BillFitzmaurice.info

Hey guys,

I need to make some line level attenuator pads because todays DJ mixers will overload the line inputs on my stage mixer with my gains all the way down. I've never done this, so I was hoping somebody with experience could check my work.

I came across a few good articles here: http://www.uneeda-audio.com/pads/

and here: http://www.theatresupply.com/Data/Docum ... C_TECH.pdf

They state for line level it is best to find the impedance of the input side of the connected devices (in my case the line level inputs on my stage mixer), and design the pads input impedance to approximate that. In this case the manual for my Allen & Heath PA20-CP states the mono line level input impedance to be >30k ohms.

I found a simple calculator here: http://www.sengpielaudio.com/calculator-Lpad.htm that I can use to determine my resistor values for an L-Pad, and I will convert them to U-Pad values since these will be for balanced audio signals. This appears to be as simple as dividing R1 in half.

My first question is what value should I input for the impedance? If my mixers line level inputs are stated as "greater than 30k ohms", should I use 30k or some higher value?

Lets say I do use 30k and want to attenuate by 18db. The calculator gives me my resistor values as R1 = 26223.23 and R2 = 4320.72. For a U-Pad I divide R1 in half and so R1 then becomes = 13111.615. Obviously I would have to approximate to standard values when I buy the resistors.

If I'm on the right track and this appears accurate to you all, my next question would be if 1/2 watt resistors are enough to handle this sort of attenuation or should I jump to a higher rating?

Thanks for any input (pun)!

isnt this what compressors/limiters are for? forgive me if im mistaken on thisone.

You're making it far too complicated. This is all you need for line level attenuation:

I don't desire it to be variable, nor do I want a knob hanging between my signal cable and my mixer. I don't think 3 resistors soldered to some TRS or XLR points is complicated. I just want someone to verify to me that I have how this works understood correctly.

You say the line inputs of your stage mixer are overloading.....are you plugging into a mic input with an xlr? Or, are you truly using a line level input?

If you are going from a +4 line level output from DJ mixer the to the mic input on the stage mixer, you could easily be overloading.

Or, are you hammering the outputs of the DJ mixer? Something just doesn't sound right here.

How about one of these?

If it's balanced line, it's not as easy as sticking a control on the line if you wish to keep the line balanced. What is the make and model of the source gear, and what is the make and model of the input you are trying to drive? without knowing this, it's anyone's guess as to how much attenuation is required.

la malta wrote:I don't desire it to be variable, nor do I want a knob hanging between my signal cable and my mixer.

Obviously you would not hang it on a cable, you'd build a box with an input jack, an output jack and a pot for each channel. You may not need it to be variable, but better to have that capability and not need it than the other way around.

If it's balanced line, it's not as easy as sticking a control on the line if you wish to keep the line balanced.

In that case you'd use a stereo pot.

You say the line inputs of your stage mixer are overloading.....are you plugging into a mic input with an xlr? Or, are you truly using a line level input?

It's not unusual for mixers to use XLR for mic inputs, 1/4" for line level, and for there not to be any method of padding down the XLR input gain to handle line level.

KISS. I would never use the variable function of the pot, period, so I'm not using one. I have all the parts to make inline cable adapters with ample room for the resistors, so I'm not going to buy more parts I don't need to build boxes I don't want to build, for pots I don't want because they will easily get fiddled and knocked to attenuation points I never desire. I can use the per channel gains on my mixer to adjust the signal to whatever point I desire, ONCE I attenuate the incoming signal to a point it doesn't clip my meters with the gains all the way down.

Honestly, it doesn't even matter if I choose to use a pot or not. I still don't know what impedance I should use and what wattage rating I should use for the resistors (or pot for that matter), which was all I really need to know anyway.

Bill Fitzmaurice wrote:

If it's balanced line, it's not as easy as sticking a control on the line if you wish to keep the line balanced.

In that case you'd use a stereo pot.

No. Nonononono. The tolerance between tracks (especially on logarithmic pots) is going to be crap compared to what you want for balanced input impedance matching.

It would still work alright if the input stage is a pair of op-amps, one inverting and one not. But, you're going to completely wreck the advantages of balanced lines that way. Summing inverted pairs gives you 6dB of noise rejection. Better than single-ended, but a good input stage will float the virtual ground between + and - inputs so (typically minor) impedance mismatches are equalized. Using a cheap 5% tolerance pot is going to throw that balance way off and noise rejection will suffer. Since the back of most racks is a pile of signal and power and speaker cables, take all you can get.

OP: You might consider making a couple of pairs: 6dB and 12dB. Stick them together if you have to, but if you're having to cut 18dB, you're probably using the wrong input. Plan B might be to incorporate a slide switch that lifts one of two shunt resistors for 6 or 12dB.

For the resistor value, I'd just go with a standard 47k if the input impedance is ">30k". Although that makes me wonder, because I would've figured on a balanced line input it would be much lower. Either way, use 1% resistors, or better if that's a cost-effective option.

la malta wrote: I can use the per channel gains on my mixer to adjust the signal to whatever point I desire,

Once again, are you plugging into a mic level input (xlr) on the mixer? Or, the line level (1/4").

If you are plugging into a mic level input - that's your problem. This may not be the case, but so far there is nothing to indicate exactly what equipment you are using or which inputs you are using. Without that, it's hard to offer a good solution.

But, if you are using reasonably new equipment - there is no reason that a +4 line level output would be overdriving a +4 line level input.

My gut feeling is that something is hooked up wrong.

Yes, I'm using the 1/4". These DJ mixers have high outputs. My stage mixers mic ins are XLR, and the line ins are TRS. Trust me, I use the TRS's almost all the time as I do sound for DJs almost all the time and this happens pretty frequently. I'm not the DJ, and you know what that means as far as high gains, cranking the main outs etc. The obvious thing to do would be to turn the main output knob on their mixer down, but unfortunately that is not always possible for a couple million reasons. My stage mixer does not have pads. All I'm trying to do is make my own.

I was going to use 1% tolerance resistors because both articles said to for balanced line, I just wasnt sure the wattage. The second article has guidelines on when to use 1/4w and 1/2w, but putting in my numbers in the l-pad calculator yielded some big numbers, I got scared and just wanted to be sure, hence this post. Another thing I will need to figure out is if I need an O pad or just the U.

My Allen & Heaths input impedances:

XLR mic ins: 2k
TRS mono line in: >30k
Stereo line ins, 2 track in etc: >10k

Ahhhh....I thought you were in control of the first mixer. I guess telling these guys that getting into the red will make it sound like S#%t, doesn't matter to 'em? Yeah.....thought not.

Oh well, good luck.

Bruce Weldy wrote: I guess telling these guys that getting into the red will make it sound like S#%t,

Doesn't help that first red is already +10db above unity!

CoronaOperator wrote:

Bruce Weldy wrote: I guess telling these guys that getting into the red will make it sound like S#%t,

Doesn't help that first red is already +10db above unity!

Good Lord!

What crap.