Quote from: phatt on March 18, 2018, 04:39:00 AM
Who cares what it looks like,, it works Great to hear. :dbtu:

Re the tone controls; Here is a picture of the wiring.
As should be obvious if you want the mid in the middle you will have to work that out.
This way the wires just jump to the next pot.
If you notice a lot of the old valve fenders have the mid on the end easier to wire up I guess and less chance of mistakes.
Phil.

Excellent! that should clean up some of the wiring mess. Perhaps I can try bypassing the board's tone stack and moving it after U3 so that it's output goes straight to the volume pot hence saving another wire to the board (guess I'll need a coupling cap). That should make it easier to troubleshoot too

Well, the amp is finished and I'm quite happy with it considering it's my first build, I mean the thing is a mess but it works which is more than what I expected  :lmao:

I ended up using that gain pot in the first opamp as Phil suggested instead of adding another stage, & another board for the overdrive, just to keep is simpler, and I like the overdriven tone, it's definitely better than the OD of that MPF-102 JFET I built before.

The tone stack still needs work, not sure what but I'm missing something. It's usable anyway. The bass control works fine, although it doesn't have too much range. The mid works great but the treble is still behaving like a plain volume control. I'll see if there's anything I can do about that.

Thanks once again to all of you who helped me pull this off, specially Phil.

Here's a pic of my brand new Ghetto Casino 12. Please don't laugh

Quote from: phatt on March 07, 2018, 12:29:14 AM
WTF? a 3 inch speaker has little chance, a banana might work better.  :lmao:

Re changing the response curves.
From what I understand you are building a preamp to run into a small low wattage power chip that is running on maybe 12Volts.

If so, then when you max out the low freq at the preamp then that little power chip has no chance of producing a balanced response. All that you will get is early distortion and mud bass when you crank it up loud. xP

If you want big bass you need a power amp with headroom and that means big PSU.
and you need to get a real guitar speaker of at least 8/10 inch in a cab otherwise you are getting a false impression of the real sound/tone produced.

To give example of what is going on;
Take a 1,000Watt pa setup,, It takes 700W to drive the low Fq, 200W mid band Fq, 100W for hi Fq.
It takes a lot of energy to drive low freq.
That is why a guitar player using a 30 Watt amp can sound as loud as the bass player using a 400W rig.

The classic Marshall Amps from years ago had little low Fq below 100hZ and took a dive up around 3 or 4khZ. that is called tone focusing.
All the energy is focused in that central area and gives much better clarity/Note definition.

Now most guitar crap has way too wide bandwidth and once you add all the dirt pedals it becomes too cluttered with harmonic crud. If you want great guitar tone limit the bandwidth. 8|
Phil.

Damn, makes complete sense.
The power amp is a 15W TDA7297, 32dB of voltage gain, driven by a  12 to 15V / 5A power supply which I think should be plenty. I initially planned on using a much smaller TDA7267A, but then I found out about that other one and went with the more powerful TDA7297. Not sure if that changes anything?

Unfortunately I don't have access to a proper cabinet to test the amp at high volume, but I might be able to borrow one

Any chance you can share the Spice file for the circuit please?

BTW, I tested my crappy 3'' speaker response using the function generation app and anything bellow 250 Hz sounds like crap. No wonder the coupling caps mod seemed to make no difference to the listening. So I wired the output to take stereo headphones and plugged some good ones with proper bass, and man, this little monster sounds great!

Success! I eventually upped all C1, C4, C6, C9, C13 and C15

Result:

50Hz: 0.8VV
100Hz: 1.6V
200Hz: 2.3V
300Hz: 2.8V
400Hz: 3.2V
500Hz: 3.5V
600Hz: 3.7V
700Hz: 3.8V
800Hz: 3.8V
900Hz: 3.9V
1KHz: 3.9V
2KHz: 3.9V
5KHz: 3.4V
10KHz: 2.1V

So it seems the -3dB threshold is somewhere around 150Hz when it was at a little over 300Hz before.

I've been playing it for a while today and it seems to lack some low end. May be the speaker, but I decided to try something: built an 8 ohm dummy load with a few 10W resistors in parallel, then used a function generator app and measured the RMS voltage at the output of the preamp. Here' what I got:

50Hz: 0V
100Hz: 0.3V
200Hz: 1V
300Hz: 1.8V
400Hz: 2.6V
500Hz: 3.2V
600Hz: 3.2V
700Hz: 3.9V
800Hz: 4V
900Hz: 4V
1KHz: 4.1V
2KHz: 4.2V
5KHz: 3.6V
10KHz: 2.2V

I've read I can increase the value of the coupling caps to get more bass, those are C1, C4, C9, C13 and C15, right? or is C6 also a coupling cap?

EDIT: upped C6 to 220nF and lost most of the signal, so that's going away

Quote from: phatt on March 05, 2018, 10:52:24 PM
Don't worry the best learning tool is a mistake. :tu:
It's hard to blow up opamps so you will be fine.
With high current circuits like discrete power amp stages you have to be much more careful. 8|
Phil.

Good to know, it wouldn't be a huge deal if I blew up an opamp, other than it probably would have me scratching my head trying to figure out what went wrong.
Anyway, I rewired the pots but it still isn't quite there. So I think I'm giving up on debugging this particular board. The tone stack is still all wrong. I'll try bypassing it, and increasing the negative feedback resistors (at least U3's) to experiment with those things and see how much more gain I can get from it, so I'll use it as a pure gain module. Then I'll breadboard the tone stack and that will go in another board somewhere. Probably where the power supply is right now, since the tone stack wires seem to be picking up an awful lot of noise from it and I'm running out of real state for the OD stage anyway.

But before I do any of that I'm going to do some reading on opamps

I think I found the problem with the tone stack. I got the wire colors all wrong, duh!

Bypassed the tone stack and it works, it's louder too as expected, but doesn't seem loud enough. If I put my bazz fuss pedal in front of it, it gets much much louder, so I believe there's plenty headroom in the preamp. Right now I have 1M for the input impedance.

On a side note, I like the tone a lot!

I'll check component values and wiring.

Quote from: phatt on March 05, 2018, 09:12:44 AM
Turn the test board/circuit around the other way and turn the schematic up side down.
NOW re track all the wiring again.
WHY? simple it forces your brain to reconnect all the nodes in a different way.
A good way to see if you missed something.
Phil

Great suggestion, I'll do that, thanks Phil. Maybe I can also try to bypass the tonestack to check if the 3rd amp stage works as it's supposed to. I guess I should get tons of gain without the tonestack attenuation. That way I can narrow it down to a specific section of the circuit and double check everything there.

I don't have a problem redoing the whole thing if needed be. It's a complete mess anyway and this is about the learning process more than the end result

Quote from: phatt on March 05, 2018, 07:39:41 AM
I assume you have power to u3?  I did not draw everything as power and ground are often left out for clarity.
If using 2 dual opamps then you have to power the other opamp.

Tone pots are correct you can bridge top and middle of bass and mid if you wish.
Treble is a voltage divider while bass and mid are wired as a variable resistor.

Quick check with a meter on power pins and also check the 3 positive inputs,, they should read close to half voltage,, your bias voltage.
Phil.

Yes, I powered U3 and also "disabled" the other opamp in the the second IC putting bias V on the + input and connecting the - input to it's output.
I'll check voltages, and also the wiring again.

This is what the pots do right now

Treble:  does nothing at all
Mid: behaves as a volume
Bass: Like the Mid pot but doesn't roll of the volume completely

OK, it's finished and it works. Well, sort of. The tonestack is all messed up and I don't get much output at all.
Are these pot lugs actually supposed to be disconnected? (see attachment)

Quote from: phatt on March 03, 2018, 09:23:35 PM

Quote from: dazz on March 03, 2018, 01:55:24 PM
Phil, what's the taper of the volume pot please? I take it log/audio, right? I think I know the tone stack's: 250K's are logarithmic and the 10K is linear, correct?
Also, can I use a different value for the volume pot or is the 10K critical?

Yes not over critical but Audio taper is likely better.
Re Tone; Treble can be linear,, bass Log,, mid linear.

output volume; not critical,,, up to 100k if you want.
Phil.

Got it, thanks.
Well, it's almost ready, if I don't manage to screw up wiring the last opamp I should have it ready to rock today. My prototyping sucks though, the thing is a mess, hahaha. I've done some reading on perf-boards and learned that parasitic capacitance can cause some issues in audio circuits (cut higher frequencies). We'll see how it sounds, hopefully in a few hours (fingers crossed)

Phil, what's the taper of the volume pot please? I take it log/audio, right? I think I know the tone stack's: 250K's are logarithmic and the 10K is linear, correct?
Also, can I use a different value for the volume pot or is the 10K critical?

Quote from: phatt on March 02, 2018, 07:55:00 AM
All good,,,Yes I figured you meant preamp Input.

No because when the OD circuit is disconnected you would have to readjust the 1 meg pot to maintain levels, I doubt that would be user friendly.
Why passive?
Because if you draw a schematic of the guitar (passive volume) and the preamp input you will see that you then have 2 passive pots in the signal path. So as you need a level control on the output of the OD anyway,, add it there where it would be of more use.

Anything in front of the First Active element of an amplifier is effectively a passive component, So in a normal Electric guitar all the PU's and pots are passive as there is no amplification.
If you have Active PU's then that changes things a bit.
You might need to have 2 inputs like professional rigs have.
others here may have better options but in my Experience,,, I hate Active PU's :grr
Phil.

Phil, by how much would you calculate R2 could be increased from 470K if I fed the amp say 15V instead of 12V?
Also, I'm gonna try a few OD's, one of the candidates is this JFET Professor Tweed. I take it will color the sound somewhat even if the gain is low enough as to not clip it's output nor the preamp's? because if it doesn't I guess I could do without the OD bypass switch