The nerds' corner

[RobA]

Simple circuits reduce regulator noise floor

The "capacitor multiplier" circuit looks really interesting. Wonder how viable of an idea it is to replace large power supply caps with say a 10uF ceramic, a resistor and a transistor.

Interesting stuff there. I wonder how the other characteristics of large caps come in to play and how these multipliers work with it. It seems it would be useful for power supply applications in SMD settings if it works well.

I did some playing around with using an op amp based capacitance multiplier to make the high shelf circuit of a parametric EQ movable. I remember it working pretty well. Which reminds me that I need to go and play with that some more.

[midwayfair]

I forgot to post this little bit of experimentation here back when I went through it.

First, here's an extended discussion.
http://www.diystompboxes.com/smfforum/index.php?topic=104341.0

Here's the results of my experiments:

I got some time to do some experimentation with the FETs I have here. I used a 10K for the source resistor for the tests just to get a sense of what they did. The supply voltage was 18V and the gate bias resistor was 2.2M. No positive bias on the gate.
I tested:
2n5457 (fairchild from Smallbear and the mystery ones from Tayda)
2n5458
2n5485
2n5952
J201 (fairchild from Smallbear and the mystery ones from Tayda)
MPF102 (fairchild from Smallbear and the ones from Tayda that seem to be the real thing)
2SK30
2SK170

MPF102 ended up with the highest source voltage, about 3.5V (all were above 3V out of the ones I had). I was a little surprised by this based on the datasheets.

The second best was 2N5458, about 2.5V.

2n5457, 5485, and 5952 all ended up around 1.5V-1.7v, despite the differences in Vgs on the datasheets. 2SK30/170 were a little closer to 1.5. The supplier didn't seem to matter in this usage even though I know that many of the Tayda ones have less output when used in a booster.

J201 was all the way down at .7V! And I could indeed hear it distort with some humbuckers.

MPF102 is fairly common, but the 5458 is a little cheaper.

I went back to an MPF102 on the breadboard to mess with the source resistor some, and no matter how big I make the source resistor it never got above a certain level (about 3.6V). None of the other FETs I have on hand get anywhere close to that.

Okay, I know that 3.5V is VERY BIG for a guitar signal, and anything in front of the buffer that might be creating a signal that big is probably a dirt pedal, so any buffer distortion is not likely to be too noticeable or objectionable. But it's certainly nothing like the half supply you guys have mentioned. I don't exactly have a huge collection of FETs, but these are a fairly wide range.

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Also, Brian at Runoff Groove suggested 2N3819, which ends up with an even higher emitter voltage (over 4v).

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Hope some people find this useful. In particular, you might want to consider swapping out J201s in your input sections for a different FET as noted here. For instance, I've been using MPF102s for buffered bypasses (one in my Fatpants 2013 and one in my Zen Drive because that circuit sounds like ass unbuffered).

[kothoma]

Thanks Jon, that's very useful.