I want to drive low-z (32 ohm) headphones using op amps.

For various reasons I need:
1) Quad amps to be used, there's a requirement for other op-amps in the
units.
2) Single rail, around 12-15 V, I guess most modern op amps will do that
easily.
3) Not too current hungry, I want to power a number of these units from one
simple 12VDC transformer supply.
4) Lo-fi will do so long as distortion is not too gross. Intercom quality is
OK.
5) Reasonable cost.

I built a prototype using a LM324 that happened to be handy. Result was
gross crossover distortion when the output had to supply current to the
low-z earphones. I know these amps can be biased into class A operation but
I want reasonable power economy. Would be nice to just drop a pin
compatible device into my prototype, say a LM 387 ? Except the spec says
600ohms load.

Advice/comments please ?

"TonyL" wrote in message


I want to drive low-z (32 ohm) headphones using op amps.

For various reasons I need:

1) Quad amps to be used, there's a requirement for other
op-amps in the units.

That one is probably a non-starter, for reasons I'll detail below.

2) Single rail, around 12-15 V, I guess most modern op
amps will do that easily.

Yes.

3) Not too current hungry, I want to power a number of
these units from one simple 12VDC transformer supply.

The application defines the current drain.

4) Lo-fi will do so long as distortion is not too gross.
Intercom quality is OK.

No need to compromise sound quality.

5) Reasonable cost.

I built a prototype using a LM324 that happened to be
handy. Result was gross crossover distortion when the
output had to supply current to the low-z earphones.

Read the spec sheet and do the math. What sort of voltage will you need to
drive headphones? How much peak current does this net out to be?

I know these amps can be biased into class A operation but
I want reasonable power economy. Would be nice to just
drop a pin compatible device into my prototype, say a LM
387 ? Except the spec says
600ohms load.

Advice/comments please ?

Quit pulling numbers out of your memory from days long ago, and do what
professional engineers do when they have a task like this: Go to several op
amp manufacturer sites like TI, National, Maxim, NJM and Philips, and see
what they recommend for the purpose.

There are even such things as op amps that are designed to drive low
impedance loads and work from single-ended supplies.

The odds that they will be quads is about zero, since headphone drivers are
typically used in pairs.

"Arny Krueger" wrote in message
...
"TonyL" wrote in message


I want to drive low-z (32 ohm) headphones using op amps.

snip

There are even such things as op amps that are designed to drive low
impedance loads and work from single-ended supplies.

Indeed there are. Since 32ohm is now the standard for headphones there are
plenty of heaphone amps intended to drive 32 ohm h/phones on the market.
Mostly they are designed to work from single-ended supplies of around 3-12V

The odds that they will be quads is about zero, since headphone drivers
are typically used in pairs.

Agreed. I know of no quad op-amps that will do the job. I think Tony should
accept that he will need a separate h/phone amp chip.

David.

David Looser wrote:

The odds that they will be quads is about zero, since headphone
drivers are typically used in pairs.

Agreed. I know of no quad op-amps that will do the job. I think Tony
should accept that he will need a separate h/phone amp chip.

All comments noted. Thanks.

The *only* problem with the LM324 in this application is the crossover
distortion. It drives low-z headphones fine. So, I'll try plugging in a
TL074 and see what happens before looking to a purpose designed hadphone
driver.

In article , TonyL
scribeth thus
David Looser wrote:

The odds that they will be quads is about zero, since headphone
drivers are typically used in pairs.

Agreed. I know of no quad op-amps that will do the job. I think Tony
should accept that he will need a separate h/phone amp chip.

All comments noted. Thanks.

The *only* problem with the LM324 in this application is the crossover
distortion. It drives low-z headphones fine. So, I'll try plugging in a
TL074 and see what happens before looking to a purpose designed hadphone
driver.



Would one of those SSM -whatever -there -called series line drivers do
this quite well?..
--
Tony Sayer

tony sayer wrote:
In article , TonyL
scribeth thus
David Looser wrote:

The odds that they will be quads is about zero, since headphone
drivers are typically used in pairs.

Agreed. I know of no quad op-amps that will do the job. I think Tony
should accept that he will need a separate h/phone amp chip.

All comments noted. Thanks.

The *only* problem with the LM324 in this application is the
crossover distortion. It drives low-z headphones fine. So, I'll try
plugging in a TL074 and see what happens before looking to a purpose
designed hadphone driver.



Would one of those SSM -whatever -there -called series line drivers do
this quite well?..


Don't know. But if I can't use the spare section on the existing quad op-amp
to drive the headphone then I may as well go for a purpose designed heaphone
amp as suggested by others in here, or add a bipolar complementary pair.

TonyL wrote:

David Looser wrote:

The odds that they will be quads is about zero, since headphone
drivers are typically used in pairs.

Agreed. I know of no quad op-amps that will do the job. I think Tony
should accept that he will need a separate h/phone amp chip.

All comments noted. Thanks.

The *only* problem with the LM324 in this application is the crossover
distortion.

A consequence of its internal design.

Graham

On Tue, 13 Jan 2009 08:22:26 +0000, Eeyore
wrote:



TonyL wrote:

David Looser wrote:

The odds that they will be quads is about zero, since headphone
drivers are typically used in pairs.

Agreed. I know of no quad op-amps that will do the job. I think Tony
should accept that he will need a separate h/phone amp chip.

All comments noted. Thanks.

The *only* problem with the LM324 in this application is the crossover
distortion.

A consequence of its internal design.

Graham

It has a poorly biassed output stage - virtually pure class B.

d

Arny Krueger wrote:

The odds that they will be quads is about zero, since headphone drivers are
typically used in pairs.

Very true.

Graham

"TonyL"

I want to drive low-z (32 ohm) headphones using op amps.

For various reasons I need:
1) Quad amps to be used, there's a requirement for other op-amps in the
units.
2) Single rail, around 12-15 V, I guess most modern op amps will do that
easily.
3) Not too current hungry, I want to power a number of these units from
one simple 12VDC transformer supply.
4) Lo-fi will do so long as distortion is not too gross. Intercom quality
is OK.
5) Reasonable cost.

I built a prototype using a LM324 that happened to be handy. Result was
gross crossover distortion when the output had to supply current to the
low-z earphones. I know these amps can be biased into class A operation
but I want reasonable power economy. Would be nice to just drop a pin
compatible device into my prototype, say a LM 387 ?


** Huh ??

The LM387 is a long obsolete, dual low noise pre-amp.

How about you try a TL074 - exact same pin out as the LM324 and no x-over
issues.

Will deliver about 20mA ( = 13mW peak ) into a 32ohm ear phone, which is
plenty loud.




...... Phil