audio-talk

Testing the amp with music now and I’m well impressed. The musical and sonic realism has jumped up another level. I don’t think there’s anything magical about why this is. It’s almost certainly down to to a combination of reduced distortion from a better balanced LTP, less DC offset at the output, r...

It all works. DC offset at the speaker terminals is down from 15-22mV to 5.2 mV on one channel and 5.8mV on the other and no longer varies with the position of the volume control. I’d call that a result. :) Gave it a quick blast with speech off the TV and it plays on both channels. I’ll see how it s...

Boards reinstalled and all connections made: New sil pads on output transistors.

Right! Well...carrying on from the other thread about feedback caps, the postie delivered four Nichicon ES bipolar caps, courtesy of Nick, who had prepared two of them with a copper foil shields and earth wire. This of course meant that I had to tear down the amp and get the boards out, in order to ...

Loved her.

Aye, its something else that seems logical, now I’ve realised with your help what is going on.
As The Korgis said, 'everybody’s got to learn sometime.'

I’ve also seen other circuits where the feedback resistor matches the input resistor. They all have input DC blocking caps I can see why the resistors match now.

Cheers Nick,
Things are starting to make sense now.

I would guess in the absence of of a cap on the input the resistance looking into the pot from the LTP base is whats causing the change. What I would suggest trying is a resistor to ground on the front of the amp so the resistance to ground on the +ve LPT base is the same as the resistance seen to ...

Interestingly the DC offset on the output stage varies with the position of the volume control. As the pot is turned towards its maximum clockwise travel, the DC offset falls.

OK.

Cheers Nick,
I’ll look into that, when I get going on the installation of the caps.

Cheers Nick, that would be great.

Fair enough. I would have thought the different conditions on the two sides of the LTP would lead to a DC offset on the output. I know this thread is ancient history but I tried a 3u3 cap on the inputs of my amp boards last week. It led to a -100mV DC offset on the output. The speaker cones could b...

I’ve had an issue with this build for a while now, where the sound disappears from the left channel. One culprit was a duff cable where the centre conductor had snapped inside the phono plug. I thought I had cured the problem (well you would wouldn’t you.) Then the bloody thing reared its head again...