Can I use BS170's in my BSIAB?

[scott1568]

-want to try something different...

[petemoore]

  Mosfet has different bias needs than Jfet.
  RG has a Mu article, IIRC there may have been mention of Mosfet / Mu amp at GEO.
  WGTP has done some Mosftet distortions, perhaps you can dig something up that way...

[earthtonesaudio]

-want to try something different...

You could sub a triode wherever you see a JFET.  Tube emulation of a solid state circuit.  Might need a bit higher supply voltage though...

[Vaum]

Without any modifications, you can use a MOSFET in the upper position of the mu-amp circuit.

The rest of the story:

A little knowledge is a dangerous thing. You can indeed *sometimes* sub a MOSFET for a JFET, but only when the rest of the circuit is such that the several volts of difference in where the MOSFET gate needs to be from where the JFET gate is held does not matter.

Any time a JFET is biased with resistor to ground and the source is biased above ground with a resistor, you can not sub in a MOSFET and have the circuit work the same way.

An enhancement mode MOSFET like the BS170 needs a bias voltage of from 0.8 to 3.0 Volts more positive on its gate than its source (see much, much more on this topic in "Designing MOSFET Boosters at GEO". A JFET is a depletion mode device, meaning its gate has to be one to several volts more *negative* than the source. Only when the rest of the circuit makes this not matter can you sub an enhancement mode MOSFET directly for a JFET.

In the typical mu-amp/minibooster, the input JFET gate is tied to ground. You can mod this by setting up a bias voltage voltage above ground for the MOSFET. All that droning on I did about MOSFET biasing in the "MOSFET Boosters" article explains this in excruciating detail. You'll have to add an input capacitor to keep the bias voltage from being present on the signal input line as well.

R.G.


http://www.geofex.com

[MikeH]

You could substitute BS170s for the Mu amp stages, but it would just end up sounding and looking an awful lot like a zvex Box of Rock.

[WGTP]

http://www.diystompboxes.com/analogalchemy/sch/obsidian.html

The last version of the one that started it.   

http://www.aronnelson.com/gallery/main.php/v/WGTP/Mosfet_Incenerator.jpg.html?g2_imageViewsIndex=1

I don't think the BOR uses mu amps does it?

[MikeH]

I don't think the BOR uses mu amps does it?

No- that's what I meant.  It uses MOSFETs

[WGTP]

Doh.  Got ya.

I have gottne BS170's working in a Mu configuration (I think).  Had to do some weird biasing stuff that I don't recall to get it to work, so I'm not sure it maintained it's Mu operational characteristics.  IIRC I like the sound of the Jfets better, at least in that application.