Very interesting comments - thank you! You have me lusting after the HE500... again :evil:
I believe the 'b' has the same broad specification except the external/power supply and wholly metal finish. There may be other component variations...
Your comments about high vs. low are interesting. I tend to go with the setting that 'sounds best' on a particular track but I think I might be being a bit of a philistine here :blink: e.g. both the Shure 940 and Grado RS-1 have more 'body' on the high setting despite lower impedance transducers. This may well be at the expense of accuracy. Low has less hum... as you indicated!!
I'm still frustrated generally with RS-1 and K701. The RS-1 sounded amazing with a friend's Cyber 30 but have never sounded that good with any amp I've owned. The Cyber 20 is probably no worse than the GS Solo Ultra Linear and has a better soundstage for RS-1.
I will get around to addressing the mains hum issue (I don't want to be parted from it). For ref. this is the suggested approach I got from my dealer. I checked the frequency of the hum and it is definitely 50hz UK mains....
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[COLOR=0000CD]There are a few possible sources for mains hum to break through, it could be insufficient smoothing of the HT supply, insufficient smoothing (or even AC) on the heater supply or a lack of adequate screening between the power stages and the audio stage (either due to location of transformers or wiring screens etc). The first two are relatively easy to sort out as long as we have sufficient space inside - it’s been a while since I’ve looked inside one but I think we should be ok without having to build a separate power supply box.[/COLOR]
[COLOR=0000CD]If the transformers a mutually coupling (too close together) then that’s probably when we would have to consider a separate power supply box, but there may be another less intrusive solution (a small increase in feedback, although this will reduce gain a little too). Two boxes can cause other problems as we end up with multiple earth points (causing more hum) and a difficulty of getting the high voltage supply between boxes completely safely – neither is insurmountable but I’d always try to work on the one box solution if at all possible.[/COLOR]
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I'm afraid a separate power supply box is unlikely to solve the problem. I have the two-box Cyber 20, and it also hums. I sent mine back to the manufacturer, thinking it was faulty. Opera returned it to me, telling me that the hum was normal, and that this was something to do with the output transformers. Personally, I think it may be AC on the heaters - I haven't been able to test this, though.
For what it's worth, if you use a low-sensitivity can like the HE-500, the hum is so faint as to be virtually inaudible!
Best
James