Process for calibrating console?

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jdurango

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I am trying to calibrate line in/out on a freshly refurbed/recapped/gone through Sound Workshop Series 40 console. The VCA's have been bypassed with regular audio faders and cap values doubled. Otherwise it's stock.

I was given a process which doesn't make sense to me, but I will summarize thusly in case it may at least guide down a helpful path. Might be worth skipping over all this though....

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1. Set sig gen to line level (1.23VAC RMS between ground and tip @ 1k).
2. Set a channel to tape in mode (which monitors the line in BEFORE the channel fader), with the tape level pot cranked and pan pot all the way left.
3, Run the line level signal into the tape/line input.
4. Monitor the tape path on the master section control room out with the console master fader all the way up (0 marking).
5. Take note of what the left master VU meter reads (let's say it's +14)....doesn't matter if this is accurate for now. Just trying to establish a level.
6. Switch the channel to listen in line in mode (same exact input source, but now monitoring post fader) and hook the channel direct out up to a VU meter or computer to monitor level (I'm actually using a true RMS DMM and calling 1.23VAC between tip and ground line level/0dB/+4dBu)
7. Now MOVE THE CHANNEL FADER to whatever arbitrary position makes the signal on the left master VU the same as before (so +14 in this example).
8. Now here's where I get lost....we've set the channel fader to some arbitrary position to get +14 on the left master VU. Now we adjust the line trim cal pot to see line level on the DMM monitoring the direct out (which is post fader).....so we've just set the direct out to line level, great.....but only at whatever arbitrary position the fader happened to land on in step 7....assuming that doesn't happen to be 0, when we move the channel fader to the 0 marking, the direct output will now be different from line level/0dB/+4dBu.
9. But let's continue. "Now you can calibrate 0dB on the meter"...I'm guessing this means adjust the LED's to indicate 0dB....but there is another cal pot on the meter which also affects post fader level (TK cal, just regulates level going to the output card which feeds the direct out). So if I adjust this, it'll negate the line level we just set in step 8.
10. Apparently now the channel is calibrated and you move on to calibrating the master section (which also doesn't seem to work) then use this "properly calibrated" channel to verify your master section is correctly calibrated, then you can move on calibrating the rest of the channels in the same fashion, except using the master section VU meters as your line level reference now instead of an external DMM or DAW or VU meter.

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Assuming that doesn't lead anywhere and I need to find a process starting from scratch, here is a link to a PDF with relevant info: https://drive.google.com/file/d/1pHLLHGsSWBg3DNyJGFTvmsqpmszsa_jf/view?usp=sharing

Page one: Block diagram showing basic signal flow in the console.
Page two: The only tiny piece of info I could find about calibration in the precious little Series 40 documentation I have.
Page three: Input module schems (you'll see the tape/line cal pot on the OL gain of the tape/line driver IC)
Page four: Output module schems (the TK cal pot seems to have the same effect as the tape/line cal pot on the channel card).

I know it's a PITA but any help or direction with this is GREATLY appreciated! Thanks fellas!
 

jdurango

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The closest I can get is simply running a line level signal in with the chan fader @ 0 and adjusting line trim pot til I see 1.23vac on the direct out, which yields about 1vac @ the balanced insert out and 1.32vac at the bus/track out (even with TK cal pot all the way down.....can't get it any lower). Then I can adjust LED's to read 0 while the fader is @ 0 and all outputs are at least in the ballpark of 1.23VAC (wish there was a seperate adjustment for insert out level).

Then I can adjust the master section CR output to also be 1.23v...only problem is I can't get the master VU LED's cranked down any lower than +10 without turning down faders or level pots elsewhere....which then of course lowers the CR out below 1.23v.

Again, all VCA's have been bypassed with audio faders....so the 0 markings on the channels are probably just approximate. The 0 on the master is at the top of travel, should effectively be the same as the VCA (a near short).

This process almost seems TOO easy and straightforward....and levels still aren't matching up perfectly. Something still seems off....
 

PRR

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TL;DR

Not gonna start without a level diagram, stock AND as-modded.

You replaced VCAs with pots. VCAs often give gain. Pots don't. Is it possible the level structure is all messed up?
 

Brian Roth

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Looking at the linked block diagram, it appears the internal operating level is -2 dBu.  That shows up at the insert point and at the direct out (unless the optional 1:2 transformer is installed which boosts the direct out up to +4 dBu).  Start from there.

Bri
 

jdurango

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Brian Roth said:
Looking at the linked block diagram, it appears the internal operating level is -2 dBu.  That shows up at the insert point and at the direct out (unless the optional 1:2 transformer is installed which boosts the direct out up to +4 dBu).  Start from there.

Bri

Thanks Brian! That certainly sheds some light into things. It definitely uses -2dB for the internal signal but output is listed in the specs as +4dBu. I assumed that applied to direct outs too! Thanks a ton!

So I'll set the line cal pot to - 2dB at the insert point or direct out, then the bus outs to +4 using the gain pot on the output/meter cards. I'll try this tomorrow. Hopefully this will also allow me to match up the LED meters more closely!

Thanks a ton!
 

PRR

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> uses -2dB for the internal signal but output is listed in the specs as +4dBu.

There's a transformer on that main output. I assume the extra 6dB is just in there.
 

jdurango

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PRR said:
> uses -2dB for the internal signal but output is listed in the specs as +4dBu.

There's a transformer on that main output. I assume the extra 6dB is just in there.

Yeah it has 4 API xformers in the master section for outputs....but shows 6 xformer balanced outs (3 stereo) in the diagram. I'm guessing the one for CR and Studio is shared over multiple windings, with the 2-tk having its own dedicated xformer?
 

Brian Roth

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jdurango said:
Yeah it has 4 API xformers in the master section for outputs....but shows 6 xformer balanced outs (3 stereo) in the diagram. I'm guessing the one for CR and Studio is shared over multiple windings, with the 2-tk having its own dedicated xformer?


????????? Shared over multiple windings?  That makes no sense to me. 

Thinking back to a similar desk....MCI JH-636.....they mounted output iron for the CR and Studio outputs down in the belly.  2 track outs (IIRC) were electronically balanced circuits on the master module.

I don't have any brilliant ideas.

Bri
 

jdurango

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Okay, one last question....I think.

With a +4 line in, if I adjust the direct out to -2 (0.615v), I'm getting about 0.49v on the insert (about -4dBu).

If I set the insert to -2 (0.615v) then I get 0.767v on the direct outs (just under 0dB).

Should I go with the former? Latter? Split the difference? Thanks fellas!
 

Brian Roth

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Looking again at the block diagram, the direct out is affected by the fader pot setting.  The flow chart implies the fader's nominal operating point is 12 dB below "fader full", with a 12 dB makeup gain stage post fader.  That may be messing up your measurements if the fader is set to some point other than 12 dB down.

Bri
 

jdurango

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Brian Roth said:
????????? Shared over multiple windings?  That makes no sense to me. 

Thinking back to a similar desk....MCI JH-636.....they mounted output iron for the CR and Studio outputs down in the belly.  2 track outs (IIRC) were electronically balanced circuits on the master module.

I don't have any brilliant ideas.

Bri

Dude, once again you are correct! There was a xformer option on the 2-tk outs, but mine is xformerless and electronically balanced with little amps on the back of the output/meter card, exactly as you said. So the 4 API's are for the CR and Studio!

Sorry, I thought some xformers had mutiple primaries and multiple secondaries....my bad on that!
 

jdurango

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Brian Roth said:
Looking again at the block diagram, the direct out is affected by the fader pot setting.  The flow chart implies the fader's nominal operating point is 12 dB below "fader full", with a 12 dB makeup gain stage post fader.  That may be messing up your measurements if the fader is set to some point other than 12 dB down.

Bri

Aaaaand once again, hit the nail on the head!

Okay. We're damn close if you'll bear me with a bit longer.

If I set the direct out to +12dBu at the top of fader travel, I get near perfect +4dB on the direct out at the fader 0 marking (so of course there is really only 8dB between the 0 and +12 markings....not a big deal). Only other problem is that now the insert point is reading about +2.7, so without measuring voltage on the actual traces inside the channel, it seems like we're straying off quite a bit from that -2dB nominal internal operating level. Let's call this option A.

Playing around a bit, I think the next best option is setting 0 on the fader to 0dBu (via direct out). This yields nearly perfect -2dBu on the insert and about +7.3 at top of fader travel. Let's say this is option B.

I can always change the physical markings, so that's not a huge issue. Which one would you suggest? Or perhaps an option C?

Thanks again! I really really greatly appreciate the help!
 

Brian Roth

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jdurango said:
Sorry, I thought some xformers had mutiple primaries and multiple secondaries....my bad on that!

Indeed, multiple winding xfmrs do exist, but all the audio running through the windings will be related to a single audio source.  CR and Studio outputs will typically be totally different audio feeds.

Bri
 

Brian Roth

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jdurango said:
Aaaaand once again, hit the nail on the head!

Okay. We're damn close if you'll bear me with a bit longer.

If I set the direct out to +12dBu at the top of fader travel, I get near perfect +4dB on the direct out at the fader 0 marking (so of course there is really only 8dB between the 0 and +12 markings....not a big deal). Only other problem is that now the insert point is reading about +2.7, so without measuring voltage on the actual traces inside the channel, it seems like we're straying off quite a bit from that -2dB nominal internal operating level. Let's call this option A.

Playing around a bit, I think the next best option is setting 0 on the fader to 0dBu (via direct out). This yields nearly perfect -2dBu on the insert and about +7.3 at top of fader travel. Let's say this is option B.

I can always change the physical markings, so that's not a huge issue. Which one would you suggest? Or perhaps an option C?

Thanks again! I really really greatly appreciate the help!

Once again, the "normal" fader operating point should be 12 dB below any level measured with the fader wide open.  The panel markings sound like they were chosen to match whatever fader the factory originally installed and thus not matching the taper of the faders currently installed.

The block diagram shows that, and I just crunched the numbers from the schematic at the fader makeup amplifier.  With the resistor values shown in the schemo, I arrived at just under 12.24 dB gain in the fader booster amp stage.

I believe that you don't have the optional output 1:2 iron at the direct out?  If so, direct out will be at -2 dBu when the insert out is at -2 dBu AND the fader is 12 dB down from wide open.

The schemo you posted is pretty fuzzy.  One Wildcard I just saw was a balance return stage at the patch return.  That may throw a monkey wrench into my numbers because it seems like there is some gain in that stage, but the schemo is tough to read.  I'm just going by the numbers in the block diagram for the "fader at - 12" observation....which is typical/sensible.  -10 dB was common and -12 is still in that ballpark.

Bri

EDIT: I squinted again at the schemo, and the balanced input stage at the insert return is unity gain.


 

jdurango

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Brian Roth said:
Once again, the "normal" fader operating point should be 12 dB below any level measured with the fader wide open.  The panel markings sound like they were chosen to match whatever fader the factory originally installed and thus not matching the taper of the faders currently installed.

The block diagram shows that, and I just crunched the numbers from the schematic at the fader makeup amplifier.  With the resistor values shown in the schemo, I arrived at just under 12.24 dB gain in the fader booster amp stage.

I believe that you don't have the optional output 1:2 iron at the direct out?  If so, direct out will be at -2 dBu when the insert out is at -2 dBu AND the fader is 12 dB down from wide open.

The schemo you posted is pretty fuzzy.  One Wildcard I just saw was a balance return stage at the patch return.  That may throw a monkey wrench into my numbers because it seems like there is some gain in that stage, but the schemo is tough to read.  I'm just going by the numbers in the block diagram for the "fader at - 12" observation....which is typical/sensible.  -10 dB was common and -12 is still in that ballpark.

Bri

EDIT: I squinted again at the schemo, and the balanced input stage at the insert return is unity gain.

Bah, yeah sorry that schem got degraded when I made the PDF as a quick reference. Here's the original scan, much better:
https://drive.google.com/file/d/1QMJVEhI-YnayH57ng0pd_fN-rD93ADJL/view?usp=sharing
And here's all the schems I have scanned: https://drive.google.com/open?id=1MjloWdlX-kBqwyUSl6ev-U9hAJu_9Sb2

Anyhow, put the channel on the bench and verified that a +4dB line in signal calibrated to spit out 0.615v (-2dB) directly from the tape/line amp IC yields -2dB at the insert point as well. So good there, -2dB internal and -2dB at insert. But then with the fader cranked all the way, the max I can get is about +7.9 at the direct out, so 12dB below that puts the direct out level at -4.1dB, well below the -2dB spec. I tried this on multiple channels and they're all in the same ballpark. With insert @ -2, could ALMOST hit +8 with the fader cranked.

And of course if I crank the cal pot to +10dB with the fader at max, this puts the -12dB direct out level at the -2dB spec, but also raises the operating level and insert level. Bah.....not sure what to do, but I feel like in lieu of changing feedback resistor values on the fader gain amp, I should probably set it for proper internal operating level and do my best from there with new position markers and some tech notes posted....unless you've got any other tricks or recommendations up your sleeve? Thanks again for the massive help!! I really, really appreciate it!
 

Brian Roth

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Just curious....What do you read at the Direct Out with 0 dBu into the insert return and the fader wide open?  If I'm understanding the design correctly, you should see just a tad over +12 dBu at the Direct Out.

Bri
 

jdurango

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Brian Roth said:
Just curious....What do you read at the Direct Out with 0 dBu into the insert return and the fader wide open?  If I'm understanding the design correctly, you should see just a tad over +12 dBu at the Direct Out.

Bri

Insert return is returning pretty much the same levels as above using the line in. So 0dBu via the insert yields about +3.4 via direct out with fader cranked and cal pot set for -2dBu internal operation given a +4 input. 0db line in yields the same +3.4 direct out with fader cranked.

And +4 via insert still yields just under +8dB on direct out fader cranked whether going into insert return or line in.

Tried multiple channels, even a different function gen and DMM to check.

I'm looking at the diagram and it looks like the insert return BTA sees - 2dB but has no gain listed, then out to the fader which is almost a dead short (14 ohms) at top of travel, then fader makeup booster adding +12 yields - 2 at the direct out (I suppose unity fader is implied here)....so wouldn't -2dBu @ unity + 12dBu of gain yield +10dBu at top of travel.....with maybe a couple dB lost from the 14 ohms on the fader? Without doing the math, that doesn't seem right.

Bah, I think my mind is mush from all these vac, vpp, dBu and dBv #'s.
 

Brian Roth

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Bah.....LOL.  I like that comment!

The reason I chose "insert return to direct out" was to travel through the shortest path I could find through the input module.  Indeed, according to the block diagram and the schemo, you SHOULD see a 12 dB boost through that path with the fader wide open.

Spoooo.....I'll sit back, take a sip of beer and ponder......

I'll ask about what generator(s) and meters you are using to eliminate some sort of instrumentation errors.

Many function generators have an unbalanced output....."low" side is grounded which MIGHT be an issue if the generator is powered from AC mains with a typical 3 pin AC power connector.

Are you feeding the generator into the insert return using tip and ring on the connector (and floating sleeve)?  In theory that should be the correct hookup.

The "bta" in the block diagram (and schematic) should be unity gain.  Thus, I would expect 12 dB gain with a signal fed into insert return, fader wide open, and measured at the direct out if the module does NOT have the optional transformer for the direct out.

I'm certainly puzzled by your results.  It's tricky trying to do this by "remote control"  <G!>

I also ASSume that someone hasn't done any mods to that signal path.

Bri  .... who is scratching his head
 

jdurango

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Brian Roth said:
Bah.....LOL.  I like that comment!

The reason I chose "insert return to direct out" was to travel through the shortest path I could find through the input module.  Indeed, according to the block diagram and the schemo, you SHOULD see a 12 dB boost through that path with the fader wide open.

Spoooo.....I'll sit back, take a sip of beer and ponder......

I'll ask about what generator(s) and meters you are using to eliminate some sort of instrumentation errors.

Many function generators have an unbalanced output....."low" side is grounded which MIGHT be an issue if the generator is powered from AC mains with a typical 3 pin AC power connector.

Are you feeding the generator into the insert return using tip and ring on the connector (and floating sleeve)?  In theory that should be the correct hookup.

The "bta" in the block diagram (and schematic) should be unity gain.  Thus, I would expect 12 dB gain with a signal fed into insert return, fader wide open, and measured at the direct out if the module does NOT have the optional transformer for the direct out.

I'm certainly puzzled by your results.  It's tricky trying to do this by "remote control"  <G!>

I also ASSume that someone hasn't done any mods to that signal path.

Bri  .... who is scratching his head

Beer to the rescue! Mystery solved!! I'm using a calibrated HP function gen that I trust, but the BNC to TT cable was wired ground to SLEEVE not ring. Fixed that + used some better probes and got rid of an adapter I was using on the output side to my Agilent meter (which I also trust) and BLAMMO! With internal operating level set to -2dBu, a 0dBu signal @ the insert return yields exactly +12dBu on the output with fader cranked!

Aw man! I'm super pumped!! Thank you so much!! Do you accept tips via Venmo or Google Wallet or anything? I really, greatly appreciate the help! Least I can do is buy ya some good beer! Thanks Brian!

Jonny
 

Brian Roth

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LOL...a contributor on this forum...John Roberts....makes comments from time to time..."I need to soak this in beer".  <g>

My main effort was to work out the levels through the input module before following that signal through the rest of the desk...ie multitrack program outs/2 mix/aux buses/monitor outs/etc.  I've spent 45+ years dealing with analog desks.  Never worked with a SW series 40, but the signal flow looked VERY familiar compared to others from that time frame.

Also glad to know you have some very decent test gear instead of a Rat Shack $5.00 meter and "whistling into a mic" for a signal source <g>. 

I now need to study the SW docs for the rest of the desk to see how they dealt with internal levels and adjustments. 

I consider myself semi-retired but still take on a few projects. Happy to be of assistance!

Bri


 
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