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Thank you Zoldar and Kachui for your feedback. Unfortunatley, HD800 and K701 are not what I would consider high sensitivity cans, and I find it difficult (maybe too risky) to extrapolate... I reckon Graham Slee himself did acknowledge on its forum that the U/L could be a little noisy with some highly sensitive cans...I guess I will go for the SRG II as a starter
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Thanks for reminding us. Just read the following explanation from Graham's forum:
"So far I have two "complainants" regarding Solo Ultra-Linear noise...
If you swap from a high impedance phone to low impedance phone without swinging the volume down to where it should be, you will hear noise...
This is very reminiscent of valve noise...
So I guess the Solo Ultra-Linear does the valve job here too... Great!
Unfortunately, the one thing I or anybody else couldn't do all those years ago, is make valves noiseless (and still can't), but hey, it didn't matter because the sources then were not noiseless neither.
Since then we have noiseless CD and all sorts of other digital "music". If you read up on how digital "music" works you'll see how the noise floor is cheated by "bit shifting"... you can't shift bits if you don't have any bits, and you don't have bits in analogue...
The Solo Ultra-Linear is a purely analogue amp that does not use great lashings of negative feedback to achieve ultra low noise, so, as the transducers (phones) are strapped in close proximity to your eardrums - when you wind up the "wick" - you will hear hiss and hum (like valves...!)
Now please don't play music whilst the volume's right up there or you'll probably kill your hearing, but turn it down to your usual listening level, then listen again for the noise...
...did it go much quieter? Thought so...
You see, this is a thing called signal to noise ratio (SNR) and with the Solo Ultra-Linear it is -79dB. How do I know? I measured it with a thing called an AUDIO TEST SET. Audio test sets are precision analysers that do all the measurements such as noise, distortion, frequency response etc...
image courtesy Thurlby Thandar
This is our new one (not much change out of £6,000), and the old one was a Wayne Kerr AMS1. We also have oscilloscopes like this one...
image courtesy Farnell
-79dB is the level below the actual musical signal where the noise appears. If you park the volume control up full when it needs to be at say 12 o'clock to blow your eardrums out, then the noise will be heard. But if you turn it to a comfortable (rockin') level then listen to the noise it will be sufficiently back (79dB back, or 10,000 approx. times back) that although you will be able to detect it, it will be of no worry to you.
Now, the argument will now, I'm sure, go something like this...
"But I have a valve power amp/integrated amp and that isn't as noisy full up"
And my answer is this...
A speaker amp is designed for 8 Ohms. It will have enough latitude to drive 4 or 16 Ohms and that's a ratio of 2:1 or 1:2. With the Solo, the latitude is much more (much more). It spans 25 Ohms to 300 Ohms going on the sort of headphones in use today. That's a ratio of 12:1 or 6:1 and 1:6.
Therefore that's 3 times more latitude than a speaker amp obtainable on the Solo volume control. In fact it's more than that because the Solo is designed to do 8 Ohms to 2000 Ohms giving a ratio 125:1 or 1:125.
Now I hope it can be seen that with signal to noise being a relative thing (relative to the music) that the Solo has a much bigger job to do, impedance wise, than a speaker amp?
Conclusion
If the reader still thinks that the Solo should be completely noiseless, and I say it can't be, and the reader wants no compromises, then the reader
must not buy it.
If such a reader has already bought one and is not happy with the way nature works, then, prior to leaving planet earth to find a utopian planet where all things are fantastic all the time, the Solo Ultra-Linear can be returned for a refund.
The difference between the quietest Solo we ever made and this one is 10dB which means the Solo Ultra-Linear sounds twice as noisy as a 2007 Solo.
(-79dB will hardly be noticed by anyone with a vinyl source)"
Also it does not have adjustment for output level...hum...like to hear others' experience with efficient phones