JAY X

Testing input and output levels
« on: October 29, 2016, 05:10:43 AM »
Hi!

As i am testing my summing mixer, i have to test the maximum input and output levels it can achieve.

¿is it ok to test this levels with a  0dbu test signal level?

¡Thank you!

jay x


ruffrecords

Re: Testing input and output levels
« Reply #1 on: October 29, 2016, 08:59:38 AM »
That depends on how much gain it has and what maximum input and output levels you expect it to be capable of. If you ost a schematic that would make it easier to answer.

Cheers

ian

JohnRoberts

Re: Testing input and output levels
« Reply #2 on: October 29, 2016, 10:21:31 AM »
Hi!

As i am testing my summing mixer, i have to test the maximum input and output levels it can achieve.

¿is it ok to test this levels with a  0dbu test signal level?

¡Thank you!

jay x
0dBu is a nominal 0VU operating level. Good practice is to handle line level signals 20 dB larger (or more). It is not unusual for output levels +26 dBu with active differential (dual) outputs.

JR
« Last Edit: October 29, 2016, 11:59:27 PM by JohnRoberts »
John Roberts
http://circularscience.com
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Re: Testing input and output levels
« Reply #3 on: October 29, 2016, 05:39:15 PM »
0dBu is a nominal 0VU operating level.
0 dBu is a nominal -4 VU operating level.

JohnRoberts

Re: Testing input and output levels
« Reply #4 on: October 29, 2016, 11:58:31 PM »
0 dBu is a nominal -4 VU operating level.

Yes

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

Re: Testing input and output levels
« Reply #5 on: November 01, 2016, 05:55:33 AM »
As i am testing my summing mixer, i have to test the maximum input and output levels it can achieve.

¿is it ok to test this levels with a  0dbu test signal level?

If you are trying to determine the maximum input level it can handle, don't just declare "0dBu" - actually try a range of levels and see where your summing device overloads. Using "0dBu" is pre-supposing the answer you seek - it might be 12.3dBu, 17.8dBu or any of a number of possible values, depending upon your summing device. Also note that this input overload point might be hard to measure if your summing device is a pile of resistors - in that case, do you mark the maximum input as the voltage where smoke starts to pour out of the summing resistors? ;-)

Joking aside, you will want to apply a signal to your summing device, and then increase that until your summing device distorts. That's the maximum input level. Then, measure the output level right at the edge of distortion. That's the maximum output level. Isn't that what you'd want to know, given a maximum input and output spec?  Why do you need this spec in the first place? If you think about what the spec means, it'll be easy to think of how to obtain it. :-)

JAY X

Re: Testing input and output levels
« Reply #6 on: November 02, 2016, 04:50:41 AM »
Hi!

Ok, the fact is that i try to measure the maximum input level with the mixer in the  true RTA anlyzer loop, and this does not work.
Because i'm limited to the maximum input level that my audio interface (behringer UMC 1820) can cope. Red lights lit with:
1469mv input RMS / 8.58dbu peak / 5.58dbu RMS. So, I have to measure the output voltage and distortion right at the output of the first stage.

¿Is there a formula to calculate the input headroom of an amplifier?

jay x


Andy Peters

Re: Testing input and output levels
« Reply #7 on: November 02, 2016, 02:30:22 PM »
Is there a formula to calculate the input headroom of an amplifier?

Headroom is simply the difference between the maximum signal level before clipping and whatever level you feed into the thing.

if the thing clips with a 1 V input and you feed it a 100 mV input, you have 900 mV headroom, or 20 dB.  If you feed it a 500 mV input, you have 500 mV headroom or 6 dB.

I suspect that you're asking the wrong question.
"On the Internet, nobody can hear you mix a band"

JohnRoberts

Re: Testing input and output levels
« Reply #8 on: November 02, 2016, 03:01:27 PM »
Hi!

Ok, the fact is that i try to measure the maximum input level with the mixer in the  true RTA anlyzer loop, and this does not work.
Because i'm limited to the maximum input level that my audio interface (behringer UMC 1820) can cope. Red lights lit with:
1469mv input RMS / 8.58dbu peak / 5.58dbu RMS. So, I have to measure the output voltage and distortion right at the output of the first stage.

¿Is there a formula to calculate the input headroom of an amplifier?

jay x
I will ASSume you are not asking a trick question.

Headroom in dB is a simple as subtracting the nominal level 0VU from the max signal level.  So a +26dBu max output would deliver 22dB of headroom above a +4dBu nominal 0VU.

"Input" headroom could involve a slightly different metric namely what is the hottest signal the input could accept cleanly with the input gain trimmed all the way down.  I suspect this is not what you are asking...

JR

PS: gain staging issues, like active outputs with voltage gain may confuse actual clean headroom available.  I used to run consoles -6dB internally so the internal paths would clip at the same level as the outputs with their + 6dB voltage gain. 
John Roberts
http://circularscience.com
Tune it, or don't play it...

abbey road d enfer

Re: Testing input and output levels
« Reply #9 on: November 02, 2016, 04:01:13 PM »
¿Is there a formula to calculate the input headroom of an amplifier?


You started with a simple question (maximum level), and made it complicated  ;D
As others have mentioned, headroom implies two values, one measurable and the other arbitrary.
A typical input stage has its max input level governed by the rail voltage and its gain. An opamp with +/-15V rails can deliver +20dBu; at unity gain, the max input level is thus +20dBu. Make it 10dB gain, and the max level drops by 10 dB, make it 6dB attenuation, max input goes to +26. That's the easy and measurable part.
Headroom involves the definition of a nominal operating level.
If the nominal is +4dBu (0VU), the headroom is 16dB for unity gain, but if the operating level is the "Tascam" level (-10dBv=>-7.8dBu), the headroom becomes 27.8dB.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
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JohnRoberts

Re: Testing input and output levels
« Reply #10 on: November 02, 2016, 04:12:22 PM »
You started with a simple question (maximum level), and made it complicated  ;D
As others have mentioned, headroom implies two values, one measurable and the other arbitrary.
A typical input stage has its max input level governed by the rail voltage and its gain. An opamp with +/-15V rails can deliver +20dBu; at unity gain, the max input level is thus +20dBu. Make it 10dB gain, and the max level drops by 10 dB, make it 6dB attenuation, max input goes to +26. That's the easy and measurable part.
Headroom involves the definition of a nominal operating level.
If the nominal is +4dBu (0VU), the headroom is 16dB for unity gain, but if the operating level is the "Tascam" level (-10dBv=>-7.8dBu), the headroom becomes 27.8dB.

A really really really minor point but  -10dBV  is generally designated with a capital V suggesting a dB reference relative to 1V (so -10dBV is around 0.31V.)  dBv with a small v is sometimes confused with dBu a voltage ratio relative to 0.775V so not widely used.

I think I mentioned this recently but back in the '80s I wrote a column comparing +4dBu vs -10dBV systems, and the -10dBV did surprisingly well for most metrics (including being cheaper).  But real men use +4dBm  ;D ;D (kidding)

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

JAY X

Re: Testing input and output levels
« Reply #11 on: November 03, 2016, 05:11:12 AM »
Hi All!

Ok, thank you very much for all the answers. Now i understand it better.  Yes, sometimes i complicate myself too much  ::) when indeed things are a bit more simple.!!

Jay x