What shoud i use to make very low gain preamp?

[login721]

 Hello .I want to make a low-gain preamp (~0.01) to feed the output of guitar effect into input of electret microphone.The preamp gain should be around 0.01 .That should i use to make the preamp ? invertopamp or jfet ?
Thanks in advance !
p/s : Sorry for my bad English.

[merlinb]

I want to make a low-gain preamp (~0.01) to feed the output of guitar effect into input of electret microphone.

Do you mean you want to plug the signal into a microphone preamp? (Not into an actual microphone?)
In that case you could just use a potential divider, e.g. 1M and 10k. No amplifier required.

[login721]

Do you mean you want to plug the signal into a microphone preamp? (Not into an actual microphone?)
In that case you could just use a potential divider, e.g. 1M and 10k. No amplifier required.

Thanks you!
Yes , i want to plug the signal into the microphone preamp.           
The reason i dont like to use resistor is there is will be loss of high freq .

[bool]

My suggestion is to use a buffer with a TL061 / TLE2061, and feed a resistive divider made of 10K and 100 Ohm resistors with it.

That should satisfy multiple criteria - low parts count,  low power consumption, high input impedance and low enough output impedance ... together with your specified attenuation.

[Kipper4]

DI box ?

[midwayfair]

Do you mean you want to plug the signal into a microphone preamp? (Not into an actual microphone?)
In that case you could just use a potential divider, e.g. 1M and 10k. No amplifier required.

Thanks you!
Yes , i want to plug the signal into the microphone preamp.           
The reason i dont like to use resistor is there is will be loss of high freq .

Why would you need to cut the signal to 1% of the original, and how on earth did you determine that you needed that exact gain? That will make for a very, very tiny signal.

Or are you mixing up the gain with the input impedance of the microphone preamp? Microphone preamps will usually have low impedance, but if you have a guitar pedal between the guitar and the preamp, you very likely already have a low-impedance signal for the preamp. Have you tried just plugging it in yet?

[R.G.]

Hello .I want to make a low-gain preamp (~0.01) to feed the output of guitar effect into input of electret microphone.The preamp gain should be around 0.01 .That should i use to make the preamp ? invertopamp or jfet ?
Thanks in advance !
p/s : Sorry for my bad English.

I'm pretty sure that your English is better than my mastery of your native language!

If I understand your technical issue correctly, you want to take the output of a guitar effect, run it through some electrical setup to lower the level a lot without losing treble content.

First, it's good that you recognize that a high-ratio divider causes treble loss. This is an issue in high-impedance volume controls.

The simple thing to do is to use either a JFET follower or an opamp follower (it doesn't matter which one, at least to a first approximation) and drive a low impedance resistive divider. Let the "preamp" have a gain of 1.0, then use a low impedance network to do the attenuation.

Like this:  signal -> follower -> resistor divider -> output.

The resistor divider is driven by a low impedance follower; this lets you use, for instance, a 10K+100R divider to get your 1/100 signal conversion. The 10K is so small that the treble losses are not an issue.

[login721]

Why would you need to cut the signal to 1% of the original, and how on earth did you determine that you needed that exact gain? That will make for a very, very tiny signal.

Or are you mixing up the gain with the input impedance of the microphone preamp? Microphone preamps will usually have low impedance, but if you have a guitar pedal between the guitar and the preamp, you very likely already have a low-impedance signal for the preamp. Have you tried just plugging it in yet?

The image above is my USB microphone, i cut out the electret mic , and feed it with e-guitar signal .I pluged directly output of my cleanBooster into it and get a lot of distortion even with low gain and low volume setting .As i know , the output of electret microphone is quite low ~ xx mV , output of some distorion pedal can go up to ~ 4V .So i need something to trim down the signal to 1/100~1/50 to avoid overdrien the microphone preamp .

I'm pretty sure that your English is better than my mastery of your native language!

If I understand your technical issue correctly, you want to take the output of a guitar effect, run it through some electrical setup to lower the level a lot without losing treble content.

First, it's good that you recognize that a high-ratio divider causes treble loss. This is an issue in high-impedance volume controls.

The simple thing to do is to use either a JFET follower or an opamp follower (it doesn't matter which one, at least to a first approximation) and drive a low impedance resistive divider. Let the "preamp" have a gain of 1.0, then use a low impedance network to do the attenuation.

Like this:  signal -> follower -> resistor divider -> output.

The resistor divider is driven by a low impedance follower; this lets you use, for instance, a 10K+100R divider to get your 1/100 signal conversion. The 10K is so small that the treble losses are not an issue.

Thank you !
I have one more question , what should i use for "preamp have a gain of 1.0" , an invert opamp with gain = 1 or an unity gain non-invert opamp ? I will use TL071 .

[R.G.]

The image above is my USB microphone, i cut out the electret mic , and feed it with e-guitar signal .I pluged directly output of my cleanBooster into it and get a lot of distortion even with low gain and low volume setting .As i know , the output of electret microphone is quite low ~ xx mV , output of some distorion pedal can go up to ~ 4V .So i need something to trim down the signal to 1/100~1/50 to avoid overdrien the microphone preamp .

The simple thing to do is to use either a JFET follower or an opamp follower (it doesn't matter which one, at least to a first approximation) and drive a low impedance resistive divider. Let the "preamp" have a gain of 1.0, then use a low impedance network to do the attenuation.

I have one more question , what should i use for "preamp have a gain of 1.0" , an invert opamp with gain = 1 or an unity gain non-invert opamp ? I will use TL071 .

It is less clear what unity-gain buffer (that's what a "preamp with a gain of 1.0" is) ought to be. A lot depends on other issues, like how big the signal is, how much power supply voltage you have for powering the preamp, what input impedance the preamp needs, and so on.

An opamp set up as a non-inverting follower (gain = +1.0) is a good choice in general, and especially if you need high input impedance.  An inverting opamp follower (gain = -1.0) has a generally lower input impedance. Since you're driving this from an effect, that may not be an issue. Both opamp versions will have issues with not being able to swing from the most negative power supply to the most positive power supply. A TL071, for instance, can only get to within 2V of the power supplies. So if you power from 0V and +9V, the output could only swing from +2V to +7V. That's 5V, and it may be OK if your signals never exceed 4V, but it's something to know about.

A bipolar transistor (NPN or PNP) can do the job as well, and will have a wider swing, but will have a lower input impedance - which, again may be OK.

A JFET could be used, but I don't recommend it because of the difficulties of biasing for widest signal swing.

The opamp versions *will* work. The bipolar version will work but has some tricky things to solve, and the JFET version is touchy.

[PRR]

> i dont like to use resistor is there is will be loss of high freq .

With the suggested value of 1Meg+10K, you could run 30 feet (10 meters) of cable from the divider to the microphone input with "NO" loss of treble. (Upper -3dB point above 15KHz, guitar doesn't go far past 5KHz.)

And the 1Meg seen by the guitar is plenty high enough for full response.

1:0.01 seems like a lot. I would be thinking 470K+22K divider. That's still fine for the guitar, and fine for a reasonable cable on the microphone-input side.

And a pure passive divider avoids all the trouble of wiring more than 2 parts, and the significant trouble of powering a buffer.

[snap]

Watch out for DC on that electret input, or use it.

[login721]

Thank for advices !
I will go with opamp buffer and trimpot at output for vvoltage divider.

[Gus]

Snap made a good point
I would check the voltage at the input of the circuit with a meter set to DC volts. 
Powered off USB unplugged check the input to + for resistance or trace it.
I would guess the circuit used a microphone with a built in JFET so there might be a resistor from the input(drain) to a +supply. 
I would also check what voltage the circuit works at 5VDC or 3.3VDC you could do that with a meter set to DC volts

If there is a  drain to + supply resistor you might be able to use the resistor with a device P channel JFET or PNP BJT to build a follower on the board
And then you could use a resistive divider(could be a potentiometer) as the input to the follower

I would use a active stage because you/we don't know the circuit it might have the input going direct to AD converter and a passive divider might not work correctly

[bool]

If you use low enough resistances in the divider, like the suggested 10K/100R, you don't have to worry much about the "electret phantom" powering, because the electret powering resistor is usually in KOhm values, so it will get effectively nilled by the low output resistance of your buffer/attenuator.

What you will have to look for is to any potential noise caused by cables and mechanical contact noise from the jacks (it WILL be there if you have DC across the jack)!

My suggestion is to solder the 10K/100R divider directly in your ex-mic-gizmo. Just doing that will greatly reduce the contact noises - even if you use a jack socket at that point (ie into the divider/attenuator). Then you're free to decide where exactly you want the buffer phisically.