Amplifier Distortion, DC-Offset, and You!
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.
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 openness 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
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.
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...
Last edited by EchoWars : 08-23-2004 at 03:23 PM.