Yamaha DSP A2070
 

In article ,
Jim Lesurf wrote:
It's a pretty basic home made bench supply - just using a pair of old
RS cards. The connector on the PCB is a standard type and I had a
suitable plug so made up a loom to use that. And as I said one pair of
amps is fine, but not the other. I'd say my loom is in fact shorter
than the one in the amp from the power supply. But I can easily add
some smoothing caps.

FWIW I would always use caps neat the amp board to stop the rails being
waggled at HF. A bench PSU and wires from it can have too large an
impedance at RF to keep the rails presented to the amp steady.

I've tried this and it's sorted it. All that I now need to know is why the
other identical amp doesn't suffer from it. ;-) And is it normal to have
to mod a design they made thousands of?

Removing one pair of output transistors stopped it oscillating without the
additional smoothing. They checked out OK on my tester. Didn't have any
identical spares to try. They aren't listed in my Towers so didn't know if
I would have had equivalents.

--
*Some people are alive only because it's illegal to kill them *

Dave Plowman London SW
To e-mail, change noise into sound.

Yamaha DSP A2070
 

On Thu, 19 Nov 2009 18:12:10 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
Jim Lesurf wrote:
It's a pretty basic home made bench supply - just using a pair of old
RS cards. The connector on the PCB is a standard type and I had a
suitable plug so made up a loom to use that. And as I said one pair of
amps is fine, but not the other. I'd say my loom is in fact shorter
than the one in the amp from the power supply. But I can easily add
some smoothing caps.

FWIW I would always use caps neat the amp board to stop the rails being
waggled at HF. A bench PSU and wires from it can have too large an
impedance at RF to keep the rails presented to the amp steady.

I've tried this and it's sorted it. All that I now need to know is why the
other identical amp doesn't suffer from it. ;-) And is it normal to have
to mod a design they made thousands of?

Removing one pair of output transistors stopped it oscillating without the
additional smoothing. They checked out OK on my tester. Didn't have any
identical spares to try. They aren't listed in my Towers so didn't know if
I would have had equivalents.

It looks like they have pared cost back to the barest minimum. It
would probably be no bad thing to put in the speaker stability bits
and some extra decoupling caps for the supply rails on the amp board
itself - leads as short as possible. Look in the application note to
see what you don't have already.

As for why the other amp doesn't do it - Sod's law, I guess. Slight
differences in lead length and layout, probably.

d