Patchwork Reloaded: Circuit Optimization and Board Layout.

[AndrewT]

Hi,
your sound card shows a significant 3rd harmonic at 180Hz and another at the 9th.

Why does your amp show hum and buzz all the way through the spectrum?
Is this a test set up artifact or is it coming out of the amplifier?

[MJL21193]

Originally posted by AndrewT
Hi,
your sound card shows a significant 3rd harmonic at 180Hz and another at the 9th.

Why does your amp show hum and buzz all the way through the spectrum?
Is this a test set up artifact or is it coming out of the amplifier?

Hi Andrew,
The soundcard noise is shown below. This is the soundcard itself. Everything is lower than -100db.

The hum or buzz you refer to may be some interference - the DCX2496 is still connected to the remaining channels of the amp and powered, the connection cable is a 2 metre piece of unshielded telephone cable. The amps are very quiet, so no discernible hum or buzz.
I could do everything all neat and scientific, but the picture is clear enough - the amps are performing as predicted.

Compare it to the recent tests of my finished K10A - the magnitude of the hum and buzz is much stronger in that one, yet it is very silent. These peaks are way down.

[AndrewT]

Hi,
are you able to measure the output noise (with shorted input) of both these amplifiers?
I get the impression from your graphs that it is >>0.05mVac

[MJL21193]

Originally posted by AndrewT
Hi,
are you able to measure the output noise (with shorted input) of both these amplifiers?
I get the impression from your graphs that it is >>0.05mVac

I'm too lazy to connect to my scope and I don't have a millivolt meter, but here's what I did - I recorded the output of an amp with it's input shorted. The result is attached and backs up the RMAA result. I used the same telephone cable attenuator rig as before (I need to make something better).

This doesn't show the voltage magnitude but in the next post I will attach a sound clip of this "noise".

[MJL21193]

Restrictions on the attachment size (100kb) makes for a short clip.
It's attached, a 3 second clip. You may need to listen with headphones.

[GK]

Originally posted by MJL21193
Compare it to the recent tests of my finished K10A - the magnitude of the hum and buzz is much stronger in that one, yet it is very silent.

"Very silent" was the intent, along with the simplest possible HV power supply.

Cheers,
Glen

[GK]

Originally posted by MJL21193
Soundcard THD = 0.0019, therefore the amps THD = 0.0002% (0.0021 - 0.0019 = 0.0002). Very good!

THD's don't add and subtract that way.

[MJL21193]

Originally posted by G.Kleinschmidt

THD's don't add and subtract that way.

Yes, well I figured something was up, but it can't be a whole lot more. What is the actual calculation?

[GK]

SQRT ( (dist1*dist1) + (dist2*dist2) )

So, 0.0009%, not 0.0002%

Cheers,
Glen

[MJL21193]

Originally posted by G.Kleinschmidt
SQRT ( (dist1*dist1) + (dist2*dist2) )

So, 0.0009%, not 0.0002%

Cheers,
Glen

Thanks Glen,
Another factoid to file
I hark back to the early pages of this thread to
this post from darkfenriz:

Going below say 0.02% THD at 20kHz in a design like this is hardy doable. I don't know where these 0.0000x% figures came from.

I guess we did the un-doable.

In any case, mission accomplished. The amps in the 6 channel case are on constantly - haven't been shut off in months. Totally quiet, stable and they sound sweet to boot.