How do I adjust amp bias?

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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.
 
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. ;)
 
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!
 
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.
 
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.
 

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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
 
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.
 
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sreten said:
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?

sreten said:
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.
 
fotzepolitic said:
T
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). T

I bet those devices are the ubiquitas 2sb817 and 2sd1047 complementary pair. The are 140vce 12 amp 100watt devices probably from Sanyo.

I have seen them in some amps and they are usually biased to 10mv into a single 0.22 emitter resistor. (Thats about 45mA per device).

If you are like me and want to get naughty


:devilr:

Turn it up to 22 or 23mV (100mA-115mA) but be sure the heatsink does not go above 25C over ambient or 50C absolute.

This will give you some headroom. But do this at your own peril! I wish you good luck.... you are playing with fire here. :hot:
 
millwood said:

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.

Steven said:

Dr. Leach also has a derivation of the optimum bias for class AB in his article titled "Can 'Transconductance Doubling' Occur in Class-AB Output Stages?", which can be found here: http://users.ece.gatech.edu/~mleach/papers/classab.pdf. It's a good read, not only for the optimum bias derivation, but for debunking the "gm doubling" myth.
 
Hey, thanks andy_c, for this nice reference to the Leach article on class AB biasing. I have been busy on paper to derive these equations myself, at least for BJTs, but hadn't found the step from the exp-function to the sinh-function. Interesting.

Steven
 
tube_dude:

Thanks for lifting off another veil of darkness. But I don't quite have enough info to do this yet:

1. Without having adjusted anything, I measure the four .22ohm resistors as between 3.3mV and 3.5mV. (The other two shown off to the side measure 0.00!). Does this give me a clue as to the appropriate bias current? Otherwise, how do I know what my bias current is, to determine whether the resistors are at their optimum voltage? If schematics are needed for this, they're not available - so can this value be estimaed based on an educated guess?

2. There are 2 bias adjust pots, but 4 beige emitter resistors (6, if we count them all). If I turn one bias trim pot, do I measure all 2 (or 3) resistors controlled by that bias trim pot, and can I expect the voltage to change equally across these resistors, as I fiddle with the pot? Otherwise, how exactly does one check the output of the bias trim pot, when it controls more than one resistor?

3. Which direction increases voltage on the bias trim pot? (clockwise/counterclockwise)
 
Sreten:


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

Thing is, I don't know for sure the amp is currently running at the optimum bias point; it is about 12 years old, after all (but in clean and impeccable condition, I must say). My intention is to first take a reading of what the voltage is on the emitter resistors, and then listen to the changes made by adjusting the bias trim pot. And if I don't like what I hear, I return the bias trim pots to their original location, as determined by my DMM.

> 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.

I wasn't intending on going hogwild with the bias thing (although I'd love to run it as hot as my MF A100). Because I'm thinking of long-term reliability. It has one heatsink (but a large one, see photo), but it is after all, a -budget- audiophile amp. What do you think running it that hot will do to the lifespan of the parts inside? That's my main concern. But at least I can say the amp isn't ever played at high levels.


K-amps:

> Turn it up to 22 or 23mV (100mA-115mA) but be sure the heatsink does not go above 25C
> over ambient or 50C absolute.

Well, I measure 3.3-3.5mv for each of these .22 ohm emitter resistors. Are you suggesting I adjust the bias pot until they read 22-23mV?? That seems like quite a leap. Oh, and how exactly do I measure the temp of the heatsink; do I place my temp probe on the heatsink itself, just above it, or...?

Millwood:

> I think john curl and somebody else mentioned that the optimum bias point
> is to get about 25-30mv drop off the emitter resistor.

Does that mean adjust the bias pot for 25-30mV -less- than the emitter resistors currently read?
 
Honestly guy...you are running yourself in bloody circles trying to measure bias current. Forget measuring across the emitter resistors because you just don't have a handle on where to do the measuring.

Take your measurement from the emitters of the transistors! Find the 'D1047' device for one channel, and put your positive lead on its emitter. Find the 'B817' device for the same channel, and put the negative lead there. Now you cannot get it wrong!!!!!!

Decide what you want to set bias to. I would suggest you simply get it up to an acceptable level for now...like about 50mA. 50mA across both emitter resistors (.22 ohm, right?) will give you a voltage drop of 22mV (11mV across each resistor, and 22mV across both resistors, which is what your looking for since you got the leads right on the emitters and you are looking at the voltage drop across both resistors). Set it where you want, but if you go much higher than 100mA (which is probably too much) be prepared to face the consequences.

It is not unusual to see an amp that hasn't been adjusted in many years have almost non-existant bias current. They usually sound like @ss.

Here's a pic of what might be your devices...should help you find the emitters.
 

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fotzepolitic said:
tube_dude:

Thanks for lifting off another veil of darkness. But I don't quite have enough info to do this yet:

1. Without having adjusted anything, I measure the four .22ohm resistors as between 3.3mV and 3.5mV. (The other two shown off to the side measure 0.00!). Does this give me a clue as to the appropriate bias current? Otherwise, how do I know what my bias current is, to determine whether the resistors are at their optimum voltage? If schematics are needed for this, they're not available - so can this value be estimaed based on an educated guess?

2. There are 2 bias adjust pots, but 4 beige emitter resistors (6, if we count them all). If I turn one bias trim pot, do I measure all 2 (or 3) resistors controlled by that bias trim pot, and can I expect the voltage to change equally across these resistors, as I fiddle with the pot? Otherwise, how exactly does one check the output of the bias trim pot, when it controls more than one resistor?

3. Which direction increases voltage on the bias trim pot? (clockwise/counterclockwise)

Hi fotzepolitic!

About points 1 and 2 take Echowars advice. he his right!

About point 3 it depends wow the pot is conected in the PCB circuit...you must try tourning it slowly and seeing the efects!

Cheers
 
fotzepolitic said:

K-amps:

> Turn it up to 22 or 23mV (100mA-115mA) but be sure the heatsink does not go above 25C
> over ambient or 50C absolute.

Well, I measure 3.3-3.5mv for each of these .22 ohm emitter resistors. Are you suggesting I adjust the bias pot until they read 22-23mV?? That seems like quite a leap. Oh, and how exactly do I measure the temp of the heatsink; do I place my temp probe on the heatsink itself, just above it, or...?


If I were to "overbias" my amp, I would take it to 23mV per emitter resistor or 46/47mV across both. While this will seem high, it is the bias voltage recommended by the following gentlemen (others may disagree):

1) Randy Slone
2) Douglas Self
3) John Curl
4) Nelson Pass among others...

The key is to have a heatsink large enough to avoid thermal runaway.

Also for BJT's in a emitter follower topology, it is not the bias current that lowers distortion level but the actual voltage drop across the emitters. (Goes against conventional wisdom, I know). It also means that for biasing purposes of reducing distortion (not running into class-A), the value of the emitter resistor does not really matter as long as there is 23mV across each resistor or 47mV across the pair.

Looking at your heatsink, I see it is not huge, therefore I'd get a temp probe or temp gun or something and pace it close to the output devices (not directly on them) and keep it under 50C. This may mean reducing bias lower than 47mV but it is more important to have a sustainable temp max.

As suggested by others on this thread, do it slowly and very carefully. It may take up to an hour to get an optimal bias/temp level .:hot:

Hope this helps.

By the way, for these devices, typically 10-11 mv on average(or 13mv +/- 6mv) across both emitters is a normal range for Japanese manufacturer's suggested levels.
 
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