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Simple High Performing Headphone Amp
Simple High Performing Headphone Amp
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Old 7th May 2018, 09:46 PM   #1
TimS is offline TimS  New Zealand
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Default Simple High Performing Headphone Amp

Hi All

I just wanted to share this unit gain headphone amp circuit I designed and built it on Sunday.

So far it has exemplary performance and specs considering its simplicity, the built version exceeded my expectations:
Even thought I show a cap in the circuit, if the JFET's are matched the circuit is DC coupled input to output. I built 2 channels and 1 channel has 4mV DC offset andthe other has an 3.4mV DC output offset
The frequency responce is crazy I measured the responce to be almost perfectly flat with 0dB from 10Hz to 1MHz into a 33R resistor.

The specific JFETs isn't critical as long as IDSS is greater than 3mA and they are matched. I simulated with 2sk170GR, 2sk246GR/BL but I built it with 2sk117Y's (the model of the 2SK117 wasn't good).
The output bias current it set by 0.65/R75+R76, in the built version I went with R75 and R76 5R and R50 10R so it didn't run too hot.

I put it in a box last night and listening to Leonard Cohen with it now.

Have to give credit to Nelson Pass for his constant current stage and Erno Borbely for inspiration.

Cheers

Tim
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Old 7th May 2018, 09:56 PM   #2
Mark Johnson is online now Mark Johnson  United States
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Simple High Performing Headphone Amp
The positive power supply rail is named "VEE" and the negative power supply rail is named "VCC" ??

Most folks do it the other way.

edit- also you might want to call out a specific part number for Q1. Perhaps the MPS8599?
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Old 7th May 2018, 10:12 PM   #3
TimS is offline TimS  New Zealand
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Quote:
Originally Posted by Mark Johnson View Post
The positive power supply rail is named "VEE" and the negative power supply rail is named "VCC" ??

Most folks do it the other way.

edit- also you might want to call out a specific part number for Q1. Perhaps the MPS8599?
I did indeed mix up my supply notations, sorry about that.
The VEE supply I went with +12V and VCC -12V.

For Q1 I used a BC557 but I am pretty sure most jelly bean small signal PNP bjt's will do.

Cheers

Tim
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Old 7th May 2018, 10:16 PM   #4
TimS is offline TimS  New Zealand
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Also before anybody asks the 'BIAS' label doesn't do anything.
I had an integrator set up on another page to remove output offsets, but it wasn't needed for this circuit.
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Old 12th May 2018, 12:52 AM   #5
TimS is offline TimS  New Zealand
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Default Measurements

After listening to the amp for a week I thought I should make some distortion and square wave measurements.

First the distortion measurements.
The equipment used is an m-audio FireWire 410 on its rear line terminals. Levels calibrated against my DER EE DE-5203 true rms dmm.
Software used is the Visual Analyzer v.2014.
All measurements were done at 1kHz

Unloaded at 0.969vrms 0.0034% THD
25R load 0.960vrms 0.0034%
10R load the signal clipped about 2% THD

Reset the input level to Limit the current to 120mA into 10R so 1.2Vp or 0.846Vrms.
Unloaded 0.846Vrms 0.0034% THD
10R load 0.827 Vrms 0.021% THD

0.115% THD @ 0.87Vrms 10R

The harmonic content was predominantly 2nd.
I then measured the sound cards own output and it measured 0.0034%.

Now the square wave measurements.
Equipment used is a kenwood ag-203 source and an Iwatsu 100MHz analog scope.
First image is at 1kHz 0.5v/dev
Second is at 10kHz
Third at 100kHz

After taking the photos I then check the output of the ag-203 and they were exactly the same.
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File Type: jpg 92DCDE20-A262-4A1C-A195-AE48698C44A9.jpg (810.9 KB, 1978 views)
File Type: jpg CD89A095-D150-4CCE-BE59-F56757ABC80E.jpg (688.0 KB, 1593 views)
File Type: jpg 2B197CFE-DE14-4395-8E94-0CA387467900.jpg (726.5 KB, 1488 views)
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Old 12th May 2018, 12:53 AM   #6
TimS is offline TimS  New Zealand
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I apologise for the rotated images.
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Old 12th May 2018, 04:51 AM   #7
xrk971 is online now xrk971  United States
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Simple High Performing Headphone Amp
Neat design, TimS.

Do you have photos of the amp to share?

Although 5 actives is not exactly simple, I also have a 5 actives headphone amp based on Aksa Lender preamp and it doesn't uses any 2Sk170 JFETs. Not DC coupled output, but then it has advantage of single rail. Still 5 actives and very similar low THD figures and dominant H2 and lower H3, and has gain.
Click the image to open in full size.

More info here
Aksa Lender HPA

The amp core looks like this:
Click the image to open in full size.

Which plugs into this for supporting PSU and input/output caps:
Click the image to open in full size.
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Last edited by xrk971; 12th May 2018 at 04:54 AM.
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Old 12th May 2018, 10:12 PM   #8
TimS is offline TimS  New Zealand
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Hi xrk971

The whole point why I shared this design is because it is DC coupled, as well as low distortion and wide bandwidth, with only 5 parts! Usually with simple designs I would add an integrator servo to remove output offsets (I really dislike output electrolytics).

Sure I used unobtainium (the 2sk170/2sk117's) in my design and not everybody has them in their part box but the design sims ok with BF265B, which are still currently sold at element14, just buy a bunch and match them.

When I designed this circuit I didn't need gain (I have just got a Dragonfly Black for work listening and I wanted to see if it sounded better with an external amp) but it doesn't mean the circuit can't have gain while being DC coupled (second image). R81 could be a 1k pot to zero the last millivolts since this will have a larger DC offset like about 30mV.

I am shy about sharing actual pictures of the circuit since it was both a prototype that became the final design built in ugly style and I have hidden it in a nice box. It is nowhere as nice as the Aksa Lender PCB's.

Cheers

Tim
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File Type: png Headphone amp BF256B.png (9.4 KB, 420 views)
File Type: png Headphone amp with gain.png (10.6 KB, 496 views)
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Old 12th May 2018, 11:02 PM   #9
Mark Johnson is online now Mark Johnson  United States
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Still missing a part# for PNP transistor "Q1".
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Old 12th May 2018, 11:06 PM   #10
Bigun is offline Bigun  Canada
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Simple High Performing Headphone Amp
Really like that topology Tim, very effective
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