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Amplifier Distortion, DC-Offset, and You!

I was at the Big Lots this afternoon, looking for a picture frame, but went down the tool aisle and ended up getting a cheap $5 multimeter. Got it home and checked out my amps, using the 200mV setting.

Pioneer SA-510 My old favorite.
Right: 5.2 mV
Left: 7.9 mV
Both settled down well and varied by only 0.1mV after 10 minutes. I've got a service manual, and was relieved to see these measurements low because there's no adjustment pots in this low-end model. Also confirms that I've not been crazy in thinking the old bird still sounded groovy.

Pioneer SA-608 Pretty much the same as a SA-6800, a recent eBay purchase which I worked on (cleaning dirty pots & switches) this weekend. Have only tested with cans; have not attached speakers yet.
Right: 26 mv
Left: 35 mv
Both never settled down good, after half an hour they were still moving up and down within a 1.5mv range. Guess I ought to figure out how to adjust this one before I hook up speakers.

Panasonic RE-7670 1970 low end reciever.
The suprise of the bunch. Both outputs varied from 0.7 mv to -0.7 mv. Hard to keep the probes steady on the RCA speaker outs, but I think I got good measurements. Perhaps there is something in the design of this old thing to keep the DC in order.

My Pioneer SA-9800 has been running TV sound for the family since I got home with the meter, but I'm rather sure that Terry dialed it in when he had it earlier this year. I'll get it measured once I have a chance, just to make sure it has stayed stable since it was worked on -- Foe
 
I just picked up a pretty cherry Scott 350R receiver. It's super clean inside and out. The pots don't even sound or feel dirty.

DC offset seems somewhat high, though. On the B terminals, I'm getting 40-50 mV and about 100mV from the A terminals. Anyone know if there're adjustment pots? Inside, on the board, I do see some pots, but I'm not about to start turning things willy-nilly. If a picture would help, I can take one and put it up here.
 
Sansui AU-555A (1973 Model)

All parts original.

DC-offset

8Ohm load
Right: 1.1mV
Left: 1.4mV

no load attached (int. resistor R49,R50 = 150Ohm/5Watt, tol. 5 or 10% Micron)
Right: 280mV
Left: 285mV

Regarding keeping the distortion as low as possible ?
I have adjusted the bias for 80mA each channal, f
or my use of the amp. a good compromise betwin performance vs. thermal loss :naughty:

Ps. dont just wank the bias up, themal runaway leading to great expences could be the result :smoke:
 
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Pioneer SX-880

Just measured an SX-880 at roughly 20mv on each channel..... not too shabby.

Will check again after a thorough DeOxit cleaning see if the results are differrent.

I suppose a service manual is in order so I can get in there and tinker with the pots.... and really foul things up so that it has to go to EW for an overhaul! LOL
 
Capacitor output

You guys mentioned capacitor output, why is my jbl drivers jumping for the heavens when I power up my amp. It has capacitor dc offset(cap output if you chose). all caps on the amp boards were changed, all transistors were changed(all six of em lol), still have the carbon comp resistors they are still tight. I don't get a thump but the drivers sure move in and out quite far a couple of times durring power up. just incase your currious this is a st120 dynaco amp, sweet as sweet gets to me anyway.
 
You pretty much asnwered your own question...as the output cap charges, it passes current to the woofer. As long as it is not a large excursion and low enough in frequency to be inaudible, no problem.
 
re cap

yeah it is quite a large excursion, I would guess almost to the speakers limmits, and it floats in and out a couple times. It will be a very sad day when I have to replace those big beautiful dyna caps with some tiny ugly modern cap, I think the writing is on the wall. Is their some way of filtering the output even more?, if I increase the capacitance will this help with the woffer blues or make it even worse. Digging out these caps is a real pain just to keep the nice st120 look. :worried:
 
carver 2000

My recent acquistion measures :

right channel 0.6 mV
left channel -3.3 mV

These are the values after power up and after 8 hours. It appears I'm doing alright in this area. Now to clean it up and see what needs to be fixed.

regards

Dave
 
dyna said:
yeah it is quite a large excursion, I would guess almost to the speakers limmits, and it floats in and out a couple times. It will be a very sad day when I have to replace those big beautiful dyna caps with some tiny ugly modern cap, I think the writing is on the wall. Is their some way of filtering the output even more?, if I increase the capacitance will this help with the woffer blues or make it even worse. Digging out these caps is a real pain just to keep the nice st120 look. :worried:
Click to expand...
The output caps are likely not the problem. Dig out your schematic, and replace C3, C4, and C5 on the driver boards. Digikey sells BC axial caps that would work great.
 
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thanks didn't get em from digikey, got some surplus stuff helped quite a bit. I am happy with the outcome and can live with a little driver diving lol, after all 60wpc is hitting the speakers pretty quick. I made a sonic improvment with the new old caps, got some pretty seimens(orange axial) stuff, looks even better, I increased the capacitance across the board a minumim of 10 mfd, and maintained a constant voltage that is within the limits of the transistors(I use all the old can type called for in the tip mod, very sweet but changed the output trans to mj802). just a quick note silver wire makes a noticible difference in this amp, I wired the entire amp with it even the power supply(stop laughing), a nice set of rhodium rca's and some REAL gold speaker binding posts will set you back a couple hundred bucks or more depending on your source but get ready for some nice, real nice music magic(very class A). Long live dynaco.
 
What is the problem when one of the measurements will net settle. The other channel settles right within a millisecomd. The other continously flucuates. Any idea here EW? Thanks for any insight you might have. Man, how you find the time for all of us i'll never now, but your one helluva guy!
 
DaWoofer said:
What is the problem when one of the measurements will net settle. The other channel settles right within a millisecomd. The other continously flucuates. Any idea here EW? Thanks for any insight you might have. Man, how you find the time for all of us i'll never now, but your one helluva guy!
Click to expand...
I second the motion! :thmbsp: EW for President!
 
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DaWoofer,
A certain amount of sloppiness is to be expected...fluctuations of +/-5mV is pretty normal. If you have more than that, you may have damaged transistors or old electrolytics that could use replacement.
 
Only problem is, if you only have a DMM (like me), it's sometimes hard to tell just how much it's fluctuating. Sometimes you need an analog meter just to see what's going on.

I do, however, have an oscilloscope. That's overkill, but I need an excuse to play with it anyhow! And, I'm kinda curious about the really eratic readings I got from the au-9500. It didn't seem to swing wildly out of spec or anything, but like I said, it wasn't too easy to tell just what the range was.

Is 60 Hz ripple...say, from week filter caps, a possibility? Or is it pretty much only a thermal response kind of thing when we see that?
 
john_w said:
Is 60 Hz ripple...say, from week filter caps, a possibility? Or is it pretty much only a thermal response kind of thing when we see that?
Click to expand...
Residual ripple may indeed be there, but if should be of very very low ampltude (<5mV). It should not affect offset.
 
I once had the same problem with an older DMM. Turned out to be old cables. If possible, recheck your readings with new ones.
Ron
 
Just adjusted the DC offset on my Marantz 2265

This is an amazing thread! i learned quite a lot reading it. Thanks guys.

I just adjusted the DC offset on my Marantz 2265. There are two trimmers for that. I got close to 0 V on both channels. Can't get to exactly 0, but very close. Now I am listening to it. Yes, there is a difference. I had readings of 39 mV and 70 mV before that. Now, one channel sounds "fuller" than the other one. I had a problem with that (I started a thread for that). It looks like it is coming from the pre amp section. It is like if I lost the rounded sound characteristic of the receiver. I had a creaky bass on one channel at one point. Could that be one of the cooupling caps gone bad?

Many thanks.

Eric
 
jblmar said:
I once had the same problem with an older DMM. Turned out to be old cables. If possible, recheck your readings with new ones.
Ron
Click to expand...

Well, the other amps do hold steady - offset just decreases gradually for a couple of hours as I monitor, then it holds pretty well. Otherwise, I'd really suspect that's the case, since it is a relatively old unit. It's a nice Fluke 77, but I suppose the cables could be starting to show their age by now.

Thanks for the tip! And as always, for the info from Echo as well!

- John
 
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