How do I adjust amp bias?

[fotzepolitic]

I'm thinking of adjusting the bias on my Rotel (class a/b) amp to try to improve the sound. The amp does not heat up even after hours, so it seems like there's enough leeway to increase the bias. So I'd like to know if there's a real simple way to do it, as I don't have an electronics background. I did measure the DC at the speaker outputs with a voltmeter; they read about 58mV for one channel, about 64mV for the other. My question is, can I go by this speaker output measurement to assist me in knowing how far to go with adjusting the amp's bias trim pot, or do I really need to be measuring the legs of an emitter resistor etc? And if I can use the speaker output measurement, which would presumably change as I adjust the bias trim pot, what voltage should I be looking to acheive, or what voltage should I stop at, to avoid blowing my amp (which I believe has no thermal protection for reasons of sound quality)?

btw, I don't have schematics and not interested in acquiring them; wouldn't know how to read them anyway.

[EchoWars]

I did measure the DC at the speaker outputs with a voltmeter; they read about 58mV for one channel, about 64mV for the other. My question is, can I go by this speaker output measurement to assist me in knowing how far to go with adjusting the amp's bias trim pot,

No, you're measuring DC offset, which should be as close to '0V as possible. Your measurements show a slight mismatch in the input pairs, for which there might be an adjustment on the driver board.

or do I really need to be measuring the legs of an emitter resistor etc?

Yep...to set bias, you need to measure from emitter to emitter on the output pairs, assuming a common emitter output stage.

And if I can use the speaker output measurement, which would presumably change as I adjust the bias trim pot, what voltage should I be looking to acheive, or what voltage should I stop at, to avoid blowing my amp?

Once you find the emitters, look at their value. Use this value to determine what voltage to measure. Most bjt output stage amps have a bias current in the range of 20 to 75mA, with 50mA being reasonably common.

I'd only recommend setting the bias to whatever the manufacturer recommends, but it's your amp, so best of luck.

[fotzepolitic]

Thx. I'm not really concerned with maintaining the original manufacturer's specs (which I don't have anyway), because my goal here is to adjust the bias trim pot so I can run the amp a little hotter, and hopefully improve the sound. It doesn't seem much of a risk, since the amp hardly puts out any heat however long its on (even covering the air vents with thick padding, the heatsink still doesn't get very warm).

There are only two adjustment pots on the PCB, and they're both for BIAS. So if I understand correctly, the DC Offset measurement I took at the speaker output with my auto-range DMM voltmeter will not change with an adjusment of the BIAS trim pot, that the pots only change the values at the Emitter Resistors (ER).

Attached to the large heat sink, there appear to be two pairs of large emitter resistors (inside pair marked: DE 1047 8L2, outside pair marked: BE 817 8M4) and one pair of tiny emitter resistors (DF ?? 8K). There are 3 legs per resistor, which makes 18 legs in all... I presume its 3 ER's per channel (2 large, 1 small). So I'm not sure where should the probes go (ie. which of the legs of which ER), and if I should take the reading on my DMM voltmeter at DC setting? Thanks!

[homer09]

I am in the same problematic situation. I am adjusting the bias of my amp, but i have no idea where to find the emitter resistors. What do these things look like, are they normal looking resistors (ie. two leads, with colour bands)? What are the emitter resistors usually connected to? my amp is a technics su-c03. once i have located these resistors, i simply measure voltage drop across to obtain quiscent current?

any help appreciated

the only question i can answer fotzepolitic, is yes you have to make voltage measurements with DMM at DC setting.

[EchoWars]

This is a pic of a dual emitter resistor...most emitter resistors have only two leads, but this is the best I could do for now.

fotzepolitic, the numbers you gave and the description sound like the output transistors...emitter resistors aren't generally mounted to the heat sink.

[SY]

See this thread for additional thoughts:

where is the emitter resistor?

[fotzepolitic]

If emitter resistors aren't mounted to the heatsink, then I guess they would have to be the output transistors that I described. Can you or anyone else take a look at this pic (below) of the inside of my Rotel, and spot the emitter resistors? If single emitter resistors have 2 legs, then I figure they'd have to be those large beige resistors, situated near the output transistors (there are 4 of them in a row, and 2 more off to the side). The markings of the beige resistors are: 2WJ, 8NA, 8NO and all are rated .22 ohms. The only other part that looks significant has 3 legs and large black plastic top (the left corner rounded), marked with a "+ -", (PBL403). There are two large capacitors and this device can barely be seen located behind the right cap. It looks very much like a pic I saw of a sealed, multiturn trim pot, so I'm guessing this is not a dual emitter resistor (plus it isn't very near all the output devices).

The other thing I'd still like to know is if and when I find the emitter resistors, where exactly do I attach the probes to determine the bias value, and what sort of readings might I be looking for?

PICTURE OF THE INSIDE OF MY Rotel 820A AMP:

http://www3.sympatico.ca/stilyagi/rotel820interior.jpg

PICTURE OF ONE TYPE OF 2-LEG (SINGLE) EMITTER RESISTOR:

http://www.globalsemi.com/cat99/passive398.html#s2w-r1

[Tube_Dude]

Hi fotzepolitic!!

Yes the emitter resistors in your photo are the beige cilindrical 0,2 Ohms resistors!
You must mesure the voltage at the legs of this resistor...for 100 mA bias current you must have 0,02 Volts .
For 50 mA halve that value!

Regards

[sreten]

my goal here is to adjust the bias trim pot so I can run the amp a little hotter, and hopefully improve the sound.

Generally speaking minor adjustments to the bias will sound
worse always for Bipolars, compared to the optimum bias point.

To accrue any significant benefit you have to run the amplifier
hot, around 50 degrees C. This is a temperature that feels hot
but not painful, human touch is very sensitive in this region.

It may sound crude but adjust the bias to the level you can
just keep a finger on each heatsink without discomfort.

If you can measure the emitter resistor voltages this will help.
update : you'll need to do this as you only have one heatsink.

(The better the heatsinking the more bias you will be able to run)

update : replacing the 0.2R emitter resistors with 0.1 R will help
for all biasing cases, V across the emitter R matters not current.

The amplifier will sound better if you listen at low levels, if you
are a volume fiend then the standard biasing is preferable.

sreten.

[millwood]

Originally posted by sreten
Generally speaking minor adjustments to the bias will sound
worse always for Bipolars, compared to the optimum bias point.

isn't it always true that you will get worse sound once you get off the optimum bias point? That's by definition, right?

Originally posted by sreten
To accrue any significant benefit you have to run the amplifier
hot, around 50 degrees C. This is a temperature that feels hot
but not painful, human touch is very sensitive in this region.

sreten.

I think john curl and somebody else mentioned that the optimum bias point is to get about 25-30mv drop off the emitter resistor. an old HP patent or something like said it, as I recall. it was discussed a couple months back.