can a ss amp suffer degradation in performance through accidents?

hi, gents. not really a diy topic but i couldn't think of a forum with more knowledgeable people than here so here's my question- i have a chinese tube preamp. mingda, lousy design- hums and cackles from the tweeters even with the volume set to zero. also makes a startling thump sound through the amp when turned off. for this reason, i've always been cautious to turn the amp off before this mingda preamp. my amp- sansui d907limited. a marvelous beast of an integrate. dead silent at the tweeters only until the volume is cranked all the way up. this is the issue i have a problem with.

i had the mingda hooked up to the sansui for the firs time yesterday. enjoyed it till too late after midnight when i became ready to stumble my way to the bed. already half asleep, i forgot to turn off the preamp before the amp. got that startling thump sound from my speakers. today, i turn the amp on and now the white noise is much more pronounced at the tweeters. what happened?

could one accident with the dc surge (i'm guessing that's what the preamp did) caused the amplifier's parts to wear out?

the sansui has a function to decouple its preamp and the power amp section. the power amp by itself is dc coupled. i had the mingda hooked up through the dc coupled input. but the surge from mingda seems to have affected the sansui's internal preamp that was never in use. the speakers are dead quiet when sansui's back to the power amp only mode.
 
Hi,
it's usual for Tube/Valve equipment to use a polar supply.
This forces the audio signal to sit on a very high DC voltage.
Due to this, I think all Tube/Valve equipment have a DC blocking capacitor before the output sockets. Check your's.

If the output is DC blocked then you next stage does not need extra DC blocking.

A very high voltage pulse, at switch on or switch off could easily damage the input stage transistors and other components. It depends on what types of protection the manufacturer has included.
Yes, the first sign of damage could be excessive noise. The next sign is wandering output offset and finally a dead amplifier.
 

unclejed613

Member
2006-12-28 12:19 am
large voltage spikes CAN damage input transistors, but having said that, it's just as likely or MORE likely to damage the input transistors with the amp power SHUT OFF!!!! it depends on the input circuit design, but within the last year i've read a few articles on the subject, one of them being an Analog Devices article about op amp input stages (if you haven't noticed, most power amps are gigantic op amps in disguise)...
Rarely Asked Questions... | Analog Devices

it's easier to exceed input stage voltage limits (usually expressed in terms of exceeding the input stage rail voltages) if the input stage is not powered on. if the input stage isn't powered, then the rail voltages are 0V rather than +/-50 (or whatever the rail voltages of the amp happen to be). on the other hand, the turn off thump from the preamp coupled through the amplifier can be bad for the speakers if it's big enough, and if the amplifier protection isn't fast enough (or in many cases nonexistent) to protect the speakers.

can you determine whether the noise you hear is from the preamp or the amp? one easy way to tell is to plug 1kohm terminated RCA plugs into the amp (or shorted plugs if that's easier, but many manufacturers use 1k terminated plugs in their noise testing). the noise you are hearing is more likely from the preamp unless you have a very badly damaged input transistor.
 
the amp is actually an integrate. PG:AU-D907_Limited

let me clarify. i used the tube preamp with the amp's power amp section, bypassing it's own preamp. the tube preamp was always noisy and troublesome. the amp's own internal preamp was always dead quiet. no white noise at the tweeters. until yesterday, when the tube preamp decided to surprise me with the voltage pulse. after that, i took the tube preamp back to the storage, and toggled the amp back to the integrate mode. the internal preamp for the amp now hisses. white noise in the tweeters that's as pronounced as a cheap yamaha receiver that i have laying around. i hope i explained it better this time.

are you saying that the internal preamp should naturally have a white noise?

also, could my speakers have suffered same kind of degradation? how could such a damage present itself when the speakers are still in working condition? less sensitivity perhaps?
 

unclejed613

Member
2006-12-28 12:19 am
it's quite possible the amp input stage is going. the test for this would be to put shorting plugs on the amp input with the internal preamp bypassed as you had it with the tube preamp. if the amp still hisses with shorting plugs right at it's input, then it's the amp that's noisy. if it's both channels i would lean towards the power supply that feeds the input stage (this is usually the same power supply that feeds the rest of the amp, but not always). another possibility is that if the "thump" had enough high frequency energy, it might have opened up the resistor in the Zobel network, and the "hiss" you're hearing is actually oscillation. if this were the case the amp would be running noticeably hotter than normal. try the shorting plug test first.
 
sorry i dont really get the point here ... seems like there is a long discussion about the DC behavior of the preamplifier and the OP is talking and working with an amplifier that is dated from 1979....hmmmm that is about 30 years old

it is obvious to me that 30 years old amplifier could produce a gozillion of failures on its own ...

so ... get our shelf a working amplifier to evaluate the status and/or performance of your preamplifier ... after that you may think what is actually needed to bring the sansui back to life ...

ts ts
 

unclejed613

Member
2006-12-28 12:19 am
there are a few possibilities here:
1) however unlikely, the preamp somehow damaged the input stage of the amp (both channels with the same failure at the same time is very unlikely, but possible.
2) an outside source of the behavior of both the preamp and the amp, such as an RF source. this is something that should be investigated.
3)something in the power supply of the amp causing the noise. amps of that vintage used transistor regulators with a zener reference. one source of noise here is the bypass cap for the zener drying out, and the regulator transistor amplifying and passing the avalanche noise along to the regulated supply.
4) the diff amp transistors, being 30 years old are beginning to show signs of aging, which in early stages shows up as noise.
5)catastrophe theory. the noise was always there, and the OP never noticed it before another failure (the bad behavior of the preamp) called his attention to it. this still doesn't rule out number 2, 3, or 4