AMZ Jfet Buffer-Splitter

[amz-fx]

I just posted a new article in the AMZ Lab Notebook with the schematic and perfboard layout for this simple splitter design.

http://www.muzique.com/lab/main.htm

Discrete transistor fun. Enjoy!   

regards, Jack

[gaussmarkov]

I just posted a new article in the AMZ Lab Notebook with the schematic and perfboard layout for this simple splitter design.

Cool.    Isn't that a veroboard layout? 

[amz-fx]

Actually I always saw them called stripboard, but yeah, vero.  That's what I get for writing at 6am 

Thanks, Jack

[Gilles C]

The generic name is a stripboard.

A Veroboard is a stripboard manufactured by the Vero company...  Not a lot of people use Veroboard 

http://www.verotl.com/products/index.cfm?content_id=2424A965-4AF2-433B-9589A039D35545F5

Just like a Klenex that in reality is a facial tissue. 

Gilles

[gaussmarkov]

The generic name is a stripboard.

A Veroboard is a stripboard manufactured by the Vero company...  Not a lot of people use Veroboard 

http://www.verotl.com/products/index.cfm?content_id=2424A965-4AF2-433B-9589A039D35545F5

Just like a Klenex that in reality is a facial tissue. 

Gilles

waddya know!  learn something new every day.    

[mars_bringer_of_war]

Mr. Orman:
There's one thing that's got me confused about the buffer. You address the possibiity of the signal being too bright, recommending lowering R2 and R3 to 470k. You then go on to say that raising those resistors to 10M or higher will make for a more neutral (to use your words) sounding buffer. Wouldn't raising those result in even more brightness?

[brett]

I think the comment relates to "fixing" the sometimes muddy sounds of humbuckers (esp cheaper Korean and Chinese ones).
IMO it is sometimes worth lowering the input impedance even further (R2 and R3 could be as low as 220k each).  Particularly for "bright" single coil pickups and wah pedals.
cheers

[John Lyons]

Handy little circuit there, thanks Jack!

John

[speed12]

Hi folks,

Just got a couple of questions about this circuit which could make it EXACTLY what I am looking for!

I'm building a board up at the moment and although most of the pedals are TB, a couple are buffered (Boss TU-2 and DD5 and Line 6 Verbzilla). My plan is to put these pedals in TB loops and then have a dedicated buffer at the start of the chain and then finish the chain off with a Barber Launch Pad to fire the signal to the amp.

What I would like to do with this circuit is have two outputs, one for the main signal to the board and a second to go to the Tuner. I would also like to implement the mute switch Jack talks about for the main output to allow silent tuning for the tuner. It is this I have a couple of questions about:

1) It says to put an SPST across R9 so that when open everythings normal and when closed the signal goes to ground and its muted. If the signal goes to ground when the switch is closed, then why is the resistor there anyway (without the switch it is effectively "closed" all the time)?

2) How would I implement an LED on the mute switch so that the LED goes OFF when the output is muted and ON when the output is normal? Or even better which I just thought of, how would I use a two-colour LED to have green for normal and red for muted?

Cheers in advance!!!

[Processaurus]

Nice, I probably wouldn't have thought to use one bias network for all of the jfet gates.  Speaking of which, would there be any advantage to using noiseless biasing from a decoupled V1/2 supply?

[R.G.]

Speaking of which, would there be any advantage to using noiseless biasing from a decoupled V1/2 supply?

There is nearly always a noise advantage to using a decoupled bias string and a series resistor. It eliminates the current noise component of the bias string, and also decouples any supply noise that exists. It's less of an advantage with single JFETs, but it's still a worthwhile consideration.

Nice, I probably wouldn't have thought to use one bias network for all of the jfet gates.

This does bring up the question of why multiple JFETs are needed for the outputs. The point of a splitter is to (a) drive multiple outputs while (b) not letting what's happening on any of the outputs affect any of the other outputs. One way to do that is the one presented, using multiple JFETs. The JFETs being coupled only at their gates and power supplies means that any loading on one output can't get back to the joined inputs, so it can't affect the other outputs.

However, it may not be necessary. If you posit that the things being driven are relatively high impedance, say like guitar amps at 1M, then it's sufficient to have the source impedance of the driver be less than 1/10 of the combined impedances being driven. If that is true, a wide variation in loading on one of the loads will not affect the others significantly. So if you're driving four 1M guitar amp inputs, the buffer sees a combined load of 250K. If it's output impedance is less than 25K (as a rule of thumb) then one of the driven inputs can change a lot and it will make very little difference to the signal the others see because the single buffer can drive the load with no notable change in output voltage; it can supply plenty of current.

The output impedance of a JFET source follower is typically about 1/gm. Most small signal JFETs are in the range of 200 ohms to 2K ohms in this circuit.  Cable capacitance is probably the issue to be solved. 

A 1k output impedance will drive a capacitance of C = 1/(2*pi*1000*20000) = 7nF to full audio range. Since guitar is not a full range device and many effects deliberately cut this back, you could call this 15nF and get away with it fine. Common coax cable has about 25pF per foot of cable, so you could connect up to 300 feet of cable to a single JFET buffer and not have the cable capacitance cause more than a 20kHz rolloff point. So a single JFET buffer probably gives you most of the value of having a single JFET per output.

Of course, if you needed to,  you could use multiple JFETs and put up to 300 feet of cable on each one. Talk about stereo separation... 

[speed12]

I was thinking about my two questions above earlier and realised I can actually answer them myself!

1) I misread the text and thought it meant put a switch so that R9 is totally taken out of the circuit. This is of course wrong! It is a switch ACROSS R9 so that when open some of the signal is pulled to ground through R9 but when closed, the signal bypasses R9 altogether and goes straight to ground, thus muting the signal.

Would this method cause a pop?

2) as for the LED question, it is a simple use of a DPDT switch. One set of poles would be used for the switch across R9 and the othe rset of poles would control the LED. For a two colour LED three lead LED, you would simply put the common lead to power and one of each of the others on each pole.

So there we go, please correct me if I'm wrong!!!!

[petemoore]

  thank you again Mr. Orman.
  This is a piece I didn't know how much I'd like until I built it, I knew I was overdue for having a splitter.
  I have it splitting / driving 2 PB's, 2 amps and speakers, Works great and added a ton of flexibility, very easy to wire up [I fully wired only two outputs, the third is right there just in case I decide I want it].
  It affords me a very useful/valuable amount of new utility.

[gaussmarkov]

  thank you again Mr. Orman.
  This is a piece I didn't know how much I'd like until I built it, I knew I was overdue for having a splitter.
  I have it splitting / driving 2 PB's, 2 amps and speakers, Works great and added a ton of flexibility, very easy to wire up [I fully wired only two outputs, the third is right there just in case I decide I want it].
  It affords me a very useful/valuable amount of new utility.

hi pete!  it sounds like you are setting yourself up for some ground loop problems.  or have you isolated the grounds for the two amps somehow?  --gm

[deeno]

Hi everyone, sorry if I'm resurrecting this post, I wanted some suggestions on small changes to make to this project.
the idea is to split the signal into 3 to have 3 potentiometers which will be used to level the volume differences of 3 basses.
Each potentiometer can be activated with its footswitch.
What potentiometer value do you suggest?
the 3 signals are then mixed into a single output that will go into the pedal chain.
To do this, will a resistor between each potentiometer and the output be sufficient?
if so, what value?
Thank you very much, I hope I was clear.
 Greetings from Italy!

[antonis]

[deeno]

Thank you very much Antonis!

[deeno]

antonis, sorry if I take advantage of your availability, do you think it would be better to put the switches before each buffer or after?
the idea is that when turned off they function as mute of each individual channel. Thanks again

[antonis]

In theory, it makes no difference..

I'd suggest to place switches right before respective JFETs Gates..

But I don't know the possible behavior of an open Gate & unbiased JFET (ahhh those        devices..) so better use DPST switches for Gate and OUT ON-OFF..

If it's not getting too complicated, I'd proceed with DPDT switches..
(for grounding both IN & OUT of bypassed channel..)

P.S.
I might have confused you without a wiring diagram but I'm not able to draw it for the time being..

[deeno]

Thanks a lot antonis.



if I put a DPDT switch immediately after the 10k resistor as in the diagram you see, and use the other half of each switch for the respective LED, it should work.
so I send the output of each buffer to ground without touching the gates of the transistors!
consider that they will never play the basses at the same time, only one at a time.