Combo build for pedal board core: Pre-boost, Clean/drive channel, cabsim, noise-gate, post boost.

[Paul.Ruby]

I've spent an inordinate amount of time creating a combo build with 3 footswitches in a 1590DD case. The picture tells most of the story. I'm happy to share info about it if anyone is interested... I'm wondering if anyone else has any interest in such a thing.... To make this all fit, I had to make custom PCBs, such as making the ZUUL (actually a Muzzle) as small as possible.

Guts pic... Ordinary soft-touch controllers are used for pre-boost and post-boost. The channel selector module is more complex and described below. The little hand-made PCB on the back of the ZUUL limits the the supply to 9V DC since the ICL7660 that makes negative voltage has a 10V max. Everything else in the pedal can accept up to 18V if desired.

It's very modular, despite being hard-wired in an individual build. The Zuul (actually a Muzzle) and Spark are always there with the Zuul always on. The full LA2 in the middle can be any LA2. The PCB has all the component options on it, but I really like the M2 as the core of my board. I have several different preboost PCBs that all fit the same three knob holes: Prince of tone, TS-808, OCD, Plexi Drive, Landgraph OD. I also have a REVV G2 PCB (Actually a Malachite) for pre-boost but that needs two more knob holes. Above the pre boost is an alternate drive channel. I have a PCB that can make an AMT B2, C2, E2, K2 or M2 as a stand-alone drive module to fit that spot. And I have a send/return for a 3rd drive channel as well. The picture below shows this in action to compare the various LA2 drive channels. M2 and K2 are installed in the main pedal and then B2, C2 and E2 in separate modules for comparison. But, any OD pedal can be put in the 3rd option as the drive channel. My buddy is going to put a Fortin NATAS pedal in that spot.

The channel select footswitch has an ATtiny84 as it's controller to manipulate 3 latching relays. It always switches between clean and drive but, if held down for a half second, it will switch to the next available drive channel option: M2, then other internal (K2 here), then external (C2 here) and back to M2. Once you see the one you want, release the switch to make it the drive channel. It also senses if there is an external drive channel plugged in and will skip that if nothing is being returned there.
This will eventually be paired with a pedal board power amp with integrated DI with XLR out. That's two pedals, plus tuner and wet effects for a complete setup.

 

[Paul.Ruby]

By the way, I should say... The AMT LA2 series of pedals sound great. I've compared the M2 to recent modeling pedals like the TC Ampworx 800 and V550. These old AMT analog pedals sound better compared side by side. My favorites are the M2, E2, C2 and K2, which are similar in tone but still distinctive. In the process of finding schematics and tracing some out myself, I learned that these also happen to all share the same topology with the fewest number of gain stages of all the LA2 pedals. The S2 and P2 both have two more gain stages, for example.

The clean channel is really good, despite having no EQ control of it's own (other than the mid-boost switch). It pairs really well with OD pedals to make them sound much more amp-like. The REVV G2 (Malachite on PedalPCB) sounds much better and really great in front of the LA2 clean channel.

Finally, the LA2 pedals pair really well with a pedal-board power amp. Have been using an M2 and TPA3118 based power amp as the core of my live rig for a couple years now, Keep trying newer stuff (like modeling pedals) and they still don't stack up. and the cabsim in the LA2 is great. I will be building new power-amp pedals with integrated DI and XLR out.

Anyway, seemed worth spelling out why I tackled this... Having recently discovered Kicad and cheap PCB services, I had to go down this rabbit hole and it worked out really well. Now I have a box full of little modules to make combo pedals with the LA2 being the anchor. This combo pedal is what I've wanted for a long time now.

 

[xefned]

This kind of build is what I love most about DIY: It's completely custom and there's no other pedal exactly like it on the planet.

Nicely done. It has everything you'd need for a live gig all in one box.

 

[Paul.Ruby]

This kind of build is what I love most about DIY: It's completely custom and there's no other pedal exactly like it on the planet.

Nicely done. It has everything you'd need for a live gig all in one box.

Yep! Modular but still custom and compact. This is what my buddy wants for his rig, right down to the satin finish instead of gloss.... He has a Fortin NATAS that might be his external drive option.

 

[Paul.Ruby]

A couple TPA3118 Power Amps with my Volume-Only preamp and integrated DI are finished today... The combo pedal, this power amp plus a tuner and some wet effects are a complete system on a pedal board.

 

[Paul.Ruby]

I've been doing a lot of ear testing and found the low end was lacking. Spent some time hacking at it but finally decided to do direct comparison of frequency response between a real LA2 pedal and mine. Lots of work comparing sub-circuits. Finally found the primary culprit. The cab sim had the wrong low end response. Although the component values are a match between the two, they were significantly different creating the low end resonance peak. For reference, this is the cabsim in the LA2 pedals.

CB8, CB9 and RB10 are the components that create the low end response. Original values in a real LA2 are 1nf and 750k. Shown above is what I landed on. Some results... I've also noticed the gain of the stages everywhere using 2SK208R is slightly lower than the real LA2. Notice in these curves that the shape of the real M2 and the 2nf of mine match very well but the overall level of the real one is a bit higher.

 

[Paul.Ruby]

Another tweak. RB16 controls the mid scoop. The original value of 51k has too much scoop. Changed to 100k. This is the full frequency comparison of real versus mine with the tweaks. Ignore the noise. Takes way too long to get the full spectrum. The real curves would be smooth. Red is real, Blue is mine. Yellow is the input signal.

 

[airbud]

this is such a sweet design. I'm planning my first combo pedals right now, and this is excellent inspiration!

 

[Paul.Ruby]

If I were doing this again from scratch, I would have done the frequency analysis ahead of time and probably made the cabsim a separate PCB. Although I did compare drive and clean channels behavior and tone, I didn't study the cabsim enough before doing the full builds. Then had to figure out why the full build didn't sound right. Also made some dumb mistakes on the PCBs because I was just learning KiCad, so there's a few bodge wires here and there. And, the components most likely to be tweaked should be on the bottom of the boards, which is facing up when installed. Removing the boards to remove a resistor was a huge pain. Fortunately, I did make individual PCBs for stand alone clean channel and cab sim and used those, with component sockets to get them dialed in before hacking at the fully built pedals.

If anyone wants any of the PCBs, I have a box full now. The point of these PCBs is to get as small as possible to make all this fit in the case, so they use alpha 16mm pots, 1/6W resistors and some surface mount transistors and ICs.

Pre-boost: Prince of Tone; Tube screamer; Landgraph OD; OCD; Malachite (Revv G2).
Alternate Drive: M2; B2; C2; E2; K2
Primary board can do any LA2 pedal and has the clean and cab sim on it.
I've designed an M2-only version of the full board. Not yet here but on the way.
And also the muzzle and mini spark.

And then the power amp volume-only or TMB for a separate 125B case.
And I have the volume-only with ATtiny soft-touch and XLR out.

I can do SMD soldering as well if that is needed.

 

[Paul.Ruby]

By the way, In using LA2 pedals now for years live and recording, something learned is that it's fine to have the cab sim right after the dirt and prior to all the wet effects. Cab sim does not need to be at the end of the chain. I've been trying to explain this to some guitar players and they aren't understanding why this this doesn't need the 4-cable-method. Because there is no send/return. The output of the combo pedal is everything needed prior to wet effects, including cab sim. Output goes to wet effects and then to the power amp and DI. The output of the combo pedal *is* the send. "Return" is at the end of the wet effects. This is the topology of the pedal and have lasered on to the bottom plate: