Simple (FET) preamp schematic needed that goes from unity gain to about +4dB

[Speeddemon]

I want to put some modulation effects in a True Bypass Loop box,
but I want the output to be raised, so I figured I could put a small FET or IC preamp inline with the FX-return from the TB Box to the footswitch and add a potmeter to vary the boost.
Is there such a small, simple circuit, with 1 volume knob and that starts at unity gain (NO level cut!!!) ands boosts about 3 or 4dB?

[d95err]

If I understand correctly, you want to get from gutar level to line level. That would require a lot more than a 3-4dB boost (as a reference, +3dB is approximately the smallest change in volume a human can detect).

Line level is +4dBm or more; guitar level is a lot lower, something like -10 to 20dBm (don't know exactly). So you need something that can provide perhaps 20-30dB of clean gain. In addition, you may need more than 9V to get enough headroom for +4dBm line level. (I suppose the voltage required would depend on the technology used).

Some suggestions that could do what you want (though higher voltage may be required) are:

* Adjusticator (www.geofex.com)
* AMZ Mosfet Boost

The adjusticator can run 18V without modification. The Mosfet Boost should be able to run more than 9V as well, perhaps with some minor modifications.

[Speeddemon]

No, no !

I don't want line level!

What I want is to place a slightly weak sounding effect (as in 'output is below unity gain') in a True Bypass Loop box,
but with a BOOST possible on the output, but just between unity gain and +3dB or so.
I know how to build a Micro Amp or a Rangemaster, but that's too much and too complicated.

I figure there's gotta be some small FET-based preamp circuit (with like 8-10 components max.) that can do this.

The boost should be placed between the FX-Return jack (of the TB Loop box) and the 3PDT switch, so that the OUTPUT of the effect (in the TB Loop) is raised a bit. Not the input, because that could overload the effect or just saturate it, without gaining any extra output volume.

[Joecool85]

How about an LPB-1?

http://www.tonepad.com/getFile.asp?id=68

Just don't bother with the millenium bypass, that makes for 8 components total counting the pot!

[d95err]

Sorry! I misunderstood your whole post. I guess I'm getting sleepy.

Since it seems like you want a transparent clean boost that won't color the sound, an opamp booster would be idea. Very few parts, and you can set the gain of the opamp stage to be exactly what you need (rather than amplifying a lot and then bring the level down with a voltage divider pot). I think there are some opamp boosters in the layout gallery.

If you really need to use discrete FETs, you could try something like this:
http://www.till.com/articles/GuitarPreamp/index.html

[d95err]

How about an LPB-1?

The LPB is not completely clean, it adds some distortion, and in this case I think clean is the way to go...

[dosmun]

How about Dragonfly's Sparkle boost minus the Gain pot.

[d95err]

A few simple boosters from the layout gallery...

http://aronnelson.com/gallery/album18/DOD_201_JFET_PREAMP_VERO

http://aronnelson.com/gallery/album18/DUENDE_JFET_BOOSTER_VERO

http://aronnelson.com/gallery/TheBigMan-Layouts/preampboost

[Ben N]

How about something like the recovery stage of a Big Muff Pi?  Or you can search other commercial device schematics for output buffer/recovery stages.  Getting modest gain like that should be a simple adjustment.

In any event, for a single transistor device, I would say to use a low-noise BJT rather than an FET for lowest output Z.

HTH,
Ben

[Joecool85]

Sorry! I misunderstood your whole post. I guess I'm getting sleepy.

Since it seems like you want a transparent clean boost that won't color the sound, an opamp booster would be idea. Very few parts, and you can set the gain of the opamp stage to be exactly what you need (rather than amplifying a lot and then bring the level down with a voltage divider pot). I think there are some opamp boosters in the layout gallery.

If you really need to use discrete FETs, you could try something like this:
http://www.till.com/articles/GuitarPreamp/index.html

Wow, that looks great.  I may use that.

[Speeddemon]

If you really need to use discrete FETs, you could try something like this:
http://www.till.com/articles/GuitarPreamp/index.html

This looks very nice!
But it doesn't have a potmeter in the circuit.
I've read the article before, but I couldn't deduct which resistor I can replace to make it go from Unity Gain to +3dB.

If someone could help me with this...  

[brett]

Hi.
Check out the stratoblaster at General Guitar Gadgets.  It is a 1 JFET design with variable boost (up to about 10dB), and no discernable distortion.
It's my favourite boost.
cheers

[Speeddemon]

Hi.
Check out the stratoblaster at General Guitar Gadgets.  It is a 1 JFET design with variable boost (up to about 10dB), and no discernable distortion.
It's my favourite boost.
cheers

Hey Brett, is there a way to limit the max amount of boost for that?

[scaesic]

you want a gain of 1.22, 100k rin, 100k rf in series with 22k pot.
inverting op-amp circuit, less than 10 components, easy.

[Speeddemon]

but how do I do that?
Either with the Till-FET preamp, DOD 201 JFET or the Stratoblaster?

[lovekraft0]

It should be easy enough to simply build a single JFET preamp with plenty of gain and add some negative feedback to control it, just like an opamp - simply put a pot in series with two resistors, one on each end terminal, connect one resistor to the output of the coupling cap, the other to the input cap, and connect the wiper to the gate of the FET. The gain is the ratio of the feedback R's impedance to the input R's impedance (within the limits of the circuit's available gain) - adjust the resistor values until one end of the pot is unity and the other end is +4dB. Of course, it might be easier (and more stable) to use an opamp, if you've got room. 

[d95err]

but how do I do that?
Either with the Till-FET preamp, DOD 201 JFET or the Stratoblaster?

You could add a simple volume pot at the end. Add a trimpot (same value as the pot) between the pot and ground. Adjust it until you get unity gain with the pot turned all the way down.

The AMZ Mosfet Booster will work exactly as you want in the non-true bypass configuration. It will act as a buffer with the volume pot all the way down.

I still think the simplest solution would be a non inverting opamp stage. It will be easy to set gain and it can't go lower than unity gain. Add input and output caps and some resistors to get a bias voltage and you're done.

Non-inverting opamp gain stage:
http://ourworld.compuserve.com/homepages/g_knott/elect253.htm

[Speeddemon]

So I could use a TL072 or something like that?

And have R1 be 10k and R2 2.k2 to get the gain of 1.22?
Input cap somewhere around 100nF and output 10uF? But how would this be variable? Should the R2 be a 2k pot with 1k resistor in series then?

[d95err]

So I could use a TL072 or something like that?

And have R1 be 10k and R2 2.k2 to get the gain of 1.22?
Input cap somewhere around 100nF and output 10uF? But how would this be variable? Should the R2 be a 2k pot with 1k resistor in series then?

AMZ has some good info about boosters in general (including opamp versions):
http://www.muzique.com/lab/boost.htm

[scaesic]

So I could use a TL072 or something like that?

And have R1 be 10k and R2 2.k2 to get the gain of 1.22?
Input cap somewhere around 100nF and output 10uF? But how would this be variable? Should the R2 be a 2k pot with 1k resistor in series then?

if you do that you wont get unity gain when pot down.

with R1 set to 100k, and R2 set to 100k, you could put a resistor in series with R2 thats 22k, and as it is varied you'd go from gain of 1 to gain of 1.22.
any values with the same ratio will do.
TL071 would suffice for this, TL072 is just a dual op-amp version.