Croft specs - too much gain?
I’ve recently acquired a Croft 25R pre/phono and 7R power amp to use with my Harbeth M30.1. My sources are a Technics SL-1200G with Audio Technica VM540ML (5mV) and a Pro-Ject DAC (2V output). With no music playing I’m getting very noticeable hiss through the speakers, when I am playing music I get very little range of use on the volume control. Not had any hiss with my Croft integrated phono, and seem to have a bit more range on the volume control on that amp too.
I’ve been hunting around and found some published specs for these amps on the Audio Affair website. See below for the product name and it’s specs:
Croft RIAA: 2mV for 0.5V output
Croft RIAA RS: 2mV for 0.5V output
Croft 25 line: 0.15V for 0.5V output
Croft 25 phono: 1mV for 0.5V
Croft 7 power: 0.5mV for 45W @ 8 Ohms
Please correct me if I’m wrong here, but if the Croft 25 pre will output 0.5V to the Croft 7, the 7 will give its full output of 45W @ 8 Ohms. So, this output only requires 1mV from a cartridge, or 0.15V from a digital source. Any more than that and you’ll start cramping your volume control, right?
Most MM carts are around 5mV and most digital sources are around 2.0V, which in my limited understating would seem to be vastly too much output for a preamp with these specs. I assume this is the cause of my limited volume control range.
The other issue is the hiss/noise. With the given specs it seems the 25 and 7 are perfectly matched, with the 0.5V preamp’s output and the 0.5V power amp’s sensitivity - so why the hiss when no music is played?
I tried my Technics into the 25R phono input, then output the Croft’s line output (which bypasses the volume control and mute switch) into my NAD integrated and had no hiss at all. Sounded fine.
I must add that the 25R/7R do sound absolutely wonderful in my system, notwithstanding the above issues.
Can anyone offer any technical advice on this please?
Analogue: Thorens TD 124/II - SME M2-9R - Ortofon SPU Wood A - Sculpture A Mini Nano
Digital: Roon Nucleus - Limetree Network
Amplifier: Naim NAIT 50
Speakers: TBC