preamp gain

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
i would do the following changes:
r3,r4 = 10kohm
r1,r2 = 10kohm
with those changes you will have an input buffer at each input, input resistance 10KΩ and gain from 1 to 1,5.
the gain will never be less than 1 so this circuit is not much usable as preamplifier.
if you want a gain from 0 to 1.5 then the potentiometer must be replaced with 15KΩ resistor and the pot must be placed between the two opamp (from pin 7 to ground) then the r5 is feeded from the potentiometer. hope this helps.
Last edited:
[1] Do not use crâp diagrams.

[1a] … both the input jacks and output jacks are "wired" wrong. Ground doesn't hook to the center pin. … what idiots…

[1b] C 5 and 6 are both undefined. LOL

[1c] BOTH input jacks are labelled 'L-in', tho' the top one feeds to R-out. LOL

[2] It already is capable of 1.5× output. That's what the variable resistors VR1-A and ?VR2-A are about. Adjust them to lower output.

[3] Do not follow ioannidis suggestions. They are patently wrong.

Joined 2007
Paid Member
The second opamp is configured as a Baxandall active volume control. The gain goes down to zero as the pot resistance decreases to zero. If you retain the pot (50k) then making R5 and R6 into 22k will give an overall voltage gain of around 1.5 at max resistance of the pot.

I must admit that some of the resistor values seem a bit unusual. R9 and R10 should (imo) be much lower, say 100 ohm.
On an unrelated observation, R9 and R10, shown as 4.7K are, unusually high in value. They effectively set the output impedance of the preamp at 4.7K. High enough to cause audible treble roll off in conjunction with interconnect cable capacitance. Both resistors should probably be reduced to about one tenth their shown value or less, but not to zero.
for best noise performance the input stage should have some gain, or all the gain, not attenuation as shown.
you can lower the input R1,2 to reduce its noise contribution but not too low as to load down the preceding stage, what ever that is? need to know what drives this pre-amp so that it can be optimized accordingly.
if you want input R to be 47K then suggest to make the gain of the input buffer the 1.5 that you require 47kX1.5 = 70K and change R5,6 to be 47K or 50K, so the second stage is unit gain.
R9,10 should be 4R7 not 4K7. add local decoupling 104 mlcc(x7r) to each supply lead as close to the device pins as possible.
both opamps are set up as inverting.
Some Designers like this mode of operation.
But be aware of the low input impedance due to this.
Increasing the input impedance by increasing the input resistor increases the noise of each stage.

The gain of each stage is set by the resistors
Stage 1 gain = R3/R2 = 33k/47k = 0.702times (-3.07dB)
Stage 2 max gain = Vr1max/R5 = 50k/10k = 5times (+14dB)
total max gain = (-3dB) +14dB = +11dB
Minimum gain = Vr1min / R5 ~ 1ohm/10k = ~-80dB
Stage 2 with the variable gain will suffer variable stability margins. Expect the amplification to be all over the shop as gain is adjusted.

Oh, and the VR going open circuit, even monentarily, can destroy speakers. Max gain with wiper lifted is ~+120dB.

Can you gather that I tend to using non inverting mode? Despite the alledged higher common mode distortion.
Last edited:
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.