I can't understand where is your problem....
I will repeat, i had used 12K in the ltp, 5k6+5k6 in the vas for +/-50v.
Play nicely in stereo mode.
I thought maybe it was the trimmer at the input. It was adjusted to about 28K to zero the offset so this morning I set it to 47k thinking that would work but no such luck. Each channel will play music alone but hooked up together they go into overload. There is something wrong but I just can't find it. I will put up a schematic with voltages today and maybe that will shed some light.
Yes, I have separate ground wires from power ground and signal ground. Here is a picture of how it is wired.
Attachments
Connect gnd from R channel input rca socket to gnd L input rca socket.
Use one cable to connect this common gnd to power supply 0v
Connect the rca sockets R&L using coaxial cables to the amplifiers boards as usual.
Do not use any other cable from signal gnd to power supply 0v.
Please look on the Apex schematic carefully!
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I had a pair of hybrid valve/tda7294 amps that had the same problem.
Worked fine on their own but had violent oscillation when both powered up on same supply.
There must be some interaction between them to become unstable.
Maybe a bad star ground problem where the output is getting back into the input somehow through a bad ground or through the power rails.
Worked fine on their own but had violent oscillation when both powered up on same supply.
There must be some interaction between them to become unstable.
Maybe a bad star ground problem where the output is getting back into the input somehow through a bad ground or through the power rails.
You can try compensation... use 47pF from base to colector of VAS transistor...
I tried this. Same problem. This is two separate sets of boards doing the same thing. I will go no further with this until I get another tone generator so I can look at the scope. At this point I am just chasing ghosts.
Thanks
Make sure there are no ground loops when connecting two amplifiers.
The only common connections should be at the smoothing caps at zero volts,55 volts and - 55 volts.
If you get noise in the ground line it can modulate the input signal which drives the amplifier which in turn modulates input signal more.
I designed a USB mixer that had ground problems and had 1VAC of signal on output with input shorted !
The only common connections should be at the smoothing caps at zero volts,55 volts and - 55 volts.
If you get noise in the ground line it can modulate the input signal which drives the amplifier which in turn modulates input signal more.
I designed a USB mixer that had ground problems and had 1VAC of signal on output with input shorted !
Yes Terry i can confirm that is an oscillating amplifier, unfortunately.
I haven't a scope test but i measure 35mV a.c 137kHz. at the speaker out.
This oscillate only when speaker or dummy load attachment. Without a load is ok.
You can kill oscillation if you short the input.
I haven't a scope test but i measure 35mV a.c 137kHz. at the speaker out.
This oscillate only when speaker or dummy load attachment. Without a load is ok.
You can kill oscillation if you short the input.
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Hi Thimios, I did see oscillation for the short time my signal generator was working. Now I can't get it to power up with both channels hooked up. Single channel plays but doesnt sound right. I put 47p on the vas but no improvement. Miles says many of these have been built and used but maybe with different transistors than I used.
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I connect ground like in schematic from post =1266 and I use MJE340 for VAS Outputs was 2sk1529/2sj200 and irfp249/9240 in several pcbs there is no any bias or osccilation instability.