calling all ath-ad900 owners!!!!!

Not a lot of people know this but in some conditions headphone amps can go unstable: if you're unlucky enough to drop on the "wrong" impedance combined with the "wrong" cable capacitance and a headphone amp that runs out of a little-understood thing called "phase-margin", then you get another little-understood thing called "ringing", or even worse: "high frequency instability" otherwise known as oscillation.

With headphone impedances ranging from 16 Ohms to 600 Ohms (in the real world) and cable capactances ranging from 250pF to 1,000pF, it's very hard for any headphone amp to perform at its best, and with some combinations its phase-margin just gets too small.

The resulting instability causes brightness, or bass loss.

It is quite possible that the 40 Ohm ATH A900 was not a good match for the amps the reviewers used. I have never found the A900 bass shy. The A700 maybe, but not the A900.

As John and crew here will tell you, I spent a considerable amount of time testing the phase-margin on the SRG in all sorts of impedance combinations, using the SRG's new dual negative feedback network. Each manufacturer uses (hopefully) some form of stabilisation. However, it has been noted elsewhere that some amps favour one end of the impedance spectrum or the other, which basically means at one end they're stable, and the other end they are not.

Live-listening response...

Equipment: Rega Planar 3; Roksan Tabriz; Ortofon Salsa; Prototype Revelation MC using SM-PS; Cusat50 ic, Solo SRG using SM-PS; Audio Technica ATH A900.

Music: Edvard Grieg-Peer Gynt - Gothenburg Symphony Orchestra and Chorus - DG DDA commercial pressing.

Side 1 includes "In the Hall of the Mountain King" with a quiet kettle drum beating out the bars, building up - most of which loudspeakers find difficult to reproduce (eg: simply not heard). Skins resonate at around 5Hz. At least they are heard at the usual level using the A900, but not the resonance at 5Hz which the HD250 does. HD650 next... pretty much the same! The HD250 would be the way to go IMHO except that Sennheiser stopped doing them...

Next up, my ancient Grado SR60 as a low impedance comparison... (very quick to acclimatise to although a bit too "dark" for my taste). Slightly veils it.

My reference HD250 II: Skin resonance clearly defined without listening hard for it!

Now how many reviewers don't give the HD250 a second look?...

Edit: 5 minutes later replaced prototype Revelation MC with EXP into a Gram Amp 2 SE.... prototype Revelation MC must be a tad over-damped.... you won't have any problems with the A900! It did the 5Hz drum-skin thing!

Edit 2: The old CDP (old broadcast TEAC on-loan from John Cadman) playing Iron Hand by Dire Straits. If that bass extension isn't good enough then try siting on top of an active volcano! (OK, the HD250 gets a tad lower...)

Sash "Stay"... after a long intro just after the shout "Stay", the synth "korgs!" (my slang for deep organ stop electronic bass - I know what I mean even if it's totally out of context...). OK, this Solo SRG may need a bit more burn-in as it doesn't "korg-out" to the depth the HD250 goes, but we're talking about the sound of a "show-man's amp" here (a guy here called Leo used to make these things for Carlsboro). The next track up, "Sweat" does actually "korg-out"

PS. The Korg reference was coined after I heard Cilla's backing trio, at a charity midnight matinee, which included keyboards - a little Korg synth, but boy could it plough at depth! (and obviously the PA could get that low!)


Edited by Graham Slee - 05 Jan 2009 at 2:45am

mrarroyo wrote: Grahm, I found a place in the USA which claims to have the HD250 ii for about $155. Is this a good deal? See: http://www.abcconf.com/store/pc/viewPrd.asp?idproduct=2201

$155 roughly = £110 which was the trade price a few years ago. I'd go for it.