Adding dual module support to single module amps

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Apr 15, 2015
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I've had a couple of requests for info about the RM20 amp that I modded to accept dual channel modules, so I thought I'd share all the info here on this thread. I don't intend to do any of these mods for folks myself, but feel free to use the info yourself or share it with a tech who can do the mod. I can't provide step-by-step info for every amp type or switching situation, but I can outline the basic concepts.

I'm sure Rob (JF) could whip something together for folks if he has the bandwidth.

The Basic Premise
Single channel modules are enabled (switched on and off) by pin 7 on the module connector. If a +5V signal appears on pin 7, the module is on. If pin 7 is grounded, it is shut off. The switching for dual channel modules is the same, only pin 7 activates channel A and Pin 8 activates channel B. For a combination of modules and channels, only ONE "enable" pin (either 7 or 8 ) should be activated with a +5V signal. The rest should be grounded. The basic concept is simple, but the switching logic and methods are where things can get complicated.

The RM20
The simplest amp is the RM20, with only one single channel module. To accept dual modules, you need to be able to switch the +5V from pin 7 to pin 8. The unused pin, either 7 or 8 must be grounded. The simplest way to do this is to just use a DPDT switch to rout the +5V to either 7 or 8, while simultaneously routing ground to 8 or 7 (the opposite pin). This can be done by connecting one switch pole to +5V and the other switch pole to ground. The throws are then connected to each other in an "X" pattern (throw 1A to 2B and 2A to 1B). One throw pair is then connected to pin 7 and the other to pin 8 on the module connector.

On the RM20, the +5V is created by sending the unfiltered +17V rail to a dropping resistor, but I found that pin 8 drew less current than pin 7 so that the signal voltage was higher when pin 8 was active. To fix this, I removed the dropping resistor connected to pin 7 (to disconnect pin 7 from a constant positive voltage) and took a +5V signal from a 5V regulator used in the boost circuit. So basically, I could have stopped here and had a DPDT toggle switch for channel A/B on the front of the amp. This mod would cost less than $5 for a good switch and would only need a single hole drilled into the chassis. However, I wanted A/B to be foot switchable, so it got a little more complicated.

The easiest way to make the A/B foot switchable would have been to add a 4-pin jack in the back of the amp and run a quad cable to a footswitch housing a DPDT/3PDT switch (like almost every stop box switch). This would basically act like a wire extender for the internal switch I discussed above. However, I would no longer have a front panel switch and the footswitch would need to be plugged in for either channel to be enabled. No switch, no sound. This could be fixed however using a switching jack that defaults to channel A when no footswitch switch is plugged in, however, you will need to find a good 4 pin switching jack. I'm not aware of any off the top of my head. If it exists, then this is the easiest mod to implement. Without a footswitch, Chanel A could be defaulted to, but you would still have no access to Channel B without the footswitch.

Instead I chose a more elegant solution using a programmable IC and a relay. Expat Audio( is a two-man part-time company that offers a pre-programmed switching IC geared toward studio gear, but it works great for channel switching in an amp as well. Basically, it is a three channel logic IC that can switch up to three relays with momentary contact switches. The beauty with this system is that all you need to do to switch your relay is to ground the appropriate IC input for a split second and the relay changes state (switches from A to B, or B to A depending). This means you can have a switch on the front of the amp, another plugged into a jack in the back, another plugged into a second jack on the primary pedal with another cable going backstage, etc. You can have 1000 switches connected in parallel, where if any of them are pushed, the channel will change. And if you have no switch connected, the amp will fire up with the default channel enabled. The signal output of the switching IC is sent to a FET which controls the on/off state of the relay. Schematic for this are shown on the Expat site in the Mic Pre Frontend manual. The only difference is that the relays in their design switch phantom power, a pad, and mic polarity. Instead, I just used a single DPDT relay in the way that I described above to switch the enable pins on the module card.

So in summary, I used the Expat IC connected to a DPDT relay to switch the enable signal and ground on pins 7/8. I needed +5 volts for the IC that I borrowed from the boost circuit, and +12V for the relay that I borrowed from the tube heaters. I put a momentary contact switch (normally open) on the front of the amp, and added a 1/4" jack to accept another momentary switch in a foot pedal.

I didn't add an LED to the footswitch because the LED indicators on the module were enough for me, but I could have easily. To do this, use a stereo 1/4" jack instead (or any other three pin jack, DIN, XLR, etc), and connect one lead to pin 8. This would have a +5V signal appear at the pedal whenever Channel B is selected. Add an LED and the appropriate dropping resistor and you have a Chanel B indicator on the pedal. The only downside to this design is that a less common momentary switch is needed on the pedal, but many good switches are available like the Boss FS-5U. The Boss FS-6 even allows for both momentary and latching with the flick of a switch. For a DIY switch, use any of the momentary stomp switches from Small Bear:

And finally, one drawback (or feature if you are me) is that if a single-channel module is installed, switching to channel B mutes the module because there is no channel B. There is a sense pin on dual channel modules to announce the presence of a dual (pin 9 is grounded on duals, floating on singles). That could probably be used as the ground reference for your momentary switches so that you couldn't switch a single channel module, but I haven't tried it yet myself. It should work though off the top off my head.

The RM22, RM50 and beyond
The same premise holds true for amps with multiple single modules. You just need to get that +5V enable signal to the pin that will enable the module or channel you want, and ground the rest (all others) of the unused enable pins. This can be done by cascading relays, which is what I did for my DIY modular bass amp. However, you will need a switching relay for every module (12 for an RM1250U) in addition to whatever module selector switching you are using. It can get hairy really quickly as the number of modules increase.

But the good news is that it is easy to implement cascading relays for a two module single channel amp. Just feed the switched module signal from one switching relay (module selector) to two channel switching relays. These relays can be switched from a single button for global A/B like I did for my DIY bass amp, or with two buttons for separate module 1 A/B and module 2 A/B states. Fortunately, the Expat IC allows for three switches. This allows you to switch from super clean Module 1 channel A to kill-yo-momma gain on Module 2 Chanel B with one stomp.

Switch 1= Module 1/2
Switch 2= Module 1 channel A/B
Switch 3= Module 2 channel A/B

Since a schematic is worth 1000 words, I'm attaching the schematic I made for my two-button switched DIY bass amp to the thread. Just ungang the two channel switching relays and add one to it's own output on the Expat IC (with the proper FET circuit of course).

But what about MIDI?
Well, truth be told, I'm allergic to MIDI. Simplicity is my mantra, and I used to play most gigs with a '78 Marshall 2203 and a pedal or two if I felt like getting fancy. However, I do respect those with more elaborate systems, but I can't really help out much in that area.

I know that there are plenty of DIY MIDI projects out there on the web, with plenty of them geared towards switching stuff and control voltage outputs. If you remember the basic goal of getting +5V to the right pin and grounding the rest, I'm sure that somewhere there is an IC geared towards DIY MIDI to analog that can be bought for a reasonable price, possibly with an entire PCB that will be almost turn-key.

My final disclaimer
You've heard it all before, amps can kill you. No BS, they really can. You can also easily destroy an amp or a module if you aren't careful. If you don't understand everything about what I said in this thread, you should probably print it out and hand it to a qualified technician. I make no guarantees about any of this info. I may have made a mistake or two and I can't help ANYONE with a step by step guide. I don't have drop in PCB's designed for these mods, and wiring up perfboard without a good layout is about as fun and easy as doing root canal on yourself, and I don't have that layout. I just winged it twice and it worked despite the agony.

But on the plus side, feel free to use, share, post, host, print, archive, or memorialize this info in any way you see fit. Just don't sell the info, otherwise you are a bad person. I don't mind though if you use it and sell mods or products based on it though. Just give me a freebie or discount if you are feeling generous. ;)
Is this guide also valid for RM4? I suppose it is but i'm asking because i've always heard that the only one that could be modded is the RM100, and now you say that (with patience and skill) basically the all RM series is moddable, right? Thanks for posting this valuable infos
This looks like it has potential.
i am not sure I understand the limitations of the RM4 and the M4
compared to the newer Synergy stuff so must ask maybe somewhere else
but bookmarking this to come back to.
Yes he is saying that it is possible to run 2 channel modules on a rm 20,rm 50,ect....I'm going to attempt this on my rm 50 combo in the next few weeks so check back if you want the results of my adventure.