AMZ MOSFET Booster query

[davebungo]

re. http://www.muzique.com/schem/mosfet.htm
I have asked this of Jack Orman but alas no reply.  I am puzzled by the use of a relatively high current MOSFET (the BS170) in a pre-amp circuit.  The device is biased at (relatively speaking) a very low current right on the knee of the transfer characteristic.  What is the idea of this? - I mean why not use a lower current device like the BF245 or similar?  Am I missing something or is it just the sound of the device in this circuit ?
P.S. I haven't tried building it yet - has anyone else tried it out?

[donald stringer]

that circuit is one of the best sounding volume boosters there are. I would probaly think maybe everyone on this forum has built it. I used a 2n7000[I prefer them or you can use the 170 or radio shack 510.

[bwanasonic]

I mean why not use a lower current device like the BF245 or similar?  Am I missing something or is it just the sound of the device in this circuit ?
P.S. I haven't tried building it yet - has anyone else tried it out?

Your assignment is to build the AMZ Mosfet Boost stock with a BS170, and then try a BF245 and write an essay on the difference. Pulling yourself away from your guitar once you try the stock circuit will be the hard part. BTW- I have no idea what a BF245 is or how it would sound in this circuit.

Kerry M

[Ben N]

Sounds like Mr. Bungo is getting a bit of a brush-off.  I, for one, wouldn't at all mind hearing a more cogent response than, "Hey, it sounds good," perhaps from Jack, RG, Zach, or somone else who actually knows what they are talking about.  Knowing WHAT sounds good (and how to build it)is great; knowing WHY is even better.  Of course, the empirical approach of socket & compare is certainly valid.
Ben

[bwanasonic]

From what I can see, the  BF245 is not a MOSFET, but a JFET.  

Kerry M

[mikeb]

Sounds like Mr. Bungo is getting a bit of a brush-off

It might look that way, but possibly it's because no-one knows the answer to his question, or that the importance of the high-current capability / low current use in the circuit is minor compared to the final sound, especially as davebungo says he hasn't built it yet? In the AC world of audio, we aren't concerned with the current handling characteristics of the device we use; and the bias is usually calculated to give maximum headroom before clipping (or intentionally setup to give asymmetrical clipping and so on).

Mike

[RDV]

BF245 is definitely a JFET.
http://www.semiconductors.philips.com/acrobat/datasheets/BF245A-B-C_2.pdf
The BS170 is an excellent choice for the Mos-boost and does not actually draw much current(a 9v battery will last a long time).


Regards

RDV

[davebungo]

Thanks for the replies.
Yes, I know the BF245 is a JFET and no I am not suggesting the substitution of this part into the AMZ circuit - that would be stupid.  What I am asking is why the choice of the BS170 /2N7000 MOSFET and why run it so close to pinch off (other than it results in a low current drain.  I'm just trying to understand why the choices were made.  I can understand the approach of "if it sounds great why question it", but then again, I am questioning it.

BTW I finally received a reply from Jack after posting this thread - may not have been a coincidence I don't know.  Here is Jack's reply (which doesn't really help much - no disrespect intended):
"
You cannot go by the  current ratings on the datasheets...  they are
typically for switching applications and often represent maximum limits
on the device.

A BF245 is a jfet...  the BS170 is a mosfet.  They are different type
devices.   Mosfets may be used because they can give more gain than a
single jfet.

There are many things that have to be balanced in designing a circuit,
such as gain, current consumption and specification limits of the
components being used.

regards, Jack
"

I'm not going to press too hard to extract an answer to my question - at some point I just say "what the hell" and build it, use it, enjoy it and accept it, but it won't stop me from experimenting with other designs and questioning existing ones, that's what makes all this stuff enjoyable IMHO.
:wink:

[ErikMiller]

I'm not going to press too hard to extract an answer to my question - at some point I just say "what the hell" and build it, use it, enjoy it and accept it, but it won't stop me from experimenting with other designs and questioning existing ones, that's what makes all this stuff enjoyable IMHO.
:wink:

That's it, Dave. You've passed an important threshold. The one where you take a look at someone's design, and ask yourself "why didn't they do it another way?"

Most of the time the best way out of it is to see what happens if you try it the way you would have done it. I've tried asking the original designers of circuits I've built why they did this or that, and the answers I get are usually less satisfying. Jack's right: there are so many factors that go into the decision to take this or that path. Sometimes I can't even remember why I did it this way and not some other way. I recall that one of Jack's designs uses a particular oddball resistor value because that was what he had in his parts bin that day.

One of my favorite ways to inspire myself is to go through my parts bins and see what suggests itself from what I have on hand. If I have a CA3240 and use that, does it mean that a 4558 wouldn't have worked just as well? Who knows? If I'm trying different parts to see what sounds good, and I hit one that really excites me, I usually stop and go with the one that makes my motor run.

One thing I can say about that damn MOSFET booster is that it's a pretty purpose-optimized design. I've been trying off and on for months to develop a clean booster I can sell commercially, and I have looked hard at that circuit. There is very little I've tried on it that didn't wind up making it sound different, and usually not as pleasing.

I'd be interested to see what happens if you try a different device in it.