maybe I wasn't clear, I was testing according to this note under FAQ on Hoppe's Brain
"I recommend checking every individual emitter resistor and taking an average. Don’t bother with the test points. Connect the negative probe of your meter to the speaker output binding post, and then probe each emitter resistor."
"I recommend checking every individual emitter resistor and taking an average. Don’t bother with the test points. Connect the negative probe of your meter to the speaker output binding post, and then probe each emitter resistor."
Whilst I agree with the former part (as I've recommended this very thing earlier tonight in antoher thread i.e. taking an average ) the latter part is completely wrong.
You need to measure across the (a) emitter resistor !
There will be some natural variation on each one, that's why you check them all (per channel) and you average the value.
Unless there's some huge variation it's normal.
You need to measure across the (a) emitter resistor !
There will be some natural variation on each one, that's why you check them all (per channel) and you average the value.
Unless there's some huge variation it's normal.
Measure between speaker positive and each transistor's emitter pin. All the emitter resistors have a common connection to the output. This is the same as putting your meter directly across each emitter resistor, but you only need to poke around with one probe, lessening the chance of an accidental short.
For the bias adjustment, it makes no difference if the input is shorted or not. Shorting the input simulates having the input of the amp connected to a low-impedance source like a preamp, and so the DC offset you measure with the input shorted will be very close to when it is actually hooked up.
For the bias adjustment, it makes no difference if the input is shorted or not. Shorting the input simulates having the input of the amp connected to a low-impedance source like a preamp, and so the DC offset you measure with the input shorted will be very close to when it is actually hooked up.
I actucally read post 21 as connecting the other probe the neg speaker terminal, but anyway connecting to the + (output) terminal:
But you're adding in the output to output terminals wiring, when you're measuring a voltage across a 0.22r resistor this can add some error into your calculation.
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There is no quiescent current across the length of wire that goes from the output modules to the positive speaker terminal in back. Putting your meter probe on the positive terminal gives the same reading as putting it on the spot where that wire attaches, along that sort-of buss-bar PCB track that all the emitter resistors connect to. The only voltage drop you will see that will cause any error, is caused by the transistor's quiescent current across that buss-bar. It's insignificant to the compared to the voltage drop across the emitter resistors.
The emitter resistors are rated at 5% tolerance, but in my experience, they are usually within 2% of their stated value, so you can take the voltage drop across them as a pretty accurate reading of the relative beta of that transistor at idle.
You'll need a 4-wire meter, or an LCR meter to take an accurate ohm reading on them.
And yes, MJ21193G and MJ21194G are an excellent choice for the GFA-555! They tend to match very closely if you get them all from the same batch. The consistency of these devices is amazing.
Well technically, you just need to replace them in sets of 4 NPN or PNP, so you could have for example, a set of old Toshiba NPN's happily co-existing with a set of new On Semi PNP's... but for the sake of symmetry, I would replace all of them. It hardly matter though. NPN and PNP "complementary" transistors don't rrreally match very well anyways; nature of the beast.
Maybe you're already done this, but check those driver transistors too!
Maybe you're already done this, but check those driver transistors too!
I've updated the language on my FAQ to be more clear about measuring voltage drop across the emitter resistors. Good feedback, thanks!
ADCOM FAQ | BIAS Adjustment
ADCOM FAQ | BIAS Adjustment
At least we agree just go across the resistor(s) directly.