ADCOM GFA-555 DC Offset Stupid question

[slipstream55]

Hi, group.

I have an Adcom GFA-555 that has developed distortion in the left channel. First I thought it was the speaker (Polk SDA-1C) but swapping the amp for a receiver verified the speaker was OK.

My question...can the DC offset on the Adcom amp be checked without hooking the pre-amp to the amp. The reason I'm asking this is it is not convenient to unhook the pre-amp and everything that's hooked up to it to check the DC offset.

Thanks for your responses in advance.

Mark

 

[EchoWars]

Sure.

But one odd thing about the 555 is that it has a DC connection to the preamp, even though it uses bipolar inputs (a DC connection is far more common with a FET input, where you can make sure there's no current flow from the pre to the amp).

At any rate, you can certainly measure the offset, but I'm not convinced that it'll tell you a whole lot about the cause of the distortion.

 

[slipstream55]

Thanks, Echo.

I know the 555 doesn't have speaker protection and it at first sounded like a blown speaker (very static sounding music coming from the tweeter or mid speakers) and I was sweating bullets (big ones) that I hadn't lost the speaker. Between the speaker or amp I was hoping the problem was in the amp. That I think I can fix. Can't replace the Polk driver that I know of. I thought a check of the DC offset would be the first place to start looking.

Thanks for your quick response.

Mark

 

[EchoWars]

I failed to complete my thought here...

But one odd thing about the 555 is that it has a DC connection to the preamp, even though it uses bipolar inputs (a DC connection is far more common with a FET input, where you can make sure there's no current flow from the pre to the amp).

That DC connection also means that any DC offset coming from the preamp is sent straight through to the speakers (and depending on the condition of the caps in the 555, can even be amplified a bit).

 

[kenvirginia]

When I checked my Adcom 555 DC offset, I got the following results;
I checked it at three times; 10 minutes; 30 minutes; one hour...warmup.

W/O preamp connected;
L: -37.8 RT: -53.7mv

W/ Adcom GFP55 preamp, set as directed by EW;
L: -19.3 RT: -20.6mv

W/ HarmonKardon HK825 pre;
L: -14.2 RT: -11.6mv

Explain that to me in simple terms...I'd like that.

-Kenny

 

[EchoWars]

I thought I already had...the 555 has a bipolar input differential stage which is DC coupled to the preamp (or whatever source), and thus can source or sink current from the preamp, changing the offset.

Amplifiers with DC connections to a preamp usually use a FET input stage, since a FET has such a high input impedance that it cannot source or sink current. The input impedance of a bipolar transistor is orders of magnitude less than a FET, and thus a bipolar transistor differential input stage is nearly always DC blocked with a capacitor at the input to the amp to prevent sourcing or sinking current from the preamp. So the DC coupled bipolar differential input stage of the Adcom 555 is pretty unusual.

 

[kenvirginia]

Thanks, Glenn. I got most of that...alot more than I would have a month ago....lol.
So, I deduce from that, that the HK-Adcom combination, in this case, is slightly better than with the Adcom Pre, correct?

Of course, I will recheck the Adcom after I replace the small caps.
-Kenny

 

[EchoWars]

So, I deduce from that, that the HK-Adcom combination, in this case, is slightly better than with the Adcom Pre, correct?

It's just a matter of luck.