Moseley TFL-280 FM Broadcast Audio Limiter Modded for the Home Studio

Introduction:
My local music consignment store, Spaceman Music, hosts these garage sales a couple times a year. They usually try and get rid of overstock, half working, or just plain old guitars, pedals, amps, and studio equipment. These sorts of sales are absolute treasures for finding hidden gems such as this Moseley TFL-280 Audio Limiter. When I first picked this thing up, I had no idea if it worked, but it sure looked impressive. The thought of smashing a drum signal through an FM broadcasting unit got me excited and I made the purchase. Here is a picture of the insides:

I quickly searched for the manual and found a copy at this website here. Cited is a description of the circuit: “Audio limiting as used in broadcast transmission, is used to apply a definite ceiling to the level of modulation. By allowing the limiter to be active or to work an appreciable portion of the time, an increase in the received signal volume may be achieved.” If you are familiar with how compressors and limiters work, this is no different. The amplitude peaks are attenuated so the overall volume of the audio can be raised.

As with older audio equipment, input transformers are used in the signal path. If you’ve spent enough time around audio engineering forums, you know that the “transformer tone” is caught after and desirable. 

The attack time is fixed at 50 microseconds, which is considerably fast and great for heavy compression. The limiting knob lowers the threshold of the detection circuitry. The recovery knob is the same as a release knob on a standard compressor, setting the time the unit takes to return to 0 dB of gain reduction.

I tested the unit by sending a line level input. The pre-emphasis and de-emphasis switches were great at changing the overall equalization of the output signal. The pre-emphasis boosted the high end considerably while the de-emphasis decreased the high end. These switchable filters in the signal path are used by FM broadcasters to make increase intelligibility of the modulated signal. When limiting the audio signal quite hard, a lot of high end would be brought out by the limiting. By switching in the de-emphasis, these were tamed and would give as more natural response. Needless to say, it was a shame these switches were on the rear of the unit. Luckily we know how to solder and it’s a simple modification!

Modifications:
There were two modifications that I needed to see through for this unit to be usable in a home-studio setting. 

• 1/4 inch jacks as the inputs and outputs instead of the screw terminal blocks
• Move those pre-emphasis and de-emphasis switches to the front panel so I could change them on the fly.

I/O screw terminals.

Switches I want to move to the front planel.

Switches on the back panel with their cable shielding fixed to enclosure.

Photo showing the beautiful cable management, zip-tied and routed around the PBC.

There are three switches on the back plate, but I only want to move two of them. The noise reduction switch deals with switching in an external noise reduction unit, which I don't have and have no use for. The cabling going to the switches are nice and tidy, grouped together with zip ties. I don't desire to splice and re-solder the wires to the right length. As shown later on, I'll just be bending them neatly so they are out of the way.

Luckily, there is a lot of free real estate on the front panel where it's only natural to have those pre and de-emphasis switches. The wires going to these switches are shielded and the shielding is connected to the enclosure with a simple screw and nut. I had to drill a hole in the bottom plate to fit this enclosure ground terminal.

Photo of inside of front panel showing good free real estate for switches.

Switches mounted to front panel with ground shielding still unconnected.

Extra hole drill on bottom of enclosure to secure screw ground terminal.

Cable neatly bent with blue zip ties being the ones I added.

Photo of front panel with newly moved switches.

All the blue zip ties are the ones that I've added to make the wiring nice and neat. I wanted to avoid having them rest on any components or but any unwanted stain of those flimsy tantalum capacitors, for example.

All in all, moving the switches was fairly easy. You just have to make sure you solder back any connection you had to de-solder when moving to the new location. 

Next up was connecting the input and output jacks. Since this unit was going to be mainly used as a channel insert, with my set-up, I only need unbalanced connections. I decided to use the two newly vacant holes where the switches were and enlarge them to fir the 1/4inch jacks. Then it was as simple as soldering the jacks to the block terminal in parallel. This way I can use both the screw terminal and jacks if need be. I still wanted the signal to go through those input inductors for the added filtering, so I made sure to solder the jacks as if I was connecting the input and output cables from the outside. 

Photo of behind the screw terminal with filter inductors in series with connections.

Used vacant holes from switches to place I/O 1/4inch jacks.

Routed jack connections to back of the screw terminal and soldered in place.

Photo of input and output jacks on back plate.

Important note: this unit has a balanced input, hot, cold, and ground, but I wanted to send and unbalanced signal to it. I tested two configurations with different noise results:

• (Tip) --> hot 
• Ground (Sleeve) --> cold and ground
• This configuration had significant 60 Hz hum!

and 

• Signal (Tip) --> hot 
• Ground (Sleeve) --> cold 
• Hum eliminated!

By sending the single ended signal into the differential input (hot and cold) of the limiter's input, I got a nice and clean sound with no added hum. To make this possible I need to isolate the sleeve from the  1/4inch jacks from the metal of the enclosure. This was done using heat think tubing around the shaft of the 1/4inch jack where it would be touching the enclosure. Furthermore, I used rubber washers before and after the enclosure so none of the metal from the jack is touching the enclosure in any way. I of course used a multimeter to ensure this isolation was stable.

Important note 2: Since I was using unbalanced inputs and outputs, I hooked up the unit to a scope and ensured that I was getting the right polarity between the input and output. The last thing I would need is to have the output be 180 degrees out of phases with the input signal.

But How Does it Sound?
Preliminary tests with this limiter confirm it to be an absolute UNIT. The attack time is incredibly fast, perhaps faster than you would normally want, but it adds something nice. The recovery knob is really essential to adjust from situations to situation. I've tried it on drums and vocals. Really great at smashing drum bus or room mic. On vocals, it can really push the sound right up front.

As for the switches, the pre-emphasis ads this incredible sparkle to the sound and just lifts up the presence of any signal I put through it. When you start to really compress a drums signal hard, the cymbals really start to get a bit much with this unit. The de-emphasis control takes that down and brings it down to a more usable sound. Although counter intuitive, when I engage both the pre-emphasis and the de-emphasis, there is a nice mid range bump happening somewhere. Moving these switches to the front means I can easily switch between a more bright or dark sound and see if it works with the track.

I'm thinking of posting some audio examples sometime soon, stay tuned!

First DIY Synth

One of my most recent endeavours was in the world of synthesizers. As of today I've made a pretty interesting and unique little oscillator. 

I must admit, the enclosure is not the the prettiest, however it does get the job done as I needed to put this thing in a box quickly for an upcoming gig. 

The schematic was from this one youtube video (https://www.youtube.com/watch?v=yZSkNASlMjM)

I made a few modification to the circuit. First off I made two independent oscillators make it sound even fatter. The circuit has one main square wave oscillator. Underneath it is a Low Frequency Oscillator, same circuit as the main oscillator except at a lower frequency. This controls the Pulse Width Modulation or PWM for short. In the original schematic, the frequency was fixed. This means the LFO was only oscillating at one rate. Wanting to experiment I figured it would be really cool, and easy enough, to add a pot to be able to change the rate. Of course this added a new dimension to the sound and I'm super glad I did this cause it just adds to the versatility.  

I also realized that this is a resistance based oscillator, as opposed to the classic control voltage which is a standard amongst synths. Being resistance based, I figured anything with a resistance could control the pitch of the oscillator, even sweaty palms! This lead me to the crazy idea of somehow integrating this synth is a new and exciting way. But first here are some pics of the inside:

 Not my cleanest work but it needed to get done. Still solid though, nothing has gone wrong...yet...

 Yes! Those are key switches! Why not have some fun, am I right?

Being resistance based, I decided to put three pots in series to control the pitch. The range knob is a broad pitch and the fine is very minute, for tuning the two oscillators to each other. Onboard there is the main pitch control with the fancy silver knob. Below that knob is a switch which switches between the red trigger button or the key switch.

Here is the the patch work for the oscillators. The output is exactly as it says, the output. But the input, which was probably mal name, is a TRS jack. With a flip of the switch, the circuit get rerouted to this jack. So instead of the circuit going to the Pitch pot, it goes out through the Ring part of the TRS to one end of a resisting source, then back to the tip. So the input jack is really just a external pitch control. Now you might ask what it is that I'm using to external control the pitch? Great question!!

Being a Rush fan, I've always like the idea of bass pedals. To be able to trigger a bass synth with my feet is both a dream and very useful in a band situation. One problem was actually getting bas pedals. If you search on Kijiji, someone somewhere will be giving away an organ for free, guaranteed! My dad and I pick up this organ and promptly take it apart for it's pedals. The mechanism is quite simple:

Press down on the pedal and it will close the circuit. Now all I had to do was wire up a trim pot to pedal so they can each have different pitches.

So the signal flow now goes out of the synth, into the trim pot, then is switched on my pressing down a pedal. Then the closed circuit is sent back to the synth. All and All this is just one big resistor. 

 This only works if you have a TRS as it needs to have have an in and out.

Add a nice wooden frame and you're golden! The plexiglass, salvaged from the organ, made a nice cover for the trim pots. If ever I want to re tune the pedals, I just have to readjust the trimpots. I also have the range and fine pitch control which is in series with the 'input' jack. Having this means I can change the pitch of the pedals if I so choose. 

It has been a while since I built this, so I'm missing all the little intricacies and problems I had to fix during the construction process. A fair amount of time was spent on the breadboard for this one though. The goal was for sure to make bass pedals as well as an independent synth. So I had to really figure out how I would route everything. 

Well thank you for taking the time and perusing my latest project. I know it isn't a guitar pedal, but I had a great deal of fun and this is something that I think really interests me at the moment. 

Feel free to ask me any questions as I'd love to help!

As always, happy soldering!!

GCB-95 Crybaby Wah MODS

Another big one! Here is the famous crybaby GCB-95 wah pedal. A lot of people have this one, therefore, even more people are looking to mod it. A simple google search delivers a wide array of mods that is enough to boggle the mind of anybody trying to choose which ones to do. 

I'm not doing anything revolutionary here but I did some common mods that, in my opinion, sound great! Some resistor values are lowered to increase the gain going into the transistor, effectively increasing the gain of the circuit. This however gives a great sounding wah sound!

Here are some pics of the changes I did to the board:

There's a nice little picture of the PCB. There are some variations between the different years. I believe this is one of the most recent GCB-95's. 

R5: Changing this will change the Q of the effect. The Q is the width of the bandpass filter. Lower resistance, more subtle wah sound. Higher resistance will give you a sharper more vocal wah.  

Stock: 33k

Mods: -47k

           -68k  -  (This is the one I choose) 

           -100k

R1: Increasing this will smooth out the transition between heel to toe. Sometimes the stock wah can transition quite rapidly and doesn't sound too good. This should smooth out that transition and increase the mids.  

Stock: 1.5k
Mods: -1.8k
           -2.2k  -  (This is the one I choose)
           -2.7k

R9: This resistor change will increase the gain of that transistor and add more bass response.
Stock: 390ohm
Mods: -270ohm
           -300ohm
           -330ohm - (The one I choose)

            

R15: This is the input resistor. Therefore, if you decrease it, more signal will pass through the circuit and as a result, it will give it a little volume boost
Stock: 68k - (I stuck with this value because I found it had no huge difference with the 47k)
Mods: 47k
       

List of mods done on my wah:

• Resistor       Stock        Mod
• R5               33k           68k
• R1               1.5k          2.2k
• R9               390ohm    330ohm
• R15             68k           68k

Everybody has their own preference of wah pedal. I like the way mine turned out, but I encourage you try different values and find out what works well with your rig. In general, all of these mods increased the gain going into the transistors. You can definitely hear the difference, my wah sounds more vocal, louder, and more gainier.

Happy modding!

OffBoard soldering the BOX oF ROCK!

I finally finished it! I built the ZVex Box of Rock and it sounds amazing! This post is to show you all the off board soldering I did in this pedal. I'm pretty pleased with the way it turned out, the wires are fairly neat. Well...here it is

 I always start with soldering the LED first

Then the power and ground. For me it is always a chalenge to fit two wires in the small lugs of the power jack. What I like to do is twist them as small as I can then put them both in at the same time.

I feel like I could have done a better job soldering the 3PDT switch but it works just fine.

Here I have the small board for the LED. This board holds the resistor for the LED. I made it as small as possible so it would fit anywhere.

And since a lot of the ground is connected to the jacks, I do that next. As you can see I usually always have the ground connections with black wire just for a visual representation.

 It's always really hard to fit both wires into the small hole but I did it. Since the second red wire goes to the LED. Next time I'll just solder another wire to the board where the 9v is coming in

Hope this help a little bit!