Quick note on A&S and noise:
I’ve had a Mogwai SE as my daily for soon to be two years now, and I can confirm that what @perogie is saying is definately not out of thin air even though he might not have hands on.
Now there might be many reasons for why this has come to be, but I think it’s fair to say that the origins of A&S amps are that of repurposed speaker-gear. Albeit for super sensitive speakers, still doesn’t really fit the bill for sensitive headphones.
The Mogwai SE i have is has around 2 times the noise floor of what is listed on the revised version. There is indeed audible noise on it depending on what you’re doing, but also ways around it for the most part once you know the amp well enough.
IME: What are the factors for when this audible in practice?
- Sensitivity of the headphone (disregarding impedance in ohm) being the main factor in combination with the OI on the tap you’re using.
- Dropping the output effect (lower wattage tube rolls) mitigate a lot.
- Gain doesn’t seem to affect it much (ie going from a 4V RCA input or dropping in low gain input tubes).
- Flat impedance curve headphones seem less affected aka planars.
In practice sticking with the 8ohm tap, and it’s a non-issue for what i drive on it mostly to begin with. The 300 will always have noise no matter what i do to it, but the 32 is also “fine” with a low wattage tube roll config on stuff like Sennies and ZMFs.
It’s important to note that the original A&S amps only had 32 and 300 jack and were usually presented and reviewed with the full 8W tube rolls (KT88 or 6550 with GZ34/5AR4), thus presenting 2 quite noisy options. My Mogwai was the first one to get an 8ohm tap back in may of 2018 according to Justin, if it didn’t have it I would not be happy with the product to be honest.
I’d consider it an extra thought if i had a desire to run very sensitive headphones. But for most normal headphones you’d typycally run I’d personally not be worried, esp given the noise ratings of the current line. I would not pick up old 32/300 only models though.