Hey Hey!
Forgive the question that's probably been asked before a million times. I'm just not finding it...
I've got new Focal Twin6 Bes and an SPL 2Control replacing my Mackie HR824s (Which are for sale, btw) and an A-Designs ATTY.
So now I've got monitors without input trims and I'm running the 2Control at about 8:00-9:00 (-25 on the label) and want to pad down the XLR +4 lines to the Twins so I can get up another 20dB or so into a better range of control.
The 2Control has 75 Ohm outputs and the Focal Twins have 10k Ohm inputs.
So hey, can one of you old schoolers who actually know your math/circuit design tell me what I should do to pad the outputs of the 2Control/Inputs of the Twins about 20dB without degrading fidelity?
Thanks,
Archmart
Pads!
Hey Hey!
Yeh, how've ya been? How's NYC treatin' ya? How's the demand for mrdibs?
Ok, so I'm trying to strum up what little I recall from my EE days, reading through this Uneeda document, and coming up with the following:
* I need to use a balanced "U" pad with two 337.5 Ohm resistors for R1/2 and a 75 Ohm resistor for R2. (matching the 75 Ohm output of the 2Control)
* 1/4 watt should be fine?
* I happen to have some 5% carbon film resistors. I have no 75s or 337.5s, of course, but I've got 27s and 47s (to put in series to make 74 Ohms - close enough) and I've got 220s and 120s (to put in series and make 340 Ohms).
* (I do also have 330 Ohm resistors, perhaps that's close enough? Likewise I've got 68 and 82 ohm resistors. Close enough? If so, better over or under?)
* Or should I really be looking for 1% resistors? If so, should I look for a particular type other than Carbon Film?
(As long as I'm at it, I might as well do it right and learn a thing or two...)
Any advice will be most appreciated.
Thanks,
Archmart
Yeh, how've ya been? How's NYC treatin' ya? How's the demand for mrdibs?
Ok, so I'm trying to strum up what little I recall from my EE days, reading through this Uneeda document, and coming up with the following:
* I need to use a balanced "U" pad with two 337.5 Ohm resistors for R1/2 and a 75 Ohm resistor for R2. (matching the 75 Ohm output of the 2Control)
* 1/4 watt should be fine?
* I happen to have some 5% carbon film resistors. I have no 75s or 337.5s, of course, but I've got 27s and 47s (to put in series to make 74 Ohms - close enough) and I've got 220s and 120s (to put in series and make 340 Ohms).
* (I do also have 330 Ohm resistors, perhaps that's close enough? Likewise I've got 68 and 82 ohm resistors. Close enough? If so, better over or under?)
* Or should I really be looking for 1% resistors? If so, should I look for a particular type other than Carbon Film?
(As long as I'm at it, I might as well do it right and learn a thing or two...)
Any advice will be most appreciated.
Thanks,
Archmart
Hi. I don't really know the specs on the 2control thing, but it sounds to me like its output impedance is 75 ohms, but that doesn't mean that it works best into 75 ohms, or that you need to match that. In general you want to make a bridging connection which means low impedance output into high impedance input. The 2control will probably have lower distortion and possibly slightly better headroom and bass response into a higher impedance.
That's OK actually, because it doesn't sound like what you've got will actually load the 2control with 75 ohms anyway...more like 800 ohms. Not bad, but you could go higher.
You don't need precisely 20dB attenuation, but I assume you'd like to have your left and right channels be attenuated by precisely the same amount, and also keep your lines well balanced. So you'd be best off using 1% resistors even if you end up ballparking the actual resistance values to standard 5 or 10% values. Or you could take a bunch of your 5% resistors and an ohm meter and match them up by hand.
For easy values, I'd suggest a pair of 1.5k resistors and a 330 ohm resistor to make each of the pads. That'll give you 20dB atten on the nose, and you can find those values in either 1% or 5% and I guess you might already have them. It will give a pretty healthy input impedance of about 3.3K to load the 2control thing while still feeding your monitors with a good low impedance source.
Or you could us two 4.7k resistors and a 1.2K resistor to load the 2 control with 10K ohms, which might actually be what the recommended load is.
OH......I just realized that when you said you wanted to match the 75 ohm output you meant you wanted the pad's OUTPUT impedance to be 75 ohms. That makes more sense. But I still think you don't need to go that low and would be better off with a higher input impedance.
Ned
That's OK actually, because it doesn't sound like what you've got will actually load the 2control with 75 ohms anyway...more like 800 ohms. Not bad, but you could go higher.
You don't need precisely 20dB attenuation, but I assume you'd like to have your left and right channels be attenuated by precisely the same amount, and also keep your lines well balanced. So you'd be best off using 1% resistors even if you end up ballparking the actual resistance values to standard 5 or 10% values. Or you could take a bunch of your 5% resistors and an ohm meter and match them up by hand.
For easy values, I'd suggest a pair of 1.5k resistors and a 330 ohm resistor to make each of the pads. That'll give you 20dB atten on the nose, and you can find those values in either 1% or 5% and I guess you might already have them. It will give a pretty healthy input impedance of about 3.3K to load the 2control thing while still feeding your monitors with a good low impedance source.
Or you could us two 4.7k resistors and a 1.2K resistor to load the 2 control with 10K ohms, which might actually be what the recommended load is.
OH......I just realized that when you said you wanted to match the 75 ohm output you meant you wanted the pad's OUTPUT impedance to be 75 ohms. That makes more sense. But I still think you don't need to go that low and would be better off with a higher input impedance.
Ned
Hey Hey!
Thanks!
So, yeh... The 2Control is 75 Ohms and the Focals are 10K Ohms. I don't frankly know exactly what I need, that's why I'm asking!
So your 4.7k and 1.2k option will give me 20dB and precisely 'respect' the impedances of both devices, yes?
And if I'm having to order the 1% resisters anyway, I might as well get the best kind... What do you recommend? But I could also just measure with my meter and match them as close as possible from the 5% variety, yes?
Thanks much,
Archmart
Thanks!
So, yeh... The 2Control is 75 Ohms and the Focals are 10K Ohms. I don't frankly know exactly what I need, that's why I'm asking!
So your 4.7k and 1.2k option will give me 20dB and precisely 'respect' the impedances of both devices, yes?
And if I'm having to order the 1% resisters anyway, I might as well get the best kind... What do you recommend? But I could also just measure with my meter and match them as close as possible from the 5% variety, yes?
Thanks much,
Archmart
Last edited by Archmart on Sat Feb 28, 2009 2:29 pm, edited 1 time in total.
I think the 4.7k and 1.2k thing would be good. 1k output impedance is a little on the high side, but not anything to worry about I think in this case. If you want to be fussy, just be sure to put the pads at the speaker end rather than the control box end.
Assuming the input impedance of the monitors is 10K which I think is what you said, then yeah, the 4.7K and 1.2K set should give you pretty much exactly 20dB (actually 19.8dB if all the values are exact).
You don't need to be that concerned about low noise in this case, so carbon film will be just fine. I think in real life you could probably just use random 5% resistors and you'd never know the difference, but if you want to be fussy then cherry pick them or order 1% metal films.
The whole pad will only be absorbing a few milliwatts between all three resistors, so you could use 1/2 or 1/4 or 1/8 watt resistors or whatever, but ideally a little bigger can't hurt. I'd go with 1/2 watt if I had them on hand and if I didn't then whatever.
Ned
Assuming the input impedance of the monitors is 10K which I think is what you said, then yeah, the 4.7K and 1.2K set should give you pretty much exactly 20dB (actually 19.8dB if all the values are exact).
You don't need to be that concerned about low noise in this case, so carbon film will be just fine. I think in real life you could probably just use random 5% resistors and you'd never know the difference, but if you want to be fussy then cherry pick them or order 1% metal films.
The whole pad will only be absorbing a few milliwatts between all three resistors, so you could use 1/2 or 1/4 or 1/8 watt resistors or whatever, but ideally a little bigger can't hurt. I'd go with 1/2 watt if I had them on hand and if I didn't then whatever.
Ned
Hey Hey!
Excellent. Thanks.
It worked out perfectly. I had 4.7k 1/2 watts and 1.2k 1/4 watts, which I wired into the XLR Males on the monitor end. They were only 5%, but cherry picking found well every 5% I could find to be within 1%. In fact most were less than .5% off.
A funny thing, though. I went about it very meticulously, and then once it was wired up I began some listening and checking out what my new standard level setting might be. While doing so, I was hearing an out of phase thing, but I was so convinced I'd been so meticulous about it that I convinced myself that I must just be tired or now finally hearing some other problem in my system. I was worried what else might be wrong. Sure enough, when I looked closer into it the next morning, I had one wired out of phase.
Thanks,
Archmart
Excellent. Thanks.
It worked out perfectly. I had 4.7k 1/2 watts and 1.2k 1/4 watts, which I wired into the XLR Males on the monitor end. They were only 5%, but cherry picking found well every 5% I could find to be within 1%. In fact most were less than .5% off.
A funny thing, though. I went about it very meticulously, and then once it was wired up I began some listening and checking out what my new standard level setting might be. While doing so, I was hearing an out of phase thing, but I was so convinced I'd been so meticulous about it that I convinced myself that I must just be tired or now finally hearing some other problem in my system. I was worried what else might be wrong. Sure enough, when I looked closer into it the next morning, I had one wired out of phase.
Thanks,
Archmart
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