Fæpbœx II: 2-Band Preamp

[EricssonB]

While planning for the Bœx series of pedals, my friend requested to build and older-style preamp to contrast the power of the more modern 3-band that I was also building. Found schematics to an older MM preamp and built to that. Glad he did, as the frequency range on this one is phenomenal! Not quite as powerful as the 3-band that I built, but it makes up for that in tone.

This thing is great for pushing an overdrive pedal -- wow.

Like FBIII, these'll be cracked in half, thrown into 1590 enclosures with a bunch of rad LEDs and a 3PDT switch. I just primed the enclosures and am waiting to buy some rattle-cans to finish them up when I get payed again.

I don't have money to buy electronics for all of my instruments (2 guitars, 2 basses), so building what would normally be internal preamps and sticking them into enclosures is a damned-good compromise.

Hopefully soon I can shutter the factory and get my studio back into full summer production; finish the ol'album that's going on seven years.

Spækerbœx: 27.2Hz High-pass filter
Fæpbœx III: 3-Band Preamp




...how romantic. Not only do they share boards, but currently share a power supply!

Edit: Currently! Hah! Electricity pun!

[Pollinator95]

Originally Posted by EricssonB

Edit: Currently! Hah! Electricity pun!

Are you going to leave the board like that or snap it for a smaller enclosure?

[EricssonB]

Originally Posted by Pollinator95

Are you going to leave the board like that or snap it for a smaller enclosure?

1590s, sir. Standard generic pedal size. Definitely be updating when I get them some. Heck, I got payed today! To the paint store!@

Originally Posted by EricssonB

Like FBIII, these'll be cracked in half, thrown into 1590 enclosures with a bunch of rad LEDs and a 3PDT switch. I just primed the enclosures and am waiting to buy some rattle-cans to finish them up when I get payed again.

[Pollinator95]

*Facepalm* Must have skipped that paragraph to the link or something.

[EricssonB]

Working on the layout. Any suggestions?

Yeah, it's going to be polished aluminum under black burlap. Yeah. It's going to be rad. Red and orange LEDs, sort of a Halloween theme.

[Stealth]

I like the perfboard layout a lot, and I applaud your effort with the boexes.

There are a couple of things I'd suggest, though. I just went through my files and found a schematic of the old '76 MM preamp (or, rather, Bajaman's schematic of it and SGD's corrections) which yours is based on, and I found a few things that differ.

In the SGD corrected schematic:
R4 = 220kΩ (bass pot bias resistor)
R6 = 2.2MΩ (quiescent current resistor)
C9 = 10μF (output cap)
His schematic didn't account for the LED on the R6 - is the LED the reason you dropped the R6 so low?

[EricssonB]

Originally Posted by Stealth

I like the perfboard layout a lot, and I applaud your effort with the boexes.

There are a couple of things I'd suggest, though. I just went through my files and found a schematic of the old '76 MM preamp (or, rather, Bajaman's schematic of it and SGD's corrections) which yours is based on, and I found a few things that differ.

In the SGD corrected schematic:
R4 = 220k? (bass pot bias resistor)
R6 = 2.2M? (quiescent current resistor)
C9 = 10?F (output cap)
His schematic didn't account for the LED on the R6 - is the LED the reason you dropped the R6 so low?

Bassically. I plan on having a few always-on LEDs, so I figured I'd put a test LED on the board to make sure that it's on. I'll probably update the drawing when I figure out how many will be plugged in around the enclosure and how much throttling they require.

[EricssonB]

Two months later and a lot of life happening between the first two of the Fæpbœxes are done, with two more to go.

Now, a problem I'm having with Fæpbœx II is that there's a gigantic pop when switched on and off. I'll place this on it being designed for on/being used or off/not being used, as it's a design from active basses.

I've tried pulldown resistors all the way down to 20k Ω, on both the input and output sides of the circuit and it doesn't help.

Any ideas? Kinda fed up with it and may just say "DEAL WITH IT" as it's a preamp and it likely won't be switched very regularly.

[alec]

Originally Posted by EricssonB

Now, a problem I'm having with Fæpbœx II is that there's a gigantic pop when switched on and off.

Isn't that normal for a 3PDT switch?

From Iron Ether's website:

Q. What is “relay-based true bypass”? Is it really true bypass?

A. Yes, it is absolutely true bypass. This means that when the pedal is bypassed, the signal goes directly from the input jack to the output jack via a mechanical switch and does not go through any buffers, electronic switching or any other circuitry that could affect tone. What makes it different from the true bypass found on most boutique pedals is that instead of the standard 3PDT switch, we use a mechanical relay meant specifically for audio-type signals. It is a more expensive and complex solution, but we use it because it provides several advantages: no build up of static electricity and thus no loud pops, a quieter & smoother switch, greater reliability, and the ability to automatically go into true bypass if power is ever lost to the pedal, which can be a life-saver on stage.

[Skrogh]

Nahh if things are wired correctly, the 3pdt should not make any loud pops. maby a slight "puf" ´, but no pops. How is the switch wired up? Are you turning on a lot of LED's directly at the switch? That could lead to a pop.

[EricssonB]

Added a 33k Ω resistor and between the internal side of the input capacitor and ground.

All unwanted electrical noise is gone.

-- That's the fix if you want to run full voltage (18V) with bright LEDs.
-- If not, lower the voltage. 9 or below works.

[SGD Lutherie]

Originally Posted by EricssonB

Added a 33k Ω resistor and between the internal side of the input capacitor and ground.

Use a 1megohm resistor to the input side of the first coupling cap, and output side of the last coupling cap. That's a pull down resistor, and bleeds off the stored charge on the coupling cap which is what causes the pop.

33k will set too low of an impedance to the front of the circuit and dull the tone.

Shown in blue:

http://www.muzique.com/news/pulldown-resistors/

[SGD Lutherie]

Originally Posted by Skrogh

Nahh if things are wired correctly, the 3pdt should not make any loud pops. maby a slight "puf" ´, but no pops. How is the switch wired up? Are you turning on a lot of LED's directly at the switch? That could lead to a pop.

The coupling caps are popping because the circuit was not made to be bypassed.

[EricssonB]

Originally Posted by SGD Lutherie

The coupling caps are popping because the circuit was not made to be bypassed.

Yeah that's what I ended up figuring out. I tried pulldown resistors, but they really weren't doing it until I moved them to the inside of the coupling cap. That did the trick.

[SGD Lutherie]

Originally Posted by EricssonB

Yeah that's what I ended up figuring out. I tried pulldown resistors, but they really weren't doing it until I moved them to the inside of the coupling cap. That did the trick.

You have to keep in mind that putting a 33k resistor inside the front coupling cap is going to affect the treble control. You are lowering the inout impedance of the preamp, and loading down the treble control which feeds into the input of the op amp.

When I do these with bypasses I use the pull downs on the other side of the caps.

[EricssonB]

Originally Posted by SGD Lutherie

You have to keep in mind that putting a 33k resistor inside the front coupling cap is going to affect the treble control. You are lowering the inout impedance of the preamp, and loading down the treble control which feeds into the input of the op amp.

When I do these with bypasses I use the pull downs on the other side of the caps.

Yeah, and I actually did some experimenting with the values and where the pull down resistors were placed. I'm not terribly scientific about this sort of thing. I really needed to find a balance between tone loss and that house-rumbling pop of switching this thing on and off. It was BAD.

33k didn't seem to affect tone, and I'm overwhelmingly glad that the pop is going. The treble could afford to be rolled down a bit.

It's screwed closed and I don't want to open it up again. Ever. Ever! EVER!