locally... What are the options!? John
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Post by JohnH on Jan 14, 2011 7:50:43 GMT -5
The electrolytics will be fine for this, just put the - end towards ground -ve. Also, If you wanted to use a larger value for the 330nF, you could go up to an electro, say a 1 or 2,2uF electro. I just tend to use whatever is in my spares box so long as it is large enough, if I want a flat response.
On your tone stack, another matter for choice is the crossover frequency - maybe aboy 500Hz would be good? (ie a bit higher)
cheersd
John
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Post by ashcatlt on Jan 14, 2011 11:36:09 GMT -5
From what I've heard, tantalums have reliability issues of their own. Also, they're not always as clearly marked for polarity, but far more sensitive to reversed situations. If the thing is still around when the electros go bad...well you might be too old to replace them, but whoever you will it to can use whatever new technology they find appropriate.
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Post by ozboomer on Jan 15, 2011 7:28:44 GMT -5
I've had some successes today, in that I think I worked-out a "good" design for a Big Muff -style tone stack for this booster... and I've built-up a prototype that doesn't sound too bad. I've also worked out a nice lil' layout for the 2nd stage booster (but that's something yet to be built, etc). Using the ever-famous Duncan's Tone Stack Calculator, I came-up with the following design; thanks to JohnH, this time I remembered to make the adjustments to suit the output/input impedances of the BoostA/BoostB stages:- I then built-up the design on a small breadboard unit and connected it up as: Guitar -> booster -> tone stack breadboard -> amp... and, as I mentioned in an earlier posting, it sounded pretty good. I then tried dropping it from the amp and plugging it into the UCA202 interface into the computer... and ohhh, it sounded like garbage -- too quet, too muddy -- so in this environment, the limitations of the tone stack became painfully obvious(!) Therefore, I "re-tooled" the chain of elements... and remembering another ol' trick from JohnH, I connected the guitar up to the computer thus: Guitar -> RC-2 (looper pedal) -> tone stack breadboard -> booster -> UCA202 interface -> Audacity. Now, it won't sound exactly the same but it gives a good idea, I think, of how the tone stack is working. So, to a couple of sound samples, all done with my Bullet Strat, with SimpleMod-f installed, middle pickup only, fully-"bright" tone controls and full volume:- - BMTS-HighCut.mp3 - something that works like the 'normal' tone control on the guitar. Makes the sound less bright
- BMTS-MidCut.mp3 - the more unusual effect of cutting out some of the lower frequencies, "by implication"(!) making the sound more bright
In both cases, I've started with the "middle" setting on the tone stack, and rotated the knob to the "more bass" or the "more treble" side(s), while playing through a rhythm sequence... and there's a definite change in the sound... which isn't too bad. Anyway, just thought I'd post a "success status" message on the tone stack... as I haven't had any joy (yet) in working out what I need to do for the cap+pot gain control on BoostB... *mutter,mutter* John
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Post by JohnH on Jan 15, 2011 15:12:14 GMT -5
The samples sound good to me.
For that gain control on BoostB, I see we already worked out that a 10uF cap was a good value for getting max boost with full frequency response. 5Spice says that the addition of the cap will increase gain from about 8db up to about 23db, with less than a 1db drop at 60Hz compared to higher frequencies. Any cap from there, up to say 100uF would also be OK. What the cap does is change the ratio between source and drain impedances, for ac signals. Without it, the gain is determined by the ration of resistors 10k/3.3k. Actually, it is a bit less than that, 10k/(3.3k+Rsource), Rsource is a characteristic of the JFET. (I'm not sure if that is the right term for it). But for these JFETs, I reckon it works out at about 750 Ohms. So bypassing the 3.3k resistor with a high-value cap changes the gain form about 10/(3.3+0.75) to 10/0.75, ie a gain change from about 8db up to 23db. Putting a resistor in series with the cap controls the increase. i reckon the value needed to get say 12db gain will be around 3.5k to 4k, so a 10k preset would be a good choice, given the variability of JFETs, and then adjust by ear.
John
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Post by ozboomer on Jan 16, 2011 5:37:47 GMT -5
The samples sound good to me. Great... So, we'll go with a "freeze" on that aspect... Ooo.. and many thanks for the clues on the "bypassing" and gain calculations, John -- that certainly makes things a lot clearer and more understandable...! However (there's always a 'but'...)... ...I now find I have a few more questions. Referring to this diagram (apologies for the fuzziness):- - Section A as shown is what I have in my current booster. Referring back to your original design, it seems I've been using the wrong component to "tune" the bias voltages; I should have been doing it on R13, NOT R4. I would think that my thinking was that as R13 determines the output resistance(!?), I didn't want to fiddle with that too much... but with hindsight, it's probably an advantage if R13 is tuned lower to give a lower output resistance... but as the booster works, maybe it doesn't really matter? ...but for future builds, I should really adjust R13 with a trimpot, eh?
- Given all that, let's say "I'm lucky"(!) and we'll say that the design I have working has R4 = 3.3k, which would give a gain of 8dB as shown in the diagram notes.
Putting a resistor in series with the cap controls the increase. i reckon the value needed to get say 12db gain will be around 3.5k to 4k, so a 10k preset would be a good choice, given the variability of JFETs, and then adjust by ear. Referring to Section B, do you mean that, with R4=3.3k and R13 changed to give the 5.5V-6.0V on the JFET drain, should I then add C6 and a 10k trimpot (VR2) into the circuit and adjust VR2 to get 5.5V-6.0V on the JFET drain again? ...or am I simply trying to make an adjustment on the trimpot to get the 12dB gain value set? If that's the case, I guess I'm listening (by ear or by signal level) for "no change" in the output level when the tone stack is switched in and out, eh?
- Section C - An on-going question that I've never quite resolved... For some reason, I added "R99" into the booster I now have. I think I read somewhere that you need something like that to "tie down" or "stabilize" the output? I'm guessing it's not really necessary?
Again, many thanks for all the assistance... and I'm hoping we can get these details ironed-out some, not only for my current project, but as the effects of a booster are so profound, I think we should probably look to make a "sticky" topic so it's one of the first projects people have a go at(!). John
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Post by JohnH on Jan 16, 2011 6:17:20 GMT -5
John - you can set your bias with either r4 or r14, and it doesnt matter which in principle (though it makes some differnce on the setting of gain of course). For this BoostB stage, you would set the bias first, before setting Vr2. The bias is all about steady-state dc conditions, so the C6/Vr2 chain plays no part, because C6 blocks the dc. Then set the gain, and yes, do it by ear to to make it sound right - the theory just gets it into the ball park.
Section C of the circuit is good to have on the output, to stop spurious charges in the output cap causing a pop wen you plug in, but you can omit it is desired.
cheers
john
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Post by ozboomer on Jan 16, 2011 21:16:47 GMT -5
Thanks for the explanation about how to set-up the C6/VR2 arrangement, John... All these facets are slowly getting organized in my head Now, I've been thinking again (ohhhh, not AGAIN, I hear you say!)... and I've been wondering about how the "compensation" for the signal loss in the tone stack is going to work... Let's say we have BoostA with 2x (or ~6dB) gain. Whatever signal comes into BoostA will get increased by 6dB. More often than not, that signal is at a low-level (other than the buffering, that's probably why we wanted the booster in the first place), so let's say we start with a -3dB signal... which gets a 6dB boost, giving a +3dB level. If the tone stack is next, which we worked-out has a 12dB loss, we're back down to a -9dB level. Now, before anything else, as the level is "so low"(?), what if it's so quiet that all that remains is noise? Having a 12dB fixed gain for BoostB isn't really going to help, is it? So, shouldn't we design BoostA to ALSO have as much gain as will be lost in the tone stack? Another thought: As a general concept, wouldn't it be "better" to have the attenuation (output level control) on BoostB? This way there'd be minimal variation in tone, etc as the volume was changed...? ...and of course, from our vast (VAST!) experience with the control stack on our guitars, I'm guessing we'd also do some sort of "treble bleed" on the volume control of this box, no matter where it is? ...and I haven't even STARTED to think about incorporating a variable gain control yet(!) Thanks a heap for any thoughts... as always.. John
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Post by JohnH on Jan 16, 2011 22:17:51 GMT -5
You don't want to compensate for the tone-stack loss before it has happened, otherwise, you'd be boosting to a level which causes clipping. But if there is an overall volume control, it might be best after BoostB, so BoostA can always present a constant impedance to the tone stack.
If this gadget is at or in a guitar, treble bleed is not needed, but if it is a separate box, then it would be wanted on the guitar (if you like that sort of thing)
J
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Post by ashcatlt on Jan 16, 2011 23:02:21 GMT -5
JohnH answered all the questions, but I had things I was going to type here, so I did.
As long as Boost A remains in it's linear operating range, any noise inherent in the original input will maintain its level relative to the guitar signal. It will be boosted the same 6db, attenuated the same 12, and boosted back by the same amount and the S/N ratio from that portion will remain exactly the same. Aside, that is, from whatever frequency dependent stuff happens in the tone stack.
Boost B will be amplifying the noise generated in the Boost A section, and the tone stack itself. It is a concern, but I honestly think that this added noise will be minimal. You probably won't hear it over even a well shielded guitar.
Theoretically speaking, as I said above, it is best to get as much gain as you need as early as practical. In practice, though, sometimes it's just not a big enough deal to worry about.
If you can get 18db boost from Boost A without pushing it out of its linear zone it would be best to do so. Like JohnH said, though, I'm afraid you'll be pushing it. You're talking about 8x gain. A 1V peak input voltage would push an 8V output. I don't know how close these things will come to the rails before they get squashy, but I'm thinking this would be pushing it. Of course, it usually takes a couple coils in series (like a humbucker, you know) to get 1V out of a passive guitar, but again I don't know how close these things will get to the rails...
OTOH, if you was to bump up the supply voltage to say 12 or 18V, you'd have more room to play around. First you'd want to check the rated voltage of your caps. Then probably re-figure some of the resistor values, which might cause a change of some cap values.(?)
Just to put a finer point on what JohnH said re: treble bleed: It ain't an issue with these low-Z active outputs.
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Post by ozboomer on Jan 17, 2011 2:19:04 GMT -5
Another few Golden Facts in these postings, thanks... If you can get 18db boost from Boost A without pushing it out of its linear zone it would be best to do so. Like JohnH said, though, I'm afraid you'll be pushing it. You're talking about 8x gain. A 1V peak input voltage would push an 8V output. I don't know how close these things will come to the rails before they get squashy, but I'm thinking this would be pushing it. Actually, this is an interesting and important point. My digital multimeter doesn't really go as low as millivolts, so take what follows with a grain of salt... but I measured the tiny AC coming out of my Strats and the highest output was about 30mV. After going through the booster, it went up to maybe 120-150mV, so that's 4x or 5x (12dB-14dB) for the circuit... maybe. Still, as long as I don't start heading for the 20dB point off one boost stage, I think things will probably be Ok. ..but as has been said, I think it's a matter of using the ol' ears to see when the distortion is being hit and to pull back some from that point. Still, step by step for now.. I think I have a pretty good idea of what I need to do, so I'll head off to build-up a Big Muff Tone Stack "on a pot"... and then have a go at the "fixed gain booster" (BoostB) with a volume control. Thanks a heap, once again.. John
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Post by JohnH on Jan 17, 2011 20:44:23 GMT -5
With these issues of headroom, I think it is all about the transients, which may be too quick for a meter to register. +/-1V is quite easy to get, or more.
When you record into Audacity or similar, you can see your waveform and the peak heights. I once worked out a crude calibration, based on putting some on/off pulses from a known 1.5V battery into the input, then comparing that to a buffered guitar input. My LP was getting 1.5V transients on a heavy strum, and my 'all 3 in series' Strat was peaking at 3V!
cheers
John
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Post by ozboomer on Jan 23, 2011 7:34:29 GMT -5
I added the tone stack to the built box this weekend... and although the box is still ugly as heck (with it's weirdo knobs and switches and things -- I won't subject you all to that at the moment...), it works Ok... but Dang! I should've included the tone bypass in this prototype - I'm back to a low-level output now until I build the 2nd booster... Anyway, the concepts are working-out pretty well, I think. I have yet to breadboard the 2nd (adjustable) boost stage but I have it designed. Even though it's a fairly simple circuit, it would have to be one of the more complex breadboards I've done so far (and I've already modified it a tiny bit since this diagram was first drawn), viz: ...and I've been thinking again about how the elements might all fit together. With a booster/tone control such as this, I wonder how... "useful"... it might be to actually bypass the entire device, viz:- Any thoughts? The build continues... John
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Post by ozboomer on Jan 25, 2011 6:48:52 GMT -5
I had a go at building a breadboard version of "BoostB" tonight.. and it works pretty well, I'm pleased to say... but it didn't all go exactly to plan... Again, with reference to JohnH's MagicCircuit TM:- ...and my rough'n'ready breadboard build:- I originally had a 10k trimpot in place for "R13"... and found that it didn't give me enough adjustment. At (nominally) 0k, the bias voltage at the "drain" terminal of the JFET (under no load) was more-or-less the same as the voltage at the battery, being 9.90V. When the trimpot went up to about 9.6k, the drain voltage went down to 6.92V, which was still a bit too much. After a bit of resistor swapping, I finally settled on a 12k resistor for "R13", which gave a drain voltage of 6.01V, which I'm guessing is near-enough. So then, it was off to the trimpot sitting between C6 and ground. In this version, I have a 10k trimpot plus C6 as a 10uF electrolytic cap (and C2 is another electro at 0.47uF)... but otherwise, the circuit is more-or-less the same as that shown above. Heck! the output from this unit is HUGE. From my Bullet Strat and straight into this "BoostB" stage, I was rattling the windows through my Frontman 15R.. and the amp was on "2" and my guitar was on "1.5"(!!!)... So, I know the circuit is Ok and the components all work alright... and it's just a matter of flinging everything onto some stripboard and maybe using a 20k trimpot for R13(?)... and then adjusting the gain to suit the "BoostA + Tone Stack" section and (the prototype) is done Dang, I get excited by simple accomplishments John
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Post by JohnH on Jan 25, 2011 18:54:21 GMT -5
Looks like youve come up against the variability of JFETs!
With that JFET wanting a drain resistor of 12k, it means that it is also getting some higher gain, based on the ratios we discussed before. It may not need much more to get you to what you need for BoostB. For the source bypass cap/series resistor, maybe try some values from the spares box, or rig up a pot to experiment before deciding.
cheers
John
Happy Australia Day - we are heading for a temperature of 40C here
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Post by ozboomer on Jan 29, 2011 2:11:41 GMT -5
An interesting turn of events today... but ultimately all very instructive... I took the breadboard version of "BoostB" and hooked it up as-is to the previously assembled "BoostA" and "Big Muff Tone Stack". It's slack but I just wanted to see how the "tuning" with the trimpots would work, to balance-up the signal levels with the (tone stack + BoostB) bypassed and included in the signal path. In short, it all worked pretty well... although, the trimpot for the "Gain" amount on BoostB was a bit peculiar (see the layout/circuit/photo above). "R99" was a 10k value trimpot and turning it anti-clockwise got the loudness up a fair way, so that was promising... but when I turned it clockwise, the loudness of BoostB was still too much; that is, (BoostA + tone stack + BoostB) was still louder than (BoostA). For whatever reason(?), I tried using a 20k trimpot... and again, turning it anti-clockwise got things nice and loud.. but this time, turning it fully clockwise got the levels down such that (BoostA + tone stack + BoostB) was just the same loudness as (BoostA); although, I still haven't quite nutted-out the way the trimpot is affecting the gain... Sidebar: As I'm not too flash at hearing subtle level differences and if a signal is distorting, I enlisted the help of a signal analyzer VST plugin and some other bits of gear in my testing.
I started by recording a sine wave oscillator signal, running at 440Hz and making that an MP3. I then dropped it into a player and set it to "repeat play" and plugged it into the "BoostA" unit. I'd dismantled some of the wiring in the "metal box" so I could test the tone bypass, etc... and I connected the "BoostB" breadboard. The output from "BoostB" was then connected into the computer and into some software which was hosting the VST plugin.
Here are some images of what the signal analyzer showed... starting with the signals with all elements in the signal path (BoostA + tone stack + BoostB):-
...and with BoostA only (the tone stack and BoostB bypassed):-
The levels matched-up very well... but I noted an extremely slight phase-shift in the output.. Meh..
Then, I tried turning the tone control to the "full bass" direction:-
...followed by the "full treble" direction:-
A comparison of these two extremes:-
Not unexpectedly, it seems we get a decrease in level when the "high pass" passive filter section is fully selected (max. "treble")... but how do we get an increase in level when the "low pass" passive filter section is selected (max. "bass")? I'm not quite sure what's going on with that....!? Still, the tone control sounds Ok, which is the main thing.
-- o0o -- All in all, I think i've worked-out most of what I needed to know... and so, I'll probably go ahead and work-out a final design/board layout/etc. I may see about trying to tweak a little more gain out of "BoostA", 'coz I'd like a bit more *oomph* when coming through the tone control... and the configuration of "BoostB" would seem to be able to cope with it, I think. For now (and for simplicity), I've cut-down the current build to be just "BoostA", as that seems to work well for a "basic" boost and to clean-up the guitar tone going into my computer/recordings. More to come, when I've re-worked the design to combine all the elements into the one board, etc John
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Post by ashcatlt on Jan 29, 2011 11:08:05 GMT -5
Take a look at your pic above from the calculator. When you get to frequencies a bit below the crossover point the full bass curve is up from the center position, while at full treble it's a bit down. Had your test frequency been 1K you may have seen the opposite action.
All of this leads me to believe that you're crossing over a little bit above the 500Hz listed in that pic.
You're probably aware, but 440Hz is 5th fret on the first string.
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