DC leakage from power amps

I found this link doing a search.
http://audiokarma.org/forums/index.php?threads/amplifier-distortion-dc-offset-and-you.5634/

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..)

Note, I personally do not know if the guy's method for testing for DC offset at an amp's output/s is correct or not.
.

One obvious question, how loud are you listening to your music?

@crazyeddy,

Not sure if it is relevant to the discussion but what are you using for speaker cables? Brand, type? Approx length? Wire gauge?
.

I have a vintage Marantz 2252B I used to checked for DC offset at it speaker outputs. Note I have not powered up the receiver for about a month.
The 2252B has two sets of speaker outputs, system 1 & system 2. I have a a pair of speakers connected to #1 so I used #2 for my first testing. system 1 set to off.

multimeter is a Fluke87. Set to dc mv range.

I followed the procedure outline by the guy in my previous post.
Powered up the receiver and allowed it to warm up for 10 minutes.
Initial test results.
L (Left) ch 1.4mv
R (Right) ch 10.5mv

I then checked again after the receiver had been power up for 20 min.
L ch 1.2mv
R ch 9.6mv

After 30 min I checked again.
L ch 1.4mv - 1.5mv
R ch 9.2mv.

All the above measurements were taken without any load connected to the outputs. I had read on one website to get a more accurate measurement a load needed to be connected to the outputs. The guy said to use an 8 ohm resistor.
I don’t have an 8 ohm resistor for a load. I do have the speakers that are connected to speaker system 1.

With the the Marantz you have the choice to power speakers outputs 1 or 2 or both.
(By now the receiver had been powered up for about 25 - 30 min.)
Here are the test results.

L ch without connected load 1.5mv. With load 1.4mv - 1.5mv

R ch without connected load 9.2mv. with load 4.0mv

Not sure how the measurements would be using an 8 ohm resistor connected to the same output terminals as the meter test probes.

.

@crazyeddy,

What are you feeding the two amps with? I assume each amp has its’ own 120V 20 amp dedicated branch circuit. Roughly what is the distance, length, of the wiring from the electrical panel to the wall receptacle outlets? What are you using for receptacles? Did you use #12awg bare minimum wire size or increase the wire size to #10awg? Are you using the stock power cords that came with the amps? If you are using after marker power cords, made by who, model?

It would appear these amps need plenty of mains power if you are pushing them hard. If the mains voltage is dropping due to VD (Voltage Drop) on the branch circuit wiring it may be having an impact on the amp’s ability to deliver their full rated power.

You can check the mains voltage at the wall receptacle with your multimeter.
First check the voltage with the amps at idle.
Check the voltage again playing music at a moderate level.
Check the voltage playing music loud at the level you like to hear it at.
Is the voltage lower but holds steady?
Is the voltage lower, bouncing around, with the beat of the music?

Specs
http://bryston.com/PDF/brochures/SST2_875HT_BROCHURE.pdf


https://www.youtube.com/watch?v=TOof73b60E0

Review
https://www.stereophile.com/solidpoweramps/108bry/index.html

You may find these posts of Charles Hansen of interest concerning "fully balanced from input to output".
https://www.audioasylum.com/forums/amp/messages/22/220523.html

Jim