Is having too low of a bias dangerous or anything?

[maxp779]

I lowered the bias of my Marantz PM8005. It was sucking ~40w sat doing nothing and now I've got it at 27w now after following this service manual for the PM8004 (couldn't find the equivalent page in the 8005 one):

Idling Current Alignment - Marantz PM8004 Service Manual [Page 33]

Marantz PM8004 Manual Online: Idling Current Alignment. Digital Voltmeter L ch T.P. ADJ J707 R707 P701 Adjustment Procedure Set the power voltage to rated voltage for this adjustment. (1) Adjust the Idling Current with the variable resistor R707 and R708 on the PWB P701. (2) Turn off...

www.manualslib.com

I used a Fluke 106 to measure the mV from each channel, I got it to around 10-11mv, the manual seems to recommend 12.5 +- 0.5. Originally they were at 20mv. Basically did what this guy did with his PM7000N which looks a heck of a lot like the PM8005 internally:

Are there any downsides to doing this? It's still playing music. Sounds aite. Hasn't gone on fire, the top feels cooler as well after idling for a while. But like I said, I know very little about this kind of stuff. I do know when a PC gets too high or too low voltage for certain things all kinds of bad things can go wrong, but no idea about amps. Would be good if an expert could weigh in on this kind of adjustment

 

[MaxwellsEq]

You have probably increased the crossover distortion. In general, increasing the bias increases the amount of time each power transistor spends active until both are always on, which is Class A. With less bias, the amp goes from Class A at low levels to Class B more quickly as the volume is increased.

It's you are happy with more distortion, it should be OK.

 

[valerianf]

A long time ago, when I was a fresh EE, I remember lowering the bias of a Kenwood AVR (5.1).
I measured (voltmeter) the inflection point where the power transistor starts conducting (beginning of the straight conduction gain) and adjusted the potentiometer.
I do not thing that most modern AVR have any more an adjustable potentiometer.
@maxp779 You are lucky that the PM8005 allows you to do that.

The result was satisfying with less heat when the AVR was doing nothing.
Sound quality was the same as far as I was able to hear it.

 

[Doodski]

Wow! So easy and convenient to calibrate. Did you check the DC offset too @maxp779.

 

[maxp779]

You have probably increased the crossover distortion. In general, increasing the bias increases the amount of time each power transistor spends active until both are always on, which is Class A. With less bias, the amp goes from Class A at low levels to Class B more quickly as the volume is increased.

It's you are happy with more distortion, it should be OK.

That's cool, I can't hear a difference so it's alright by me.

A long time ago, when I was a fresh EE, I remember lowering the bias of a Kenwood AVR (5.1).
I measured (voltmeter) the inflection point where the power transistor starts conducting (beginning of the straight conduction gain) and adjusted the potentiometer.
I do not thing that most modern AVR have any more an adjustable potentiometer.
@maxp779 You are lucky that the PM8005 allows you to do that.

The result was satisfying with less heat when the AVR was doing nothing.
Sound quality was the same as far as I was able to hear it.

I had a wee look around some other amps internal pics on google images and a few of them do seem to have these adjustment pots, im not sure if they are also for idling current or something else though. Marantz lower end PM6007 has them same as the PM800x series. Rotels seem to have blue colored pots and even the over complicated looking Cambridge Audio CXA81 has some kinda pot on it. So it looks like some brands/models still offer this.

Wow! So easy and convenient to calibrate. Did you check the DC offset too @maxp779.

Hmm what's DC offset? I haven't messed with it no lol. I didn't know it was a thing but I can see the other 2 adjustment pots for it. Might have a look just to see what it is.

 

[maxp779]

So I had a look at the DC offset, the LCH was 2mv and the RCH was 5mv. I've adjusted them to be as close to 0mv as possible. Jesus those pots are sensitive! Bit of a turn and it swings 20mv in either direction lol, need to be extremely delicate with them.

If I understand the service manual correctly it seems to indicate 0mv is a desirable DC offset? Anything +-3mv from 0 seems to be within operating parameters.

 

[MaxwellsEq]

So I had a look at the DC offset, the LCH was 2mv and the RCH was 5mv. I've adjusted them to be as close to 0mv as possible. Jesus those pots are sensitive! Bit of a turn and it swings 20mv in either direction lol, need to be extremely delicate with them.

If I understand the service manual correctly it seems to indicate 0mv is a desirable DC offset? Anything +-3mv from 0 seems to be within operating parameters.

Speakers don't like to see DC; they are designed to operate properly with only AC signals. DC is obviously silent, so you can't hear it, but if high enough the speaker motors are heated by the DC and can damage them.

 

[Zek]

If I understand the service manual correctly it seems to indicate 0mv is a desirable DC offset?

Yes.
Leave the amplifier to warm up and measure the values again after 15-20 minutes.

 

[Doodski]

So I had a look at the DC offset, the LCH was 2mv and the RCH was 5mv. I've adjusted them to be as close to 0mv as possible. Jesus those pots are sensitive! Bit of a turn and it swings 20mv in either direction lol, need to be extremely delicate with them.

If I understand the service manual correctly it seems to indicate 0mv is a desirable DC offset? Anything +-3mv from 0 seems to be within operating parameters.

If there is DC offset at the speaker terminals then the speaker driver will be forced in or out a little bit and held there for as long as that DC remains. This condition will remain when playing music and it causes issues. If you take a 1.5V DC AAA battery and connect it to a speaker driver you can test this yourself by connecting the battery to the speaker for a short while and you will see the driver going in or out. So zero volts is the most desirable calibration.

 

[Piere]

But don't overdo. DC plus / minus within 50mV at the output doesn't harm. Most amps will drift within 10mV after being heated up to steady operating temperature.