ADCOM GFA5500 bias voltage needed

[dballes15]

Does anyone know the bias voltage for an Adcom GFA5500?

My friends GFA5500 heatsinks get too hot to touch after a few minutes running, even at low listening levels.

 

[Alfatipo]

You get an answer to this anywhere?

I have those figures but I have a little wine in me right now so I'm not too inclined to walk upstairs.

You should be able to keep your hand there for 15-20 seconds before it gets really uncomfortable.

High bias is good but too much will out run the heatsinking and damage stuff.

25 degrees over ambient is about right.

Jef

 

[DCPreamp]

The service manual says:

Let the amp idle for about 5 minutes to let it warm up.
Connect a millivoltmeter across emitter resistor R52 for the left channel (R51 for the right).
Adjust bias pot P2 until the meter reads 50mV +/- 5mV.

My GFA-5500 runs quite hot too. Plus, I've read on this site that the MOSFET outputs, the IRFP240 & IRFP9240, must be passing a moderate amount of current in order to get them into their linear range. This certainly explains the amp's running hot, but also explains their great sound.

Despite my amp running hot, I've never had it go into thermal protect mode even after many hours of listening.

 

[jimangie1973]

Thread back from the dead...

I opened up my 5500 only to discover the bias settings were not even close to spec. I suspected this because my unit was never really heating up. One channel was only biased to 15mV.

There are 10 source resistors per channel. After I set the bias correctly, the bias voltages across 8 of the 10 resistors (couldn't access the other 2) for each channel are:

Left.......45.9, 52.6, 53.6, 55.4, -, -, 53.3, 38.7, 52.9, 53.5 -> Avg 50.74 mV
Right.... 48.6, 46.4, 47.2, 58.0, -, -, 45.6, 49.4, 51.2, 59.0 -> Avg 50.68 mV

There's quite a variance to the bias voltages. Poor matching? Now the heatsinks get nice and toasty and the amp sounds great.

Jim

 

[birdyman]

Adjust way up until Jef at #2 post said. That is what Nelson Pass recommends.

 

[ryanjoz]

I recently adjusted the bias on my GFA-5500. The bias voltage prior to starting was 7.9mA for the left channel and 7.9mA for the right channel. The factory service manual states these should read 50mV +/-5mV.

I now notice static more than I did previously. Did I miss something in the manual? I disconnected the pre-amp and speaker connections to make the bias adjustments. I work for a company that has calibrated meters so I brought one home from work to adjust the amplifier.

The DC offset measures 4.5mV (L) and 5.4 mV (R) so both channels are in the spec from Adcom.

This is the first time the amplifier was opened up as I bought it new about 3 years ago.

Thanks in advance.

 

[Ian Finch]

There are wide variations in bias settings among different series of the same ADCOM models. I know there are at least 4 widely different ranges for different series of the GFA 555.

Just be certain you haven't been given wrong bias info. for your version of GFA 5500, if that is an issue, as it seems the previous setting of ~8mA in both channels is way too much of a coincidence, since you are the only owner, to be in error.

 

[ryanjoz]

I mistyped, I measured 7.9 mV across the resistors to determine the bias, not mA.

 

[Ian Finch]

Typo

It happens to all of us.
Even so, the the fact that both channels measure the same, just 3 years down the track, with no interference from unknown persons is just too much of a coincidence. Wouldn't you conclude that it left the factory set like that?

 

[ryanjoz]

Yes, I assume it left the factory that way. The bias was in question because the amp has never been warmer than room temperature even during long listening sessions at high volume. The bass response has been weaker on this amp than the previous Adcom amp I had previously (5503).