PPM meter driver - should i?

[flav]

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]

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]

.

 

[PRR]

.

 

[flav]

Thank You ! I will do some reading now and I will keep you posted.

 

[flav]

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]

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]

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

 

[warpie]

Looking good!

What's the "slugged" jumper?

 

[flav]

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 

 

[pvision]

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

 

[flav]

Hello to all,
One week ago i received the pcb's from China. So far so good, i got some time and populated one of each. Without a signal generator i powered on the first board (AM20/3) and looked good - no smoke. I terminated the input with 600ohm and set the 0 trimmer. I was delighted.  same with the Surrey based board.
After another day i borrowed from a friend a signal generator and started the test.  The needle was resting left on any signal..not good.. and both drivers behave the same.
I switched the meter polarity after following the schematics using an oscilloscope and checking every stage of the signal... and finding negative voltage on the meter + terminal. Duhhh... I failed to pay attention on the meter polarity and assumed wrong based on the schematic position (-10v above the meter, 0v below).
The meter drivers are working awesome , both of them.  In order to mount the board on the meter i had 2 choices - first to rotate the driver 180deg , but it would not fit the case.  Second choice - to make some brass spacers (given that the parts are now on the meter side of the board... )
I made a passive monitor controller and included this nifty gadget. Now i'm happy.

I learned a lot doing this project  and now i feel a little bit more confident on my electronics skills (i usually design buildings ).
Same link as above, i put some pictures and videos of the build.  Also, as previously stated, if anyone wants a free pcb (i have available 8 of each) please pm the mailing address - i'll be happy to post them.

PCB

Testing

 

[Mictester]

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

PPM 6 was peak level.  Anything over 6 was ruthlessly chopped by the limiters! 

It was interesting to compare the different BBC radio networks (only four of them in my day) - Radio 4 was usually between 4 and 5 with occasional peaks to 5½ or so.  Radio 3 was usually around 3 - 4  with occasional bigger peaks.  Radio 2 was usually 4 - 5 with speech peaks to 5½ or so, and Radio 1 was nailed to 6 with just a slight quiver of the needle to show that it wasn't tone!

You're right about the efficacy of PPM - they're good for both line-up tone levels and for dynamic programme.

Mt

 

[neil.johnson]

Nice piece of work there!

I've been restoring a DDA D-Series desk that came with PPM meters (I think it had been used in a theatre company).  As two of the meters were not working I took the opportunity to reverse-engineer the driver circuit, and uploaded the result to my restoration project page.
Schematic: https://www.njohnson.co.uk/img/DDA/DDA-PPM.png
Project page: https://www.njohnson.co.uk/index.php?menu=2&submenu=4&subsubmenu=16

Two things I like about the DDA circuit are:
1/ Totally floating module, just needs 18V across its power pins
2/ Balanced inputs, capacitively coupled (see #1)

One is already fixed (bad TL072).  The second one is under the scope probe...

Cheers
Neil

 

[pvision]

Nick

PPM 6 was peak level.  Anything over 6 was ruthlessly chopped by the limiters! 

Memory is a fragile thing....

Nice job on the PCB design, Flav

Nick Froome