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PreSapian 3rd September 2010 11:22 AM

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.

AndrewT 3rd September 2010 11:54 AM

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.

PreSapian 3rd September 2010 12:12 PM

the preamp has a dc coupled cap on the output terminals- 1uf, 630v DC. could the voltage pulse still have damaged the amplifier circuit? would i be able to spot the damaged components with a visual observation.

unclejed613 3rd September 2010 12:20 PM

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.

PreSapian 3rd September 2010 12:51 PM

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?

AndrewT 3rd September 2010 01:02 PM


Originally Posted by PreSapian (
the amp's own internal preamp was always dead quiet. no white noise at the tweeters. ..............and toggled the amp back to the integrate mode............ the internal preamp for the amp now hisses.

I interpret this as damaged input stage of the Power Amplifier.

unclejed613 4th September 2010 12:26 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.

PreSapian 4th September 2010 06:02 AM

thanks. i'll diy a phase plug as soon as i get the parts. what do you think could have happened to my speakers though? can any performance loss be experienced from the voltage pulse?

unclejed613 5th September 2010 07:50 AM

if they had you probably would have already noticed.

east electronics 5th September 2010 10:48 AM

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

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