Clandestine Preamp

[caiofilipini]

I've been busy with a whole bunch of other stuff, so not building many pedals, but here's a super fun project: the Clandestine Preamp (Chase Tone Secret Preamp).
I was thinking about buying this one for a while, and I ended up getting one about a month or so ago. Opened it up, it looked pretty cool, with Orange Drop caps, carbon film resistors and whatnot. Very well built too.

I decided to try and trace it. I got 98% of it done, then I messaged @PedalPCB asking if he'd be interested. He helped me figure out a few details around the power supply section. After a little back and forth, he sent me the prototype boards and I just finished building it. Here it is:






I decided to go overboard and used CDE Orange Drop caps, KOA Speer carbon film resistors, Nichicon audio electrolytics and a Fairchild 2N5457. Sounds delicious! To top it all off, I put it in a Gørva C65 white enclosure and used a hip looking synth pointer knob from Smallbear. Super happy with the results!

Huge thanks to @PedalPCB for trusting me with this one and helping me figure out the power supply stuff.

 

[caiofilipini]

@fig I guess now you understand what all those Orange Drop caps you saw on the picture of my breadboards were for!

 

[fig]

That looks really nice, inside and out! Those caps set it off and provide all the the ooziness needed.

 

[caiofilipini]

That looks really nice, inside and out! Those caps set it off and provide all the the ooziness needed.

Thank you!

 

[jjjimi84]

I am looking forward to this release! Looks great!

 

[caiofilipini]

I am looking forward to this release! Looks great!

Thanks, Dan!

 

[JamieJ]

Secret verification of a secret preamp. Very nice as always @caiofilipini

 

[caiofilipini]

Secret verification of a secret preamp. Very nice as always @caiofilipini

Thank you, Jamie!

 

[Boba7]

Very nice!!
Any chance to see the schematics @caiofilipini ?

 

[caiofilipini]

what is the optimum voltage for the jfet?

My original measures 12.46V, with a Vref of around 22V.

 

[Boba7]

My original measures 12.46V, with a Vref of around 22V.

Usually I prefer to bias at 2/3, and I remember an Echoplex schematic that showed a bit more than 14v for the jfet.
But it shouldnt make any difference

 

[Boba7]

I wonder if he's using the 500k pot as a voltage divider (like a standard volume pot) or as a resistor to ground (like the echoplex does).

Also with 1N5817 the charge pump should output around 26v I believe, not 22v. Curious to see the schematics

 

[caiofilipini]

Also with 1N5817 the charge pump should output around 26v I believe, not 22v. Curious to see the schematics

There's a 1N4748A Zener diode used to bring the 26V down to about 22V at the end of the power supply.

 

[Boba7]

There's a 1N4748A Zener diode used to bring the 26V down to about 22V at the end of the power supply.

Right, of course! Thanks

 

[cooder]

Super sweet build! Good on ya guys for getting the schematic figured out, looks a treat and I bet it sounds too!

 

[Preverb]

I ordered one of these PCBs. I am wondering if the 3 hole predrilled 125b from Tayda would line up since there are no 2 hole versions?

 

[caiofilipini]

I ordered one of these PCBs. I am wondering if the 3 hole predrilled 125b from Tayda would line up since there are no 2 hole versions?

Not sure, maybe @PedalPCB could chime in.

 

[Onkelmike]

Hey can anyone tell me where the R5 (240k) resistor is in circuit? Wondering how much difference a 221k would make there.

 

[mdc]

I don't think the schematic for the secret preamp has been shared yet, but here's the echoplex it's based on.

The 220K resister "references the output to ground, and keeps the output from floating positive if left unconnected." I don't think the value is too critical there, but I could be entirely wrong. The Tillman preamp is similar and uses a 51K resistor in the same spot.

 

[Onkelmike]

I don't think the schematic for the secret preamp has been shared yet, but here's the echoplex it's based on.

The 220K resister "references the output to ground, and keeps the output from floating positive if left unconnected." I don't think the value is too critical there, but I could be entirely wrong. The Tillman preamp is similar and uses a 51K resistor in the same spot.

Thanks!