Preamp - 18 dB Gain too low?

The discrepancy between 1V sensitivity (for full rated power output) and 4V is 12dB -- huge! Grant Fidelity and Shuguang really need to get in sync on that one -- it's very confusing/misleading, on such an important spec that can make or break a system.

A 25Watt amp with 4V sensitivity WAY on the low side -- I don't know what the hell they intended that amp to be used with. If would be great for high-output pro sources > 4V. The issues with a typical 2V hifi digital source are one thing; a low-output MC cartridge would be almost impossible to step up enough to coax a satisfying output from those amps.

As a comparison, the Rogue Apollo KT90/KT120 tube amps have a sensitivity of 1V for 250 Watts output. That's 22dB more gain than the Shuguang 845! Yep, they're on the high side of gain.

Another example on the (very) high side: the old Eico HF-87 amp (runs a quad of EL34; cathode bias; 35W/ch), which was spec'd (according to the assembly manual) at 0.34V for +3dB over full output (70 Watts). That's a whopping 26dB over the Shuguangs!

Both the Eico and the Rogue use a 12AX7 input tube...

my 845 amp is very high, 200mv. I wonder if I am losing any performance since my volume is at 9pm for normal listening. it sure sounds good though despite the touchy volume control. the manufacturer said I could swap out two resistors to make it less sensitive, which might do.

Being ignorant how you calculate 22 dB more gain with Rogue Apollo? Any websites or references are appreciated.

Nguyen787,
A doubling (2x) in voltage is (almost exactly) 6dB. A 10x in voltage is exactly 20dB. So for example if you have a 20x factor in gain then you could say: 20x = 2x * 10x = 6dB + 20dB = 26dB

Note that dB values are always added together (since dB values are really exponents), while their corresponding multiplication factors are multiplied together.

When comparing power: a 2x in power is approximately 3dB, while 10x is exactly 10dB.

The precise formula for converting Voltage ratios to dB is: dB = 20 * log(V1/V2), where V1 and V2 are two different voltages, and the log function is base-10. Note that if you flip V2/V1, you get the same magnitude dB value but with a different sign. For power ratios, it's: dB = 10 * log(P1/P2), where P1 and P2 are two different powers in the same unit (e.g. Watts).

So in my example with the Apollos, you start out by comparing Voltages:
20 * log(4V/1V) = 12.0412 dB

Thus, the 4V amp will require a 12dB higher input signal to achieve its "full output" (the typical definition of amplifier sensitivity), versus the 1V amp. Next we have to compare the difference in "full output" between the amps:

10 * log(250Watts / 25 Watts) = 10.0 dB

So the amp that puts out 250W will be 10 dB above the 25W amp when both are at full output. Add the two together: 12 + 10 = 22dB difference in amp gain.

--
Mike

An input sensitivity for an amp of 1v is quite normal as nearly every amp I've seen on the market has been .5v to 2v for max rated rated output (to clip/distort).
An input sensitivity of 4v is wrong/bad and I would call the amp broken or seriously badly designed.
You have a problem ask the manufacture to give you a fix to make the amp/s 1 to 2 volt input sensitivity, as the standard is.

Cheers George