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Old 7th March 2010, 02:56 PM   #1
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Default help setting up idle current

afternoon everyone

i need a bit of help setting up the idle current on my old teac dc servo A9 amp, it has how to do it in the schematic, but it involves some serious test equipment! a AF ocsillator, an attenutaor, distortion meter and.. an oscilloscope!

now just being an odd diy'er i dont have this gear, ive never tried to do this before and am no expert, all ive built is a few cmoys, a phono stage and a power supply, as well as recapped a few old amps, my electronics experience is very minimal.

i came across this post on TNT audio:

Adjusting the Bias Of Your Amp [English]

but i dont think im understanding the full jist of this, when its stated:

"Then, put one wire on one side and the other on the other side of an emitter resistor, noting its value. Say it is 0.22 Ohms and say you have a voltmeter reading of 10 mV. The quiescent current is obtained by dividing the voltage with the resistance, i.e. 0.01:0.22=45.45 mA. "

so would that mean shove a high wattage reistor (8 ohm?) at the speaker output terminal then measure the voltage from one side of the resistor to another? its just in the teac schematic it shows the resistors as partially grounding the speaker signal, the bulk of it going to the test equipment.

and would this method even work for a dc servo amplifier? and if so what ideal current value would i be looking for?

many thanks
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Old 7th March 2010, 03:02 PM   #2
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Originally Posted by lensmanMk2 View Post
afternoon everyone
"Then, put one wire on one side and the other on the other side of an emitter resistor, noting its value. Say it is 0.22 Ohms and say you have a voltmeter reading of 10 mV. The quiescent current is obtained by dividing the voltage with the resistance, i.e. 0.01:0.22=45.45 mA. "

so would that mean shove a high wattage reistor (8 ohm?) at the speaker output terminal then measure the voltage from one side of the resistor to another? its just in the teac schematic it shows the resistors as partially grounding the speaker signal, the bulk of it going to the test equipment.

and would this method even work for a dc servo amplifier? and if so what ideal current value would i be looking for?

many thanks
Dont connect a speaker in case there is a DC offset to confuse matters.

Just put your meter across the 0r22 set on mV range.
Adjust the bais to the meter reads 10mV.
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Old 7th March 2010, 03:04 PM   #3
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Originally Posted by lensmanMk2 View Post
so would that mean shove a high wattage reistor (8 ohm?) at the speaker output terminal then measure the voltage from one side of the resistor to another?
No it doesn't mean that at all. I would leave well alone as you can do more damage than good messing about with the bias. Unless you have replaced output transistors etc there is no need to reset the bias.

The Teac manual is on about setting it the correct way by monitoring distortion with test equipment. The TNT site is bad information.
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Old 7th March 2010, 03:19 PM   #4
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Originally Posted by richie00boy View Post
No it doesn't mean that at all. I would leave well alone as you can do more damage than good messing about with the bias. Unless you have replaced output transistors etc there is no need to reset the bias..
thats the problem richie i have replaced 4 transistors in the power amp section that were fried, the amp works, but i would like to get it set up to spec as its not sounding as good as it did before.

i tried taking it to a few audio repair shops, none of them will touch it , one guy even said "look for a grey haired old guy, cos theyre the only ones who will touch it..." really helpfull..
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Old 7th March 2010, 03:25 PM   #5
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Originally Posted by lensmanMk2 View Post
thats the problem richie i have replaced 4 transistors in the power amp section that were fried, the amp works, but i would like to get it set up to spec as its not sounding as good as it did before.

i tried taking it to a few audio repair shops, none of them will touch it , one guy even said "look for a grey haired old guy, cos theyre the only ones who will touch it..." really helpfull..
What voltage do you get across the 0r22 ?

If its much above 10mV then you might end up damaging your amp again.

If its a lot less then you will be getting crossover distortion.
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Old 7th March 2010, 03:32 PM   #6
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havent even got that far yet nigel as i was unsure how to set it up,but i'll have a trip to maplin and get a high wattage 0.22ohm resistor

i must admit i didnt really understand the TNT post saying to ramp it upto 100ma no matter what the amp, wouldnt that be biasing it towards class A and screw up the components?
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Old 7th March 2010, 03:35 PM   #7
godfrey is offline godfrey  South Africa
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If you post the schematic of the amplifier we will be able to see where to measure.
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Old 7th March 2010, 03:39 PM   #8
godfrey is offline godfrey  South Africa
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Originally Posted by lensmanMk2 View Post
havent even got that far yet nigel as i was unsure how to set it up,but i'll have a trip to maplin and get a high wattage 0.22ohm resistor
No, wait. You don't need extra resistors.
The idea is to measure the voltage across resistors that are already inside the amp.
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Old 7th March 2010, 03:47 PM   #9
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here we go:
Download Teac_A7_A9.pdf from Sendspace.com - send big files the easy way

thanks for the help so far everyone
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Old 7th March 2010, 04:08 PM   #10
CBS240 is offline CBS240  United States
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Originally Posted by richie00boy View Post
No it doesn't mean that at all. I would leave well alone as you can do more damage than good messing about with the bias. Unless you have replaced output transistors etc there is no need to reset the bias.

The Teac manual is on about setting it the correct way by monitoring distortion with test equipment. The TNT site is bad information.
Right on, that TNT site is a bit misleading. The whole idea is to maintain a constant Gm vs current across the zero current crossing. The amount of current for optimal bias depends on the type of output devices, Zout, and the transconductance curve. Too little or too much bias can increase distortion in a class AB follower circuit, MOS or BJT.
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