Icon Audio HP8 Mk2 Tube Headphone Amplifier
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.
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.
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.