AMZ jfet guitar buffer. Will 6v battery work?

[LeeYoo]

Hi ickefes.
Maybe....
Use a FET with low cutoff voltage, like the J201.
You might have to adjust the values of R2/R3 for symetrical clipping.
It will work, but you haven't got a lot of headroom with a 6volt supply.
Guitars can put out several volts top/top when played hard. Your circuit will (soft)clip.
Current draw is only 0.5mA. If you want it small, a 12volt car remote battery might be a better option.
Leo..

[zvukofor]

This booster with BF 245a not working with anything less 7V, tested practically. So yes, as LeeYoo said, try J201. But IMO it is much better just to use classic 9V battery.

[ickefes]

LeeYoo and zvukofor: Thank you both so much for helping out. I understand what you say that the headroom will be to low if using a less powered battery and this 12 volt car remote battery might be better so I will check that one out. Thank you again for the detailed explanations.

The swedish store that I will order from does not carry the j201 and I found on a site somewhere that a 2N5457 TO-92 JFET N-ch 10mA can be used as substitute. Is that correct? Regards.

[LeeYoo]

Hi ickefes.
According to the datasheet, the 2N5457 has a cutoff voltage of about 2.5 volt.
This means that the source (R4) is always 2.5volt higher than the gate (R3/4).
Your circuit uses two equal resistors at the gate (R3 and R4).
This puts the gate at HALF the supply voltage (3volt on a 6volt battery).
The source floats 2.5volt above that, so DC voltage on R4 is 3+2.5=5.5volt.
A soundwave on R4 can only go 0.5volt up (and 5.5volt down).
So there is only 0.5volt clean positive headroom if you're lucky.
Not enough if you play hard.
Increasing the value of R2, or removing it, will drop the gate voltage (and the source voltage).
There will be more positive headroom now.
Idealy, you will have about 60% of the battery voltage on R4 for symetrical clipping.
Controlled with R2.
Changing R2 will also change the input impedance, which is about 1Meg ohm now.
If you want it back to 1Meg, you also have to adjust R3.

Low noise J201 fets have a low cutoff voltage, so less problems on low voltage circuits like this.

Buy JFET Transistors Transistor,JFET,J201 Vishay J201-E3 online from RS for next day delivery.

And as said, a standard 9volt block battery is economic and lasts long with this circuit (500-1000hours).


ESD Tip: stick a bare, very thin wire through and around the legs of the FET, so all three pins are shorted together when handling/ soldering.
Remove that wire when all the parts are on the board.
The FET has a much higher survival chance this way.
Leo..

[ickefes]

Thank you LeeYoo but that site says the item is out of stock. All sites I find says there have no stock. I know it might be a lot to ask but can you find a suitable replacement over at 2N | Electrokit ? I wouldappreciate the help so much. Regards.

[LeeYoo]

I have sent you a PM.

Read the datasheets online.
Any small (<10mA) lownoise FET with a Gate-Source cutoff of less than half the supply voltage can be use.
But some have a cutoff of up to 8volt!!
That means buying 10 of them, and selecting.
Leo..

[tINY]

Remember, too, that V(gs)off is not a tightly controlled spec - The J201 spec says it can be anywhere from -0.3 to -1.5 volts. So some fiddling may be required....

On the battery issue, why not stack 3 or 4 of these?
Buy - Bluecell 25 Pcs CR2032 Lithium Button Cell Battery 3V for Watch Toy Calculator + Bluecell Cable Tie




-tINY

[LeeYoo]

In my experience it's never as bad as listed in the datasheets.
My last batch of 10 J201s were all between -0.5 and -0.8 volt.
Seems the TO-92 is not in production anymore.
SMD version (SST201) is still available.
http://octopart.com/sst201-t1-e3-vishay-122633
Leo..

[ickefes]

Tiny, that is a great idea. They won't take up much space. Thanks!

[mus1k_freak]

Probably not a helpful reply but I built one of these last night in a small enclosure with a j201 running on 9v and it sounds great! Love what it does to my tone. Playing through a Marshall jtm60 with a les paul

Sent from my Galaxy Nexus using Gearslutz App

[ickefes]

mus1k_freak: Where have you put the circuit? In the guitar? In an in-line device or as a pedal on the floor using a short cable from the guitar? Regards.

[LeeYoo]

Hi ickefes.
I have made buffers like this with a very short (low capacitance) lead to the guitar.
With phantom power and/or battery power.
With a rotary switch with various capacitors, to shift the peaking frequency of the guitar pickup between 2 and 6 Khz.
And a rotary switch with various load resistors to control the dampening of that peak.
Two 12-position rotary switches to quickly dial back exact settings.
Some pickup explanation here. Look at the graphs.
BuildYourGuitar.com :: The Secrets of Electric Guitar Pickups

I use a >1mA main FET, and a trimmed 0.5mA FET constant current source instead of R4.
This however requires measuring and selecting the FETs.
Leo..

[mus1k_freak]

Originally Posted by ickefes

mus1k_freak: Where have you put the circuit? In the guitar? In an in-line device or as a pedal on the floor using a short cable from the guitar? Regards.

I built it in a real small pedal. 1090 maybe? The ones that you can barley fit 2 jacks in. I find with some pedals you don't need it on, like a tube screamer has a buffer built in but some pedals don't so I wanted it to be switch able.

Sent from my Galaxy Nexus using Gearslutz App