Wanted: distortion free soft clipping circuit

[jan.didden]

It looks like you build a compressor. It woiuld be of interest to see how the distirtion changes when the conmpressor becomes active.
BTW Organising the gain structure so that the preamp clips before the poweramp can also be of interest.

Jan

 

[EUVL]

As to a low leakage diode - did anyone mention the FDH300? I did extensive testing for my autoranger protection .....

Jan

Jan,

Did you try BAS716 ?
How does it compare ?


Patrick

 

[dadod]

This amp has two parts (it is integrated type), preamp with no GNFB and OPS with GNFB and gain of two. The volume control is after preamp part.
This anti saturation circuit is to prevent OPS to saturate. The distortion is quite low and start to increase wen zeners start to conduct of course.
Why is important how distortion changes when anti saturation circuit (I would not call it compressor) become active?

 

[mvs0]

I've come up with an unusual amplifier topology involving boostrapped stuff, and that seems to have, shall we say... non-optimal clipping behavior. So it looks like I'm going to need a pre-clipping circuit.

Of course it should not distort at all until it does.

I've sketched a number of attempts, all involving bootstrapping the clipper diodes to reduce distortion by keeping voltage across them constant, which avoids nonlinear capacitance effects, leakage, etc. The bootstrap voltage is itself clipped by another pair of diodes, so when it reaches the limit, the other side of the diodes no longer follows the input signal and they begin doing their job.

The four ones on the left use diodes or transistors as diodes. They're strictly at the input of the amp, so they need an opamp buffer for the bootstrap voltage. The one on the right deletes the opamp by using the amp's output voltage (the power amp is VCVS E1) and using BJT emitter followers as diodes. Basically the two transistors' bases follow the input signal so they do nothing, until the diodes on the right conduct, at which point the transistors limit the input signal.

On the left is a sinewave with a smashed top.

View attachment 1071619

And here is THD vs input level. Simulations are sometimes slightly optimistic, but it looks like "no distortion before it has to begin clipping" is achievable.

View attachment 1071625

Thoughts?

I made this softclipper with simple LED's:

https://www.diyaudio.com/community/...-8-ohm-how-to-limit-power.421941/post-7890159

https://www.diyaudio.com/community/...-8-ohm-how-to-limit-power.421941/post-7890258

Its TDH < 100dB upto -9dB with softly increasing distortion above that, very happy with the solution and you can see when it triggers 🙂

 

[jan.didden]

This amp has two parts (it is integrated type), preamp with no GNFB and OPS with GNFB and gain of two. The volume control is after preamp part.
This anti saturation circuit is to prevent OPS to saturate. The distortion is quite low and start to increase wen zeners start to conduct of course.
Why is important how distortion changes when anti saturation circuit (I would not call it compressor) become active?

You would want to know how rapid the distortion rises at the onset of overload. How much better is it than just clipping?

Jan

 

[benb]

It appears to me that in the circuit of post #150, transistors Q13 and Q14 operate in the reverse active mode.

• NPN Q14 has (VBE = -10V) and (VCE = -10.5V)
• PNP Q13 has (VBE = +10V) and (VCE = +10.5V)

This is a bit unusual for audio circuits, and I worry that transistor behavior in this mode is likely to have a much greater variance, both from unit to unit, and from low temperature to high temperature. Simulation DC bias result shown below.

Indeed, looking at datasheets for BC327 and BC337 (several manufacturers) show VBEo MAX as 5V. I've seen reverse-biased B-E junctions of transistors used as noise generators in the 5 to 10V range where they have a zener response, but somewhere I've read advice against doing this, that it damages the transistor/reduces HFE. Then again that may not much matter in this circuit.

 

[dadod]

You would want to know how rapid the distortion rises at the onset of overload. How much better is it than just clipping?

Jan

You can see from rounded edges when clips that distortion is not abrupt, and the main advantage is that OPS does not clip, as there is no saturation.

 

[dadod]

If I insert a resistor in series with those zener diodes the clipping is much softer.
First one is just on the brink of the clipping, second one is dip in the clipping.

 

[jan.didden]

From the graphs it apperas that with the series resistor there is less gain, and that results in less clipping.
It is very hard to judge things from a scope screen with many other factors not known.
The best is a graph of distortion versus output level in the various conditions.

Jan