Patchwork Reloaded: Circuit Optimization and Board Layout.

[danielwritesbac]

Originally posted by MJL21193
You're slipping again Dan! Didn't you take your meds today?

We are talking about power supply ripple rejection of the amp circuit. The resistors and caps form a filter to drive the effects of this ripple from the power supply down in audibility.

What happens when you try to make an adder with caps? That does a lot of harm to a signal instead of just a little bit. You want DC right?

[Nico Ras]

Last night looking at the old amp revealed PSR of about 70 dB.

If you filter the power line with a simple RC network the PSR would improve by theoretically but you are loading the filter so the improvement will be less than theoretical.

Adding a pass transistor and filtering the base, the power supply remains low impedance. Okay you do get a volt drop across the transistor, but you could achieve quite high PSR this way.

The picture shows a filter 36 dB down at 100 Hz and this means the amp PSR would be 106 dB (add amp PSR of 70 dB).

[MJL21193]

Originally posted by Nico Ras
Last night looking at the old amp revealed PSR of about 70 dB.

If you filter the power line with a simple RC network the PSR would improve by theoretically but you are loading the filter so the improvement will be less than theoretical.

Hi Nico,
I employed the simple RC low pass filter to improve the mid/high range PSRR. As shown in my plot, this gives good simulated results. This also gives good real world results without too much complication. If I want to improve this further, I would split the frontend of the amp from the main supply and use a separate, regulated supply for the front end. For my tastes, my ears and my expectations, this is not necessary.

Over in my Abomination thread, I was shown how to do a simple test that demonstrates the audibility of the power supply ripple. In the test, detailed here, I could not hear anything from the speaker.

It is good to shoot for the absolute lowest, but in most cases it is not necessary. Numbers are misleading and real listening is what counts.

[Nico Ras]

That was an interesting thread

[MJL21193]

I have taken another look at the "bootstrap" resistor, prompted by Nico. I have found that it isn't making the difference that I first experienced. Granted, a lot has changed in the design since I first employed it, and the optimizations that have been done may have made up for the deficiency it was covering.
I have decided to delete it, rather than leave it for "old time sake" It doesn't do anything bad, but it also doesn't do anything good.
I have updated the plan:

[Nico Ras]

No you haven't. Bootstrap resistor is there in all its glory

[MJL21193]

Originally posted by Nico Ras
No you haven't. Bootstrap resistor is there in all its glory

Hi Nico,
Not the VAS bootstrap. The frontend resistor to the feedback loop - R28 on the old schematic. Remember, the one that you said made no difference?

[Nico Ras]

Ah.... I am getting old

[MJL21193]

Originally posted by Nico Ras
Ah.... I am getting old

No problem, we all are.

[jacco vermeulen]

Originally posted by MJL21193
we all are.

Oh Johnny Boy,

can't be, your ears haven't ripened enough to distinguish between head and tail of a wire.