Audire Forte output transistors and DC offset

Hi.

I just got an Audire Forte. I checked the DC offset and one side is solid at about 10mV while the other is all over the place between 95 and 130mV. Keeps changing around and it doesn't settle anywhere. I tested the amp and it sounds fine. No hums, no pops. I'll be doing a recap on the driver board soon so I hope that will take care of the DC offset problem. I know this amp uses the MPQ6600 and I don't want to start looking for one. Any suggestions?

The channel with the good DC offset seems all original. The output transistors are the originals MJ15022 / MJ15023 (3 pairs). The channel with the DC offset issue has output transistors 2SD555 / 2SB600 pairs. These look like an old replacement and you can also see that some of the caps on the driver board are different. My question is: if they are working OK do you think I need to replace these for original MJ15022/MJ15023 pairs? Could these be causing the DC offset issue above?

Thanks.
Elias
 
You should be looking at the input stage instead, where DC offset problems are much more likely to originate. Either there or there is some other stage with a duff transistor that's resulting in much reduced overall loop gain. Typical power amps have so much loop gain at DC that the output stage has essentially nothing to say when it comes to output DC offset.

You may get lucky and find it's just a leaky coupling / feedback cap indeed, these amps are not entirely new any more after all. I see what you mean about the MPQ6600 - though if in doubt I'd think it should be possible to make a suitable replacement using some thermally coupled BC547 / 557 or similar.

A review mentions an input impedance of only about 1.6 kOhms, now that would be pretty low. I'd verify that.
 
Thanks sgrossklass for the reply. I did replace the caps but the high wandering DC offset is still there on the left channel. Interesting though, as the Forte has a switch for each channel, when I turn on the left channel, the DC offset hovers around 60mv, but as soon as I turn the other one it goes up to about 120mv. It ten keeps wandering around between 100 to 150mV.

From what I read I suspect the MPQ6600 may be going bad. I found the MPQ6600 on the AmericanMicroSemi.com web site. Only about $10 a pop. Any issues with getting from americanmicrosemi? There are a few on eBay, but they are MPQ6600A which looks like it has double of the current gain. Not sure if it matters.

I spoke with Mr. Julius Siknius and he said he may be able to provide me with the schematic.

Thanks again.
Elias
 
Update and nudies

Got the schematics from Mr Julius Siknius and the MPQ6600 from American Micron Semiconductors. Actually it's a vintage Motorola MPQ6600, I think from 2004 (426 on the chip??). Replaced the MPQ6600 and the DC offset is now stable around 9 mV :cheers:

The Audire Forte is now driving my double pair of KLH Model Nines. Sounds great, more coherent and great authority on the bass.

Here are some nudies of the Forte:

IMG_1237.jpg

IMG_1238.jpg

IMG_1239.jpg

Elias
 

pottedman

Member
2017-08-31 5:30 pm
Audire Forte

Hi.

I just got an Audire Forte. I checked the DC offset and one side is solid at about 10mV while the other is all over the place between 95 and 130mV. Keeps changing around and it doesn't settle anywhere. I tested the amp and it sounds fine. No hums, no pops. I'll be doing a recap on the driver board soon so I hope that will take care of the DC offset problem. I know this amp uses the MPQ6600 and I don't want to start looking for one. Any suggestions?

The channel with the good DC offset seems all original. The output transistors are the originals MJ15022 / MJ15023 (3 pairs). The channel with the DC offset issue has output transistors 2SD555 / 2SB600 pairs. These look like an old replacement and you can also see that some of the caps on the driver board are different. My question is: if they are working OK do you think I need to replace these for original MJ15022/MJ15023 pairs? Could these be causing the DC offset issue above?

Thanks.
Elias


POTTEDMAN
Hello from Germany,
could you supply me with the SCHEMATIC for my AUDIRE FORTE Amplifier?
That would be great.


Best Regards,
Wolfgang