I sold a pair of hybrid amps.
It has SRPP front end and TDA7294 chip amp on output.
The buyer says the amps work great separately but if he connects both to the power supply they give out horrible squealing noise and get hot.
He says he has used a star ground.
The power supply is a 450va transformer and 40,000uf per rail.
The voltage is +/-35VDC.
The power supply leads are short.
He admitted he doesn't have the amps connected to a source so the inputs are floating. The input resistor is 1meg but I have a 47k and 100pf RF filter on front end.
I can only guess the amps are amplifying RF then each amps is passing it on to the other via the air?
It has SRPP front end and TDA7294 chip amp on output.
The buyer says the amps work great separately but if he connects both to the power supply they give out horrible squealing noise and get hot.
He says he has used a star ground.
The power supply is a 450va transformer and 40,000uf per rail.
The voltage is +/-35VDC.
The power supply leads are short.
He admitted he doesn't have the amps connected to a source so the inputs are floating. The input resistor is 1meg but I have a 47k and 100pf RF filter on front end.
I can only guess the amps are amplifying RF then each amps is passing it on to the other via the air?
Unless it's a musical instrument amplifier and he has to have 1M, instead try a 100k or lower
right at the input stage, and change the 47k to 4.7k. Are the two input leads from each RCA
input socket twisted together tightly? Doubt that it is RF.
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Its a music system so it should really always be connected to a low impedance source. He hasn't even got anything connected to the input, its just the amp input floating.
Given its audio frequency oscillation then I guess you are right saying it isn't RF.
Its just going to amplify anything it sees on the input and radiate it out through the valve to the other units input.
When I did the first revision of the pcb I had oscillation with that due to the low gain being out of spec for the tda7294. The tda oscillating fed back into the valve and caused another feedback loop. The tda and the valve were both oscillating !
I fixed it with a feedback capacitor between tda + and - inputs.
He is saying the amps are barely warm.
My experience of oscillation is usually the demise of the chip amp or it gets very hot.
He says the wires are short and thick.
The circuit is just an SRPP valve front end into a tda7294.
On the one I got working I had a gain of 8 on the tda which is out of spec but I found putting 1000pf across tda + and - inputs stopped the oscillation.
The tda gain was reduced to adjust for valve gain of about 8.
He does say he got the tda from Little diode so maybe he has got hold of fake parts ?
Its maybe a bit of a false test using his finger to give the amp a signal but it shouldn't shriek when he does it. Sounds like a bit of instability in there.
Its hard to remotely fix an amp. I would probably first check to see if its the valve stage or tda that is unstable.
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