bpdude
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Post by bpdude on Jan 20, 2008 15:52:34 GMT -5
(Moved to the proper forum by sumgai, 1/20/08) Hello! This is my first post on this forum. I came up with what I think is a good way way to modify my Epi Les Paul so I can blend the signal coming from the pickups. Normally when I play I always have the pickup selector in the middle and then use the individual pickup volume controls to mix the pickups. I find a lot of in-between settings to be much better sounding than the regular one PU/other PU/both PU's setup. The problem with this is that I can no longer use the knobs as volume controls. This inspired me to rethink the current control elements in the guitar electronics, and modify it so it fits my needs better. Along the way I decided to make it possible to have different coil configurations (within pickups), and a phase reversal option (global). I also added an active preamp circuit to have more consistent tones. | So the controls on the guitar would be like this:
- 1) and 2): Regular Les Paul tone-pots, 1. for the neck pickup, 2. for the bridge.
- 3) Master volume control, also a push-pull phase switch. It inverts the bridge pickup, and sets it to the inner coil when 4. is in series mode.
- 4) and 5): three-way 3P3T rotary switches controlling individual pickup wiring. 4 is for the bridge pickup, 5 if for the neck pickup. In the first position the two coils of the humbuckers are wired in series, then in parallel, and in the third position a single coil is used. It also increases the pickup output level when in series or parallel mode.
4. will need another hole that will have to be drilled in the body.
- 6) A blend pot that mixes the two pickups together
The knobs are not actual size. In reality they are much less cramped.
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I drew the following circuit schematic for this task using PSpice student. (Download the schematic from here: www.mediafire.com/?4qoklad04m8) I didn't include the third pole of the rotary switches that can be used to increase the gain of the pickup preamp when it is in split/parallel mode. I'm thinking of using precision trimmers for this. I also used an inverting buffer to switch the phase of the bridge pickup because I couldn't find a 3pole push-pull switch that would change to the inner coil when the bridge pickup is in single mode. I think this is important so the hum is canceled out instead of added together when both pickups are in single mode. As you can see from the diagram, the circuit is far from being complete. This is where I ask help from forumers who are more experienced with opamps/guitar circuits. Here are my questions: - What values should i choose for the feedback resistors R3/R4; R5/R6; R7/R8? I think unity gain would be enough for the serial mode, but apart from the ratio of the resistor pairs, what else should I consider when choosing their values?
- Is the summer at the output correct?
- I should probably use some kind of decoupling. I'm sorry but I know less about op-amps then I should to design such a circuit. Where exactly do I need capacitors, and what value?
- I want to use it with a 9v battery. Will a pair of resistors be enough to provide the 4.5v ground?
- How should I wire it together with the output jack plug so it turns off when I unplug the cord?
- What else might be wrong?
- Also, any recommendations on good, low noise quad op-amps that would be suitable for this circuit (buffering guitar pickups, running from 9 volts)
( Edited by sumgai to show the schematic, instead of a nearly invisible image acting as a link.) (Size should be good now)
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Post by sumgai on Jan 20, 2008 22:42:11 GMT -5
bippy, Hi, and welcome to the NutzHouse! ;D First things first, could you please reduce the pixel size of your image, and repost it? And for the record, we do prefer to see the whole thing while we're discussing it, as it now appears. Linking off-site as you did is OK, but not as user-friendly, if you get my meaning. Thanks. EDIT: Ah, much grass! Lookin' good!Answers to your specific questions will have to wait for one of our enterprising geeks to come along. ;D sumgai
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Post by JohnH on Jan 22, 2008 6:54:22 GMT -5
Hi bpdude and welcome. I see your post has been sitting for a couple of days. I think that is probably because round here, there is not a lot of active electronics discussed, and I do not recall any op-amp circuits posted here before. I’m not experienced with them myself – I last built a project with op-amps 30 years ago. Clearly you have put a lot of thought into your design, and I think it would basically work, possibly with coupling capacitors between stages to avoid amplifying dc offsets. But I don’t think you need 4 op-amps to accomplish what you are doing. In fact it is quite possible with just passive wiring – or with just one active stage. I have built a few buffers into guitars, and I tend to use JFETs, for their simplicity, nice tube-like character and the potential for much lower current consumption than op-amps. Heres what I know about them: guitarnuts2.proboards45.com/index.cgi?board=schem&action=display&thread=1159677238&page=1On the diagram, it shows how a stereo jack can be used to connect the power when you plug in a mono plug. You can indeed do a phase switch, which also reverses the selected single coil, with just two pole. Just do a two pole switch to swap hot and ground leads of series-connected coils – then do the coil cut by independently grounding the central connection between the coils. Swapping the phase moves the position of the coils relative to hot and ground, hence a different coil is cut. That will save one op-amp stage This design for an LP has that feature, and also a JFET buffer: guitarnuts2.proboards45.com/index.cgi?board=schem&action=display&thread=1169255966&page=1Getting smooth blending is one of the trickier problems, and there has been several threads on it recently. Given that you only want blends, and are willing to use active electronics, I would be inclined to have passive wiring from the pickups and switching, similar to what you have but without the two first-stage buffers. Then feed that to a 250k linear pot, with no grounded lugs and one outer lug to each pickup, and the central lug to the input of a single high impedance buffer stage, with an input impedance of at least 2M. This is easily obtained either with a JFET stage or an op-amp stage. In the central position, each pup will be feeding through a 125k resistance, which will cause negligible loss when feeding a 2M input, giving you both pups equally blended. At the ends of the blend pot travel however, one pup will be directly connected to the buffer, while the other is connected via a 250k resistance. The second pickup will be largely suppressed by the lower resistance through the first pup and be inaudible. At in-between settings, all the blends will be found. Here’s another consideration for your LP. Usually these need long-shaft pots to get through the thick top. Such pots are not available from most electronics stores, and usually are only available in standard values as used on Gibson-type guitars. Finally, you should note that active buffers change your tone – making it clearer and brighter. It is often an improvement but it is a personal choice. I have buffers in three of my guitars, but not my LP, because I like the sound of it better without. Good luck John
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Post by RJB on Jan 22, 2008 13:16:43 GMT -5
Welcome It looks like you're well on your way to being a resident with the rest of the nutz here. I agree with JohnH that the coil selecting and phase can all be handled in switches. I just can't think of exactly how off the top of my head. And not enough time to work it out at lunchtime. You might visit Wolf's site for some ideas. You mentioned using the 3rd pole of the coil selector switched for gain control. I assume you're planning on using it to switch in different values for R4 & R6? This would be to "level" the volume between pickup modes? This will also eliminate any interaction between the individual tone controls. Connecting the outputs of the 2 buffers to 3rd through a pot as JohnH suggested and a pair of coupling caps. This will blend nicely, and the caps will block DC from the pot. Less scratchy noise that way. If the intention was not volume leveling and/or tone control isolation then a single buffer stage at the output would suffice. When we nail down the topology I'll help you with the part selection. Ron
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bpdude
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Post by bpdude on Jan 22, 2008 16:34:15 GMT -5
A load of thanks to both of you suggesting that there is no need for an inverter. I feel a bit silly for not finding that out Oh yeah, I don't mind active electronics in this guitar. Sometimes I feel like I'd like to turn the tone knob above 10. I feel that a buffer will help me with that, and the parallel switching ability is also a must for me. The 1 linear pot vs. blend pot is also a step forward in terms of simplicity. I still happen to think that I would benefit somewhat from buffering the pickups separately. Wouldn't lowering the bridge tone control have an effect on the neck pickup's tone? And I don't want to get into the FET vs. chip war. I guess both would be equally transparent. My most important concerns are battery consumption and circuit board size. Maybe I'll make both opamp and discrete FET preamps and choose which works best. You mentioned using the 3rd pole of the coil selector switched for gain control. I assume you're planning on using it to switch in different values for R4 & R6? This would be to "level" the volume between pickup modes? This will also eliminate any interaction between the individual tone controls. Yes, that was my main intent. For example, if I fully turn the blend knob towards the neck pickup, and then switch from single to parallel, it would be great if there weren't any change in the volume. So right now the diagram looks like this. This is plan b) with the passive pickup summer before the buffer. (the component values are default) And here's the third version. Should I ground the third lug of the tone pot the way I did in this picture? Is there any need for an inverting mixer at the output at all?
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Post by RJB on Jan 22, 2008 22:31:22 GMT -5
In option b you will get interaction between the tone controls. With the buffers in place (option c) there will not be any. As far as a final buffer after the blend pot. My preference would be yes. I would think you would get a smoother blend function, and the benefit of low impedance drive at mid blend, rather than 125K series from the pot.
If you wire option c as shown, I would include a 470 ohm resistor in series w/ each buffer output. This eliminates a virtual short circuit on the out if blend is fully one way, and master is all the way down.
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Post by JohnH on Jan 23, 2008 3:24:50 GMT -5
That all looks good, subject to cap values etc. Id also agree with a buffer after the blend pot. In fact, Id put it before the vol pot too, and have a low value volume pot, 50k or 100k at the output, dependent on how grunty the output buffer is. That will allow the blender to always feed a very high buffer input resistance, causing no level losses due to the blender.
The b) will indeed have some tone interaction between pups, although much less than for conventuional wiring due to the sepertaion provided by the blend pot. Buffers will cut this out however. JFET buffers with no gain, running at less than 0.1mA each would be particularly suitable for these initial buffer stages. Then maybe the output buffer could be an opamp, with configurable gain that you select with the switching if you wish, or maybe another JFET stage such as the Fetzer valve from runoffgroove.com
John
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bpdude
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Post by bpdude on Jan 23, 2008 8:58:33 GMT -5
My head's starting to spin. Now I've calculated the mixing law of the single linear blend pot and I've found out that it's just the opposite of what I'm looking for. It kinda follows a hyperbolic curve, being louder in the edges (fully neck or fully bridge), then quickly dropping a lot towards the middle, but nearing the other end, the pickup that is further from the potmeter (that is supposed to be off) is not a lot quieter than the other. Looks like it'll be back to the dual-gang blend pot. Or I may have seen some other kind of mixing method...lemme check. edit: Yep, here's another alternative that I've seen on some mixers: Gives the opposite of the circuit before. That is, pickup A is completely off at one end, then it quickly climbs (-1/x function like), the two pickups meet about 1dB below maximum output at the middle of the pot, and it gently increases another decibel on the last half. That all looks good, subject to cap values etc. Id also agree with a buffer after the blend pot. In fact, Id put it before the vol pot too, and have a low value volume pot, 50k or 100k at the output, dependent on how grunty the output buffer is. That will allow the blender to always feed a very high buffer input resistance, causing no level losses due to the blender. Sounds great to me. 3 gain stages then. The b) will indeed have some tone interaction between pups, although much less than for conventuional wiring due to the sepertaion provided by the blend pot. Buffers will cut this out however. JFET buffers with no gain, running at less than 0.1mA each would be particularly suitable for these initial buffer stages. Then maybe the output buffer could be an opamp, with configurable gain that you select with the switching if you wish, or maybe another JFET stage such as the Fetzer valve from runoffgroove.com John The 0.1mA FET buffer is probably the winner. Lol, I could just wire it to be on, and the battery would rot before it drains! Though it's the pickup buffer's gain that I'd like to change. I could also add an attenuator after the buffer for the series connection I guess. I don't know where I would be without you guys! I'm very grateful for your help.
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Post by JohnH on Jan 23, 2008 14:48:35 GMT -5
My head's starting to spin. Now I've calculated the mixing law of the single linear blend pot and I've found out that it's just the opposite of what I'm looking for. It kinda follows a hyperbolic curve, being louder in the edges (fully neck or fully bridge), then quickly dropping a lot towards the middle, but nearing the other end, the pickup that is further from the potmeter (that is supposed to be off) is not a lot quieter than the other. Im not agreeing with you there! - I reckon youll get very consistent volume across the mix, and, particularly with the 3x buffer design, youll get the full blend range. The key parts are the low output resistance and high input impedance of the buffers. With blender all one way, one pup will be feeding from a very low impedance, while the other will be via the 250k pot. The relative outputs of each pup in the mix will be in inverse proportion to those effective impedances. In the mid position, if you have 250k blender, and a buffer with 2M input resistance then you should get about 97% of the signal voltage, which is a negligible loss of db's cheers John
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bpdude
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Post by bpdude on Jan 23, 2008 16:29:06 GMT -5
That's true, and I'm not concerned about that 3% loss for full on. The problem is that even if a pickup is fully turned off, it will not be a lot quieter than the other pickup. For this circuit, when the top output is fully turned off by the potmeter, it's level at the buffer will be R3+R2/(R2+R3+R1) * R4/((pot CCW = 250k) +R4) * input. With values R1=R2=470 R3=250k and R4 =2M the resulting loss at full attenuation is only 0,887 = -1,04dB ( I'll double check this) I'll stay with the pot being connected to ground for now. Here's an updated schematic. Including whatever I've learned so far. It's starting to look a bit tight. I'll substitute the op-amps with FET's when I find out how that is done.
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Post by JohnH on Jan 23, 2008 18:26:37 GMT -5
Not trying to push, but I don’t want you to lose out on a workable solution for mathematical reasons.
The linear blender action, when fully towards one pup, works mainly by the action of the output of the dominant buffer acting effectively as a ground for ac signals coming from the reduced pup.
So if we reckon the buffers output resistances are R1 and R2, then the suppressed output is reduced by a factor of :
(R1//R4) / (R1//R4 + R2 +R3) where // indicates ‘in parallel with’, with appropriate math
R1//R4 is almost equal to R1
If R1 and R2 are 470 and R3 = 250k, R4=2M, the reduction is: x 0.0019, or a -55db reduction
In practice, the buffers might have an output resistance of say 2k, so effectively R1 might be 2.5k. Reduction is still x 0.01 = -40db
John
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Post by RJB on Jan 23, 2008 21:54:12 GMT -5
Ok I got out my simulator software tonight. I picked a generic cmos opamp, and setup 2 signal sources (pickups) each into it's own non-inverting amp. I used 4.7k for both feedback and shunt resistors on the opamps (Av=2). I'll need to check, but I think that makes the amplifier look like R+Rf .. 10K. I'll have to get my textbook out.
I took each output, cap coupled, to either end of a 250k pot, and fed another non-inverting amp from the wiper. The simulation showed a fairly even, <1.8 db variation from 10%-90%. With a distinct additional +2db at 0 & 100%.
This confirms JohnH's blend action, I thought it was sound as well. This will save you at least a handful of components and using a ganged pot.
The down side is that my test only showed a 18db attenuation of the "unselected" source. I think we can improve on that with some component selection. Give me a couple of days to tinker a bit.
What kind of gains are you looking to get overall. Or just minimum gain and buffering action only?
Ron
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Post by sumgai on Jan 23, 2008 22:24:17 GMT -5
bippy,
While your circuit certainly "looks" busy, it fact it's quite simple, and easily manipulated where and when desired. The only thing I'd change would be in the two tone circuits (R18/C1 & R17/C2).... I'd remove the grounded tab on each pot.
Just to conform with the more-or-less standard "best design practices", you understand. ;D
sumgai
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bpdude
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Post by bpdude on Jan 24, 2008 10:34:38 GMT -5
Not trying to push, but I don’t want you to lose out on a workable solution for mathematical reasons. You weren't pushing. Now I can see where I was wrong. Bad math is not a "mathematical reason". (I'm just a guitarist fer christsakes)Ok I got out my simulator software tonight. I picked a generic cmos opamp, and setup 2 signal sources (pickups) each into it's own non-inverting amp. I used 4.7k for both feedback and shunt resistors on the opamps (Av=2). I'll need to check, but I think that makes the amplifier look like R+Rf .. 10K. I'll have to get my textbook out. I took each output, cap coupled, to either end of a 250k pot, and fed another non-inverting amp from the wiper. The simulation showed a fairly even, <1.8 db variation from 10%-90%. With a distinct additional +2db at 0 & 100%. This confirms JohnH's blend action, I thought it was sound as well. This will save you at least a handful of components and using a ganged pot. The down side is that my test only showed a 18db attenuation of the "unselected" source. I think we can improve on that with some component selection. Give me a couple of days to tinker a bit. What kind of gains are you looking to get overall. Or just minimum gain and buffering action only? Ron That sounds promising! -18dB is low enough to start with. What software do you use? I couldn't really get the hang of PSpice yet, and I was wondering what else might there be. I was thinking about a gain of 1-4 set by the variable resistors in the feedback of the pickup buffers for the single and parallel modes, and 1 for series. I don't need an overall boost, since I've never ran out of gain on any amplifier I tried so far. Thanks sumgai, I'll change it to that.
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Post by RJB on Jan 24, 2008 12:54:51 GMT -5
I'm using a program called TINA. It was a free download from Texas Instruments web site. Not the best I've used, but had what I needed for a project at work.
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Post by JohnH on Jan 24, 2008 14:53:35 GMT -5
I previously used PSpice student, but couldn't get it to run on my current pc. Now I'm using 5Spice: www.5spice.com/Its much better, and its free for non-commercial use, with limitations only on the size of the drawing area - which is fine for everything I want to try. Last night I tried modeling the 3x buffer circuit, using three JFET stages - no gain though, of the type that I show in my buffer thread. With both input buffers with the same signal, I got blend output level consistency of within 0.3db across the sweep. With one input only, I was able to suppress the active input by -38db. This is based on db for power = 20log (V1/V0). One thing I found, which I hadn't expected and which I think would apply to op-amps as well, is that the coupling capacitors after the first stages need to be quite large, to make sure the blender is able to fully suppress one pickup across the full frequency range. I found that about 2µF was about right. John
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Post by RJB on Jan 27, 2008 21:01:45 GMT -5
Ok First JohnH thanks for the 5spice link. I REALLY like this program. I think I'm going to have to register for it.
I ran a simulation with an OPamp I use all the time. Low power Rail-Rail input and output, CMOS. The big thing I noticed is the bypass cap for the "midrail" voltage divider. If it's too small the attenuation begins to fall below -40db. I'll draw up a neat schematic and post in a day or two. I'll set some values for 0db gain for humbucker mode, and some gain to match in split mode.
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Post by RJB on Jan 31, 2008 9:45:56 GMT -5
Ok This is what I would build. 0db gain in humbucker mode, 6db in parallel mode, and 10db in single mode. I only drew one gain control switch cause I'm too lazy. Good luck w/ the build. Ron
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Post by sumgai on Jan 31, 2008 16:21:43 GMT -5
Ron, Good job, but I've got two observations to make, if you'll allow....... C2 and C5 are redundant - they block DC from the summing mixer, but then C3 does the same job, just after the mixer control. C1 and C4 introduce a large phase shift - not very good for the tone, and not truly necessary. (However, if there's a popping noise when the pickups are switched on or off, then this is the first place I'd insert a capacitor, in an effort to quell that noise.) Otherwise, nice design. I'm also impressed with the (relatively) new LMC6484, looks at first glance to have been made with the needs of on-stage musicians in mind (ultra-low battery drain, low noise, etc.). It would appear that I may have some 'sperimentin' to do here. ;D HTH sumgai
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Post by RJB on Jan 31, 2008 21:31:12 GMT -5
SG No problems with the comments. I used 3 caps around the blend pot to keep all DC off the pot, because the input of the third stage can also be a source of leakage. This makes for a noisy pot. The 3 are just considered good design practice. This OpAmp has low input/output offset voltages, but hey once an engineer always. The 2 input caps are there to block leakage from the DC bias on the input from flowing back through the pickup coils and forming an electro magnet. And again to reduce popping when switches are thrown.
All can be eliminated, with varying degrees of degradation/improvement. That's why we build prototypes and experiment.
This particular opamp is a standard that we use all the time. Rail-Rail input and ouputs and can source into a 600 ohm output w/ minimal voltage drop even at 3V. Designed for low power battery operated portables. Like most R-R there is a slight non linearity that shows up at higher gains/frequencies though.
BTW - Did you notice the shorting of the feedback resistor, to change the device to a unity gain stage. This gets around the +1, in Rf/R +1. Better headroom with the humbuckers.
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Post by JohnH on Jan 31, 2008 21:40:25 GMT -5
Nice work. Whats the thinking with the db gain values? paticularly para and single modes being different. I guess it would be good to fiddle with it in practice to hear what sounds best.
Also, with that opamp, what current do you reckon the circuit will require?
Finally, an observation from buffered designs with pickups in series and parallel modes: Once they are feeding into a buffer, rather than a cable, the sounds end up quite similar to each other, separated only by volume. It will be interesting to see if this occurs here.
John
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Post by RJB on Feb 1, 2008 9:49:53 GMT -5
I came up with the Humbucker and Single values based of DiMarzio's site (~300mv MedHot Hum, 100mv Single). The Middle value I just guessed, split the difference.
Quiescent current is in the micro amps, the spec sheet claims 5V rail, Vo= 2.5V into 1M all four amps (quad) 2.8mA. It is a push-pull driver good for 20mA source, after all.
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bpdude
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Post by bpdude on Feb 1, 2008 13:55:30 GMT -5
Thanks for the schematic and the tips! I guess the other leg of C7 is going to 0V, am I right? I found most of the parts available in my junk box with a few differences: Can I substitute 4.7uF caps instead of 2.2uF? I only have those. In the shop where I asked they said that they have never heard of the LMC6484 chip. They didn't even have any quad opamps, so I'll go with a pair of TL082's that I had laying around, at least for this experiment. I'll also use trimpots as variable resistors for setting the gain.
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Post by JohnH on Feb 1, 2008 14:06:37 GMT -5
Since Im here this morning as you posted, Ill chip in until Ron checks it out. I think you are OK with what you said about the caps value and grounding C7. I dont know about the chips but Id guess another opamp would work the same - at least good enough for a test.
John
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Post by RJB on Feb 4, 2008 8:17:48 GMT -5
Oops, My bad. Yes it goes to 0V Yep.
The TL082(084 quad) is a very good opamp. It was a standard used for years, IIRC this is the buffer used in the Roland hex-pickup interface. It has R-R JFET inputs, but Darlington output. Which reads - High output drive with reduced headroom about 1.5V from either rail max, and about twice the current draw. It have does have better linearity though. For your application this means changing the battery at 5V instead of 3V.
Good Luck! Ron
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bpdude
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Post by bpdude on Feb 5, 2008 16:29:27 GMT -5
The circuit is on perfboard, and I have all the switches and pots to do this. I only need to slice up the pickups and run four wires from them, but that'll have to wait for 2 more days (band rehearsal tomorrow night)
I'll post some before / after sound samples when I'm done.
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bpdude
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Post by bpdude on Feb 11, 2008 18:59:45 GMT -5
I'm almost done, but I have an important question. Should the body/pickup shield be at 4.5V or 0V? Could there be any hazards either way? I also noticed that on the drawings I wired the volume pot backwards - awkward.
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Post by sumgai on Feb 11, 2008 22:10:16 GMT -5
bippy, It's called "ground" for a reason... 'cause there ain't no voltages on it. There probably wouldn't be any safety hazard to you, but I can almost guarantee that you'd fry everything under the cover, if you put your shield at 4.5vDC, and then somehow an accidental short materializes, for reasons known only to your personal friend, Mr. Murphy. Jean-Luc Picard says "Make yours so". sumgai
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bpdude
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Post by bpdude on Feb 12, 2008 4:23:35 GMT -5
Alright thanks, and what about the pickups' signal ground and the tone pots? If i ground that to 0V won't it send the output to the negative rail? (I wired the shield in the pickup cables separate from the coils)
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Post by sumgai on Feb 12, 2008 12:16:30 GMT -5
Alright thanks, and what about the pickups' signal ground and the tone pots? If i ground that to 0V won't it send the output to the negative rail? (I wired the shield in the pickup cables separate from the coils) Uh oh, I think there's an open can of worms in here, and they're getting loose! We've bandied about so many circuits, and parts of circuits, that I wanna see a quick re-cap...... what's the latest incarnation of your idea here? Please. I ask because I don't wanna be speaking of one kind of power supply, and you're gonna use another kind..... a wrong assumption here might well lead to a handful of smoking parts. sumgai
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