SSL Calibration and Tone Oscillator questions

[MarconeMusic]

Hi Guys,

I have a quick question for yall. I did a quick search and didnt find anything quickly about this.

Im playing around with the Digital Converters i've been using for years... They can be set for 14-20 dbu. I have them set at 18dbu in and out. (Im using antelope Orion 32 Converters)

I was messing with the DBu in and out for fun to see what happens and what would be best.... I still dont know about that part..

heres my Q.

I've been under the impression that the voltage of the oscillator should be 1.226v or somewhere near there... .which should relate to +4DBu which is 0VU Correct?

When my 4k Oscillator is at the Cal position it measures 0.9v ac. ( i dont have a meter that does terribly fine details but thats too low? )

When i inject the oscillator to a channel line in, look at the centre section and i get -4vu on the master meter when channel is centred and when i pan to the left or right i get 0vu. Seems correct....

If i inject the same tone Osc to the console master section (post or pre) vca position i get approx +3vu? Odd

If i send a tone signal from Pro Tools at -14db 1k tone out to a patch point it measures 0.8v ac (similarly close to the console cal but not the same)

When i Inject the oscillator to a channel line in, i get 0vu on the channel meter.
When sent to the master and the pan is left or right i get -1vu on the master meter. When channel pot is centred i get -5vu. Seems normal. Given the voltage difference may be giving the -1vu but the same results from the same channel different osc.

If i inject the signal to the master Pre or Post vca position i get approx -3vu...? Weird..

What am i missing here. Shouldnt the osc come out at 1.226v firstly? i know theres a trim pot on the osc. I dont want to touch it incase it is set right ... but i've recapped the console in the last year. so maybe someone set the pot last through bad caps ? Dunno...

Why would pro tools be outputting a .8v I'd assume it be outputting a 1.226 properly but i dont know...

I'll be calling our tech, but i would love to know if this is something that is way out or im doing it wrong first.

thanks!

[amillar]

First question is what sort of meter you are using! Most meters don't work brilliantly at 1kHz, something like a Fluke 8060 (my personal favourite) will be fine. Worth trying different frequencies to see what happens.

Yes, you're right that 1.226V is +4dBu in SSL terminology.

After that - welcome to the 4k gain structure! 30 years ago I could have explained to you for hours and hours what was going on...but certainly the gain structure of the 611 compared to the 651 inserts does take some getting your head around - it is not obvious at all. I think what you are seeing is what it's supposed to do, but I may be wrong.

Cheers,

Andy

[MarconeMusic]

Hey Andy! Thanks for the reply.

I’m using a basic tech shop meter. Velleman dvm850bl
It’s not a great one.
I did, during my testing, use a 1K tone, but tried to use the 440 and 220 for the exact reason you mentioned.

All results were the same.  0.8 v

[Chris Simon]

I'd say it depends on how you're measuring, and where. The on-board oscillator should measure +4dBu (1.228 V) when measured on the other end of the bantam cable; if it doesn't, then adjust that first. Now, if that same patch cable is patched to a line in, you'll measure 0dBu (0.775 V) at the insert send, as the level is reduced internally. That same signal, with the channel VCA fader at the "0" mark, will drive the Quad bus to a level somewhat lower level than 0vu on the Quad meters; I've never actually seen documentation that says what that level is supposed to be. Remember that the Quad bus would be overloaded in a hurry if any individual channel was able to drive the bus at unity gain.

Do not trust your meter. If it's a typical non-audio multimeter, the response may be a little better at 100 Hz, but not as good as the classic Fluke 8060A (I have one myself). Don't trust the Pro Tools oscillator, as it's response is different in Peak and RMS modes. Don't trust your interfaces until they're calibrated, inputs and outputs. I believe the industry standard is -18dBFS = +4dBm, so confirm that an external signal that's known to be 1.228 V will drive the Pro Tools inputs to -18 when in calibration mode.

Don't worry. It may turn out your system is just fine, but without proper test equipment you can't be sure.