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RJM Audio Sapphire Desktop Headphone Amplifier
RJM Audio Sapphire Desktop Headphone Amplifier
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Old 20th November 2014, 03:27 PM   #311
HaroldHill is offline HaroldHill  Canada
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repente, I use the Sapphire as the main driver for my HE-6. In my case, I replaced R4 with a 33k ohm resistor. The gain is just about perfect, with my volume pot just passed 9 o'clock. (I tried a 66k ohm resistor, too, but the gain was just too high.)

I've never hear the Beta22, so I can't compare the two, though!
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Old 20th November 2014, 10:43 PM   #312
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Richard Murdey
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RJM Audio Sapphire Desktop Headphone Amplifier
You can read about Amb's beta22 amp here. I haven't heard it either, but from the schematic it looks pretty cool.

As for improving the Sapphire, the hard part, and why I was asking repente about the impressions, is figuring out what's wrong with it. I'm very happy with the way it is. If anything I suppose I could wish for a little more "mellowness", as I agree that the dominant impression is of power/control/clarity rather than warmth. How one might obtain a more relaxed sound that without making the amp worse is, however, not immediately obvious to me.

Thoughts:

There's the op amp, an obvious weak point but making something better than an OPA134 from discrete parts is not self-evident, since we must answer "why is the OPA134 deficient?"

There's the output stage. Again no glaringly obvious route to improvement. And there's the voltage regulation, which, here too, what is "better" and what is "worse" from the endpoint of tuning the sound is far from clear.
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Old 5th January 2015, 05:57 PM   #313
vexus79 is offline vexus79
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Hello together,

I just finished my Sapphire Amp and I use it also as active Preamp for my Desktop system.
I'm blown away by the resolution and power it delivers.

Thank you very much for this fabulous amp.

Regards from Switzerland
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Old 6th January 2015, 02:47 AM   #314
ammel68 is offline ammel68  United States
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RJM Audio Sapphire Desktop Headphone Amplifier
Richard, what sets the Sapphire amp apart from the Lehmann design?

I see a lot of similarities from the op-amp input stage to a diamond buffer output stage.

I'm not knocking the amplifier in any way, just curious why it would sound any better than the Lehmann?
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Old 6th January 2015, 12:05 PM   #315
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[deep breath]

So, would you believe I had no idea Lehmann even made a headphone amplifier? I did not pick it up in my many, many Google searches for "diamond buffer" or "headphone amplifier circuit schematic".

For the record my design can be dated Feb 2, 2010 (proof), while the Lehmann is at least a couple of years before that. So its a lack of due diligence on my part rather than the other way around.

Anyhow, the fact is though that the concept (op amp voltage stage + diamond buffer) is self-evident and I don't think the circuit elements can't be appropriated by anyone.

I don't know the Lehmann circuit, so to answer your question I will assume it closely resembles this "clone" published on head fi: here.

You can see that all the main elements are basically the same, right down to the op amp and output transistors. I use BD135,6 transistors throughout the diamond buffer, the 'clone uses smaller BC550,560 on the input.

Here are the differences.

The Sapphire is dual mono, single op amps are used, and nothing is shared between channels. That's a big difference.

Also the clone uses LM317/337 voltage regs. Blech. Boo. Hiss. Sapphire has Zener regulated BD135,6 pass transistors. I think that makes a big improvement.

The clone runs at 15 V rails, the sapphire about 10.6 V. Sapphire is biased to 30 mA. I don't know what the clone runs at, you can't tell from just looking at the circuit, but given the 10 ohm emitter resistors it's probably the same or a bit less.

So, looking at it, the big thing is that the Sapphire has completely independent power supplies and signal paths for each channel due to its dual mono (one board per channel) construction.
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Old 3rd February 2015, 12:20 AM   #316
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Lately I've been wondering if some soft of damping was needed in the Sapphire circuit between the driver and output transistors of the diamond buffer.

I added an RC filter stage (475R 1nF low pass) before the output transistors limiting the buffer bandwidth to 335 kHz.

The voltage drop across the resistor lowers the bias current in the output stage. I hadn't planned for that, and the side effect of considerably less class A output power is unwanted. In future this will have to be compensated for by adjusting R9, R10.

The effect of the RC filters on the sound is surprisingly large, and not a result of the change in bias current (I already confirmed independently what low bias current sounds like.) It really does seem to take the "edge" off things. I think I may have overshot it a little, and a smaller filter or perhaps just damper resistors alone might give the best balance - but the interesting thing is that the position does really seem to be a "pain point" for the circuit, so its worth looking into it in some depth.
Attached Images
File Type: jpg DSC_7111_02.jpg (337.3 KB, 235 views)
File Type: jpg DSC_7120_05.jpg (324.4 KB, 231 views)
File Type: jpg DSC_7122_06.jpg (326.2 KB, 227 views)
File Type: jpg DSC_7126_09.jpg (460.7 KB, 234 views)
File Type: png Sapphire 20f5 mod 01.png (29.6 KB, 247 views)
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Old 3rd February 2015, 01:10 PM   #317
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Adjustment to R9,R10 can be calculated easily enough, assuming the base of the output transistor draws about 0.06 mA, and the emitter of the driver pair sources 10 mA.

R9 should increase about 1 ohm for every 133 ohms of resistance added before the base of the output transistors.
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Old 4th February 2015, 11:07 AM   #318
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Default Sapphire 3.0

I'm starting to ramp up things towards the next spin.

Upcoming features being considered:

1. Gain switch, optional, configurable, hi/lo.
2. Current sources on the driver emitters.
3. Better optimization of the bias current ratios.
4. A well-defined, resistor-programmable output bias current.
5. Some sort of damping between driver and output stages - the exact circuit is currently under evaluation.
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Old 5th February 2015, 12:27 AM   #319
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A word or two about bias currents.

The diamond buffer is a sequence of two emitter followers, the driver and the output. Both have minimum h_FE current gains of 100. (BD135/6)

The output drives headphones, the "design maximum" output current into the headphone load is about 50 mA. So the output base needs 0.5 mA, provided by the driver. The driver runs in class A, so the standing bias has to be slightly in excess of this value, say 1 mA minimum, to avoid clipping. 2-3 mA is a reasonable headroom. In my rev. 2 version, it was 9-10 mA.

In terms of impedances, 30 ohm headphones translates into an output driver input impedance of (30xh_FE) 3 kohms. The driver stage emitter resistor (R7,R8) was 1 k in rev.2, a substantive load, for a total of 750 ohms, translating to an input impedance of the buffer of (750xh_FE /2) 37.5 kohms.

It's ok, but the circuit is working unnecessarily hard to put a large current through the driver transistors. This lowers PSRR and increases distortion, as well as lowering the input impedance. The solution is to replace the 1k emitter resistors with a) a larger value or b) a current source.

A current source would allow a large current AND a large impedance, at the expense of noise and complex distortions. A larger resistor means lower bias currents, unless the driver stage power rails are increased of course. I haven't fully committed to one or the other solution yet.

As the input impedance of the output transistors is not especially high (particularly with low impedance headphones) its probably not a good idea to put large resistors in from of the base. A maximum of about 220 ohms seems reasonable. The small "boost" resistors on the driver emitters (R9,R10) need to be included in this total. They are 4.7 ohms in rev. 2, they could be made significantly larger with no real detriment to the performance.

In the schematic below I've made some tentative changes, adding the switchable gain feature, reducing the driver bias currents, and adding small resistors to the base of the output transistors. I haven't replaced the driver stage emitter resistors with current sources yet.
Attached Images
File Type: png sapphire 30a 00.png (19.9 KB, 138 views)
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Last edited by rjm; 5th February 2015 at 12:30 AM.
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Old 5th February 2015, 11:33 AM   #320
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Richard Murdey
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The "nightly", LTSpice sim attached. This looks pretty good, the problem is LTSpice transistors are not likely to be the ones I'll actually use.
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File Type: png sapphire buffer 3 02.png (71.6 KB, 115 views)
Attached Files
File Type: zip Sapphire3 buffer trial 02.zip (1.3 KB, 10 views)
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