Parallel headphone amplifier

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JAY X

Well-known member
Joined
Jan 9, 2009
Messages
683
Hi All!

Attached to this post is a pdf containing screen captures of a parallel opamp headphone amplifier.

it is build as follows:

1 zero impedance fader amplifier + 3 buffers with 150 Ohm output resistors in parallel. (50 ohms output impedance).  This way it nearly matches the impedance of 55ohm studio headphones.

I first build this amplifier on a test board with a quad opamp : MC33079. It worked well. No problems.

Then i build a pcb version with two dual opamps and one quad: MC33078/MC33079.

I placed two 220uf35v electrolytics in the supply rails, to act as reservoir caps.

After all checkings i powered the little beast.... only one channel worked...  more checks.....layout is ok all tracks checked with the multimeter...etc... new test...still sounds only one channel.... then......booomm the output caps popped!! :eek:

Well, ok so i pull out the 220uf 35v output caps....replaced for a piece of wire.... and still only one channel works..... :p

Today i try to make it work with the output jack gnd connected to circuit gnd to see if this makes a change....noo.....worse...i played an old discman, and only i could hear sound when i switch down power of the headphone amp..... ??? hmm...¿ a time constant...? no clue really....just guessing...

FADER AMPLIFIER FEEDBACK LOOP: 4k7 / 2k2 to ground

fader resistance: 10k linear.
Input capacitor: 2u2 63v.          orientation:  input  +_____-
Output capacitor: 220uf 35v. orientation: output -_____+

BUFFERS: 150 Ohms at their outputs. 3 in parallel.


Any comment is wellcome!!

Thanks a lot!
JAY X.


 
JAY X said:
BUFFERS: 150 Ohms at their outputs. 3 in parallel.

Any comment is wellcome!!

MC33078 data sheet tells use that its output short-circuit current is round-number 30 mA max. And then you put 150-ohm series resistors on the outputs so when paralleled they don't fight. So I don't see the point.

OK, I realize that that a) I'm gonna sound like a broken record, and b) the chips cost an order of magnitude more, but why are you doing this?  MC33078 is a fine device, but not as a headphone driver. Use something like OPA551, which gives you high current output, more than enough to drive your headphones to earwax removal levels.

My guess as to why only one channel worked? You blew up one of the amplifiers. Look at the unloaded output on your 'scope to see if it wiggles as expected. If it does, connect the load and repeat.

Oh, regarding your simulation: I see that it shows NE5532 instead of the 33078, which is what it is, but you don't show any load connected to the output. You have to hang the 64-ohm (or whatever) on it to get the simulation to reflect reality.
 
Hi!,

Finally i simplified the circuit with just one dual opamp. I changed the output resistor from 150ohms to 68ohms. I think i can go lower... Sounds good to me!.

Jay x

 
user 37518 said:
Why does the DC offset determine the smallest limit? is it due to the current that would be flowing from one output to the other due to DC offset voltage?
The DC offset will cause current to flow between the two op amps at idle. That is not good for the planet or the op amps if too much current (kind of worst case for dissipation).

I do not expect 68 ohms to be a problem.. since modern op amps do not have large DC input errors.

JR
 
> Why does the DC offset determine the smallest limit?

Take a wild case. One output, supposedly "zero", at +0.5V DC, the other at -0.5V DC.  1V difference.

With two 68r resistors, just sitting there we have 1V/138r or 7mA "waste" current flowing. If each opamp current-limits at 20mA, then two can only deliver 33mA.

A half-Volt offset among modern opamps properly biased is obscene. Maybe if you rigged an audio gain of 100 and had no DC blocking so the ~~5mV input offset of older opamps multiplied-out to 500mV. But any high-gain audio amp needs careful control of DC. And you would not want any such part of a Volt of output offset across most dynamic drivers. Modern opamps are reliably under 2mV (maybe far under), you can roll-off DC to unity gain, so you shouldn't be anywhere near a problem.
 
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