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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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phatt

Maybe add a 10~47uF cap on the supply.
With such a small case you have to watch out for inductive coupling which can cause all sorts of problems.
Also if the battery is old that might cause problems.
Better minds here may know more.
Phil.

dazz

#61
Quote from: phatt on February 19, 2018, 08:19:11 AM
Maybe add a 10~47uF cap on the supply.
With such a small case you have to watch out for inductive coupling which can cause all sorts of problems.
Also if the battery is old that might cause problems.
Better minds here may know more.
Phil.

Thanks Phil. I already have a 100uF cap at the supply, per TDA7267A schematics. Pretty sure the battery is at fault, it's fresh but a cheap one. Measures 9.5V unplugged, goes down to 8.8V as soon as I connect it to the amp, and down to 7.7V when I strum a chord at full volume.

Well, sorry to be such a pest but I have yet another question about the larger amp (Casino 12 + TDA7297). I bought me a power supply, one of those universal laptop chargers that runs at anything between 12 and 24V which seems to work really well and dead silent after grounding the DC negative output.
I don't think I want anything over 15V through the TDA7297 since that's when it reaches 10W and it starts to distort (according to the datasheet), but let's say I add a DC input jack so that I can run the amp off the internal 12V psu or this external supply: that would put 15V (or more) at the output of the disconnected internal 12V supply. Can this damage the 12V supply? Also, what's the maximum Vdd I can feed your Casino 12 please?

phatt



Quote from: dazz on February 19, 2018, 08:57:22 AM
I don't think I want anything over 15V through the TDA7297 since that's when it reaches 10W and it starts to distort (according to the datasheet),

whatever the max supply voltage reads on the data sheet,, don't go above that.

Quote from: dazz on February 19, 2018, 08:57:22 AM
but let's say I add a DC input jack so that I can run the amp off the internal 12V psu or this external supply: that would put 15V (or more) at the output of the disconnected internal 12V supply. Can this damage the 12V supply?

Sorry but without a drawing there is little to go on.
If the internal suppy is *Disconnected* then I assume there is no problem powering from an external supply.
Most pedals have a switch built into the DC socket so when you insert an external wall wart the battery is disconnected.

Quote from: dazz on February 19, 2018, 08:57:22 AM
Also, what's the maximum Vdd I can feed your Casino 12 please?

Whatever the data sheet reads for the chips you are using.
Most will run on 30VDC or 15-0-15VDC.
Phil.

dazz

#63
Yeah, I should have figured those out myself, sorry about the dumb questions.
I knew in the back of my mind that 3rd lug in the DC socket was there for a reason *blush*

dazz

I'm afraid I have another question (hopefully this one is not as dumb as the previous ones). I don't have many 10K resistors right now so I'll be using something higher for the bias resistors. I understand R7, R17 and R18 are bias resistors, but what about R14 in the 3rd opamp? There was a 10K resistor there in the original schematics so I'm not sure if that needs to be matched to the value of R7, R17 and R18

phatt

As long as R17 and R18 are the same value it will still give you half voltage at the bias node. R2, R7, and R12 give each input a ground reference to work from so the signal can swing evenly pos and neg of the bias point.
(opamp basics, the positive input always needs a DC path to reference, in this case it's the half voltage bias node)

R7 can be a bit higher or lower than 10k,, lower values may pull down the gain, too high and it may effect the filter network in front of it.
R12 needs to be as big as possible, lower values there will effect how well the tone controls work. Try 2meg if you want.

R14 along with R13 effects the gain of U3, higher values at R14 will lower the gain while higher values of R13 will increase the gain.
R15 and C16 pull a bit more treble from that stage.

If you use a bread board to test you can tweak the values till you find the best outcome for you needs. 8|
Phil.

dazz

Thank you for the detailed explanation and your patience too, Phil  :cheesy:

dazz

The parts finally arrived and I'm already prototyping the first stage of the preamp on the breadboard.
And of course I have more questions  :-[

I've been reading about caps and how ceramics are not the best for audio since their non linear, but the smallest film cap I have is 330pF. There's a bunch of 50pF in the circuit, can I use ceramics for those or should I shop for smaller film caps?

Also, the 1uF electrolytics I have measure about 5% ESR. For what I could gather this is normal for 50V 1uF, but what if I parallel two 47nF to half the ESR? is there any benefit to that?

dazz

Well, I get no sound from the first stage straight to the speaker. I've double checked the wiring and it seems to be OK

(see attachment)

EDIT: nevermind! I run it through that tin can amp I built and it's definitely working and amplifying! woohoo!

dazz

Quote from: phatt on February 14, 2018, 09:51:58 PM
R2 can be anywhere from 220k up to 1Meg.
The input Z will always be a lower number than the value of R2,, but not much.
The higher Z you go the more sensitive the input but high Z can be problematic if you have lots of hi gain pedals.

Try a few values and see/hear for yourself,, most will pick 1Meg because you get a bit more perceived volume but at the expense of early distortion in the power amp.
As your power chip is small and only running 12 volts then there is not much headroom.
Phil.

Something just occurred to me. What if I swap R2 by a 1M pot and use that as a volume instead of the 10K VR4 as in the attachment circuit? That way I can control the level of the signal at the preamp output to avoid clipping there, right? I plan on adding an overdrive stage before your Casino preamp, probably some JFET based pedal, with a switch to bypass it for pure clean sound

phatt

Hi Dazz,,Yes you could do that but when the OD circuit is bypassed that 1 meg pot becomes passive, much like the volume pot on your guitar and I'm not sure if that is a good idea but try it out first before you commit to a build. :-X

Normally an effects pedal has (or should have) a level pot to control the output level to control the output strength.
Phil.

dazz

Quote from: phatt on March 02, 2018, 03:48:32 AM
Hi Dazz,,Yes you could do that but when the OD circuit is bypassed that 1 meg pot becomes passive, much like the volume pot on your guitar and I'm not sure if that is a good idea but try it out first before you commit to a build. :-X

Normally an effects pedal has (or should have) a level pot to control the output level to control the output strength.
Phil.

Sorry, I said the idea is to "control the level of the signal at the preamp output" when I meant at the preamp input, obviously. But anyway, what do you mean by "that 1 meg pot becomes passive" please? Wouldn't it simply act like a variable input sensitivity or input impedance at the Casino preamp? I'll try it and see how that goes. The thing is that I'd like to keep volume and gain pots to a minimum but also have that preamp input signal control working both when the OD circuit is switched on and off, so I can have the best of both worlds: maximize sensitivity (and volume) or lower it for hotter/active pickups

phatt

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. :-X
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.

dazz

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. :-X
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.

Understood, thanks so much Phil. No, I don't own any guitars with active pups, it's just that that previous post of yours got me thinking about input impedance because I'd like to maximize the amp's output but also avoid clipping at the preamp. I have a strat with fat 50's style pups that are quite hot, so I'll use that as a reference and pick the highest R2 so that the amp still stays clean. I guess I'm overthinking it a bit since for what I know half the input impedance / wattage is just 3dB quieter. At most I might put a switchable resistor in parallel with R2 for low/high impedance

So yeah, that settles it then: no low input impedance input and no volume pot at the preamp input.  :tu:

dazz

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. :-X
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