This week on the Breadboard: The Vivie Minerva (Wise Sheep)

[Chuck D. Bones]

This pedal has received some rave reviews in the Build Reports forum, so I had to breadboard it and see what it was all about.

The circuit starts out simple, with the first two stages looking pretty similar to the BluesBreaker. After that, there is a whole lotta EQ. At low GAIN settings, all of the clipping comes from D1 & D2 and we get only odd-order harmonics. When the GAIN is turned up, the first stage (IC1A) starts to saturate and then we get some additional even-order harmonics. R1, R8, C5 & C6 provide a very broad mid-cut centered at 1KHz. The TONE pot, R24, R2, C6 & C7 form a 2-stage low-pass filter. Then comes the LEVEL control, followed by a 9dB gain stage (IC2A). Now it gets interesting. The 1st gyrator (IC3B) creates a narrow -7dB notch at 5KHz. I'm guessing that's supposed to mimic a resonance in a tube amp's output transformer. The 2nd gyrator (IC3A) creates a narrow peak at 1.4KHz. The TUBE knob varies the height of the peak from 0 to 9dB. The net effect is that the 1.4KHz peak brightens the tone and emphasizes certain harmonics while the 5KHz notch reduces higher hamonics. There is a lot of interplay between the TONE and TUBE knobs. And then there is Amp Select switch. In one position, there is no additional filtering. In the other position, there is an additional 8dB of treble cut at high frequencies.

This circuit sounds great in stock form and I only have 3 complaints:
1. The reverse voltage protection (D3) has no current limiting and could burn up traces if the voltage is reversed. Sheepylove fixed this in his version.
2. R16 & R19 do absolutely nothing. It's bad enough that there is one resistor doing nothing, but Vivie saw fit to put a pair of do-nothing resistors. If you build this board, don't bother installing them.
3. The Q of the 1.4KHz peak is too high IMHO and does not sound good when TUBE is dimed. There are a couple of simple remedies for this.



Here's my breadboard. I made a few simple mods.

Knobs (L-R): LEVEL - TUBE - TONE - GAIN. The MODE switch is to the right of the LEVEL knob and is shown in the OFF (no additional treble cut) position.


1st mod was to correct the reverse voltage protection. It's now wired like the PPCB boards with a series 1N5817 diode.

2nd mod was to delete R16 & R19.

3rd mod was to retune the peak gyrator (IC3A) in order to reduce the Q slightly. Changes are:
C14 = 12nF
C19 = 6.8nF
R27 = 1.5K
R32 = 100K
I also changed the TUBE pot to C10K for a more uniform sweep.

4th mod is to increase the range of the TONE circuit by decreasing R24 to 1K and increasing the TONE pot to B25K. It now goes brighter and darker. The darker end of rotation obviates the need for the MODE switch. At the same time, I increased the LEVEL pot to B250K, returned pin 1 of the LEVEL pot to Vref, deleted R9 & R37 and jumpered C2 & C4, saving 4 parts.

5th mod is to increase the impedance of IC2A's feedback path so that it doesn't push as hard on Vref. Changes:
R20 = 150K
R21 = 82K
C12 = 470pF

Mods 1 & 5 are highly recommended, the other mods are optional.

The only other mod worth considering is to replace IC3 with a pair of emitter-followers. Saves power and one opamp.

 

[temol]

What about changing the opamp order? IC1A -> IC2A -> IC2B -> IC1B

 

[Chuck D. Bones]

I would not use the same device for the 1st and last stage because I want to keep input and output as far apart as possible. Many builders use a crazy order, but they're increasing the risk of oscillation.

 

[temol]

I would not use the same device for the 1st and last stage because I want to keep input and output as far apart as possible. Many builders use a crazy order, but they're increasing the risk of oscillation.

Yeah, I know.. but it's a bit easier to have a neat signal path/layout this way.

 

[Chuck D. Bones]

IC3 can be a TL072, then you can swap opamps between all 3 devices. Not recommending it, just saying it's possible.

 

[bluedmc777]

Where can one find the original project I can’t find it in the shop.

 

[leadfoot]

Where can one find the original project I can’t find it in the shop.

You can find it here: https://sheepylove.com/products/wise-ass-yabb

 

[HamishR]

I have built this on a Sheepylove PCB believing it to be a variation on a Bluesbreaker theme. It sure doesn't sound like one anymore! It's a very interesting sounding pedal. Higher gain than I would generally use (a LOT higher gain!) but some cool sounds. Any way to reduce the gain a bit without massive redesign? I'd be interested to hear how it might work at lower gain.

 

[Chuck D. Bones]

It bears a passing resemblance to the BB. But the EQ at every stage is different. The gain distribution is different. There is no resistor in series with the clipping diodes...

Reference the schematic above. To lower the gain, first thing I would try is increasing R12. Change it to 3.3K for starters. To keep the EQ the same, we need to lower C10 to 22nF. In the original circuit, the R12-C10 time constant is 68μs. We want to keep that the same, so if we make R12 = 3.3K, then 68μs / 3.3K = 21nF. 22nF is plenty close. I would leave R3 and R10 alone. They are well-matched to the GAIN pot resistance and taper.

A 2nd option is to increase the 2nd stage clipping threshold. We can try LEDs or stacking diodes. I already know which option you prefer.

A 3rd option is to reduce the 2nd stage gain by reducing R7. Try 220K and increase C7 to 2.2nF.

Try any mod, or combination. But do them one at a time so you know how each change affects the tone.