Is it typical for too high input signal to result in blown output at high volume?

I suspect the title is not terribly clear, so here's the situation:

I am repairing a Luxman L-430 amplifier. It came to me with a blown channel and required several output transistors and an electro cap with a hole in its side replaced. I considered that the electrolytic was the likely culprit and have replaced all electros in the amp as a precaution.

I had it running at up to moderate volumes for several days without trouble (and it sounded very nice).

Source has been a professional CD player with 2V line output, full signal input for the amp is 200mV. This is not ideal but it has been only a minor annoyance with a number of other amps over the last ten years.

Today my partner went to play a CD while I was not home and as she is not used to the new system had not realised that I had turned the amp off earlier (the lamps are all blown and I haven't fixed them at this stage). So without looking she turned the volume up nearly full, realised it was not on and turned it on at full volume. That immediately blew an output device, with just a tiny pop of sound out of the speakers.

That's not the behaviour I would have expected from the amp. I would expect a clipped signal (due to the signal level mis-match) and enough volume to make you wet your pants, but not self destruction.

Am I missing a problem somewhere that higher power may be showing up? Is this actually normal behaviour that I'm unaware of because I don't normally listen at 120dB?

Thoughts are sought, output transistors are a good deal more expensive than fuses and I'd like to rule out doing this frequently.
 

Mooly

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2007-09-15 8:14 am
A few possibilities come to mind.

1/ You do mention that the amp appears to have input sensitivities more in line with amps from pre CD days and so yes, that could be a factor if the incoming audio content was really high.

2/ The amp has basic current limiting protection provided in the normal way by sensing volt drop across the emitter resistors of the output pair, and using that to pull the drive voltage down by means of two pairs of transistors.

It is not foolproof, if it were we would see far fewer zapped amps. Even chip amps with their supposed inbuilt comprehensive protection can expire at the drop of a hat.

3/ Did you use the correct replacement output devices or did you substitute for modern parts... not that there is anything wrong with that... but we need to be sure of what you fitted and its provenance.
 
The PA amp trick against overvoltage input is to first put diodes to + & - op amp voltages (or first stage voltage) to clamp the input to those voltages. Then you put a 1000 ohm resistor between input and those diodes, to protect the diodes against a 75 W guitar amp output in the input. If the resistor is 1/10 watt, it burns out, cheaper than a fuse. See PV-1.3k schematic on eserviceinfo.com
I would look at changing the feedback resistor on the first stage to cut the gain. Output to minus on an op amp input (usually), collector to base on common emitter transistor input. As you are lowering the value, you can usually solder a parallel resistor on top of the old feedback resistor instead of pulling the old resistor and possibly damaging a pcb land.
 
What happened here is probably the worst-case scenario when it comes to secondary breakthrough. At idle, supplies are near max.

Which output transistor was it that blew - an original one or a new one?

I vaguely remember something about these old Luxmans requiring some reworking for long-term stability... extensive resoldering maybe? New electrolytics can't hurt though, these have been noted for the big ones leaking. It looks like bias current also likes to drift up somewhat (spec is 50 mA).... but I guess that would have been reset during the repair here.
 
Did you use the correct replacement output devices or did you substitute for modern parts... not that there is anything wrong with that... but we need to be sure of what you fitted and its provenance.

Original specced parts were used, sourced from WES Components in Sydney, who should be fairly reliable in terms genuine components.

Were the speaker´s impedance withing the amp ratings/specifications?

Yes. Also a pretty good match for power ratings. Soundcraft studio monitors from the 90's. They are capable of taking a pounding, I wouldn't expect that they presented any particularly out of spec load even under these conditions.

The PA amp trick against overvoltage input is to first put diodes to + & - op amp voltages (or first stage voltage) to clamp the input to those voltages. ..... I would look at changing the feedback resistor on the first stage to cut the gain.

I perhaps prefer the first of those options, as a turntable will likely be hooked up before too long and it will conform to the expected line level the Luxman was designed for. What are the sonic consequences? And I imagine the scheme needs to be somewhat different as the inputs are not balanced. (Although I have not checked if - is hooked to ground or not.)

Which output transistor was it that blew - an original one or a new one?

A new one. One of four, the others appear to have survived.

I vaguely remember something about these old Luxmans requiring some reworking for long-term stability... extensive resoldering maybe? New electrolytics can't hurt though, these have been noted for the big ones leaking. It looks like bias current also likes to drift up somewhat (spec is 50 mA).... but I guess that would have been reset during the repair here.

I'd love to find the details of that rework. I didn't change the very large PS filter caps, mostly as they are expensive.

Bias was reset, with a much longer warm up time than the service manual advises, so a colder bias than spec.
 

Mooly

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2007-09-15 8:14 am
After reading all that I suspect at the end of the day this may just be an unfortunate occurrence that little could have been done to prevent... it happens and solid state amps (particularly old designs like these) were somewhat fragile.

I suppose you could experiment with simple attenuators on the input sockets and depending how they are physically wired you have a lot of scope how to accomplish that.
 
I perhaps prefer the first of those options, as a turntable will likely be hooked up before too long and it will conform to the expected line level the Luxman was designed for. What are the sonic consequences? And I imagine the scheme needs to be somewhat different as the inputs are not balanced. (Although I have not checked if - is hooked to ground or not.)
If the input is not balanced, only the hot side needs clamping to the power supply with diodes.
The sonic consequences of diode clamp of input are that when an overvoltage signal is present, it sounds like a siren with all the tops cut off. When the signal is in spec, my CS800s with diode input clamp has .03% HD.
Don't understand comment about turntable hookup direct to the amp input. Records have the RIAA preemphasis which needs to be countered by a preamp; else bass is missing. My preamp has 50x gain which means for a MM cartridge to get to 1.6 v standard line level, the signal is ~32 mv. If the owner is using an obsolete ceramic phono cartridge, I believe those had about 1 v output. Some receivers had both a phono input and a line level input. You are not saying you had the CD player plugged in the phono input jacks, are you? Line level input of 200 mv max is weird. The dynaco power amps, which had huge market share in 1965, used 1.6 v.
 
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When the signal is in spec, my CS800s with diode input clamp has .03% HD.

That's somewhat more than the rest of the amp, from memory.

Don't understand comment about turntable hookup direct to the amp input

The amp has an integrated phono pre-amp, which by many accounts is very good. If I reduce the gain of the power amp I am still left with a signal level mis-match between anything with a modern line level output and the phono.

After reading all that I suspect at the end of the day this may just be an unfortunate occurrence that little could have been done to prevent...

I may have to run with some administrative controls rather than hardware fixes... Mostly I am being mindful to make sure that I'm not missing something in the overhaul of the amp that leaves it more susceptible to catching fire than it should be. I'm no expert at this and I sometimes suspect I know enough to be dangerous and no more.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
I may have to run with some administrative controls rather than hardware fixes... Mostly I am being mindful to make sure that I'm not missing something in the overhaul of the amp that leaves it more susceptible to catching fire than it should be. I'm no expert at this and I sometimes suspect I know enough to be dangerous and no more.

Provided any safety components are not compromised or replace with unsuitable parts then there should be minimal fire hazard risk (minimal as in it still meets all original safety specs).

For example the manual lists some resistors as 'Rf' types which mean flame proof. So you shouldn't replace these with normal types.
 
If you repair an amp, you need to drive it into hard clipping into the rated dummy load for a minute or two. Ideally this would be at 10KHz+ but frankly many, especially older amps will never pass this test, so keep it down to 1KHz for older amps. I corrected the shoot through problems that killed many older amps years ago when I did that for a living, but you have to understand the circuit better than the maker did. Look for instability bursts and rail sticking when the amp is clipping. Both are big trouble that will kill the amp pdq. Keep an eye on the temperature too. If you don't test the amps ability to survive abuse, it will die in the hands of the customer.