flav

PPM meter driver - should i?
« on: December 14, 2018, 01:07:08 PM »
Hello guys,
I got my hands on a Sifam 22AF , BBC PPM scale (1-7). i paid for it, you would not believe, 25Lei (less then 5 pounds).
I set myself the task of driving it the  proper way so i begun researching. I found some options:
 - Rod Elliott's VU / PPM Circuit  - http://sound.whsites.net/project55.htm
-  surrey ppm3 driver - http://www.saturn-sound.com/images%20-%20cct%20dia/surrey%20electronics%20ppm3%20-%20cct%20dia'.jpg
-  ssl ppm - https://www.ka-electronics.com/images/SSL/SSL_PPM.JPG

So, i would like to design a PCB so i can DIY'it (i am very crafty at pcb design but i only have medium/limited electronic knowledge).  I plan to do a passive monitor controller and include this meter....
Forgiving my ignorance, i would appreciate some advice to choose one path.

p.s - i allready found and ordered 1s44 diodes on ebay - as used in ssl schematic above, i presume they are germanium


pvision

Re: PPM meter driver - should i?
« Reply #1 on: December 14, 2018, 06:16:33 PM »
I think the Surrey design will be what you want but also suggest you have a look at the source:

http://www.bbceng.info/ti/eqpt/ME12_10.pdf

Nick Froome


PRR

Re: PPM meter driver - should i?
« Reply #2 on: December 15, 2018, 01:17:53 AM »
.

PRR

Re: PPM meter driver - should i?
« Reply #3 on: December 15, 2018, 01:18:36 AM »
.

flav

Re: PPM meter driver - should i?
« Reply #4 on: December 15, 2018, 07:59:44 AM »
Thank You ! I will do some reading now and I will keep you posted.

flav

Re: PPM meter driver - should i?
« Reply #5 on: December 16, 2018, 12:32:58 PM »
Hello again,
After more study, and drooling at the BBC paper (thanks Nick), i decided to first give a try to Surrey PPM3 schematic.  The  op-amps used are obsolete/hard to find so i designed the PCB using TL08* .  After changing op-amps, i had to ditch some of the parts (frequency compensation capacitors C4,C5,C6, and R11 which i guess could also be for freq. stability). I plan to do a home etch proto next week so somebody please stop me if i'm on a wrong path with the modification.  I also changed transistors to similar ones that i already have - BC309 is nowhere to find.

I may also protoboard the small transistor schematic (thanks PRR) and give it a try, but for sure i would not mess with the tube  version  ::)


I attached the schematic
link to eagle&pdf files - http://www.axaprospect.ro/electro/ppm/


pvision

Re: PPM meter driver - should i?
« Reply #6 on: December 16, 2018, 07:11:42 PM »
FYI, many PPM driver PCBs have two holes in so they can be mounted directly on the back of the meter. The holes are plated so the meter connects to the board

I imagine they're sized so the meter can be dropped into a hole in a panel with the PCB attached. At least, that's what I'd do

You'll find an SSL PPM driver schematic out there as well. They were an option on the 4000E and, from memory, used Sifam meters

Nick Froome

flav

Re: PPM meter driver - should i?
« Reply #7 on: January 07, 2019, 07:28:19 AM »
I decided to allow myself multiple choices. As a consequence, having some spare time this year end, i designed also a pcb using the schematic from the BBC paper.  I also refined the surrey ppm3 clone and sent gerbers to a Chinese fab house. I plan to have one of each populated, tested (hope i did not mess the designs) and come back with the result.  If the designs will work as expected i will have a bunch of extra boards for both designs. If anybody wants one i will be happy to give them away for free  ;) ... if they work...

I also updated the content (eagle files) on the link for anyone interested.

BBC AM20/3 pcb


Surrey PPM3

Re: PPM meter driver - should i?
« Reply #8 on: January 07, 2019, 09:09:53 AM »
Looking good! :)

What's the "slugged" jumper?

flav

Re: PPM meter driver - should i?
« Reply #9 on: January 07, 2019, 02:08:19 PM »
Both surrey and BBC schematics have a SPDT switch  labelled normal/slugged mode.  In slugged mode it switches a pair of caps in parallel with the meter witch i believe dampens the motion of the needle.  I really don't know the use of this, maybe there is someone here who does  :D


pvision

Re: PPM meter driver - should i?
« Reply #10 on: January 07, 2019, 07:31:30 PM »
The BBC spec required a certain decay time for the meter needle. The pointer is driven to peaks and decays about three or four times more slowly. IIRC this is to enable the eye / brain to catch up with the peaks and register them. I believe the slugged setting would be the slower decay

The BBC carried out a lot of research into metering and the human / meter interface. I think it was useful - once you've used BBC-spec PPMs for a few years all other metering systems seem rather pointless

PPM 4 was "zero" and each graduation was 4 dB so PPM 7 was +12 or peak level. I think the gap between PPM 1 and 2 was more than 4 dB - I don't remember

Going over PPM 7 might get the studio manager a slap on the wrist. Put an ex-BBC person in front of some PPMs and ask them to push it over 7 - you'll see them wince unconsciously

Nick Froome


 

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