Line to micpre pad

Hi,

I made a line to micpre pad to use my Great River MP2NV for analog summing. Dan Kennedy suggested this on another forum:

Out L Hot-----2.2K --------------NV L In Hot
.....................................|
...................................470
.....................................|
Out L Cold----2.2K--------------NV L In Cold

Other channel same as the first:

Out R Hot-----2.2K --------------NV R In Hot
......................................|
....................................470
......................................|
Out R Cold----2.2K--------------NV R In Cold

Click to expand...

My (stupid) question is: What is that 470ohm resistor doing?


Thanks

Richard

I was just reading Self's writing on balancing, he mentions using mic amps for line inputs.

Check out the bottom of section 5 on this page:
http://www.dself.dsl.pipex.com/ampins/balanced/balanced.htm

Hope it helps,
hejsan

As a matter of fact Self says that the way you are doing it is bad in regards to CMRR and suggests a better way.

[quote author="Hayman"]My (stupid) question is: What is that 470ohm resistor doing?


Thanks

Richard[/quote]


Its doing the padding. Its partially shorting the two legs of the balanced signal together so that some of it is lost, but not all. So you have a pad!
The 2k2s sets the input impedance of the pad, which he has made fairly high so that anything can drive it without too much trouble.


M@

The 470-ohm resistor is doing two things. First, it's attenuating the input signal. This network will give you an attenuation of about 23dB. The other thing it does is lowers the source impedance seen by the preamp. Without this resistor, the preamp would see about 4500 ohms source, rather than the 150 ohms it's expecting. That would be less than ideal in terms of noise performance. Not a real big deal, but since you need the attenuation (that's why you're using a pad, right?) it's convenient to optimize the values. If you had asked me, I would have recommended higher-value resistors to keep the load seen by the line out up around 10k, and go for about 34dB of attenuation. But Dan knows a whole lot more about this stuff than I do, and an awful lot of what I do know I learned from him, so I won't argue with his suggestion (unless he intended it for a specific application which may differ from yours).
While we're discussing it, how exactly is this pad going to be useful "for analog summing"?

you might also consider an H pad if you will be building the pad into a stand alone box and dont need to make it conveniently built on an on/off switch. I always have found the H pads I have built to sound different the U pads I have built and I always find that whenever I say that someone suggests Im insane or full of shit, take your pick. Maybe Im fooling myself, but Im usually try to be pretty unbiased about stuff like that. Anyhow, if you have some time to experiment, check out H pads. I use U pads on virtually every project I do as they are less complicated to switch in and out with a relay, however for permanent installation I would probably go with an H pad.

dave

If you're driving transformer-input circuits, and the H-pad has a different output impedance from the U-pad, then the transformer is seeing a different source impedance with the two pads, so yes, they should sound different. So you're not crazy -- or, if you are, this isn't evidence either way.

Peace,
Paul

[quote author="ulysses"] While we're discussing it, how exactly is this pad going to be useful "for analog summing"?[/quote]

Sorry, I did not mean summing. Don't know why I wrote that.
I'm just experimenting with running mixes through my MP2NV with analog compression and back into my DAW.


Richard

Doesnt the MP2NV have a built in pad already?


M@

H-Pads can be useful if you're going for constant impedance and you want the input and output impedances to be matched. In today's world, that is almost never the case. You almost always now want a high load impedance and a low source impedance. So think of a "U-Pad" as just another H-Pad, except the last two resistors are zero ohms because you don't want to raise the source impedance any more than you already have. In voltage systems (low source impedance, high load impedance) it's already impossible to build a resistive pad that has a small attenuation value without degrading either the input or the output impedance. Adding those two extra resistors would only make things worse. For this reason, yes, the H pad should sound different. Nothing wrong with sacrificing some noise performance and resistively raising source impedance to get a different sound, but generally the point of a resistive pad is to get the same sound as with no pad at all. That's hard to be objective about, because clipping-vs-not-clipping is of course a major audible change. Besides which, pads allow you to use different gain settings which also changes the sound of an amplifier. But in principle I believe the pad itself shouldn't alter the sound of the circuit.

Not sure if it's been linked to before, but this site explains and does the math for you as well.

http://beradio.com/notebook/passive_attenuators/



While we're here, what is the public opinion on a lattice type attenuator?



:guinness: :guinness:

Chris

[quote author="producer4000"]
While we're here, what is the public opinion on a lattice type attenuator?

[/quote]

I've never seen that before. I'm going to have to stare at it for a while, going, "what the hell?" before I can develop an opinion. Though at first glance it seems like it would exacerbate the problem that you can't build a low-attenuation pad with high input impedance and low output impedance. That is the "cold fusion" of resistive attenuation.

with variable resistor, that one can attenuate all it's way through to reversing phase.. :grin:

[quote author="mattmoogus"]Doesnt the MP2NV have a built in pad already?


M@[/quote]

Nope. It has low gain settings though, and rarely needs a pad in normal use.

Hey Richard, if you build the pad, use 1% metal film resistors and match them further by hand (w/ a meter) if you can. I threw one together out of 10% carbon resistors I had lying around once, and I was shocked at how much noisier it was than a normal, well-matched H pad.

Chris