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

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

"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.

S.

--
http://audiopages.googlepages.com

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.

On Mon, 30 Mar 2009 16:59:11 +0100, "TonyL"
wrote:

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".

Just to clarify, mine doesn't swamp the reactance with resistance, it
actually removes the reactance totally - the load is purely resistive.

If you are driving many loads, what about configuring it as a proper
distribution amp. Use a separate 50 ohm resistor to feed each cable,
and load the far end of each with 50 ohms. Everything is properly
loaded for flat response, nicely buffered from everything else, and
guaranteed stable. You get a net 6dB loss, which you can make up
simply in the feedback network.

d

Don Pearce wrote:
On Mon, 30 Mar 2009 16:59:11 +0100, "TonyL"
wrote:

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".

Just to clarify, mine doesn't swamp the reactance with resistance, it
actually removes the reactance totally - the load is purely resistive.


Assuming terminating resistor=cable characteristic impedance, yes.

But for various practical reasons I'm using twin-core twin-screen stereo
cable with an undisclosed characterisic impedance and a capacitance of
around 100pf/meter. My fault I didn't mention this....sorry.

If you are driving many loads, what about configuring it as a proper
distribution amp. Use a separate 50 ohm resistor to feed each cable,
and load the far end of each with 50 ohms. Everything is properly
loaded for flat response, nicely buffered from everything else, and
guaranteed stable. You get a net 6dB loss, which you can make up
simply in the feedback network.


If I were starting this project from scratch I well might do that.

I'm fairly happy with the stability now. I've tried adding large amounts of
capacitance at various places in the cable setup and the system remains
stable, the only effect of any added C being increased HF attenuation.

How can that work, you would not have any signal left for the real load,
surely..
Brian

--
Brian Gaff -
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"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