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Old 17th September 2011, 08:19 AM   #11
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Richard Murdey
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R14 = 33 R would do the trick. The penalty you pay is a small reduction in maximum output power, and a relatively large increase in output impedance.

Note that an shorted load will not blow the output devices unless there is a AC signal present (i.e. playing music.) The DC offset is too small to generate significant current even with the BOM value for R14 = 1 ohm.
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Old 17th September 2011, 08:36 AM   #12
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So, this afternoon I put my 14s1 Sapphire on the bench and gave it a thorough going over with the voltmeter. Before I post some photos and lose your attention for good, let me post some numbers and comments.

- with the rev 14 layout, putting the output and driver transistors on the same heatsink, I solved the "reverse thermal runaway" issue that, in the rev 10 prototype, caused a significant reduction in output bias. I'm getting within 10% of the worksheet values, that 10% difference is most likely because the calculation assumes the devices are at the same temperature when in reality the drivers are a little bit warmer as the thermal coupling is not 100%.

- every other test point is spot on.

- in my build I have R10,R11 = 360 R and R6 = 13k. All other values as per the BOM.
- Talema 62062 transformers, but I'm running on 100 VAC line (Japan) so the effective secondary rating is 10.7 V rather than 12 V.
(I adjusted my version of the worksheet to reflect those changes before comparing the measured and calculated values)

Unrectified DC is +/-15.7 V. Regulated voltage rails are +/-10.8 V.
Measured driver current is 28 mA [calc=measured]. Measured output bias is 19 mA [calc=measured*1.1]. With my HD600 headphones, 23 dB gain and 64 mW of class A output power (calculated).

-The only surprising thing, to me, is how hot things got. Total heat for the stereo amp is 5 W, 0.5-1 W per heatsink. Overall the whole case gets comfortably warm when left on for a while. The heatsinks with two transistors against them are just cool enough to be able rest your finger against. Probably about 55 degrees C, or 20 C degrees above ambient (it's still hot here). My conclusion is that I would not want to increase the bias currents much above what they already are without a complete rethink of the thermal design of the amp.

Final word: Sapphire rev 14 works as designed, and is pretty well optimized at the current circuit values.
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Last edited by rjm; 17th September 2011 at 08:40 AM. Reason: additional info
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Old 17th September 2011, 08:43 AM   #13
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Sapphire Desktop Headphone Amplifier

(after fitting the new rev 14 boards)

- R6 is 13k tantalum, I grabbed them at hifi DIY store in Osaka.
- R10,R11 is the 360 ohm resistors from my proto built and were spliced in there after I mistakenly fitted 330k ohm resistors in there by mistake.
- The heatsinks are the wrong size, but they'll do until I get the new ones.
- The bridges, for the record, are 400V 50A each.

[Nikon D40, Ai micro-Nikkor 55mm F3.5]
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Last edited by rjm; 17th September 2011 at 08:47 AM.
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Old 18th September 2011, 02:29 AM   #14
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above should have been: R7 is 13k tantalum (gain set resistor, giving +8dB over the standard BOM config. to better match my 300 ohm headphones.)
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Old 18th September 2011, 09:18 AM   #15
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Btw, where do you source those nice enclosures from? I want one of these

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Venci.
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Old 18th September 2011, 01:13 PM   #16
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Here.
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Old 18th September 2011, 06:56 PM   #17
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Quote:
Originally Posted by simonov View Post
Btw, where do you source those nice enclosures from? I want one of these

Regards,
Venci.
Ebay is another source. I saw this there several times.

By the way RJM, the Sapphire looks good. Good job!
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Old 30th September 2011, 09:41 AM   #18
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I was scouring the internet for headphone amplifier circuits similar to the Sapphire : a current buffer running open loop, with either a discrete or integrated voltage front end. Other than the 47 labs model 0147 I haven't found anything. Dozens of diamond buffer variants out there, all complications of the basic LH0002 caliber - I think I've seem most of them now. Several "diode biased push-pull transistor pair" output stages stapled into the op amp feedback loop. Nothing I could really point to and say "hey that's pretty much the same as the Sapphire."

I'm interested because I'd like to know how other people approached a similar design, what they did differently, and why, maybe learn some new tricks.

Anyone care to point me in the right direction?
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Old 1st October 2011, 01:20 PM   #19
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BUF634 is a diamond buffer too. It uses CCS instead of emitter resistors for improved thermal stability.
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Old 2nd October 2011, 07:08 AM   #20
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True, there's that. Still available too, I though it was discontinued for some reason. Unfortunately it is fixed at about 6 mA quiescent current, so it more-or-less has to be used inside the feedback loop.

I was searching instead for example circuits where the output buffer is run open loop.
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