Icon Audio HP8 Mk2 Tube Headphone Amplifier

Tyll Hertsens's picture

Icon Audio HP8 MKII Headphone Amplifier ($999)
I find it interesting that I had never heard of Icon Audio before. They had been around for well over a decade and had earned considerable accolades from various British Hi-Fi mags. Yet somehow the brand flew completely under my radar until roughly 6 months ago when I stumbled across their website. I actually had my eye on their Stereo 20 PP which is a small integrated amp with headphone out capability, but then I saw the HP8 MKII and all bets were off.

Interacting with head engineer David Shaw, I get a distinct "classic HiFi" vibe. David mentions how the team at Icon loves the sound of vintage gear like McIntosh, Leak, Dynaco, and Quad. He feels that despite modern advances in audio reproduction, the shortest path to really good sound involves relatively simple tube-based designs with quality parts. This idea certainly isn't unique to Icon but it's the execution that matters and each company has their own version of what that should look like.

With Icon, it follows a pretty straight forward design theme: point to point wiring rather than printed circuit boards. Easily available tubes. No opamps anywhere in the design. Everything should be reliable but also repairable if needed - a two year warranty helps reinforce that notion. Again, none of this is completely unique to Icon but it does place them in a good company with the likes of Woo Audio and Decware in general terms and particularly with VTL in some more specific areas.

Design
The Icon Audio HP8 MKII captures all that talk of "classic tube design" and distills it into a mid-sized chassis measuring 14 inches deep, 4.3 inches wide, and about 6 inches tall at the highest point. It's a relatively simple unit - a single RCA input, power switch, volume knob, and impedance selection switch allowing the user to choose output impedance. Build quality is very solid - the unit weighs about 15 pounds which is rather unexpected for a device of this size, especially if one is accustomed to dealing with solid state amps which tend to weigh a lot less. Little touches like the Teflon tube sockets quietly assure that Icon isn't cutting corners. I find the aesthetic quite appealing in a laid back, no-nonsense sort of way, though I wouldn't mind seeing Icon switch to a different power indicator - the current LED is blindingly blue.

This lack of nonsense continues to the internal design with a 12AX7 used for the gain stage and the venerable 6SN7 double triode driver tube applied in a single-ended parallel configuration. The 6SN7 is working in the middle of its characteristics in pure Class-A giving exactly the power needed - this particular tube was deemed superior to the popular EL84 pentode which proved too powerful for the job and charged a penalty in terms of noise. A strong power supply with plenty of filtering - no wall warts here - comprises a good portion of the total weight. This is a transformer coupled design using custom hand wound output transformers which Mr. Shaw claims are key to the resulting sound quality. Apparently Icon had numerous prototypes sent to "recycling" before arriving at their current method of layering the windings, resulting in the desired sound balance and detail levels. Mr. Shaw referred to it as a "black art" but the science behind it shows how variations in output transformer design and construction have measurable significance - though any talk of one method sounding better than another brings us back to the realm of subjectivity.

These multi-tap output transformers allow some flexibility in terms of output impedance. The user can select low, medium, or high - which translates to 32, 300, or 600 ohm output impedance - on-the-fly via a front panel knob. The output impedance is very broad within each band due to feedback within the output transformer and a small amount of global feedback. Because of this, Icon recommends the low setting for loads as low as 8 ohms and as high as 300 ohms, and so on with the medium and high settings. In my listening I found that I overwhelmingly preferred the low setting with a few exceptions. I have heard forum chatter about people enjoying the Sennheiser HD800 with higher output impedance so those folks will appreciate having options. In my case I did like the high setting with the beyerdynamic T1 which is listed as having a 600 ohm impedance but in reality climbs as high as 1400 ohms.

Somewhat conspicuous from a "classic tube-design" standpoint - the HP8 uses solid-state rectification. I discussed this in depth with David Shaw, who clarified how he is actually a major proponent of valve rectifiers. Many of his designs use valve rectification including his CDX1 player (how many CD players use valve rectifiers?) but in the HP8 it was determined to be untenable for several reasons. First, it would require increased size in both chassis and power transformer - aside from the physical space needed for the extra tube socket, the high current AC heater would generate noise which would require either a separated partition in the case or an outboard power supply. Further, it would ideally use a choke filter for optimum performance, which of course leads to extra space and additional cost. For the intended size and price of the HP8, solid state was deemed the better way to go, and I can't say that I disagree.

Power
David Shaw was kind enough to humor me and measure the HP8 at the various loads I feel are reflective of real headphones. The highlights:

Low setting - 451mW at 32 ohm, 720mW at 50 ohm, 667mW at 150 ohm
Mid setting - 735mW and 10.5Vrms at 150 ohm, 653mW and 14Vrms at 300 ohm
High setting - 403mW and 11Vrms at 300 ohm, 482mW and 17Vrms at 600 ohm

As you can see, the HP8 swings a good amount of voltage into higher impedance loads. It's also fairly potent (for a tube amp) in terms of current delivery in the critical 32 to 50 ohm range which is where all the power hungry HiFiMAN and Audeze planar magnetic headphones fall. Damping factor aside, the impedance selection switch allows the HP8 to work in the optimal range for a given headphone, be it low impedance or high.

Company Info
Icon Audio (UK) Ltd
351 Aylestone Road
Leicester
LE2 8TA
0116 2440593
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Comments
donunus's picture
 This looks like my hd800

surprise This looks like my hd800 plan is nearer to reality.Unless were talking they are only as good as the woo 6 or something in that league. 

lithium's picture
comparison to woo

Hi  John,

Thanks for the great and succint review. I feel small companies like this can really benefit from marketing particularly by interacting with people on Head fi which can be a really cost effective way to raise sales. I feel more headphone centric companies often do this and this is often neglected by other general audio companies. 

Also, did you get a chance to compare it to the WA-6 SE and the WA-7? They are probably most popular in this particular design topology for headphones and also in the same price range. Thanks.

John Grandberg's picture
Woo

I've heard the WA-6 SE on several occasions. It's a nice amp overall but I believe I like the Icon HP8 better. Icon is more balanced sounding and less "tubey" if I'm allowed to use a generic term like that. It really depends on the system though, including the tube choices.

The WA-7 is very new so I haven't yet heard it. 

ultrabike's picture
Great revew John.  It seem

Great revew John.  It seems like a great headphone amp. I also found your link to the "Measuring Output - Transformer Performance" very useful and valuable. Thanks!

I feel the 32 ohm lowest output impedance for a tube amp seems pretty decent, but I find it odd to recommend (Icon Audio) even such a setting for 8 ohm loads. That aside, power numbers are phenomenal, and your impressions are very well presented and thoughtful. From your review, I could see this amp as a perfect mate for the likes of a DT990-600ohm (which I though sounded very good out of a Schiit Valhalla.)

It is also interesting to see some few similarities with the 120 ohm Bottlehead Crack OTL. While this one uses the 12AX7 for the gain stage, the Crack seems to use the 12AU7. From the little I've read it seems they are pin compatible but the 12AU7 has lower gain. Any idea as to how the Crack compares to this?

John Grandberg's picture
Thanks!

I really dig the Bottlehead Crack from the limited exposure I've had. Couldn't say how it stacks up to the Icon though, as I haven't had enough experience. I think the Crack I heard had the Speedball upgrade too, so I don't know how big of a difference that makes. Apples to oranges.

 

lithium's picture
Thanks John for your comparison

This has been a really enjoyable review....perhaps you should review Decware as well for inner fidelity. 

elmura's picture
Sounds good.

The impedance switch is a great way to suit a variety of phones.

donlin's picture
Thanks or the review!

Great review John.This amp was unknown to me until I picked one up from Music Direct about two months ago.  It's a great amp and drives my LCD-2's with more authority than anything else I've tried including the powerful Burson Soloist.  Icon had a great sounding speaker setup at Axpona in Chicago last weekend.  I spoke to David Shaw who seemed very surprised that I already owned one of their amps.  He was a really nice guy.

BrianR's picture
What Were You Using as a Source?

John,

I decided to buy an Icon HP8.  It arrived yesterday and I hooked it up to my Musical Fidelity M1 DAC being fed by a Windows laptop.  Big disappointment:  I get HUGE amounts of computer noise coming through the amp.  It's completely unlistenable.  As soon as I turn off the computer it's dead silent.

Do you think that this is unique to the DAC that I am using?  Did you use any DACs during your review?

The amp sounds great with a CD player as a source.  But, my intention was to use this amp as part of a desktop system with a computer/DAC as my source.

I have a Halide DAC HD on order and have my fingers crossed that it will not pass the noise that the the Musical Fidelity does.  If it does, sadly I will be returning the Icon because it will not work for me.

John Grandberg's picture
Hmmm...

Seems like that would be an issue with your DAC rather than the amp. The DAC shouldn't be passing along any type of noise. Is it audible with any other amps you may have tried?

I have noticed the HP8 as being rather sensitive to ground loop issues. I had some success using various power conditioner with noise filtering. That allowed me to use multiple DACs as well as a laptop or CD player as source, all without noise issues. 

Hopefully your new Halide DAC takes care of the issue. As a USB powered device maybe it helps the situation. Feel free to report back when you have it. 

timits's picture
Suitable for IEMs?

Hi John,

Thanks for an informative and engaging review.

I would value your opinion on whether the HP8 would be suitable for a pair of Final Audio Design Piano Forte XIII earphones?  

You also mentioned that the Audio Technica W1000X is not ideal because of the slight midbass hump. Would you say the same applies to the W3000ANV?

John Grandberg's picture
Sorry...

I have not had the pleasure of trying anything from Final Audio, nor have I gotten my hands on the W3KANV yet. I plan on trying to find the latter one of these days though, but probably not soon enough to help your decision. 

 

Sorry!