This Week on the Breadboard: The Barber Small Fry (PPCB Tater Tot)

[Chuck D. Bones]

This one has been in the queue for a while. It's everything people say it is. Huge range of tones, very smooth and amp-like. The breadboard was built to the schematic, but then I made two little changes.

1. On the TONE pot, I broke the connection between pins 1 & 2 and connected D5 to pin 2 of the TONE pot. This configuration is known as the AMZ "Stupidly Simple Tone Control." It keeps the volume from changing when TONE is rotated. Other than that, it sounds the same as the stock config.

2. I moved the LEVEL pot ahead of the last stage. With the toggle switch in the middle position and DYNAMICS at zero, the last stage can saturate. I fixed that by disconnecting C7 from IC1-5 and replacing R10 with an A500K pot (pin 1 to Vref, pin 2 to IC1-5, pin 3 to C7). The out pin goes directly to R7. Now when needed, we can reduce the LEVEL so the last stage does not saturate.

Sounds good with all of the trimpots at noon. I have not spent much time fiddling the trimpots.

I should mention that I wired the DYNAMICS pot in accordance with the Small Fry manual, i.e. resistance increases as the knob is turned CW. This is the reverse of the Tater Tot schematic.

I didn't have JRC4559D, so I subbed JRC2068D.

 

[BuddytheReow]

Aren't the BASS and MID controls very interactive with each other since they're tied together?

 

[Chuck D. Bones]

Not as much as you might think, but yes, there is some overlap. All of the action of the BASS trimpot occurs below 300Hz. The MID trimpot affects everything below 6KHz, but is strongest between 300Hz and 4KHz. These trimmers are for matching the pedal to the amp (or whatever comes after the Tater Tot). Think of it this way: the 1st stage cuts off a fair amount of bass and lower midrange. The 2nd stage puts some of it back. The trimmers let us control how much bass and lower mid get added back in. The BASS trimpot has a much stronger effect. If I was going to put one of the trimmers on the front panel, it would be BASS.

 

[BuddytheReow]

Curious. What is the fundamental difference between the PRES and SHAPE controls as per the PPCB schematic? They are identical from a topology standpoint. Are they just different corner frequencies to adjust similar to how a Rat has 2?

 

[Chuck D. Bones]

Those two overlap a lot. SHAPE varies the gain above 200Hz. PRES varies the gain above 800Hz.

Because these trimpots are before any of the clipping diodes, they have a strong effect on harmonic content when BURN is above noon.

 

[BuddytheReow]

It might help others understand what we're talking about if there's a schematic close by.

I have 2 more "circuit analysis" questions on this.

1. What is the purpose of D8? Is it just half of a hard clipper pair?
2. What does C6 do in the hard clipping section? Does this create a low pass filter with the DYNAMICS control or does it do something else?

 

[Chuck D. Bones]

When engaged, D8 provides asymmetric clipping. It works in conjunction with all of the other clipping diodes.

C6 works with D9, D10, R6, and the DYNAMICS pot to smooth the clipping a little.

 

[cooder]

I had an original one in a former life, which I sold when I started building stuff, as you know.
Sounds like I need to build one again now with the Boneyard mods. Maybe put the bass and pres dials on the dash to make it a 6 knobbie thing?

 

[Chuck D. Bones]

I'm still evaluating the utility of moving one or more trimpots to the front panel.