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October 05, 2022, 05:21:51 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

Circuit link below. Sorry for the poor quality. This amp was not popular and this is the best I've been able to find.

Background: I have some basic understanding of SS components and circuits. But I'm really a newb. Trying to increase my knowledge and perhaps mod this amp.

Signal comes in and through IC A / 4558 and moderated by the gain control POT 1. It then flows down into the other half of of the 4558, IC B (IC B is switchable by panel or footswitch through Q1). From there, the signal runs past a pair of Red LED clipping diodes LD1 & LD2. But, they are unswitched/directly grounded.

Are they always clipping? Or are the Red LEDs chosen because they only conduct over a higher gain signal coming through?

Possible Mod: What I'm thinking is that if the diodes themselves had a switch to ground that I might be able to get additional clean headroom with the primary gain control. But, I'm also thinking that by running the signal hotter (no clipping at all) that perhaps it is going to overdrive the poweramp... and maybe there's not much clean headroom to be added because of that (?). Perhaps the clipping is a "required limiter" or sorts?  Any danger to the other circuit components by switch-controlling the LED's?

Also, there's a capacitor C9 in parallel with the LEDs. Is it shown there simply for circuit drawing convenience in the schematic? Or would it need to be switched along with the LED's if I switch them?



EDIT: Schematic cleaned and waxed: Zoomable circuit diagram: https://i.imgur.com/WjfO8n1.jpg

 



Enzo

Well, before deciding what they do and designing a switch circuit.  If you wonder what they do, unsolder one end of each and lift it out of the circuit.  Now you'll know.

An LED has a junction voltage.  Seems to me they start at something over 1 volt, and like blue ones up to well over 2v maybe even 3v.  I don't mess with them so I am not up to date on current LED types.   But until you hit that voltage, they don't conduct, so they only will clip when your signal level gets up to the 1.2v or whatever it is in your case.

Just my opinion, but I suspect they are not there as clippers, they are there as limiters.  I could be wrong.

SemiConductive

Thanks for the insight.

I'm OK with lifting them to test the sound... I'm just looking for confirmation that that's an OK thing to do. If they are limiters to subdue the signal of the dual 4558 amps and I'm going to fry the 1875 power amp by letting too large a current in, then I clearly wouldn't want to do it.

Also, I'm not sure what to do with C9 - leave it or lift it with the LED's. I don't know enough to understand, but I know enough to ask first :-).

Enzo

They limit the signal by not letting it get larger than the LED drop.  They don't act as an attenuator.  Like your car seatbelt limits you to not going through the windshield in a crash.  The only time they have an effect is on peaks.  They will lop off peaks when they get too tall.

Tassieviking

In my opinion, and it is only my opinion based on my understanding.
If you ignore the LED's,R13 and C9 would be a low pass filter, the LED's will clip the signal when it reaches the voltage to make the LED's conduct.
Look at the LED's while using the amp, full gain and the switch on overdrive and if the LED's light up its working as a clipper.
Q1 changes the gain of the op-amp, it puts R5 in parallel with R6.
If you want more clean head-room, you could add a germanium diode in series with each LED.
That would slightly increase the clipping voltage.
You could try different led's or different diodes in series with the led's to experiment until you are happy with the sound.
I think removing the LED's might not be a good idea, too much signal might get through.

Which amp model do you have ?
It appears that there might be 2 models using that circuit, the watts available depends on which power transformer is used.
There are no stupid questions.
There are only stupid mistakes.

phatt

I doubt you will blow the power amp chip by removing the leds.

I tend to think these little chips blow because quite often the modern metal shred players use very high out put pu's and thrash these chips to death. Often adding pedals to make it even louder.
Add to this it's a small amp and more often than not they have tiny heat sinks which are not up to task, so the combination of above will fry the chip.

At sane bedroom levels which these small amp are designed for you won't have issues.
So As Enzo mentioned lift the 2 Leds (or one at a time) and you will get more clean head room but as it's a small power chip it may not be much more because the power section will distort.

you can if you wish add a pot in series with the leds and that will allow you to dial in how early the leds clip.
(Leave C9 bridged from R13 to ground)

You could try better speakers but that will get expensive and still may not improve the SPL by much.

If you want more clean head room then you will need a bigger amp. Sorry
-------
Regards the C9 Q;
The cap is there to wipe off excess hifreq crud.
Valve amps wipe of hi freq by design while SS amps you have to add some hi freq roll off.
A lot of budget small amps don't bother to tweak this issue and hence sound very harsh Especially when in distortion mode.
Again lift it and hear the difference.
You will certainly have heaps more treble.
If the Amp is already bright then Leave it in place and just lift the Leds.
If you want, you can try different values on C9 to fine tune the top end. (lower values = more treble. higher = less bright)

One thing that may happen when you remove leds and engage the distortion.
The signal going to the reverb may start to distort the reverb drive. so you might have to raise the value of R201.

Just a note on the schematic;
Unless I'm missing something??? IC1B is labeled wrong.
Pin 5 is the Positive input and is normally grounded in this configuration,, It looks like it's + & - input labels are flipped. I can't read the label Numbers,, they both read 5 to me.
Hope it helps
Phil.

SemiConductive

Thanks all.

Makes sense. I had a home-built distortion pedal I made that had a pot just like that... controlling the amount of clipping. I'll probably try that at least as a test to find the balance between clipping and power amp distortion.

Thoughts on the proper value for the pot to reduce LED throughput to zero? 10K? 100K? 250K? I know I can just head to the big side, but if ya'll have a suggestion, I'll start there.

Thanks for the insight on C9. I thought it might be a filter with that resistor, but I'm weak in that regard... and I definitely don't understand that tone stack. Different than anything I've seen or in Duncan's Tone Stack Calculator. Also, I am familiar with the "harsh treble" issue for SS amps! All to common. This one works though - the amp has very nice tone / range.


phatt

You may only need a low value pot for the leds ~ even 1k would likely be enough but use the lowest value you have on hand.
Yes odd ball tone design but if it works then leave it.
Phil.

Tassieviking

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

Enzo

I just don't think they are there as an effect.  Just don't want to overdrive the reverb or whatever.  The power amp can only put out what the power supply rails allow.  Drive it too hard and it just clips.  I do agree with insufficient heatsinking.

Lift them to see what difference it makes, if any.   No point in designing a circuit modification if it doesn't do anything.   Find out.

Loudthud

The clipping LEDs make the amp a little more bedroom friendly. When operated with the Master Volume turned down so as not to allow the power amp chip to clip, the LEDs give the effect of more preamp gain and allow the Master Volume to be turned up a little farther past the point where the volume jumps from nothing to too loud.

SemiConductive

Quote from: phatt on April 06, 2022, 09:42:26 PM...

Just a note on the schematic;
Unless I'm missing something??? IC1B is labeled wrong.
Pin 5 is the Positive input and is normally grounded in this configuration,, It looks like it's + & - input labels are flipped. I can't read the label Numbers,, they both read 5 to me.

Phil.

Yeah, this is actually the schematic from the manufacturer too. LOL.

From what I see, you are correct. Pins 3 & 5 are positive inputs. Pins 2 & 6 head to ground.

You're saying they'd normally flip the positive the other way when using both halves of the 4558? I'll look when I'm in there in a few days to see how it's really wired. 

RufusGartz

A better quality schematic for the Epiphone Regent 20,50R is available from

https://gibson.jp/support/schematics-and-manuals

direct link -
http://images.gibson.com/Lifestyle/Support/Files/Schematics/Regent20,50R.jpg

The schematic is readable - IC1B 4558 is pin5 positive and pin 6 negative.

phatt

The circuit is drawn right but labeled wrong, just to clarify a pic may help;
You cannot view this attachment.


phatt

#14
Quote from: SemiConductive on April 07, 2022, 12:06:30 PMYeah, this is actually the schematic from the manufacturer too. LOL.

From what I see, you are correct. Pins 3 & 5 are positive inputs. Pins 2 & 6 head to ground.

You're saying they'd normally flip the positive the other way when using both halves of the 4558? I'll look when I'm in there in a few days to see how it's really wired. 

Pins 2&6 are Neg inputs,, just don't assume that because they are Negative they go to ground.
As a rule of thumb the Positive input always needs a ground reference, often a resistor.
If you notice the Pos on IC1A has a resistor to Ground/Common.
While IC1B the Pos input goes directly to Ground/Common.
These chips are nearly all differential inputs, IC1A is non inverting while IC1B is wired as an inverting stage. both need the Pos input to have a DC path to Common.
Phil.