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Building a TDA7267A guitar amplifier with overdrive

Started by dazz, January 04, 2018, 08:53:12 AM

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dazz

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?

phatt

Opamps were designed to work within a window of psu voltages,, sometimes mentioned on data sheets. That is why they can be used in pedals (9Volts or less) and up to 30Volts or more. Some chips are designed to work down to 3 or 4 volts. (TL062 is one such chip) So 12 or 15 volts makes no difference, just use what value works for you.
Anywhere from 220k up to 1 meg, the higher the value the more sensitive the input.
Phil.

phatt

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.

dazz

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)

dazz

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)

phatt

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.

dazz

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

phatt

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

dazz

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

dazz

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.

dazz

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

phatt

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.

dazz

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

dazz

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

dazz

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.