Croft specs - too much gain?

[Wegamus]

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?

[Audio Al]

Send Glenn a email and ask for his advice

glenn@croftacoustics.co.uk

[Wegamus]

Send Glenn a email and ask for his advice

glenn@croftacoustics.co.uk

Thanks Allen, I’ll do that. Sounding fab here otherwise - really impressed.

[Pharos]

The problems of level matching are cropping up numerous times on the forum, and they are easy to resolve if a clear understanding is obtained.

Of course it would be much better if the industry standardised levels more, as is the case in broadcasting where numerous pieces of equipment are swapped into service all the time, necessitating interchangeability, and hence compatibility to facilitate this.

All amplifiers have a range of level operation which gives an optimum performance, the lowest level limited by quiescent noise in the amp, and the highest limited by design, this involving gain structure, devices, and power rail voltages.

Optimum matching between pre and power amps is achieved when their respective ranges are matched; that is, when the lower level limits are operating together, and the higher ones also, but a bit of headroom is always a bonus on a preamp.

When these ranges are matched, the best S/N ratio is achieved, and the volume pot will give the best range of volume over its rotation; none of the 'crammed up at one end' and easily too loud*, or turned up full, but too quiet, because the power amplifier is not being fed enough voltage to drive it to produce the desired volume.

*Many pots are also poorly matched at the lower end of their range, resulting in possible unequal levels from each channel at that end of the pot range.

The CD player industry likes to have O/Ps of much higher voltage than most other line level sources - at a guess they consider that this level is best high so that it does not in any way limit performance; it is always good to have level and/or gain in hand to 'play with'.

Many line level sources have about a 150mV O/P level, tuners for example, and so a CD player fed into a preamplifier will, compared with this, overload the I/P by about 21 to 22dB. This is problematic when switching from one source to another for obvious reasons, and so attenuating the higher CD O/P is sensible.

In line attenuators are convenient and negate the need for fitting internal components either in the CD player O/P or in the preamp I/P ccts.

The vinyl problem is that cartridges are very individual in their characteristics, this resulting from the designer's attempts using innovation, to reach higher performances, and as a result characteristics, including O/P levels vary, as do the needs or not, for capacitative loading, hence the adjustment available in many preamps.

[Barry]

What is the position of the volume control for comfortable listening? Does it have to be set too low, or too high?

[Wegamus]

What is the position of the volume control for comfortable listening? Does it have to be set too low, or too high?

I’m barely cracking it open playing vinyl, Barry. Digital I just lower the DAC output by 30% before it gets to the pre.

[Barry]

Feeding 4mV (from your cartridge) into a preamp having an input sensitivity of 1mV for full output, ought not cause any problems, as the overload headroom should be something like 20x the nominal sensitivity (or 26dB). The fact that you barely have the volume control from minimum, suggests there is something wrong with the disc input of the preamp.

As Al has suggested, contact Glen Croft for advice.

[Wegamus]

Feeding 4mV (from your cartridge) into a preamp having an input sensitivity of 1mV for full output, ought not cause any problems, as the overload headroom should be something like 20x the nominal sensitivity (or 26dB). The fact that you barely have the volume control from minimum, suggests there is something wrong with the disc input of the preamp.

As Al has suggested, contact Glen Croft for advice.

From what other Croft preamp owners have said, they all have limited range on their volume controls. I’ve emailed Glenn now so hopefully it’ll be sorted soon.

[hifi_dave]

Cen be easily corrected. Don't worry.

A common problem - have you ever used any Naim ?

[Wegamus]

Cen be easily corrected. Don't worry.

A common problem - have you ever used any Naim ?

Yes it sounds like a change to a ECC82 in the 7R might do the trick. A previous owner of nearly the same setup, except he had a 7 rather than a 7R, was recommended by Glenn to use an ECC82 which solved all issues. I’m curious as to why the 25R/7R has such a volume control issue in the first place. But an electronics expert, I am not!

And no, never owned any Naim. What are their volume controls like?