SIFAM Director PPM14 and Presentor 19B Difference

Hi!

I have just purchased a Sifam Presentor 19b meter to use as a cheaper alternative in my second GSSL build. It is 1mA fsd.

Before installing it into the new build, I decided to check it out "in situ" in a known working unit that is all running to spec... so I swapped out the director ppm 14 , for the presentor 19b , and ran some tones..

Both meters are linear scale 1mA , but they do not behave the same!

The cheaper Presentor meter is reading higher , i.e 4dB on the director looks more like 6dB on the presentor , and 8dB reduction appears more like 10dB...etc

I have checked the meter specs , and it appears that the internal resistance is different...or "terminal resistance" as sifam put it (I'm assuming this is the same thing)

Director ppm14 1mA - 68 ohms

Presentor 19b 1mA - 50 ohms


I'd like to keep the dB scale as similar as possible to the standard director ppm14 "dB compression" scale!

So, can I simply adjust this discrepancy with an appropriate shunt resistor?

Or should I be toying with fine tuning the value of the on board meter resistor??

Cheers y'all

nEon.

Measure the ACTUAL gain reduction, and adjust the series resistance to make it match.

The reality of it all is that neither will likely be perfectly spot on, and if you want 20dB to ACTUALLY indicate 20dB GR, you will probably have to fine-tune the value.

I use a trimpot to set mine, then measure the final value and replace with a fixed resistor, or series combination of two resistors.

By the way, 90% of all GSSLs probably read incorrectly, but nobody ever calibrates them. -If full scale equals -18dB, most users don't care, and set them by ear anyhow...

Likewise, when I measured the actual RATIOS of most clones which I set up/calibrate/adjust for colleagues/friends, they have been so far off that I started a whole thread on the subject... Again, the 'official' front panel numbers are arbitrary, so who really cares all that much?

Keith

Hi Keith , thanks for the advice!

I'll do exactly that!..infact looking closer at my 1st gssl , I've a 2k2 in there,which is why it's pretty much spot on with the ppm14...I must have fine tuned it at the time!

But now I'll replace it with the cheap presentor meter , and free up the Director ppm14 for my ULTIMATE SSL!!!

Fired up the ultimate yesterday for the first time , and oh what joy to hear that dual sidechain at work!!!

Great work on that Keith!...I reckon my next step will adding your new turbo board to my first (gyraf)gssl , as soon as they are available. It will be a new and interesting experience to do something on a pre fabbed board , as i've only been self-etching everything, so far.

Cheers!

nEon.