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Tom Christiansen Audio HPA-1 Headphone Amp Review

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Cool. My suggestion of using a 30-second sweep and slower pot rotation still stands.

Tom
Looks like we need Adriano stepper motor controller attached to a servo in a jig to let you do this in an automated way.

Tom are you using a Linear pot?

I a/b HPA-1 vs Monoprice THX 887 today Single-Ended, it no contest HPA-1 is much nice AMP working a broad range of headphones Dan Clark ( Mr. Speakers) Aeon Flow Open, Hifiman Ananda (25 Ohms), Rosson Audio RAD-0 (29 Ohms), Audeze LCD-4 ( 200 Ohms). I had zero problems driving any of these headphones with HPA-1 without adding gain. THX no gain is level 1, I found I need Level 2 gain with THX with LCD-4 to volume match at a moderate level of volume.

With Rosson Audio RAD-0 I had 1/4 Single-Ended to HPA-1, and XLR out to Headphone from Monoprice THX 887 4 pin xlr jack, I still preferred the output of the HPA-1 Single-Ended. You have plenty of volumes, a very clean signal, and great dynamics.

Loving the AMP more and more
 

tomchr

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Looks like we need Adriano stepper motor controller attached to a servo in a jig to let you do this in an automated way.
Feel free to build one and ship it to me. :) The thought did occur to me. I'd probably implement some sort of slip clutch so the stepper motor doesn't rip the pot apart. Then again, there are many other things I'd rather spend time developing. A filter so I can better measure THD would be good. As would a headphone dummy load with interface to the APx525.

Tom are you using a Linear pot?
No. Logarithmic.

I a/b HPA-1 vs Monoprice THX 887 today Single-Ended, it no contest HPA-1 is much nice AMP working a broad range of headphones Dan Clark ( Mr. Speakers) Aeon Flow Open, Hifiman Ananda (25 Ohms), Rosson Audio RAD-0 (29 Ohms), Audeze LCD-4 ( 200 Ohms). I had zero problems driving any of these headphones with HPA-1 without adding gain. THX no gain is level 1, I found I need Level 2 gain with THX with LCD-4 to volume match at a moderate level of volume.

With Rosson Audio RAD-0 I had 1/4 Single-Ended to HPA-1, and XLR out to Headphone from Monoprice THX 887 4 pin xlr jack, I still preferred the output of the HPA-1 Single-Ended. You have plenty of volumes, a very clean signal, and great dynamics.

Loving the AMP more and more
I'm glad you like it. Thanks for sharing.

Tom
 
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Feel free to build one and ship it to me. :) The thought did occur to me. I'd probably implement some sort of slip clutch so the stepper motor doesn't rip the pot apart. Then again, there are many other things I'd rather spend time developing. A filter so I can better measure THD would be good. As would a headphone dummy load with interface to the APx525.


No. Logarithmic.

Gs> Thanks.

I'm glad you like it. Thanks for sharing.

Tom
Simpler solution might be tape a string to the knob and wrap a few loops around the knob then out to a winder it releases the last revolution

I am starting to develop a dummy load box. Moser dropped box load of parts. Maybe I look at microcontroller now to change values
 

trl

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Finally... The measurements for the HPA-1 are now up. You'll find them in the Performance Graphs on the HPA-1 product page: https://www.tomchr.com/products/hpa-1

I'll go through and proofread tomorrow. I'm sure I can tweak a few sentences for clarity. The measurements can speak for themselves, though. I may add a few measurements as time goes on, but what's there is pretty comprehensive.

Tom
What I like most is the transient response and stability with regular cans. This is something that usually most manufacturers don't show to their customers:

"The oscilloscope screen shot below shows the transient response of the HPA-1 when presented with a 10 kHz square wave and loaded by 300 Ω in parallel with 470 pF of capacitance. This load is representative of a pair of Sennheiser HD-650 with the stock cable and a 2-3 m (~10') extension cord.

HPA-1_R1p0_Transient_300R470p.jpg
"​

Of course, testing it with 10nF capacitance (equivalent to 100 meters headphones cable) really proves that this headamp is stable:

"Finally, when loaded with 10 nF, the amplifier finally shows slight overshoot and ringing. Note, however, that to obtain such capacitive load in actual use would require approximately 100 m (300') of extension cable on a typical pair of headphones. It is also noteworthy that the HPA-1 remains stable even with 100 nF capacity load. Needless to say, I have full confidence in the stability of the HPA-1.

HPA-1_R1p0_Transient_300R10n.png
"​
 

tomchr

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Yeah. Those measurements used to be in every hifi magazine review. These days, you're lucky if you get much beyond sappy verbiage.

Tom
 

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