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 Post subject: AM TRANSMITTER
PostPosted: Feb Mon 12, 2018 12:31 am 
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Joined: Mar Fri 07, 2014 10:20 am
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I am looking to source a am transmitter to be able to play cds on my tube radios. I would like to either build or buy one. Thanks from a0e 1k0 nl Canada


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Mon 12, 2018 3:24 am 
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There's the Talking House transmitter which is advertised on the site, and from what I've heard it's more than sufficient.

There are also a plethora of transmitter threads you can go off of (however, don't go off mine, the schematic's incomplete for now!)


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Mon 12, 2018 11:18 am 
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Perfect but I have read some info and there are many different opinions on this. Where could one buy a kit to build yourself?


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Mon 12, 2018 2:48 pm 
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Ebay has hundreds of DIY kits for this purpose!


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Mon 12, 2018 3:20 pm 
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Any better than others.


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Mon 12, 2018 3:44 pm 
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Location: Arlington, TX, USA
And... this is the problem. I know of no objective tests or reviews of any low power AM transmitters, only anecdotal reports besides the Hobby Broadcaster "transmitter shootout" which only tested for signal tange. That leaves us to roll the dice and hope we buy something that works well. If I were equipped and funded I would do these kind of tests, but at present I am not, but I am curious to know which designs perform well and which do not as far as measured performance. And by that I mean audio quality such as hum/noise level, frequency response and distortion in addition to signal level and/or range, frequency accuracy and stability.


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Mon 12, 2018 5:17 pm 
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spsquires wrote:
Perfect but I have read some info and there are many different opinions on this. Where could one buy a kit to build yourself?

If by "kit" you would accept a parts list and schematic, you can build two highly regarded transmitters developed by members of this forum. A HUGE benefit is that you don't have to rely on getting assistance from one seller, as you have many forum members' experience with these as a resource.

TUBE- Norm Leal's 6888 transmitter viewtopic.php?p=495438

OR

SOLID STATE - Tom's (35Z5) LM386 transmitter viewtopic.php?f=12&t=193042

Moderator, is it possible to have a sticky for favorite circuits whose uses keep coming up all the time?


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Mon 12, 2018 7:08 pm 
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Location: Oblong, IL
I'm interested in Norms transmitter. The link takes me to Mike Toon's thread, but non of the schematic links work. This thread is over 10 years old. Any help appreciated. I am reworking a mike osc. from 1953. I doubt it will have the best quality sound or range tho. Been studying a Heath diagram. The only thing that concerns me about building one is getting (or making) the right osc coil and tuning cap. I want a little more power than 10 or 20 ft, and good sound quality. I live in the country so a little too much grunt wont be a problem.

I too like the idea of a sticky. Been searching for hours with very limited results.

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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Mon 12, 2018 7:29 pm 
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Location: Saskatoon
Here is Norm's original schematic for his 6888 transmitter.
Attachment:
6888_trans.jpg
6888_trans.jpg [ 31.55 KiB | Viewed 1756 times ]


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Mon 12, 2018 9:00 pm 
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BobWeaver wrote:
Here is Norm's original schematic for his 6888 transmitter.
Attachment:
6888_trans.jpg


Here's a poorer copy that includes a power supply.


Image


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Mon 12, 2018 11:03 pm 
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Hum in any small BCB transmitter is almost always antenna location/orientation or ground issues... AFAIK unless there's a P/S problem, none of the transmitters we've built here on ARF have a inherent hum issue...

Different power loads within the dwelling can cause problems... I can turn on lights in my garage upstairs and cause hum, turn on lights downstairs and it's gone... Modern switching wall warts are the bane of AM radio, my Amazon Firestick destroys the band... I have it's wart connected to a switched extension cord, cut off if not in use..

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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Tue 13, 2018 4:14 am 
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Location: Arlington, TX, USA
35Z5 wrote:
Hum in any small BCB transmitter is almost always antenna location/orientation or ground issues... AFAIK unless there's a P/S problem, none of the transmitters we've built here on ARF have a inherent hum issue...

Different power loads within the dwelling can cause problems... I can turn on lights in my garage upstairs and cause hum, turn on lights downstairs and it's gone... Modern switching wall warts are the bane of AM radio, my Amazon Firestick destroys the band... I have it's wart connected to a switched extension cord, cut off if not in use..


Exactly. Hum is a grounding problem, either due to a ground loop created by the audio input device or just trying to ground through noisy house wiring unless of course the power supply is just that dirty as far as ripple or diode switching noise. Transmitter range is infinitely more affected by the antenna/ground system than the transmitter itself. No one can expect more than a few feet of range off a wire hanging off the back of the transmitter. The electrically short antennas that are practical for low power AM transmission need to be matched at least with a loading coil for best results. And the transmitter HAS to be connected to a ground for any sort of range at all.


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Tue 13, 2018 3:06 pm 
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Erich Loepke wrote:
If I were equipped and funded I would do these kind of tests, but at present I am not.

I am so equipped, because I am equipped to do AM broadcast transmitter audio performance measurements, using a Belar AMM-3 modulation monitor, an HP 651B test oscillator and an HP 334A distortion analyzer. The only limiting factor for testing Part 15 transmitters is in amplifying the Part 15 output to a level that will drive the Belar mod monitor, which needs 5-10 volts input. I am set for transmitters on 540 and 1480 as I own two Belar RFA-2 RF amplifiers, factory tuned for those frequencies.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Tue 13, 2018 6:20 pm 
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Dale H. Cook wrote:
I am equipped to do AM broadcast transmitter audio performance measurements, using a Belar AMM-3 modulation monitor, an HP 651B test oscillator and an HP 334A distortion analyzer.
Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA

HI, Dale, I'm envious. Like many on this forum, I just have a scope, a wideband AM receiver and my ears. I don't want to be like the tone deaf person who doesn't know he can't sing, so I presume I must be missing something by not having such additional professional equipment. With respect to Part 15 transmitters, can you tell me exactly what I'm missing?


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Tue 13, 2018 8:08 pm 
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Location: Roanoke, VA
Macrohenry wrote:
With respect to Part 15 transmitters, can you tell me exactly what I'm missing?

One thing that you are missing is decades of experience working with and measuring the performance of AM transmitters as part of earning a living. :-) An AM modulation monitor is the standard instrument for measuring modulation, and its demodulated output has minimal envelope distortion, suitable for noise and distortion measurements. For an introduction to how measurements are made on AM broadcast transmitters see the instructions on my proofs page.

Proofing a Part 15 transmitter is another matter. An AM modulation monitor consumes a small amount of power tapped from the transmitter's output stage, but that is not practical with a Part 15. That is where the Belar RFA-2 amplifiers come in.

Over at Hobby Broadcaster Bill DeFelice (who, like myself, is a career broadcast chief engineer) and I have begun discussing a relatively simple home-brew AM modulation monitoring system that could also be adapted for Part 15 frequency response, noise, and distortion measurements. Those would still require a low-distortion audio sine wave generator and a distortion analyzer.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Tue 13, 2018 8:55 pm 
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rxradio wrote:
Ebay has hundreds of DIY kits for this purpose!

spsquires wrote:
Any better than others.

Erich Loepke wrote:
And... this is the problem. I know of no objective tests or reviews of any low power AM transmitters, only anecdotal reports.

Exactly. Even with as close to an "industry standard" as we can get, with the Talking House 5, or the SSTrans 3000, there will still be varying reports. I had one of the former, and sold it because of it's background hum, and yet others on the forum have posted that it's transmission is just fine. So maybe a difference in antenna placement, specific radio receiver, or even the individual's hearing ability is the determining factor.

I had replaced it with an SSTrans 3000, but that also didn't give me acceptable transmission. After getting some education on the board here, I re-located it, so the antenna was away from vertical house wiring, and voila, it's behaving just bitchen now.

There are simply too many factors to arrive at a "one size fits all" determination.

Here's a kit I built from an eBay seller in Japan, which works reasonably well. Not as good as the SSTrans (which is no longer available), but at 10% of the price, shouldn't be expected to;

https://www.ebay.com/itm/DIY-655kHz-131 ... Swk6pZfyS3

He also sells tube transmitters.

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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Fri 23, 2018 5:01 am 
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I've tried several transmitters and IMHO this is the best, by far. It's not cheap, but this is one of those cases where you get what you pay for. Clear, strong and easy to tune to an open area on the band. Designed by a guy with decades in broadcasting. I bought two in case he decides to retire from making them.

https://www.ebay.com/itm/AM-BROADCASTER ... Swf~9Za1YP

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I know just enough to be dangerous.....
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 Post subject: Re: AM TRANSMITTER
PostPosted: Feb Mon 26, 2018 2:53 pm 
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Location: Warner Robins, GA
Here's what I've found concerning transmitter hum.

It is best to have the transmitter grounded and add a switch to permit lifting the ground as some radios hum less with the ground conencted and others hum less with the ground lifted.

Proximity of the antenna to electrical wiring also plays a part in how much if any hum is present.

Also better control over the hum can be achieved by using a 1:1 audio transformer on the input keeping the input jack isolated from the transmitter ground.

For mine I installed an Edcor WSM15K/15K balanced/unbalanced to balanced/unbalanced transformer on my two tube transmitter and used an isolated 1/4" jack for the input. Reduced the hum level some and eliminated the effect the audio cable and device had on the hum level.


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 Post subject: Re: AM TRANSMITTER
PostPosted: Mar Sat 24, 2018 9:46 pm 
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Joined: Mar Fri 07, 2014 10:20 am
Posts: 2451
There is some good ideas here for sure but has for building one , I would love to but do not have easy access to parts to start and I have not enough knowledge either. But to buy one I am not sure what to get , to many choices.


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 Post subject: Re: AM TRANSMITTER
PostPosted: Mar Sun 25, 2018 3:30 am 
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Location: Beautiful Downtown Burbank CA
Tube Radio wrote:
Here's what I've found concerning transmitter hum.
Also better control over the hum can be achieved by using a 1:1 audio transformer on the input keeping the input jack isolated from the transmitter ground.


Yes, one benefit of Norm's 6888, aside from boost, is input isolation. :wink:


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