simple buffer gainquestion

[maximebodson]

hello you all !!
I am in the process of building myself a blender/preamp with separate eq  channels for my acoustic guitars. This is very promising at the moment as the sound of the unit is very nice indeed.

Here's my question: how to build a single opamp buffer that has variable gain (6-9dB) possibilities and a 1 MOhm input impedance?

for now I am using a unity gain buffer (using the positive opamp input), but I woud like to hit the eq stage at a higher level and gain a bit of s/n ratio.


concerning my project: my guitar has a sunrise magnetic pickup and a K&K pure mini piezo type pickup. for now the design consists in two buffers feeding two channels of fixed 3 bands eq and a blender circuit . I used different eq bands for each pickups  (especially the mid band which are centered at  800Hz for the sunrise and 1.5k for the k&k), and the results are nice indeed.

any ideas

cheers

Maxime

[GibsonGM]

My 1st idea is to use a TL072; first section as buffer, fed to second section, which will provide you with the gain you want.   This will guarantee you keep your high input impedance.  Not that it can't be done with 1 stage, but as you know, you start increasing gain and you'll change the Zin, and could end up with different tones as you change gain settings.  2 stages will give you more consistency.  Just my opinion....it's a $1.00 difference and 20 minutes more work, lol.   

[R.G.]

1. Take the standard opamp circuit for non-inverting gain, which has:
- a bias voltage midway between the V+ and V- terminals; the 10K/10K/10uF bias generator will work
- V- being used for signal ground external to the circuit; +V a 9V battery
- (+) input tied to the bias voltage with a high resistance, 1M to 2.2M
- feedback resistor from output pin to (-) input
- gain setting resistor from (-) input to a capacitor
- the capacitor to either V- or the bias voltage
- output capacitor from output to signal out
- a pull down capacitor of perhaps 1M on the output

2. Make the feedback resistor be 22K
3. Make the gain setting resistor be a 10K pot in series with a 10K resistor

Gain is  1+(feedback resistor/gain resistor); max is 1+22K/10K = 3.3x or 10db. minimum is 1+22K/20K = 6.4db

[R.G.]

Mike, you posted just as I hit the post button. 

That way works too.

[GibsonGM]

LOL, your way explains it better, R.G.!

[maximebodson]

thanks for your replies,

R.G., If I get it right the circuit should look like this: (where R1 is gain resistor-10k+10k, and R2 22k)



I'll try as soon as I am back at the breadboard.

GibsonGM,  it seems your way is the nice way, I just want the unit to be as compact as is possible, and to include the smallest number of parts.
and this is not so easy to do, with already 6 gyrator circuits on the board (2 preamps of separate 3bands eq) and a blender..

thanks again,

best

Maxime

[R.G.]

Yes, that is the essence of it.

However, there are some details having to do with the DC levels that are missing.

You need an input capacitor to block any incoming DC. That means you also need a large-value resistor from the + terminal to your reference voltage - about which, more in a minute.

You need an output capacitor to block any outgoing DC.

Because the circuit will also produce DC gain as well as AC gain, you need a capacitor in series with R1. This forces the circuit to have a gain of unity at DC.

Opamp circuits commonly don't show power supplies. You need those as well, so there is an unshown V+ pin on the opamp. This connects to the most positive power supply, usually +9V dc from a battery. There is also a V- pin, which usually is connected to the minus terminal of a 9V battery for effects. We also usually use the - terminal for both signal and DC ground.

That last produces a problem for us in that the opamp needs its + input pulled to the middle between the most positive and most negative power supply pins to work well. This is usually done by putting two equal value resistors between +9V and ground, and then using this newly created "bias supply" to connect through the high value resistor to the + input. A capacitor from the midpoint of the bias divider to signal ground keeps that point quiet.

You may want to read "What are all those parts for?" at GEOFEX for an explanation of why there are additional parts that don't seem to be in the simple opamp circuits.

In your circuit, the capacitor to the input + pin can be between 0.1uF and 1uF depending on the value of the resistor you take from it to the bias voltage, and the others can be 10uF. Be sure to get the + side of these caps toward the opamp pins. This happens to get the DC polarity right in this circuit.

[maximebodson]

thanks a million for your insights, R.G.
Sure I 'll go for it  and follow your clever tips !

happy diy to you all

maxime