Chris Cuff C-Quam AM Stereo Transmitter Discussion

Adding an update.

I decided to try a laptop power supply on this transmitter. I was told the higher the voltage the power supply provides the better.

Using a Liteon brand, AC to DC laptop power supply Model PA-1650-02 putting out 19 Volts 3.42 Amps 65 Watts provided me with a clean NO 60Hz hum.

So far so good.

I will keep you all updated as my progress with this project continues.

So far, feeding stereo audio from a portible CD player into the transmitter, is producing a stereo carrier, with stereo audio separation on the Realistic TM-152 Receiver.

Working now on tuning things up and making adjustments a bit at a time.

Range has increased somewhat, still have to work on a loaded broadcast band AM antenna that works well with 1250KHz, but first, I need to get the transmitter working well, then move on to the next project. Placing it in a metal cabinet for better PC board shielding and looks.

Bruce.

Excellent project!  Keep us posted.  I want to hear about the end result with matching antenna, range achieved and any problems you run into!

I've been studying CQUAM AM stereo as well, and think Chris Cuff's transmitter is great, it seems to cover every base needed for stereo transmission in an accurate way.

It's really a variation of the transmitter by Alfredo, minus the audio processing section that Alfredo had put in it, but is pretty much the same otherwise.

Other kits seem to be based on straight phase modulation of the carrier by L-R with a 25 hz pilot tone mixed in, like some of the versions using tubes on.. You Tube.

From what i have seen, most of these low power c-quam transmitters don't use a proper vector modulator, which would lower distortion of the am envelope on the receiver.
I have been told that this method is fine for c-quam decoding, and for a mono envelope detector distortion is minimal, compared to the full spec modulator.

I have several c-quam encoder/modulators, when i get some time, i will have to check this out.

Paul. 

A side thought while pondering on this project is what if I do place this PC board into a metal enclosure, either keeping it still mounted inside the plastic case and using screws to keep it in place, once it is mounted inside the metal case or simply using brass stand offs to hold just the PC board minus the plastic case, when it comes to placing a SO-239 coaxial connector into the metal chassis for external antenna, will the PC board ground cause an effect with the metal cabinet if a short piece of shielded RG-58 50 Ohm cable is used and that cable shield solders to the SO-239 shield solder lug mounted on the cabinet chassis?

In otherwords, should the PC board ground of the transmitter be isolated from the metal chassis using coupling capacitor/resistor networks?

I think the last time I worked on CB radios which are low frequency, the main PC boards and the final stage to SO-239 connectors were isolated from chassis ground using capacitors and in some cases, even resistors. Been a while since I did repair on those LF devices.

Thoughts?

Bruce.

 I love the old AM stereo modes and I 

remember when they were all over the

AM broadcast "dial."

There are still a few stations around and

there is one here in CT I can get during

the daylight hours.  The radio is a Sony SRF-A100.

I really like the picture of your tuner with the

stereo light operating.

Keep us posted Bruce.

Brooce, Part 15 Hartford CT

Are/is anyone still manufacturing stereo AM for cars?

Chris Cuff's transmitter is vector. A 4x frequency RF oscillator is divided by a ring counter by 4, and In-phase and a Quadrature signals are picked off from it at the station's frequency.

The I signal goes to an AM modulator circuit, which is modulated by L+R, the Q phase signal goes to a balanced modulator, with L-R plus pilot modulating.

The output of the two modulators are combined to make a QAM signal that's then passed through a hard limiter to provide a carrier source to the final modulated RF stage, modulated by L+R.

The pilot is generated by a 555, with a 4017 divider that produces a stairstep output at 25 hz, that's filtered to sine with a 4.7 uf cap.

Now Alfredo, the originator of the circuit, has audio conditioning on L+R and L-R after the input matrix, with 3 bands on each, strapped together. He also used a 10 khz filter followed by notch filters for a steep rolloff.

Chris Cuff's is "Alfredo Lite", minus the processing stages. You cand see a construction video on Yutube, and I think the audio input is covered in video 2 or 3 of the series.

Nice Mr. Bruce, and thanks for the images, it's always nice to see what people have, and your AM only tuner is the coolest.

I wouldn't know if you'd need a metal case unless you're getting RFI, and it would probably be fine by itself, maybe not right up against a big loading coil, which is true of any circuit.

WDCX, I don't know about AM stereo car radios, but radios can come with lots of variations, like short wave bands and such, and there is a chance with those having HD sections, at least some home tuners with HD have been known to lock on AM stereo stations.

For the last few days, I have been busy moving my transmitter board into a metal enclosure.

I have removed the RCA jacks from the transmitter PC board and have added shielded wires in place of the thin wire originally used to connect to the board mounted RCA jacks. I plan on adding RCA jacks to the metal case for enternal connection of auio equipment.

I am using an old, gutted, Asus mid tower computer tower as a case for the transmitter, it conveinently has air holes on the side cover and I will be using this mid tower case laying on its side so the access panel with air holes in it, will now be the top cover.

I have used some of the towers welded brass stand-offs that were used for the former mother board as my PC board mounting locations for the transmitter.

On the front side of the case, I have added a piece of soft metal plating that was the former bottom access cover of an old VCR. The original front cover area of the tower was a plastic cover with an access door for DVD burner and other assessories, I will not be using the original plastic face plate. I needed a flat surface, so I cut a piece of thin metal, that covers that whole exposed area, no longer leaving the former unsightly looking inner metal case area visible. 

The back side of the case, where formerly there were mother board add in cards slots exposed as well as the 3 pronged power supply recepticle was, Is where most of my external connections will be made.

I can not use the power supply that came with this mid tower for this project, because the filtering is very poor in this PSU, so instead, I am going to be using a LITEON PA-1650-02 laptop power supply instead. It has, so far, shown to have very clean power in the transmitted carrier, no signs of AC sinewave.

The power brick on that LITEON power supply is simply a plastic box with the PSU on the inside, with a cord on either end. This presents a problem with a loose power supply being on the inside of the case, possibly floating around, breaking loose etc.

My original plans were to add a 3 prong AC power cord recepticle to the case from an old computer PSU.

Doing things that way, will put the case on the 3rd prong of Earth ground of the AC mains.

Doing this could have one of several results:

1) Adding an AC mains Earth Ground could cause a ground loop and AC ripple to the signal.

2) Adding an AC mains Earth Ground could cause the transmitter to be non-compliant.

3) Adding an AC mains Earth Ground could cause the trasmitter to work better and still be part 15 legal.

I am now considering using the original DC power recepticle of the Chris Cuff transmitter on the metal case, where the DC power cable of PSU connects to the transmitter, rather then requiring me to add an AC power mains 3 prong (chassis grounded) electrical recepticle to the cabinet and cutting the AC power cord plug off of the power supply's AC power cord to attach to the new 3 prong recepticle on the inside.

Be mindful, the AC power cord side of the LITEON power supply has a triangular configured removable power cable, that only fits the laptop type PSU. The DC power output cable is hard wired into the PSU.

Now, as I think about what exactly is needed, I realize the back area of the cabinet has the most items to be installed.

1) Power supply input (DC 19 volts 3.42 Amps 65 watts)

2) SO-239 coaxial connector for external antenna connection

3) RCA stereo jacks for LEFT and RIGHT audio input channels.

Because of the low RF power output expectantcy of this trasnmitter, I have to find and add a PA stage that complies with Part 15 AM. Working on that part right now as I type this. Although my only course of action is to use a slightly over the limit PA, I will do whatever can be done to limit the output to 100mW. Under current conditions and as is well known by those who own this transmitter, the RF power output stage is very, very, very limited and is far below the maximum allowable RF power input/output under part 15 rules. Something has to be added to the output stage to increase this output to a usable level. 10 foot distance from an 102" antenna with EXCELLENT GROUND and perfectly tuned coil, is very poor even for a personal yard caster!! 

So far, the only parts I need and have for the front cover is the power switch and power on LED. Most of the hardware is required to be at the back side of the transmitter out of view from the front for neatness.

I have a multi-LED stereo audio meter that came from a gutted stereo VCR unit, that contains the LED driver chips and audio detection chips that I want to try and wire into this transmitter unit.

I know the LED VU meter lights up when DC power is supplied to it, I just have to figure out how to add the audio circuits of the transmitter to it and calibrate it so it reads the actual RMS of the audio being applied. (if I worded that part right that is.)

I hope to use this LED VU meter as a reference to audio level input and peak reading levels to avoid over modulation/saturation issues.

I just have to find a sweet spot in the audio circuit to wire this meter in for the best results.

This should prove to be an interesting project, if I can get it to become an actual physical part of the Chris Cuff transmiter

I wish to add new pictures as I go along hosted on this site, although, at the present moment, I can no longer add newer photos to this topic. I'll see if that can be changed, so I can add newer photos to this topic as they are needed.

Stay tuned for more updates.

Bruce.

On the Aspisys asmax, it also uses a vector modulator.
What initially lets the asmax down is the pilot tone is a badly rc filtered squarewave, and looks rough on the scope.

I have fitted a bandpass filter to clean up the waveform, now looks good.
The reason i did this is that the dirty pilot caused added lf noise on the receiver through the hifi.

The asmax good points are no microphonics of the phase modulator, and the pilot is synthesizer derived, so accurate in frequency.

I also have the Sean Cuthbert exciter, seems to have a basic phase modulator, and vfo pilot, and the 4x pll osc coil picks up microphonics quite bad.
But i don't expect perfection given the price.

Paul.

Nate,

Interesting you should describe the pilot tone oversampled waveform of the c cuff circuit, i have recently bought a c-quam exciter from Cyprus.
It is marked Lesvos Electronic, it looks to use 3 modulators, so looks vector type, and uses a 555 and a 4017 divider to produce the oversampled pilot waveform too.
Scoping the cap shows the stepped waveform, so still might need additional filtering, will look in to that !

Looks like parts of other peoples designs are being cherry picked here !

Paul.

That Realistic AM stereo you have is proabably a rare gem....never knew they made one and the first time I saw an AM only tuner in stereo. Remember some Heathkits way back when that were AM only but not stereo.

Don't think many were made.

As for the power supplies the brick style ones for laptops or the 12  volt ones for the LCD monitors are the best for clean no hum power as they are a lot better than the normal wallwart kind.

Mark

Radio Snack was always a company that tried to jump on the latest tech, and even pushed for some things to happen, like FRS radios.

I think the Realistic stereo AM tuner TM-152 was 'new for '86', and you can probably find it at the Radio shack catalogs site. When AM stereo finally seemed to get its groove, there was a good bit of excitement about it, every electronics experimenter magazine, broadcasting journals and even some mainstream publications were all talking about it, The New Sound Of Your AM Radio.

The tuner might have sold well in places where there was a good AM music station that was popular, and if the station talked about AM stereo on the air. It seems that many didn't care to mention it beyond their promo materials and station ID, like saying "AM stereo 1490."

BOARDMAKER, other transmitters probably have borrowed from Alfredo and Chris Cuff's circuits. If you're dealing with that pilot generator, have a look at this instructional video. The explanation of the circuit should be clear enough to see if it's like your transmitter's generator.

The pilot section:

If you follow along in the video series, it breaks down the different sections of the transmitter, and it's a good to learn from.

His modulator ICs are NE602. The classic 1496 should work, that's what my shop has.

Mr. Bruce, good that you're mounting the kit in that old computer case, that's a nice idea, recycling that way, and it sounds like you're on the right track. I don't know how much you'd have to worry about the RF ground being single point, I haven't had to worry about it much on the AM band.