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Croft specs - too much gain?

Nagraboy said:
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?
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Going by those specs, the power amp has a voltage gain of around 31.5 dB, and the line stage has about another 10.5 dB of gain. So 42 dB in total on the line input. That's a huge amount of gain! Most systems have a lot less.
 
Nagraboy said:
Glenn Croft suggested to me to put in a 12BH7A which is on its way to me now.
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That should do the trick for the hiss, I imagine you have a noisy valve in the power so replacing with whatever one Glenn recommends should work as there shouldn’t be any hiss as standard even using 25r/7 with the stock ecc83. The question of travel on the volume is a different point though, my experience is the 25r doesn’t need much movement of the volume control, even with the matching 7 power amps, so it’s normal and not a concern. Changing the valves from the stock type to others might have a very small influence but perhaps at the sacrifice of other aspects of sound quality, but the preamp can be modified to change the line stage if it is a problem or if you want to use it with a different power amp with lower input impedance.
 
The Bish said:
That should do the trick for the hiss, I imagine you have a noisy valve in the power so replacing with whatever one Glenn recommends should work as there shouldn’t be any hiss as standard even using 25r/7 with the stock ecc83. The question of travel on the volume is a different point though, my experience is the 25r doesn’t need much movement of the volume control, even with the matching 7 power amps, so it’s normal and not a concern. Changing the valves from the stock type to others might have a very small influence but perhaps at the sacrifice of other aspects of sound quality, but the preamp can be modified to change the line stage if it is a problem or if you want to use it with a different power amp with lower input impedance.
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I tried a different ECC83 in the 7R today and it made no difference, still hissing.

Just now I tried a vintage GE JAN ECC81 I found and again no difference, still hissing.

It’s puzzling certainly.
 
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