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Post by frets on Oct 24, 2022 17:01:58 GMT -5
Happy Monday Guys😸😸😸 I’ve got a little project that I know you all can help with. I have been challenged to build a simple guitar amp that can run on 12 volts and provide sufficient guitar noise to an old large 80 watt box speaker in my trunk. The amp has to be small enough to put in a little pedal box for portability. It would connect to the car battery (or if that would kill the battery) an auxiliary car battery. I am using parts in my junk drawers. So this is what I have so far. Now remember, the purpose of this amp is to wail and is not intended to be anything for the serious audiophile. My main goal is loudness. I don’t know if I’ll get that with this setup. I don’t even know if it will even work. Any input is appreciated😸  |
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Post by sumgai on Oct 24, 2022 19:06:08 GMT -5
frets,
Wouldn't hurt my feelings any to learn just where you got this diagram. Not sayin' anything here, just sayin', ya know......
sumgai
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Post by frets on Oct 24, 2022 19:12:21 GMT -5
Sumgai,
Its not mine.
I didn’t mean to infer that. What I did was I found a video of a guy making an amp just by soldering the components directly together. I sat there and watched it five times and drew a little diagram from it. If there is a diagram of it, I don’t know.
I have the parts and it looked easy until I really started to draw it out.
The guy didn’t have it on a battery and he was using it for music. I thought it might work and have some loudness to it.
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Post by thetragichero on Oct 24, 2022 22:40:48 GMT -5
gotta find my notebook (i think i know where it traveled in the move). i breadboarded what started as a triode-to-jfet emulation of a supro amp and tweaked until i liked the sound through one of those $3 chip amp boards from ebay. one thing you'll want to make sure to do is limit the peaks of your output signal as these power amps meant for audio reproduction don't clip gracefully (crude way to achieve this is to figure out what input the power amp wishes to see and string together diodes to shunt any signal bigger than that)
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Post by sumgai on Oct 25, 2022 11:08:14 GMT -5
frets,
I didn't mean to imply that perhaps it was your circuit, I already know that you are still in your early-learning phase of this branch of electronics. Still, I did think that you would've at least known that sticking a battery directly across the speaker would do nothing more that produce a pretty noticible thump, and then proceed to sound like crap, no matter what kind or how strong/weak a signal was fed into it (because the cone will be "bottomed out" in one direction or the other).
And while I'm warming up to the topic.... since when does a single wire both feed input to a circuit, and also carry the output away from the circuit? That'd be a thing of beauty, if it could happen in the real world. (And yes, for those of you who are more advanced in these arcane arts, we do that all the time... in the digital realm. But on a strictly analog basis? I don't think so.)
So, frets, let's skip over that, I'll not insult the originator of that circuit. Instead, let me assure myself with this question; Are the two chips shown actually in your parts bin right now, and you'd like to not purchase anything else if you can help it?
sumgai
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Post by frets on Oct 25, 2022 14:21:17 GMT -5
Well,
It’s not my best effort, I admit. So maybe we should just forget this circuit. But I swear he had that strange power setup going on. I just thought it was some weird inverse thing. But as you stated, I am new to the world of amplifiers. I think Trag helped me with one before for a stereo.
I’ve got so many parts Sumgai, I’m sure one of you could recommend some part and I’d have it or I could get it from a friend. I had an uncle who ran an electronics store and I got so much stuff from him. He’d just put it in boxes and say “here ya go, might be something in there you can use.” It was right before he passed. I’ve got a lot of IC’s op amps and transistors. And I had those parts. It took me two weeks to organize everything!
So “no”, I’m not wed to the parts. If you guys tell me something else, I’ll look for it and by chance if I don’t have it, I’ll get it off of EBay.
I just want it small, loud and able to run on 12 volts. That’s all😸😸😸
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Post by unreg on Oct 25, 2022 17:13:18 GMT -5
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Post by frets on Oct 25, 2022 19:23:24 GMT -5
Thanks Unreg, I appreciate you looking😸. I really want to build this amp myself and I think I’ll go with plain speaker terminals. But thanks for trying to help me out😸
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Post by thetragichero on Oct 25, 2022 21:17:59 GMT -5
 that's pretty much exactly as built besides the diodes on the output (should've done that but didn't). you're welcome to bypass the source resistors with capacitors if you're needing more gain (10uf will give you full range boost, use lower values if bass gets too woofy) |
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Post by frets on Oct 25, 2022 22:08:22 GMT -5
Trag,
I really appreciate this. Thank you so much. I’m going to diagram it out on the computer and post it for you to look at. I’ll put “?” marks at components I need values and notation for; e.g., the transistors, the diodes, et. al. I’ll try and do it in electrical notation rather than my literal diagramming. Again, thank you. This is great😸😸😸
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Post by frets on Oct 26, 2022 18:19:54 GMT -5
Trag, I really appreciate your help with this. I know how busy you are with the renovations and your business; so, thank you. I did the best I could with the diagram. I made an assumption that the amp was a combination of various transistors; however, I have zero knowledge regarding this discipline. I have marked in question marks where I did not see a description of the component and again, I made assumption pertaining to the symbol that looked like a transistor. I also did not see the voltage input. I really hope you can see and read the diagram. This is the first one of its type I have done.  I had a general question regarding this type of diagramming using electrical symbols and their corresponding adjacent component connections. In particular, pots. I have a snip of the diagram that illustrates my question.  Is the pot facing up with the shaft pointing upward or is it upside down? Disregarding orientation for a moment, does the pot connect to the 100k on both the outside and sweeper lugs? Thanks again. |
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Post by thetragichero on Oct 26, 2022 18:52:15 GMT -5
that's a trimmer setup as a rheostat so it's set wherever gives you 2/3 of your supply voltage (9v) at the drain of that particular fet, so it doesn't really matter which outside 'lug' is connected to the wiper (i suppose it would make most sense to have minimum resistance fully counterclockwise as that would have the largest voltage but once it's set you shouldn't need to change it). what i did since this was already on my breadboard (hence different values than the schematic i 'borrowed' for inspiration) was measure the resistance once it was set right and then use the closest standard resistor instead of a trimmer on my final perf build (since this is a geetar amp and not a rocket ship or reverb hifi amp, precision doesn't really matter). since two transistors with the same part number from the same production run can vary wildly, it really has to be set based on the particular transistor for the fets use whatever n-channel jfet that's decent for audio (as in folks have used with success in pedals or whatever, need not be some audio-specific fet that probably costs $$$). i wouldn't waste any stash of through-hole j201 you may have on this edit for clarification: Disregarding orientation for a moment, does the pot connect to the 100k on both the outside and sweeper lugs? yes, it's being used as a rheostat upward-pointing triangles in my drawing go to supply voltage. i did not include any supply decoupling capacitors but they would be a good idea for the diodes i would use whatever has a forward voltage equal to about half of the desired input signal to the power amp. so if it's looking for a 1V input (likely) a standard silicon diode should be just fine (1n4148/1n914, 1n4001, whatever). if it's fine with a few volts on input you could use leds |
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Post by thetragichero on Oct 26, 2022 19:02:02 GMT -5
are you sure that symbol is for an n-channel jfet? looks different than what I've seen
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Post by newey on Oct 26, 2022 21:34:11 GMT -5
are you sure that symbol is for an n-channel jfet? looks different than what I've seen Because it's the symbol for a MOSFET, not a JFET. frets, the right symbol is like the one in Tragichero's handwritten schematic. Arrow pointing inward is the N-Channel one; arrow going out indicates a P-Channel JFET. |
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Post by sumgai on Oct 26, 2022 23:32:11 GMT -5
trag,
What's the reasoning behind having two diodes connected in series between Vss and ground? Seeing as Vss is some positive voltage above 1.3VDC, this sure looks like a short circuit to me....
sumgai
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Post by Yogi B on Oct 27, 2022 3:36:06 GMT -5
What's the reasoning behind having two diodes connected in series between V ss and ground? Seeing as V ss is some positive voltage above 1.3VDC, this sure looks like a short circuit to me.... Well it is a short, but the other way: from ground to the supply (i.e. not a short, assuming positive supply voltage). (Also, likewise, don't you mean V dd?) Though I'm not saying the diode arrangement is right either... for the diodes i would use whatever has a forward voltage equal to about half of the desired input signal to the power amp. The configuration you show won't do anything to limit the upward swing of the output signal beyond what has already been done by the JFETs / output cap. Since the volume control is a DC path to ground the output is biased to 0V (when the output cap is charged) and the theoretical maximum output (assuming +12V supply and disregarding V TH of the JFETs, and lower diode) is ±6V (or, before the cap is charged, 0V to +12V.) Thus always less than the diode drop above the supply voltage that's required for the upper diode to conduct. The limits you want are a diode drop either side of ground, i.e. both diodes (anti-parallel) from output to ground. are you sure that symbol is for an n-channel jfet? looks different than what I've seen Because it's the symbol for a MOSFET, not a JFET. The symbol frets used is also the most confusing symbol variation for a MOSFET: it depicts a solid channel, despite (usually) representing an enhancement mode device; plus there's some confusion online about which arrow orientation is correct, since it points 'backwards' to that in the symbol which does not omit the body (it maybe visually pointing in the other direction, but it still points towards the source terminal — though maybe it's the typical depiction of the internal connection between the source & body that is the actual cause of this confusion).
that's a trimmer setup as a rheostat so it's set wherever gives you 2/3 of your supply voltage (9v) at the drain of that particular fet Although this does depend on the threshold voltage of a particular JFET, in general won't that set the bias 'hot' (closer to the supply rail, than to V S+V TH). In the circuits that I've paid attention to it, this does seem to be typical, buy why? Assuming we are clipping and only on one side: of the two, wouldn't it be more preferable have the softer clipping from saturating the JFET rather than hard clipping against the power rail by turning the JFET off? |
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Post by thetragichero on Oct 27, 2022 9:07:37 GMT -5
What's the reasoning behind having two diodes connected in series between V ss and ground? Seeing as V ss is some positive voltage above 1.3VDC, this sure looks like a short circuit to me.... Well it is a short, but the other way: from ground to the supply (i.e. not a short, assuming positive supply voltage). (Also, likewise, don't you mean V dd?) Though I'm not saying the diode arrangement is right either... gotta think about what happens at the diodes during the positive and negative swings of your audio. positive swing anything above the diode's Vf gets shunted to the positive supply. negative swing anything about the diode's Vf gets shunted to ground. i've seen it in many solid state amps (here's a sunn beta lead and a peavey xxl. the peavey is silly with em). basically a very crude limiter   |
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Post by sumgai on Oct 27, 2022 10:45:28 GMT -5
Ah, you know what? I got dyslexic for a moment, and my widdle biddy brain saw the diodes backwards. That's been happening more and more lately. Now it's spilled over into my scribblings here on the forum. Makes me want to scream.
I try really hard to follow cynical1's advice (Never complain about growing old, it's a privilege denied to many.), but for the life of me, not complaining is becoming less and less of an option.
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Post by frets on Oct 27, 2022 11:20:16 GMT -5
Okay Fellas, As you know, I’m a noob to the amp diagramming and understanding of the circuits. I think I have it right. I just want to clarify what 3 of the pots would be called and where additional capacitors would be needed. I went ahead with 1N4148’s. Will those be able to take the 12 volts? Are those pots I’ve asked about in red just trimmers? Are J111’s okay for this? I have others. Thank you😸  |
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Post by thetragichero on Oct 27, 2022 13:32:36 GMT -5
I'd be happier with a j113 over a j111 if you have em. this article may be helpful, or it may muddy the waters even more: sound-au.com/articles/jfet-design.htmby power supply decoupling cap i mean a big old electro cap from the supply rail to ground (say 47-220uf. I'd probably go with 220 myself). preceding it by a small resistor (say 100-680r. I'd go with at least a half watt resistor here simply because it would dissipate the most current in our preamp circuit. nothing wrong with overspecing parts in the power supply) would provide for more supply noise reduction |
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Post by Yogi B on Oct 27, 2022 14:11:01 GMT -5
gotta think about what happens at the diodes during the positive and negative swings of your audio. I am, and those example circuits make it easier to reason about (no cap between the gain stage & the diodes). Again: for the diode going to the positive rail to conduct, the voltage on the output would need to be at least diode drop above +15V; and for the diode to the negative rail to conduct, the output voltage would need to be at least a diode drop below −15V. This will never happen due to the op-amp (or similar stage) — the output of which is already limited by the supply rails to ±15V (at least for those simple stages without any form of supply bootstrapping). Why "silly", because it has them both before and after the gain? That should be a hint you're missing something. Yes the diodes do form a limiter, but it's not about limiting the output signal. Where are these diodes placed? Yes around gain stages, but between them and what? Input / output jacks — the outside world: full of bandmates, klutzes, the ill-informed, the intoxicated or other fallible meatbags with access to voltages greater than 15V in magnitude, which if applied to the pins of the op-amp will very likely cause damage. The aim of these diodes is to protect the op-amp from all this and instead shunt large incoming voltages to the power rails. Now although this solves the immediate issue of protecting that specific pin of that specific chip, of course now you've opened the flood gates. Shunting the potentially harmful voltage on the power rails means you now have the inherent problem of potentially harmful voltage on the power rails. If they have some form of overvoltage protection across them akin to a Zener diode then you may be fine, but it's probably a better idea to use Zeners directly at each input so as to nip to problem in the bud (these can also then have a breakdown voltage lower than the supply voltage, as it's unlikely that you actually need to accept input signals all the way up to the magnitude of the supply.)
Further reading: Input Protection Circuits (Another sound-au / ESP link). |
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Post by frets on Oct 27, 2022 14:24:00 GMT -5
Thanks Trag for the clarification and the web page. It is very interesting but over my head. Here is Trial # 3 of the diagram. I am assuming the pots off the power rail to each JFET are trimmers and not pots (you know what I mean). I am confused as to Yogi’s discussion pertaining to the location of the diodes. As it practically applies to the diagram. Should I add a diode at the input? I am not seeing that configuration in the article I read. Nonetheless, is it a best practice to add such a component at the input juncture; and if so, is that at the juncture of the Voltage input or Audio input? I assume from Yogi’s post it is at the Voltage input.  |
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Post by MattB on Oct 27, 2022 16:03:32 GMT -5
A J113 will work, but it probably won't have a lot of gain.  This is just a model of the first gain stage. Supply voltage is 12V and the drain for each JFET is at 8V. Input level is 200mV peak to peak.
If you add a source bypass cap a J113 has plenty of gain.
 For more gain, use a bigger drain resistor. This will lower the drain voltage, so you also need to increase the size of the source resistor. That reduces the current flowing through the JFET, which means less voltage drop across the drain resistor.  |
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Post by thetragichero on Oct 28, 2022 8:40:53 GMT -5
Here is Trial # 3 of the diagram. i should've been more instructive: your small resistor and the electro cap go between your supply voltage and your trimmer resistors |
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Post by frets on Oct 28, 2022 10:32:14 GMT -5
Are either of these correct? 
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Post by thetragichero on Oct 28, 2022 12:31:34 GMT -5
#5 is close but i doubt you have -12v on there (you're using a car battery so I'm assuming your power rails are 0v and 12v)
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Post by frets on Oct 28, 2022 12:59:53 GMT -5
So I just remove the -12 juncture? Is that what you mean? I was just thinking of the another ground for the battery but I see now that is redundant. Thank you for your help.
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Post by sumgai on Oct 28, 2022 13:34:19 GMT -5
frets, Stick with #4. If nothing else, it's easier on your budget. But now that I've said that..... Do keep in mind that this is nothing more than a pre-amp.... you'll still need to build a small power amp to drive the speaker. For that, I usually recommend using a LM386, which is good for a couple of watts at reasonably low distortion. But there are other good small power amp chips, you might start comparing what's in your bins and drawers with what's usable in circuits that will meets your needs. Still, there is one thing you need to consider, seeing as how a battery is involved.... Output power will have a direct effect on playing time. More power will equal a shorter battery charge duration. Think of it as a real-life example of Ohm's Law.  HTH sumgai |
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Post by frets on Oct 28, 2022 14:51:26 GMT -5
Sumgai,
Thank you for your help.
No, I had no idea this was just a preamp and that I would be needing an amp circuit as a companion. I’m now depressed. I want this to be loud enough to run a good sized speaker with some power.
I have no idea how this would even hook into an amp. Does the output run to the input of the amp?
So now I have to find an amp circuit with sufficient power that runs on a 12v.
This is more complicated than I thought it would be.
Thanks to all that helped.
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Post by frets on Oct 28, 2022 15:11:45 GMT -5
Okay, I found an amp. It runs on an LM1875 but only takes 9 volts of power. What do I need to get this one to work with 12v? First, I should have asked is this an appropriate circuit for what I have in mind?  |
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