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Amplifying a Raspberry Pi

Posted: Thu Aug 29, 2019 11:43 am
by OldskoolPirate
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.

Re: Amplifying a Raspberry Pi

Posted: Thu Aug 29, 2019 12:59 pm
by Albert H
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.....

Re: Amplifying a Raspberry Pi

Posted: Thu Aug 29, 2019 2:47 pm
by OldskoolPirate
Yeh I thought it would be lol. Was just wondering. Thanks

Re: Amplifying a Raspberry Pi

Posted: Fri Oct 11, 2019 12:57 am
by biggiedan
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.

Re: Amplifying a Raspberry Pi

Posted: Wed Oct 30, 2019 7:44 pm
by Gigahertz
Yep as Albert has mentioned I wouldn’t amplify the raspberry Pi let alone use it on its own.

I’ve recently tried Pi_Fm_Rds file and the output of the Pi can be heard on other spot frequencies across the broadcast band!

I found with roughly 80cms of wire it would broadcast up to 500metres and I measured the output on a Lake electronics power meter “ how accurate it is???” about 25mW at 76MHz and about 10mW at 108MHz.

Rather put the Pi to better use....

Re: Amplifying a Raspberry Pi

Posted: Thu Oct 31, 2019 3:30 am
by Albert H
I've been using a Pi Zero W and an "Audio Injector Zero" card as a streaming server for an internet radio station.

I installed the cut-down version of Raspbian ("Lite") and added the drivers for the Audio Injector - a bit trickier than their website would suggest, but I got it working OK. I then installed Darkice and Darksnow (the configuration programme for Darkice) and configured the stream info as I wanted. I enabled SSH but didn't bother with VNC as I don't want to be running the desktop if I don't need to, and I've always found VNC to be pretty flaky!

The initial set-up was done with a little USB hub connected, a monitor plugged in, and a keyboard and mouse plugged in. Once I was satisfied that it was working OK, I disconnected all the peripheral bits, rebooted to the command line, and used Putty on my desktop machine to talk to the Pi. I set up a Chron job - to run on boot - to use the saved configuration file in Darkice, and another to set the "mixer" input setting to 56 - which seems to be about the right sensitivity for 775mV into 600Ω. I soldered terminating resistors on to the little RCA socket input board that came with the Audio Injector, and hard-wired the output of a little limiter board to the underside of it.

The Raspberry Pi Zero W with its connected Audio Injector card were housed in a plastic instrument case (from Farnell), along with the basic stereo limiter and level indicators. The whole lot is powered from a cheap "wall wart" linear 15V 1A power adaptor (from Rapid), and there is a 7805 providing the power for the little computer board, and a 7808 powering the bar graph indicators.

On boot, the R Pi begins streaming after about 30 seconds - the Zero W is very slow compared to the full-fat big brother R Pi. The audio quality is superb, and it sips current (compared to a complete desktop computer being wasted for the same job!). The whole thing cost about £35.

My next project (possibly this weekend) is to use the same hardware to build a stereo Outside Broadcast TX and RX pair. I'm going to try to include talkback if possible!

Re: Amplifying a Raspberry Pi

Posted: Thu Oct 31, 2019 8:19 am
by neckertude
Interesting stuff, Albert. I've been meddling with a Raspberry Pi 3b and Hifiberry card, its pretty darn good for the cost / performance. Would be very interested to read about your next Pi project :D