Mesa Walkabout power amp - inherent flaw?

[Mick Bailey]

I've worked on a few of these and all of them have one thing in common - high DC offset. The one I have right now has +180mv. Last one was over 100mv too. I've also noticed that when hot they can shift either way.

Anyone notice the same thing?

Walkabout.pdf

[loudthud]

Well, it really has no control of the output offset except MOSFET matching. Some circuits of this type have a servo to zero the output, but some don't. This may be the only Mesa product with a bias adjustment.

[Mick Bailey]

I suspect many of these are running with a high offset without anyone noticing. This one needed pots cleaning but I checked it over anyhow. I like simple, but it seems to me this is too simple.

[J M Fahey]

True but no big deal.

Output rail is centered depending on D1/D2 R19/R20 matching.

Fets also play a little part in it.

Not worse than the classic transformer driven bipolar output, which was never adjustable either, and depended on matching of a string of resistors and also some influence from transistor Vbe.

Used for decades and never caused trouble.

[Dave H]

True but no big deal.

Yes, why worry 180mV offset is only 4mW with an 8 ohm load.

[Enzo]

I agree, 180mv may be more than average, but I don't really consider it excessive until maybe a quarter volt.

[Mick Bailey]

Agreed, no big deal, but interesting to hear opinions. Though for a relatively modern amp design this is high when we're so used to seeing just a few mV offset. I've previously swapped diodes to get the offset down, but that's a game and not really worth it given the amount of time involved in removing the board.

I got fixated on offset a while back when I redesigned the Sessionette PM-120 power amp to include adjustable bias and offset. The result was nice in theory - perfect bias and zero offset. Worth it? NO!

But I can sleep at night.