View Full Version : AM25 - How about an AM25A?
Given the size of the antique radio collector market, including yours truly, I'm wondering if there is a possibility that Ramsey would be developing a new and improved AM25 - e.g. the AM25A? I had heard before that the AM25 only goes up to 3000Hz in the audio response - fine for modern junky AM radios that only go up to 3000 HZ, but not appropriate for a vintage Heathkit JT-11 which goes up to 9000 HZ down only 2.5 db. Or a lot of typical tube radios that made it up to 5000 or 6000 Hz OK. [Also not OK for a Superradio III or its RS clone, both of which are still made and sell rather well to AM fans]. Perhaps some of the same brick wall technology of the STC-1 could be used to create an AM25A which would go up fairly flat to 9000 Hz, then crash off to avoid hetrodyne problems. AM stereo would be nice, to be sure, but the antique radio market could really be tapped into with a relatively HiFi transmitter! Perhaps if their are some other antique radio buffs who are interested, they could make their views known!
02-27-2002, 10:22 PM
Ideally we would have two new AM transmitter kits to enjoy: an AM-25A with enhanced audio frequency response and lower distortion, and an AM-100, based on the Alfredo Torrejon/Chris Cuff AM Stereo transmitter, for the ultimate in Stereophonic AM audio quality.
I don't think any artifical limiting of the audio response should be included, at least not as a mandatory feature. Even many ordinary AM radios have some degree of audio response well past 10 kHz, especially for older radios that do not use any ceramic filters. And there are still quite a few wide-response AM radios available, including the many car radios with AM Stereo, as well as the home and portable radios by GE, Optimus, and Philips-Magnavox.
FCC Part 15 devices, such as these low-power transmitters would be, are not required to abide by the NRSC 10 kHz AM audio bandwidth restriction, so therefore as long as you choose a clear frequency, transmitting even full 20 kHz audio bandwidth wouldn't cause any trouble, and you surely would enjoy the excellent quality that "hi-fi" AM radio has to offer.
In any event, it would be nice if the folks at Ramsey could at least verify the audio frequency response of the AM-25 transmitter, and tell us whether it has any built-in treble pre-emphasis or not. (Commercial AM stations use the same 75 microsecond treble pre-emphasis as FM in order to improve audio clarity and increase the S/N ratio.) I have used the erstwhile AM-1 "toy" transmitter and its audio response is almost unlimited (at least 15 kHz) so I don't see why the AM-25 should not also offer true high-fidelity audio transmission.
Quite some time ago, before the Ramsey forum was revamped and the old posts were thereby lost, I had inquired about the AM25's frequency response. I did get a response from a Tech at Ramsey - 3000 Hz. I'm wondering what type of receiver they used on the testing end, because if it was a rig limited to 3000 hz, that would not really be a fair test. But if it was used with a receiver with a wide bandwidth - [hey, in a pinch use a crystal radio!] then I'm sure its accurate.
How about no frequency response limitation, but an AM version of the STC-1 that instead of having a 15,000 hz brickwall has a 9000 hz brickwall? Or whatever?
12-06-2012, 02:49 AM
I've tested an AM-25 Rev. 1.1.
Using a sine wave signal generator, I pumped well over 10khz into it and the AM-25 modulated it. In fact, using a Sony wide-band receiver I readily heard the tone (so did every canine in the neighborhood).
12-06-2012, 03:35 AM
I ran an audio frequency response test on the AM-25 by monitoring the audio going into the final RF amplifier and the -3dB points were 20 Hz and 20 kHz. When the signal is received with a radio with 16 kHz IF bandwidth it sounds very good.
Typically this transmitter can produce up to 90% modulation with little distortion.
12-08-2012, 04:21 PM
balanced/ unbalanced 600 ohm stereo inputs, CQuAM, large low loss toroid/capacitor output bypassable for 50 ohm output. DDS synthesizer, asymmetrical capability -99%/+130%, 100mW DC input, isolated output stage so tuning doesn't affect rf power.
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