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Old 2nd August 2014, 11:19 PM   #1
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Default My LM5532 Headphone Amplifier project

I purchased a gross of 5532 op amps back when I was interested in building Douglas Self's 5532 Op-Amplifier (Elektor) project. I never got around to building it, but I now find myself wanting to build a headphone amplifier. Adapting some aspects of Self's amp to my headphone amplifier project seems like a great use of my existing op-amp stash.

This amp is probably slightly different than some others out there. I will be driving the input from a preamp that has some good voltage gain so the headphone amplifier can be unity gain input to output. The design is basically a buffer, a bunch of (16) amplifiers in parallel with current sharing resistors, and a DC servo. The input buffer 5532 alone can have some DC offset - up to about 50mV for the values I am using. I happen to have a bunch of OP-07s around that I can use for the servo, so I threw it in there just in case eliminating the offset is actually useful.

I borrowed the unbalanced input from Self's 5532 amplifier and some of his other published work and tweaked a value or two as needed. The buffer is mainly to allow for AC coupling and to provide drive for all 16 parallel inputs.

When dimensioning the current stage I assumed a load of 32R, which I guess is the low end for what you can expect from headphones. This load would result in each op amp seeing just over 500 ohms, which should keep distortion low and the op-amps happy. I didn't bother to include an output zobel.

I will power all 5532s from a well-heatsinked +/-15V 317/337 regulated power supply.

Below is the schematic for one channel. The other is identical. Comments welcome, especially if I have overlooked something.
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Old 3rd August 2014, 12:10 AM   #2
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The last headphone amp I made had a dual op-amp, one driving each stereo channel.
I think I put 100R in series with each output.
I used the headphone amp on a disco and it worked really well even with the disco on full volume.
Its amazing what you can get away with.
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Old 3rd August 2014, 02:54 AM   #3
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Interesting project. One gasps.

Your 10 ohm output/loading/matching resistors are 51 ohms in "Burr-Brown Corporation AB-051" (google including quotes).

A possible value of 5 ohms is listed in "Intersil AN1111.1" (google again).

Apparently there's some flexibility.

Just to to toss in 2 cents.

The data sheet says the NE5532 can drive 600 ohms (per amp). This suggest to me that:

2 drive 300 ohms.

4 drive 150 ohms.

8 drive 75 ohms.

16 drive 37 ohms, call it 32, which is pretty much as low as headphones go. It seems you've covered every headphone known to man.
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Last edited by bentsnake; 3rd August 2014 at 03:00 AM.
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Old 3rd August 2014, 03:09 AM   #4
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Thinking about it, I wonder if bias current might not be a problem.

Worst-case bias current is 1ma in the data sheet. This would require the buffer to supply 16ma, which is pushing things about to the limit. Although certainly worst case times 16 is unlikely, just a thought.
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Old 3rd August 2014, 03:10 AM   #5
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Have a look at this. With just a minor extra complexity you can achieve somewhat more interesting result What "dead bug" can do? : diyaudio
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Old 3rd August 2014, 03:30 AM   #6
Art M is online now Art M  United States
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Quote:
Originally Posted by bentsnake View Post
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Worst-case bias current is 1ma in the data sheet.
I see worst case input bias current (Ib) as 1uA on T.I data sheet ?
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Old 3rd August 2014, 03:36 AM   #7
sangram is offline sangram  India
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I suspect you may need the k versions of the regulators. Depending on the input voltage, the dissipation may be too much to handle for the plastic ones. The 5532 is not gentle in idle consumption. Just 3 of them in my active crossover are enough to make TO220 regulators run toasty. I unfortunately have a very high input voltage, but still. Better to be safe.

Edit, 600 ohm performance of the 5532 is extremely poor. Opamp loading distortion was tested - author's name escapes me - and distortion at low loads of most common opamos was found to be terrible. I think the paper recommended a load of no less than 5k per opamp.

Last edited by sangram; 3rd August 2014 at 03:40 AM.
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Old 3rd August 2014, 04:27 AM   #8
agdr is offline agdr  United States
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Quote:
Originally Posted by sangram View Post
Edit, 600 ohm performance of the 5532 is extremely poor. Opamp loading distortion was tested - author's name escapes me - and distortion at low loads of most common opamos was found to be terrible.
Samuel Groner's epic op amp distortion paper has it on page 352(!) of the pdf link under "op amp distortion" here:

SG-Acoustics Samuel Groner IC OpAmps

The THD+N graph of the NE5534 at 10KHz with a 600R load and +/-15Vdc rails (30V) vs. output amplitude is around 0.05% at the best. Extrapolating the plot in my head I would take a guess of somewhere around 0.1% at 20KHz.

Last edited by agdr; 3rd August 2014 at 04:45 AM.
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Old 3rd August 2014, 04:35 AM   #9
Mihkus is offline Mihkus  Estonia
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I would add 3 transistors and one opamp so i could drive even 16 ohm earphones ..
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Old 3rd August 2014, 04:52 AM   #10
sangram is offline sangram  India
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Quote:
Originally Posted by agdr View Post
Samuel Groner's epic op amp distortion paper has it on page 352(!) of the pdf link under "op amp distortion" here:

SG-Acoustics Samuel Groner IC OpAmps

The THD+N graph of the NE5534 at 10KHz with a 600R load and +/-15Vdc rails (30V) vs. output amplitude is around 0.05% at the best. Extrapolating the plot in my head I would take a guess of somewhere around 0.1% at 20KHz.
The actual plot shows no better than 0.2% at any frequency or output voltage into a 600 ohm load. At the test impedance of 2.2k ohms, distortion drips to below 0.01% as the output approaches 0dBu. That's twenty times as much distortion.

Caveat, I only have a very old copy, mine has the 5532 results on page 172 or something.
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