Leach Amp Hiss

[kennethrl]

I bought a used Leach amp that was built by Electronic One. The amp sounds beautiful. However, a very low level hiss can be heard when no music is on. I have had other amplifiers before and never notice this hiss at "normal" volume levels. Could this be a sign of a component in the amp going bad or out of spec? Maybe this is typical for this design?

[mastertech]

it could be a component going bad is this amp old
it could also be hum some amps have higher levels than
others, if it is not too annoying dont worry too much
about it

hope this helped, some of the other members could be
more helpful

cheers

[AndrewT]

Hi,
with normal sensitivity speakers the amp should be quiet.
If yours are above the mid 80s then this would exaggerate the hiss.

Could you check voltages at the transistor collectors and emitters? You would need a DMM and a PCB layout so that we know which voltages you are quoting. I am suspicious of a failed semi (zener) causing a wrong voltage and overloading a transistor that has not yet blown.

Zeners have a reputation for being noisy. So the noise could be getting into the front end from the four Zeners if one of the two caps have gone bad. They will be cheap to replace and you could add a very small cap (10nF to 100nF) in parallel, soldered to the electrolytic pins underneath the PCB, or soldered piggy back to the Zener legs on top if this position suits the cap size & pin pitch.

It is possible that a transistor has gone/is going but it will be more difficult to remove and there are a lot of them. I would try replacing both the cascode trs first then follow with the LTP.

THE BIG Q. can one half of the complementary input be switched off to let just half the VAS operate? This could isolate the problem to upper or lower half. Any comment?

[djk]

That thing is at least 25 years old. Replace all the electrolytics. Check and see if you have continuity from the RCA ground to the speaker ground.

[richie00boy]

Surely switching off half the input stage and VAS would result in the output stage going to the opposite supply rail?

[djk]

"Surely switching off half the input stage and VAS would result in the output stage going to the opposite supply rail?"

Not if you put in a pull-down resistor to act like a CCS.

It's probably noise from power supply instability. The 100µF caps on the front end will be all dried up, as well as the 22µF caps for the output section. The cap in the feedback loop dries up and the gain goes away (loosing the bass first).

Replace the caps first, they need it anyway. Check the input grounding. If that doesn't fix it, then throw out the plastic transistors.

[pooge]

That amp is based on the original Leach amp. It would not have a cascode input.

[kennethrl]

The cap in the feedback loop dries up and the gain goes away

I thought the didn't have a feedback loop?

What is the best type of capacitors to use for replacement (without going esoteric or new age)?

[AndrewT]

Hi,
if you have any small electrolytics say 4u7F and below then consider replacing them with plastic film.
Use ceramic as PSU decoupling.
Replace mylar, polycarbonate and polyester with polypropylene.
Consider bypassing all big caps with a better quality small cap of value between 1% and 5% of the larger.

Q, do interference supression polypropylene caps perform well in audio circuits?
They are now commonly available at sensible cost.

[djk]

"if you have any small electrolytics say 4u7F and below "

None.

"Consider bypassing all big caps with a better quality small cap "

They already are.

" I thought the didn't have a feedback loop?

What is the best type of capacitors to use for replacement (without going esoteric or new age)?"

It has a feedback loop, Panasonic caps work well.

" It would not have a cascode input."

It doesn't.