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!

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!

Phase 90 mods

In the summer on 2012 I was looking into a phase pedal. The obvious choice was the Phase 90 by MXR. Now they have a huge selection of phaser pedals. Sadly the one I wanted, script reissue with LED and DC power jack, was pretty expensive. The regular phase 90 was a lot less expensive compared to the script ones. I don't know how but I stumbled across this thing call the script mod. So I've heard of pedal mods before. But I never knew that it was that simple. The Script Mod was basically removing the R28 resistor from the circuit. All I had to do was cut off one of the legs of that resistor. I really like the idea of being able to have a more vintage sounding phaser, but what if I wanted to switch back to the original design if I didn't like it or if the song demanded it. This is where I got the idea of putting in a switch. I'm really glad I put in this switch. To this day I still switch between the stock and script switch to change up the sound. I did this mod last summer and since then I decided to do another mod to the Phase 90, but I'll get into that a little later. Currently my Phase 90 has 2 switches on it. One is the script mod and the other is a tremolo mod. Here are some pictures of my modded phase 90. I'll also give some tips I learned along the way with modding. 

So here it is in its beauty

The V represents vintage and that drawing is just to say what I have done with the circuit. I can switch between having the R28 resistor in or out.

Taking out the PCB isn't that difficult but putting it back in is

Here is the solder side of the PCB. Don't mind the two wires, thats for the tremolo mod

You have to take off all of the nuts before you can take out the board...

....even the jacks

So here I have two tightly fit switches between components

This I believe is a bias knob. DO NOT TURN IT. This will change the sweep and make it sound uneven

Here is the switch for the script mod. I put electrical tape around it to make sure it doesn't short circuit.

There is the placement of the R28. Just above the output jack.

As you can see I desoldered one leg of the resistor and lifted it up.

What you want to do is have a SPST switch. I had to just snap off one leg so when it's in that position the resistor is removed. Have one wire that goes from one lug to the empty whole. Have the next wire that is soldered to the second lug be soldered to the other end of the resistor. This diagram explains it pretty well...I think.

The placement of the switch is important. You want a space that has a low profile so the switch can fit without putting stress on the board.

Next you would want to drill a whole accordingly. 

This is the next mod I decided to do, it's called the Tremolo mod. I found this in the Wampler book of pedal modifications. It's a great book, thats where I found the mod for my Marshall Guv'nor. This mod does as it's supposed to do, tremolo. But in my experience, it has kinda like a univibe characteristic to it. It does phase but it also swells the volume like a tremolo. The one thing that makes this mod not perfect is that at the peak of volume it almost distorts a little. But I can't really tell in the moment.

The mod was to put a 0.1uf cap to touch these two points in the circuit.

For the switch, what I did was have the .1uf cap flush against it for minimal space occupied

As for the placement of the two wires, one wire is attached to one lug of the switch, the other wire is soldered to one leg of the cap, and the other leg of that cap is soldered to the other lug on the switch. I hope thats not confusing. So I have the switch itself on top and the solder on the back. 

 So I can very easily take the two wires and bend the around the corner place where there is room.

Then the switch rests in-between some transistors

A vey snug fit!

So this is my phase 90. I never thought that I could do two mods but look at it now. It's a very versatile pedal for only one knob!

If you have any questions or comments about these mods, please leave them in the comment section below. 

Marshall Guv'nor mods

The marshall Guv'nor was the first pedal I ever bought. And to be perfectly fair it is a very nice overdrive/distortion. I stumbled upon book, made by Brian Wampler, with a lot like I mean  lot of modifications to practically any pedal. Luckly it had the guv'nor on the list. One the the suggested mods are diode changing. The diodes that are originaly in there are red LEDs. Wampler suggested replacing them with 1N4001, the ones I used in the Distortion plus build(see previous posts). Another cool thing about this mod is that it makes the clipping section asymmetrical. There is two types of clipping symmetrical and asymmetrical:

This is a pretty good diagram showing the different kinds of diode clipping. Symmetrical has two diodes in parallel and asymmetrical has 1 diode on one side and two in series on the other side.

Anyways, this mod book says that this mod makes the clipping more responsive to pick attack, what the stock guv'nor lacked in. 

Switching diodes is a very easy way to modify any of your overdrive/distortion pedals that include a diode clipping section. This post will go through the desoldering process and replacing of the diodes with step by step instructions. 

Marshall GUV'NOR Plus

 Between my MXR Distortion Plus build (in previous posts) and now, I've gotten a new soldering iron. It's got a controllable wattage knob, not necessary but nice. A huge improvement from my last soldering iron.

Once you have opened up the back of your pedal and spotted the diodes you want to replace, the next step us to desolder. Before you even touch the soldering iron, make sure that your not replacing the wrong diode. Thats why I like using the mod book, it has a visual of the positioning. What I used to desolder is desoldering braid. I know desoldering pumps are a lot more effective, but it wasn't working for me for some reason. So I went back to basics. Anyway, it work out for me very well. If you want to learn how to use soldering braid I'm sure there are many sites on the internet and youtube for that.

 To be honest I am a little disappointed in the pump, but the braid is there for me

Make sure when you do apply your soldering iron you don't overheat the tracks. If you are able to see through the wholes / be able to fit the component through, you are ready for the next step

For the diodes, I am replacing them with 1N4001. The first one is just the single diode. I used the vertical diode technique o fit it in the right holes. Before you solder it in, make sure it's facing the right way. The line on the diode is facing the same way as the line on the schematic is facing. 

You might be able to see the schematic symbol on the PCB in this picture. But it will most likely be there, so put you diodes in the right way

The next diode I need to take out is replaced with two diodes in series. I good technique for soldering these together is bending one in a circle and putting the other one through the hole. This makes for easy soldering. Helping hand really help to hold the diodes in place.

 Make sure the diodes are facing the same way, like the picture below

 Because the holes were origonaly for an LED, I did need to be the diodes in a fancy way. Bending it this way really worked for me. You can bend it in any way, but just make sure the leads aren't touching any other components.

 What I like about this bend, it has a low profile so the excessive amount of wire in this pedal doesn't put pressure on the diodes.

Once everything is in position all you need to do is solder it in. PCB's have a very densely packed board so be extra carful about soldering.

I didn't have a lot of time to test out this mod, but from what I heard and played, it did make it more pick responsive, like the book said. I would say that this is a success

If you have any comments or question please leave them in the comment section below