What is Q4 doing??? (Calling JFET experts!)

[digi2t]

Hi all.

Been digging into the Spaceman Mercury IV booster lately. Here's a prelim scheme (not sure if it's right yet).



What in the heck is Q4 doing? Voltage controlled resistor of sorts? Attenuator? What? So many JFET's. So little knowledge.

[thermionix]

Too fuzzy to read.

[digi2t]

Too fuzzy to read.

Sorry. How's this?

[Prehistoricman]

Looks like a current source to Q5 and Q6 differential amplifier.

[digi2t]

So I'm assuming the cap is to block DC noise from the Harmonics pot?

[Rob Strand]

Q4 is the upper transistor of a mu amp.
http://www.muzique.com/amz/mini.htm

Q5 and Q6 are the *both* the lower transistors of the mu amp.
Q5 and Q6 form a mixer and amplifier.
The inputs to the mixer are from Q2.  The two input signals are out of phase.

The whole is idea of the circuit is to cancel out the fundamental and leave the second harmonic.
When clipping comes into play the signal is more complex but it still tries to cancel the fundamental.

So that's why that signal chain is called "Harmonics".

[Ben N]

A Mercury booster ought to be called Redstone or Atlas. Just sayin'.

[digi2t]

A Q4 is the upper transistor of a mu amp.
http://www.muzique.com/amz/mini.htm

Q5 and Q6 are the *both* the lower transistors of the mu amp.
Q5 and Q6 form a mixer and amplifier.
The inputs to the mixer are from Q2.  The two input signals are out of phase.

The whole is idea of the circuit is to cancel out the fundamental and leave the second harmonic.
When clipping comes into play the signal is more complex but it still tries to cancel the fundamental.

So that's why that signal chain is called "Harmonics".

Once again, my inexperience has led me to draw this out.... well.... not as well as I could have if I knew where I was going  . It's now much clearer. Thanks for the link too. I kinda had a handle on the mixer/amplifier angle, and the odd/even harmonics stuff, but Q4 was throwing me off. I've just never really played with JFET's enough to really understand them and the applications. Took me a while to get my head around transistors, guess I'll have to expand my horizons. I can clean up this schematic a bit more now.

Thanks for the guidance Rob.

A Mercury booster ought to be called Redstone or Atlas. Just sayin'.

Redstone! Brilliant! PM me your address Ben. I'll launch a board your way when we get this one done.

[Rob Strand]

led me to draw this out.... well.... not as well as

I'd probably draw Q7, Q8, Q9 more like the top two circuits on this page,
http://guitarwork.ru/electronic/firm/Distortion/BOSS%20Blues%20Driver%20BD-2.jpg

It forms an opamp with a +input, -input and output.
The boost pot is like a the variable feedback resistor on an opamp ckt.
I'm sure you will see the pattern.

[digi2t]

led me to draw this out.... well.... not as well as

I'd probably draw Q7, Q8, Q9 more like the top two circuits on this page,
http://guitarwork.ru/electronic/firm/Distortion/BOSS%20Blues%20Driver%20BD-2.jpg

It forms an opamp with a +input, -input and output.
The boost pot is like a the variable feedback resistor on an opamp ckt.
I'm sure you will see the pattern.

Rob.... my savior! Muchos gracias amigo. PM me your addy as well. I'll send a board out to you when it's done.

[Rob Strand]

Rob.... my savior! Muchos gracias amigo. PM me your addy as well. I'll send a board out to you when it's done.

No problem, you are welcome.
PM sent.

[Ben N]

Dino, you are too kind, although I confess that (a) the premise of this pedal is very intriguing, (b) most of it is way over my head, and (c) there is zero chance that I will build anything with that many discretes myself without an RTS-PCB. So, mucho gracias.
Edit: PM sent.

[Prehistoricman]

I was intrigued by this circuit so I breadboarded just the harmonic section and found it very disappointing. No octave type effect at all. After much playing about, I found that switching out JFETs for Q5 and Q6 did produce a bit of the 2nd harmonic.

So it was a matching problem. I found two JFETs that were 0.01mA different and the effect was much better. However, to make it work with a greater selection of JFETs, you could put in a 10K trimmer between the drains like this:



As you turn the pot, you can hear the non-inverted to one side, inverted to the other, and it goes quiet in the middle where the two signals 90% cancel out.
Doesn't work with very different JFETs. I found my least two matched ones (of the same part number) and the effect wasn't so great.

[digi2t]

I was intrigued by this circuit so I breadboarded just the harmonic section and found it very disappointing. No octave type effect at all. After much playing about, I found that switching out JFETs for Q5 and Q6 did produce a bit of the 2nd harmonic.

So it was a matching problem. I found two JFETs that were 0.01mA different and the effect was much better. However, to make it work with a greater selection of JFETs, you could put in a 10K trimmer between the drains like this:



As you turn the pot, you can hear the non-inverted to one side, inverted to the other, and it goes quiet in the middle where the two signals 90% cancel out.
Doesn't work with very different JFETs. I found my least two matched ones (of the same part number) and the effect wasn't so great.

From what I've read (and experienced) with this circuit, I don't believe octave is the goal. Turning up the "Harmonics" simply introduces more even order harmonics.

Here's an updated (read "cleaned up") version of the schematic;

[Rob Strand]

Here's an updated (read "cleaned up") version of the schematic;

Looks nice.

[digi2t]

Here's an updated (read "cleaned up") version of the schematic;

Looks nice.

Good guidance = nice schematic.