The MURF-BOX

The new Attenuator kit from W2IHY

Thanks to WV4R for the picture of the kit

Before reading about the Murf Box below, you may want to know that Julius W2IHY has made kits and fully assembled units available for purchase. They are real slick and make constructing the box a lot better and easier. All the parts are included in the kit, even the little project box. If you want to see a glimpse of the construction manual on line, please click here. This file is a PDF file and you will need the Acrobat Reader to view it but the reader is available free online on the internet. Other pictures of the finished box can be found on the web page of WV4R Click here. You can contact Julius at 1-877-739-2449 to get prices and have it shipped to your QTH. I doubt that you could assemble the components yourself and make the box any more inexpensively. Please read below if you would like to do it on your own. Below is not the same box, exactly and it is not as neat, but it will work just fine.

There are several ways to match an audio chain to a rig. You could go out and buy a commercial transformer  like a Jensen or their ISO-MAX. You can get one of several types from Whirlwind. There are others out there, but they run from $50  to $250 or more. The Murf Box I describe here does the same thing but at a fraction of the cost. Some of the audio guys will recommend a commercial box because they have never used a Murf Box and don't really know what it is, and have already invested a lot of money in another device.  The transformer that is used in the box has been swept down to 50Hz up to beyond any frequency we are going to be using in amateur radio. Some guys will say you don't need one. Well in some cases like when you are going into some of the ICOM radios, I guess that is true in that you can go into the balanced modulator and I understand that this is at line level and the radio will handle this, but  I do not really know as I have never had a rig that I connected in this fashion. But, having said that, I would still think you would want to put at least a good quality audio transformer in line to stop the possibility of hum sneaking up on you. If you do not use a step down box like the Murf Box, you may be able to get around it by riding the mic gain and output of the next component in line and keep the levels somewhat in line, but it will be very touchy and you will have no headroom at all in mic gain before you start to over drive. Be sure you know if you are going in at mic level or line level and then decide if you need this box. In any case it would not hurt and  make any connection much better. Remember, you are getting two (2) functions in this one box. Isolation from the chain and associated possible AC hum, and a step down matrix. Be careful in that some of the before mentioned devices only do one thing like isolation and you have to get another to do the step down function. If you are going to do other things like recording, you will need more transformers, not necessarily step down matrixes, but if you are going to have to spend $70 to $200 for each one, you had better pick the correct method.

The Murf Box, was designed by Julius, W2IHY the same ham who makes the W2IHY equalizer and noise gate, for Murf, WV4R.  The Murf Box will do isolation and step down to mic level in one little inexpensive box. The circuit is pictured below. All components can be purchased at your local Radio Shack store with the exception of the transformers. Don't be tempted to use the ones from Radio Shack here as they are not up to passing enough frequency to be usable in the circuit. The proper transformers are Stancor TTPC-2 or TTPC-8 Telephone Coupling Transformers. The address on the box is 1401 Pullman Drive El Paso, Texas, but I got mine out of the Allied Electronics catalogue. Do a search on the internet and you will find this company. They are less than $5 and well worth the money. If you are going to do other things in audio like recording, you will need more, so getting a couple spares is not a bad idea.  If you want, you can also contact Julius at 1-877-739-2449 or go to his web site at www.W2IHY.com He can provide you with the transformers and just about any part you need for this box. If you insist he may even sell you a completed box, even though he is not really set up to build them, but being the accommodating individual that he is, he just may build you one.

Put the circuit in a metal box and make sure that everything is floating. You can use 1/4 jacks or RCA jacks or DIN plugs or just about any arraignment you want to interface this box to your rig and rack. It just depends on how you want to go from your radio to the rack. Again Radio Shack has a lot of cables that have different jacks on either end so you can do it however you want. Below is a picture of the circuit in a box. Don't buy the real small project box at Radio Shack, as I have found that putting the circuit in the small box is not worth the space you may sacrifice in your shack by using the larger box. 

This box will step down the signal between the rack and radio, by about 23db. For most applications this will be enough. The great thing about this box is that it works, and it can be put together for less then $15 if you buy everything brand new. Many of you will find some of the parts in your junk boxes. The picture above shows 4 resisters and the circuit diagram shows 3. This was done, because there was not a resister with the exact value needed and had to put a couple of resisters in series to bring value up to what was required. You may not have to do this if you can find resisters of the exact value shown in the diagram. REMEMBER the receptacles  must not be in contact with the box. If you can find some older ones with plastic shafts, they are better. If you can only find the ones with metal, shafts like most newer ones at RS, be sure and put tape or something between the box and the receptacle housing. The commercial boxes do a good job and I have one here in the shack, but it cost me $80 and does not do as good a job as this one.