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Transformer coupled interface at Plasma Music

Many musicians, engineers and producers appreciate why signals should be kept as 'hot' as possible. High signal levels can reduce your noise floor and of course a lot of professional, studio equipment is designed to run at +4dBu (about 1.25 volts). What many might not appreciate is the virtue of also running balanced signal lines.

Running an analogue audio signal over a balanced line severely increases immunity to noise. I won't go into the physics of just why that is yet, although I'll no doubt end up adding to this post at some time in the future.

Unfortunately there's a lot of older gear out there including stuff we all love dearly, which has unbalanced +4dBu outputs. The Marshall JMP-1 that was featured in a recent post, is a prime example. The Roland RSP-550, an excellent nineties multi-effects processor, is another example.

Roland RSP-550 outputs
Unbalanced (switchable) +4dBu inputs and outputs on a Roland RSP-550

So, the conventional way to convert an unbalanced signal to a balanced signal is to run it into a D.I. box. We're all familiar with those little boxes that cover many a studio and stage floor, right? They work just great but while they balance the signal, they also knock down the level of the signal to a few millivolts which means that you have no option other than to run the other end into a mic. pre-amp, either as a separate unit or built into a desk. That's not always convenient and it seems stupid attenuating a signal, only to amplify it again, anyway!

Have you noticed another downside to trying to use a D.I. box to solve this problem, yet? Earlier I gave the example of the Roland RSP-550. It's an effects unit. It has +4dBu inputs! You ain't easily gonna be able to send signal from your desk to it via a D.I. box! The inputs on the RSP-550 are looking for either -20dBV or +4dBu. The microphone level output from a D.I. box just won't be enough to drive the unit. On top of that, the D.I. box output is balanced. The inputs on the RSP-550 are unbalanced. 🙁

To convert an unbalanced signal to a balanced signal and maintain the level of the original signal was a problem I decided to solve once and for all, many years ago. I developed the transformer coupled interface (TCI). It's not rocket science (as they say) but boy, do these things come in handy.

Transformer coupled interface schematic (basic)The TCI is a non-powered device (like a passive D.I. box) which basically comprises a single transformer on each channel. I designed the transformer myself as not only did I want the best linearity I could get over the entire audio spectrum but I also wanted minimum phase distortion, especially down the bottom end. Anyway, I managed to get a bunch of these things made here in the UK and the TCI was born.

Similar to a passive D.I. box, one hidden benefit to using the TCI, is that it offers something called 'galvanic' isolation which means that the signal path between your source and destination equipment is 'broken'. With the ability to easily lift the earth on the output (balanced side) of the TCI, these things just great for getting rid of ground loop problems!


All equipment has what’s called an input impedance and an output impedance. Impedance, like resistance and is measured in Ohms but differs in that it is frequency dependant. The impedance varies depending on the frequencies in the signal. Having said that, due to the nature of things, a common resistor is often used on for example, the output of an electronic circuit to provide the output impedance of a device.

Most guitarists will be quite aware that guitar electronics is high impedance and that the first thing you plug your guitar into should have a very high input impedance. This reduces the ‘loading’ on the guitar pick-ups and allows for a good frequency response.

Unbalanced outputs such as those found on a lot of consumer electronics such as hi-fi gear, have a high impedance, perhaps in the order of thousands of Ohms. Balanced outputs in contrast, are always low impedance; just a couple of hundred Ohms. Again, I won't go into the physics suffice to say that this why amongst other things, low impedance outputs allow for long cable runs.

Having said that, unbalanced outputs on a lot of ‘studio’ gear can be low impedance. This makes life a lot easier!

The TCI has a low input impedance and while it will technically work on something with a high output impedance, you’ll notice some high-end roll-off. Basically, you’ll need to check the output impedance of your gear before considering a solution like the transformer coupled interface. That’s why D.I. boxes are so popular. D.I. boxes have a high input impedance which means you can plug in just about anything. A passive D.I. box will also have a transformer to convert the input to a balanced line. The turns ratio of the transformer however, is such that while presenting a high impedance at the input and low impedance at the output, the signal voltage is greatly attenuated.


Marshall JMP-1 serviced at Plasma Music

Here’s a Marshall JMP-1 MIDI valve pre-amp that I’ve just serviced.

I’ve just changed the valves and PSU capacitors, cleaned up the metal work and implemented my ‘Live-for-Ever’ battery mod’.

The serial number indicates that it was built in early 1992 which not only makes it twenty-eight years old and one of the first off the production line.

The first MIDI valve pre-amp (I think), the Marshall JMP-1 has always been a really under rated bit of kit, despite the fact that named artists such as Billy Gibbons (ZZ Top), Phil Collen (Def Leppard), Iron Maiden, Megadeth and many more, still use them even today. In production for well over ten years, that’s got to say something about it.

The JMP-1 is not a modeller like modern stuff; it’s the real thing, just with MIDI. Run into a valve power amp (or two), it just sounds amazing and the quality of the speaker emulated outputs is definitely good enough to run straight into a desk or your DAW. Note, that alkthough the outputs can be switched to +4dBu, they're unbalanced so you might want to think about either running them into a D.i. box or a balancer (in fact I have just the thing), first.

They now go for between £400 and £700 (yes, £700). If you can pick one up nearer £400, then I personally think it’s well worth it.

Custom designed electronics at Plasma Music

Over the past couple of weeks, I’ve been designing a gadget to switch on a power amp when an audio signal is detected and to wait fifteen minutes to switch off the power amp when no signal is present on the input.

The audio signal comes straight from the sub bass output of an A/V amp and is therefore about -10dBV (approximately 310 mV) unbalanced. So apart from auto switch-on / switch-off, the circuit also amplifies the input signal to +4dBu (about 1.25 V) balanced so as to properly drive the power amp.

After testing a prototype for several months, this evening I ordered the PCB (the  design of which is pictured above) for the final version.


Well, the power amp in question, drives a passive sub-bass system on a home cinema. It's a big, professional amp and definitely wouldn't come with auto power-on / power-off which of course, the main (consumer) A/V amp does have and which 99.9% of sub-bass units have built-in.

When the home cinema system is switched on, the power amp comes on when it detects a signal. When the system is switched off, the power amp switches off a few minutes later... PERFECT!


I don't see the point of reinventing the wheel, especially if someone has already designed something really good that works. Hence, the switching circuitry of this gadget is based on a design by Rod Elliot of Elliot Sound Products(see project 38). Rod puts up some fantastic, in-depth explanation into how the circuit works and how it can be tweaked to your specific requirements.

Roland VK-7 transformer power supply fix at Plasma Music - keyboard repairs in Hertfordshire

This Roland VK-7 belongs to my mate, Alex Richards. The transformer is mounted upside-down on the inside of the top chassis. Guess what? It fell off! The machine  was plugged in at the time so the accident caused considerable damage.

To avoid this happening again, I mounted this heavy little component to the bottom (steel) chassis and laid new connections to the power supply board.

Twenty years old, has taken quite a few knocks, it plays and sounds great.

Nektar MIDI Controllers - best tech support ever from Nektar - keyboard repairs in Hertfordshire

Customers’ units come first so when my own stuff needs repairing, it kind of gets put to the back of the que, especially if it’s not a critical system.
Anyway, a few weeks ago, the attack slider on my Nektar Impact LX61+ MIDI keyboard started playing up. I tried to do a quick fix but the potentiometer was definitely duff. I fired up a support ticket and received the most amazing response from Nektar. The company basically just sent me a whole new PCB! All I really wanted was the slider pot’.
Cost-effective, versatile and easy to program, the Nektar Imact range of MIDI controllers seem perfect for any project studio set-up. Now that I’ve experienced the company’s support first-had, I really can’t recommend them enough.
Mine’s a 61-note which means it’s only got USB out. With a USB switch though, I can run it into my DAW or to a USB MIDI host which allows me to drive my MIDI synths.

Of course I don't just do Marshall repairs on Mondays but what a great way to start the week; a gorgeous 4502 JCM900 50W Dual Channel Reverb on the bench. There's nothing too wrong with this amp. It just came in for a re-valve and a tune-up (bias check) after the customer complained that it sounded a bit dull and lifeless.

In my humble opinion, the JCM900 Dual Channel Reverb was the last 'proper' Marshall. Personally I'm a 100W head freak but the 50W heads and combos do sound sweeter. Put a 50W through a 4 x 12 and you get a whole lotta' beef underneath that sweetness.

Marshall 4502 re-valve and bias at Plasma Music - amp repairs in Hertfordshire


I get a lot of people ask me about impedance matching valve amps and speaker cabs so sometime over the next couple of weeks, I'll be posting some bumf on how it all works and best practices. Stay tuned!

Yamaha DM2000 User Defined Keys at Plasma Music
General Purpose Interface under the User Defined Keys section on a Yamaha DM2000

Virtually all high-end Yamaha mixing desks such as the DM series, PM5D, LS9 and CL / QL series to name but a few, are equipped with the Yamaha  General Purpose Interface or GPI. The picture above shows the GPI buttons under the 'User Defined Keys' section on a Yamaha DM2000.

It's a great idea and can be so incredibly useful in so many situations. Unfortunately, very few people know what GPI is all about and Yamaha only offer limited implementation options.

Often used to interface a desk with video equipment, the Yamaha GPI standard can also be used to switch on and switch off other equipment such as amplifiers, computers and outboard gear which might be located some distance from the mixing desk. For recording and live applications, this facility could be so incredibly useful.

To take advantage of the Yamaha general purpose interface, I build a distribution box and various gadgets which will allow the GPI on your Yamaha mixing desk, to do just that.

Pictured below is a 3-channel mains power switcher in a solid steel case, capable of switching three loads totalling 10 amps. This particular interface switches three power amps located in another room in a recording studio. Also pictured, is the breakout box which splits the GPI from the desk to eight individual lines. Channels 7 and 8 of this particular switch for example, are used to switch on two computers.

Yamaha GPI Custom Distribution and Mains Switch at Plasma Music
Do magic things with GPI switching and breakout systems at Plasma Music

If you have a Yamaha mixing desk equipped with GPI and want to remotely switch gear on and off from it, then don't hesitate to get in touch.

Fireface 800 Power Supply repair at Plasma Music - audio repairs in Hertfordshire

Many will know that I'm a mega-big RME fan. Stylish, reliable, oozing sonic excellence in abundance and very easy to use, RME interfaces like the Fireface 800 are just so transparent.

Every six or seven years however, the power supplies on these systems pack up. Delivering a reliable but significant output, they do get hot and despite the use of high-temperature tolerant components, time takes its toll. Symptoms range from flashing lights on the front panel, to  the unit not powering up at all.

Anyway, I can fix 'em. For a flat £50.00, I can get your FireFace 800 or similar generation RME box, up 'n' running again. It's well worth it. These interfaces are just great. Just contact me.

Fireface 800 Power Supply repair at Plasma Music

Click here for more information on RME interfaces.

Plasma Music Sale 2019

I'm currently downsizing my recording facility and so a lot of stuff is now up for sale.

It's not just studio gear but drums, guitar and keyboard stuff, video cameras and more, computer equipment and live performance gear like PA systems, amps, etc.

It's all been lovingly looked after while I've had it and most of the stuff is in pristine condition. Please do check it all out: before it all goes on eBay!