This is not just another gainclone

[Bricolo]

I checked my outputs with a scope, and had weired results

the signal is clean, at low volume
when I turn the volume kno a little higher, the curve is like the one shown on my drawing


If I disconnect the other speaker (from the other channel, not monitored on the scope), I can push the volume higher before seing this distortion



where can it come from? (input signal is clean)

[Fred Dieckmann]

It looks like high frequency oscillations.

How close are you decoupling caps from the amp chip?

Does the amp have a gain of 10 or higher?

Does the amp have a zoble network?

The more information about the amp construction and design the better. This should not be that hard to fix if you provide enough imformation.

[mhennessy]

Just thought I'd jump in here and show you what I got up to at the weekend

You'll note the careful choice of chassis material - luckily, it doesn't sound too "grainy" [groans]

These are inverting LM3875T's, with RF=120K, RI=4K7, and the non-inverting input connected to ground via 1K. All standard 1% MF... The input cap is 10uF Nichon Muse bi-polar, PSU caps are 2800u 63V BCC devices rescued from a very high quality telecomms PSU. All have been tested and are in very good health...

Both channels share a 120VA torroid - was going to use two, but one of them buzzed mechanically - obviously slightly unwell. Rectifiers are standard bridges...

There's no sign of any hum or oscillations, and noise is ok (although I'd prefer to use lower value feedback components here). They power up/down cleanly with no pops and bangs. Currently there's no Zobel...

The plan is to build up to 4 of these in monoblock form - I have enough cases, heatsinks, caps and other components for this. I plan to use them for surround-type applications, in addition to some decent "overkill" monoblocks for the main LR channels. But, even in this rather basic form they are a decent upgrade on what I'm currently using, so that makes this project even more worthwhile. At the weekend, I should get the chance to start experimenting with different component types

If you'd like to look at some more pictures, check out this page on my website

Finally, thanks to everyone here for the info and inspiration

[fedde]

Originally posted by mhennessy
Just thought I'd jump in here and show you what I got up to at the weekend

You forgot to mention how it sounds

Looks cool,

Fedde

[Bricolo]

I'm sooooooo stupid


Trying to fix my problem (cf previous image), I added 1 more 2200µF cap per rail (so, 2*2200µF total)

stupid as I am, I wired the cap's - to gound.
Guess which rail I was wireing....



I plugged the AC rail, got big HuuuuUUUUU, BOUM!
One of the 2 negative rail's caps not only explosed, but flyed in the room (it was soldered!) and the tiny wire it was soldered to, had this insulator melted



I rewired the caps as realier (1 cap pre rail) with the 2 that were on the positive rail
it works, very little hum


but now at the output, with the scope, I have 2v peak to peak oscillation, at 666kHz

Is my amp possessed by the ?

[moamps]

Originally posted by Bricolo

but now at the output, with the scope, I have 2v peak to peak oscillation, at 666kHz

Is my amp possessed by the ?

Hi,

try zobel network RC on output (like 10ohm/2W + 100nF/63V MKT)


Regards

[Bricolo]

I haden't this oscillation before fireing this damned cap




So, for now, I'm more affraid of having fired something else, that having oscillation going to the speakers


I haden't it before, so it should work without a zobel, like before

[moamps]

Hi,

you have oscillation before...
Zobel could help...
How you wish....
Look Freds question few posts up...

Regards

[Bricolo]

Originally posted by moamps
Hi,

you have oscillation before...

Regards

I hadn't

I thought I had, because my chips were hot and drawed 300ma at iddle, but the scope showed no oscillation



Now I have



I've tested something 2 minutes ago:
on a dummy load (10R resistance): no oscillation
on a speaker: 666kHz oscillation

when I disconnect the other's channel speaker: no more oscillation

[Bricolo]

Originally posted by Fred Dieckmann
It looks like high frequency oscillations.

How close are you decoupling caps from the amp chip?

Does the amp have a gain of 10 or higher?

Does the amp have a zoble network?

The more information about the amp construction and design the better. This should not be that hard to fix if you provide enough imformation.

It's oscillations
I've got a beatifull sinudoid, at 666kHz

0.1uF caps are directly on the chip
2200uF on the bridges (I use a 2 bridge setup, one for each windings), the 2 are used for both sides

I use the minimised gainclone shematic: gain=22

no zoble, and I'd prefer doing without