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Amplifier Distortion, DC-Offset, and You!

I have a question about adjusting the DC-offset on a power amplifier:

I measured the DC offset on the Power Amplifer which is a Rotel RB-870BXD and it is:

-22.8mv Left -10.7mn Right

When I opened it up I saw 2 blue potentiometers which I figured were for this adjustment, however I was wrong (they don't change the value of the DC offset).

http://img345.imageshack.us/my.php?image=10060520zh.jpg
 
wdug said:
Hi-
I have two questions about testing DC-offset that I wasn’t able to find answers to within this thread:
1) Two of my receivers (the modest Sansui R-30 and the mighty Onkyo TX-8500) do not have Aux inputs on the input selector switch per se. They have FM, AM, and phono choices on the switch, and tape monitor buttons. Is it a valid test of DC-offset to set the input selector switch to FM (or AM) and engage the tape monitor button?
Click to expand...
Sure...
2) Is it a valid test to check DC-offset on a power amplifier with no source connected to the unit?
Click to expand...
Sure...

:D
 
rulerboyz said:
I have a question about adjusting the DC-offset on a power amplifier:

I measured the DC offset on the Power Amplifer which is a Rotel RB-870BXD and it is:

-22.8mv Left -10.7mn Right

When I opened it up I saw 2 blue potentiometers which I figured were for this adjustment, however I was wrong (they don't change the value of the DC offset).

http://img345.imageshack.us/my.php?image=10060520zh.jpg
Click to expand...
Those are very likely bias pots, and fooling with those is a quick way to let the factory smoke out. Don't take this wrong, but I've said in about 50+ posts in this thread: Don't go twisting pots unless you know what they do and how to measure the change they are making!

Find a manual, and adjust the bias...unless this is just a practice unit you don't care about.
 
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DC-offsetting Sansui au-9900

I didn't get much AK-help when my much belowed Sansui au-9900 amp started to give me trouble: http://audiokarma.org/forums/showthread.php?t=47493. I then ordered the service manual and I'm now ready to set the DC-offsets and the Bias Currents straight.

The symptoms are still the same: the protection kicks in after about one hour (also when the amp is unloaded). The right channel still measures about 50 mv when the protection lamp turns red.

My question is (still!): is it likely that a DC-offset of 50 mv is able to turn the protection on?

Peter, Denmark
 
pqrannes said:
I didn't get much AK-help when my much belowed Sansui au-9900 amp started to give me trouble: http://audiokarma.org/forums/showthread.php?t=47493. I then ordered the service manual and I'm now ready to set the DC-offsets and the Bias Currents straight.

The symptoms are still the same: the protection kicks in after about one hour (also when the amp is unloaded). The right channel still measures about 50 mv when the protection lamp turns red.

My question is (still!): is it likely that a DC-offset of 50 mv is able to turn the protection on?

Peter, Denmark
Click to expand...

Peter, I am not an expert, so this may not be a definitive answer to your question, but based on what I have gathered from reading this and other similar threads on AK, a 50mv offset value seems unlikely to be the cause of your problem.

It seems like an offset value in excess of 200mV is typically required before the speaker protection relay would trip.

Hopefully someone else with more technical experience can chime in with a more complete and authoritative response...
 
I just dug this up from early in the thread. The guy Echo is responding to had something like a 230 mv offset tripping the relay.

EchoWars said:
An unbalanced pair could generate distortion as well as trip the DC protection (but so could a dozen or two other things..) When troubleshooting, let the failure occur (protection circuit) and start measuring voltages. Start with the DC offset, measured before the relay, of course. :) Suspect the driver transistors, input pair, and the VI limiter.
Click to expand...

I was trying to find Echo's previous description of what it takes to kick in the protection. I seem to recall that it varies quite a bit from model to model, but 50 mv is definitely way too low to be the source of the problem, in any model! When the amp is working properly, it should take well over 100mV DC in any amp.
 
Peter - Here's another interesting factoid about Sansui protection circuits. (I suppose you'd need the service manual to know if yours has this feature.)

CDFixer said:
On some of the other Sansui amps the outputs have a thermal switch (located on the heat sink) that is incorporated in the protection circuit. If the output heat sinks get above 125 Deg, the protection will kick in. I have noticed that the bias current adjustment will make a huge difference in the temp of the outputs.

A 9090DB I had was running about 75 ma on one channel (should be 30) and it got really warm quickly, reduced the bias current to normal and the heat went away. I don't know if the BA is the same or not.


Hope this helps.
Click to expand...
 
The symptoms are still the same: the protection kicks in after about one hour (also when the amp is unloaded). The right channel still measures about 50 mv when the protection lamp turns red.
Click to expand...
Once the relay 'kicks out', you should measure nothing at the terminals. If you still do, the relay may be 'fused' (contacts melted together from a previous failure).

My question is (still!): is it likely that a DC-offset of 50 mv is able to turn the protection on?
Click to expand...
No...most amps require a volt or two to trip the protection. In all honesty, it sounds like there are multiple problems that may require a professional to address.
 
Thx to hpsenicka, John_w and echowars.

I feared that the 50 mv wasn't enough to trip the protection. And I noticed that the DC at the terminals drops to zero once the protection takes over. And as I've descripted in the Sansui forum the au-9900 ends up (after about an hour) going into and coming out of protection mode, again and again.

Well, I've properly got to wait for the local tech to find time for me and my amp.

But the substitute, my Sansui Eight, and the au-5900 doesn't reach up to knee caps of the au-9900!

And staring at page 3 ('Adjustments') in the Service manual (that I've received) I really don't understand where to put the minus (-) clip of my otherwise nice handescope measuring tool. I'm never going to be the tech I dream of.

I've got to persuade the tech let me look over his shoulder when I get that far.

Peter, Denmark
 
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I hope I'm not intruding on Echo Wars territory too much, especially since I'm not anywhere near his level of expertise. But, my 1 1/2 cents' worth:

pqrannes - If you really want to learn this stuff, I would suppose that you could ground the "-" clip about anywhere on the metal chassis, which is just a huge ground plane, electrically.

Also look at the instructions for checking the bias, especially if the manual mentions a thermal protection circuit as mentioned by CDFixer.

This may or may not reveal anything, but at least you would have some experience poking around and taking measurements.

On the other hand, poking around in a powered-up system requires a LOT of caution and at least some of those Sansui manuals "speaks the English not so good". Maybe take a look at the bias part of the manual and see how confident you feel about it. If this is your only amp or an especially prized possession, you might just want to take it in anyhow.
 
Question about DC offset / capacitively coupled amp

I recently bought a H/K 330B. This output of this receiver is capacitively coupled. With no load on the amp I read .5 volts DC. With an 8 ohm resistor across the speaker terminals this drops to 3 mv on each channel. A recent post in this thread indicates that this is as it should be. I thought, however, some earlier posts indicated that even with no load there should be no (or minimal) DC at the speaker terminals.

Can someone please clarify this for me?
 
Not sure if this topic matches this thread (and I give my sincere apologies if it doesn't) but is it possible to measure the saturation (clipping) level of an amplifier using a multimeter INSTEAD of an oscilliscope? My reason for asking is that I have just purchased a 10wpc tube amp and want to know how far I can crank it up without damaging my speakers :scratch2: ......
 
kenwood_lover said:
Not sure if this topic matches this thread (and I give my sincere apologies if it doesn't) but is it possible to measure the saturation (clipping) level of an amplifier using a multimeter INSTEAD of an oscilliscope? My reason for asking is that I have just purchased a 10wpc tube amp and want to know how far I can crank it up without damaging my speakers :scratch2: ......
Click to expand...


No a multimeter won't work for that measurement. Takes a scope, or a distortion meter would show a sharp increase in distortion as clipping approches.

Lefty
 
I pulled my Pioneer SA-9800 from use last week due to it no longer sounding as good as it used to, and tested it out today. DC offset won't stay steady; I'm testing it with my cheap digital multimeter and with the pre-power jumpers pulled. At one point I had a channel that would slowly build up to about 100mV then drop quickly to around 0mV, then start the cycle over again. Hasn't happened since, but I can no longer adjust it to zero DC offset and get it to hold steady there like I used to; either after turning it off and back on, or letting it sit on idle for a time. Both channels seem to drift in sync with each other; I zero both, and when I measure again they are both around the same measurement, sometimes as high as 150mV.

Time to get the old bird recapped? Anything else that may need replacing? -- Foe
 
Sansui au-9900 revisited

Since no one did comment on my au-9900 issue in the Sansui forum I fear I'm being annoying in some way I can't see my self. But stubborn as I am, I'll give it another try. The issue is my previous mentioned (in this thread too) troublesome Sansui au-9900.

This is what I wrote in the Sansui forum:
As I wrote here: http://audiokarma.org/forums/showthread.php?t=47493 my belowed au-9900 amp started to give me serious problems some weeks ago. After being turned on for about one hour it started to go into and out of protection mode. I set it aside waiting for my tech aquiantance finding time for me, replacing it with a eight reciever that I find a bit boomy compared to the au-9900.

But as those who also follow the tubes audio forum might have noticed I'm very impatient (fired up a vintage Braun tube amp without a variac or something alike - it turned out allright and I love my little tube amp).

Yesterday I thus checked and set the dc-offset and the bias on the au-9900 hoping - in despite of the good advice I've got from other AKs - that might solve the problem. The newly aquired service manual told me how to do it. I turned on the amp and it stayed turn on for hours (no red protection light). But back in the living room the au-9900 sounded horrible. After another hour (still no red protection light!) it sound even worse. Loose and with a lot distortion I think (my "audio memory" is not good, but it didn't sound good at all).

What went wrong? Setting the bias? I measured the bias between the measuring point pointet out in the SM and the red wire that I took off the measuring point. Was that wrong? The bias was set precisely as the SM said, 50mA. But I noticed that the amp was running kind of 'cold', not much heat on the top. Maybe I used the meter wrong and got the bias wrong, too low I guess, that could explane the distortion!

Got to open the amp again and redo the measuring. Any hints on getting it right or any explanations of the distortion are welcome.

I took the amp back to the basement and took a second look (measuring) of the bias'es. They have decreased to about 40 mA which could explane the distortion I heard. I got them back to 50 mA and I have left the au-9900 in the basement playing now and again. I have checked the biases and the DC-offset many times and stabilized both to the values given in the SM.

But still I wasn't quite satisfied with the sound. Could I really still detect some distortion? Or were my ears fooling me?

My tech aquaintance who still doesn't have time to go thru the amp suggested that I raised the biases a bit and I did so to about 55 mA. I think it sounds better now. But I also think it is running a bit cold, that is; it is not as 'hot' on the heatsinks as it use to when I thought it sounded allright.

I'm not quite sure that my multi meter (basically like this one: http://www.china-electronics.com/Products/meter/DT-9201.htm) is ok and is getting the mA's correct. Is there any way to check that?

I happened to have a old Handyscope HS 1 (http://www.tiepie.nl/uk/products/16) measuring tool that connects with my lab top. It does the W measurements just fine but I can't get it to measure A's. I'm sure it does a lot of things too, I could use if I only new how to. Any AK's got experience with the HS 1?

Many Questions. Again! I hope y'all will forgive me.

Thank in advance.

And greetings from Denmark on yet another Indian Summer day (October!)

Peter
 
Sometimes a Post just needs a bump to be seem at the right time. (We all get busy and sometimes we all get busy at the same time! this weekend was aboutb as dead as I have seen AK based on how few Emails I got on threads I'm subscribed to....)


Multi meter.

Use it to check your AC line. See if it reads abnormally high or low. If it does, try replacing the meter's battery. I thought I had 2 that went out of spec, but it was just low batteries and the meters were "measureing" about 30% high! :no:
 
pqrannes said:
Since no one did comment on my au-9900 issue in the Sansui forum I fear I'm being annoying in some way I can't see my self. But stubborn as I am, I'll give it another try. The issue is my previous mentioned (in this thread too) troublesome Sansui au-9900.

<snip>
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This is one of the Sansui's where you directly measure current by pulling a lead and setting the meter to read milliamps. IMHO, a dangerous way to get the job done, but if you are careful it ought to be OK.

Ths bias is not the source of your problem. If it were that touchy, SS gear would be unlistenable. As it is, there is a fairly wide range that will allow the amp to sound quite good due to the correction of negative feedback.

Whatever you got going on isn't related to bias adjustment. Check the offset, and if that's OK, then you will have to start somewhere else (clean all the switches, for one). You may have to wait till your friend has a chance to look at it...a sine-wave source and a scope would tell a LOT about what is going on.
 
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