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Amplifier Distortion, DC-Offset, and You!

gonzothegreat said:
Is the offset stable in either channel? Does the protection trip in a regular and repeatable manner? Will it idle (i.e no music) for long periods and not trip? See if you can find the amp output before it gets to the protection ckt and see what is amiss. Can you disconnect the preamp from the amp?

EW - Could this be a protection circuit issue? Or is it more likely an amp problem?

Theoretically it could be something as trivial as dirty pots. Swap 'em out for multi-turns and if that isn't the issue, you'll be glad you did anyways.
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The offset is not stable in either channel. The protection does not trip in any sort of pattern nor in response to output level. It will sit idle with nothing hooked up to it and not trip. I can uncouple the amp/preamp sections. I'll need to seek the help of a knowledgable friend to track down the output before the protection circuit. I've got the schematics but aside from the bloody obvious I couldn't find my :butt2: with both hands. Thanks

mac
 
Dirty pots...probably need replacing.

...although the relay circuitry is known to have problems on these, I'd vote the pots or a busted ground wire to the driver board.
 
EW, 4 9090's here! What for, I don't know :scratch2:

Well, one of these 9090's isn't too bad so I did the ofset and bias. BIG DIFFERENCE IN THE SOUND!! Just one question. Service manual says to set the bias at 50 ma with leads at the fuse?? Seems that everyone else is setting voltage, not amps. And they are setting at 30 or less? Which is correct??

BTW, You can fix the DB! Just gotta figure out how to safely ship it to ya. Thats the one that kept blowing my precious JBL drivers. I will soon send you a PM with Phone number for an estimate. :thmbsp:
 
Follow the book procedure. It's a PITA, but while I usually measure the voltage at the emitters and calculate the current, it means identifying the emitters and getting the leads on them, which isn't easy and the risk of shorting something is great.

Be interesting to see what's up with that db unit. I remember the description and it's a bit strange.
 
Measured up a broken Sansui Eight

Halfway reading this very, very interesting thread I could help myself and started to measure up some of my Sansui stuff.

My main amp, an AU-9900, came out (cold) with a 15 mV vs. 2mV which I take is ok.
An old and dusty receiver 661 was in the 30s in both sides.

But then I put the meter to my broken Eight Receiver! :worried:

I need to explain that this nice item and its problems have been discussed in this thread Sansui Eight - serious problems . Apparently the preamp section is working (with external power amp section it played nicely), but something keeps popping two (to the right on the picture included here) of five Danish Demko fuses when the Eight is turned on.


The DC of the Eight (without the two burned fuses, though) was about 145 mV in both sides! But then: what was I looking at? At a wrong measuring because two fuses were missing? Or at the reason that the fuses keep blowing? :scratch2:

Peter, Denmark
 
I'm unfamiliar with the internals of this one, so anything I'd say would be a guess. I'd suggest that the root problem (blowing fuses) be addressed before worrying about anything as trivial as offset.

Sorry, but that's about the best advice I can offer knowing as little as I do about the unit.

pqrannes said:
The DC of the Eight (without the two burned fuses, though) was about 145 mV in both sides! But then: what was I looking at? At a wrong measuring because two fuses were missing? Or at the reason that the fuses keep blowing? :scratch2:

Peter, Denmark
Click to expand...
 
Hey, ew..

Any experience with the Realistic Sta-2200? I have one, just got it, and it seems to be working nicely... Only thing is... It seems to be putting out some noticeable heat from the top, right above the amp modules... at low vloume... and with high volume, the heat does NOT increase much.. Now, I THINK i did an offset measurement on this, before I put it in my system, but that was with no load, and COLD.. Do I need to do this again, after the receiver has been on awhile?I only looked at the insides a little, and so, since this one uses an FET amp,can I do the offset adjustment the same way? After all, it STILL does have a complimentary amp(2sj49/2sk134) pair.


BTW... As for measuring and setting idle current .. In the past, I have had to do this, and usually I just put a current meter on the AC input,a voltmeter on the base of the outputs, and adjusted idle Bias, till the minimum current was showing, with no signal..And I usually had about 0.4 volts or so on the base of the output(idle) at this point.Is this ok to do?
 
If it's a DC coupled amp, then offset is still an issue regardless of whether it uses bipolars or FET's at the output. Check offset after it's been on for an hour or so.

Your method of measuring bias by looking at AC input current is pretty useless. All you're measuring is power consumption, which is related to the bias current draw, but you can't even guess what the actual bias current might be from that figure within a factor of 10. If you really want to know what the bias is, then you will need to locate the source resistors on the FET's and measure across them.

As far as the heat, there is likely a voltage regulator inside that generates a bit of excess heat. Also, if this is a FET amp, bias is generally high on these, and even at idle there is a fair bit of heat generated.
 
Bias current..

Thanks for the advice.. I should tell you, that this is how the techs at Tandy did it, And I guess I just picked up on it. what you say DOES make sense, though.

As for MY Receiver, It DOES use Mosfets for the outputs, and has a pair of oversize-looking sinks on them. SO.. THATis the reason for those huge sinks in the STA-2200, 'eh?
 
I just got around to reading this thread and its real interesting. I've only got the SX-1980 near me so thats the one I tested. 1.4 mvolts on the left and 5.5 on the right. :yes:
 
Suitable substitutions for 2SA620's?

EW or other techs/modders,

I'm looking to replace an input pair of 2Sa620 pnp transistors. The paramaters for these guys are 30/.05/.2/250/120. They have -56-59 volts on the collectors (?) so going up in voltage would be good (?). In reading through this thread and searching a bit a few possible substitutions have come up:

Trying to keep the Ic value close to the original:
2N5087 (50/.05/.6/250/40)
2SA872 (90/.05/.3/250-500/90)
2SA992 (120/.05/.5/500/100)

or, can I use the ZTX795A devices with the .5A Ic?

If the 2N or ZTX's can be used they can easily go on the next donation to Digikey :-) The others will take a bit more scrounging, but do appear to be available.

thanks for any suggestions,
Steve.
 
I prefer the Zetex ones. Very high gain, and the larger die necessary to handle the 500mA of current leads to less thermal modulation of the die (don't ask...)

The problem is that you need to match a pair of them, unless your amp has provisions for adjusting DC offset. In order to get two matched pairs, you likely will need to order about 20 transistors. Wouldn't be such a big deal except we're talking about $20 worth of transistors...
 
Thanks Echowars!

I can match for hfe on one of my meters, but I'm not sure how to match for beta if that is required. There is an offset adjust on this amp so I may be OK. If it only takes another $20 on this amp I'll be happy!!

thanks,
Steve.
 
SteveP said:
Thanks Echowars!

I can match for hfe on one of my meters, but I'm not sure how to match for beta if that is required.
Click to expand...
They are close to the same thing...good enough.
There is an offset adjust on this amp so I may be OK.
Click to expand...
Then matching isn't near as critical. I'd buy 10 of 'em, and find two pairs that were reasonably close to each other and call it a day. :thmbsp:
 
What an awesome thread!! :) My complements to EchoWars for all the work. :thmbsp:
I haven't read it all yet but I have printed all 26 pages for future reference. :banana:
 
D.C. Offset

I checked my Kenwood Eleven reciever and it checks in at 3.8 and 4 or right in between there. Not bad? :banana:
 
Kenwood Eleven

By the way, I have a service manual and it shows where to adjust for the bias, but its states nothing about how to adjust the D.C. Offset. So, do you have any info on these Kenwoods regaurding the adjustment for Offset? Thanks for your knowledge and any input you might have. Bryan :scratch2:
 
A lot of the Kenwoods simply don't have an offset adjust...and that may be the case with yours. If you are measuring 3.8mV and 4mV (you didn't state the voltage range you were on) on the channels, there's nothing you need to do except sit back and listen. :)
 
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