Possible stupid question on the EP preamp pedals…but imma ask it anyways!

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Drayve85

Active member
Alright, so here it is. I e been racking my brain for a few days trying to figure it out, but I just can’t think of a reason for it. Apologies if this has been asked before, but I couldn’t find anything about it.

In the Chasetone Secret Pre circuit and the Catalinbread Epoch Bias circuits, the Clandestine Preamp and Eschaton Preamp respectively on PPCB, the pedals use a voltage multiplying power supply circuit to bump the voltage to around 22volts, but the JFETs are only biased to less than half that. The Clandestine I built is biased at around 7 volts, and the Eschaton has a 22k resistor at the drain, which I just measured, biases to 4.12 volts. It also has a Bias knob that connects to the Source. But, why have the voltage multiplying power supply, if it only uses half or even a quarter of the power that is being made??? Sorry if this is a dumb question, but I can’t see any point in it. Wouldn’t the circuit sound the same with a standard ‘ole 9 volt power supply? I’ll attach the schematics too. Thanks in advance! -Don
 

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Curious about this too. I know the original EP-3 preamp circuit used around 22V, and they do intentionally include this in pedal recreations, but I'm not sure how it's being used if the transistors are being biased lower than half that. I'm tempted to see what the transistors in my real Echoplex are biased at....
 
iamjackslackof said:
Curious about this too. I know the original EP-3 preamp circuit used around 22V, and they do intentionally include this in pedal recreations, but I'm not sure how it's being used if the transistors are being biased lower than half that. I'm tempted to see what the transistors in my real Echoplex are biased at....
Click to expand...
I’d be interested to know what they’re biased at also! If/when you measure them, could you let me know, by chance??
 
Drayve85 said:
…Nobody?? Anybody??lol @Chuck D. Bones would you mind schooling me on this, if ya have a sec.
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Chuck D. Bones

Thread 'JFET Biasing - part 1'

I spent a fair amount of time trying to make this as concise and readable as I could.

Intro
In theory, setting the bias on JFETs should be easy. In practice, not so much. Here are the details on why that is and what we can do about it.

Let's review some theory and get the math out of the way.

In may ways JFETs are ideal devices. At audio frequencies, the input and output impedances are so high we can treat them as infinite. Once set, the drain
current and gain are very stable. The gate current is essentially zero. That means any current that flows into the drain lead has...
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Ok, so just to clarify, I’m guessing the higher voltage contributes to the higher impedance and higher current, which contributes to the signature sound of the circuit!? I’m a troglodyte when it comes to all the math and electronic jargon in that thread, but I am learning. I’m also reading and re-reading that post in order to glean as much as I can from it.

@benny_profane , thank you for that reply. I figured there was a reason for the 21 volts in the circuit, but I couldn’t figure it out!lol
 
Your post is a little confusing to me—it seems that the word ‘bias’ is applying to multiple parameters of the JFET device.

Both circuits you’re working on are source follower amplifiers. The first section, Setting the Bias, in Chuck’s post discusses this type of arrangement.

The trimpot in the clandestine is setting the drain resistor (Rd), while the eschaton has a fixed gain resistor and a pot for the source resistor (Rs). Both gates are kept at DC ground via the pulldown resistors and coupling capacitors.

You can either measure the JFETs you’re using or, if that’s not an option, reference the 2n5457 datasheets to get nominal figures for the relevant characteristics to run the calculations in Chuck’s post.

Using the results of those calculations, you’ll be able to see the affect of the increased Vcc (via the charge pump) on headroom/clipping as well as the gain of the JFET amplifier.
 
I was just wondering why both circuits were bumping up the voltage to ~21v when the JFETs are only using ~7v in the clandestine and ~4.5v in the Eschaton. I didn’t know if there was a reason to have the voltage increased for a reason that I wasn’t seeing or understanding, or if the designers of the circuits just kept the voltages high because that’s what the original EP-3’s had. I’m assuming the original ep-3 had another use for the 21v somewhere else in the circuit, ie. the dc motors that run the tape, or something like that.
 
Those bias voltages seem off to me. Where did you source your JFETS? Anyway, the guitar signal can potentially swing from 0V to 22V, minus some losses. The drain voltage just defines the midpoint of this output swing.
 
lowpitch said:
Those bias voltages seem off to me. Where did you source your JFETS? Anyway, the guitar signal can potentially swing from 0V to 22V, minus some losses. The drain voltage just defines the midpoint of this output swing.
Click to expand...
Ok, so I’m sure it was due to ‘operator error’, but now the eschaton is running at 18.6v with the bias knob all the way up, and 5.3v with all the way down. I must’ve measured it with the knob all the way down.lol eh! What do ya want from me, I ain’t the brightest colored crayon in the box!😂😂 I’m running an SMD 2n5457 in it that I got from PPCB. In the Clandestine I’m running a 2n5952 that I ordered from Mouser a long while ago. The trimpot in it is letting thru 8v, which is what the JFET is getting, obviously. They both sound great!
 
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