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I recently asked about the general setup of our Talking Newspaper recording studio (Undestanding a recording studio configuration).

From the answers given, it seems that without buying extra compressors, the audio chain is probably as well set up as it can be. I spent the evening at the studio, and found the manuals for our various bits of equipment. For some reason, reading manuals always seems to leave me more confused rather than less!

Our mixing desk is a Soundtracs Topaz 14-4 - the manual for this says the Master Outputs (balanced) are +27dBu

The mixer feeds into our compressor, a Drawmer DL251 dual spectral compressor - this states the XLR inputs can be set as +4dBu or -10dBV, with the output being the same.

The compressor output feeds into the distribution amp - a Chilton +26 (2 in/12 out in pairs) - this states max input +26dBu, preset to unity gain for +4dBu input and get the screwdriver out to change the variable resistors inside if your input isn't +4dBu

The distribution amp provides signal to (currently) 4x Tascam 130 cassette decks (input 97 mV, 50k Ohms), a monitor speaker, and computer via an M-Audio Delta 44 sound card, which again takes either +4dBu or -10dBV balanced/unbalanced inputs.

If I understand correctly, the compressor takes a +4dBu signal, and outputs a +4dBu signal after doing it's magic to make the quiet bits louder and the loud bits quieter… The distribution amp similarly takes a +4dBu signal, and makes sure it's still at +4dBu when it is split to six different devices, which all expect to receive a +4dBu signal.

The bit that troubles me is that the mixer says it is outputting a +27dBu signal. Am I fundamentally misunderstanding something in the figures? Or, is this usual? Does the +27dBu carry through the chain, or should the compressor still only output +4dBu?

As the system has been set up like this for several years, I am at least hoping that if there is something "wrong" with it, it is only affecting the sound quality of the final recording! As a small charity, buying a whole load of new equipment isn't really an option, but it would be really nice to properly understand what we have, and to make sure it is working properly for us.

(NB as was pointed out in an answer to my previous question, the mixing desk doesn't have master inserts, or in fact group inserts so to make real use of a compressor it has to be on the master ouput, or limit it to just working on two of the five mic channels we run.)

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I helped answer your last question as well, and nothing is wrong with your setup.

I understand your confusion, but take from this that +27dBu definitely does not mean you are outputting +27dB. The dBu numbers correspond to the nominal operating level of your equipment, and in the pro audio world -10dBu devices require more gain to reach the same nominal level of +4dBu devices, which in some cases results in more noise(!!) (because when you turn the signal up, if there is noise in the signal, you turn the noise up also). There is a pretty straight-forward article about decibels at Sound On Sound if you like.

To be honest, it is unlikely that you are driving your mixer as to clip and I assume your master fader is somewhere around 0dB... so it is safe that you are arriving somewhere close to that +4dBu that your compressor is expecting.

dBu means decibels unloaded which is an RMS (average) Voltage measurement taken to rate pro audio equipment input and output capacity, regardless of impedance (which also manipulates the variable "load"). RMS levels differ from peak levels in the sense that peaks levels are considered maximums (peak-to-peak) and RMS is a measure of the average between them.

(RMS=avg, VPK=peak, VPP=peak-to-peak)

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I still encourage you to consider an 8I/O sound card for your setup in the future so you can run it mostly from your computer. You would have enough inputs for your mics and enough outputs for your devices, plus you could utilize software compressors and EQ's independently on virtually as many channels as you would like. This could replace your compressor and mixer with more viable routing options.

I hope this helps with your questions :)

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  • I still don't quite get what the mixing desk manual is referring to when it says +27dBu output - is that a theoretical maximum rather than a standard working value, or is it to do with the overall gain you can achieve from source to output?
    – OwenM
    Commented Feb 21, 2014 at 16:13
  • Regarding an 8I/O sound card - when we next update the computer, this is probably the way to go, though a lot of our volunteers are from the days of recording solely to tape and get a little panicky around computers. A virtual mixing desk would be a step too far for some of them!
    – OwenM
    Commented Feb 21, 2014 at 16:16
  • It's sort of like the reasonable maximum power before driving your console into clipping. A removed definition could include "headroom" or "continuous power". All of the analog audio signals that run into your console are just another form of electrical energy. The electrical energy outputs a waveform, and of that waveform, in picture-form, the dBu level is proportional to the area underneath the wave over a period of time(think of the sine wave example). Your +27dBu output can tuck more energy under that sine wave, leaving you a better signal above the noise floor, and within your headroom.
    – Chris Bolseng
    Commented Feb 22, 2014 at 2:19
  • It is a lot to grasp, especially by my loose definition but you could look into RMS Voltage if you want a deeper view of it. Don't confuse RMS with PEAK levels *(your compressor is likely set at -3dB peak). And I understand about changing to software, haha. It might take some time but it might be advantageous for you guys to consider in the long run. I hope this helps.
    – Chris Bolseng
    Commented Feb 22, 2014 at 2:25
  • Your comments are helping a lot, oddly RMS is something i have a reasonable grasp of in electrical terms. I know cheap speaker manufacturers like to express power in PMPO because it gives big numbers, and people buying at that price trend not to know the difference! All the variants in how levels are expressed feel like trying to cook a meal with one recipe giving the temp in Fahrenheit, another centigrade, the next "gas mark" and your cooker is marked celcius (dBu and dBv are the same, so i can have Celsius/Centigrade!). Then just because they can someone else uses the subjective "low heat"
    – OwenM
    Commented Feb 22, 2014 at 2:54

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