Solid state amplifier biasing - Need a tutorial

kbhuvi

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Hello FMs.

Greetings and hoping all is well.


Inspite of owning a couple of vintage amplifiers, the practical of how to bias an amplifier has eluded me. Youtube has not been of much help. I know the science behind it but still have not been able to understand where the probes of a multimeter have to be placed for taking the bias reading.

I am placing a request if any FM can post a video explaining the biasing on a schematic first and then carrying it out on a amplifier which does not have dedicated biasing points.

Regards
 
No responses yet so I am tagging some FMs. @Aniket, @shaan, @mpasanthosh, @manniraj @Kana711104, @anilva @sachu888

I hope one of you gentleman can cook up a video. No pressure intended but I have tagged you just in case you missed the thread.

Regards

This is a big topic and a debate as well, the biasing procedure is different for each and every amplifier with varied test points.
Ideally the biasing test points would be across the emitters of NPN and PNP transistor pair in most solid state amplifiers, where in DC Voltage is being measured or calibrated.
The other principle for Biasing procedure requires adjustments in current and not necessarily voltage for which you would need a ammeter to calibrate.

Ideally there would be no need to calibrate the bias of a amplifier, unless the amp has aged components which are weak or the bias has deviated due to damaged potentiometers.
The best way to go about this is to restore the amp to a good condition and then bias it accordingly, or change the biasing potentiometers if they are damaged and replace them with new sealed type bourns potentiometers.

You would also need to look at DC Offset calibration and not just bias, for some amplifiers the service manuals would not be readily available and this makes proper accurate bias adjustment a challenge.

A video will surely not help as the procedure will be different for different amplifiers.
There are other factors that you would need to consider apart from just biasing, that would keep the amplifier healthy over long periods of use.

1. Weak diodes & Resistors
2. Fake Transistors or replaced transistors that are not in original spec
- It is literally impossible to source the original output transistors once they blow, all ebay transistors are made in China and are replicas.
3. Thermal Greasing of transistors in heat sink
4. Capacitors with drifted ESR's which are no longer in spec
5. Biasing transistors to be in good shape
6. Health of Filter capacitors

Just biasing will not help you obtain maximum performance from your vintage amplifier.
This is more of a technicians job and should not be attempted with ease.
A higher bias will yield more heat which results in high temperatures , with improper thermal greasing you will damage the transistors as well.

It is best to have a service technician get this checked for you.
 
@mpasanthosh Superbly explained Santosh.

Biasing is more critical to tube than SS amps. With SS amps, the variation comes mainly with aging components.

The procedure however varies from one amp to amp, and unless mentioned specifically in the user/service manual, technical help must be sought.

I have biased quite a few amps successfully based on information available either in the manual or from information obtained on the internet.

I have had bad experiences as well. I blew a of vintage JVC amplifier where one channel was a bit too off from the listed rating.
I just blindly followed the procedure listed in the manual, with the effort ending up in smoke.

Yes, if you get an used amp in hand and see the bias a little too off, it is better to check first for failing components around it.
Another issue I have observed is when some component associated with biasing is failing, even after adjustments it keeps falling off.
This too needs technical intervention.

Coming to the OP's topic of procedure involved, there are usually two methods, biasing the current or the voltage. Biasing the current is a bit more tricky as you need to put a multimeter (with current measuring feature) in series to the amp.

Another equally important factor is adjusting the DC offset which is measured between the speaker terminals in idle state.
Some amps provide for fine tuning this setting as well.

Here is an example as listed in the manual of Pioneer GM-X944
"ADJUSTMENT -
Setting Idle Current

Setting the idle current to the following ranges will require you to measure the voltage across the indicated resistors and adjusting the indicated variable resistors to set the required voltage range.
This adjustment is performed with no loads on the outputs with no signal input.
Specification: 45 mA ± 5mA
Voltage Range: 4.0 mV to 5.0 mV
1. Measure voltage across Resistor R433 and set VR501 to Voltage Range.
2. Measure voltage across Resistor R434 and set VR502 to Voltage Range.
3. Measure voltage across Resistor R435 and set VR503 to Voltage Range.
4. Measure voltage across Resistor R436 and set VR504 to Voltage Range.
Notes: The Idle current should be set when the amplifier has not been run for awhile, and set at room temperature. "
 
Agree with everything posted. Just want to add that current measuring is usually done by measuring the voltage drop across a known resistance value.
 
Agreed with @Kannan And @mpasanthosh
Biasing in one word is a sweet spot of you ear which varies design to design. Every design has its own tuning and biasing procedures. You must look up for author's documentation about biasing. But over all in one word "sweet spot".
 
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