piezo buffer mod for boss GE-7 eq pedal

[Jimbo78]

Hi all, found an old ampage thread about adding a high impedance buffer to the GE7. The author describes the mod as follows:

"I wanted to use a common Boss GE-7 pedal for a piezo preamp. It has an input impedance around 500kohms, one tenth of that which would be a good match for a bare piezo. Unfortunately, the input shunt resistor is part of the IC, so it wasn't possible to simply replace a 470k resistor with a 4.7 megohm unit. R.G., Gus, and others suggested adding an FET buffer to achieve the desired high input impedance.

Well I finally got around to doing the surgery on a GE-7 pedal. The buffer is a 10kohm source follower, using a 2N5457 with a 4.7meg gate resistor and a .02uf gate isolation cap. My prototype circuit was built on a perf board, about half the size of a postage stamp. There's just enough room to sandwich it between the input jack body and the top of the case. "[username 'Doc']

Well, AFAIK, the pedal has a standard 1M imput impedance, and I'd like it to be 5M or 10M. Main problem is, I can't quite picture the circuit described above. Can anyone give me a hand, please? The author thought it worked well. They also described it as being on as long as a jack was inserted, whether pedal was on or bypassed.

Here's the GE7 schematic for reference:http://www.hobby-hour.com/electronic...-schematic.png

Jim

[J M Fahey]

Substitute parts values suggested by "author" in:

*or* substitute R29 on GE7 with 2M2 or 3M3 and give it a try.
It should reasonably work, although we do not know what IC5 is. Do you?

[Jimbo78]

Hi, thanks Jim, you're prolific!

IC5 is some kind of (proprietry?) opamp device, in an SIP package called HA1457W. I can't find a datasheet for it anywhere.

I'll try that mod and post an update when I've tested it.

cheers

[J M Fahey]

HA1457 - Google Search

About 8,410 results

Although they do not specify input impedance or bias current, from what I see (non-darlington single bipolar input transistors) it will have problems with high value input bias resistors; the factory recommended value is 100K; 470K is probably as high as it will take, so discard my suggestion of using 2M2 or 3M3 resistors there.
If you dare (I would ) , and space permitting, of course, I would "dead bug" mount an TL071 there, which allows for an up to 10M input resistor.

[pdf64]

Also see
Stop Tone Sucking cheaply and easily in your wah pedal
for ideas as to how you could implement a JFET buffer (change the 1M to 10M for your application).
Pete.

[Jimbo78]

buffer schematic draft

Hi all, thanks for your comments.

Here's a draft of the circuit I plan to build. Any comments?

1. I'm confused about the decoupling...do I need an output cap when it's feeding straight into the GE7's input cap?

2. And I should measure voltage across point marked 'x' to ground? I can't remember if it's drain or source voltage. I remember it should be roughly half supply voltage (9V), 5-7V, and adjust resistor to match?

Finally, any suggestions on which JFET is best? I use adaptor with pedal, so current drain less important that performance. I'm using with a piezo on an acoustic guitar, so I want something that won't clip/distort easily. I know there's another way to configure the input resistor with "Vr" suppy. Is that likely to be necessary?

Jim

[J M Fahey]

.do I need an output cap when it's feeding straight into the GE7's input cap?

If you build it "fixed" inside the GE7, the input cap is all you need, it's already there.

2. And I should measure voltage across point marked 'x' to ground? I can't remember if it's drain or source voltage. I remember it should be roughly half supply voltage (9V), 5-7V, and adjust resistor to match?

Drain voltage (X) will be +9V, since it comes straight from that rail.
Source voltage will be 4.5V+1.5 or 2V, according to Fet used.

Finally, any suggestions on which JFET is best?

Use any N Fet you can get.
R2=4k7 to 10K

[thehallofshields]

I was considering a project like this for the GE-7 for quite sometime. I even tried to contact the guy you quoted in the original post to see how it all worked out. Didn't get a response...

My plan was to put a simple J-Fet buffer circuit on a switch between the Input Tip and the PCB (I think it is usually a brown wire, Pad#18) and, of course, tap the 9V power supply to feed the J-Fet. But I decided against it because I was led to believe that the cable capacitance would still load-down the PZT. -Is this correct?

[Jimbo78]

Yes, I've thought about that too. I think for 1M to perhaps 3M a short 2ft cable might be OK, but I'm using 10M, so I've actually built a buffer in a box and mounted it real close to the guitar for the internal pickup. Sound is good, smooth and warm, perhaps too much bass. I'm going to blend with mic anyway, to get the high end.

So, I think a preamp/buffer on-board might be better than modding the EQ pedal (already has Monte Allums mod--huge difference).

Does anyone know at which impedance & cable lengths cable capacitance becomes an issue?

My circuit is not the same one posted here, I'll post shortly. Still breadboarding some other configurations to see what is best.

By the way, a question for all: for an input impedance of 5M or 10M, acoustic guitar, piezo pickup; how do I choose the 'correct' input and output capacitors? I've seen preamp/buffers with input decoupling caps ranging from 0.022uF to 1.0uF, generally MKT type, and outputs from 0.1uF to 10uF, generally electrolytic. What's the relationship between input cap + resistor (to ground after cap, before gate) and bass roll-off?

Cheers

[Jimbo78]

prototype schematic--any suggestions?

Hi all,

here's the circuit I built. It sounds OK, but I want to be sure it's within sensible parameters.
I'm not sure about the 'reference voltage' set-up running from the base of R1. I adjusted R2 and R3 to get 5.5V across source resistor (R4). Is that what it should be?

Further questions: (sorry, I'm struggling with theory of how these things work!)

• what is the output impedance? Is it the same as R4?
• what is the bass frequency rolloff with C1/R1?

Is the input impedance 10M?

What I want is....a nice, clean piezo buffer for acoustic guitar that:

doesn't clip
frequency roll-off starting around 100-80Hz.
runs off a 9V battery with sensible power consumption
5-10M input impedance
1-5K output impedance or whatever suits running into 10k line inputs
preferably uses JFETs J201 / MPF 102 / 2N5458 as I have a bunch...

Thanks for your help! If someone felt like posting a couple of formulas using this circuit's values as examples...well, then hopefully I'd be able to design my own circuits in future!

[J M Fahey]

Yes, it's fine, will work properly.
The input impedance is 10M.
The output impedance is 1/Gm (Fet transconductance), in parallel with the source resistor.
Without going through the Fet datasheet (which by the way is approximate, given the huge parameter spread Fets have) you have around 500 ohm to 1K5 there, low enough to drive quite a long cable.
As of low frequency cutoff, you should know your Piezo's capacitance.
Many multimeters have such meters; if yours does, post results.