amp clipping earlier than usual?


I have a pair of mono amps with clipping indicator LEDs. One of the amps' indicators is lighting up for the first time. I've never been able to make them light up before. But it only happens on one of them. It lights up way sooner than as loud as I typically max them. I don't hear any distortion. So I'm thinking it's not an output transistor. These are LSR&D Superamp monos, 300wpch. Anyone? TIA
csontos
Does the system sound OK and the channel to channel balance is good?
If so, it could be a problem with the indicator itself.
Hey Ralph. Actually, that amp seems to be down about 2db. I switched sides to check. I have several of these sets so I'm certain.
Hey Ralph. Actually, that amp seems to be down about 2db. I switched sides to check. I have several of these sets so I'm certain.
You switched amps, so it follows the amps? (2dB isn't much but if you didn't use a sound pressure meter for that I bet its a bit more.)

If yes that suggests some sort of damage to the amplifier.
OP: you said you switched sides. Do you mean switch preamp connections to the amp channels with respect to the preamp L+R channel outputs?
Unless they were rebuilt, these amps are close to 40 years old, so I totally agree with Ralph, although, " worn out " might be a better description ( such as capacitors ). Enjoy ! MrD.
Thanks guys. No pre, Oppo CDP direct. So yes, switched ICs and speaker cables. These amps are completely rebuilt about 4 years ago.
Ralph- yeah, more like 3db, noticeable. But if there's no distortion, and not a blown output(maybe yes?), what could cause that? My main concern is the transformer.
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Hi Elizabeth. I should mention the transformer in that amp is 120deg F while the other is 112deg at idle
Transformer runs hotter when filter caps are failing 
Well, the only caps not replaced are the large ones in the psu which I just happen to have Kendeil replacements for. But is there a way to be sure?
So the amp was " not " completely rebuilt " ? I would replace those caps for sure, as it cannot hurt.
Right. They seem to be the ones that are usually kept in place during rebuild. I guess they are more robust than the smaller ones.
You can visually inspect for bulges, leaks, test for esr and traces of 60 HZ downstream
or just get on with the business of it...
remember the voltages you are playing with can be fatal
get help versus poking around with a fork....
Sounds like a trip to an electronics shop is in the future for both amps, if not for anything more than an inspection and tuneup.
Have the amp checked out before blowing money on caps and such. It could be as simple as the bias in the amp being off (too high or low). 
I have some limited skill in electronics, mostly with good direction. I can replace the caps no problem. I have LTP transistors on the way anyway. Offset is a bit high for my liking so I will take care of both issues at the same time. It's just that I would rather not change the big caps unless I'm sure they're the culprit.
Ralph- yeah, more like 3db, noticeable. But if there's no distortion, and not a blown output(maybe yes?), what could cause that? My main concern is the transformer.
The fact that the power transformer is running warmer is a bad sign and I agree with others that the filter capacitors in the power supplies are suspect (and cause this sort of thing) especially given the age. If left unchecked it could lead to transformer failure.

It is also a good idea to check the bias points. This is not particularly hard to do; with the cover off a tech can do it easily in five minutes.

So there are the two things that really should be sorted and it could well mean that they sort out the problem too. But even if they don't this sort of service would be prudent.


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I set the bias to 120ma as per spec. However the late Dr. Marshall Leach was aware a hotter setting is preferable to some and did state it could be safely set as high as 200ma. So there is a wide safe margin. The psu caps are good quality Sprague and show no signs of malfunction. I have a hunch there's something else going on.