Do I need to worry about 70-75mV DC Offset?

nightcat23

New Member
I have a yamaha cr 420 and decided to do some dc offset measurements as per this:

http://audiokarma.org/forums/index.php?threads/amplifier-distortion-dc-offset-and-you.5634/

I got between 70 - 75 mV DC offset (it wouldn't sit and stay at one number). Both channels were about the same amount. Now, according to that thread, it's not enough to damage my speakers or anything, but I am just worried that its making the sound quality worse.I'm also not sure if it is damaging to the amp or anything like that. I know it can be adjusted on some receivers with a pot, but I already have little knowledge on this stuff and don't feel comfortable doing it. My understanding that this measurement is the amount of dc that's getting to the speakers, but would it effect the record output? headphone jacks? I figure without touching that adjustment pot the only way to reduce the offset is to replace those large capacitors by the transformer, but i'm not sure if that would totally remove it. This receiver is from the late 70's so I figure it being slightly higher really isn't out of the norm. So, what the best thing to do at this point be? just keep using it, and when I do need servicing when something breaks I should mention the dc offset? Maybe i'm just worrying too much. I don't hear distortion and I'm not sure if I would be able to anyways.
 
70-75mV is at that level where you start to ask questions. No it will do no damage to the amp and won't be audiable.
The CR420 has no adjustment for dc offset. Replacing the differential pair, transistors TR403,,6 normally fixes the problem.
An adjuster could be fitted. If you are not comfortable with working on a live unit then don't. Lethal voltages inside. I'd go
with the plan that the next time it needs work then mention the dc offset.
 
It absolutely will affect sq. Right now there's an invisible finger holding itself against the drivers of your speakers. Low level resolution is what is being affected. The air/ambience/decay is diminished. You can hear the difference going from +/- 5mv to +/- 20mv easily. Even though lots of amps are within spec at +/- 20mv, they can still be tweaked to 5mv or less with a significant increase in performance. This has been my experience.
 
You can hear the difference going from +/- 5mv to +/- 20mv easily. Even though lots of amps are within spec at +/- 20mv, they can still be tweaked to 5mv or less with a significant increase in performance. This has been my experience.
:bs:
 
oh well, the offset is keeping the woofer out of its "lazy" hysteresis position, who knows, maybe it sounds better :D

Anyway, offset depends on the voltage derived from the input current of the first transistor pair base resistor.
Higher hfe -> lower base current -> lower offset

Now, maybe transistors are deteriorating so maybe check offset again half a year later and then decide.
 
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I feel like this is something that I won't get a straight answer on. Much like capacitors, I keep hearing different things from different people. To me the receiver still sounds good, but I bought it from someone, so I am not the original owner. I have no idea if it sounded different/the same when it was new. My main concern is that I am in the process of digitzing my vinyl records, and I really like the sound of the yamaha pre. That's why I am concerned if DC offset (if it is affecting the quality) would be diminishing the quality of my recordings, and it's not something that I'd like to have to re-do. Probably just going to leave it as is, as I do have a few other amps/receivers I would like to get before deciding if its worth throwing cash at.
 
Whether you can hear 20mV will ultimately come down to the sensitivity of your speakers. 20mV equates to 50 micro Watts, nominal 8 ohm load.
I would not be comfortable with 70mV, not happy about 20mV either, but I have the basic skills to do something about it. Rather than risking life and limb
or damaging the amp, tend to agree to leave it be. The CR420 is ok, but suggest moving a bit higher up the food chain. Things get interesting at CA-800 or
CR8xx and beyond.
 
hat's why I am concerned if DC offset (if it is affecting the quality) would be diminishing the quality of my recordings, and it's not something that I'd like to have to re-do.
If your just using rec out. The only active electronics used for LP transfer would be the MC amp. Or the MM amp. So no, it wouldn't interfere with your archiving plan. The DC offset is either in the final amp or possibly in the pre-amp. Those are down at the other end of the circuit. And not in your record signal sound path.
 
play some music for 20 minutes at moderate volume then immediately check offsets again . it might well change one way or the other .
 
You'll hear a bunch of different opinions on what to do just because there are a lot of options.

With 75mV you won't damage your speakers and might not notice any sound degradation. That leaves you with the do nothing option open.
You could also go after the diff pair and try to get it down which might cost say $100+ from your local tech depending on how much time it takes. That's the do a little option.
Realistically with a receiver that old it would benefit from a full restore while you're in there but that's likely $$$ and more than the receiver is worth. This is the most expensive but most likely to get you the best sound/highest reliability option.

You just have to decide which option you're comfortable with.
Most of what I own wouldn't be worth having a tech do a full restore and I don't like running gear on borrowed time so I learned to fix it myself. This cost many, many hours of time between reading books and several hundred dollars (more than that most likely) of test and repair gear but I enjoy doing it and I can use some of the gear for my real job which offsets the costs a bit.
 
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