AMZ Mini Booster

[Don]

I'm building an AMZ Mini Booster again. I built one a couple of years ago, wasn't in love with it and am building another. My needs are a bit different now.

I'm going to include a tone control (I might make it switchable).

Has anyone here built the buffered version? I'm interested to know if there's a sound difference.

[R.G.]

Has anyone here built the buffered version? I'm interested to know if there's a sound difference.

I haven't looked at that in a while. Did Jack post a buffered version?

In any case, the issue with buffering is that the output of the unmodified mu-amp (which is what this is) is quite high impedance, and any loading on it lowers the gain you were trying to get. I dug through some of this in a couple of articles at geofex. The first one looks at the development of the mu-amp:

Foolin' with FETs

And the second looks at a one-resistor change to get the mu-amp to buffer itself from any loading issues.

Mod Your Mu-Amp

The SRPP mod (second article) is particularly easy.

To answer your question: yes there is a sound difference if what you're driving is much less than 1M impedance, and no, there's no difference if what you're driving has a big input impedance. This last amounts to buffering the mu-amp all by itself. The frequency response of the mu-amp all by itself does not change when buffered unless you goof up the buffer.

[d95err]

I'm building an AMZ Mini Booster again. I built one a Has anyone here built the buffered version? I'm interested to know if there's a sound difference.

Do you by any chance mean the AMZ Mosfet Boost? It has a buffered version shown in the article. I have built it (buffered) and it has a permanent place on my pedal board. There's always a use for it.

[Don]

He has a schematic for the buffered version of this booster-
Guitar Effects - AMZ Mini-Booster

It basically adds an NPN tranny with a 4.7k resistor after the other trannies.

[R.G.]

He has a schematic for the buffered version of this booster-AMZ Mini-Booster
It basically adds an NPN tranny with a 4.7k resistor after the other trannies.

The link doesn't show the buffer. unless I'm missing something. Compare the "Mini-Booster" to the mu amp in the article. The unbuffered mu-amp has a quite high output impedance and is susceptible to loading.

A single NPN transistor (if that's the buffer) may or may not be good enough to buffer it. It's certainly better than nothing, I guess. A JFET source follower would help more. Even better is inserting a 1K resistor between the source of the top JFET and the drain of the bottom JFET, taking the output from the source of the top JFET. The 1K isolates the top JFET source, letting it buffer the output without loading down the lower JFET drain. It's a nice trick.

I'd expect a simple NPN buffer to color the sound, but I can't really say without seeing the schemo.

[Don]

I'm sorry. I meant to say that this was a schematic to the regular mini-booster and to describe the buffer.

I'll see if I can post a link to the buffered version. I don't want to accidentally post any link that Jack sells.

[JoeM]

The link doesn't show the buffer. unless I'm missing something. Compare the "Mini-Booster" to the mu amp in the article. The unbuffered mu-amp has a quite high output impedance and is susceptible to loading.

A single NPN transistor (if that's the buffer) may or may not be good enough to buffer it. It's certainly better than nothing, I guess. A JFET source follower would help more. Even better is inserting a 1K resistor between the source of the top JFET and the drain of the bottom JFET, taking the output from the source of the top JFET. The 1K isolates the top JFET source, letting it buffer the output without loading down the lower JFET drain. It's a nice trick.

I'd expect a simple NPN buffer to color the sound, but I can't really say without seeing the schemo.

R.G,

There is a version of the minibooster with a buffer (a single transistor). But when I purchased the PCB from Jack he gives a link that he requests not be publicly posted. Otherwise I'd share, but will respect his request.

[R.G.]

There is a version of the minibooster with a buffer. But when I purchased the PCB from Jack he gives a link that he requests not be publicly posted. Otherwise I'd share, but will respect his request.

No biggie. Jack's secrets are his own. I'm not terribly interested in a "mini booster" with or without a buffer. I was just trying to help Don.

With no more info, I would ditch any NPN buffer on a mu-amp. The second and third illustrations in the "Mod your Mu-amp" articles provide superior performance to any single NPN buffer. When I wrote that article, I did a lot of simulation work on the proposed variations and then breadboarded them. They really help keeping the loading from losing the high voltage gain you tried to get by going with a mu-amp.

In fact, from the work I did to write that article, I'd never consider building a bare mu-amp for my own purposes. An SRPP is by far superior to the mu-amp in terms of not being susceptible to outside problems, and costs only one more resistor.

You'd have to use another resistor anyway with a buffer NPN, and then you'd have to worry whether the external NPN was doing funny things to the mu-amp.

If I had to use an external buffer, I'd put a JFET in instead of the NPN (second circuit in the Mod Your Mu-Amp article) and get much higher buffering.

But then that's just me.

[JoeM]

I'm building an AMZ Mini Booster again. I built one a couple of years ago, wasn't in love with it and am building another. My needs are a bit different now.

I'm going to include a tone control (I might make it switchable).

Has anyone here built the buffered version? I'm interested to know if there's a sound difference.

Why not try it both ways, with or without the buffer? I still havent completed mine yet, I built the buffered version since I already had a suitable transistor.

[Don]

I think I'll build it without the buffer. I'll just tack on the jumper for the third tranny rather than solder it through the holes so it's easier to remove. That way I can add the buffer later.

[Don]

There's a big difference with the buffered version! It's got a lot more gain and it's quite a bit brighter. Maybe it would sound good pushing 100 feet of cable and a bunch of effects but it's way too much played alone into a tweed Deluxe clone!

The non-buffered version is just right. I like it with the tone control. It takes a bit of the edge off, even with the tone control all the way up, compared to my last build without the tone control.

[R.G.]

There's a big difference with the buffered version! It's got a lot more gain and it's quite a bit brighter.

That's another way of saying "With the buffer, the gain is not pulled down by loading, and the treble is not removed." Same thing, different words.

[Don]

I had the feeling that I was hearing the booster as it really sounds- without two 10 foot long cords, 4 jacks, a volume pot and a tone pot.

That's not always better.

[Mark Hammer]

Mine does not have a buffer, and quite frankly that's probably a good thing.

When amps are pushed hard enough to break up, you don't really want a lot of top end going into the amp, since that will just produce a harsher breakup. One of the reasons why many ostensibly "clean" boosters come with tone controls to tame the treble in preparation for pleasing amp breakup.

The context where having the buffer would be preferred would be those cases where the mini-booster is being used to optimize S/N ratio by taking advantage of the clean headroom in a signal path. Or, for example, if you were feeding a line input direct, on a mixer or power amp. Presumably, in those cases, anything that conserves bandwidth is a good thing. If you're overdriving a subsequent stage, though, full bandwidth is NOT what you want.

I suppose the most practical compromise would be to have a buffered mini-booster with a treble rolloff control so that all contexts could be properly anticipated.

[R.G.]

I suppose the most practical compromise would be to have a buffered mini-booster with a treble rolloff control so that all contexts could be properly anticipated.

It would, indeed. And this can be easy - series cap out from the buffer, series resistor of about 4.7K, then a cap in series with pot of about 50K to ground. The pot is set up as a variable resistor. That is, the classical guitar tone control to eat treble.