X-Altra HPA-1 Class A Headphone Amplifier

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#261
Thank you for your suggestions, c214/215 are right there at the supply pins of opamp, it might be difficult to see from picture.

All transistors will be bolted down and ground line will be moved as you suggest.

Will report back when I built it.
 
#262
In action, all I can say is this amp is really great. I prefer this over many diy and commercial hp amps. Thank Bonsai for this great work.

I am using toshiba 1837/4793 and bc550. PSU is normal 317/337 at +/-16v. Bias measures from 1e resistor is 90mA and rising slowly. Not sure when it stop. Heatsink underneath is really hot, I need to change that.
20220825_163419.jpg
 
#263
Glad you like it! The bias current stability is not critical as long as it’s within +-10%. However, if it’s running too hot, that isn’t good - you will need to increase the size. The heatsinks I use are 4C/W but you can go to 8C/W - that will give you about 22C above ambient temperature rise.
 
#267
Bill, just seen your post. I have not experienced any problems with my headphones (Audio Technica, Koss, Bose) and the output coil does a good job of isolating any capacitive load so you should not experience any problems. I would recommend all headphone amps use an output coil + Zobel
 
#270
I've just built the HPA-1 and added it into my pre-existing preamp. On trying it out, my musician son's words were "Wow!!"

My 74 year-old eyes, aided by my inspection headset, had no problems hand soldering the SMD components. After hand soldering 0805 and smaller components on other projects, the 1206 sized SMD parts seem huge.
 
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#274
In preparation for the forthcoming series of "Strictly Come Dancing", I've double checked my HPA-1 is still working. Yes, it's ready to go 👍👍👍and provide much needed aural entertainment :hphones:while SWMBO hogs the TV for the Saturday show and all its daily spin-offs
 
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#276
wirewiggler said:
We’re some headphones reacteding badly, or are you recommending it to all amp for all headphones. I am using Hifiman Ananda with planer drivers at 26 ohms.
Click to expand...
Apologies for this VERY late response.

On the first iteration of the HPA-1 I used a low value resistor (3.3 ohms IIRC) to isolate the output from any capacitive load. But, large planar low impedance load headphones don’t like that and affects the bass response I assume because you’re have to move a lot of air at LF. In other words, for a good headphone amp you really do want a low Z out. So the 3.3 ohms series resistor was thrown out and a 1-2 uH coil in parallel with 2.2 ohms used instead. The Z out is now about 2 orders of magnitude or more lower and the bass response perfect.

This BTW is not something you would see with a measurement using a resistive load - I came out of listening tests. I realised from this that the interaction between amp, headphone diaphragm and ears is pretty complex. YMMV

😊
 
#277
Bonsai said:
Sorry for the late reply. There shouldn’t be any reason why you couldn’t use an SMPS. 👍
Click to expand...
I am currently using a 10 watt smps module for either rail. The power supply itself works but I noticed one of the output transistors is getting extremely hot, and i beleive because of that the psu's overload is shutting it down as it will constantly turn on and off almost like it's oscillating. Any reason why it could be happening, as I didn't use essentially the same design with my older amp which is also the x-altra but it was powered with a torrodial transformer and hence was massive.
Regards
 
#278
Hi izerotwo,

The only thing I can see here is that either the supply rails are too high leading to excess dissipation in the HPA-1 output stage, and/or there is a startup issue with the SMPS. There is a hefty capacitive load (1000uF with 1 Ohms series resistor) that the SMPS must startup into and that could be causing your problem. In principle, there should be no issue powering the HPA-1 off an SMPS.

Check the following
1. what is the no-load output voltage of the SMPS? should be an absolute max of +-18V and a minimum of +-12V
2. what is the current draw of the HPA-1? With both channels on and no load it should be about 180mA
3. Check that the output offset of both channels is not more than +-4mV. If you have a large offset, you will have problems that require addressing
4. if that is all ok, I would temporarily remove the 1000uF capacitors and then try to start it with the SMPS. If it starts ok and the rails hold up, then it seems to point to an SMPS startup issue

Let me know how it goes.

rgds

Andrew
 
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#279
Thank you for the reply, I was also wondering if the large initial load is to blame. I will surely check it out, the total combines capacitance is around 6000uf each rail. I did test the power supply section prior to connecting the amplifier. The smps module produced 24v stable and after the active low pass, it was around 21v and after the lm7818 ldo it was a stable 18v this was with the combined capacitance load of 4000uf in the unconnected psu.

For the rest I shall check and get back to you.
Again thanks for the reply!
 
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