Can this be sim'd?

[digi2t]

Had a slick gent send me his Saturn V booster for tracing. Never seen a circuit like this, but a little digging brought me to the topic of using N-channel depletion mode MOSFET's as amplifiers. I'm wondering if this is indeed what's going on here. Can somebody sim this, or at least tell me if I'm on the right track?



As for the weird ass resistor values... that's a Spaceman thing, as usual.

[thermionix]

Well that's hilarious.  3K01 + 1K69 = 4K7.  Somebody's messing with us!

[PRR]

> Can this be sim'd?

Not of much use unless you have part-numbers for the depletion MOSFETs, a less-common part in different sizes and flavors. (LND150??)

> Somebody's messing with us!

That, or.....  there's variability in MOSFETs and no bias stabilization. Perhaps the idea is: build it with 3K+1.7K, test it. If Drain sits low, jumper-out one or the other resistor. The sample seen by digi2t apparently biases OK with all the resistance. If the MOSFET ran 1.5X this current with the same diode bias, then the 3K alone would be good; if 3X the current then the 1.7K alone would work.

I'm not in love with this thought. It is usually easier to snip-out than to jumper. A scheme like 4.7K||2.7K makes 1.7K, 2.7K, or 4.7K with one snip at most. But maybe the Designer came to this from another path. Or maybe the PCB allows rapid jumpering.

"3.01K" instead of "3K" just means 1% parts. The nominal values of 1% parts have extra digits so that your workers can't easily use a 10% part where you want a 1%: the digits are not the same. Obviously if the real variance is a 3:1 spread of MOSFETs, this is pointless. However the price of 1% parts today makes it cheap folly.

[thermionix]

Not knowing anything about the brand or designer, I was figuring they were just trying to throw cloners off.  Look at the 6K19 resistor, in series with a (20% tolerance?) 100K pot.  I'm sure a 6K2 would do just fine, given that pots don't usually hit dead zero when turned all the way down.

I guess another possibilty is that they scored a deal on some oddball resistor lots.  I still have a big bag of 3K92 resistors that I got somewhere long ago for next to nothing.

[bool]

These look like standard E96 values.

http://www.logwell.com/tech/components/resistor_values.html

[digi2t]

Guys... I get this everytime I dissect one of these Spaceman units. Whether it's utility or show, it's part of their schtik. All their creations come standard with bizarre values and some fancy dancy mil-spec components. It's the Spaceman "aura".

The values you see here are taken right off the components. The 3k01's are TRW 3011F resistors, which the datasheet actually lists as "3.010K". Yeah, I know 3k01 + 1k69 = 4k7, but when you look at the layout.... the symmetry is actually quite beautiful. Possibly useless in some cases, but beautiful nonetheless.

[Gus]

Are the transistors LND150? (PRR posted about the LND150)
https://www.futurel.bg/datasheets/4/LND150.pdf

or are the transistors any in the following https://aboutspice.com/models/supertex/supertex.lib

Are the transistors the same part numbers? I ask because of the different drain voltages.

[digi2t]

Another Spaceman tactic; major components have the part numbers sanded off.

As for the different drain voltages, I'm thinking maybe different part numbers, maybe the lower vF of the BAT diode, or maybe a combination of both? Maybe? Dunno.

[Mark Hammer]

I'll say this, VERY nice build quality.

[Gus]

Interesting the first stage is closer to cut off and the second stage is closer to saturation.
Might have a nice distortion change with signal level
Diode bias with a 1N4148 and then  lower voltage bias from the Schottky

I would breadboard a circuit fragment
9VDC supply
4.7k drain resistor
a diode in the source try both
and a resistor from gate to ground
then try a LND150 and other mosfets and measure the voltages and compare to what you measured

[jubal81]

Reminds me a bit of the Runoffgroove Peppermill.
With that setup, they've got to be FETs of some kind. JFETs are depletion mode. If the numbers are sanded off, I'm wondering what clues led to depletion mosfets.

[digi2t]

Interesting the first stage is closer to cut off and the second stage is closer to saturation.
Might have a nice distortion change with signal level
Diode bias with a 1N4148 and then  lower voltage bias from the Schottky

I would breadboard a circuit fragment
9VDC supply
4.7k drain resistor
a diode in the source try both
and a resistor from gate to ground
then try a LND150 and other mosfets and measure the voltages and compare to what you measured

Exactly my next step. All the surrounding cast of characters are already on the board. LND150's and some other similar mosfets are ordered.

It is a VERY interesting pedal. You can either push the boost, and push the amp, or push the drive, and let the pedal do the dirty work. There's lots of texture in between too. I'm quite enamored with this little circuit.

Reminds me a bit of the Runoffgroove Peppermill.
With that setup, they've got to be FETs of some kind. JFETs are depletion mode. If the numbers are sanded off, I'm wondering what clues led to depletion mosfets.

The depletion mosfet clue came from my DCA55. I managed to get a reading off the Q1, but not the Q2. I'm guessing though that it might be something similar for Q2, just by the almost mirror topology.

[Rob Strand]

Out of the box MOSFET models supplied with simulators are terrible.  They are especially terrible for effects as the MOSFETs operate at low currents.  The supplied models are tweaked for higher currents with no consideration for low current behaviour - that goes even for small MOSFETs.  The only solution is to measure the MOSFET characteristics then fit the model parameters to the measurements.

[digi2t]

Got it. Here's the final trace. I have it on the breadboard, and the voltages are extremely close, and the sound is almost identical. I attribute any sonic difference to some minor component value differences in a couple of my caps. I didn't have the same values as the unit, so I picked the next closest. Here ya go folks. It's a great booster/drive.



Many thanks to Duff Man for lending me his unit. Gonna get this off to PMowdes for a PCB creation. Definitely something to add to the arsenal.

[imJonWain]

nice work!

[thermionix]

Awaiting demo video 

[Plexi]

Great..!

Gonna get this off to PMowdes for a PCB creation.

They design the pcb layout?

[Kipper4]

Great work Thanks Dino.

[deadastronaut]

funny, i was looking at this the other day....only because i wanted the saturn v name...

too late as usual.

nice one dino..

[digi2t]

You're welcome all. It's been a while since I had a trace come together easy peasy like this. It was as refreshing as the sonic qualities of this circuit.

Awaiting demo video 

Won't have video of the original, since I promised the owner that I would have it for the minimum of time required to trace it. I did however record some high quality sound bytes of the original in the studio for reference. I'll upload those and put up the link and info, maybe tonight.

Great..!

Gonna get this off to PMowdes for a PCB creation.

They design the pcb layout?

Yeah PMowdes and I have been working together for a wee bit now, me feeding him circuits, he designing PCB's, and both of us figuring out where the hell we went wrong sometimes. Anywho, PMowdes is the genius behind the Harvey Dent, Atomic Duck, and other boards that we've been selling off of late. We're putting this one in the cue. Right now, I'm waiting with great anticipation for the Schumann Lion X boards to show up. I've added an expression pedal add-on to it that I think will be off.... the .... hook.