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Polaris balanced outputs for the Marshall JMP-1 is modular

Polaris balanced outputs for the Marshall JMP-1 gives our favourite MIDI valve pre-amp something that it should have had thirty-two years ago!

So why didn't Marshall do this in the first place? Well, perhaps for the same reason that many manufacturers at the time,  didn't put balanced outputs on their gear. Yeah, I'm none the wiser either!

Anyone with professional music performance or recording experience, will be aware that balanced audio connections aren't just used extensively in any big live set-up or recording studio but are pretty standard in those environments. In my own studio, EVERYTHING is balanced.

Balanced signals provide increased immunity to external noise, deliver a signal that's +6dB hotter than the unbalanced equivalent and are low-impedance so can run long distances.

Polaris Balanced Outputs for the Marshall JMP-1
Component density looks low but some components, like the bi-polar electrolytics are on the underside of the PCB.

The Marshall JMP-1's outputs already have the option to switch to +4dB so to have them balanced makes sense.


Firstly, unlike a normal (unbalanced) connection which comprises a single signal conductor and a return conductor (usually a screen), a balanced connection has three wires; an in-phase signal carrying conductor, an out-of-phase signal carrying conductor and of course, a screen.

Looking at a normal audio connection, any noise can be picked up by the single signal conductor, even through the screen. This will be passed on to the input of the destination device, like a mixer, for example.

Effect of noise on an unbalanced signal
Crude representation of a simple sine wave over an unbalanced line, mixed with noise.

On the other hand, any noise picked up by the two signal conducting lines in a balanced system, will appear equally in amplitude and phase on both lines. When fed into a differential amplifier (balanced input) all signals common to both lines, are removed and the only signal that's common to both lines is noise that's been picked up between the outputs of our sending device and the inputs of the receiving device. That's fundamentally, how balanced lines works.

Noise is the same on both lines in a balanced system
Notice how noise is the same as it affects both in-phase and out-of-phase signals in a balanced line system.

There is a third advantage to using balanced lines; balanced systems are low-impedance and without going into even more physics, they're able to drive signals over much longer distances than high-impedance systems.

Polaris Balanced Outputs for the Marshall JMP-1 Sockets

Polaris balanced outputs for the Marshall JMP-1, uses the same principles and components as my Nebula balanced outputs jack-board for the Roland MKS-70. A no-compromise design, Polaris has precision high impedance buffers on its inputs and dedicated balanced line drivers on its outputs, thereby guaranteeing the cleanest and most phase-coherent signal possible. With lots of attention to detail, the design includes for example, excellent separation between the audio tracks and the +/-15V rails. The feeds to Polaris from the JMP-1 main-board are via screened cables, thus ensuring minimal pick-up of noise from within the JMP-1. Polaris even uses gold-plated jack sockets!

All ICs have decoupling on both supply lines and all inter-op-amp connections are via high-quality unpolarised electrolytic capacitors, thus ensuring that no unwanted DC gets to the outputs. The signal path is minimalistic and very tidy indeed!

Polaris balanced output test
Here's a screen capture of my first test of Polaris. As you can see, with dedicated differential line driver stages, the phases are perfectly opposing.

When designing Polaris, the biggest challenge was how it would physically integrate into the JMP-1, especially in such a way that anyone with a good level of competence, can install it.

Of course the original two-pole sockets have to removed and (of course) you can't just connect three-pole (balanced) sockets in their place. My solution was to make a small, light-weight PCB that would mount using the rear sockets. Further connections to power and audio are made via Molex headers.

Polaris balanced outputs for the Marshall JMP-1 installs easily
Polaris tucks in nicely into the Marshall JMP-1 with power taken from conveniently available +/-15V points.

Marshall should have at least made the speaker emulated outputs balanced as they will inevitably be used for recording or sound reinforcement. Indeed Polaris was originally supposed to replace just those outputs.

Polaris however, soon evolved as I considered the master outputs and here's why...

Quite simply, more and more users are choosing to record from the JMP-1's master outputs. Feeding them into any one of a million impulse response speaker / cabinet simulation plug-ins, provides artists with considerably more options and in many a view, considerably better results than recording from the original speaker emulated outputs.

A versatile solution, Polaris gives you the option to have either one pair of outputs or both emulated outputs and master outputs, balanced. In fact, you can just do one pair now and fit another Polaris whenever you want.

Polaris will fit into both outputs of the Marshall JMP-1
Convert one or both pairs of outputs to balanced. Do one at a time or both simultaneously.

In a dual Polaris set-up, you only need to connect one of the boards to +/-15V on the JMP-1 PCB. A simple 'POWER LINK' connection between the two Polaris boards, delivers power from one module to the other. Coolamundo, dude!!!!

Polaris incorporates PowerLink for easy power connection if you have two Polaris modules installed
Polaris incorporates PowerLink for easy power connection between two Polaris modules, giving peace of mind if you wish to upgrade at a later date.

HEY, AND DON'T FORGET... With Polaris installed, you can still connect your JMP-1 to unbalanced inputs. Of course you won't get the benefits of a balanced line connection but things will still work just fine.

Over thirty years since its launch, the Marshall JMP-1 seems to be as popular as ever. Sirius balanced outputs for the Marshall JMP-1 ensures that using our favourite MIDI valve pre-amp with a sound reinforcement system or in a recording environment, will yield the best possible results with improved immunity to noise and hotter signals.

Polaris costs the same as a professional studio grade D.I. box. Being active however, means that it doesn't 'colour' the sound and has excellent phase-coherency over the entire audio spectrum. Of course, being integrated into your Marshall JMP-1 also means that there's one less box to carry around. 🙂

Check out all of my other JMP-1 stuff here. or you can buy Polaris here:


I design a lot of upgrades / mods for various pieces of vintage music technology but... I really don't like hacking stuff.

What I mean, is that I don't like designing things that require holes to be drilled or any other mutilations to be performed on our beautiful vintage gear. Indeed, all my upgrades and modifications are fully reversable. They can be quickly and easily removed allowing equipment to be returned to its original factory condition. Whilst sometimes a design challenge, for me, this is a big deal as I really value this old tech'.

And lastly... Occasionally, I get asked to design mods that will change the sound. Well, I'm afraid I don't do that. My feeling on the matter is that if you don't like the sound of something, get something you do like the sound of! What's the big deal, guys?