Buffer + Bypass "Quasi-IC" Module

[MattG]

I also posted this to DIYStompBoxes, but it's not getting much attention over there, so I thought I'd see if it piqued anyone's interest here.

In my pedal builds, for bypass switching, I generally prefer a buffered electrical bypass scheme (similar to typical Boss pedals). I've been doing this with a CD40106 for footswitch response and state management, and a CD4053 for the actual audio signal switching. I've used this circuit as a standalone module as well as directly integrated into the PCB of the effect itself.

Integrating it directly with the PCB is frustrating with through-hole components, because it uses up so much precious PCB space. The standalone module works fine, but I don't like all the added board-to-board wiring.

I got to thinking, the ideal would be if all this functionality was contained in a single integrated circuit. I don't think such an IC exists. Having custom ICs fabricated is unlikely to be accessible to hobbyist budgets any time soon. So I figured the next-best thing is a "quasi-IC", basically a small (i.e. using SMD components), standalone PCB that can be directly-soldered to the actual effect PCB. My other observation is that all the new and improved ICs are generally only available in SMD format.

Attached is the design of this "quasi-IC" circuit so far. An overview of the design:

• TLV9301 for virtual ground buffer: available in ultra-small SC70 package, 150uA quiescent current, can drive capacitive loads
• AP7375-50 5v LDO: tiny sot-23 package, 3uA quiescent current, up to 45v input voltage
• DG413 for audio switching: replaces the CD4053 I previously used; quad SPST switches, two normally open, two normally closed; one is unused in this design, the remaining three exactly mimic Boss-style JFET audio switching; 35R on-resistance
• SN74HCS74 for state-management; this is a dual D (data) flip-flop with Schmitt trigger inputs (think CD4013 with Schmitt inputs); one flip-flop is unused, the other is wired as a T (toggle) flip-flop (/Q tied to D); replaces the CD40106 in my previous design; available in ultra-small SOT-23-THIN package
• OPA1652 input and output buffers: hifi-grade opamp with super-high impedance JFET inputs, tiny bias current, rail-to-rail capable, overkill specs

In the schematic, the "offboard wiring" will be castellated PCB edges, so the actual effect PCB ("motherboard") would have corresponding solder pads (turning this bypass mini-PCB into a "daughterboard").

The current draft PCB layout is about 38mm X 23mm. Not quite IC sized, but dramatically smaller than the through-hole equivalent.

Any thoughts/comments/criticisms? Notice any errors or gotchas I likely overlooked?

Edits from the original DSB post:

• I just noticed I have R22 as 22k, that's a typo, should be 22R. (Only off by 1000!)
• I also realized the input cap C18 plus the R4 Vbias pullup resistor will have a time constant around 10 seconds. I'll think on this a bit, but 1uF or maybe even 470nF is probably fine here.

 

[MattG]

Here's an updated schematic. I'd like to think I'm getting close to a prototype fabrication run. Changes from the previous schematic:

• Changed the buffer opamp from opa1652 opa2197; besides being cheaper, the opa2197 is rail-to-rail output and input, and also has some built-in RF protections
• Put a small RF filter at the circuit input
• The circuit output now has a 1M pulldown resistor
• Added p-mos reverse polarity protection (instead of series 1n5819)
• Added a SN74LVC1G14 Schmitt-trigger inverter on the footswitch (the SN74HCS74 flip flop triggers on a rising edge, so I originally had the footswitch wired directly to the 5v supply; I didn't like this, so in order to tie the footswitch to ground but still have the effect/bypass trigger on a press, I had to drop in an inverter)
• Several other small tweaks

This is what I have for documentation so far:

• Power supply: the module assumes the parent PCB has at least 22uF of bulk power supply decoupling
• The +9v power supply pin is intended for any voltage within 9 to 18 volts
• The status LED cathode pin has only 4.7k of current-limited resistance; this is provided only for bare-minimum current limiting on this PCB; this is a reasonable value for standard LEDs and 9v operation, but you will generally want to add current-limiting for ultra-bright/high-efficiency LEDs and/or 18v operation.
• VREF is provided only as a high-impedance bias reference; it should be loaded only with large resistances, preferably 100k or greater; do not use VREF as a supply rail, LED reference, virtual ground power node, or return path
• The footswitch should be a 2-pin normally-open momentary SPST switch. Connect one pin to FOOTSW and the other pin to the motherboard's 0v/GND. Footswitch leads should be as short as possible (<10cm ideally), and tightly twisted.
• The footswitch debounce timing is approximately 10-15ms on switch press, and 100-250ms on release (press debounce timing may vary with switch model, age, dirty contacts, etc, as well as actual component tolerances)
• EFFECT_PCB_IN and EFFECT_PCB_OUT are AC-coupled; the effect must provide its down DC biasing (VREF is available, but cannot be loaded)

 

[jwin615]

Cool idea.
Why not just put this on an IO board with jacks?

 

[MattG]

Cool idea.
Why not just put this on an IO board with jacks?

That's a likely next-evolution of this circuit. Main reason I don't do it now is that I hand-drill my enclosures and I don't feel that my consistency/precision is good enough for an I/O board. Quoting myself from this thread:

I think the next evolution of this idea - and I've seen other people here do exactly this - is to make this quasi-IC into an I/O and power module, where you solder the power and I/O sockets directly to it. Then use a ribbon (or JST) connector to go between the I/O and power module and the actual effect PCB. The only reason I haven't done this myself is that I hand-drill my enclosures - wired I/O and power jacks makes for very forgiving enclosure drill holes. But it looks like all the board-mount DC jacks are rectangular, and I don't know how to cleanly make a rectangular hole. So I feel like I'd need some kind of robot-based drilling service to make these precision holes for me to realize such a module.

 

[szukalski]

Another vote for the IO board. It removes another component completely from your build, and removes the need for jack in/out pads.

Personally, I don't care about the rectangular hole thing. I just drill an oversize hole for the power, it gets filled by the power cable anyway.

 

[MattG]

Another vote for the IO board. It removes another component completely from your build, and removes the need for jack in/out pads.

But it also removes flexibility... making it into an I/O board basically forces a specific layout. Do you want top-mount 125B, top- or side-mount 1590B, what about 1590A? Etc etc.

The idea is to think of it as an IC for much of your "pedal boilerplate" needs, for use with custom effect PCBs. It will be awkward to use for readily available commercial boards (e.g. PedalPCB, Aion, etc).

So taking the I/O board concept further - why not make the entire effect PCB direct-soldered to I/O and power jacks? That's a big part of the thinking behind this module: it is supposed to facilitate more freedom/flexibility in the PCB design process.

Personally, I don't care about the rectangular hole thing. I just drill an oversize hole for the power, it gets filled by the power cable anyway.

Clearly you're not as neurotic as I am. Even though I know it doesn't really matter, big round hole vs rectangular jack... I'm getting anxious just writing about it!

 

[MattG]

I should also add... I have mixed feelings about I/O jacks being direct-soldered to a PCB anyway. It certainly makes for neat and tidy internals; and it's convenient for building. But the jacks are one of the highest physically-stressed parts of a pedal. So having them isolated from the PCB with physical wires provides some protection for the PCB. It also makes high-wear parts more readily serviceable.

 

[LukeFRC]

So taking the I/O board concept further - why not make the entire effect PCB direct-soldered to I/O and power jacks?

there was a thread a few weeks ago where someone had done that.

I like the idea of yours as a wee IC like plug in module - and possibly common breakout boards with space for it.

 

[szukalski]

You can make it compatible with 125B/1590B/1590BB and that covers the majority of use cases. Though you need to compromise on length with 1590B, and clearance can also be a challenge.

It depends on what your use case is, I guess. Mine is for a more enjoyable building experience, so the fewer modules and cables, the better. Add plugin connectors and I’m even happier with assembly!

I’m not sure there’s a problem with durability of onboard jacks. There are plenty of manufacturers doing it, and Ive never heard of it as a common fault.