DELTA console - balance trim method

Archived from groups: rec.audio.pro (More info?)

In article <115855eslb9u617@corp.supernews.com> franks.pacifier.com@pacifier.net writes:

> Each of the stereo pgm outs of the master module of my Soundcraft Delta
> 200 have a 1k trim pot across the active balanced output stages (in series
> with a 1k resistor on each leg). The wiper is tied to ground.

LIke this?:



|\
-------| ------=====---------+------------->
|/ 1K |
I
I
I<---+
I |gnd
|\ |
-------| ------=====---------+------------->
|/ 1K


> I am rechipping and recapping and assume these trims should be tweaked
> once the new parts are in.

Not necessarily. I suspect that it's to even out the difference
between the 1K resistors. Unless you're replacing them (or any other
gain-determining components) you probably don't need to tweak this.

> I'm thinking I could Y
> the + and - legs of the output together, feed a signal through (a sweep,
> perhaps?), and tweak the pot for maximum cancellation (scope or perhaps
> even by ear at high gain).

That would probably work. I don't guess the output would mind being
shorted since it's protected by those 1K resistors. The real deal for
a balanced output is that both sides have the same impedance to
ground. A better test would be to actually measure the impedance of
each leg by measuring the voltage drop when you connect a known load
resistor to each side of the output. That way even if the voltages
aren't equal on both sides (which isn't a requirement for a balanced
output) you can trim the source impedance.

But you can probably just leave it where it is.

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Archived from groups: rec.audio.pro (More info?)

mrivers@d-and-d.com (Mike Rivers) writes:

>In article <115855eslb9u617@corp.supernews.com> franks.pacifier.com@pacifier.net writes:

>> Each of the stereo pgm outs of the master module of my Soundcraft Delta
>> 200 have a 1k trim pot across the active balanced output stages (in series
>> with a 1k resistor on each leg). The wiper is tied to ground.

>LIke this?:

Not exaclty, the "leg" 1Ks are right off the pot, not in series with the
output.

I've modified the drawing:


> |\ IC1A
>-------| --------------------+------------->
> |/ |
> /\/ 1K
> I
> 1K I<---+ gnd
> I
/\/ 1K
> |\ |
>-------| --------------------+------------->
> |/ IC1B


>> I am rechipping and recapping and assume these trims should be tweaked
>> once the new parts are in.

>Not necessarily. I suspect that it's to even out the difference
>between the 1K resistors. Unless you're replacing them (or any other
>gain-determining components) you probably don't need to tweak this.

Does the move of the 1Ks to the trimmer "legs" change that supposition?

My thought was that if the driver op amps weren't pretty close this pot
would need to be tweaked when changing those amps. Then again, given that
the two amps are in the same package, I'd *hope* they'd be rather
well-matched. <g> My first go-around is using BB OPA2134s


>> I'm thinking I could Y
>> the + and - legs of the output together, feed a signal through (a sweep,
>> perhaps?), and tweak the pot for maximum cancellation (scope or perhaps
>> even by ear at high gain).

>That would probably work. I don't guess the output would mind being
>shorted since it's protected by those 1K resistors. The real deal for

Good point; as you can see above, the outputs would be shorted together
in the "Y" approach, though I have some 910 ohm 0.1% Holcos I could use to
protect the outputs and be assured of a pretty close match in the test
jig.

>a balanced output is that both sides have the same impedance to
>ground. A better test would be to actually measure the impedance of
>each leg by measuring the voltage drop when you connect a known load
>resistor to each side of the output. That way even if the voltages
>aren't equal on both sides (which isn't a requirement for a balanced
>output) you can trim the source impedance.

Ah. Interesting point.

>But you can probably just leave it where it is.

That would be nice, as I have to have the L/R master boards dismounted
from the module frame and laying out of the console to get to the timmers,
which means I need a bench connector for this beast, which I haven't had
time to build yet. One more thing on that (long) do list...

Thanks, Mike, for your advice.

Frank Stearns
Mobile Audio

--
.

Archived from groups: rec.audio.pro (More info?)

That trim is to balance the outputs so they deliver equal outputs. Not
the most elegant way to do this, (an adjustment in the feedback loop
would avoid the signal attenuation from this approach). Unless you
replace the sip resistor arrays, you don't need to tweak these. If you
do replace the arrays with discrete resistors (not a bad idea) you will
need to adjust the trims. Take 2 20k ohm resistors. Match within .5%.
Connect one from each output pin 2 and 3. Then tie the resistors
together. Send in a 400 hz signal and adjust the pot for 0 signal at
the junction of the 20 k resistors. Pat yourself on the back.

Keep in mind the Delta has poor power supply decoupling, local .1 uf
mono ceramic caps should be added from each power pin (pins 8+4) to a
local ground point. Replace the 47 uf coupling caps to 220 uf for
tighter bass.

Remember, they don't work if you let the smoke out!

Jim Williams
Audio Upgrades

Archived from groups: rec.audio.pro (More info?)

Frank Stearns wrote:

> Each of the stereo pgm outs of the master module of my Soundcraft
> Delta 200 have a 1k trim pot across the active balanced output
stages
> (in series with a 1k resistor on each leg). The wiper is tied to
> ground.

> I am rechipping and recapping and assume these trims should be
tweaked
> once the new parts are in. One channel at a time I'm thinking I
could
> Y the + and - legs of the output together, feed a signal through (a
> sweep, perhaps?), and tweak the pot for maximum cancellation (scope
> or perhaps even by ear at high gain).

> Is this an acceptable method, or is there a better procedure?

This strikes me as a pretty strange setup. I guess some design
engineer thought that balanced lines work better if the two halves
output the identical same magnitude voltage. This isn't true. Balanced
lines work better if the source impedance for the two outputs are
identically the same. Ironically, this scheme could be a step away
from impedance balancing.

The final answer would depend on the details of circuitry that drives
this lash-up.

Archived from groups: rec.audio.pro (More info?)

Arny Krueger <arnyk@hotpop.com> wrote:
>
>This strikes me as a pretty strange setup. I guess some design
>engineer thought that balanced lines work better if the two halves
>output the identical same magnitude voltage. This isn't true. Balanced
>lines work better if the source impedance for the two outputs are
>identically the same. Ironically, this scheme could be a step away
>from impedance balancing.

Are you guys even sure that this control adjusts the output balancing?
You sure it isn't a DC offset control? It might be worth checking
the manual for the board.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Archived from groups: rec.audio.pro (More info?)

Thanks, Jim!

Great advice.

Frank Stearns
Mobile Audio

jwilliams3@audioupgrades.com writes:

>That trim is to balance the outputs so they deliver equal outputs. Not
>the most elegant way to do this, (an adjustment in the feedback loop
>would avoid the signal attenuation from this approach). Unless you
>replace the sip resistor arrays, you don't need to tweak these. If you
>do replace the arrays with discrete resistors (not a bad idea) you will
>need to adjust the trims. Take 2 20k ohm resistors. Match within .5%.
>Connect one from each output pin 2 and 3. Then tie the resistors
>together. Send in a 400 hz signal and adjust the pot for 0 signal at
>the junction of the 20 k resistors. Pat yourself on the back.

>Keep in mind the Delta has poor power supply decoupling, local .1 uf
>mono ceramic caps should be added from each power pin (pins 8+4) to a
>local ground point. Replace the 47 uf coupling caps to 220 uf for
>tighter bass.

>Remember, they don't work if you let the smoke out!

>Jim Williams
>Audio Upgrades

--
.