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Amplifier Distortion, DC-Offset, and You!

Kenwood KA-9100

Before
Right -15.0
Left -35.0

After
Right -0.0
Left -0.0

Both +/- about 5-7 due to dirty? pots.
Simply pushing down on the Left channel nearly brought it down to 0.0 so I figure they need a good cleaning.
I didn't see a BIAS adjustment, does the Kenwood have one?
 
These are worth their weight in gold for connecting to components, and help to make sure that a slip of the hand does not result in a blown amp...see if they will fit the leads of the meters that RS has...
 
Originally posted by EchoWars
These are worth their weight in gold for connecting to components, and help to make sure that a slip of the hand does not result in a blown amp...see if they will fit the leads of the meters that RS has...
Click to expand...
Okay I got the 29 Range Multimeter and the mini connectors. The mistake I made was bringing my kids. I wound up buying some play station memory and another controller so my son can kick my butt at NBA Shootout. I will try to test my Technics SA-303 tomorrow:D
 
Adjusted My SA-8800...

I finally got around to testing and adjusting the DC-Offset for my Pioneer SA-8800. Previously, I didn't have a multimeter sensitive enough to perform this test, but since I bought a DMM for my SX-850 project, it was only a matter of time before I got around to this.

My buddy Ryan (justforfun52), came over tonight and at his urging we tested the 8800. Wow, I hadn't noticed any problems but she hovered around:

-90mv (right channel)
-110mv (left channel)

****. That sucked. Well, we popped the top hoping the DC offset adjustment pots would be easy to locate. The problem is there are like 6 pots on the power-amp board. Hesitant to mess with it not knowing what pots to turn, we were about to put the top back on (despite Ryan's uring me to 'go for it') when I had an idea.

We hooked up the DMM, powered the amp up and checked our readings again. Same as before. I gently tapped one of two 'suspect' pots. Well, the DMM dropped down about 10mv just by touching it. We waited for it to settle down again, and tapped a second time just to be sure we weren't mistaken. Well, to make a long story short(er), we found the right pots and were able to get the DC-Offset down with our fingers so it was fluctuating between -1 and 1 mv on each channel. Damn that's good hey?!? Well, it sure felt good after that initial shocker.

We hooked it back up and turned it on. Sounds great! We didn't get a chance to turn it up because it was past 12, but we'll definitely get around to that soon.

Tomorrow we're going to adjust my 7800 which tested at about 10mv on each channel. We'll also test and adjust the SX-1050, SX-535, and SA-7500II.

Looking forward to it!!!
Thanks EW, this is a great thread!!!

Stew.
 
Re: Adjusted My SA-8800...

Hey Stew.... just a word of caution, don't always assume you may get the right pot without having the service manual, I would urge you to purchase one if you haven't already. You may not be so lucky the next time around.
Just my 2 cents.
SX-980





Originally posted by stewey
I finally got around to testing and adjusting the DC-Offset for my Pioneer SA-8800. Previously, I didn't have a multimeter sensitive enough to perform this test, but since I bought a DMM for my SX-850 project, it was only a matter of time before I got around to this.

My buddy Ryan (justforfun52), came over tonight and at his urging we tested the 8800. Wow, I hadn't noticed any problems but she hovered around:

-90mv (right channel)
-110mv (left channel)

****. That sucked. Well, we popped the top hoping the DC offset adjustment pots would be easy to locate. The problem is there are like 6 pots on the power-amp board. Hesitant to mess with it not knowing what pots to turn, we were about to put the top back on (despite Ryan's uring me to 'go for it') when I had an idea.

We hooked up the DMM, powered the amp up and checked our readings again. Same as before. I gently tapped one of two 'suspect' pots. Well, the DMM dropped down about 10mv just by touching it. We waited for it to settle down again, and tapped a second time just to be sure we weren't mistaken. Well, to make a long story short(er), we found the right pots and were able to get the DC-Offset down with our fingers so it was fluctuating between -1 and 1 mv on each channel. Damn that's good hey?!? Well, it sure felt good after that initial shocker.

We hooked it back up and turned it on. Sounds great! We didn't get a chance to turn it up because it was past 12, but we'll definitely get around to that soon.

Tomorrow we're going to adjust my 7800 which tested at about 10mv on each channel. We'll also test and adjust the SX-1050, SX-535, and SA-7500II.

Looking forward to it!!!
Thanks EW, this is a great thread!!!

Stew.
Click to expand...
 
:saywhat:

Well, that's why I tapped the pot before actually adjusting it. We had done a little research before-hand and had a hunch as to which pots to adjust.

Not such a big gamble as I see it.
 
Pioneer SA-7500 II
44mv left
46mv Right
(booo, Hiissssss)

Pioneer SX-535
~21mv left and right

I'm going to adjust the 7500 when I get a manual.
Same with the 535. Can't find one though.
Tried to locate the pots on the 7500 and couldn't find any so I'll have to wait.
 
SX-980

EW I want to thank you vary much for all the help this post has given me.
Just did the DC offset on my SX-980.

Before 18.7mv left , 22.5mv right

NOW 4.3 mv Left , 5.6mv Right

The difference is like night and day.
cleaned the switches and pots to, not a job i would want to do on a reg bases.

Thanks again
Ron
 
Very Interesting. It's a new world to me but I've been a good student and read the entire thread and followed EW's instructions on how to measure the DC Offset on an amplifier.

Luckily I have 1 of those multimeters at home. THIS ONE

Firstly I set it to '200mV', the closest down to your recommended 300mV.

I have quite a few Sansui amps in my home but for now I only measured the DC Offset to my beloved Sansui AU-919.

(I let the amp warm up for 10 minutes first)

Now here goes:

AU-919

Right channel

13,3

Left channel

02,1

And I think this is excellent! Only perhaps the 13,3 could be a little lower.
 
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Hi there,

I measured my Sansui AU-D9 and look at the results!!

SANSUI AU-D9

--------------------------------------------------------------------
Right Channel

Fluctuating between

-49,6 and -50,4

Left Channel

Fluctuating between

-49,8 and -51,0

--------------------------------------------------------------------

Now I'll have to learn how to correct this problem!!
And when I do I'll post the results.
 
my best amp

Results for Sansui AUX1111 Mos Vintage
------------
Right

07,2

Left

19,6

-------------

For this upscale equipment the DC-Offset should be perfect right, so I'll have to do something about it.
If it was only my au-d9 with those results the future would be bright.
The only problem here is, the rarity of this Sansui, therefore the difficulty to find a service manual.
 
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Sansui AU-5500 measurement


This was tricky. I couldn't make the measure on 200m scale so I did it in 2000m scale

Right channel then reads

300

Left channel then reads

350

Is this like '3000' and '3500' on 200m scale? Is she sick??
 
From what you're saying that's 300 & 350 millivolts. According to EW's first post that is far too high and could damage your speakers. Perhaps someone else can chime in with instructions on how to adjust that amp. Otherwise you'll have to pick up a service manual or take it to a professional and bring these numbers down.
 
Nawww...not enough to damage speakers, not even enough to trip the protection. But 350mV is waaaay more than enough to destroy sonic integrity.
 
Just picked up my Sony STR-7025L receiver (late 70s 18 wpc)

DC offset:

Right: 15 mV
Left: 12 mV

After 10 minutes:

Right: 12 mV
Left: 9 mV

Pretty good, huh?

- Harald
 
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