Amplifying a Raspberry Pi

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OldskoolPirate
no manz can test innit
no manz can test innit
Posts: 142
Joined: Thu Feb 18, 2016 9:32 pm

Amplifying a Raspberry Pi

Post by OldskoolPirate » Thu Aug 29, 2019 11:43 am

I’m guessing amplifying a raspberry pi configured to broadcast fm is going to be a sproggy mess, but has anyone tried it ?

A raspberry pi offers audio streaming, rds and fm oscillation all in one.

Albert H
proppa neck!
proppa neck!
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Re: Amplifying a Raspberry Pi

Post by Albert H » Thu Aug 29, 2019 12:59 pm

Please don't do this!!!!

The "FM" output from an R-Pi is actually a harmonic from one of the outputs. It's truly filthy! You can use the R-Pi as a streaming receiver and stereo coder (with a fast sound card add-on), and for bonus points, you can generate RDS too!

An FM exciter can be a fairly simple thing - especially if it's built for one frequency. There are plenty of good, working circuits around. Take a look at the old NRG circuit with the balanced oscillator. If you get it accurately balanced and loaded correctly (not like the "Veronica / Arreff" mess), you can get a really pure Band II source with a ½f output to feed to a cheap and simple CMOS PLL. A couple of well laid-out and filtered stages of amplification will get you to a really clean and stable Watt.... You can build one for <£20 if you shop around for the parts.

A friend of mine is using a variant of this circuit with an SAA1057 PLL with a 2MHz crystal and ½f input to the PLL. He has a pair of BF199s in the oscillator followed by a further pair of BF199s in parallel to drive a 15 Watt Mitsubishi FET to 12.5 Watts - just three stages to a useful power level. His PCB is double-sided (the top is just groundplane) and it's about 12cm X 8cm. The frequency required is burnt into a little 8-pin PIC, so changing frequency isn't trivial, and there are no adjustments on the board whatsoever - tuning is accomplished by bending coils a bit and by selection of a couple of the capacitors. I've looked at his board on a spectrum analyser, and there's nothing nasty - it's really clean. He bought a big batch of components and manufactured 200 of the boards. He's selling them in the 'States and into South America, and the first batch sold out in a couple of weeks! The only returns he's had are from people who needed a frequency change.....
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

OldskoolPirate
no manz can test innit
no manz can test innit
Posts: 142
Joined: Thu Feb 18, 2016 9:32 pm

Re: Amplifying a Raspberry Pi

Post by OldskoolPirate » Thu Aug 29, 2019 2:47 pm

Yeh I thought it would be lol. Was just wondering. Thanks

biggiedan
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big in da game.. trust
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Re: Amplifying a Raspberry Pi

Post by biggiedan » Fri Oct 11, 2019 12:57 am

OldskoolPirate wrote:
Thu Aug 29, 2019 11:43 am
I’m guessing amplifying a raspberry pi configured to broadcast fm is going to be a sproggy mess, but has anyone tried it ?

A raspberry pi offers audio streaming, rds and fm oscillation all in one.
Maybe this could be an alternative solution.

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