Buff n Blend adding gain. How can it be done.

[nguitar12]

Hello everyone just wonder if anyone know how to add some gain to signal as I found that there is slight volume drop when using this circuit. Thanks for helping.

[induction]

Try another jfet instead of the MPF102. In my experience a 2N5457 will get you much closer to unity. The higher the transconductance, the lower the volume drop.

I'd just use an op-amp, but that's me.

Edit: By the voltage gain of a jfet buffer like this one is calculated as 1/(1+(1/transconductance*R3)), so it will always be lower than unity. But with a higher transconductance it might get close enough not to notice. An op-amp buffer has a voltage gain of exactly* 1.

* Or close enough not to notice.

[nguitar12]

Try another jfet instead of the MPF102. In my experience a 2N5457 will get you much closer to unity. The higher the transconductance, the lower the volume drop.

I'd just use an op-amp, but that's me.

can it be done by changing the resistor value? MPF102 supposed to have a boost function isn't it? By the way I am using J201.

[induction]

can it be done by changing the resistor value? MPF102 supposed to have a boost function isn't it? By the way I am using J201.

In principle it can, but that will change the idle current. When the idle current gets too low, sound quality may be affected. It's worth experimenting, though. I hope you're doing this on a breadboard to start out.

IIRC, J201's have very high transconductance compared to most other commonly used jfets. If that doesn't get you there, I'd try an op-amp.

What are you using it for? If it's always on, then a little volume drop won't matter. If it's an input buffer to a circuit with gain, you can make up the difference in the latter circuit. But if it's a stand-alone buffer that you turn on and off, that's a different story.

[nguitar12]

In principle it can, but that will change the idle current. When the idle current gets too low, sound quality may be affected. It's worth experimenting, though. I hope you're doing this on a breadboard to start out.

Can you suggest me a way to make the changes then?

What are you using it for? If it's always on, then a little volume drop won't matter. If it's an input buffer to a circuit with gain, you can make up the difference in the latter circuit. But if it's a stand-alone buffer that you turn on and off, that's a different story.

I am using it for delay pedal.

[induction]

Here's some good reading from someone much smarter than me.

Jfet tolerances are incredibly wide, so there are huge differences between different devices with the same part number. I'd try a 2N5457 if you have one. Or try five and choose the one that gives the best result.

If you calculate the voltage gain for both the maximum and minimum expected transconductance (gm), you'll see that you only get large improvement on volume from increasing the source resistor if gm is fairly low. For example with gm=0.001, voltage gain = 0.69 with an R3 of 2k2, and voltage gain = 0.91 if R3 = 10k. But if gm = 0.005, the gains are 0.92 and 0.98 respectively. So I'd try to find the best jfet first, and then put a 10k pot wired as a variable resistor inline with your source resistor. See if dialing up the resistance helps without adding noise. Find the best balance and measure the combined resistance of the pot setting and the source resistor. Use that value for your new source resistor.

Or just use an op-amp.

[induction]

MPF102 supposed to have a boost function isn't it?

Forgot to answer this part. A jfet can be used to amplify voltage to greater than unity in a common source amplifier, but that has a different circuit topology, and its output impedance is set by the drain resistor. In practice, its output impedance will be too high to use it as a buffer.

[nguitar12]

MPF102 supposed to have a boost function isn't it?

Forgot to answer this part. A jfet can be used to amplify voltage to greater than unity in a common source amplifier, but that has a different circuit topology, and its output impedance is set by the drain resistor. In practice, its output impedance will be too high to use it as a buffer.

Thanks you your help. I will take some time to read the information you give.

[merlinb]

Hello everyone just wonder if anyone know how to add some gain

You can't. You need a different circuit if you need gain.
Are you using R4? If so, jumper it and also get rid of R5. This will at least reduce the loss in the circuit.