Croft specs - too much gain?

[Macca]

for what it's worth, I don't think I've ever had a Croft amp hiss (unless there was a faulty valve somewhere). they are remarkably quiet, even compared to SS designs (which can hum). I've had limited usable volume knob travel in some combinations, but even with modern DACs I can usually comfortably get to the 3 o'clock notch. & in my experience that was never correlated to any form of hiss. so my vote goes to the people saying there must be a fault in the signal path.

Could it be a dodgy valve? I used a Croft pre-amp for several years and it had no hiss or hum. No amplifier should exhibit hiss or hum whether it is valve or solid state. if it does it is either faulty or rubbish.

[Wegamus]

Could it be a dodgy valve? I used a Croft pre-amp for several years and it had no hiss or hum. No amplifier should exhibit hiss or hum whether it is valve or solid state. if it does it is either faulty or rubbish.

Late last night I tried the Croft preamp outputs into my NAD integrated and there wasn’t a trace of hiss. So it seems it’s the power amp. Someone told me the pre has 10.5dB of voltage gain and the power amp another 31.0dB which is a huge amount and must be the route of the issue.

[hifi_dave]

Did you buy the units new or s/hand ?

The power-amp is normally, virtually silent.

[Wegamus]

Did you buy the units new or s/hand ?

The power-amp is normally, virtually silent.

It was pre-owned and about 18 months old. When something’s not brand new you never know if there might be a fault, but I’ve heard from someone who bought a new 25R/7 4 years ago and had the same issue from the start via the Harbeth M30.1 like I have here. Glenn sent him an ECC82 to sort it which did the trick.

After the sale, the seller of my 7R told me he had hiss problems with the 25R/7R too, but when used individually they didn’t cause hiss. Eg DAC feeding the 7R caused no hiss, but DAC volume had to be ramped down to -50dB for normal listening because of the high gain of the 7R; the 25R also gave no hiss problems when used with a different power amp. He told me he used attenuators and a 12BH7A to solve it, and offered them to me.

[Pharos]

A typical gain of a power amp may well be between 20 to 30dB, and if its I/P sensitivity is about 0.5V which is typical, that would produce an O/P voltage of approximately 5 to 15 V, 10V across 8 ohms giving an O/P power of about 12W, and 15V about 28W.

That all seems reasonable to me.

I've been pondering the possibility of a feedback loop which had become open cct, and has resulted in a jump in gain.

[Wegamus]

A typical gain of a power amp may well be between 20 to 30dB, and if its I/P sensitivity is about 0.5V which is typical, that would produce an O/P voltage of approximately 5 to 15 V, 10V across 8 ohms giving an O/P power of about 12W, and 15V about 28W.

That all seems reasonable to me.

I've been pondering the possibility of a feedback loop which had become open cct, and has resulted in a jump in gain.

Why would a power amp have an input sensitivity of 0.5V? Is it because most of the circuits were basically designed before 2.0V digital sources appeared so no one wants to change them? I’ve noticed old Quad amps are 0.5V too, and seen modded ones where it’s been changed to 1.5V presumably for use in a modern setting. My old Nagra power amp had a switch to change it from 1V to 2V which was very handy. I don’t understand the wide variation and the purpose of it when we’re all using the same sources - 2V for digital and a few mV for vinyl. It’s like volume controls too, they’re all 9 o’ clock at best for normal listening when there’s a whole pot to use, just why?

[hifi_dave]

I think the comments by the seller are a clue. The amp is faulty, probably a noisy valve and Croft will sort it out.

Normally, they are totally silent with ear up to the speaker and yes, I too use Harbeths from P3ESR to M40.2.

[Pharos]

Why would a power amp have an input sensitivity of 0.5V? Is it because most of the circuits were basically designed before 2.0V digital sources appeared so no one wants to change them? I’ve noticed old Quad amps are 0.5V too, and seen modded ones where it’s been changed to 1.5V presumably for use in a modern setting. My old Nagra power amp had a switch to change it from 1V to 2V which was very handy. I don’t understand the wide variation and the purpose of it when we’re all using the same sources - 2V for digital and a few mV for vinyl. It’s like volume controls too, they’re all 9 o’ clock at best for normal listening when there’s a whole pot to use, just why?

The (arbitrary) 0.5V was arrived at through the years of the early 'evolution' of amplification, and became a default standard, as so many other things in society have done, and I presume that this level was regarded as sufficiently high to be relatively immune to external interference. It is very easy to with hindsight, criticise earlier things when a greater understanding has been achieved.

In broadcasting a voltage standard of 0.775V was a standard derived from a power level of 1mW in a 600 ohm resistor, this derived form maximum power transfer theory in telecommunications, and became the standard. But power transfer efficiency is not a real consideration with signals in which low currents are involved, but maximum S/N ratio is very much so. Arguably since there are metres of cable in broadcast studios carrying signals at this level, it may be advantageous to use a higher level to improve S/N ratio and crosstalk.

[Pharos]

From another thread a link was posted to PFM which was;

"I plan to use a Croft Basic preamp with a Quad 707 power amp. I have read that the gain of the Croft can be reduced by changing a resistor per channel. I hope this combination will provide power from a 150W power amp and the magical midrange from a valve pre amp. "

[Wegamus]

It’s all things to think about. If the new valve doesn’t work out as planned I’ll ask Glenn about resistor changes. Would be a quick job I guess.