Convert Electret Powert Barcus-Berry 3000A

[joenoone]

Hi,
I have a BB preamp/EQ that I want to convert to use for a piezo and was wondering if anyone has converted one of these. Electret mics need power, and on the board is labeled a resistor that supplies that, so I was wondering whether I just remove it or replace it with a straight wire to remove the power to the microphone input. Anyone done this before?

 

[Rockin John]

There's more to this than meets the eye. Get a circuit diagram from the web is the best plan, then folk can better help. The safest option is to lift one end from the circuit board. That way the part is not in circuit but it won't get lost, either so can be put back in the future.

For the piezo to work well you need a high input impedance. Some say 1M ohm but I like 10M. You'll probably need to change the chip and some components around it, too, because a mic does not need such high values of input impedance. Also, such high impedance will easily pickup hum and rubbish so proper attention to screening will almost certainly be needed.

Best option is not to mod it at all, IMHO. But get a circuit, anyway.

Best.

J

 

[joenoone]

Originally I thought this didn't work after I bought it used and stored it away, but the problem was I was trying it with a piezo pickup. Recently I got some cheap electret elements to use to test this and it does work when I plugged one in, so I un-soldered one leg of the RX1 resistor and put between that and the board a "1-Bit Slide Type DIP Switch Module" and voila - it works with the piezo when I turned it off and works with the electret mic when it was turned on.

I figured I'd ask if anyone had tried this before doing it myself, but looks like I got to be the guinea pig.

 

[byacey]

What you've done is fine. The RX1 resistor is supplying the 9V B+ for the electret. I'm not sure this has a high enough input impedance for good piezo response.

 

[Rockin John]

Sure. All understood. It looks like you've sorted it. Glad it works.

 

[Ash Hines]

What you've done is fine. The RX1 resistor is supplying the 9V B+ for the electret. I'm not sure this has a high enough input impedance for good piezo response.

Just spoken to the Man who actually makes both the 3000A and the 3000AE at Piezo Kinetics (how's that for detective work!? - and I live in Taiwan, to boot!) and he confirms that clipping out the RX1 resistor should do the trick.
The problem is Barcus Berry - since they were bought by Jam Industries, Barcus Berry is not communicating clearly enough with its distributors - the 3000AE is NOT meant to be used with piezo transducers; Barcus Berry still make the 3000A - but many music stores are now buying the 3000AE thinking that it is an upgrade from the 3000A (Bluestar Music, for example).
The 3000AE is not an upgrade! It's made for an electret mic..

 

[RobbieK]

so I un-soldered one leg of the RX1 resistor and put between that and the board a "1-Bit Slide Type DIP Switch Module" and voila - it works with the piezo when I turned it off and works with the electret mic when it was turned on.

I know this is an old thread, but FWIW, you could have left the resistor there, but put the switch where that zero ohm resistor is above the opamp. (That's the beige resistor with the single black band. Essentially a jumper wire.) Or if it was too close to the battery frame, you could put the 6.8k Rx1 there instead, and put the switch in Rx1's original position.

Also FWIW, the LF442 is a fet input device, so the preamp's input z is essentially the value of R6; 2.2M. Yeah this is a bit low for most piezo pickups, but let your ears decide of course. If it sounds a bit dull in the treble and lacking punch in the bass, you could increase this resistor to say 4.7M.

If you only ever use the pre with a piezo, you could also replace the input cap; C4, with a jumper wire. Piezos won't pass DC so I never bother with a coupling cap, and this might give you a deeper tone. I'd also check the value of C6. You could probably remove this altogether for a clearer treble, but keep the wire from the piezo short and use good quality cable with a braided shield.