Pad calculations driving me nuts!

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matta

Well-known member
Joined
Jan 11, 2005
Messages
1,640
Location
Cape Town, South Africa
Guys,

Level reference continue to confuse the crap out of me: dBu, dBV, add in +-4dBu and -10dBu and INPUT/OUTPUT impedances not forgetting the various TYPES of pads (T,L,O,U etc) and you get a whole lot of crazy.... at least for me....

I'm trying to get to the bottom of this and would love some help... here is what I DO know... in a balanced system a typical "U" pad will work, albeit asymmetrically but this is fine for attenuating a LINE LEVEL signal INTO a MIC INPUT LEVEL.

Ok, so in a typical situation where you would have a LINE LEVEL signal, -10dBV / 10K OHMS from a prosumer signal source you would need to ATTENUATE/PAD the signal to match the MIC INPUT source impedance? Correct? Typically this is 30-40dB?

What happens in this situation where I have a +4dBV output from a PRO AUDIO sound card with an output impedance of 50-100 Ohms (like line level out of a mic pre I would guess)?

The way I understand it 'typically':

INSTRUMENT LEVEL = +1M OHMS
MIC LEVEL = 100+ OHMS
+4dBu BALANCED LINE LEVEL = 50 OHMS
-10dBV UNBALANCED LINE LEVEL = 10K OHMS

The weird thing is there doesn't seem to be and standard with manufactures RE giving these details away, some manuals have them others just broadly speak of +4dBu and-10dBV with no concern/indication of impedances etc.

Here is the predicament I find myself in, a client/friend has asked me to build him a pair of cables with prebuilt pads coming OUT of his Pro Tools hardware interface (according to the spec it is balanced +4dBV / 50 Ohms) INTO a Great River Mic Pre (can't seem to find the input specs on them) to add coloration to his stereo mix, recording the output back into Pro Tools.

So how does one go about working out how MUCH of a pad would be needed in a situation like this? Again sorry if it seems like such a basic questions but levels confuse me... still waiting for Kev's know it all primer!

Off tangent...

What confuses me is in a typical situation of a console less environment, it is more and more common for guys have racks of outboard gear and run a mic into a pre, then into an EQ then into the DAW... well if levels/impedances are correct then the output of the mic pre would be around 100 OHMS but the input of say an API 550EQ would be 10K Ohm and the output back to 50-100 OHMS when entering the DAW/Converters... weird and perplexing... doesn't this cause impedance mismatching/issues?

Thanks in advance!

Matt
 
[quote author="matta"] a client/friend has asked me to build him a pair of cables with prebuilt pads coming OUT of his Pro Tools hardware interface (according to the spec it is balanced +4dBV / 50 Ohms) INTO a Great River Mic Pre (can't seem to find the input specs on them) to add coloration to his stereo mix, recording the output back into Pro Tools.

So how does one go about working out how MUCH of a pad would be needed in a situation like this? [/quote]

Matt, do some refresher reading on bridging impedances. Don't over think what the output drive impedance is in this situation. It doesn't affect any calculations with regards to pad values. In short, calculate a U pad based on matching resistance of the preamp and the desired loss. You are only worried about MATCHING the input impedance of the preamp in this case, to make it 'look' like a typical mic OUTPUT DRIVE impedance. So, you have your matching shunt resistor (middle of the U pad) value which is most important, and you have two build out resistors which determine the losses beyond the 6dBish MATCHING loss dictated by the shunt resistor in the middle. The 2 series resistors can be seen as build out FROM the drive impedance, effectively raising it. Many devices have build out resistance internally as protection, and those resistances + the actual drive resistance behind them determine the printed spec (if provided at all). So, the build out resistors reduce the drive impedance/current, while the shunt fixes a match at the receiving end. That's my quickie half-baked version for now.

Determining output impedance when you really need to know:

A 6dB loss of output level means you've arrived at a matching condition, which means the resistive value that achieves this IS the output impedance under test. You can do this with tones and a dB meter comparing unloaded condition with variable loaded condition. Use a rheostat and fiddle about.
 
Matt,

First things first, we are working with modern studio gear where interconnections between gear rely on "bridging" impedances in order to get good voltage transfer. So, we tend to aim for an impedance for the input of the receiving gear to be 10X or so of that of the output impedance of the sending gear. Draw a voltage divider if you like with typical values and see how much of the source voltage arrives at the destination depending on the two impedances. For old-school matching impedances, only have of the voltage will arrive; -6dB.

Don't worry about the dBs. It's always a ratio which is expressed in simple numbers which are simple to add. I'd rather say "Matt, give me 60dB on that amp" rather than "Matt, give me 1000X amplification". So, dealing with different versions audio level dB scales can be a bit frustrating when you need to convert them, but at least for dBu and dBV we only need to remember that one is a factor of 1 volt, and the other is a factor of 0.775 volts. So, +6dBV = 2V, and +6dBu = 1.55V.

The amount you want to pad by is going to depend on what the nominal level coming from Protools is (0dBu for -18dBFS?) and how much gain you want to nominally set these mic preamps to. There may be a preference in how you have the preamps set - eg. driving the transformers hard with low gain, or perhaps the opposite. You will also need to consider the level coming out of the preamps which you want to achieve. This will most likely be 0dBu (or otherwise +4 or -10dBV) which should correspond to somewhere between -20 and -12dBFS at the actual converters/software. I'd guess -18dBFS. (0dBFullScale is the digital clipping point for any digital recorder - we line them up so that our chosen nominal input level will hit something like -18dBFS so that we have 18dB of headroom to play with for safety.


[quote author="matta"]
Ok, so in a typical situation where you would have a LINE LEVEL signal, -10dBV / 10K OHMS from a prosumer signal source you would need to ATTENUATE/PAD the signal to match the MIC INPUT source impedance? Correct? Typically this is 30-40dB?[/quote]

Sounds like a reasonable guess.

What happens in this situation where I have a +4dBV output from a PRO AUDIO sound card with an output impedance of 50-100 Ohms (like line level out of a mic pre I would guess)?

The input at the mic preamp is 14dB hotter and you'll want to dial in less gain or pad more.

The way I understand it 'typically':

INSTRUMENT LEVEL = +1M OHMS
MIC LEVEL = 100+ OHMS
+4dBu BALANCED LINE LEVEL = 50 OHMS
-10dBV UNBALANCED LINE LEVEL = 10K OHMS

You need to specify input and output impedances. Output impedances for line outs (eg. Protools out, preamp out) will be say 50 - 100 Ohms; low. Mics will be 50 to 600; similarly low. Mic preamp inputs will be typically 1K to 2K - high enough to "bridge" the voltage from a 200 Ohm mic source. Line inputs will be typically 10K to 20K (there are variations, but that figure will do).

The weird thing is there doesn't seem to be and standard with manufactures RE giving these details away, some manuals have them others just broadly speak of +4dBu and-10dBV with no concern/indication of impedances etc.

Levels do vary. For impedances of modern gear, you can usually assume.

Here is the predicament I find myself in, a client/friend has asked me to build him a pair of cables with prebuilt pads coming OUT of his Pro Tools hardware interface (according to the spec it is balanced +4dBV / 50 Ohms) INTO a Great River Mic Pre (can't seem to find the input specs on them) to add coloration to his stereo mix, recording the output back into Pro Tools.

Have a bash at working out a rough pad figure and take into consideration of the levels at each part of the chain. Building/designing the pad itself is fairly simple.

Roddy
 
Which Great River mic pre? The original ones were remarkably low-coloration devices; if the client is looking to add color, they won't do much. The NV series...that's another story.

Figure you want to set the pres for about 40dB of gain, a reasonable proposition, and you want to drop the ProTools output the same amount so the overall system gain is unity (0dB). The preamp probaby has an input impedance around 1.35k (1.5k || the phantom power resistors), and wants to be driven by a 150 ohm source impedance. As a first approximation, a U pad with two 7.5k resistors in the series legs and a 150 ohm resistor in shunt will have a source resistance of 148 ohms, close enough. However, the 1.35k of the preamp will load the shunt resistor down so that it looks (to the series resistors) like 135 ohms, so the actual pad is about 41dB. If you can live with that, you're home free. The exact pad value is less critical than showing the right source impedance to the mic preamp's input transformer.

This isn't about impedance matching; modern gear doesn't use matched impedances (unlike the stuff of yore). It's about levels and light loading.

Peace,
Paul
 

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