Tech Question regarding headroom on mic pre's.

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Mr Funk

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Hi there

I know that specs can be very missleading as they can be dipicted in a number of ways, some which look more impressive than others.

I remember reading that mic pre headroom is very important in some circumstances (recording drums/brass etc) and that a figure of +22db is about average, with some pre's going up into the +30's.

I have been investigating Line Audio pre's and this is the reply I got regarding headroom:

'If 0dB is considered 775mV (0dbU) then the headroom is +8dB (8dBU) which is very high for a signal coming from a microphne'.

Does this seem low, or is it just the way the specs are presented?

Thanks
 
The first numbers you quoted are for the output stage of the preamps, +22 is average, >30 exceptional

The 8dbu figure is for the signal going into the preamp, and is indeed not too bad.
 
Thanks Dan

Below is more of the email from Roger Jönsson of Line Audio, just in case you want to comment or are intersted in their pretty low priced gear?


'The main goal is to make it as transparent sounding as possible.
Personally I do not understand the idea of adding non-reversable colouration to the sound being recorded'.

'If 0dB is considered 775mV (0dbU) then the headroom is 8dB (8dBU) which is very high for a signal coming from a microphne'.

'We chose this to give the preamps a good margin, whilst keeping the noise low even when the -20dB PAD is activated. +8dB is a good margin for most any mic'.

'We chose to not have the gain potentiometer in the first amplifier stage. The first stage works with a fixed gain. This way we reduced risk of that potentiometer making noise and distortion as it ages. So there are two gain settings, resulting in a max level of 180mV and 1.8V RMS with the PAD activated. Of course in most situations noise in no problem at all at high levels, but max level is what we went for'.

'If you state it in your order you can have the -20dB PAD changed into -25dB or -30dB, then you will have a headroom of up to 18dBU. The signal to noise of the normal non-PAD setting is not affected at all and this is the setting which is used for noise critical events'.

Here are the specs:

Power requirement 15-18VAC DMP:7VA / OMP:15VA
External 230VAC transformer included
(can be excluded on special order)

Frequency resp. 15Hz - 100 KHz ± 0.5dB
Gain 60dB (Gain at max setting)
Mic inputs Electronically balanced
Input impedance >10Kohm
Input level max 1.8VRMS (180mV RMS with -20 disabled/out)
Outputs Electronically balanced
Output impedance 600 ohm (minimum recommended load 600ohm)
Output level max 15V RMS (electronically balanced)
Output DC offset max 1mV at 40dB gain, max 10mV at 60dBgain
Harmonic distortion: <0.05%
Input noise: 128 dBm/dBu (100ohm termination 20KHz B/W)
Input noise: 133dBV (input shorted 20KHz B/W)
XLR Polarity Pin 1 ground, pin2 hot +, pin 3 cold -


Does the output level of 15V RMS equate to a db rating (ie. like the average 22db)?

These guys have been really helpful in answering my questions and seem to be quite flexible etc. I wonder how good and transparent the mic pre's are?
 
No. Those +22 and >30 figures are for "Max. Input Level" specs.

A +8dBu Max. Input Level, in my opinion, would not be all too good if considering a preamp to use with high output condensers close-miking drums or a few other occasions.

However, with that said, I suppse I should mention that +8dBu is traditionally not bad.

Please refer to the bottom of this thread for some preamps comparisons regarding this:
https://homerecording.com/bbs/showthread.php?threadid=84959
 
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R.E. Thanks for the info, I will run off to have a read of that post in a mo!

The DUAL pre amp in question is only £170 BTW!! Here is more info supplied by Roger at Line Audio:

Output is max 15V RMS which equals +26dBU (reference 775mV) or +22dBm (reference 1,25V).

In reality it can output 2 dB more if the load is not very tough, but I settle for the above figures to have some margin.

Really, Is there a need for more headroom!?
The following apparatus must be able to handle it too, without distorting, for it to be useful...

All the best, Roger Jönsson.

What do we think about the specs then???
 
I think we need to clear up whether we are talking about input or output tolerances?

It appears that the DMP has a mic input headroom of +8dB (over 0dB) and an output of +26dBU. It also appears that the other figure quoted in other threads are for the output also, as they are all around +22dBU.

So the headroom for the Line Audio DMP is somewhere between a Mackie VLZ Pro and the RNP! For £170, this seems pretty good? Anyone know what the input figures are on the RNP?

The figures given by Line Audio are also RMS, I'm not sure about Mackie or FMR's?

I am probably totally wrong in my assumptions, please put me right someone?
 
It's true... Your following gear must be able to handle what is coming out... I mean my HD24 recorder is +19dBu (6.91V) for 0 dBFS...

However, most people would say their preamps would crumble under a circumstance such as a Neumann TLM103 about 1 inch in and 3 inches above, point toward the center of a snare, directly into a preamp with NO pad.

And though rare, a singer can on occasion overload a Mackie 8 Buss preamps with its +14dBu Max. Input Level.

What do I think about the Line Audio preamp Max Input Level specs..? Well, its +7.32dBu (1.8V) isn't bad (just doesn't cover all occasions), but that figure is with a 20dB pad?! Ouch! It's -12.68dBu (180mV) with its 20dB pad disabled is pitiful!
 
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I am talking about Max. Input Levels. I'm pretty sure the RNP's +27.5dBu quote is Max. Input Level because on their site, right next to that figure, it mentions microphone output (which is what we're talking about here):
http://www.fmraudio.com/rnp/index.html

Yes, the voltage given by Line Audio are RMS. So are the voltage figures I gave when comparing those preamps in the other thread.
 
The specs say:

'Input level max 1.8VRMS (180mV RMS with -20 disabled/out)'

Disabled/out means withouth the pad engaged right?


'If you state it in your order you can have the -20dB PAD changed into -25dB or -30dB, then you will have a headroom of up to 18dBU. The signal to noise of the normal non-PAD setting is not affected at all and this is the setting which is used for noise critical events'.

Is this offer worth considering?
 
The maximum output from a Neumann TLM170 is 2.5 Volts, so the Line Audio is adequite for that almost anything from that mic. There are many preamps that will not take any more than 0dB.
 
Right. Disabled means without the pad engaged.

As for changing the pad to accommodate, it just doesn't make sense to me. It seems to conflict with even the simplest pro audio design logic. But maybe they're on to something... I don't know...
 
marcuswilson said:
The maximum output from a Neumann TLM170 is 2.5 Volts, so the Line Audio is adequite for that almost anything from that mic. There are many preamps that will not take any more than 0dB.
Click to expand...

Even with the not all that hot TLM170, you're already past that 1.8V of the Line Audio WITH the -20dB pad engaged! You've peaked with about 3/4 of the 170's max. output.

I mean, even your typical Behringer board pre has a +12dBu (3.09V) Max. Input Level.
 
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If anyone would like to ask Roger any questions (please don't have a go at him), here is his email: roger@lineaudio.se

I'm hoping he'll pop along here for a chat?
 
We should really define headroom. I don't know that I'm clear on it.

I look at the +27dbu output of an MP-2 or MP-2NV as having 23db of headroom over a nominal +4dbu operating level.

If your talking about input signal handling, an MP-2 will take up to +13dbu before clipping the output without the pad, or about +28 with it engaged. I use a nominal 15db pad.


If I were to use an output step-up transformer like the API style circuits those numbers would all go up 6db. But it's kind of pointless, as there's nothing out there that can handle that sort of signal.

As far as I can see from the OSA webpage their numbers are right in line with mine, as I'd expect.

I can't say off the top of my head what the NV will take, but it's in the neighborhood of +14 or so before the input amplifier craps out. I'll measure it and report back to class...

Is headroom the right word?
 
Well, traditionally, Headroom is the diff. between nominal level and point of distortion, stated in dB.

But the key word here is "nominal". As obviously, the higher the operating level, the lower the headroom.

However, nominal level is not what is always being referenced when someone mentions Headroom.

So Dan's reported +23dB for the MP-2 or MP-2NV would traditionally be the Headroom figure without the pad.

Then we have the issue of Max. Input Level. Now some might consider this to be the handling of the exact voltage coming out of a microphone, and while that CAN be the ONLY consideraion for this figure, many times, it is NOT. This is because many preamps have a min. gain and is considered in the figure.

Where things get really blurry is people (myself too) and companies interchanging the terms.
 
So for the Line Audio DMP...

'The output is max 15V RMS which equals +26dBU (reference 775mV), which is +22dB above (the studio standard+4dB / reference 1,25VRMS)'.


...and...

'If 0dB is considered 775mV (0dbU) then the headroom (input signal handling) is +8dB (8dBU)'.

These figures seem OK for the price of the unit?


'If you state it in your order you can have the -20dB PAD changed into -25dB or -30dB, then you will have a headroom of up to 18dBU. The signal to noise of the normal non-PAD setting is not affected at all and this is the setting which is used for noise critical events'.

So if the PAD is lowered to -25dB the input handling (without using PAD) would be +13dB or +18dB if the PAD is lowered to -30dB. Would it be wise to get one with a +13dB handling and a -25dB PAD, for input safety?
 
Yes, again, traditionally, +8dBu isn't bad... But that's with a -20dB pad. The -12.68dBu figure without the pad does throw me off quite a bit.
 
Sorry, I think you are reading the info incorrectly?

I think that it is +8dB without using the PAD? And I think if they lower the PAD to -30dB then the headroom (NOT USING THE PAD) raises to +18dB?

I will clarify this with Roger.
 
From the Specs.:
Input level max 1.8VRMS (180mV RMS with -20 disabled/out


It is saying, with the stock -20dB pad enabled/in, the Max. Input Level is 1.8Vrms (+7.32dBu). And with the -20dB pad disabled/out, the Max. Input Level is 180mVrms (-12.68dBu).

And yes, obviously, with a -25dB pad (instead of the -20dB pad) you'd get a Max. Input Level of 3.2Vrms (+12.32dBu). Or with a -30dB pad, you'd get a Max. Input Level of 5.69Vrms (+17.32dBu).

I am not misreading the above spec., but I see now, that you were.
 
Sorry RE

It appears I was reading the info wrong. Thanks for explaining. I will ask Roger if this is correct, as you are right, it seems low! Although, as the signal to noise ratio is unaltered when the PAD is in or out, what would be the downside to leaving the PAD switched in? Would the self noise of the mic and the gain needed impact on the sound quality?
 
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