Serge Auckland wrote:
"Don Pearce" wrote in message
news:49d0d0fb.430221593@localhost...
On Mon, 30 Mar 2009 14:51:17 +0100, "TonyL"
wrote:

As part of a low-fi audio system I need to drive a capacitive load.
Basically, the load is lengthy cabling with high impedances at the
far ends.
Total load capacitance with the length of cabling used is likely to
be around 50nF which I know is far beyond what many audio op-amps
will handle.
I've been staring at datasheets of ICs such as the LM8261, which
can drive capacitive loads but getting a bit punch drunk. Any
recommendations ? Main problem with the LM8261 is the packaging, I
would prefer DIP-8. TIA


Why not hang a 50 ohm resistor across the far end (assuming 50 ohm
cable, if 75 ohm cable make it a 75 ohm resistor), then you won't
need to drive a capacitive load. Plenty of op amps out there which
will be perfectly happy with that.

d

Good advice, but if for some reason that isn't possible, just use a
low power audio amplifier IC. 50nF is not a high capacitance to a
small power amp. Some of the small (1-5) watt ICs are designed for
portable radios etc, so aren't terribly hi-fi, but you did say it was
for a lo-fi application.

Thanks both, two good ideas to try. Both solutions would need a LM386 or
similar IC as the 20 far end resistors would total 2.5 ohms. I'm currently
doing this with a humble LM388 and could see some parasitics when driving
load. I'm assuming this is due to it not handling the reactance too well.

Since my last post I tried reducing the HF phase shift by simply inserting a
single 33 ohm resistor directly in series with the LM388 output. This
amounts to the same solution that Don offered...that of swamping the
reactance with resistance. This *seems* to have fixed the problem but I
need to be sure before this load of kit gets shipped half way round the
world. Last thing I want is a midnight phone call saying "This *&^% thing
you sent doesn't work".

Anyway, thanks again for the tips both.