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Benchmark HPA4 Headphone / Line Amplifier Impressions
Jun 5, 2018 at 3:10 AM Post #16 of 1,410
Tobes said:
Love the Benchmark AHB2, it's a neutral window to the music - it just sounds right to me. It's also compact, cool running and sounds great from switch on - which makes it extremely practical.
The competence of the AHB2 has given me the confidence to buy the HPA4 and I've also ordered a DAC3 L to complete the chain.
To be honest I'm sceptical whether the HPA4 will show significant improvement over the BP26, which itself has distortion levels an order of magnitude below typical hifi gear. The HPA4 boasts significantly higher S/N ratio, though whether this will translate to any practical advantage remains to be heard. At least I'll reduce my box count.
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Thanks for the update regarding the AHB2. It certainly is an amp that I'll keep on my radar.

Concerning the HPA4: I hope to improve channel tracking at low volumes. With my present pre-amp, I get the feeling that at low volumes, there is a bit of a channel imbalance because it uses a conventional potentiometer (although a high-quality Alps pot). The HPA4 seems to have an elaborate system of volume control. I like their approach. In addition, I am hearing a slight hum (even though not audible at listening distance) that I hope to remove with the HPA4.
 
Jun 5, 2018 at 3:37 AM Post #17 of 1,410
Yeah the 256 x 0.5dB step attenuator is a major attraction as it promises perfect tracking over a very wide range. Should have great adjustability with any amp/speaker or headphone setup.

Another feature of the AHB2 amp is the adjustable gain - so you can get preamp volume controls operating around the 11:00 to 2:00 o'clock position. I run the AHB2 in it's lowest gain position with my BP26 - this setting is also recommended for the HPA4.
 
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Jun 5, 2018 at 8:19 AM Post #18 of 1,410
Interesting idea: running the amp at the lowest gain would maybe allow me to increase the volume setting on my pre-amp for better channel tracking. Perhaps I don't need to buy a new pre-amp? I need to think hard about the AHB2! Interesting!
 
Jun 5, 2018 at 10:39 AM Post #19 of 1,410
Worth thinking about if you're otherwise happy with your setup.
The Odyssey Khartago has a highish gain of 30dB - so the AHB2's gain options of 23, 17 and 9.2dB could offer some useful alternatives.
I used to own a Khartago with most of the upgrades and it's a nice amp, but the high gain can be problematic particularly with high output digital sources.
I believe John Siau of Benchmark is not a fan of high gain power amps because you typically end up heavily attenuating the signal and throwing away S/N ratio.
 
Jun 5, 2018 at 1:51 PM Post #20 of 1,410
Tobes said:
Worth thinking about if you're otherwise happy with your setup.
The Odyssey Khartago has a highish gain of 30dB - so the AHB2's gain options of 23, 17 and 9.2dB could offer some useful alternatives.
I used to own a Khartago with most of the upgrades and it's a nice amp, but the high gain can be problematic particularly with high output digital sources.
I believe John Siau of Benchmark is not a fan of high gain power amps because you typically end up heavily attenuating the signal and throwing away S/N ratio.
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Interesting!!! On my GS-X Mk2 preamp, I am at low gain (I presume it‘s unity gain), and I need to turn down the volume such that it is somewhere between 08:00 and 09:00 for convenient listening, but I must say I prefer to listen at low levels anyway. There are other problems as well: I am hearing a low-level hum when I am close at my speakers, at normal listening distance I don‘t hear the hum. Could perhaps be caused by excessive gain? Not long ago, I had an ATC SPA2 power amp over from my local dealer and the hum was nearly gone, almost inaudible. This points to the Khartago being the culprit, but I suspect that the pre-amp contributes also. Digital output of my DAC is in balanced mode 4.2 V if I remember correctly. So, very good suggestion of looking into the AHB2! Thanks so much for your ideas!
 
Jun 5, 2018 at 11:20 PM Post #21 of 1,410
biosailor said:
I am hearing a low-level hum when I am close at my speakers, at normal listening distance I don‘t hear the hum. Could perhaps be caused by excessive gain?
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Not likely the cause, mains hum is usually a grounding issue. With grounded equipment, powering off the same ac point can often be helpful in reducing hum problems. Using balanced cabling/equipment will often ovoid the issue - there's a reason it's used in pro setups.
Everything in my audio setup is powered from a dedicated ac line. When I introduced a subwoofer which was located in the opposite room corner to the other equipment, I tried powering it from an ac socket off a nearby seperate circuit - introducing some low level hum. Running an ac power cord back to the ac line powering the other equipment eliminated the hum.
Running balanced from the the BP26, the AHB2 is dead quiet in my setup.

I can't really say whether or not the AHB2 would be a better move than the HPA4 for your setup but, as well as the gain options, it would allow you to run a full balanced setup.
As a previous owner of the Khartago amp, I think the AHB2 provides a solid improvement in low noise floor, neutrality and reproduction of fine detail. The more I listened to the AHB2 the more I appreciated its qualities.
 
Jun 6, 2018 at 1:59 AM Post #22 of 1,410
My apologies if I drove the discussion off-topic! Thanks for all the good advice given here!
 
Jun 6, 2018 at 2:10 AM Post #23 of 1,410
No problem from my perspective, but some people can get antsy about drifting OT, so didn't want to annoy these folks with my reply.
BTW you may find it interesting to read John Siau's perspective on the balanced vs unbalanced argument.
 
Jun 6, 2018 at 4:12 AM Post #24 of 1,410
Tobes said:
No problem from my perspective, but some people can get antsy about drifting OT, so didn't want to annoy these folks with my reply.
BTW you may find it interesting to read John Siau's perspective on the balanced vs unbalanced argument.
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Will do that, thanks for the link!
 
Jun 9, 2018 at 2:36 AM Post #25 of 1,410
Outputs for both single ended and balanced headphones

Is this like the GSX in that it requires as input a balanced signal from the DAC (otherwise only 1/2 the amp used) - or can it take unbalanced in via its RCA'S and it converts to balanced internally?
 
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Jun 9, 2018 at 1:07 PM Post #26 of 1,410
buzzlulu said:
Outputs for both single ended and balanced headphones

Is this like the GSX in that it requires as input a balanced signal from the DAC (otherwise only 1/2 the amp used) - or can it take unbalanced in via its RCA'S and it converts to balanced internally?
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From the Benchmark application notes for the HPA4:
"The 16-step gain boost ladder sits inside the input buffer's feedback loop. A second buffer drives the 256-step attenuator input, while a third buffer drives the output. Each buffer is actually a balanced pair of buffers and the entire signal path is fully balanced. The second buffer also serves as a precision fully-balanced differential amplifier. The differential amplifier delivers a voltage-balanced signal to the attenuator stage. The differential amplifier removes common-mode interference while maximizing the use of the voltage headroom in the 256-step attenuator."

An unbalanced signal to the diff amp stage should provide a balanced signal to the attenuator.
 
Jun 9, 2018 at 3:48 PM Post #27 of 1,410
Thanks for explaining the technical jargon.
This is what has always held me back from purchasing the GSX - the fact that it needs to be feed a balanced signal in order to take advantage of what it is fully capable of.
I ended up with the Moon 430HA as it can be feed with an unbalanced signal and it will internally convert it to balanced. It might be a bit slow and veiled for my taste.

The new Benchmark sounds interesting when I hear comments such as "fast and speedy"
 
Jun 20, 2018 at 1:23 AM Post #29 of 1,410
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