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Current drive headphone amp in the works
Current drive headphone amp in the works
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Old 28th October 2021, 03:42 PM   #1
abstract is offline abstract  Poland
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Default Current drive headphone amp in the works

Headphone amp nearing completion...

Overview:
The prototype is optimised for 250 ohm Beyerdynamic DT880's, but should work well across 32-600 ohms, as long as the impedance is not too peaky. It is about as feedback-free as it gets, not counting the servo feedback for DC stability and source degeneration resistors.

Audio chain:
single JFET input
N MOSFET output
--current drive output (open drain, common source)
--high performance CCS with 70+ dB of PSU attenuation and low self-noise

RC Zobel network for Beyerdynamic DT880 headphones

More features:
DC stabilizing servo feedback.
Virtual ground to eliminate the need for output caps.

Shorting relay to protect against ON/OFF thumps.

Small ARM MCU that monitors voltages, flips relays, and shines LEDs.

Power stage:
5V (from ordinary phone-style wall outlets) boosted to 24V, then regulated down to 20V.
The actual voltage swing is fairly modest, allowing for about +/-2.5 to 3V before clipping @250ohm.

3.3V for the processor.

So far, I've managed to keep the number of polarised or polymer caps at ZERO, using only SMD ceramics in the periphery, and a couple of polypropylene caps. The PCB looks like an aerial shot of a medium sized town, but the audio basically passes straight through. It has 20-or so dB of gain, but the most important factor for me is the voltage-to-current conversion with minimal distortion.
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Last edited by abstract; 28th October 2021 at 03:47 PM.
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Old 30th October 2021, 03:55 PM   #2
abstract is offline abstract  Poland
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I've attached a 2d view and 3d view of the PCB.

Still have a few finishing touches to do, but it's getting there.

I'm a bit reluctant to post the schematic just yet it's something that I've put many many hours into.

Most of the resistors and capacitors are a comfortable 1206 size, and note the SOT-23's. I saw no real point in using larger packages in the output stage when the bias current only needs to be around 10mA.

The 24V output is regulated with an LM217 in a SOIC-8 package, and that is followed by the constant current source which provides further filtering, and the other SOIC-8's are op-amps for the DC servo.

I may add some THD simulations later, but it's a bit arbitrary. From memory: 0.005%THD @1kHz, @100mVout. Although an NE5532 simmed better than that, I specifically want current output so that there's less distortion induced from nonlinearities in the headphones themselves.

Total voltage noise <7uV up to 20kHz.
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File Type: png 2d-view 2021-10-30 16-20-52.png (351.9 KB, 324 views)
File Type: png 3d-view=2021-10-30 16-19-51.png (187.1 KB, 336 views)
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Old 30th October 2021, 04:20 PM   #3
needtubes is offline needtubes  United States
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Looks like a nice project. I've also been playing around with current drive circuits, but mostly for speakers. Have planned on looking into it for headphones for quite a while now, but just haven't found the time. Looking forward to your results and listening impressions!
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Old 2nd November 2021, 03:21 PM   #4
abstract is offline abstract  Poland
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Block diagram version of the schematic, showing the left channel.
Probably a bit 'cheesy' not showing all the details, but some people might like the challenge.

My top 2 'contenders' in TINA were this one and a push-pull version that looked very similar, except for a 2nd tap coming off the JFET output to drive a BSS84 MOSFET.

Note: the 2N7002 is not set in stone, but it should be a non-ESD protected version, since the ESD protection has quite high leakage and may not play well with a sensitive RC filter in series with an op-amp that sets DC bias.

The push-pull version had marginally better THD and voltage swing, but it lost out on the PSU noise. It was sensitive to noise on the top rail, whereas this one is not.

*Errata: the input JFET also has a pull-up resistor.
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File Type: png Left-channel-blockified.png (120.4 KB, 300 views)
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Last edited by abstract; 2nd November 2021 at 03:33 PM.
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Old 3rd November 2021, 02:07 PM   #5
DontHertzMe is offline DontHertzMe  South Africa
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Current drive headphone amp in the works
Looking good . I love that it has an obtainable JFET
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Old 3rd November 2021, 02:57 PM   #6
abraxalito is offline abraxalito  United Kingdom
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Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Interesting to see, thanks for sharing. BTW - using a MOSFET buffer in your virtual GND isn't a great idea if the aim is low noise. MOSFETs are notoriously noisy, 2N7002 is among the noisiest I've seen.
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Old 3rd November 2021, 03:04 PM   #7
Mark Johnson is offline Mark Johnson  United States
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Current drive headphone amp in the works
Could you explain why you've chosen R123 = Infinity ohms, please? Thanks!


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Old 3rd November 2021, 11:54 PM   #8
PRR is offline PRR  United States
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Current drive headphone amp in the works
0% to 80% seems like a wide range for a "halfway" voltage. 60%-70% ought to cover all the outliers and leave less scope for user mess-up.

Also looks like "low Z" only on pull-up, high Z on pull-down. Double-high Z pulldown without your R123.
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Old 4th November 2021, 10:45 PM   #9
abstract is offline abstract  Poland
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Quote:
Originally Posted by Mark Johnson View Post
Could you explain why you've chosen R123 = Infinity ohms, please? Thanks!
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The BC850 should be sinking quite a high base current because of the 1k emitter load, so the fet's source is already pulled down.

The standard I wanted to beat was a 470~1000u series output cap, with its unavoidable DC voltage due to leakage.
An earlier version had an LM217 to generate 10V, but it still needed 1000u + 47 ohm on the output to reduce noise, so it wasn't ideal.
This is taking it a step further, using a ceramic cap and giving it a unity gain buffer. It can certainly be improved further, but this what I have so far.
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Old 4th November 2021, 11:10 PM   #10
Mark Johnson is offline Mark Johnson  United States
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Current drive headphone amp in the works
Great, thanks! I hope I've got the math right: BC850's emitter is at 10V and emitter load resistor is 1K so the emitter current is 10mA. Base current is emitter current divided by Beta, so max base current occurs when Beta is minimum: (10mA / 420), namely Ibase is less than or equal to 24 microamps. If that tiny current has to discharge the 2N7002's Cgs capacitance after a transient event on the 10V Vmiddle, it might take quite a long time.



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