As a lifelong professional and home hobby broadcaster I was excited to learn of a new kid on the block, the Cuthbert AM Transmitter. You can find this and other electronics goodies Sean sells on eBay if you search AM Stereo Transmitter – his seller name is sean-jcil. Sean graciously sent me his AM Stereo C-Quam model for evaluation.  I have to say I had forgotten that I owned a still-in-the-box Realistic TM-152 AM Stereo tuner from the 1980s when I worked at an AM Stereo station. Sean has a construction video which demonstrates his transmitter’s stereo capabilities when you adjust the stereo alignment which also demonstrates how well it works.  I do know, since C-Quam units have to perform to a more stringent level of modulation quality than mono transmitters, that even when listening on a mono radio this unit should be a top-quality performer and that’s exactly what I subsequently found.  (A reader recently sent me a Sony SRF-A1 Walkman AM Stereo radio and I will be posting an article on that and the history of AM Stereo soon).

The features list is a good one: from the eBay description:

Auto power control to give as close as possible to 100mW under all tuning and antenna conditions.

AM stereo C-QUAM to give high quality stereo sound with compatible radios.

PLL drift free transmitter with memory. No expensive crystals to buy. Just set the frequency and it stays there rock solid.

Built in audio compressor to keep volume constant and prevent over modulation.

Built in frequency display.

Built in antenna tuner

Built in power meter.

Ultra Hi-Q output stage for incredible spectral purity and maximum possible power to antenna.

Covers several hundred feet! 

The frequency is simple to set using just 2 buttons for up or down tuning.

Covers 1024 to 1710KHz in 1 KHz steps for both US and Worldwide AM Bands.

Comes with Power supply, Antenna wire and a 3.5mm to RCA phono cable to connect to your music source. 

Kit comes with FREE USB DRIVE containing step by step assembly instructions, Schematic diagram and an alignment video.

Unlike some other kits, all instructions are in beautiful high-resolution color. 

Components come taped to component cards for easy identification. Includes solder.

The Cuthbert is an unusually easy kit to assemble, due in part to the Component Cards which separate and identify all the small component parts. Every transistor, resistor and capacitor is clearly identified for you so there is no doubt as to which part is which, which greatly simplifies the process. The circuit board itself is an unusual design with a marked grid and matrix of holes through which you insert the component leads and solder…this is really well thought out. With reasonable care, anyone who can carefully solder should have no trouble putting this kit together.

The case is sturdy and attractive but it can be a bit tricky to open and close…it uses a snap-fit design which is strong and simple but I had some trouble getting it to snap closed completely…a bit of fiddling and I got it…a minor quibble. (One of Sean’s many videos demonstrates opening and closing the box).  I tried the transmitter with a variety of audio sources to see how it performed and found it easy to set up and use. One great feature is the large LED frequency display with simple Up/Down buttons to select the desired transmit frequency. Compared with my reference unit, which requires the use of a chart to set internal DIP switches, tuning the Cuthbert is as easy as tuning a radio. Also, the front panel power meter is especially welcome – simply adjust the front panel knob for a maximum reading and the transmitter will be optimally matched to the antenna wire. My reference transmitter requires the use of a meter with an internal adjustment…the result is the same but the Cuthbert makes it so simple by comparison, especially if you will be moving the transmitter or its antenna wire – you can instantly retune it as needed…very nice!

However, I do wish it had an audio input level control. Luckily the unit seems to have a wide range of acceptable input levels… I tried it with a variety of audio sources and got good results with all of them. It does have an automatic volume control function and it seems well suited to the needs of AM broadcasting, but how much audio compression is applied will depend on the amount of signal level you feed into it. I must say though that the automatic level control was not intrusive on typical program material and it does ensure that you will always be at maximum possible modulation with no worries of over or under modulation…for most users this is a plug and play solution and it works very well.

In use what I did was to feed in audio at a low level and slowly increase it to the point where I heard no further increase. This indicated I had reached the threshold of the audio limiter. From there you can increase the audio input level just a little further if you want the transmitter to make gentle corrections to the audio level, or more if you want it to exert more level control… knowledgeable users will soon get the feel of how different audio input levels control the eventual sound of their “radio station”.  But again, even with standard line out levels from fixed output sources or headphone outputs I found the results always seemed good.

Hum is a major issue often encountered in home AM transmitters and it is usually caused by the interaction of the transmitted signal and your home’s AC wiring. I have dealt with hum issues with every home AM transmitter I’ve ever owned and have found that with patience and diligence it can usually be reduced or eliminated by experimenting with antenna and radio location and grounding considerations for the transmitter. Often, connecting the transmitter’s ground terminal to a ground will eliminate this kind of hum. Although an actual ground rod driven into the ground or a cold water pipe are usually best you can also try using a screw in the center of an AC outlet (please be very careful working around AC outlets). If you’re really lazy you can try a cabinet screw on a desktop PC which is usually grounded by a 3 prong AC plug. This actually worked for me when I used the transmitter fed from that desktop PC’s sound card. Again, hum is an annoyance with any home AM transmitter and it sometimes requires a bit of experimenting to deal with it.

Another cause of hum is ground loops which often occur when inter-connected audio components are connected to different grounds. Although I have not had a hum problem using my desktop computer’s sound card output, I tried one setup using an AC-powered boombox as an audio source and that caused a loud hum. The cure was the simple addition of an inexpensive ground loop isolator in the audio input leads such as one of these:

https://amzn.to/2KyAkH7

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These items tend to come and go…if the one you want shows “Currently Unavailable” just search for “Ground Loop Isolator” and you will find several. In this application they all perform the same function…choose one with the types of connectors you need. Since the Cuthbert uses RCA plugs for audio input that is the configuration I bought.

When I got everything set up I set the Cuthbert to 1610 KHz and my reference transmitter to 1580 KHz and compared their sound quality – they seemed virtually indistinguishable from one another. I walked around my property with one of my best radios and also with a more basic radio and I also listened on my car radio. I was gratified to find that overall their sound and apparent coverage distance were virtually identical in every detail, which is as it should be. The compression on the Cuthbert worked well and generally controlled the volume very effectively…evidently its settings and action are very similar to the way I had set the compression previously on my reference transmitter.

Another note: The Cuthbert broadcasts a wide frequency response that is only limited by the bandwidth of your radio. In addition to most Stereo AM radios I have a few mono radios which have a wide bandwidth setting and in that mode the Cuthbert sounds as clear and lifelike as FM. The current C. Crane CC EP-Pro and vintage GE Superadio III are two mono radios which offer wide bandwidth for excellent AM sound quality. In other words, the Cuthbert will make any AM radio sound as good as that radio is capable of.

Conclusion: The Cuthbert C-Quam Stereo AM Transmitter (and the less expensive standard mono unit) are great buys on today’s market, replacing my reference transmitter which is sadly no longer available. The only negative is the lack of an input level control and level indicator which might make setup easier depending on your audio source…most users will be happy with it just as it is.  However, the list of plusses is a long one…I especially appreciated the front panel Up/Down Frequency buttons and LED readout along with the easy to use antenna matching control with its metered readout, which combine to make initial setup much easier than my reference unit. I bring my AM and FM transmitters to a yearly convention I attend and this year I’m bringing the Cuthbert because it will be so easy to set up and its performance is as good as it gets.

For info or to order contact Sean at: sean.cuthbert1@gmail.com

Recommended!

Jay Allen