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Finding unity gain

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February 25, 2005 3:12:26 PM

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

I'm setting up a mixer which has no indication for unity gain on
individual channel gainpots. I'm measuring the output of the main buss
and adjusting the channel gain knob so that I have the same level at
the main buss outs as I'm putting into the channel input. The faders
(both channel and master) have a unity (0) mark part way down their
scale. I assume that at this 0 mark the fader is still attennuating the
signal. Should I have the faders all the way up when tring to find
unity gain for the input or should I have them at their unity (0)
setting?

More about : finding unity gain

Anonymous
February 25, 2005 3:26:39 PM

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

apa wrote:

> I'm setting up a mixer which has no indication for unity gain on
> individual channel gainpots. I'm measuring the output of the main buss
> and adjusting the channel gain knob so that I have the same level at
> the main buss outs as I'm putting into the channel input. The faders
> (both channel and master) have a unity (0) mark part way down their
> scale. I assume that at this 0 mark the fader is still attennuating the
> signal. Should I have the faders all the way up when tring to find
> unity gain for the input or should I have them at their unity (0)
> setting?
>

The unity setting is supposed to represent unity gain (0 dB). The fact
that the fader is not at max is offset by the 10 dB or so gain of its
associated amplifier.
Anonymous
February 25, 2005 9:30:20 PM

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

In article <1109362346.143582.323510@g14g2000cwa.googlegroups.com> tacoma57@hotmail.com writes:

> I'm setting up a mixer which has no indication for unity gain on
> individual channel gainpots.

> The faders
> (both channel and master) have a unity (0) mark part way down their
> scale. I assume that at this 0 mark the fader is still attennuating the
> signal.

Nope, that "unity" mark is the design center for the fader position.
It may not be unity gain for the channel (depending on the gain
structure of the mixer) but that's the place where you should put them
when you

> I'm measuring the output of the main buss
> and adjusting the channel gain knob so that I have the same level at
> the main buss outs as I'm putting into the channel input.

Most mixers are designed so that there is some gain available when you
push the fader up from its design center position. But some (mostly
older designs) are attenuate-only. It's more intuitive to be able to
push a fader up than to only be able to pull it down so that's why
they build (and mark) them this way.



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However, until the spam goes away or Hell freezes over,
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February 26, 2005 5:26:36 AM

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

Unity gain for any single gain stage in a mix system happens when that
stage's oupt level matches the level of the input signal

SYSTEM Unity gain happens when you have all gain stages set so that they all
reach their clipping point simultaneously, then back off the FIRST STAGE
GAIN till the system output matches the level of the input signal at the
input jack.


On 2/25/05 3:12 PM, in article
1109362346.143582.323510@g14g2000cwa.googlegroups.com, "apa"
<tacoma57@hotmail.com> wrote:

> I'm setting up a mixer which has no indication for unity gain on
> individual channel gainpots. I'm measuring the output of the main buss
> and adjusting the channel gain knob so that I have the same level at
> the main buss outs as I'm putting into the channel input. The faders
> (both channel and master) have a unity (0) mark part way down their
> scale. I assume that at this 0 mark the fader is still attennuating the
> signal. Should I have the faders all the way up when tring to find
> unity gain for the input or should I have them at their unity (0)
> setting?
>
Anonymous
February 26, 2005 11:59:39 PM

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

apa wrote:
> I'm setting up a mixer which has no indication for unity gain on
> individual channel gainpots. I'm measuring the output of the main
buss
> and adjusting the channel gain knob so that I have the same level at
> the main buss outs as I'm putting into the channel input. The faders
> (both channel and master) have a unity (0) mark part way down their
> scale. I assume that at this 0 mark the fader is still attennuating
the
> signal. Should I have the faders all the way up when tring to find
> unity gain for the input or should I have them at their unity (0)
> setting?

The "0" mark is where unity gain should be on the faders. If
that's in the middle of the fader throw, bet it's a Mackie or a Samson,
eh? <g>

Having a tone generator that puts out a stable 0VU level is very
helpful for setting up unity gain on a mixer. You want to measure a
channel's level prefader (PFL) and trim the input gain till it shows
0VU, then measure the channel's level postfader (AFL) and make sure
that also measures 0VU. Cheap mixers don't all have facility for
metering those locations, but that is part of what makes the more
expensive mixers cost more.

Then you can send the Channel tone to your Mix Buss and make sure
it's balanced left right and at least in the ballpark; better mixers
also have output tweak screws on the channel faders you can get at
through little holes in the fader with a "greenie" to do this. You can
also send tone to the Groups and check levels there, but unless the
mixer is freshly commisioned most of the time each individual channel
feeds the various group busses at slighty different levels, if there
are twaeks for that they are inside the console and you need to put the
module on a jig to get at them.

You might be able to make sure the Groups and Stereo Buss faders
are at 0VU before tweaking the channels feeding them by injecting tone
in the Mix Buss insert return.

Will Miho
NY Music and TV Audio Guy
Staff Audio / Fox News Channel / M-AES
"The large print giveth and the small print taketh away..." Tom Waits
February 28, 2005 8:38:22 AM

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

WIll Miho sent this via email and asked me to post it to the group for
future reference. The text below is his. Thanks for all the replies.
-Andy

First your level coming into a mixer channel should be checked
_prefader_ (PFL), and trimmed so it reads 0VU on input. Make sure all
your eq's/filters are off or flat when you do that. Then you check
the
AFL (after fader level) of the channel to make sure you have Unity gain
at the fader 0VU mark. So you should have Unity gain on the
individual
channel then, or at least close enough to tweak (better console faders
everywhere have output level screws you can tweak with a greenie. But
faders with 0 marks in the middle of the throw don't - sounds like you
are using a Mackie or a Samson, eh? Hopefully you have a few metering
options on your console and a tone source with known levels though.)

Anyway then you can assign the channel, now set at Unity gain to
your stereo mains, check levels *there* PFL, and then put the fader up
and check it AFL to make sure your main output fader at the 0 mark is
also at Unity gain. Similar procedure for the Group busses, except I
usually feed tone directly into group inputs if I can, rather than deal
with the minute fader level differences feeding from a mixer channel.
And actually it is not unusual for mixer channels to feed somewhat
different levels to the various available output buss paths, unless the
board has a lot of internal tweaks and has just been commissioned.

Given that I work in TV every day, I am probably kinda overly
obsessive about unity gain and audio levels, 1/10th of a db imbalance
between left and right levels will really bug me even though we
cablecast in mono. But you dance with the one that brought ya...

Will Miho
NY Music & TV Audio Guy
Staff A1 / Fox News Channel / M-AES
"The large print giveth and the small print taketh away..." Tom Waits

apa wrote:
> I'm setting up a mixer which has no indication for unity gain on
> individual channel gainpots. I'm measuring the output of the main
buss
> and adjusting the channel gain knob so that I have the same level at
> the main buss outs as I'm putting into the channel input. The faders
> (both channel and master) have a unity (0) mark part way down their
> scale. I assume that at this 0 mark the fader is still attennuating
the
> signal. Should I have the faders all the way up when tring to find
> unity gain for the input or should I have them at their unity (0)
> setting?
Anonymous
February 28, 2005 12:29:20 PM

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

"apa" <tacoma57@hotmail.com> wrote in message
news:1109362346.143582.323510@g14g2000cwa.googlegroups.com

> I'm setting up a mixer which has no indication for unity gain on
> individual channel gainpots. I'm measuring the output of the main
> buss and adjusting the channel gain knob so that I have the same
> level at the main buss outs as I'm putting into the channel input.
> The faders (both channel and master) have a unity (0) mark part way
> down their scale. I assume that at this 0 mark the fader is still
> attennuating the signal. Should I have the faders all the way up when
> tring to find unity gain for the input or should I have them at their
> unity (0) setting?

To find unity gain you need some kind of measuring or comparison device. It
need not be very accurate, but it needs to be fairly consistent. A cheap DVM
should suffice.

A good source would be playback of an audio CD recorded with a 400 Hz tone.
You should be able to make such a thing with a computer that has a burner
and even trivial audio editing software.

First measure the voltage output of the CD player, and then measure the
output of the console. Adjust the console until they are the same.
February 28, 2005 4:11:26 PM

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

The "two or three places to screw up the gain staging *internally*" are
what I was really concerned about. I have an oscillator and a meter. I
just wasn't sure how to account for the gain or lack there of
associated with the fader positions. Will's PFL method makes good sense
to me. The other option I thought of was to send the oscillator signal
into the insert (bypassing the input stage) and read the level on the
main buss VU meters, then switch the oscillator over to the line in and
adjust the gain on the channel so that I get the same reading on the
main buss meters.


Lorin David Schultz wrote:
> "Arny Krueger" <arnyk@hotpop.com> wrote:
> >
> > First measure the voltage output of the CD player, and then measure
> > the output of the console. Adjust the console until they are the
> > same.
>
>
>
> Remembering, of course, that on even a stupid simple little mixer
there
> are two or three places to screw up the gain staging *internally*.
> Ideally you'd measure just after the preamp, after the channel fader,

> and at the main outputs.
>
> --
> "It CAN'T be too loud... some of the red lights aren't even on yet!"
> - Lorin David Schultz
> in the control room
> making even bad news sound good
>
> (Remove spamblock to reply)
February 28, 2005 5:10:18 PM

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

It seems to be right about 12.

If you'll indulge one more conjecture (more for my own understanding
than anything), could I:

First, measure the voltage difference between the level of the signal
being input into the channel and the signal appearing at the insert
output.

Second, convert that difference to dB.

Third, find unity for the channel gain as above (comparing the insert
to the line in), but using the dB number to account for whatever
operating level there might be between the two.
February 28, 2005 6:56:18 PM

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

Right, sorry, I meant the ratio.

xdB = 20*LOG( insert ouput voltage /oscillator output voltage )

Bad wording on my part.

Otherwise an ok method though?

Thanks
Anonymous
February 28, 2005 7:37:09 PM

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

"apa" <tacoma57@hotmail.com> wrote in message
news:1109625086.509030.26870@g14g2000cwa.googlegroups.com

> The "two or three places to screw up the gain staging *internally*"
> are what I was really concerned about. I have an oscillator and a
> meter. I just wasn't sure how to account for the gain or lack there of
> associated with the fader positions.

Obviously, a given set of adjustments to a mixer can only be unity gain at
one fader setting.

>Will's PFL method makes good sense to me.

It does, but frankly it is over-specfic and requires better equipment than
the minimum. Unity gain means that the level of what goes in is the same as
what comes out. 0 Vu is a nice level to pick, but unity gain is unity gain
for all reasonable levels.

> The other option I thought of was to send the oscillator
> signal into the insert (bypassing the input stage) and read the level
> on the main buss VU meters, then switch the oscillator over to the
> line in and adjust the gain on the channel so that I get the same
> reading on the main buss meters.

That can work, too providing that there is standard gain staging through the
inserts. I understand that some consoles have inserts designed to run at +4,
and other have inserts that are designed to run at -10, although I can't
name any of the latter. There is a possibility that the gain from line in
to the inserts isn't supposed to be unity, though again I know of no
counter-examples.

Often level controls have logical and even sometimes marked points that
correspond to unity gain. Some place between 12 o'clock and 3 o'clock is
common.

> Lorin David Schultz wrote:
>> "Arny Krueger" <arnyk@hotpop.com> wrote:
>>>
>>> First measure the voltage output of the CD player, and then measure
>>> the output of the console. Adjust the console until they are the
>>> same.

>> Remembering, of course, that on even a stupid simple little mixer
>> there are two or three places to screw up the gain staging
>> *internally*. Ideally you'd measure just after the preamp, after the
>> channel fader,
>
>> and at the main outputs.
>>
>> --
>> "It CAN'T be too loud... some of the red lights aren't even on yet!"
>> - Lorin David Schultz
>> in the control room
>> making even bad news sound good
>>
>> (Remove spamblock to reply)
Anonymous
February 28, 2005 8:06:01 PM

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

apa wrote:
> The "two or three places to screw up the gain staging *internally*"
are
> what I was really concerned about. I have an oscillator and a meter.
I
> just wasn't sure how to account for the gain or lack there of
> associated with the fader positions. Will's PFL method makes good
sense
> to me. The other option I thought of was to send the oscillator
signal
> into the insert (bypassing the input stage) and read the level on the
> main buss VU meters, then switch the oscillator over to the line in
and
> adjust the gain on the channel so that I get the same reading on the
> main buss meters.

While some mixers have clear markings or a detent for where a 0
Line level is on the channel inputs, many do not especially when mic
and line level inputs are changed with a switch but they use the same
pot for gain trim. And if you bypass the input gain pot totally and go
straight in the channel insert, how does that give you unity gain with
say, a tape track actually feeding a 0VU reference level in that line
input? Your mixer and your input devices need to have reference
levels that match, so you start with tone for so you confirm Unity gain
within the mixer, and then you can adjust your trims so the levels from
external devices matches.

Look, with your mixer channel's fader at the 0 mark, if the pfl and
afl levels do not match you do not have Unity gain in and out of the
channel, right? And usually the throw of a console fader is has more
resolution near the 0 mark than lower at say -30, so 0VU is a pretty
good place to check for Unity gain (and they call it a "reference
level" for a reason.)

Will Miho
NY Music & TV Audio Guy
Staff Audio / Fox News Channel / M-AES
"The large print giveth and the small print taketh away..." Tom Waits
Anonymous
February 28, 2005 8:51:43 PM

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

"apa" <tacoma57@hotmail.com> wrote in message
news:1109628618.216402.278670@f14g2000cwb.googlegroups.com
> It seems to be right about 12.
>
> If you'll indulge one more conjecture (more for my own understanding
> than anything), could I:
>
> First, measure the voltage difference between the level of the signal
> being input into the channel and the signal appearing at the insert
> output.
>
> Second, convert that difference to dB.

Point of order, you can't convert differences into dB. dBs relate to ratios.

> Third, find unity for the channel gain as above (comparing the insert
> to the line in), but using the dB number to account for whatever
> operating level there might be between the two.

Stated more simply, that would be how you determine that there is a certain
number of dB gain.
Anonymous
February 28, 2005 10:44:48 PM

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

"Arny Krueger" <arnyk@hotpop.com> wrote:
>
> First measure the voltage output of the CD player, and then measure
> the output of the console. Adjust the console until they are the
> same.



Remembering, of course, that on even a stupid simple little mixer there
are two or three places to screw up the gain staging *internally*.
Ideally you'd measure just after the preamp, after the channel fader,
and at the main outputs.

--
"It CAN'T be too loud... some of the red lights aren't even on yet!"
- Lorin David Schultz
in the control room
making even bad news sound good

(Remove spamblock to reply)
Anonymous
March 1, 2005 3:29:07 AM

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

"Arny Krueger" <arnyk@hotpop.com> wrote:
>
> [...] I understand that some consoles have inserts designed to run
> at +4, and other have inserts that are designed to run at -10



Actually, I think you'll find that most mixers are set up either 0dBu
or -2dBu at the insert.

--
"It CAN'T be too loud... some of the red lights aren't even on yet!"
- Lorin David Schultz
in the control room
making even bad news sound good

(Remove spamblock to reply)
!