Introduction, a question and a hope for some insight / help

[Lex]

Phil & Joe thanx,

the purpose of re-building this old cheap Champ amp is purely to learn, understand and improve the sound and my knowlegde, if possible. 
Im re-entering music again after many years.
This time Im gonna learn to understand and not just interpret someone elses music. Or gear..

While drawing I did not pay attention to the polarity of diodes as Im not used on their graphical appearance.
But I do assume that all diodes should be wired in the same way, probably positive (+) as input and negative (-) as output.
I think this was a wrong assumption now.

So this explains where I made a mistake:

" wiring back to back in the loop" and
  + -
<     >
  - +

I have attached changed polarity for S1 section of diodes, I hope I understood your comments gent's,

Thanx

Greetings

[phatt]

Yes much better

There are 2 basic ways to do this Distortion trick.
1/ across the signal path and 2/ in the feedback loop.
Although your setup is in the Fb loop this page might help you understand that it's not just about what fancy pants diodes are used.

This explains the finer details of how diodes work across the signal, often driven by an opamp stage.
https://sound-au.com/articles/soft-clip.htm

In your case you might do well to back off the gain of the input stage as that will slow down how early the amp goes into distortion.
That may well be easier to do that muck around with a whole lot of diodes.
Phil.

[phatt]

Oh I forgot the obvious page,
Here is a good explanation of the TS9 circuit by R G Keen of Geofex;
http://www.geofex.com/Article_Folders/TStech/tsxtech.htm
It explains how BB diodes work in the fb loop of an opamp which is the type your circuit uses.

FWIW,
I've come to the conclusion that I'd rather work with a single CLEAN channel amplifier and just use pedals for all the other FX.
My main working amp is not even a guitar amp,, it's an old Keyboard amp with treble bass and volume, but does have a real spring reverb. All the fancy tricks are on my pedal board and forgive me for blowing my horn here but half of those pedals are my own designs or modified brand name units. Took many years to work out what I wanted,, an sure many of those designs ended up as land fill but in the end I got exactly what I wanted.

WHY pedals?
Cause I'm yet to meet a player who uses multi channel amps and totally happy with all channels.
Unless you are rich and can afford to have someone custom build exactly what you want then it's likely most of the off the shelf shop rigs will leave you less than impressed.

I spent years trying to get an ALL in ONE amp to work for me but in the end I resorted to pedals. I don't like pedals but for me I'm playing many different styles of music and the pedals cover most of what I need.
Sadly pedals are an evil necessity for me,, it's a love hate thing.

Reason 2/
 It's a heck of a lot easier to swap out pedals than it is to have to rebuild amplifiers all the time until you find the sound you want.
My rant over for now,, hope it helps
Phil.

[Enzo]

Well you don't have to turn all the diodes around, just turn the two-pin connector around.

[phatt]

Well you don't have to turn all the diodes around, just turn the two-pin connector around.

Ha Ha very good.
Thanks Enzo trust you to see the simple way to do it.

BTW I've just been studying the Schematic for this Amp and it's dawned on me that the output of power chip (pin1) connects to both +&- terminals of the speaker. 
Maybe I've missed something but might be a mistake.
Also regards to the amp distorting even at low volume;
I just simulated the preamp and a 100mV signal at input does not distort until you turn up past halfway.
But I notice the FB resistor around power chip is 130k.
surely that is rather high and I'm guessing would distort even at low gain. (And that was the main reason for *Lex's* initial post)
The schematic is not very clear and I can't read the part number on the power chip.
Or maybe I'm just not tecky enough to know what I'm doin,.
Phil.

[Lex]

 ok Gent's, thanx again.
 
 I fully understand comments.
 
 Those links are very usable. I need some time to read it and understand it, its not easy.
 
 And yes I agree about the single channel amp.
 Im running álso  Laney K30 single ch. keyboard/guitar amp (their first SS amp from 4 decades ago, got it almost unused) and Zoom FX thingy.
The sound is much much better than playing through those 2 small 15W Fenders on 12 inch speaker.
 Difference like day and night.
 However, tweaking that Champ Squier remains as a challenge to me.
 
 Enzo, yeah that's obvious!
While drawing didn't recognise that solution (not used to draw circuits) but in the real life I think I would see it myself.
 Thanx for your comment.
 

Now the second reply based on Phil's last comments;
 

BTW I've just been studying the Schematic for this Amp and it's dawned on me that the output of power chip (pin1) connects to both +&- terminals of the speaker. 
 Maybe I've missed something but might be a mistake.

Unfortunately I cant see that on this schematic.
 But I suppose it's a weird construction?

 
 

Also regards to the amp distorting even at low volume;
I just simulated the preamp and a 100mV signal at input does not distort until you turn up past halfway.
But I notice the FB resistor around power chip is 130k.
surely that is rather high and I'm guessing would distort even at low gain. (And that was the main reason for *Lex's* initial post)
The schematic is not very clear and I can't read the part number on the power chip.

Out from my curiosity, how do you simulate a pre-amp ?
Some software where you put and connect all the components and then observe amplitudes .. ? (cant imagine sound)

And yes there is a slight distortion on the clean channel, in combination with a 'flat' sound.
Second Gain pot all the way turned to 0.
I did not drive this amp more than 1.5-2 on the volume pot.

If you could point to this 'power chip' so I know where and what is it, then I could make some tweaks in the photoshop to get more clear reading of schematic?

Im enjoying emojis too, they are great !!!

Greetings

[phatt]

Hi Lex, this might be helpful.
Basic opamp designs explained,,minimum maths.
https://neatcircuits.com/op_amp.htm

It's beyond the limits for most of us to give a blow by blow description of how it all works but briefly;
You signal which is an AC voltage floats on a DC voltage potential. (your DC supply Rails)
Each stage amplifies that AC audio signal.

From left to right,
U1 is the input pre,,,U2 is the clipping stage,, u3 makes up for the losses created by having a passive tone control.
U4 is likely not needed but hey they don't make a chip with 3 opamps so why not use the 4th as another buffer.
the 4th chip on the right is the power chip which is just a much higher current opamp to drive the speaker. this is the one that bolts to a heat sink. In some small cheap amps it's just a slice of alloy.

The 130k fb resistor on the pwr chip sets the gain of the power amp along with other parts. If that is what is in your amp then no wonder you can't get a clean sound.

As the schematic is likely drawn wrong, i.e. pin 1 of power chip connects to both Pos & Neg of the speaker terminals so as drawn it can't work.
Of course it obviously does work so I have no way of being able to work out how the circuit is actually wired up.

The resistor I mention looks like it reads R19- 130k but who knows what is actually in your circuit. looks like it joins pin 1 and pin 8 on the power chip.
(I think)

I've added this pic to show what I think is a mistake on the drawing.
If the red marked link was removed then the circuit makes more sense.(R19 is just to the left of that red cross.
Phil.

[phatt]

regards to sims,, FANTASTIC learning/ teaching tool for noobs like me who wish to understand what the hell goes on inside amplifiers.
And you can't  blow them up if wired wrong or burn yourself with hot irons 
Your preamp circuit looks like this with Circuitmaker,, it's not top shelf sim but it's all you need to come to grips with basic amp circuits. (even does Valve circuits if you want)
Cheers Phil.

[Lex]

Hi Phil,

thanx .. im overwhelmed with your reply and need time to read and understand.

Mostly intrigued my the last 2 schematics, as I can see you have changed the positions of 4 diodes / resistors within the circuit ?

So this gives different amplitudes (and the sound as the final output).
What does represent the 1st simulation and what the 2nd in terms of sound?

Regards

[phatt]

Hi Lex it's ok ,, i've got time at the mo as I'm waiting for a bathroom reno to be finished so can't do much else cept wait.

Although some parts might be in a slightly different position you will find it is electrically the same thing.

Pic2 just shows how the signal swing gets bigger and smaller as it passes through each section.
in this case the ,,gain 1 pot (R6) is set at 70% rotation, while gain pot 2 (R8) is set at 40% rotation.
The input voltage is 100mV and the output at U4 (Yellow trace) is still under 200mV so the preamp is basically clean until gain2 (R8) passes 60% rotation.
Which is why I'm fairly sure the preamp is working mostly clean so if you have distortion at low level then  I would look at the schematic of power amp and see that R18 which sets the gain around the powerchip is 130k and my guess is that resistor is way too large and hence the amp distorts long before the diodes conduct in the preamp. Normally R18 would be about 20k~ 50k max.
the other screen shot is the tone response curve at different points on the schematic which gives you a clue as to what frequencies are being amplified.
The tone controls are set with Bass Full up Mid full off and Treb Full up.
so the output at U4 (top of R17) is the purple trace of the frequency curve from the preamp. 
Phil.

[Lex]

Ok Phil thanx,

still looking for the time to get into it.

Tell me please, that piece of the software is 'Circuit Maker 2000' ?

Could you share with me the 'project' file so I could import after the installation and test / learn / play .. ?

Im still not so familiar with the signs so it will take me weeks figure out

Greetings all and have a good Sunday

[phatt]

No worries Lex, Happy to help.
Yes mate That is the program I use.
How fast you learn to use it depends on how much you already understand about electronic circuits.
But as I mentioned what's to loose because you can't blow anything up and you can't kill yourself by touching high voltage or burn your fingers on a hot soldering irons. 
You will still need to read up on some of the basics of audio circuits, a lot can be found on the dub,dub,dub and help is also here of course.

The program comes with a whole library of circuits for you to learn from, some of which will be useless for Audio but no matter just start using it and don't expect to understand it all in a few weeks. It took me 6 months to fully grasp what the hell I was doing and it does send you loco at times. 
Be warned it's a highly addictive hobby, And I'm just so grateful for the friend who introduced me to simulations years back. Even though i already had a fairly good grasp of how amps worked, sims just saved me years of building land fill.
As they say "a picture is worth a 1,000 words"
check your pm's, Phil.

[Lex]

Thanx Phil,

u got a msg back

Cheers

[Lex]

Hi all,
I can't report any progress as I lent this amp to a friend and he is enjoying it like it is.
So I'll be back with more stories .. if I dont sell it to him.
I'm discovering the wonderful world of amp's so I would not be surprised if I just move to something else. (this means more amp's, not less..).
Actually Im quite interested in a vintage Peavey Studio or Bandit .. well will see,
Stray strong all and away of Corona (beer is quite refreshing tho!)
Cheers
Lex