where is the emitter resistor?

[homer09]

I'm a newbie at audiophiling, and i've been working on an amp and tuner for the past couple of weeks, learning as i go along, so far with great results.

Ive decided to follow an amp tweak which involves replacing the trim pots that control amp bias. Of course this means i need to adjust the amp bias from scratch. I understand the entire procedure except, i have no idea how to locate the emitter resistor which is essential to determining the quiescent current. My amp is an old Technics SU-C03 from the 80's. The two trim pots (very cheap ones if i may add) are located on a PCB along with an STK3062 chip, 16 resistors, a dozen caps, 4 transistors, and some diodes. Which of these resistors is the emitter resistor? What should it be connected to? Once found, i could use the voltage drop across this resistor to read the quiescent current, right?

this is the tweak i am following: http://www.tnt-audio.com/clinica/bias_e.html

Thanks for any help that can guide me in the right direction. I have no squematics/service manuals.

 

[john curl]

The emitter resistor(s) will be connected, on one side, to the output. They will be very small in value, below 1 ohm.

 

[millwood]

I'm a newbie at audiophiling,

I wouldn't attemp that if I were you.

for the minimum, you need to get the datasheet for the IC(s) to figure out how everything works. Some of them may or may not have an emitter resistor (even if they don't, you can still adjust bias).

Try to get some basics as to how an amp works and get yourself comfortable with some instruments (a multimeter for example), and the PCB in question.

Otherwise, your amp may sound worse after your tweak, or damage it.

My guess is that there is already a bias trim pot on the PCB,

 

[fotzepolitic]

Gee, I think I might actually be able to help here. I saw the same article, which inpsired me to want to play around with the bias adjustment in my Rotel amp (but not actually change the bias trim pot, I think I have the "good one"). So I did a search to see what an emitter resistor looks like, and most found line drawings, but here's an actual photo:
http://www.electronicsplaza101.com/electronics/search/res/r2985504.html

The ones I have are black, not white, but they all have 3 (flat) legs, and I presume they are always situated near the bias pots. Now if that helps, perhaps you can respond to the recent query I posted about how to adjust the bias... I didn't quite understand the entire article, so if you know how, I'd like to know exactly how to read the value that changes with a bias adjustment. That is, where to place the probes and which emitter resistors to place them on to take the reading for each channel, and wheter my DMM should be on DC or Ohms setting.

 

[SY]

What the resistors look like will vary from amp to amp. As John says, they'll go from the output transistor emitters to the output (or the output network). If you can't trace it from these hints, you probably shouldn't be trying this mod- getting it wrong can release plenty of smoke.

One other trick to do is to measure the resistance values of the trimpot of each channel on each side of the wiper, then replace the trimpots with fixed resistors of the appropriate value. You'll lose the ability to adjust bias with just a screwdriver, but unless you're replacing the output transisters, this is probably not a serious shortcoming. Same qualifier as before; if this sounds mysterious to you, don't do this mod.

 

[homer09]

Re: Re: where is the emitter resistor?

for the minimum, you need to get the datasheet for the IC(s) to figure out how everything works. Some of them may or may not have an emitter resistor (even if they don't, you can still adjust bias).

Try to get some basics as to how an amp works and get yourself comfortable with some instruments (a multimeter for example), and the PCB in question.

I have the datasheet for the IC its is an STK3062. http://www.datadart.com/al/sanyo/Stk3042.pdf
And i do know how to use a DMM! Maybe i exagerated saying i was a newbie.
I am almost 100% i have no emitter resistors in my amp. You mention you can still adjust bias without an emitter resistor, how would i go about this? I also only have two trim pots responsable for bias adjust, no DC offset trim pots. Basically, i need to know how to measure the quiescent current so that i could properly adjust bias when i replace my crappy trim pots with multi turn new ones.

any indication where i can place my DMM probes to obtain quiescent current would be appreciated.

 

[millwood]

Re: Re: Re: where is the emitter resistor?

I have the datasheet for the IC its is an STK3062. http://www.datadart.com/al/sanyo/Stk3042.pdf

it looks like the IC has input and VAS circuitry but not output circuitry. Is it possible that there are other ICs / transistors as an output stage?

You mention you can still adjust bias without an emitter resistor, how would i go about this?

any indication where i can place my DMM probes to obtain quiescent current would be appreciated.

if yours has discrete output transistors, you can insert the DMM, in current setting, into the emitter leg (you can cut the trace or unsolder the emitter, i prefer cutting the trace myself).

 

[john curl]

If I were you, I would ignore this upgrade. We don't really know what the pots really do. Without more understanding, please leave it alone.

 

[sam9]

I agree with John Curl, let it be. If what you have your eye on is the bias pot, it was most likely set optimally during the final stages of the manufacturing process. The ideal way to set it is to use a distortion analyzer and set it where you get the minumum value. I can't think of any reason why a sizable manufacturer would do it any other way. I'm also unaware of any other reason for having the pot ther except to minimize crossover distortion. ( If there are other functions served, I sure they will be posted here post haste. )

I have deliberately, mis adjusted the bias on an amp I was building just to hear what happens. It sounded bad. I don't have the command over presice adjectives some high-end reviewers have so I can't be more exact, excpt to say such an amp was not something I would want connected between my speakers and my pre-amp!

Since I don't have a spectrunm analayzer I resorted to doing a SPICE simulation to see what a mis adjusted bias does. It's not pretty - lots of high frequency hash. Maybe that's what the author of the article though was improved highs!

It is possible that the bias setting has drifted from optimum since the amp came off the line. If that's the case, it can be adjusted by someone familiar (suc as a in qualified service tech) with the appropriate procedure and in possision of the appropriate equipment.

 

[mrfeedback]

Amplifiers like your Technics will not cope with high bias current settings.
Best to set it to factory spec, or maybe just a little over, but be very watchful of temperature/ventilation.

Eric.