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Setting bias on my Adcom GFA-5400
Setting bias on my Adcom GFA-5400
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Old 23rd December 2014, 11:02 AM   #1
porkchop61 is offline porkchop61  United States
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Default Setting bias on my Adcom GFA-5400

Hi all-
I just picked up a used GFA-5400, and am checking the bias adjustment. In the service manual it says:
"Prior to performing BIAS ALIGNMENT and DC OFFSET CHECK turn unit on and allow to run with rated output for approximately 5 MINUTES before attempting adjustments"
Does this mean to run the amp at full output power for 5 minutes? I'd have to get a wattage meter (or calculate the voltage across the 8 ohm power resistors on the speaker terminals).
Also, most amps I've set up I usually short the input jacks, this one says to have an input resistance of a certain value or less. Is this just putting resistors across the input jacks?
I'm a bit confused, and I want to set the bias correctly. Anyone have this amp, or similar and done this procedure?

Here's the full bias setup procedure:

TEST PROCEDURES
All tests are performed with a 115V, low-distortion (less than 2% THD), AC-power source, 8-ohm resistive load (except slew rate), and a signal source of not more than 600 ohms.
An 80kHz low-pass filter is employed during THD distortion measurements.
Signal-to-noise measurements are A weighted.
Damping factor is measured by comparing the 1-watt output voltage with and without an 8-ohm load.
Slew rate is measured with an inductive load, and is derived with a dual-time-based oscilloscope
reading the slope of a full power 5kHz square wave. DO NOT OPERATE THE AMPLIFIER AT FULL-POWER SINE WAVE ABOVE 22kHz OR FULL-POWER SQUARE WAVE ABOVE 5kHz.

IMPORTANT
BEFORE PROCEEDING WITH ADJUSTMENTS, MAKE SURE AMPLIFIER IS AT ROOM TEMPERATURE.


Thanks
Glenn
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Old 24th December 2014, 09:42 PM   #2
6L6 is offline 6L6  United States
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Setting bias on my Adcom GFA-5400
Yes, 8-ohm resistors on the output.

I imagine the requirement for the full power is only to get it warmed up. Just play some music throug it for an hour, that way it will get truly up to temp.

The 600ohm input is from the signal generator. Don't worry about it.

? What's the procedure for the setting of bias and offset?
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Old 28th December 2014, 01:56 AM   #3
porkchop61 is offline porkchop61  United States
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Thank you.

Basically setting a voltage (35mV) over a resistor via trim pot, then setting the DC output on the speaker posts to 0mV.

That's what I did, let it play for about an hour with the 8 ohm load resistors on the speaker terminals. I did remove the input signal and short the input jacks when adjusting both the bias voltage and no DC output on the speaker terminals. Any amplifier I've seen the procedure for setting bias on always has the inputs shunted to ground. Granted most of my experience has been with tube amps, so maybe SS is different?

Glenn
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Old 28th December 2014, 01:59 AM   #4
6L6 is offline 6L6  United States
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Setting bias on my Adcom GFA-5400
I would set the bias with the input shorted as well... It seems the signal(s) are for other tests.

If the bias is set properly and you have the offset to zero or as close as you can, put the lid on, attach some speakers, and let it rip for an hour or so. Re-check to see if anything drifted, adjust if necessary and button it up.
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