Pad Questions

[chris319]

I want to knock a +4 balanced output down to mic level. I'm looking to make a balanced pad with 80 dB of attenuation.

Questions:

H pad or U pad?

I assume I want to cascade two 40 dB pads one after the other?

The output impedance of my signal source (sound card) is 560 ohms, so I'm thinking of having a 600-ohm input to the pad. I want the output of the pad to emulate a microphone so I'm thinking the output impedance of the pad should be 150 - 250 ohms.

If I use two H pads, should the impedances be calculated as follows?

600-ohm input (pad 1) -> 150-ohm output (pad 1) (-40 dB) -> 150-ohm input (pad 2) -> 150-ohm output (pad 2) (-80 dB)

So I'm looking at building one 40 dB H pad at 600 -> 150 ohms and a second at 150 -> 150 ohms, correct?

FWIW the Shure A15AS has an input impedance of 1,000 ohms and an output impedance of 150 ohms but only goes to -25 dB.

Thanks.

 

[1954U1]

Why not a single 80dB pad?

 

[chris319]

Attenuators beyond 40dB are not recommended and results are unpredictable.

http://www.csgnetwork.com/decibelattenuatecalc.html

 

[sahib]

Yes,the inductive or capacitive load will affect the the fequency response of a resistive network but I don't see any problem of using single 80dB pad.

One important point. Output impedance of your soundcard is 560 ohms which is fine. But emulating 150 ohms on the load side is a bit unrealistic expectation. If you calculate your load impedance on 150 ohms but the actual load impedance is different, than your whole calculations will be out of the window. So worrying about the instability of single 80dB attenuator comes at the bottom of the list.

Also I don't understand the point you made on Shure attenuator. I hope you did not mean a connection between the source/load impedance and the attenuation. There is not. They are independent. You can have a source/load impedance of 1000/150 ohms and calculate your resistor values for 80dB attenuation.

 

[Samuel Groner]

Are you sure you need 80 dB attenuation? Mic level sounds like 30 dB to 60 dB attenuation from +4 dBu. At least me doesn't use mic preamps with 80 dB gain...

Samuel

 

[emrr]

I only have a few things with 80+ dB of gain, and as we know, S/N gets pretty poor down there. 

 

[chris319]

Are you sure you need 80 dB attenuation? Mic level sounds like 30 dB to 60 dB attenuation from +4 dBu. At least me doesn't use mic preamps with 80 dB gain...

Samuel

OK, let's say 60 dB of attenuation. The questions are still relevant as it is more than 40 dB of attenuation.

 

[emrr]

U pad.  No math required, as experimentation is so easy.  Start with the only given, the Z seen by the following stage; 150-200 ohm shunt resistor.    Your sound card may be rated 560, but it certainly doesn't want to see that.  It doesn't care, so long as it's 10x+ higher.  You might even get away with multiple steps in the series legs, and use a rotary switch. 

 

[PRR]

> output impedance of my signal source (sound card) is 560 ohms, so I'm thinking of having a 600-ohm input

It wants a hi-Z load, I am sure.

80dB is a bit stiff, also prone to HF leak-around. 60dB is usually ample.

60dB is 1000:1.

Set your output as a 100 ohm shunt resistor. Assume preamp input Z is much higher.

As an L-pad, your series resistor is then 100,000 ohms. Or for balanced, two 50K.

For 40dB loss, two 5K.

If you decide you need to stimulate a 285 ohm mike, add (285-100)= 185 ohms as two 91 ohm resistors at the output.

 

[warpie]

ok, I might be a bit off-topic but this thread is very interesting because I'm looking at buying a pair of calrec 1253 EQ/Pre.

The only problem is that these modules have only mic input with a maximum gain of 70db.

Since I own a variety of synths I'm definately gonna need line inputs and I've been told by a very experienced calrec (and not only) designer that it is possible to use a pad circuit for converting the mic input to a line input.

At the beginning I was a bit surprised but I was even more surprised when he told me that many designs (soundcraft, calrec, some neves I think) do the same thing. Is it as easy as it sounds?

Please note he's not the same the guy who's selling the modules and therefore he has no reason to 'lie'. Actually the guy has always helped me with some of my designs and I really count his opinion/input.

 

[PRR]

> Is it as easy as it sounds?

Well, yeah.

Well.... say you have a water-well which puts out 5,000 gallons a second. Your dog is dirty. You point the output of this gusher at him, he gets washed across town. You put a valve in the pipe, you can turn-down to a gentle trickle or a brisk rinse.

You have POWER and you have LOSS. Usually, Power is expensive, so you only buy as much as you need. A half-inch well. A unity-gain input stage. 

Your synths probably _have_ ample Power to drive any board or recorder input. Why did you buy high-gain preamps? OK, because you will use them for mikes, but why use them on synth? OK, because they got EQ too. And maybe because the added gain stages de-clean the synth's pure tone.

Given that you have horse-size output, and a "mike input" which hardly needs a whole Chihuahua of push, you need a major valve, brake, pad.

Build a 60dB pad using Doug's or my values. 100, 150, 200 ohm basis... whatever is in top of your resistor bin.

Say the synth outputs 1V. After 60dB attenuation, it is 0.001V. Your max gain of 70dB brings it back up to 3V. You'll probably want to run less than max gain, 'cuz you probably want to arrive back near 1V, more or less.

 

[warpie]

hi PRR and thanks for the reply. Actually I forgot to mention that apart from using it with mics, I will mainly use it as insert Eq in Logic.
Do you reckon that the whole idea is a bit of waste of money (considering mounting costs, output tranformers, etc...)?

My point of view is I can combine a quality pre-amp and eq in one module..

thanks
warpie