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Amplifier Distortion, DC-Offset, and You!

Interesting thread. I just checked my ancient Crown DC-300A.

With the input level controls set wide open I had -

Channel 1 = +98mv
Channel 2 = +154mv

So I opened it up and adjusted the input and output offsets and now I have less than +-1mv at any setting of the input level control.

I also checked an old Velleman board I have that uses a Philips TDA1521 device and is non-adjustable

Channel 1 = +24mv
Channel 2 = +30mv

Looks like they don't trim those devices all that well...
 
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sony TA-3130F amp - L-23, R-83

sony TA-3650 integrated - L-12, R-13

sony STR-5800SD receiver - L-16, R-20

no wonder the 3130F has found it's way into a box awaiting tech check. definite preference for the receiver lately

awesome info for the technically ignorant, and a nice way to do a quick check on local prospects before buying.
 
Whew, OK, read 45 pages. The HK730 and 930 came up about twice, but no successful adjustment of pots was described. These are very popular receivers. The service manual for the '930 can be seen quick and easy here:
http://manuals.harman.com/hk/Service Manual/HK930 sm.pdf


There is reference to only one related adjustment, and it is called "Audio Driver Bias Adjustment" calling for 15 mV and internal test probe points.

Does this mean that there are no DC-offset adjustment pots (rather only matched transistor replacement?) Any chance this is an unusual design? My readings are 40 mV left and 80 mV right, with no audible problem (in fact clearly better sounding A/B on a switchbox than a Marantz 4470 next to it with readings of -9.0/9.9 mV). There is however scary slow movement of the woofer when turning the '930 on and off (also on my HK630).

PLEASE someone who can read a schematic take a few minutes to look at this in the schematic/service manual above. Thanks! -sf
 
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TommyC said:
OK, someone emailed me a service manual for my H/K 730. The closest thing I can find in it to what has been described here is a set of instructions for "Power Amplifier Idling Adjustment." Is that the same thing?Thanks,Tommy C.
Click to expand...
630, 930, and 730 service manuals all are limited to this one adjustment, no description of DC-offset adustment. Any chance the dual power-supply-transformer design is relevant to that perhaps? I'm suprised if the adjustment is typically only left off the low end units, as the '930 and '730 were successive TOTL models. -sf
 
sydsfloyd67 said:
630, 930, and 730 service manuals all are limited to this one adjustment, no description of DC-offset adustment. Any chance the dual power-supply-transformer design is relevant to that perhaps? I'm suprised if the adjustment is typically only left off the low end units, as the '930 and '730 were successive TOTL models. -sf
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No...there are a lot of near-TOTL models that have no adjustment, two that come to mind right away are both Kenwood's...the KR-9600 receiver, and the 700M amplifier. Just depends on the manufacturer and their philosophy.
 
EchoWars said:
No...there are a lot of near-TOTL models that have no adjustment, two that come to mind right away are both Kenwood's...the KR-9600 receiver, and the 700M amplifier. Just depends on the manufacturer and their philosophy.
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EW, Could you please take a look at that schematic and see if there is anything unusual in the area of DC offset going on (or if you see the transistors whose imbalance would lead to a high offset?). Thank you, -sf
 
Jim H B said:
Sysfloyd - for what it's worth, I've not been able to find a DC offset pot on the HK 730 either. Jim
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J. What kind of readings are you getting? (No one has reported good readings here for any of the X30's except the 430 unless I missed something.) -sf
 
sydsfloyd67 said:
EW, Could you please take a look at that schematic and see if there is anything unusual in the area of DC offset going on (or if you see the transistors whose imbalance would lead to a high offset?). Thank you, -sf
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The 730 may work completely different than the 930. The 930 at least has a DC coupled output with no relay, so it WILL have funky cone movement when powering up and powering down. You might minimize this by replacing the old electrolytics on the driver board.

Smallest offset is achieved by a careful matching of TR601/TR602 (and TR621/TR622 for the other channel).
 
EchoWars said:
The 730 may work completely different than the 930. The 930 at least has a DC coupled output with no relay, so it WILL have funky cone movement when powering up and powering down. You might minimize this by replacing the old electrolytics on the driver board.

Smallest offset is achieved by a careful matching of TR601/TR602 (and TR621/TR622 for the other channel).
Click to expand...
Thanks man. :thmbsp:
 
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kenwood KR-4600 help!

Hey all:

Very instructive (and sometimes) confusing thread!

I have A Kenwood KR-4600 with a very soft but noticeable low-level hum coming out of one channel. The output overall seems kinda suppressed, kind of "fuzzy". From what I've read here, I'm guessing it may be a DC-ofset problem.

Some posters have referred to DC offset adjustment pots in some vintage receivers and amps. What the heck does one look like? How do I determine if the Kenwood has one? Does anybody know?

And if not, how do I proceed? Do I just start replacing electrolytics?

Thanks

Roy
 
First things first, get a DVOM capable of measuring in millivolts (mv) and measure the DC offset. It will usually have a lowest DC volts setting of 2 V or 2000mv (or something close to it).

Second, (I doubt this will do anything in your case) try reversing the plug in the wall, or trying a different circuit, the noise may be external.

If the DC offset is OK, and you still have the hum, start looking at if the 'lyitics are leaking anywhere. Replace these first.

Then see where you are.
 
Roy,
Electrolytics or diodes. Replacing the caps isn't a bad idea on a receiver that age. DC offset that's out of spec will not produce hum. It destroys woofer voive coils if voltage is present for long periods of time. Repair the hum problem first then set the offset.
Ron
 
jblmar said:
Roy, DC offset that's out of spec will not produce hum. It destroys woofer voive coils if voltage is present for long periods of time. Ron
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J., Something else to worry about then? That had not been mentioned in the previous 50 pages. How does it deterioriate the voice coil if a few millivots (ok, 80) runs through a coiled piece of wire for a long time? Once the speakers are on, that's what goes on until you turn the unit off right? What am I missing? Thanks. -sf
 
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Ron:

Thanks for the advice. I forgot to mention that when I last tested the Kenwood, it made the woofers distendvisibly on power-up and shutoff. No sound was associated with this. Is this effect the sign of a DC ofset problem?

Thanks

Roy
 
sydsfloyd67 said:
How does it deterioriate the voice coil if a few millivots (ok, 80) runs through a coiled piece of wire for a long time? Once the speakers are on, that's what goes on until you turn the unit off right?
Click to expand...
Yup.

Excess DC at the woofer heats the voice coil and shortens it's life, but you need a lot more than 80mV before you begin to sweat it.

Roypercy said:
I forgot to mention that when I last tested the Kenwood, it made the woofers distendvisibly on power-up and shutoff. No sound was associated with this. Is this effect the sign of a DC ofset problem?
Click to expand...
Why guess? Measure it and see what's up...

FWIW, units without an output relay often have some sort of method to gently start up, and that usually results in funky cone movement.
 
Somewhat intrigued by this thread, I decided to measure the offset on my homebrew "Naim NAP 135" monoblocks. 30.6 mV on the left and 22.5 mV on the right. Surprising when they've only been built a couple of months. Anyway I got the lids off, and armed with the schematics and layouts, I set about removing the 10Ω resistor that sits between the noisy ground and the input signal ground. I then drilled the amp boards and psu bords and connected an additional cable from the signal ground track on the amp board back to the star ground between the PSU caps. Offsets are now -7.6 mV on the left and -0.7 mV on the right. I'll let them settle in for a while and re-check the readings. If the left stays out as much, I may try replacing both the 2SC2547E differential input pair trannies to see if I can get a closer match.
 
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