Onboard JFET Preamp question

[guidoilieff]

I made this pcb

From Albert Kreuzer


I want to use it on a SG bass and a Precision. Do I just omit the second input (transistor 2, R4/5/6, P2 and C2)?

Can I use one of these to turn it on/off or the circuit shuts down if there is no input connected?

thanks

[GibsonGM]

Hi Guido - yes, you would omit R4, R6, and everything below it for single input operation.

Rather than use a switching pot, I'd consider using a DPDT or 3PDT to bypass the circuit when not in use, true bypass style, so you don't completely lose your guitar signal (you are right, no sound goes thru when there is no power).  Many reasons to do this, including power supply failure, etc. 

I would breadboard this first, and adjust the caps (C1, C5, C6...) to your liking for the basses you'll be using it with...

[duck_arse]

erm, if you are building it 'on-board', with its own battery, you can use a tip-ring-sleeve jack to switch off the battery when you unjack from the bass.

or use the switch pot. or the switch as per gibsonGM.

[guidoilieff]

Cool, thanks.


I just realized how jfets works hehe

[guidoilieff]

I made this one with a fixed 50k resistor

There is a little volume drop, there is no difference if I remove the 50k resistor


I used a 3,4M resistor and this as a reference too


Shouldn't volume be higher?

[Eddododo]

^ it should be higher;

jfet specs are all over the place. You'll read that here a lot, I'll let someone else elaborate.

Long story short, use a trim pot as the drain resistor, because sometimes the 'right' resistor is not right for the actual jfet you end up with.




As far as the switching for the bigger (full kreuzer version) preamp, you'll want to take steps to avoid a loud pop when you turn it on and off... a master volume, turned down before engaging the 'active' mode would prevent loud pops in the house/amp.

[guidoilieff]

^ it should be higher;

jfet specs are all over the place. You'll read that here a lot, I'll let someone else elaborate.

Long story short, use a trim pot as the drain resistor, because sometimes the 'right' resistor is not right for the actual jfet you end up with.




As far as the switching for the bigger (full kreuzer version) preamp, you'll want to take steps to avoid a loud pop when you turn it on and off... a master volume, turned down before engaging the 'active' mode would prevent loud pops in the house/amp.

So thats why... I was using a pot instead of the 2,2k resistor, Ill try in the 6.8k resistor.

[GibsonGM]

^ it should be higher;

jfet specs are all over the place. You'll read that here a lot, I'll let someone else elaborate.

Long story short, use a trim pot as the drain resistor, because sometimes the 'right' resistor is not right for the actual jfet you end up with.




As far as the switching for the bigger (full kreuzer version) preamp, you'll want to take steps to avoid a loud pop when you turn it on and off... a master volume, turned down before engaging the 'active' mode would prevent loud pops in the house/amp.

So thats why... I was using a pot instead of the 2,2k resistor, Ill try in the 6.8k resistor.

Building on Eddododo...I would keep the 2.2k for now, and put in a 4.7K or so drain resistor, and then a 10k pot in series with it to the power supply.  This will help you dial in the 'sweet spot' (usually about 4.5V at the drain when using 9V), and you then take the pot out and replace it with a fixed resistor (which would be the sum of the one you put in plus the pot reading of course).

If you use a FET that is not the one in the original design (and sometimes, even if it IS!), you often must tailor the drain resistor for the actual device you're using.   That's why you see trim pots there a lot.    If you get it right, then your volume pot will work - you will have gain.

[amptramp]

I made this one with a fixed 50k resistor

There is a little volume drop, there is no difference if I remove the 50k resistor


I used a 3,4M resistor and this as a reference too


Shouldn't volume be higher?

You can put the JFET, source resistor and gate resistor in the guitar and put the rest of the circuitry in an outboard enclosure.  This gives you a guitar with an internal buffer with a high input impedance as seen by the pickups and an output impedance equal to the outboard drain resistor, several orders of magnitude lower than just the guitar alone.  It also means the battery or power supply is external so you don't have to allocate volume and access in the guitar itself.

If you don't want to butcher your precious axe, you can build this into the cable using a cable plug with a removable top.  Either way, you can run long cables without treble loss or as much interference pickup as you could have without it.

[guidoilieff]

^ it should be higher;

jfet specs are all over the place. You'll read that here a lot, I'll let someone else elaborate.

Long story short, use a trim pot as the drain resistor, because sometimes the 'right' resistor is not right for the actual jfet you end up with.




As far as the switching for the bigger (full kreuzer version) preamp, you'll want to take steps to avoid a loud pop when you turn it on and off... a master volume, turned down before engaging the 'active' mode would prevent loud pops in the house/amp.

So thats why... I was using a pot instead of the 2,2k resistor, Ill try in the 6.8k resistor.

Building on Eddododo...I would keep the 2.2k for now, and put in a 4.7K or so drain resistor, and then a 10k pot in series with it to the power supply.  This will help you dial in the 'sweet spot' (usually about 4.5V at the drain when using 9V), and you then take the pot out and replace it with a fixed resistor (which would be the sum of the one you put in plus the pot reading of course).

If you use a FET that is not the one in the original design (and sometimes, even if it IS!), you often must tailor the drain resistor for the actual device you're using.   That's why you see trim pots there a lot.    If you get it right, then your volume pot will work - you will have gain.

It was 2,4k instead of 6,8k and 4.9v for this jfet.


thanks a lot

[guidoilieff]

I made this one with a fixed 50k resistor

There is a little volume drop, there is no difference if I remove the 50k resistor


I used a 3,4M resistor and this as a reference too


Shouldn't volume be higher?

You can put the JFET, source resistor and gate resistor in the guitar and put the rest of the circuitry in an outboard enclosure.  This gives you a guitar with an internal buffer with a high input impedance as seen by the pickups and an output impedance equal to the outboard drain resistor, several orders of magnitude lower than just the guitar alone.  It also means the battery or power supply is external so you don't have to allocate volume and access in the guitar itself.

If you don't want to butcher your precious axe, you can build this into the cable using a cable plug with a removable top.  Either way, you can run long cables without treble loss or as much interference pickup as you could have without it.

So I put the G resistor between ground and the tip of the output jack of the bass, and  the S resistor?

Can you share a schematic/drawing of what you said? I dont think I got it.

[guidoilieff]

Is there a rule like Gv must be higher than Sv or something like that?

The volume is slightly higher without this amplifier.
WTB Im using this as an external pre, I soldered some jacks at the input and output, like a stompbox.


I used a 10k potentiometer and volume goes up at 2k in the drain. I used 2,2k, just like the source resistor.


How important are the output capacitor and resistance/50k pot? I removed em and still sounds the same.

[GibsonGM]

Well, there ARE some 'rules' and tech knowledge that should be had in order to work on your (very good) questions, guido.  That simply means reading the articles at GEOFEX and AMZ, and here -  ]http://diy.smallbearelec.com/HowTos/BreadboardBareAss/BreadboardBareAss.htm]

Read the "fetzer valve" article here: http://runoffgroove.com/articles.html

Gate voltage "holds back' the amplification - acts like a valve.   0 gate voltage = highest amplification.   Smallbear talks about this.  But there are other parameters to understand first.   That's why it's good to use a known-working design at first...any of the 'proven' FET boosts should give LOTS of gain!!

Output cap blocks the DC used in running the device from following circuits (required), and also works with the vol. control to filter some audio frequencies (high pass filter; cuts some lows).  A smaller value cap there will make the sound move to 'more treble', if you get my drift.    Those filters can also be looked up      Freq of cutoff = 1/2pi R C

[blackieNYC]

The Tillman preamp, with the J201 and the 51k resistor, is only going to give you 3dB.  Not going to be observed as a lot of gain, if that's what you'd after. 

[guidoilieff]

Well, there ARE some 'rules' and tech knowledge that should be had in order to work on your (very good) questions, guido.  That simply means reading the articles at GEOFEX and AMZ, and here -  ]http://diy.smallbearelec.com/HowTos/BreadboardBareAss/BreadboardBareAss.htm]

Read the "fetzer valve" article here: http://runoffgroove.com/articles.html

Gate voltage "holds back' the amplification - acts like a valve.   0 gate voltage = highest amplification.   Smallbear talks about this.  But there are other parameters to understand first.   That's why it's good to use a known-working design at first...any of the 'proven' FET boosts should give LOTS of gain!!

Output cap blocks the DC used in running the device from following circuits (required), and also works with the vol. control to filter some audio frequencies (high pass filter; cuts some lows).  A smaller value cap there will make the sound move to 'more treble', if you get my drift.    Those filters can also be looked up      Freq of cutoff = 1/2pi R C

Thanks!


I already read some articles but they talked about how a transistor is made and very technical data that I don't need right now. Thanks for the links.

[guidoilieff]

The Tillman preamp, with the J201 and the 51k resistor, is only going to give you 3dB.  Not going to be observed as a lot of gain, if that's what you'd after.

I know its not going to be a lot, but I think it should be more than the passive bass alone.

[GibsonGM]

Well, there ARE some 'rules' and tech knowledge that should be had in order to work on your (very good) questions, guido.  That simply means reading the articles at GEOFEX and AMZ, and here -  ]http://diy.smallbearelec.com/HowTos/BreadboardBareAss/BreadboardBareAss.htm]

Read the "fetzer valve" article here: http://runoffgroove.com/articles.html

Gate voltage "holds back' the amplification - acts like a valve.   0 gate voltage = highest amplification.   Smallbear talks about this.  But there are other parameters to understand first.   That's why it's good to use a known-working design at first...any of the 'proven' FET boosts should give LOTS of gain!!

Output cap blocks the DC used in running the device from following circuits (required), and also works with the vol. control to filter some audio frequencies (high pass filter; cuts some lows).  A smaller value cap there will make the sound move to 'more treble', if you get my drift.    Those filters can also be looked up      Freq of cutoff = 1/2pi R C

Thanks!


I already read some articles but they talked about how a transistor is made and very technical data that I don't need right now. Thanks for the links.

Right..and I don't even do a lot of looking at how they're made or anything, either...but it might be kinda wise to understand that a FET is not the same as a BJT (1st is voltage controlled, 2nd is current-controlled, for instance) - just some basic operational stuff.  Not suggesting you immerse yourself in GaAs doping or quantum physics    

A LITTLE bit of "how it works" would help you solve your own questions, but in a way that you understand instead of the forum just saying "Hey, put this part here, move that, and there you go!".  It IS hard to find a good tutorial, I know; the couple I posted for you seem to give the basics pretty well (esp. the small bear article).   

1 hour of understanding voltage division and how that works with 'biasing', and another hour of breadboarding a simple FET gain stage using a pot or something to vary the bias, with your DMM, and you would be well on your way to having control over what's happening there instead of bumping into troubles. 

Just an opinion, no problem with asking how to do something 1 time and being done with it, either.  Depends if you want to do more stuff after this.

[PRR]

If you just want gain, why not use an opamp?

[guidoilieff]

If you just want gain, why not use an opamp?

I want to understand how it works but from what I read and whats happening in the breadboard there are some inconsistencys.

With that circuit is supposed to make the signal stronger for when you use long cables, like a buffer I guess... but its making it weaker...


I know its not going to be as drastic like this
https://youtu.be/uapTNlKpmD0?t=29


I read about capacitance on long cables and how buffers work so I want to fully understand this before making a preamp.

[GibsonGM]

You should have a noticeable increase in gain if you build that 1 FET simple gain stage you posted (in Reply #8). Not a ton, but some.    If you don't, that means something is wrong with the build or the component.  Or the pinout...

The way to go about this is read the "debugging" thread, but really - what voltages are present on the 3 device pins?  Mostly these problems have to do with wrong biasing for the device, solved by a trimmer at the drain. 

Hint: If you want BIG gain, use a different circuit like the AMZ Mosfet Boost.