JFET mu-amp booster: original article images!

[stm]

I had this National Semiconductor book in a pile of books I was going to throw away, while yesterday once again the subject of the mu-amp came in another thread. That made me think this book is more a relic, so I went through the article (AN-32 FET Circuit Applications, dated February 1970) and found several interesting JFET circuits which I think might be good building blocks of food for experimentation.

First, this is the cover of the book and the title of the article:



As you can see the book shows how extensively I used it during my adolescence and university years.

Now this is a collage of four amplifier circuits:



Starting from the upper-left, and following clockwise we have:

a) The circuit that started it all: the mu-amp

b) An interesting compound amplifier (also hi input impedance), but whose gain can be easily set by R1 and R2. A good candidate for a guitar booster. Are its clipping characteristics "nice" or "musically interesting"?  Notice the similarity to the FF.

c) A cascode video amplifier with essentially no feedback. You can eliminate the RF choke and use it as an audio amplifier. Will it sound good with a guitar?

d) A RF amplifier with AGC! (gain controlled by a voltage). Replace inductors with resistors and variable capacitors with shorts and you can have an audio amplifier. Maybe AGC input can be used for tremolo or compression or multiplication with the input signal (frequency doubling)?

And now we have two hi input impedance buffers, suited for following mu-amps and cascode amplifiers that require basically no loading, or as a better input buffer for an effect in case you don't want or don't have an OpAmp:



And finally, the BONUS TRACK:



Left: A phase shifting stage that resembles a JFET version of a Univibe. Many possibilities here:
- Use a dual ganged pot connected to a wah-styled pedal and you have a foot controllable Phase 45 workalike if you mix the output of this phase shifter with a replica of the input signal.
- You can also replace the variable resistors by photoresitors. Vary the amount of light into the LDR's and you already have a Vibrato!

Right: A simple voltage controlled gain stage. In my opinion the Source of the JFET should go into the (-) input of the OpAmp instead. Gain controlled effects like compressors and tremolos could be implemented with such building block.

Closing Comments:

Many circuits based on JFETs that have been dormant for a long time were presented here. I think all of them have potential use as building blocks or to develop new ideas for guitar/bass effects.

These circuits indicate tipically +12Vdc or even +30Vdc as the supply, but of course +9Vdc may be used, in some cases directly, in others with minor adaptations.

JFET names indicated may sound strange or difficult to obtain. Many substitutions are possible here. Use what you have at hand. In some cases, try to find out the approximate Vgs(off) of the original and see what's closer in your parts bin. Also you may need to take into account the Idss value in some cases to make a wise substitution. These guys were inclined to the '2N' series, however you may already have J201's, 2N5457's and MPF102's.

Best regards,

STM

[stm]

If you have interested in the images you should save them on your HD. They are hosted at tinypic.com, and some of the images I have posted there have dissappeared with time.

Does anyone know how can I create an account for the Layouts Gallery? I couldn't find it there. Maybe a PM to Aron?

[vanhansen]

Awesome stuff.  Thanks for sharing.  I saved those babies.

Just shoot Aron a PM and he'll set you up with a gallery.

[A.S.P.]

information lost...

[amz-fx]

AN-32

You beat me to it...  get the original direct from National Semi at http://www.national.com/an/AN/AN-32.pdf

regards, Jack

[aron]

stm, check your pm.

[stm]

Well, at least the image of the book cover is original. Good thing the full article is available online, which means more readable images. Hope I raised some interest in the circuits there.

There are two additional Application Notes on that book that deserve to be read, but unfortunately couldn't find them online at the NSC site:

1) AN-88  CMOS Linear Applications, dated July 1973, which is perhaps the first reference about using CMOS logic ICs for linear applications.

2) AN-129  A Linear Multiple Gain-Controlled Amplifier, dated August 1975, which has very interesting graphs showing the distortion introduced by JFETs when used as voltage controlled resistors to set the gain of an amplifier. It explains the effect of using JFETs with different values of VGSoff (V0), and contains several schematics of gain controlled amplifiers suitable for AGC, companders and expanders.

Regards.

[petemoore]

  Cool stuff, We'll have to get Maynard to try some, I think He'll like it.
  Jfet phase stage...
  Mu amps...
  Buffers
  All Good stuff...

[brett]

Hi.
Thought I might mention that the last lower-right (variable attenuator) is a very versatile circuit in my experience.  From memory, I think it is the basis of the orange squeezer.  I've also built a more "serious" compressor in this style.  Also, it makes a great tremolo if you attach a low frquency oscillator to the gate of the FET.  Note that the FET can also be in the feedback loop of an inverting amplifier or on the "earth leg" off the feedback loop of a non-inverting amplifier.
cheers
   

[petemoore]

  b) An interesting compound amplifier (also hi input impedance), but whose gain can be easily set by R1 and R2. A good candidate for a guitar booster. Are its clipping characteristics "nice" or "musically interesting"?  Notice the similarity to the FF.
  Resemblance differs though, Q2 in the FF looking circuit is an 'upside down' PNP.

[Medved]

stm, thanks, very interesting

[A.S.P.]

information lost...

[MartyMart]

Great thread   Thanks STM and A.S.P .... data downloaded !
I wonder if The K.L.F had read these notes when they sang about ...
" Mu Mu land ..... Mu Mu land "

MM.

[stm]

Hi.
Thought I might mention that the last lower-right (variable attenuator) is a very versatile circuit in my experience.  From memory, I think it is the basis of the orange squeezer.

The OS has a JFET attenuator befor the OpAmp. It is not the same principle.

I've also built a more "serious" compressor in this style.  Also, it makes a great tremolo if you attach a low frquency oscillator to the gate of the FET.  Note that the FET can also be in the feedback loop of an inverting amplifier or on the "earth leg" off the feedback loop of a non-inverting amplifier.

Then you *must* read the AN-129 article I cited above. Unfortunately it has not been found online yet. I'll try to scan it on monday if it has not appeared yet and build a PDF. I would need a place to host it though.

Regards,

P.D. Thanks for the Gallery acount Aron.
   

[amz-fx]

Interestingly (or not), here is the Linear Apps 1 databook that I have:



The book has a Radio Shack cover but inside it is identical to the blue NatSemi book - this was back in the days when RS was of some use to hobbyists. It is dated Feb. 1973 but I am pretty sure that I bought it in the summer of 1976 when I was heavily into building Craig Anderton projects.  I did not not get around to building the first mini-booster until around 1979 or 1980.

regards, Jack

[Doug_H]

Jack,

Speaking of old Radio Shack books... Do you ever remember seeing this one?



I got it back in 1974 when I was in high school. Fun hobby type stuff, has the "RCA Fuzz" FWIW, along with a treble boost I built, tremelo, theremins, wah and some other interesting stuff. Back when RS actually had parts.

Doug

[amz-fx]

Yep, I have that one too!

-Jack