Hello ... and buffer question

[GuitarPix]

Hello all, my name is Neil and I'm just getting into the electronics side of playing guitar. I've got a plans for an optical compressor and a fuzz box to start with. I've been wiring guitars and playing with electricity most of my life (and I'm in my mid 50's), but this is a bit of a learning curve for me.

My eventual goal, if building effects goes well and I don't throw my soldering iron through a window, will be to build a simple low watt tube amp and then move on to bigger amps.

Now for my pressing question. I've got an ElectroVoice R300 wireless mic (lavalier style) with BP300 bodypack. There's a way to use it for instruments and I wired up a TA4F (mini-xlr) connector to a 1/4" plug and fired it up. There's sound but its low volume and very muffled sounding. However if I put a pedal with a buffer in the path it sounds great. But, if I'm strapping this thing to my guitar strap, having my extra tuning pedal on there is kinda awkward.

So - is there a simple buffer I could wire up for this (I have started collecting parts and have some 2N5088s on hand for example), or is there a better solution?

[antonis]

Hellow Neil..!!
(and greetings to your age... )

There are many buffer circuits but the "one transistor buffer" (BJT or FET) is simple enough...

I presume that your mic has low enough resistance so there is no need for a high input impedance (like JFET or Mosfet) so if you bias the 2n5088 with a Zin of 100k total (or so..) and a couple of 100nF IN & 10μF OUT caps should be fine..
(where Zin  = R1//R2//βRe if you'll use a voltage divider for base biasing and OUT cap may be fitted according to next chain device..)

I'm not posting any schematic because there are too many of these all around..
(just a basic approach: http://www.muzique.com/lab/buffers.htm

P.S.
Many guys  will follow trying to convince you about building an Op-Amp buffer (and they will be absolutely right.. ) but I responded taking in mind our age...
(which age is almost the halfway point between Vacuum tubes and Operation Amplifiers..)

[GuitarPix]

Hello Antonis, thanks for the info - great page.

I have to admit most of your explanation lost me as I'm just barely into this subject. The into to the buffer will be from a guitar which will be a higher impedance - so would a JFet be needed rather than the 2n5088 i have on hand?

[Transmogrifox]

I have to admit most of your explanation lost me as I'm just barely into this subject. The into to the buffer will be from a guitar which will be a higher impedance - so would a JFet be needed rather than the 2n5088 i have on hand?

2N5088 will be great.  You will be able to get the impedance you need from a buffer made from one of these.

From the AMZ buffers page linked above, I would think this would be your best, and still easy/low parts count:

You may not need something as big as 10uF on the output if you can find a cap with much smaller physical size in the 0.1uF to 1uF range.  It all depends on the input impedance of what you're driving, you may lose some low end response.  Just a heads-up if physical space is constrained by the cap value then you may want to breadboard and play with this until you get something with adequate low end response but small enough cap package to fit.

This is a situation where I personally would go the Surface Mount components route due to the size.  I know SMT isn't for everybody, especially if your eyes aren't what they used to be, but a simple magnifier lamp from Wal-Mart and a set of fine tweezers  goes a long way toward making it possible.  0603 resistors and capacitors along with SOT-23 transistor packages go right down on the copper-pad perf boards and usually fit nicely with the pad spacing if it's a standard size.

[GuitarPix]

Thanks Transmogrifox

I was looking at that one and as soon as the rest of my parts get in (I ordered a bunch of variety packs of resistors, capacitors, diodes etc off of ebay so I'd have some parts to experiment with) I'll play around with it. I do have a breadboard to start with so that'll be a good place to play.

[Kipper4]

Hi Guitarpix.
Welcome to the forum. You Sir are in for a lot of fun.
Great place to start with a nice simple buffer. Before you know it your shelves will be full of little containers full of componants. Enjoy.
Rich

[antonis]

The into to the buffer will be from a guitar which will be a higher impedance - so would a JFet be needed rather than the 2n5088 i have on hand?

As Transmogrifox (what a nick..!!  ) told you, the attached buffer should be fine..

You only need to form a voltage divider with 2 X 10k to 22k resistors for Vr ( higher values combined with 510k resistor will affect divider's voltage ) or bias Q1 base straight from a voltage divider of 2 X 1M resistrors -  although such high resistor values are not commonly used for BJT biasing...
(you'll have in both cases a base current of 900nΑ and a Collector current of 270μΑ - supposing a minimum hfe of 300)

[PRR]

> form a voltage divider

Actually, since this is a battery-job (no buzz), and the signal level is very small (mike level), you could simplify to this:



This depends on transistor hFE being not-small and not-huge. At the currents we will get, 2n5088 hFE should be in the range of 300-1,000, which with _3V_ (2 AA) supply will give 0.65V-1.3V at the emitter, 60uA-130uA current. If you tap the +5V supplied for the lavalier head, current a bit higher, fine. Emitter impedance around 200 Ohms which should be an excellent "fake microphone". Impedance to the guitar side will be like 500K, which is fine.

BP-300 specs:
http://www.parts-express.com/pedocs/specs/242-6336-electro-voice-bp-300-b-specifications.pdf

[GuitarPix]

Thanks Paul - I'll try breadboarding that when the rest of my parts arrive.

[GuitarPix]

IT WORKED

I put together the simple buffer.

At The Source (what Canadian Radio Shack's are now called) found a 4-AA battery box, and only needed two batteries but figured the circuit could be fit inside the box after removing one divider. It also has a built in on/off switch so it actually looks pretty good (other than the grey cables coming out - black would have been better.

I know most of you could have put this together in the time it takes me to sneeze, but this was the first time I've done something more complicated than soldering two wires together. Amongst everything else, I had to figure out how to lay out the circuit based only on a diagram as above without having a pre-done layout.

[Kipper4]

This

[antonis]

IT WORKED

Nice job..!!! 


(and nice interference friendly plastic box) 

[GuitarPix]

Nice job..!!! 


(and nice inderference friendly plastic box) 

Yeah - wondered about that, but will be testing it in a pretty harsh place thursday, may have to add shielding - but so far its pretty quite. Though I did opt for shielded cables to and from.

[antonis]

Though I did opt for shielded cables to and from.

You've done most of the job and it's a pity to break the shielding "continuity"..

(O.K. in your case it's a buffer so it doesn't amplify anything but you create a weak point for noise "insertion" and further interaction with the rest of the chain..)

[bool]

Wrt plastic box. There are ways to make small, uncomplex (!) circuits virtually bulletproof even if they aren't enclosed in a metal box. I designed and used several buffer/preamps (a buffer with 2dB gain and +/-1 dB or so shaping) and used them in late 80's and thru 90's live and recorded on several albums (me as well as others) - circuits were enclosed in small ABS enclosures. Worked well in then-current hostile environments (stage lighting) and in 90's cellphone era. Both belt-mounted and held in-hand (which is by nature more susceptible to interference), as well as (in certain cases) even in-pocket.

To add, I even didn't use a self-adhesive conductive tape nor paint. So, it's doable.

I'm also led to believe that if in today era of RFI pollution, you stick to instruments with active PUs and keep impedances scaled accordingly, and take certain precautive measures, you can produce sufficiently RFI immune circuit on a single-sided board, in a plastic enclosure (or held free "in-air").

Of course, you can also use graphite conductive paint on the inside as well ...

[PRR]

> interference friendly plastic box

True, but the shielded wire runs right to the board, and the total area of the circuit is very small.

In radio terms, it is a "small antenna". Won't pull in much signal.

Of course I am old-school where 6-foot waves were the smallest we used for broadcast power. These days we have Giga-Hertz waves, which must be really small, stuck on everybody's ears/rears.

But the desired signal comes from a guitar. Over a foot of wiring in there, better antenna. Some guitar-guts are obsessively shielded, but many hardly bother. First-guess, the guitar is more likely to suck-up more radio than this small buffer. Of course every axe, build, and gig is different.

[antonis]

But the desired signal comes from a guitar. Over a foot of wiring in there, better antenna.

I'm not sure if a Faraday cage could be considered as an interference eliminating stompbox but it definately should be a fancy gear..!!