Some members may recall that a few years ago I published a low distortion class A headphone amplifier design on my hifisonix.com website. Unfortunately, I never built the design or provided any PCB layout or BOM.
Hopefully this thread corrects all of that. Here is that design (with a few minor tweaks).
The X-Altra HPA-1 headphone amp :-
Full construction details, BOM and DSTHP PCB's available here :-
X-Altra HPA-1 Class A Headphone Amplifier
You can get a PCB for the companion Hifisonix Standard Linear PSU here Hifisonix Standard PSU
Hopefully this thread corrects all of that. Here is that design (with a few minor tweaks).
The X-Altra HPA-1 headphone amp :-
- Operates in class A up to 13V pk-pk into 32 Ohms
- OPS standing current of 90mA
- Simple, foolproof design
- Transitions to class AB after 13 V pk-pk up to 20 V pk-pk
- Features ultra low distortion (full measurements provided)
- Compact PCB measuring 87.5 x 72.5 mm (stereo PCB)
- Can drive heavy capacitive loads
Full construction details, BOM and DSTHP PCB's available here :-
X-Altra HPA-1 Class A Headphone Amplifier
You can get a PCB for the companion Hifisonix Standard Linear PSU here Hifisonix Standard PSU
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I am warming up Paypal so I'm ready when the boards are available. I've briefly scanned the details and looks like a great build, including alternative op-amps. I was curious about the TI Burr-Brown chips in this circuit and great to see there is a section about op amps that could work.
Ive updated the HPA-1 file with additional measurements (using an NE5532).
PCB's for the HPA-1 are now available on hifisonix website
X-Altra HPA-1 Class A Headphone Amplifier
PCB's for the HPA-1 are now available on hifisonix website
X-Altra HPA-1 Class A Headphone Amplifier
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Nice design! Would have putting the VBE multiplier in thermal contact with the outputs have made for better thermal tracking? Say, a BD139 clamped onto the same heatsink as one of the OP’s. Is there any chance of thermal runaway if cooling airflow was not properly maintained?
Congrats on the super measurements.
Congrats on the super measurements.
Thanks - appreciate it.
Good question re the bias stability. . The bias controller is active in that it directly measures the voltage across the OPS degeneration resistors. This is a bit different than a standard class AB spreader where the primary feedback mechanism for control is via temperature. Due to the arrangement on the HPA-1, the controller and output devices Vbe’s and thermal gradient largely cancel (not quite because of the scaling, but to about 85-90%). I get about 5mA change on 90 mA standing current worst case from cold to warm.
Good question re the bias stability. . The bias controller is active in that it directly measures the voltage across the OPS degeneration resistors. This is a bit different than a standard class AB spreader where the primary feedback mechanism for control is via temperature. Due to the arrangement on the HPA-1, the controller and output devices Vbe’s and thermal gradient largely cancel (not quite because of the scaling, but to about 85-90%). I get about 5mA change on 90 mA standing current worst case from cold to warm.
Bonsai, would 2SC1571 & 2SA1930 be ok for Q5 & Q4? Same pinout
https://www.mouser.com/datasheet/2/408/7654-58852.pdf
https://www.mouser.com/datasheet/2/408/7654-58852.pdf
Those look very good. I’ve not used them, but they should work as is in the HPA-1.
🙂
I will order in some OPA1642’s and do a set of measurements (I’m out of these devices devices now - all used on another project) next week. We should then have the LM4562, NE5532 and the OPA1642 covered as options for builders. There are a few ADI devices I need to try as well after that.
Nice design Bonsai! But, where's the fun in designing amp with focus on low distortion with opamps?
I have a AD8599ARZ, they look good as do AD8620 but pricey:
https://www.mouser.co.uk/datasheet/2/609/AD8599-EP-1517534.pdf
https://www.mouser.co.uk/datasheet/2/609/AD8610_8620-1502217.pdf
I've updated the X-Altra HPA-1 BOM - one of the builders picked up an error - thank you for that.
Those buffer circuits have been around forever - I used a similar scheme in a pre in 1980. Unfortunately they aren’t class A, or directly regulated class A and distortion even with good opamps is rarely ultra low.
But, I think they have their place. This is the X-Altra thread so we should stick to discussing it.
🙂
But, I think they have their place. This is the X-Altra thread so we should stick to discussing it.
🙂
I have parts on order and board on the way, thanks Andrew! I do have a question regarding construction. Another knowledge gap I need to fill. If I have an external heatsink, is there a penalty for mounting the output transistors to the heatsink and using flying leads to connect to the board?
Javier, that should work - just twist the wires and keep them as short as possible. Let’s see how it goes - if you run into problems, we can work through them.
I’ve got some OPA1642’s coming tomorrow. I’ll build up another HPA-1 unit and do the measurements. Currently, this is my favorite general purpose opamp.