Use of JFET preamps

[statzern]

Hi everyone,

I am just getting started down the road of building my first JFET preamps, and I was wondering how most people are using them in their setups? I bought an AMT E2 a while back for that super modern metal sound and have been running it into my power amp insert / effects loop return of my amp, bypassing my amps preamp section entirely (the whole point, right??). I feel like there are a lot of demos online that just run these preamps into the front (instrument input) of their amps and are getting both sounds, effectively making the JFET circuit a dirt pedal, or maybe acting as a second channel for the amp.

My ultimate goal was to put together an LM3886 power amp to go with the preamps so that the entire rig can fit on a pedalboard- no amp head needed. I understand this could be accomplished with other commercial methods (tech 21 powered cab, combo amp like Marshall Valvestate, etc) however it's not DIY if you buy it!

Thanks in advance, looking forward to doing more with JFETs!

[anotherjim]

I'm of the same opinion, although I wouldn't want the power amp on the floor with the pedals.
Watch out for many amp sims that are designed purely to go into the front of a guitar amp - they lack buffer/drive at the output for feeding a line level power amp.

[statzern]

Can you give me an example of what you are talking about? Any designs come to mind with or without the buffered output? Could just be added to most outputs of those that do or have it, right?

I was hoping to try the OLC Orange Peel first. Will need to review the circuit but as far as I know there is not a buffer in the output part of that design.

[anotherjim]

Here's an unbuffered, although Q4 close to the output might look buffered at first glance.

Feeding less than 1M amp input will...
Raise the cut-off frequency of that 0.68 output cap -  less bass.
Lose excess level over those 15k low pass output filters - weaker output.
Change the feel/range of the Master vol control by loading the pot taper.

Buffered output...

[pipporan]

i intrude in the discussion, 'cos im gonna built a couple of jfet preamps in the future too...first i was thinking about Tillman preamp...
my basci idea is to use it ('cos it's the simpliest and the smallest of all) when i travel, with just guitar and headphones...is it a good idea or should i look for other circuit for this purpose (like some headphone amp)??

[statzern]

Here's an unbuffered, although Q4 close to the output might look buffered at first glance.

Thanks, Jim. Makes sense. So, I could just add to the layout or possibly use something like the GGG IC Buffer (http://www.generalguitargadgets.com/effects-projects/boosters/buffer-ic/) as a daughter board.

Thanks for the heads up, will post back when I get through the build.

[anotherjim]

i intrude in the discussion, 'cos im gonna built a couple of jfet preamps in the future too...first i was thinking about Tillman preamp...
my basci idea is to use it ('cos it's the simpliest and the smallest of all) when i travel, with just guitar and headphones...is it a good idea or should i look for other circuit for this purpose (like some headphone amp)??

General headphones have 30ohm per side. If you drive both sides together total load is 15ohm. You really need something more like a power amp, even if it's going to be under 0.5 watt.
The "Ruby" LM386 based mini amplifier might be a better basis and can be given a basic FET guitar amp simulator front end. Search around for examples.

[BubbaFet]

i intrude in the discussion, 'cos im gonna built a couple of jfet preamps in the future too...first i was thinking about Tillman preamp...
my basci idea is to use it ('cos it's the simpliest and the smallest of all) when i travel, with just guitar and headphones...is it a good idea or should i look for other circuit for this purpose (like some headphone amp)??

You can also go the single-package dual-opamp route, one op amp for left, the other for left, like this link describes...
https://tangentsoft.net/audio/cmoy/

[pipporan]

You can also go the single-package dual-opamp route, one op amp for left, the other for left, like this link describes...
https://tangentsoft.net/audio/cmoy/

yeah, i looked at CMOY, it looks nice, but i dont have the OP needed (i just have LM386, TL07x, TL06x, RC4458D, NE5532, LM358, LM324, LM13700...), so i wouldlike to do something with those! (and then, the OP2132 needed seems to be very expensive (and i cant do another order, i just did 4 in last 2 months!!!:D)

General headphones have 30ohm per side. If you drive both sides together total load is 15ohm. You really need something more like a power amp, even if it's going to be under 0.5 watt.
The "Ruby" LM386 based mini amplifier might be a better basis and can be given a basic FET guitar amp simulator front end. Search around for examples.

so JFET preamps are usefull to... to boost the signal? to be able to mount onbard?...
anyway, i could join roby amp+ jfet preamp and i should have what i want....??

[anotherjim]

A 386 actually has plenty of voltage gain and can work alone. What it lacks is high input impedance so will roll off some treble from your pickups. The Ruby uses a FET just as a high impedance buffer to get the highs back, no pre-amp boost at all.

Remember that headphones are full range, while a guitar cab rolls off steeply over 5kHz. Any distortion will generate harsh upper harmonics which guitar amps actually rely on the cab response to smooth down. So without some simulation of cab response, headphone use could be rather uncomfortable. Cab sim need not be complicated. A useful simplification is some RC low pass filtering at 5kHz on the output can be enough. That's what the 15k/2200pF network on the output of the Citrus Graphic shown above is doing.

[pipporan]

so, tu use with headphones, i can build something dedicated like this:
http://tagboardeffects.blogspot.it/2013/01/mxr-headphone-amp.html
OR, any amp (with ICs or FETs) adding a cab simulator to be able to hear upper harmonics
OR, any amp with a RC filter added just before the output

just to know, how can be "calculated" whick value of resistors/capacitors must be used to create a high pass filter?
i've just found this page:
http://www.electronics-tutorials.ws/filter/filter_2.html

so, if i have understood well:
FC: cutoff frequency (the frequency we want to begin to cut)
FC=1/(2P*res*cap*1.000.000.000)

but to do so,i should know which frequency exactly to cut down???

the double filter shown in Citrus Graphic so it's doubled to double the Db for the frequency to cut down?

[anotherjim]

Filter tool...
http://sim.okawa-denshi.jp/en/CRCRtool.php
You'll see that the Citrus (also found in some ROG schemes) works about 4800Hz.

OR, any amp (with ICs or FETs) adding a cab simulator to be able to hear upper harmonics

Upper harmonics need reducing.
Amp sim > cab sim > clean amp > full range speakers.

[antonis]

so, if i have understood well:
FC: cutoff frequency (the frequency we want to begin to cut)

Actually, the frequency at which signal's amplitude is already cut by about 30%..
(-3dB cut-off point <=> 20*log(0.7)..)

FC=1/(2P*res*cap*1.000.000.000)

One billion multiplier on denominator should only be used for net numbers of kilohms together with microfarads..

[pipporan]

i've just built this:
http://tagboardeffects.blogspot.it/2013/01/mxr-headphone-amp.html
very simple, very effective, little enough to fit in a little box to put directly on the guitar...