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October 05, 2022, 04:42:18 AM

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Help with the Clipping aspects of this SS Amp Circuit

Started by SemiConductive, April 05, 2022, 05:28:56 PM

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SemiConductive

Thanks. I'll ask you one question, as I'll need to study a bit to ask more intelligent ones. (I'm really light when we get beyond basics and 'following directions').

Why do we invert at IC1B? It it because "that's the way a 4558 rolls"? Or, is it to accommodate what the power amp wants to see and/or another inversion happening there?

SemiConductive

Update on the hands-on side: Successful test!

I lifted the diodes from ground and put a toggle in there so I could do back-to-back testing.

The diodes were definitely clipping at fairly low signal levels... as soon as the gain pot was set to above minimal. By disconnecting them with the switch, I can get a lot more clean headroom, which is what my goal is for this amp... and as expected.

Turning on the "overdrive" switch in the panel and bringing IC1-B into play bumps the volume fairly cleanly. But with that on and the gain turned up high you still get into distortion territory, again as expected.

There is a little bit of a treble cut when you switch on IC1-B, even at low levels where it stays very clean. Very slight cut, just taking off the upper edge a little. Not unpleasant but a minor change. Perhaps the resistors and caps hanging above IC1-B are doing some filtering when it's on? Or maybe it's the opposite... there's filtering when the signal runs fully through the resistors/caps above with IC1-B shut off and turning it on eliminates that? I'm a little confused by the fact that the caps appear to be "in-line" (not going to ground) and I would think they'd filter bass, if anything, in that configuration.





phatt


phatt

Quote from: SemiConductive on April 10, 2022, 02:47:27 PMUpdate on the hands-on side: Successful test!

I lifted the diodes from ground and put a toggle in there so I could do back-to-back testing.

The diodes were definitely clipping at fairly low signal levels... as soon as the gain pot was set to above minimal. By disconnecting them with the switch, I can get a lot more clean headroom, which is what my goal is for this amp... and as expected.

Turning on the "overdrive" switch in the panel and bringing IC1-B into play bumps the volume fairly cleanly. But with that on and the gain turned up high you still get into distortion territory, again as expected.

There is a little bit of a treble cut when you switch on IC1-B, even at low levels where it stays very clean. Very slight cut, just taking off the upper edge a little. Not unpleasant but a minor change. Perhaps the resistors and caps hanging above IC1-B are doing some filtering when it's on? Or maybe it's the opposite... there's filtering when the signal runs fully through the resistors/caps above with IC1-B shut off and turning it on eliminates that? I'm a little confused by the fact that the caps appear to be "in-line" (not going to ground) and I would think they'd filter bass, if anything, in that configuration.

Good to hear,
Yes at higher levels the power chip will distort,, as well the power rails might be sagging a little bit due to the bigger current draw.

You can add a small value pot in series with the diodes if you want more control.

Re R5,R6 and C6;
These are not in the signal path, R sets the gain of the stage and C sets the hiFreq roll off.
you could lower the value of C6 which may balance it out a bit better but if you want the same freq response then you would have to switch the cap value to match the resistor change.
Likely not worth messing around with it.


Just search with *Opamp basics* or something similar as there is a stack of info on the web.

Here is a site with a massive amount of info on audio electronics.
https://www.sound-au.com/index.html
Click on *Articles* and search those for info.
Some of the *Projects* also have a good explanation of how they work.
Rod has written this site for folks like you (and Me :-[ ) with limited knowledge.
Phil.

SemiConductive

Trying to understand what you said about R5/6 & C6 not being in the signal path...

So, they are doing some filtering, but the "signal" passes through pin 5(6) of IC1-B? And R5/6 & C6 are not "signal path"?

What I'm a little confused about is how the filtering is operating. I understand how an R/C network filters off treble e.g. a "tone control" (capacitor is grounded, bleeds off high frequencies, resistor limits bleed to cap). But in that scenario, the cap is always bleeding treble to ground. In this case it looks like it's bleeding it around IC1-B (?).

I'm also familiar with using a small cap to filter bass... and that looks a lot like the arrangement of R6 & C6, but again, turning on IC1-B filters more treble, not more bass. So if I carry that logic through, is R6 & C6 filtering (more) bass around IC1-B? So it's not reducing treble, it's adding bass?

Or have I missed this entirely - and R5 is involved in the filtering too... and somehow the grounding through Q1 is pulling off treble? I thought the Q1 circuit was strictly handling the foot (overdrive) switch. Does it do something else?

Thanks,


phatt

I said,
Re R5,R6 and C6;
These are not in the signal path, R sets the gain of the stage and C sets the hiFreq roll off.

*This is the FEEDBACK path Not Signal path.*

You said:
" In this case it looks like it's bleeding it around IC1-B (?). "
YES Correct ;)

C6 basically dead shorts hi freq (set by the value of Cap)
So the stage does not amplify above a set freq.
Higher values will wipe off at a lower freq.

Valve Amps used to have a Presence control built into the power amp stage.
Simply by adjusting a pot which progressively added or subtracted a Cap in the FEEDBACK path, More cap less treble.
Nothing magic Just another place to alter tone.
------------------
If the tone knobs can't give you good results then it's a matter of tweaking what I call *Whole System Tone*
The feedback path is often used to do this.

Side note;
 I did try to simulate your tone control setup but it does not seem to work well.
I can't read some of the values so I guessing.
If you can give me the right values I maybe able to work it out. 
--------------
Something that is often missed ,,well at least I did when learning how this stuff works, LOL;
When you look at a schematic you are actually looking at 2 circuits intertwined on one drawing.

There is the AC path, (signal you hear) Which floats on the DC potential which comes from your DC supply. You have to set the DC points so as to pass the best signal. 

As I mentioned before just Google stuff.
Also down load a sim program and just fiddle with circuits relating to your Q's.
Often sims come with a library of circuits to help you get started.
Meantime trawl through old book stores you may find some electronics books.

I believe you can download Art of Electronics in PDF.
I have that book and it is not too hard to grasp.
How fast you learn depends a lot on what you already know.

For me it was a lot of reading, and Bread board testing.
But once I used sims my understand went ahead very fast. 8)
When I build/design a circuit I simulate it while also breadboarding it. The advantage is you not only see a plot on the screen but then you can hear it live.
Sims are not perfect but seeing and hearing at the same time speeds up the build process big time.

with BBoard and Sims you are able to tweak stuff to taste as well as find all the problems before you commit to a PCB. 
Phil.

SemiConductive

Ah, light bulb moment... so instead of bleeding the high frequencies to ground (like a tone control would), C6 lets them bypass amplification, thereby having the same rough effect... they don't get amplified and the tone is darker (but preserving at least some of the highs in the output).

So, if I were to change C6 to a smaller value, EX. 250pf, there would be less treble loss when IC1-B is turned on.

I'll head in there in the next couple days and try to get values on the tone stack and as many other components as I can read. If you don't mind mapping it out, that would be great. I usually try to fit tone-stacks into Duncan's Tone Calculator, but this thing doesn't look anything like the half dozen models they have.

Thanks for the pointers. I'll start looking for books as well as the sim programs.

Tassieviking

#23
Some quick reading, and playing with the calculator will help to understand maybe ?
https://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php#answer4
There are several calculators on this site for easy learning, just have to look hard for them.
Another good place to learn:
http://sim.okawa-denshi.jp/en/opampkeisan.htm
http://sim.okawa-denshi.jp/en/Fkeisan.htm
There are no stupid questions.
There are only stupid mistakes.

Tassieviking

There are no stupid questions.
There are only stupid mistakes.

SemiConductive

#25
Thanks, that is a little better. The one I had was sent to me by Epiphone/Gibson support!

EDIT: Took the better diagram you referenced and spent some time in the graphics program to clean it up further. I haven't edited any component value references... that will be phase 2. But this should be easier to read:

https://i.imgur.com/WjfO8n1.jpg

phatt

Here is my edit of Epi-Regent schematic, there may well be things I've missed.
Adding a screen shot of the tone curves.
Note; looks like C11 is 220nF which seems to work in the Simulation.
Screen shows 3 plots;
Yellow trace; Treb 10, mid 1,  Bass 10
Green trace;  Treb 1,  mid 10, Bass 1
Blue trace;   Treb 5,  mid 5,  Bass 5

Could be improved if Mid dip was shifted closer to 400hZ
Phil.

SemiConductive

Thanks, @phatt. I am studying that and will compare to the sound samples I'm getting. Seems to make sense.

Couple more questions: To shift that mid dip... to 400hz as you suggest... is that a one or two component swap, or a rearrangement of the whole tone stack? I've played with stacks a bit in Duncan's Tone Simulator, but this thing doesn't really fit in there.

Back on the capacitor C6, which is effectively filtering out treble: I've installed a switch and a pot for the diodes and played with that in conjunction with the overdrive (IC1-B) switching. It really is cutting a lot of treble. I think they were going for that dark boost tone. Misses the mark.

I know I can change the value of C6 to get more treble. Is it reasonable to just life C6 and see what it's like? Just "eliminate" the bypass? I can also try some lighter values but I'm wondering if it's safe to go to infinity.


Tassieviking

G'day SemiConductive
I'm fairly new to audio circuits myself, but I have heaps of experience with industrial electronics.
The way I understand the active inverting low pass filter of IC1B is as follows:
The gain is set with R4 and R6(and R5), we don't want to mess with this, unless you feel there is too much gain with the pedal engaged. then increase R5 till you are happy with max gain. (Pot ?)

So R6 is 150k, or 46.78899k with R5 in the circuit.
My understanding is that you calculate this the same as a RC low pass filter.
Capacitance is 470pF
Look at this page, the same formula for normal RC Low Pass filter as an Active Inverting Low Pass Filter like yours. So we can use the top calculator for frequency drop off.
https://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php#answer1

With R=150k and C=470pF you have a drop off frequency of  2.26 kHz
With R=46.78899k and C=470pF you have a drop off frequency of 7.24kHz
If you want 7.2kHz with the R=150k you have to change the capacitor to C=146pF (150pF)

You can easily put in a switch to change the frequency roll off capacitance to set it were you like it, rotary switch you get heaps of otions, but a bit overboard

But maybe I misunderstood the whole thing and got it wrong, If I did can someone please give me a hard smack over the ears please, That way I will remember next time.
Michael
There are no stupid questions.
There are only stupid mistakes.

phatt

yep that's sounds close to how the maths work. you are obviously already better than me at the maths,  :-[
Just remember that this only tells you what the rolloff point is at that point in the circuit.
By the time it gets to the speaker it will have altered, often by a large amount.
Much like the Duncan tone stack app as a good example, Yes a big help but only tells one part of the whole system tone shape.
Phil.