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I have recently purchased an amplifier for my repeater. This was actually my first amplifier purchase ever and should probably have asked more questions prior to buying it. The repeater is a Motorola XPR8400 UHF R1 running in dynamic mixed mode. The amp I purchased is TPL Communications PA6-1BE-RXRFPSM UHF 100W. I've heard that some amps have issues with digital signals and some don't depending on the amp class. I tried to do research on this but there is little to no information online. Was able to confirm though through an acquaintance who never heard of that is maintaining a repeater system running a 250W TPL amplifier in DMR without any issues. What is the theory behind digital signals vs analog and repeater operations? Will this specific amplifier transmitting DMR work or will I run into problems? Lastly, if this amplifier is not suited for DMR which make and models are?

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    $\begingroup$ This is a good question, I'm looking forward to some answers. Two things come to mind immediately: the "digital" modulation probably has an amplitude component to it, which may require a more linear amplifier than just phase modulation. And digital transmissions likely involve transmitting in timeslots, hundreds of times per second, and possibly also receiving between the transmitting slots, like a cellphone does. None of these will work well with a simple hard limiting "FM" amplifier. $\endgroup$
    – tomnexus
    Commented May 19, 2023 at 23:59
  • $\begingroup$ ChatGPT gave me the best answer. Very detailed but it was unable to tell me anything about my specific amp. It’s a very common commercial amp and I’m hoping that someone familiar with it could provide some critical information on the specific amp. $\endgroup$
    – Mike
    Commented May 20, 2023 at 17:08
  • $\begingroup$ Hah, yes, maybe it has swallowed a summary of the issues. But it also hallucinates and fabricates things. What is the amplifier, perhaps someone here will look it up. $\endgroup$
    – tomnexus
    Commented May 21, 2023 at 1:39
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    $\begingroup$ The difference in the amplifiers depends on what modulation you are using. Some amplifiers take shortcuts that make them less linear. You would need a more linear amplifier for AM or FM vs. SSB and even less for CW. FM and AM need a fairly linear amplifier, so likely a digital signal would not have a problem with the same amp. If you've already hooked it up and you're not getting distortion that breaks things, likely it is fine. Duty cycle is a bigger concern -- especially for a repeater that might have 100% duty cycle. $\endgroup$
    – user10489
    Commented May 21, 2023 at 12:19

2 Answers 2

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So... really what you want to shoot for with a digital system is relative signal consistency between among the 1's and the 0's. With that in mind, recall that:

  • Class A amps are pretty linear, reproducing the input signal with the least distortion. Good for high-fidelity audio applications or amplifying complex signals... like DMR. The tradeoff is in terms of power usage, typically only around 30-50% efficient.

  • Class B and Class AB amps are more power-efficient than Class A amps, BUT can produce more crossover/distortion if they're not designed properly for the application.

  • Class C amps are very efficient, but are not linear and produce a lot of distortion. They are usually only used for signals where the exact shape reproduction is not critical.

So you'll want a Class A or a well-designed Class AB amp for best signal integrity.

If this is the product, then it looks like each stage is a single common emitter transistor feeding the signal through an LC circuit... which I believe is a Class C design (if this is incorrect, please someone edit).

Hope this helps!!

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  • $\begingroup$ What I havent been able to determine is what class my amplifier mentioned in the post is. $\endgroup$
    – Mike
    Commented Jul 28, 2023 at 17:33
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    $\begingroup$ Just updated my answer w some info, confirm that this is your product? If it's a different one, share a link to the manual/schematic and we can figure it out. $\endgroup$
    – webmarc
    Commented Jul 28, 2023 at 20:01
  • $\begingroup$ That appears to be an old manual and it appears newer versions of this amp was updated along the way. I believe this is the manual repeater-builder.com/other-mfrs/pdfs/tpl-uhf-rptr-amps.pdf $\endgroup$
    – Mike
    Commented Aug 16, 2023 at 12:27
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DMR and P25 use linear amplifiers. Using non-linear amps made for FM or CW will not work. All your digital downlink modes use a form of QAM.

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    $\begingroup$ That's not true. All of the VHF/UHF digital voice modes hams use (including DMR and P25) are basically FM (C4FM or GMSK). That's why an MMDVM and an FM radio can handle them all. None of them are QAM. $\endgroup$ Commented Feb 15 at 1:48

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