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Amplifier Distortion, DC-Offset, and You!

In the sansui AUD11 I have obtained 10mv of DC offset on each channel but it was not easy : with a little movement in the pot the offset change a lot even after cleaning.

So I'll replace these "one turn pots" with sealed multiturn pots, these devices have between 15 and 25 turns and they're not expensive : around $2.00 each. I found them in www.jameco.com
 
outlawmws said:
EW, I tried measuring the DC offest on a Sansui 5000. It almost acts like a tube amp when I tried measuring for DC once upon a time...

Is this a design that doesnt do DC offest? :scratch2:
Click to expand...
Is this a DC coupled design? Or cap-coupled? If it's a real oldie, it may be cap-coupled, and you can't get a proper reading unless you connect a load.

Find a 1K resistor somewhere and connect between the '+' and '-' speaker terminals and try again.
 
Pioneer SX-880

1. Pioneer SX-880 (it tests fully functional)

Multimeter set to 200m DCV scale:

Speakers A R -76mv L -55.3mv
Speakers B R -70mv L -51.5mv

I found the instructions for adjusting bias from a post by Pioneer Guy:

SX-880 DC Balance


After adjustment:

R -0.5 mv L -2.0 mv

A look inside

2. Marantz 2015

Main R (-5 to +5) L (-5 to +10)

The numbers keep shifting, is this significant?


3. Rotel: Preamp RTC-850 Amplifier RB-870BX

R -7.5 L -21.3
 
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Pioneer SX-880 Problem

The reason that I had a look at this receiver is that the owner told me that the sound was breaking up intermittently in either the left or right channel. I now see that the problem seems related to the Function knob. The problem affects only the AUX mode (it doesn't lock on completely). Phono, Radio and Tape play nicely. Volume knob has some slight crackling noise when I turn it.
 
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rulerboyz said:
The reason that I had a look at this receiver is that the owner told me that the sound was breaking up intermittently in either the left or right channel. I now see that the problem seems related to the Function knob. The problem affects only the AUX mode (it doesn't lock on completely). Phono, Radio and Tape play nicely. Volume knob has some slight crackling noise when I turn it.
Click to expand...

A thorough cleaning of the controls with DeOxit will likely help tremendously.....
 
Thanks for the advice, I'm waiting for a can of deoxit to come in the mail. I just compared the difference between Aux and Tape 1. Tape 1 delivers the goods, while the clarity just isn't there in Aux.
 
SX-850 Check

I just checked the dc offset on my new 850 and here is what i read:
Left: rapid fluctuation from -.05 to +2.5
Right: rapid fluctuation -2.9 to -4.8

Is this pretty normal? I followed the measurement instructions to the letter. I read elswhere in the thread that fluctuation is normal, but is a rapid fluctuation normal?
 
Some units are just twitchy...connect the speakers and it might calm down a little.

I don't think I'd lose any sleep over it.
 
Checked DC offset on m,y SX-950. I'm getting 3.1 mV in left and 2.2 mV right. Does this sound too good to be true? I checked it a few times and that's what I get.

I purchased this Pioneer on ebay about 2 months ago - and before finding aduio karma. The unit sounds very nice although the phono section does not seem to be working properly. I'm using my Proton receiver's phono section for now with tape out to the Pioneer.

While I'm on the tape out subject, I thought I would have to use the tape monitor switch to use the tape in on the Pioneer. Turns out I have to "de-select" the input buttons on front - puch two at same time so there are no inputs selected. Does this sound kosher?

Jim
 
Thanks! Are you familiar witht the Proton 940 receivers?
The selector pot must be dirty - I beleive they were known for this. After a while I loose the right channel. I tried cleaning it with some tuner cleaner but the pot appears to be sealed. It's tight to get at. If I look through the front with knob off it looks like there is a c-clamp with plastic bushing behind it. I'm assuming the only way to clean the pot is to remove the c-clamp and hoepfully the plastic bushing can be pulled off.

Since it is so tight I thought I'd as some Ak'ers for some guidance. What's the best way to remove a tightly enclosed c-clamp? Not sure if I'll have enoug rome to work a pair of needle nose in there.

Jim
 
Going back to the original post in this thread... according to EW anything less than 50mV is considered acceptable... 15mV or less is excellent!
 
I have an old Japanese 'Sound' brand amp with Sanyo transistors mounted on heatsinks on the back. It reads 200mV and 1V.

No thats not a typo I had to change the range to read the left channel LOL.
I use this hooked up to a tuner but only have one speaker plugged in (right channel) and it gets played at low volume in mono mode. Is this something that I could easily fix? do you know what I should be looking for inside this old beast? I should mention that this is not my 'hifi' its just used as a radio at my workbench. LOL.

thanks for a great thread BTW
 
If the amp has an adjustment, then adjust it. If not, then offset is at the mercy of transistor input pair matching...so you'd need to match a couple of transistors and install them.

The dangerous part is that most amps also have adustable bias, so there may be two adjustments per channel...offset and bias. Trying to set the offset while cranking on the bias pot is a sure-fire (pun intended) to fry the amp.

Caution suggested. A manual would be great.
 
THank you Echowars for the very interesting thread. A simple test like this makes the prospect of getting to know how an amp works less daunting!
At the start of the thread you asked for people's readings; here are mine for a Luxman L-308 after 10 minutes: L: 4.3 mV and R: 28 mV
The values kept going down however, after about an hour they were L: 3.2 mV and R: 26 mV. I think I'll read the DC-offset after keeping the amp turned on all day, to see if it's lower. Or is that cheating :)?
BTW, a couple of pages ago you warned against reading Amperes on your meter when it's connected to the speaker terminals, as it - I think you said - smokes the magic. I'm afraid I missed this warning before and did actually read A with the multimeter connected. Does this damage the amp? It still sounds very good so I'm hoping not. I guess I still have a lot of reading to do on this amazing site!
regards,
Willem
 
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