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Old 14th Dec 2019, 7:32 pm   #1
Ian - G4JQT
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Default AM Pine Board Project

I've had a search and not found this previously mentioned on the forum...

In 2017 Bob Heil, K9EID started the Pine Board Project which crystal-controlled valve AM transmitter built on pine board, although could of course be built using a normal chassis.

The original design had an RF output of only one watt, but a change to solid-state rectifier brought that up to about four watts.

I thought it might be of interest to some forum members. It could be fun for AM contacts across town - or further afield with a decent antenna. It might make an interesting and displayable MW 'pantry transmitter', but only if fed into a dummy load...

The trouble is that despite there being a considerable amount of information about it, there's nothing concrete about its modulation quality. No 'scope screen shots and no proper technical data, so I suspect it's fine for amateur use, but nothing else.
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Old 15th Dec 2019, 1:22 am   #2
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Default Re: AM Pine Board Project

Only thing I will add is if you decide to build it, please be careful.

Quite apart from everything, including the mains input, being exposed, it never had a discharge resistor across the HV caps in the power supply (which may have been fixed in later iterations), he was relying on the rest of the circuit to do it, which obviously doesn't happened if the B+ has been disconnected.
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Old 15th Dec 2019, 3:55 am   #3
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Default Re: AM Pine Board Project

AES still supply something similar as a kit for only $44:

https://www.tubesandmore.com/product...ss-transmitter
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Old 15th Dec 2019, 4:02 am   #4
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Default Re: AM Pine Board Project

Dodgy looking ceramic cap across the mains input

Think I might pass on that one.
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Old 15th Dec 2019, 8:11 am   #5
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Default Re: AM Pine Board Project

I could point out that all of these types of designs are just "toys".

If you want a good Pantry transmitter it requires the following:

1) A power output stage with a proper impedance match to the antenna.
2) A crystal timebase, not just a drifting L/C oscillator.
3) A Linear Amplitude modulator.
4) A proper system of audio compression (AGC) to allow for the wide range of levels with different audio source material.
5) A system of audio soft peak clipping (with a visual indication) so that the carrier does not get over-modulated.

I have mentioned all this before with posts of circuits how to do it, but most don't bother and as a result most Pantry transmitters out there are pretty terrible quality.
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Old 15th Dec 2019, 10:29 am   #6
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Default Re: AM Pine Board Project

I think the idea of the Pine Project is just to get people building something simple and nostalgic that can be used to transmit a voice. But yes, it is little more than a toy.

He makes the dangers of the circuit clear quite early on and he has some discharge wand thing and an HT bleeder resistor in a later version.

Good sounding pantry transmitters are not that hard to build, particularly solid state ones. Audio compression - which when done properly - is quite complex - and if not done properly can make matters worse! (I'm sure software can do most of the work these days.) But retransmitting a radio station from say FM or DAB the dynamic range is already somewhat restricted, not as much as is required for AM, but once the modulation is set on one of my devices, there's no over-modulation and what would normally be considered as under-modulation in quiet passages is taken care of by the relatively high field strength.

But to set this up a modulation-depth monitor of some type is virtually essential.
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Old 15th Dec 2019, 10:56 am   #7
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Default Re: AM Pine Board Project

Quote:
Originally Posted by Ian - G4JQT View Post
Audio compression - which when done properly - is quite complex - and if not done properly can make matters worse! (I'm sure software can do most of the work these days.)
No that is not correct. The software I find is problematic and useless. All it requires is the NE571 compander IC.

One IC is "just" enough. After years of experiments with this issue for AM transmitters, I found that if you cascade two NE571's basic system in series you effectively create a constant volume effect that is equally is good as any radio station transmission (it ends up taking the fourth root of the average level). Since the NE571 was designed for two channel use, you only need the one actual IC to do it for mono. I could attach the schematic if you are interested, including the circuitry for the soft peak limiting and LED display of peak limiting, last time I attached it few were remotely interested.

It works exactly as though you had a human being adjusting the amplitude of the audio signal for the same listening (modulation) level for all music recordings regardless of their original level. There is a huge difference in the levels from various mp3 recordings as one example.
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Old 15th Dec 2019, 11:19 am   #8
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Default Re: AM Pine Board Project

Quote:
Originally Posted by Argus25 View Post
Quote:
Originally Posted by Ian - G4JQT View Post
Audio compression - which when done properly - is quite complex - and if not done properly can make matters worse! (I'm sure software can do most of the work these days.)
No that is not correct. The software I find is problematic and useless. All it requires is the NE571 compander IC.

One IC is "just" enough. After years of experiments with this issue for AM transmitters, I found that if you cascade two NE571's basic system in series you effectively create a constant volume effect that is equally is good as any radio station transmission (it ends up taking the fourth root of the average level). Since the NE571 was designed for two channel use, you only need the one actual IC to do it for mono. I could attach the schematic if you are interested, including the circuitry for the soft peak limiting and LED display of peak limiting, last time I attached it few were remotely interested.

It works exactly as though you had a human being adjusting the amplitude of the audio signal for the same listening (modulation) level for all music recordings regardless of their original level. There is a huge difference in the levels from various mp3 recordings as one example.
Yes, that sounds interesting. I don't remember seeing such a circuit. Please post it for us to look at.

I've worked in broadcasting and the relevant experience here was that I operated the equipment that cleaned up HF signals ready for rebroadcasting from Woofferton. I remember the problems in the 1980s with the tech available then that improperly set up AM processing sounded dreadful - and that was a long time before any software was available.

Even in the 90s and 00s I worked with RSLs and some staff would insist in twiddling the processor controls, even though we tried to hide the devices! The most objectionable effect was the audio levels 'pumping', sounded dreadful!
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Old 15th Dec 2019, 11:59 am   #9
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Default Re: AM Pine Board Project

Quote:
Originally Posted by Ian - G4JQT View Post
Yes, that sounds interesting. I don't remember seeing such a circuit. Please post it for us to look at.
I build the attached circuit into all of my Pantry TX's, prior to the linear modulator. It works well receiving a signal from an iPod with files from multiple sources.

I also found an oddity, even though in theory the SA571 should be the same as the NE571, it is not. The NE571 is the superior part.
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Old 15th Dec 2019, 12:18 pm   #10
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Default Re: AM Pine Board Project

Thanks. That's something I might consider. Could you send the parts list? PM me with that if you'd prefer.

Thank you.
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Old 15th Dec 2019, 2:40 pm   #11
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Default Re: AM Pine Board Project

The SSTRAN AMT3000 uses a SSM2166A chip for audio processing, works well.
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Old 15th Dec 2019, 9:29 pm   #12
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Default Re: AM Pine Board Project

Quote:
Originally Posted by Ian - G4JQT View Post
Thanks. That's something I might consider. Could you send the parts list? PM me with that if you'd prefer.

Thank you.
I've got a copy of the diagram with the part values labeled, I will look for it and post it. Also that one I posted the available voltage to run it was only 11.5V (inside the SG503 sinewave generator) so it got regulated down to 8V which was about as low as it could run on. Usually in my other versions it has a 10V regulator..
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Old 15th Dec 2019, 9:36 pm   #13
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Default Re: AM Pine Board Project

If you build a pantry TX on a pine board Monterey Pine is the best

Lawrence.
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Old 15th Dec 2019, 10:07 pm   #14
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Default Re: AM Pine Board Project

I found the parts list and my original notes/diagram of the compressor part, before I added the soft peak limiter and LED indicator that follows it. On the peak limiter you will notice that instead of a diode's current limiting one polarity of the signal, a transistor's B-E junction is used and that transistor's collector current is use ultimately to drive the clip LED circuit with a pulse extender.

The cascaded NE571's work really well, its very difficult to hear any attack/decay issues and it results in very uniform average levels, so when the level is set up so there is only the occasional peak clipping visible on the LED, its fine for all source material or recordings from the ipod which can have quite differing levels. Also I found that ipods with "volume leveling" inbuilt , it doesn't actually work very well.
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Old 16th Dec 2019, 9:30 am   #15
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Default Re: AM Pine Board Project

Quote:
Originally Posted by Argus25 View Post
I found the parts list and my original notes/diagram of the compressor part, before I added the soft peak limiter and LED indicator that follows it. On the peak limiter you will notice that instead of a diode's current limiting one polarity of the signal, a transistor's B-E junction is used and that transistor's collector current is use ultimately to drive the clip LED circuit with a pulse extender.

The cascaded NE571's work really well, its very difficult to hear any attack/decay issues and it results in very uniform average levels, so when the level is set up so there is only the occasional peak clipping visible on the LED, its fine for all source material or recordings from the ipod which can have quite differing levels. Also I found that ipods with "volume leveling" inbuilt , it doesn't actually work very well.
Thank you.
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Old 16th Dec 2019, 3:49 pm   #16
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Default Re: AM Pine Board Project

Thanks from me too Hugo, I'll save that to build at some point
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Old 18th Dec 2019, 12:08 pm   #17
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Default Re: AM Pine Board Project

R&EW, page 8 - Audio limiter p.8. You can replace the op-amp/LED display with a
LM3915 IC or module monitoring the voltage across C7;

https://www.americanradiohistory.com...EW-1983-05.pdf
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Old 18th Dec 2019, 5:06 pm   #18
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Default Re: AM Pine Board Project

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R&EW, page 8 - Audio limiter p.8. You can replace the op-amp/LED display with a
LM3915 IC or module monitoring the voltage across C7;

https://www.americanradiohistory.com...EW-1983-05.pdf
The are also volume compressors (levelers) for stereo audio that became very popular for minimizing the differences in level between the TV adverts and other program material that drive people up the wall. Just like the one you cited and and mine they all use the NE571 compander IC.

However, as I noted, to get a constant level effect that I was satisfied with I had to cascade two stages, one was inadequate to fully level the source material from an ipod. One stage was a reasonable improvement, but not as good as two.

I have attached a messy graph from my archives of when I was experimenting with the leveling problem with the NE571, the blue line shows the time averaged rms output voltage from a single stage(channel) of the NE571 (the way the manufacturers intended it to be used) labelled V^(1/2) and the green line what happens with two cascaded stages labeled V^(1/4).

With just the one channel the slope was still too steep, but more importantly it failed to level adequately on a listening test. Adding more than another stage , or three cascaded stages was not that helpful. That is why I settled on two.
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