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xrk971 7th June 2018 05:29 AM

Simple High Performance DC Coupled Class A HPA with sub PPM THD
3 Attachment(s)
Edit April 5, 2019: Beautiful example of this amp in finished case by Avtech23 on post 425.


This is a simple DC-coupled high performance SE Class A headphone amplifier design by member TimS who has approved this GB. The design uses four cascoded BF862 JFETs on the input stage. They provide compensation for DC drift and the amp has stable DC offset of less than 5mV. The design has a dynamic CCS, reminiscent of an Aleph to provide good efficiency and low distortion from two P-channel MOSFETs in the output stage. Simulations show very low THD in the sub 1 part per million (PPM) range for 5vpp into 55ohms for a 400mA bias current. At this bias, the heat output is about 14w per channel and allows it to be used as fleawatt speaker amp and able to drive 8ohm speakers to 1.5w with 0.004% THD. Increasing the main source/drain resistors to 2.7ohms sets the bias current at about 115mA and this has very good performance. I built a hand etched prototype and measurements and listening shows a very powerful, dynamic, and clean sounding amplifier. The GB boards will be high quality 1.6mm thick with 1oz copper and ENIG gold finish with blue solder mask.

Here is LTSpice schematic and DC setpoints for simulation:

Predicted FFT for 5vpp into 55ohms with 400mA bias:
Here is hand etched prototype:

Measured FFT for 115mA bias 1vpp into 42ohms - note that elevated H2 distortion peak is probably a component choice of the lwo cost prototype (cheap steel leads on resistor, etc). I am quite sure that a proper amp using a real PCB and all high quality resistors and MOSFETs will be superior:

Measured 1kHz sine wave at 17.6vpp for 115mA bias and +/-15v rails for almost 1wrms into 42ohm load

10kHz 2.5vpp square wave - nearly perfect performance with no overshoot, or ringing:

This is the schematic for production PCB:

Here is the layout by JPS64:

Here is 3d render of PCB with components (bottom side):

3d render of PCB with components (top side):

How does it sound? Very dynamic, transparent, open with deep deep bass. It is DC coupled in and out so there is no capacitor to color the sound. The distortion is very low and you have to be careful not to play your headphones too loud if you are used to judging loudness by distortion. It's a great little amp that is easy to build with relatively low cost parts, yet is very high performing. Compare the predicted distortion levels with amps using 5x as many transistors. The nice feature about this amp is the rather high output power it is capable of and a rather high gain close to 20dB that allows one to drive an inefficient set of planar magnetic headphones to loud+ levels with very low distortion.

If there is interest, please add your name to GB interest list. Cost will be about $28/set and shipping will be $5 in US, $10 in Canada, $15 everywhere else.

Update July 3, 2018;

The FFT of the wall mains powered 12v to the on-board ultra low noise DC-DC dual rail PSU has noise characteristics resembling a battery. Note total absence of 60Hz/120Hz/180Hz/240Hz mains peaks and how low the "grass" of the noise floor is:

Here is the final design for production, and it is an 8 panel board that contains:
  • input and source switch board (only preamplifier version)
  • crossfeed board (with defeat Xfeed ON/OFF switch)
  • headphone jack output board (6.35mm stere TRS output jack)
  • volume potentiometer board (with the possibility of three different types of pots)
  • PSU (includes CLC, DC-DC dual rail step up, cap Mx, CRCRC filter to produce battery-like rails from simple 12v wall-wart)
  • CLC filter board (in case you want to use your own PSU)
  • ON/OFF switch board
  • amplifier board

The above boards can be mixed and matched and connected for either an HPA or a preamp following this diagram:

Schematics and other data here:
Simple High Performance DC Coupled Class A HPA with sub PPM THD - Page 10 - diyAudio

Top side:

Other side:

Mounted in a popular all CNC'd $36 Aliexpress case as a HPA, it looks like this:

Here is rear view of HPA:

As a preamp with your own machined panels, it looks like this:

Rear view of preamp:

Edit July 21, 2018: final schematics and stuffing guide here:

Simple High Performance DC Coupled Class A HPA with sub PPM THD - Page 10 - diyAudio

Update Aug. 4, 2018: Schematic Update! Please use this schematic for the PSU as the original has a few typos.

Update Aug. 5, 2018: Silkscreen on PSU PCB has error: Diode V241 has cathode and anode flipped.. Mount diode with Anode towards Drain of P channel MOSFET on cap Mx.

Update Aug. 27, 2018: verification build of GB boards completed. Amp sounds superb and measures very well. Here is FFT for 1vpp into 51ohms - very similar to earlier measurement but much lower distortion.

Update Sept 2, 2018: BOM from verification build can be found here.
Simple High Performance DC Coupled Class A HPA with sub PPM THD - Page 24 - diyAudio

DontHertzMe 8th June 2018 06:52 PM

My eyes might be deceiving me :magnify:, but I think I see a discrepancy between the sim and the schematic. In the sim, R7 is connected to vee, whereas in the schematic the same resistor (designated R112) is connected to gnd.

xrk971 8th June 2018 08:00 PM

Good catch man! That should indeed go to -Vee and not GND.

xrk971 10th June 2018 12:32 AM

3 Attachment(s)
I got the amp all boxed up. This one was actually easy to work with as IEC, power switch, RCA, 1/4in headphone jack out, volume known, and LED hole were all predrilled. I just had to mount the amp MOSFETs and PSU and trafo.

DontHertzMe 10th June 2018 09:38 AM

Looking good! :D

badboygolf16v 10th June 2018 01:01 PM

I'd be interested.

xrk971 10th June 2018 03:58 PM

Ok, good. Hope others take an interest as well. It’s a very powerful amp and hearing some detail in songs that I missed before probably due to extended resolution of not having caps. But really need to do an AB test with level matching to compare.

dabore84 10th June 2018 04:09 PM

Count me in as well, want to make a fleawatt and a hpa. Have a bunch of bf862 lying around.

xrk971 10th June 2018 05:16 PM

GB interest list for TimS Simple HPA:


Badboygolf16v - 1 pair
Dabore84 - 1 pair

funch 11th June 2018 04:16 AM

I'll take a pair also.

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