Half wave rectification causes the power trafo to deal with "standing" DC, which is bad and could easily be the source of the annoying buzz. Put a full wave bridge rectifier made from 4X inexpensive UF4007 1 A./1000 PIV diodes into the amp. Make the switch to a bridge rectifier before you try a filter choke. By itself, full wave rectification may quiet things down sufficiently.
adason, keep an eye on the B+ when you switch to a bridge rectifier. It'll probably go up, maybe a lot. It might not be a real problem given that the B+ is probably pretty low to begin with (150V or so?) One solution that I'm sure will be offered is to add series resistance before the first filter cap (increase the size of the first resistor in the ps.)
On the other hand, is might work just as well (or better) to leave the higher voltage on the plates of the 25L6's but reduce the voltage on the screens (and consequently the 12AX7) to bring things back to where they were originally. That means increasing the size of the second resistor in the ps. The extra voltage on the plates should have little effect on the idle current of the 25L6's and might actually sound better.
-- Dave
On the other hand, is might work just as well (or better) to leave the higher voltage on the plates of the 25L6's but reduce the voltage on the screens (and consequently the 12AX7) to bring things back to where they were originally. That means increasing the size of the second resistor in the ps. The extra voltage on the plates should have little effect on the idle current of the 25L6's and might actually sound better.
-- Dave
hi guys,
well, Eli, bad news, it did not work so far with full wave rectifier
no matter how I tried. The buzz was many times louder. Maybe it has something to do with separating one side of heaters from the ground. I will try to draw the schematics and explain it better.
Dave, yes, I was watching the DC, I used variac for experiments and never reached mains normal voltage.
There is a bit of a good news, I experimented with blocking capacitors, between the ground and metal chasis and it helped a bit. Now the buz on output is just few millivolts.
Maybe what I am trying to achieve is not even possible. These amplifiers were never designed to be completely quiet.
I will post updated schematics and start working on some power supply upgrade strategy.
well, Eli, bad news, it did not work so far with full wave rectifier
no matter how I tried. The buzz was many times louder. Maybe it has something to do with separating one side of heaters from the ground. I will try to draw the schematics and explain it better.
Dave, yes, I was watching the DC, I used variac for experiments and never reached mains normal voltage.
There is a bit of a good news, I experimented with blocking capacitors, between the ground and metal chasis and it helped a bit. Now the buz on output is just few millivolts.
Maybe what I am trying to achieve is not even possible. These amplifiers were never designed to be completely quiet.
I will post updated schematics and start working on some power supply upgrade strategy.
Hi Eli,
I did not separated heaters and power supply by isolation trafo before, I did not get it, I just use it to isolate the whole amp from mains. I have redone the isolation trafo thing, now the mains go thrue switch to heaters first, than there is isolation trafo, fuse and B+ supply. I tried full wave rectifier, it seems to have no affect on buz. Since the B+ is higher, I would have to change resistors in supply, so I changed it back. However, with the addition of blocking caps from the ground to the metal chasis; this time its only B+ ground, not connected to heaters; the buzz is almost unnoticable. Actualy I am listenintg to the amp right now with Lowthers and I must place the ear close enough to the speakers to hear it. Otherwise sound is nice, considering it runs straight from discman to amp and to the speakers.
Thanks for help, I believe I am almost done.
ed
I did not separated heaters and power supply by isolation trafo before, I did not get it, I just use it to isolate the whole amp from mains. I have redone the isolation trafo thing, now the mains go thrue switch to heaters first, than there is isolation trafo, fuse and B+ supply. I tried full wave rectifier, it seems to have no affect on buz. Since the B+ is higher, I would have to change resistors in supply, so I changed it back. However, with the addition of blocking caps from the ground to the metal chasis; this time its only B+ ground, not connected to heaters; the buzz is almost unnoticable. Actualy I am listenintg to the amp right now with Lowthers and I must place the ear close enough to the speakers to hear it. Otherwise sound is nice, considering it runs straight from discman to amp and to the speakers.
Thanks for help, I believe I am almost done.
ed
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