Whisper of Screams (Kasha OD Pedal Trace)  [traced]

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Cybercow
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Post by Cybercow »

For your breadboarding pleasure . . . .

I give you the "Whisper of Screams" - a trace of the Kasha OD Guitar pedal. One of the best, oddest, yet simplest, circuits in guitar pedals I've ever encountered.

This post is my contribution to the pedal building community in resurrecting this rare find.

This thing rips and has only two active components - a pair of JFETs. No diodes were used or harmed in the creation of this circuit. Yet, it can can deliver a clean 3db boost, all the way up to pleasant, punchy overdrive with about an 18db total gain - depending on the settings.

The provided schematic does not show any stompswitch wiring as that can be decided at build time. Same for any power protection and\or filtering.

Wait! There's no "tone" control! It doesn't need a tone control. It is quite transparent with regard to reproduction of a guitar's tone.

Unity-gain varies with component tolerances and the position of the 'Level' control.

The settings are described thus:

• Calm: This setting is bright, and sounds like a low-wattage tube amp but you don’t lose any of your lows. A nice 3db clean boost with the 'Whisper \ Scream' switch in the 'Whisper' position.

• Sullen: This setting is a simple boost stage, which ups your gain about 6dB. If you toggle the 'Scream' switch, then you get a full 13dB to start really working your amp.

• Moody: This setting hits your amp with 11dB of gain, and gives you added bass. The tone from this channel is absolutely HUGE! Then you flip the Scream switch, and WOW!

• Angry: With this setting you get a heavy metal type sound. The voicing gets a big, but tight bottom-end plus a few more dB’s of gain.

The original Kasha OD Guitar pedal was in a simple, tight 1590B sized enclosure with just a 'Volume', 4-way rotary and a SPST toggle switch. In my initial build, I replicated the 1590B enclosure approach. Then I had second thoughts and thought it would be a good idea to put the 'Whisper - Scream' toggle under a footswitch instead. (NOTE: the 4-way switch must be wired backwards to match the labels of the enclosure artwork.)

It sounds great thru a solid-state amp, but it really shines when used in front of a tube amp.

After the initial trace, I grew more curious about JFETs & general FET-based audio circuits and started digging into other FET circuits. Of course the phrase "Fetzer-valve" showed up in my searches. After examining "Fetzer-valve" tech, it occurred to me that applying the phrase 'Fetzer-valve' to any FET circuit is much like saying a brushless motor is "digital". So that phrase shall not be used in any of these descriptions.

I was pleasantly surprised & amused when I discovered the the Drain and Source pins can be swapped without issue. The original circuit used BF862 (2AW) FETs. Very hard to find, I started a deep-dive into auditing several different FETs for this build. Without changing a single component value, I first learned that NXP, who manufactured the BF862, also still produces the BF861C FET. These work well in this circuit with no need to change any values.

Continued auditing revealed that (genuine) J201s work perfectly in this circuit - and IMO, better than the BF861C or any other FET. I also found 2N5457s and 2SK170 (not to be confused with the BS170 - they are NOT the same) work very well in the unaltered circuit.

Other FETs (J112, J113, 2N3819 and some others) will work, but the Source resistances must be changed to higher values in order to work as nicely as J201s. But I found they don't achieve as much boost as J201s. The proper Source resistances have to be found to avoid gating and more volume. To be honest, there is a lot of technical jargon that revolves around the differences that I am not able to just recite off the top of my head. But the auditing process is pretty much fool-proof.

So, there you have it. I hope that those of you who elect to try this circuit find it as exciting as I do.

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okgb
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Post by okgb »

Looks great, straight forward thanks, you weren't tempted to put a second stage volume [ master ] at the end ?

Manfred
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Post by Manfred »

I was pleasantly surprised & amused when I discovered the the Drain and Source pins can be swapped without issue.
Some JFETS are indeed symmetrical.

roseblood11
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Post by roseblood11 »

Nice! Do you have sound samples?

Have you tried to replace the 1m resistor in front of the second transistor with a pot? Maybe this could replace the "whisper" mode?

Why do they use such high resistor values at the output?

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Post by Manfred »

Have you tried to replace the 1m resistor in front of the second transistor with a pot? Maybe this could replace the "whisper" mode?
The resistor network of the three resistors connected to the gate of Q2 together with the input capacity (Cin = Cgs + Cgd * stage gain) form a low pass filter with a cut-off frequncy of about 3.7 kHz
If the 1 megaohm resistor is replaced by a potentiometer, the cut-off frequency changes with the potentiometer setting.

Cybercow
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Post by Cybercow »

Thanks folks! But notice that this is a "traced" circuit, not a new design. It is a resurrection of a pedal that is no longer manufactured; with aunique configuration. Try breadboarding it as it is and see how it sounds. I suspect you'll be impressed.

okgb
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Post by okgb »

I haven't built this yet, [ but will ] but here a blurb apparently from a reverb listing quoting PG


Super rare and out of production! This pedal does what it promises... super transparent and powerful OD. So many OD options with one pedal. Great shape and fully functional. 3M Tape on bottom.

From PG Magazine:

The glossy black pedal features a simple design without even a tone control. Kasha says this is because often a tone control or passive filter is used to dampen the unwanted high frequencies but as a result, diminishes the tone of the amplifier.

The KOD is designed with four unique voice selections: Smooth, Classic, Hot and Melt, and a master 10dB turbo switch for all four voices. This is how Kasha describes each channel:

The first voice, Smooth, provides a 3dB up to assist in achieving that one last kick when the amp isn’t just quite there. This is a prefect setting when playing a blues riff or wanting to add a little hair on your clean channel. Engage the turbo switch for cleaner power chords and smoother lead tones.

The second, Classic, provides an 11dB up, adding a dash of bass and highs to the tone and still keeping the overall signal intact. Use it for dynamics in songs ranging from a clean crunch to balls-to-the-wall riffing. Kasha says this is where the KOD really starts to set itself apart from others by allowing Les Paul-style guitars to sound like stacked pickups on a Strat-style instrument and bringing real attitude to imported shredding machines. By applying the turbo switch in the Classic voice, the tones start capturing that vibe of pure metal carnage.

The Hot voice adds 15dB up and is perfect for turning your amplifier into a chainsaw – in a good way – by helping the player cut through a wall of sound like butter. This is ideal for solo boosting and when you kick in the turbo switch, the KOD provides violin like sustain for days.

The last voice is Melt. By providing 18dB up, this setting is for those climatic moments when “You are almost there, the crowd is on their feet, your face is cringed, head back, eyes closed, your dream tone is starting to take shape, but you need just a bit more” occasions. This voicing takes up the gain a few ticks and then adds a tight bottom end where the notes become thick without the flub and overtones.

"It is the first overdrive pedal that does not change the amps tone (and gives a) variety of useable voicing’s for many styles and tracks every note I pick. (The KOD) makes other distortion or overdrive pedals sound better and turns your amp into multiple amps with pro tones" – Phil X (Kelly Clarkson, Avril Lavigne, Tommy Lee, Daughtry)

Features of the Kasha Overdrive include:
· 4 channels with separate voicing and gain structure
· 10 dB clear boost
· Analog design
· True bypass switching
· Very low power consumption (3mA and runs on a single 9V battery)
· High gloss mirror black powder coat
· Low noise design and pro sound
· Hand made in the USA

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toneman
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Post by toneman »

don't see the schematic?? always interested in jfet OD's.......:-)
Tone-to-the-Bone

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toneman
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Post by toneman »

Realized i found it on 6-9-2020...... :hmmm:
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WhisperOfScreams-2X-JFET.jpg
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