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Old 11th March 2014, 02:07 AM   #1
smcoxon is offline smcoxon  United Kingdom
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Default Microphone Preamp balanced voltage levels

Hi, I'm looking at building a microphone preamp with balanced and unbalanced line outputs.

My question is: if I build the line output stage circuit to provide +4 dBu (1.736 V peak) signal and pass this through a unity gain balanced output opamp stage, the balanced output Hot and Cold lines will each be +-1.736 Vpeak and the receiving equipment balanced input differential voltage will be twice +4 dBu. Is this OK or should I build my output stage circuit to provide only 0.868 Vpeak to the balanced output opamp stage so that the voltage differential delivered to the receiving equipment will be +4 dBu (1.736V peak)?

Thanks in advance for any advice you can give.
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Old 11th March 2014, 04:10 AM   #2
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+4dBu is the nominal 0VU output level (across both outputs) and you'll want to allow for at least 20dB of headroom above this.

All good fortune,
Chris
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Old 11th March 2014, 10:48 AM   #3
smcoxon is offline smcoxon  United Kingdom
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Hi Chris, thanks for your quick reply.

I'm still unsure what I should be doing. Would you mind clarifying a bit more please? When you say +4dBu is 0VU output level (across both outputs) does this mean for a balanced output the peak voltage on hot and cold should be 1.736v or 0.868v?

And then you say I need to allow another 20db of headroom. So by that do you mean I need to allow the output to be varied up to 10 time the +4dBu voltage level?

I know I'm probably not seeing the obvious here. Thanks for your help.

Sean.
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Old 12th March 2014, 01:57 AM   #4
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The +4dBu "standard" for prosound connections is for a VU meter reading of 0VU. These are averaging meters, with ballistics chosen to best match apparent volumes of different sources to each other, for use in a radio station, for example. The connections have been carried over into modern pro audio use because they're robust, (and because they were already there!).

This is very different from a modern digitally sourced "standard" like 2 volts output from a CD player. Digitally derived levels are already peak outputs, and no headroom need apply.

Averaged, damped numbers need to be allowed some headroom for signal peaks. It's common for professional use to allow 20dB, or a little more if possible, headroom for recording, or often a good bit less for sound reinforcement. Good homebrew design means a +24dBu output capability. This can be split between both polarities (+18dBu and +18dBu) if you know that both will be used by the receiving end, and your metering is across both polarities (or scaled as if it is).

All good fortune,
Chris

Last edited by Chris Hornbeck; 12th March 2014 at 02:18 AM.
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Old 12th March 2014, 09:58 AM   #5
EssB is offline EssB  United Kingdom
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Quote:
Originally Posted by smcoxon View Post
I'm still unsure what I should be doing. Would you mind clarifying a bit more please? When you say +4dBu is 0VU output level (across both outputs) does this mean for a balanced output the peak voltage on hot and cold should be 1.736v or 0.868v?
Assuming both outputs are driven by opamps at the same level on each line, then for +4dBu balanced line level, the peak voltage w.r.t ground on each line will be 0.868Vpk.
Normally we don't talk about ground referenced levels on balanced lines, because they could be floating or driven single ended. Talking about peak voltage can cause confusion, just use Vrms or dBu.
Quote:
And then you say I need to allow another 20db of headroom. So by that do you mean I need to allow the output to be varied up to 10 time the +4dBu voltage level?
You don't need variable gain to crank the normal operating level up another 20dB, if thats what you mean. Headroom is max output level above operating level before clipping. If you use the +4dBu balanced and -2dBu internal unbalanced levels you'll have enough headroom.
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Old 12th March 2014, 05:45 PM   #6
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It may be useful to think about VU based "standards" by analogy to another averaging measurement, Sound Pressure Level. SPL is most commonly measured with an averaging meter, and we all know that peaks will be somewhere above the metered average.

Same-same for VU based measurements, and exactly as much headroom will be needed for any particular sound source. For example, a drummer hitting a snare hard but slowly will need very large headroom over measured SPL, an organ drone tone will need comparatively little.

All good fortune,
Chris
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Old 12th March 2014, 05:55 PM   #7
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What about for mic preamps with variable gain and/or trim?

Also, WRT to the OP's preamp, is the +4dBu output referenced to a particular standardized mic input level? (different mic's have different outputs)
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Old 12th March 2014, 06:07 PM   #8
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*All* general-purpose mic preamps must have variable gain, or trim, to optimise their own internal dynamic range. A more general name for this is "gain staging". For a homemade mic preamp, and for use in a particular setting, with a particular microphone, that rule could be relaxed.

But this is a separate issue from output level and how it's specified.

All good fortune,
Chris
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Old 12th March 2014, 06:25 PM   #9
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OK, but what I mean is that the gain adjustment range has to be such that +4dBu can be achieved at the output for all anticipated mic types, is that correct?
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Old 12th March 2014, 06:31 PM   #10
EssB is offline EssB  United Kingdom
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Yes, required headroom is variable with source. Trouble is VU and other metering standards (only PPM?) are well defined in terms of response time (ballistics) and we're stuck with them. I think best solution is LED clipping indicators. I took it for granted that a gain control will be used.

Hang on a minute; are VU meter ballistics well defined ?
ISTR reading specs for it years ago. I do know PPM is well defined.

I think we've got a bit OT - the OP was only asking about the line output stage of his mic pre.
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