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## How To Change Gain For QSXM2 and QSXM3

Started by goldkenn, November 07, 2009, 09:03:42 AM

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#### goldkenn

Due to morden low gain preamp and low sensitivity loudspeaker, plus personally I prefer passive preamp, compare with something like Lehmann Audio, I think the gain I need will be at around MM/50dB;MC/70dB for QSXM3 and for QSXM2 MM/55dB;MC/80dB.

However, I don't know how to calculate the value, may I have the equation about how to calculate the resistor value, or please anyone could simply tell me the value?

Thanks!

#### peranders

The gain is pretty high already. I was forced to lower the gain in my gear but if you want to tune up the gain a bit you must these resistors:

QSXM2
pre amp, A = R50/R51 + 1, change R50, increase it. Double the gain = 6 dB
RIAA amp Change R93, don't change the other resistors. Decreasing R93 = increased gain.

QSXM3, Change R6 and R4, lower the value = increase the gain
/Per-Anders Sjöström, owner of this forum

Homepage with my DIY hifi stuff

Thanks

#### goldkenn

Some new questions regarding fain og QSXM2:
1) The MC mode total gain is input stage + RIAA stage, which means with componets value as schemtic, it is 67dB?
2) What is the maximum gain I can (each +6dB step) reach for the input stage by changing R50/E50?

Since yesterday I have a A/B test with my friend's EAR 834P, which suppose to have 68dB gain at MC, but in his system, only swapping between 834P and QSXM2, QSXM2 with standard value of R50/E50(68R) and R93/E93(680R) as schemtic, it sound like just something at 5xdB gain only, we feel strange since even non matching impedance or else, if just 1dB gain difference, the sound level should not be so huge difference, we need to tune the volume knob from 10 o'clock (with 834P) to almost 2  o'clock with QSXM2).

Then I try to ask if he have something for replace R50/E50 since he also DIY, we found only 2 matched 330ohms so give it a trial, but heavily distored, I think we add too much gain which cause overload or unstable.

Therefore please advice the maximum value I can do for gain changing, to aviod mess up the machine, thanks.

#### peranders

#4
It's not a problem to increase the gain if you want to.

How is the max level out? Is it too low? Remember, more gain = more noise
/Per-Anders Sjöström, owner of this forum

Homepage with my DIY hifi stuff

#### goldkenn

You mean max kevel of the system or QSXM2?

Meanwhile, actually I still not really understand, as you told:
QSXM2
pre amp, A = R50/R51 + 1, change R50, increase it. Double the gain = 6 dB

Then if with componets value as schemtic
A = 68/5.6 + 1
It is 13.14, this 13.14 is "times" or "dB"?

And, change R50, increase it. Double the gain = 6 dB
Which means from 68R increase to 136R = increase 6dB, right? So increase 12dB is 68R x 3 or 68R x 4?

Also, please confirm me, at MC mode total gain is input stage + RIAA stage, which means with componets value as schemtic, it is 67dB?

#### peranders

It's correct. The gain formulas is "times", not dB. If you want dB you must first use Log and then multiply with 20.
/Per-Anders Sjöström, owner of this forum

Homepage with my DIY hifi stuff