Help! Tonemender as a Preamp

[blindsjc]

Hi friends,
I'm using a "Tonemender" from RunOffGroove as a preamp im my project, it acts
as a preamp for a "TDA2050" (Datasheet Project) power amp. The problem is, i got
distortion at very low volume, a really ugly clipping and worst, my "TDA2050" burned
after 5 minutes. The PSU is working perfectly and provides 22-0-22 (at 4ohms speaker).

What I need to work? The preamp input? The preamp gain? The preamp out?
Something on the amp like the input impedance? Thanks in advance.

Sorry about my bad english.

Ronaldo

[teemuk]

Oscillation?

[blindsjc]

Hi, thx for reply.
There was no oscillation, just a clipping sound
at low volume. Maybe a clipping from preamp
caused by a too strong signal from epiphone
les paul special?

Maybe I'm waiting too much sound from a
single TDA2050.

Thx

[blindsjc]

Friends,
The problem is resolved,
I changed the power voltage to +-15 and all the clipping and burning
chips are gone. Everything is working perfectly. I'm sure that the
datasheet says +-22, but...

Thanks for all replies. Anybody has the same problem before?

Ronaldo

[teemuk]

I still suspect it might be oscillation. Lowering the rail voltage could have removed it but the datasheets shows the chip should handle higher rails than +-15 and I doubt that they'd print a faulty "demo" design in to the datasheet. You can't always hear HF oscillation but it might cause blocking distortion. That would sound like attenuated signal with a lot of distortion. Have you verified that the circuit is stable with a scope? If you don't own a scope you can try adding more negative feedback for HF.

Besides the zobel network the datasheet amplifier circuit has no stabilization methods to prevent oscillation. Well, it has 100nF filter caps but it seems that they are located quite far from the chip's power pins to have any effect. (Did you use the datasheet layout, by the way?) The tonemender's gain stage opamp has a 47pF NFB capacitor but the gain of the stage seems rather large. In my opinion, larger gains than 50 are pushing it with most opamps.

Another question is how do you power the circuits? The supply capacitance of the TDA2050 demo design is not even near of adequate but I guess you knew that already and took care of it. How long are the supply leads and is the grounding configuration allright? What I'm also wondering is how do you power the tonemender - from a battery? If not, this could explain your problems.

[blindsjc]

I agree with all,
Let me tell the whole thing.

1) With +-22, I got a explosive TDA... changed...

2) The second TDA was clipping, a lot, at very low volume...
even with my portable cd player...was using +-22 too... changed...

3) Changed the supply for +-15. Changed for a new TDA... and bingo...
No more problems, great sound, big volume at a 4ohms 10 in. speaker.

In the PSU I used 2x2200uF caps for smoothing after a bridger rectifier.

thx