Author Topic: Yet another minibooster/muamp question  (Read 1588 times)


Yet another minibooster/muamp question
« on: December 01, 2005, 12:40:03 AM »
I am using a minibooster/muamp as a preamp into a power amp and I am having clipping issues. It seems that the muamp has too much gain and not enough headroom from a 12V supply. Are there any issues with making the source resistor in the bottom jfet large? Say 22k or even 47k?

I only need roughly 20dB of gain and I am finding that the larger I make the source resistor, the higher I can go with the input without clipping.



Re: Yet another minibooster/muamp question
« Reply #1 on: December 01, 2005, 07:12:10 AM »
You should first try a different transistor type for the bottom jfet. An MPF102 will be lower gain, for example.

Then you can begin to adjust the size of the source resistor.  Making it larger will decrease gain but also change the bias at the output.  This is okay if you can find the right gain for your circuit.

regards, Jack


Re: Yet another minibooster/muamp question
« Reply #2 on: December 01, 2005, 07:56:12 AM »
No offense, but isn't a mu-amp designed for the highest gain possible.
Isn't a single MPF102 a better joice?
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Re: Yet another minibooster/muamp question
« Reply #3 on: December 01, 2005, 09:03:51 AM »
The simplest thing to do is to load it down. A mu-amp is sensitive to loading, so a capacitor/resistor to ground on the output will lower gain directly. A lower resistor will lower the gain directly without changing the bias at all.

Er... you could also lower the INPUT by making the input resistor to ground be a 1M pot. Again, no change to biasing, transistors, etc.

A lot of times the answer to headroom problems is to turn it down. Counterintuitive, eh?

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Re: Yet another minibooster/muamp question
« Reply #4 on: December 01, 2005, 09:54:01 AM »
Jack, I though of using a 2N5458. I may try that. Is there a "safe" range of values for the source resistor, or a good way to check that the output bias is still correct?

Steben, no offense taken. One of my initial reasons for using the mu-amp was the high gain with only one jfet in the signal path. I just didn't realize how high the gain was :) Another consideration is that the mu-amp seems to handle "noisey" wall warts very well!

R.G., you know, I was already loading down the output a bit and never thought to take it a step further. I have a buffered output stage since I am driving a very low impedance load. I originaly had no resistors on the gate of the output stage but I added two 1meg resistors to help the low end loss and tame the gain a bit. I may just reduce them!


Re: Yet another minibooster/muamp question
« Reply #5 on: December 03, 2005, 02:15:49 AM »
Ok, here is what I have come up with so far:

J1 is a 2N5458 - lower gain than J201
J2 is a 2N5457 - lower gain and I have lots ;)
Q1 is an MPSA18 - IMHO this just sounds better and dosen't distort as quickly

Ignore Rload and the AC input, they are just for simulation. I also tried various bipolar trannys for J2 and distortion went way up. So if you are looking for distortion.....

As shown the preamp should work good enough for the intended purpose. I do not have access to the power amp at home, but the gain should be more than enough. It *will* distort if I push the E string hard enough. I am testing by running into my cheap mixer with the mixer gain all the way down and the volume cut back. I then use headphones to hear the distortion.

I tried it with a 24V supply (3 x 8V batteries) and it dosen't distort even when pushed hard. However, this is with my bass with the weakest output so YMMV.

 I may still increase R2 to 10k. I may still replace C1 with a resistor.