Amplifier Distortion, DC-Offset, and You!

I just tested my Technics SA8000X Quad and the highest is 22mv so I think I am good for now. I did not have a problem I just wanted to test it after reading the beginning of this post.
 
My new old sansui 9090 dc offset - I'm doing this with the speaker switch turned to off or should i have it turned to the pair I'm testing?

Right channel 43.7 fluctuates 2mv up down
Left channel 44.5 fluctuates 2mv up down
 
when i change the speaker selector to speaker A the pair I'm reading from the readings are:

R Channel 0.0 mv
L Channel 0.3 mv

I'm thinking these are the correct values since the values before I had the speaker select knob set to off?
 
redk9258 said:
Are you sure your meter is on the right scale to be sensitive enough?
Click to expand...

My bag I previously removed the the speaker knob for cleaning and I put it back on off a click. So my readings are

R Channel 43.5 mv
L Channel 44 mv

Should I bother to adjust its a 1976 sansui 9090. Anyone have pics of someone adjusting dc offset for this model?
 
Be careful. I'm not a technician, but have read enough threads here where someone slipped with a meter probe or screwdriver shorting something out. This lets the "magic smoke" out. Also, old trimpots may not respond well to adjustment and cause you to blow the amp. If it sounds fine, leave it. Some models do not have DC adjustment trimpots. They rely on matching certain transistors.
 
redk9258 said:
Be careful. I'm not a technician, but have read enough threads here where someone slipped with a meter probe or screwdriver shorting something out. This lets the "magic smoke" out. Also, old trimpots may not respond well to adjustment and cause you to blow the amp. If it sounds fine, leave it. Some models do not have DC adjustment trimpots. They rely on matching certain transistors.
Click to expand...

I think I will just check to see where my bias is set at and my try to adjust the trim pots very carefully. Just a quick question I let my receiver warm up without any load on the speaker terminals I'm assuming this is ok since its solid state?.
 
EchoWars said:
Got the amp back together today (parts finally in), and fired it up with my small Dynaudio's (can't use the Heil/Dynaudio's for this, as they are bi-amped). Ummm...it sounded like ass. OK, it didn't always sound like ass, but at low volume levels it was obvious that something was wrong. I had a pretty good idea what was going on, so I grabbed my meter. Sure enough, there was 100mV of DC offset in the left channel, and almost 250mV (!!!) in the right! 250mV is almost enough for the protection circuitry to kick in!! Not good. I pulled the driver boards out and replaced the amp input differential pairs for both channels with new Zetex HG PNP's ($5 in transistors, no biggie). DC offset is now about 12mV in both channels. An input pair being as unbalanced as the Kenwood was when it arrived probably generates 10x the distortion as a properly balanced pair, especially at low volume. If you would like to read more about the benefits of a balanced differential pair, read here.
Click to expand...

Hmmmmm.... interesting guidance however the guidance given for bias adjust on a KA 9100 is to fine tune to 0-VDC. This is if the original spec manual (owners) is used per guidance.

On another note, it was rather difficult, using a DVM to accomplish, most recently. So as an alternative, I used a known input signal, music material or otherwise and zeroed the meters built into the 9100. At first I thought this to be insignificant.. until at playback, the channels appeared more consistent (wattage and clarity).

Not sure if I did the same as per the manual or pulled a troubleshoot "okeedoke"?

Your thoughts? :dunno:
 
EchoWars said:
Got the amp back together today (parts finally in), and fired it up with my small Dynaudio's (can't use the Heil/Dynaudio's for this, as they are bi-amped). Ummm...it sounded like ass. OK, it didn't always sound like ass, but at low volume levels it was obvious that something was wrong. I had a pretty good idea what was going on, so I grabbed my meter. Sure enough, there was 100mV of DC offset in the left channel, and almost 250mV (!!!) in the right! 250mV is almost enough for the protection circuitry to kick in!! Not good. I pulled the driver boards out and replaced the amp input differential pairs for both channels with new Zetex HG PNP's ($5 in transistors, no biggie). DC offset is now about 12mV in both channels. An input pair being as unbalanced as the Kenwood was when it arrived probably generates 10x the distortion as a properly balanced pair, especially at low volume. If you would like to read more about the benefits of a balanced differential pair, read here.
Click to expand...
gadget73 said:
DC offset is something you get with direct coupled output designs. It sets the balance between the positive half of the amp and the negative half of the amp. Not all amps have an adjustment for this. The goal is to have no DC voltage at the output, but some small amount is acceptable. The specified value changes from amp to amp. One of mine allows up to 10mv.
Click to expand...

This answer was given in another recent post ... I believe it clarifies my issue and perhaps those of other posters :thumbsup:
 
tippyd said:
I think I will just check to see where my bias is set at and my try to adjust the trim pots very carefully. Just a quick question I let my receiver warm up without any load on the speaker terminals I'm assuming this is ok since its solid state?.
Click to expand...
Keep in mind adjusting trimpots that have not been moved in decades may not be a good idea. Like a scratchy volume pot this may be similar. Messing with the bias trimpot could cause it to go wide open an take out many transistors, diodes, etc. It might be a good idea to use a meter to see how the trimpots respond (with no power of course). Might be best to just replace them.

As far as I know, solid state amps do not need a load.

You might want to make a post in the Sansui section. Many experts there to offer advise.
 
I
EchoWars said:
Time for one of those long, boring semi-technical posts that no one here reads...

As a few of you know, I bought a rare Kenwood 700M amplifier a few weeks ago on eBay. It arrived with a weak channel which was taken care of by replacing a bypass cap. Since then, I have gone through the entire amp and replaced all the electrolytic capacitors with the exception of the big power supply caps (not that expensive...maybe $20 in caps).

The previous owner described the amp as 'perfect' sounding, and compared with the big Mac's and Krell's and such. Before buying it, he described his current system which was quite high-end. I assumed that since he owned expensive equipment that he knew what he was listening to.
:withstpd:

Got the amp back together today (parts finally in), and fired it up with my small Dynaudio's (can't use the Heil/Dynaudio's for this, as they are bi-amped). Ummm...it sounded like ass. OK, it didn't always sound like ass, but at low volume levels it was obvious that something was wrong. I had a pretty good idea what was going on, so I grabbed my meter. Sure enough, there was 100mV of DC offset in the left channel, and almost 250mV (!!!) in the right! 250mV is almost enough for the protection circuitry to kick in!! Not good. I pulled the driver boards out and replaced the amp input differential pairs for both channels with new Zetex HG PNP's ($5 in transistors, no biggie). DC offset is now about 12mV in both channels. An input pair being as unbalanced as the Kenwood was when it arrived probably generates 10x the distortion as a properly balanced pair, especially at low volume. If you would like to read more about the benefits of a balanced differential pair, read here.

OK, sit down for another listen. NOW we're cookin'!! Amazing night and day difference. I can honestly say that it is without a doubt the nicest amp I have ever listened to, and there have been more than a few that impressed me. The bottom end on this thing is as clear as spring water, and it has an openess that is difficult to describe. As for power, my dummy loads cannot take the power output of this thing, but I can crank it for 10-15 seconds without destroying them. Left channel - 218W before clip, right channel - 220W, this into 8-ohms. Totally cool!! I just have never heard bass like this...wow...

Bottom line...if you expect to hear great sound, you just might...regardless of the reality. The guy I bought this from was well-meaning, but did not know how to listen subjectively. His new spendy amps could be performing horribly, and his expectations of what he felt he was supposed to hear would rule out anything to the contrary.

Your own subjectivity could be suffering too, so give yourself a reality-check.

As a semi-poll, I'd like to see those on this board whip out their multimeters and take a look at the DC that is being presented to the speakers. This means..

1. Speakers disconnected (or connect the meter to the 'B' speakers and set the front panel speaker control accordingly)
2. Input set to an unusued position (not Phono)
3. Volume control at minimum.
4. Balance in center
5. Tone controls either defeated or set to mid position
6. Set your meter to read DC, and set to a low scale (300mV scale is common) Connect directly to the Pos and Neg of the speaker terminals
7. Give the amp 10 minutes to settle. Report back...I'd like to see how healthy all these old amps are.

If you read:

0 - 15mV: Damn good!! If you read '0V', you may have a capacitor output, or your meter is set wrong

16mV - 50mV: An acceptable value, especially at the lower end of this range. 2nd harmonic distortion is probably twice to four times what manufacturer's spec calls for at higher frequencies. Probably not audible, as the distortion is mostly in the upper octaves. At the upper end of this range I begin to raise an eyebrow. :saywhat:

50 - 85mV: Something is certainly amiss, and while this is not enough to put your speakers or equipment in jeopardy, the amp is running nowhere near where it should. I'd venture to guess that most of the DC-coupled amps that are in use by forum members here fall into this range.

100mV to ?: A high enough voltage will cause the DC protection to kick in. This happens at a level determined by the designer, but is usually equivalent to about a diode drop (600mV)or so. Needless to say, if you are listening to an amp with 100mV or more of DC offset, you have no idea what the amp really is supposed to sound like. Indeed, some amps without a differential input are actually designed to have a bit of DC at the outputs, but this is triple-rare, and I don't think anyone here owns one. (in my book it's piss-poor design, but if you can sell it WTH..)

Soooooo...go grab a meter and tell me what you find...
Click to expand...
Checked mine with a fluke787 got 7mvdc qx-9900 right front wont play. I set recievers to aux 2 turned midway up base and treble to mid.
 
My fully refurbished Marantz 2216 that had been working fine last year, blew a tweeter fuse on my ADS speakers the other day on the left channel. I put a multimeter on to test both channels, and while the receiver previously had been very stable at about +5 mv on both, now appears to pass a dc spike at power on (reading of 1 volt at power on) then settles to fluctuating between -2 mv and +10 mv. What is the most likely cause of this behavior? I don't want to use this beastie again until I can get her very stable again.

I think the fuse blew because this ONE TIME I forgot to leave the speaker selector switched off on startup, and whatever amount of voltage came through on power up, it was enough to blow the 1.5 A fuse on the ADSs.

It's a PITA that Marantz did not have protection relays! I love my Yamaha CR-420 for that reason, and many others.
 
I know the KR-9600 I had wasn't adjustable. I had too change the diff transistors because one side had a DC offset of 85 and the other was 150mv. I guess anything under 50 mv is acceptable. So it should be fine.
 
Not too great either. Just a few more clicks up and your moving to the adjustment zone. I'm not chearing with those numbers, but that's just me.
20 and under, 50 is not acceptable.
 
You must log in or register to reply here.

Similar threads

bob412
Measuring DC offset on a Lepai Tripath amp
Replies
1
Views
347
battradio
battradio
Tris
Sx-990 DC offset
2
Replies
34
Views
6K
gadget73
gadget73
Greyrat
HH Scott and DC offset
Replies
6
Views
3K
jasoncauthron
J
Chris Brown
DC-Offset and Amplifier bridging
Replies
6
Views
651
TerryS
T
D
Amplifier Distortion and DC-Offset
Replies
1
Views
1K
Fred Longworth
Fred Longworth
Back
Top Bottom