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Amplifier Distortion, DC-Offset, and You!

Thx to EW and Outlaw. I'm most obliged. You both definately are an important part of audiokarmas back bone!

I'll take a second look at my multimeter and give the switches another cleaning; I keep cleaning the balance switch and it keeps getting noisy again (after some hours). Maybe my de-oxit stuff isn't as good as good as the kind available on the other side of the Atlantic?

About the amp running to 'cool': I kept it turned on all night (no input, volume low) but it is still cool, not even luke warm on the top. How does that sound? Someone told me that the transistors 'peak' at 75 celsius degrees and that the tough guys adjust their biases with a thermometer. I guess that doesn't go with the old Suis and other vintage stuff!

Peter, in still sunny Denmark (where did autumn and winter go?)
 
pqrannes said:
About the amp running to 'cool': I kept it turned on all night (no input, volume low) but it is still cool, not even luke warm on the top. How does that sound?
Click to expand...

I suspect you'd need to come close to disturbing the neighbors (or use a dummy load with the volume cranked) to get much heat output from that amp.

My AU-9500 never really breaks a sweat no matter how long and hard I work it, and it's the only amp I have that has the offset reasonably within spec. It would get just a little warm after cranking it up for hours (which I did more than a few times in college). At low volumes, even overnight, it's cool as a cucumber. It seems to warm up the most if it's cranked up to around '4' on the volume control.

Sansui used to use a LOT of heavy iron in those heat sinks, probably to a large extent because they started as an industrial transformer company. I would guess that if you're getting good volume out of it (should be quite loud at '4' on the volume) then the lack of heat production is a really good sign; under normal circumstances, the cooler it runs the better!

So, as EchoWars already stated, sounds like the problem is elsewhere...
 
Problem setting bias

I got the technical manual for the Rotel power amplifier RB-870BX. I tried to follow the instruction but didn't notice any change in my readings. The instructions for setting the bias were as follows:

Instruments: DC milli-voltmeter

Notes: Prior to bias adjustment, run about 5 minutes with rated output (8 ohms) and warm up Power Transistor and heat sink. Set input off.

Coupling= Plus lead goes to TP2 and Minus lead goes to TP1. The adjustment is made on a potentiometer labelled VR601 on both the left and right side. The adjustment should me made to read 5 mv.


However after turning the potentiometer, I don't get it to deviate from 0 mv.

Another general question I have is what is the general rule for where the positive lead and negative lead of your multimeter goes when you are testing a voltage drop across a resistor.
 
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You answered your own question... You want to know the voltage drop across a transistor, that's where you measure.

Why are you fooling with the bias? You're either measuring in the wrong place, or turning a wrong pot...very dangerous.
 
"Why are you fooling with the bias? You're either measuring in the wrong place, or turning a wrong pot...very dangerous."

I was just following the technical manual, trying to make sure I turned it to the correct value...and there is only one type of adjustment pot for this unit (one for each channel). The reason I am "fooling" with it is as a precaution. I turned the pot by accident a while ago and wanted to make sure I put it back the way it was/should be (according to instructions) before I use the amplifier again. I was just surprised when the pot didn't have any effect on the value at the test points. The first logical conclusion, as you pointed out, would be that I was on the wrong pot or measuring the wrong value or making an unnecessary adjustment.
 
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I C... Well, I asked since the general gist of the thread was offset adjustments, so my inquiry was directed in an effort to ensure that you weren't wanting to correct an offset problem by adjusting the bias.

Rather than rely on the manuals procedure, see if you can get your meter leads on the emitters of the output devices themselves...one on the NPN emitter, and another on the PNP emitter. Sometimes this is difficult, but you will be able to calculate to 100% accuracy the bias current with no question (as long as you know the size of the emitter resistors).
 
I am new to this forum. I just purchased a Kenwood 600 on eBay (paid dearly for it). It is in beautiful condition. The left channel seems a bit noisy. I want to send it to someone reliable and reasonable for cleaning and adjustment. I live in NYC. Can anyone recommend a good place to service this unit?
Thanks.
 
I am new to this forum. I just purchased a Kenwood 600 on eBay (paid dearly for it). It is in beautiful condition. The left channel seems a bit noisy. I want to send it to someone reliable and reasonable for cleaning and adjustment. I live in NYC. Can anyone recommend a good place to service this unit?
Thanks.
MLMD
 
The 600 is a real piece of work, but you'll have to hope the problem isn't in a noisy FET. Disconnect the amp from the preamp via the switch at the rear, and see what happens to the noise...

...and start a new thread. :yes:
 
I read this thread a while back but have only now bought a multimeter and tested my receivers. Great thread, by the way. Thanks for all the great advice Echowars!

Yamaha CR-640:
left: 16.4
right: 4.5

Marantz 2230:
left: 0.0
right 0.0

I'm not sure how the Marantz could read 0.0 on both channels the way it does (it does occasionally jump to .2 or .4, but goes straight back to 0). It's funny, when I turn the power off on the receiver, the multimeter jumps up a bit, and there is a small reading (only 1 or 2 mv, but at least there is some movement). Then as soon as I turn the receiver on, it goes right back down to 0. The receiver does sound pretty good to me, so I'm not sure if this reading is something I should be worried about or not. I haven't been able to read all 52 pages of this thread, so I'm not sure if anyone else has had this issue or not. My understanding from the initial pages of this thread was that no reading might be something wrong. Any help would be appreciated. Thanks!

P.S. I'm not sure if this matters or not but I have the Marantz plugged into a power strip and leave the power button turned on at all times. I turn the unit on and off by flipping the switch on the power strip. I was told once that the power buttons on marantz receivers were prone to breaking and that this was a way to avoid that from happening. I don't think this would make a difference, but I throw it out there just in case.
 
Sounds like bad contact somewhere...most likely in the protection relay itself. The only units that will hold at zero are those with a servo, and the Marantz does not have one.
 
Should look a lot like this:
4PRLY-12S.jpg


Just got to get the cover off and clean the contacts. I use a thin folded piece of #1200 sandpaper whetted with DeOxit, and then flush with DeOxit before I put the cover back on.

You don't want to scrub on the contacts much with the sandpaper, as the plating is quite thin and once removed the life of the relay is much diminished.

I might add that if you aren't experiancing any real problem you might do well to wait till one develops.
 
Wow! A picture and everything. You are amazing Echowars. All that knowledge right there at your fingertips. . . .

I am not experiencing any problems with the Marantz at all, so I think that I may not touch the protection relay right now. I am using the receiver with 2 nice sets of Bozaks and the sound is great as far as I am concerned -- best system in my house. Probably better just to leave it alone for now.

Thanks again for the great advice!
 
As the relay contacts degrade, the voltage necessary to make them conduct grows higher and higher. Eventually you'll have to 'race' the volume control way up and quickly back down to overcome the crud. Right now is seems that the voltage necessary to make the contacts conduct is only a few millivolts. Eventually it will probably need attention, but for now, just listen and enjoy.
 
joenielsen said:
I read this thread a while back but have only now bought a multimeter and tested my receivers. Great thread, by the way. Thanks for all the great advice Echowars!

Yamaha CR-640:
left: 16.4
right: 4.5

Marantz 2230:
left: 0.0
right 0.0

I'm not sure how the Marantz could read 0.0 on both channels the way it does (it does occasionally jump to .2 or .4, but goes straight back to 0). It's funny, when I turn the power off on the receiver, the multimeter jumps up a bit, and there is a small reading (only 1 or 2 mv, but at least there is some movement). Then as soon as I turn the receiver on, it goes right back down to 0. The receiver does sound pretty good to me, so I'm not sure if this reading is something I should be worried about or not. I haven't been able to read all 52 pages of this thread, so I'm not sure if anyone else has had this issue or not. My understanding from the initial pages of this thread was that no reading might be something wrong. Any help would be appreciated. Thanks!

P.S. I'm not sure if this matters or not but I have the Marantz plugged into a power strip and leave the power button turned on at all times. I turn the unit on and off by flipping the switch on the power strip. I was told once that the power buttons on marantz receivers were prone to breaking and that this was a way to avoid that from happening. I don't think this would make a difference, but I throw it out there just in case.
Click to expand...


I thought I read is some thread here lately that the 2230 used capacitor coupled output stages. If that is the case, 0 mv would be the expected value as the output cap blocks DC but passes the audio AC signal.

Lefty
 
If it is cap-coupled, then 0mV is what to expect. And there certainly wouldn't be a relay.

For my part, I think I read 2330 instead of 2230. My bad.
 
Good news - fixed the DC Offset of my Marantz SR-6000DC

AK member oldtubes kindly dropped me a post telling me where I could find the trimming pots to adjust the DC offset on my Marantz, and I've been able to tame it down from 120mv to approx 1mv on the left and 1mv on the right. Those values are are acceptable, right ? :banana: :banana: :banana:
 
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