• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Evaluating Adcom GFA-2535 : DC offset question

ChrisPDX

ChrisPDX

New Member
I recently acquired a used Adcom GFA-2535, which is two GFA-535 amps (MKII version, if I understand correctly) in one unit. These amps can be run independently in “stereo” mode or in “bridged” mode. This unit does not have input-level control dials.

I wanted to check the amps' DC offsets before hooking them up, in order to make sure they were performing well and would not damage my speakers. While researching how to do this, I found contradictory advice on the state in which to test the power amps’ speaker outputs for DC offset voltage, including this informative FAQ from Adcom specialist “Hoppe’s Brain”:
How much DC offset is normal?

DC offset should be measured with the input shorted. (You can make a shorting plug from an old patch cord, just cut the wire about 1″ long and solder the wires together.)
  • GFA-535, 545, 555 MKI. Normal from the factory is about 40mV. With a pair of carefully matched input transistors, such as the ones I sell, you can expect less than 20mV. If you see much more than 50mV the amplifier should be checked out.
  • GFA-535, 545, 555 MKII, GFA-565, GFA-585
    These amps contain DC servo circuits, and so DC offset should be very close to 0V, whether you have the inputs shorted or not. Normal range is less than 5mV, and it may slowly drift around a bit.
Click to expand...

So I tested the Left and Right channels on both amps (A & B, in “Stereo” mode, warmed-up) in three states:
  1. No signal
  2. 1000 HZ tone (from iPhone signal generator app at 75% volume)
  3. RCA input shorted
Based on several rounds of tests, these are my results (averages +/- 5 mV):

........... NO SIGNAL.... 1KHz TONE . INPUT SHORTED
...AMP A -------------------------------------------
.....RIGHT .... 33 mV ...... 104 mV ........ 102 mV
.....LEFT .... -18 mV ....... 80 mV ......... 79 mV
...AMP B -------------------------------------------
.....RIGHT ..... 9 mV ...... 101 mV ......... 99 mV
.....LEFT ...... 3 mV ....... 93 mV ......... 90 mV

Any advice on how to interpret these results would be much appreciated !

Based on the measurements with no signal input, Amp B looks pretty good and Amp A looks marginal, but all of the other test results seem quite high (again per Hoppe's Brain above: the MKII "should be very close to 0V, whether you have the inputs shorted or not. Normal range is less than 5mV"). If Amp B is good to go, I would be happy to just use that one alone and not take this unit in for servicing.

( For reference, the Adcom GFA-2535 Service Manual is here. )

Secondary question: Assuming I proceed to only use Amp B, is there a non-permanent way to disconnect Amp A in order to not waste energy and to remove a potential source of electrical noise?

Thank you for your assistance! This website is a great resource, and I really appreciate the time and effort so many users devote to helping others learn.

( I hope no one minds that I took the liberty of tagging users who seem to have relevant experience: @Wolverine @Mayank @northpaw @Skywatcher @SaturationPt @Frank Sol @AdamAnt316 @SteveJewels )
 
Last edited:
Register to hide this ad
It seems from service manual there is only bias adjustment for his amp (idle current)
Adcom_2535_bias.jpg
 
Last edited:
ChrisPDX said:
I recently acquired a used Adcom GFA-2535, which is two GFA-535 amps (MKII version, if I understand correctly) in one unit. These amps can be run independently in “stereo” mode or in “bridged” mode. This unit does not have input-level control dials.

I wanted to check the amps' DC offsets before hooking them up, in order to make sure they were performing well and would not damage my speakers. While researching how to do this, I found contradictory advice on the state in which to test the power amps’ speaker outputs for DC offset voltage, including this informative FAQ from Adcom specialist “Hoppe’s Brain”:


So I tested the Left and Right channels on both amps (A & B, in “Stereo” mode, warmed-up) in three states:
  1. No signal
  2. 1000 HZ tone (from iPhone signal generator app at 75% volume)
  3. RCA input shorted
Based on several rounds of tests, these are my results (averages +/- 5 mV):

........... NO SIGNAL.... 1KHz TONE . INPUT SHORTED
...AMP A -------------------------------------------
.....RIGHT .... 33 mV ...... 104 mV ........ 102 mV
.....LEFT .... -18 mV ....... 80 mV ......... 79 mV
...AMP B -------------------------------------------
.....RIGHT ..... 9 mV ...... 101 mV ......... 99 mV
.....LEFT ...... 3 mV ....... 93 mV ......... 90 mV

Any advice on how to interpret these results would be much appreciated !

Based on the measurements with no signal input, Amp B looks pretty good and Amp A looks marginal, but all of the other test results seem quite high (again per Hoppe's Brain above: the MKII "should be very close to 0V, whether you have the inputs shorted or not. Normal range is less than 5mV"). If Amp B is good to go, I would be happy to just use that one alone and not take this unit in for servicing.

( For reference, the Adcom GFA-2535 Service Manual is here. )

Secondary question: Assuming I proceed to only use Amp B, is there a non-permanent way to disconnect Amp A in order to not waste energy and to remove a potential source of electrical noise?

Thank you for your assistance! This website is a great resource, and I really appreciate the time and effort so many users devote to helping others learn.

( I hope no one minds that I took the liberty of tagging users who seem to have relevant experience: @Wolverine @Mayank @northpaw @Skywatcher @SaturationPt @Frank Sol @AdamAnt316 @SteveJewels )
Click to expand...
DC Offset is the amount of voltage coming out to the speakers with NO signal with the "offset" between positive and negative. No "shorting plugs" are needed. They are used only to ensure no RF interference is causing a signal to be amplified. And there should be no difference in the readings of simply "no signal" and "shorted" if there is no interference. You show on your "shorted" readings you are picking up a signal from somewhere so your "test" is being done wrong. Since your "no signal" readings are small, it is "safe" to hook up speakers to both amps. A "test tone" is a signal that will be amplified and the voltage will rise so it is useless to determine "DC Offset" at idle...
 
The GFA-2535 does not appear to have the DC servo circuit that is present in the GFA-535 II. Presumably this means you can follow the guidelines generally given for DC offset, which is that <50mv is not considered harmful. Opinions may vary, however, especially if you want to consider performance issues that are less than harmful.

You can have a look at the differences in the "Circuit Description" that Adcom provides at the beginning of the Service manuals for the 2535 and 535 II. The latter describes the IC-based DC servo circuit, whereas the former describes passives that are used to minimize DC offset. Unfortunately, the schematic provided for the 2535 does not include voltage markings, so quick checks to see if anything is amiss will require a deeper understanding of the circuit. If it were me, I would not be worrying about the DC offset values you measured, but I'm no expert.
 
Hi, Chris Hoppe of Hoppe's Brain here!

Northpaw is correct, the GFA-2535 doesn't have a DC servo. It's essentially the same circuit as the GFA-535/545/555 MK1, but it has a slightly different bias circuit. 100mV seems high though. You might do better with a pair of matched input transistors. Also, there is a slightly problematic push-on connector on the 2535's bias compensation transistor mounted on the heatsink. It corrodes after a while, and could cause the bias spreader to go wonky. Just re-seating might be all that's needed. I just solder them on.

DC offset is measured with the inputs shorted, because that replicates what the amplifier sees in actual operation. The output of most preamps is something like 75-150 ohms, and that is basically a short circuit compared to the 22K input impedance of the amp.
 
FYI... the bias adjustments given above are for the GFA-535II. Service manual for the GFA-535 says to connect the Positive lead to TP1 and Negative lead to TP2 after warming up the heat syncs. For each channel, adjust VR601 to achieve 5mv (+/- .5mv).
 
You must log in or register to reply here.
Back
Top Bottom