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Old 30th July 2012, 11:42 PM   #1
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Default Diamond buffer headphone amp- can I parallel outputs?

Or will I destroy the symmetry and thermal tracking?

My scheme is to use 3 0.5 A driver transistors (locally available and cheap) with larger (30 ohm?) emitter swamping resistors, in an effort to provide some low tech, passive short circuit protection.

I hear about so many fake parts that I am leery of ordering semiconducturs on the web. Maybe somebody can hook me up with a reliable source.

I've read so much here about op amps for audio. Can anybody give me a good reason not to use the old tried and true 5532? And what op amp is suitable for the servo circuit?

Sorry no schematic. When I went to engineering school we didn't even use computers.
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Old 31st July 2012, 02:13 AM   #2
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Look here:-

1 4W 5, Other IC items in Tayda2009 store on eBay!

You can get parts so cheap there's little point in worrying about economizing, although you may have to wait for them to come.

30 ohm resistors, even 3 of them, is too much at the output. Headphone amp output impedance should be no more than a couple of ohms.

$0.2 for a 5532 is a convincing reason to use them. They're unsurpassed in most audio applications. Not all, but in most general purpose applications you won't hear the difference, no matter how much you spend.

OPA2277 is good for servos, but it'll cost more than the rest of the amplifier put together.

Age is no excuse. Learn to draw schematics in LTSpice.

Linear Technology - Design Simulation and Device Models

Last edited by counter culture; 31st July 2012 at 02:30 AM.
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Old 31st July 2012, 05:59 AM   #3
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Quote:
Originally Posted by Fast Eddie D View Post
I hear about so many fake parts that I am leery of ordering semiconducturs on the web. Maybe somebody can hook me up with a reliable source.
Err .. mouser?
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Old 31st July 2012, 11:42 PM   #4
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Quote:
Originally Posted by counter culture View Post
.

30 ohm resistors, even 3 of them, is too much at the output. Headphone amp output impedance should be no more than a couple of ohms.
I agree about the output impedance. But if the buffer is in the feedback loop, it will lower the output impedance. I am starting to think that my idea isn't necessarily the best, but 30 ohm resistors will provide short circuit to ground current of 0.5 A with a 30 volt split supply, so that part would work.

Quote:
$0.2 for a 5532 is a convincing reason to use them. They're unsurpassed in most audio applications. Not all, but in most general purpose applications you won't hear the difference, no matter how much you spend.
That's what I thought, but I have read a lot of woo about this op amp vs that op amp here and on other forums.

When I studied electrical engineering, op-amps were for illustrative purposes only. At the time, there was no op amp that even came close to being ideal or cost effective. The first op amp I worked with was the 709, which is very obselete and entirely impractical.

Quote:
OPA2277 is good for servos, but it'll cost more than the rest of the amplifier put together.
Thanks, I'm still working on that part of the concept. The servo only has to protect the headophones from excessive DC offset without being sonically obtrusive. It does not have to be super precise.

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Age is no excuse. Learn to draw schematics in LTSpice.

Linear Technology - Design Simulation and Device Models
Yeah, I'm going to check that out.
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Old 31st July 2012, 11:43 PM   #5
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Originally Posted by rjm View Post
Err .. mouser?
Thanks. I'm going to spend some time browsing that site.
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Old 1st August 2012, 07:56 AM   #6
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If your circuit is the classic diamond buffer, you shouldn't need to mess around with a servo.

You have three options, apart from a servo:

1. Just build it and live with DC offsets. (entirely practical)
2. 30 ohm series resistor as current limiter. Sadly it does influence the sound, but at last you'll never fry your phones.
3. Set up a trimmer resistor on the input to inject a compensating DC offset. Once the DC offset is trimmed in these circuits, it normally stays within tolerable limits unless your build has exceptionally poor thermal tracking.

Here's an op amp diamond buffer headphone amp I built. (link) Just for purposes of discussion, since you aren't publishing a circuit, how different is your idea from what I did?
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Old 3rd August 2012, 05:48 PM   #7
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Quote:
Originally Posted by rjm View Post
If your circuit is the classic diamond buffer, you shouldn't need to mess around with a servo.
I'm going to skip the servo.

Quote:
1. Just build it and live with DC offsets. (entirely practical)
That's what I'm going to do.

Quote:
2. 30 ohm series resistor as current limiter. Sadly it does influence the sound, but at last you'll never fry your phones
I'm going to skip that and live dangerously.

Quote:
3. Set up a trimmer resistor on the input to inject a compensating DC offset. Once the DC offset is trimmed in these circuits, it normally stays within tolerable limits unless your build has exceptionally poor thermal tracking.
I will do this.

Quote:
Here's an op amp diamond buffer headphone amp I built. (link) Just for purposes of discussion, since you aren't publishing a circuit, how different is your idea from what I did?
I am abandoning my original concept since you showed me the Mouser site. I will use BD139 and BD140 transistors. I am going to scale an old Walter Jung diamond buffer (designed to drive 600 ohms) to drive 16 ohm headphones (although all my headphones are 32 ohms). The Jung circuit (in a book, not on the web ) utilises simple active constant current sources in place of the resistors.

I will include in the chassis a passive splitter which has a 1 dB insertion loss to the system power amp, and a 3 dB insertion loss to the headphone amp (which will have 6 dB of gain). I will also include a shorting mute switch for the power amp feed to mute the speakers. In other words, the unit will be plug and play with my current system, with the addition of only one pair of RCA cables.

By the way, I appreciate your comments.
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Old 16th August 2012, 02:09 AM   #8
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Is this a diamond buffer ONLY with no loop feedback, or is it like the Sapphire with a voltage gain stage followed by a diamond buffer and a feedback loop around all of it? Since a CD player puts out 2Vrms, you don't need a voltage gain stage in your headphone amp. It can be just a power diamond buffer. Do I read your post correctly that your headphone amp is going to have 6dB of gain?
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Old 16th August 2012, 08:52 AM   #9
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you can parallel output transistors.
I used 20pairs of Phillips BC847/857 with emitter resistors.(amp with negative feedback)
My parallel bufer gives 2.8Vrms
offset without feedback - 12mV(normal value)

Last edited by stridervvv; 16th August 2012 at 08:56 AM.
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Old 16th August 2012, 02:05 PM   #10
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Quote:
Originally Posted by stridervvv View Post
you can parallel output transistors.
I used 20pairs of Phillips BC847/857 with emitter resistors.(amp with negative feedback)
My parallel bufer gives 2.8Vrms
offset without feedback - 12mV(normal value)
Twenty pairs! wow.

I'd slap some big TO-3's on there instead, but that's just me.
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