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DIY XLR Attenuator Specs Help
DIY XLR Attenuator Specs Help
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Old 3rd June 2020, 12:58 PM   #1
Javs is offline Javs
diyAudio Member
 
Join Date: May 2015
Default DIY XLR Attenuator Specs Help

Hey all,

I just need a quick check on my plan for some XLR attenuators.

I have a DBX Driverack PA2 being fed to an Emotiva XPA7. Long story short, I can hear the DSP noise floor as hiss through my 102db/w AMT tweeter from the chair and wish to pad it down a bit as I was previously doing on my unbalanced RCA MiniDSP setup (Transitioning to the PA2). Previously the Harrison Labs 12db RCA Attenuators did the trick.

I ordered some 10db attenuators from Parts Express, but they dont do a good enough (hiss reduced but not nearly enough) job plus I need to use these on the PA2 output thus I think they are actually being used backwards in how they are designed to be used (I am using F for in not M), also they are not balanced pads, so I am going to use the opportunity to use the shells and solder my own correct balanced attenuators inside them.

I want to go for 20db attenuation this time.

The PA2 outputs 120ohm through XLR. Is it ok to just have that 120ohm as the target for this attenuator? Is that the best thing to do? The Emotiva XPA7 has an input impedance of 33k ohms. The PE attenuators are 600 ohm which seems a nominal common value. I dont really need to 'convert' the impedance do I? Should I just use the same impedance through the Pad?

As you can see I am just not quite sure on which ohm value to shoot for, I otherwise was planning on an H Pad so I can use the attenuators facing either the Male or Female as the input.

A few direct questions.

1. Since I wish to be able to use either Male or Female as the signal IN (gives me options later), does that mean I have to use an H Pad and not the common U pad? Would the U pad used backwards mess up/lower the attenuation of the pad?

2. Is 120ohm the best value to shoot for since that's what my PA2 apparently outputs?

3. If using something like 600ohm in/out but an 120ohm input from the PA2, does that reduce the total attenuation? Does that explain why it felt like it didn't do enough when I tested it?

Shooting for 120ohm with an H pad,

R1/2 is 49ohm
R1/2 is 49ohm
R3/2 is 49ohm
R3/2 is 49ohm
Bridge is 24 Ohm

4. Am I ok with 0.5w resistors on an XLR cable that could see as much as 2v?

Thanks!
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Old 3rd June 2020, 03:06 PM   #2
jaddie is offline jaddie
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Join Date: Apr 2011
Quote:
Originally Posted by Javs View Post

I want to go for 20db attenuation this time.

The PA2 outputs 120ohm through XLR. Is it ok to just have that 120ohm as the target for this attenuator?

Is that the best thing to do? The Emotiva XPA7 has an input impedance of 33k ohms. The PE attenuators are 600 ohm which seems a nominal common value. I dont really need to 'convert' the impedance do I? Should I just use the same impedance through the Pad?
No. 120 ohms is the PA2 source impedance. You're not trying to match that, you're bridging it. No pro audio device made in the last 40 years is intended to drive a matching load. An H pad is an impedance matching pad. In this application you need a "U" pad.

A matched load is where the output impedance of the device is the same as the load, (600 ohm source driving a 600 ohm load). The result of this match is maximum power transfer to the load, and that's great if the system is designed for power distribution. This caused a problem in large pro audio installations because it meant output amps all had to drive only one 600 ohm load, and the performance of the output amp changed if it wasn't loaded exactly right. Pro audio moved to bridging loads and voltage distribution decades ago. In voltage distribution you have a low source impedance driving a high impedance load, "bridging" being 10X the source Z or higher. Now we have output impedances, like the Drive Rack, with 120 ohms source Z, but it can't actually ever drive 120 ohms, that's only it's source Z. It expects something much higher, like 5K or more. It's "voltage distribution" because the output voltage is transferred without concern for the power transferred. You can attache several devices to one output that way, which helped out in pro installations a lot.

So the pad becomes a "U" shape, a resistor from each output terminal of the Drive Rack, then a load resistor across the input to the amp. Since you want 20 dB, standard R values of 5K from each output leg, and 1K across the amp input would be very close.
Quote:
Originally Posted by Javs View Post
As you can see I am just not quite sure on which ohm value to shoot for, I otherwise was planning on an H Pad so I can use the attenuators facing either the Male or Female as the input.

A few direct questions.

1. Since I wish to be able to use either Male or Female as the signal IN (gives me options later), does that mean I have to use an H Pad and not the common U pad? Would the U pad used backwards mess up/lower the attenuation of the pad?
You want a "U" pad, as I outlined above.
Quote:
Originally Posted by Javs View Post
2. Is 120ohm the best value to shoot for since that's what my PA2 apparently outputs?
No. You do not want to match the output Z of the PA2.
Quote:
Originally Posted by Javs View Post
3. If using something like 600ohm in/out but an 120ohm input from the PA2, does that reduce the total attenuation? Does that explain why it felt like it didn't do enough when I tested it?
No. Again, see my above reply.
Quote:
Originally Posted by Javs View Post
Shooting for 120ohm with an H pad,

R1/2 is 49ohm
R1/2 is 49ohm
R3/2 is 49ohm
R3/2 is 49ohm
Bridge is 24 Ohm
Nope.
Quote:
Originally Posted by Javs View Post
4. Am I ok with 0.5w resistors on an XLR cable that could see as much as 2v?

Thanks!
.25W would be fine. When calculating resistor power dissipation, you have to figure power (voltage X current) not voltage alone.

I've built these pads inside XLR connectors. You can make an XLR cable and put the pad in the male end. But only if you use .25W resistors!
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Old 3rd June 2020, 10:03 PM   #3
Javs is offline Javs
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Join Date: May 2015
Quote:
Originally Posted by jaddie View Post
No. 120 ohms is the PA2 source impedance. You're not trying to match that, you're bridging it. No pro audio device made in the last 40 years is intended to drive a matching load. An H pad is an impedance matching pad. In this application you need a "U" pad.



A matched load is where the output impedance of the device is the same as the load, (600 ohm source driving a 600 ohm load). The result of this match is maximum power transfer to the load, and that's great if the system is designed for power distribution. This caused a problem in large pro audio installations because it meant output amps all had to drive only one 600 ohm load, and the performance of the output amp changed if it wasn't loaded exactly right. Pro audio moved to bridging loads and voltage distribution decades ago. In voltage distribution you have a low source impedance driving a high impedance load, "bridging" being 10X the source Z or higher. Now we have output impedances, like the Drive Rack, with 120 ohms source Z, but it can't actually ever drive 120 ohms, that's only it's source Z. It expects something much higher, like 5K or more. It's "voltage distribution" because the output voltage is transferred without concern for the power transferred. You can attache several devices to one output that way, which helped out in pro installations a lot.



So the pad becomes a "U" shape, a resistor from each output terminal of the Drive Rack, then a load resistor across the input to the amp. Since you want 20 dB, standard R values of 5K from each output leg, and 1K across the amp input would be very close.



You want a "U" pad, as I outlined above.

No. You do not want to match the output Z of the PA2.

No. Again, see my above reply.

Nope.



.25W would be fine. When calculating resistor power dissipation, you have to figure power (voltage X current) not voltage alone.



I've built these pads inside XLR connectors. You can make an XLR cable and put the pad in the male end. But only if you use .25W resistors!
Thank you that really explains a lot.

So just thinking about the pad calculators, what impedance should I be aiming for? What are your values of 5k/5k and 1k bridge roughly shooting for?

One last question, is the U shape pad reversible?

If I go female into the back of the pa2 and male to the power amp, and put the pad as you describe what if I down the track end up using the attenuators the other way around, does that do anything bad or reduce its efficacy?
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Old 4th June 2020, 05:13 AM   #4
jaddie is offline jaddie
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Join Date: Apr 2011
Quote:
Originally Posted by Javs View Post
Thank you that really explains a lot.

So just thinking about the pad calculators, what impedance should I be aiming for? What are your values of 5k/5k and 1k bridge roughly shooting for?
You’re not targeting an impedance at all, you’re building a voltage divider. As long as the load is high relative to load resistor in the attenuator, it won’t be a factor. As in the values I gave you.

Quote:
Originally Posted by Javs View Post

One last question, is the U shape pad reversible?
No, a U attenuator only work one way, when it works as a voltage divider.

Quote:
Originally Posted by Javs View Post

If I go female into the back of the pa2 and male to the power amp, and put the pad as you describe what if I down the track end up using the attenuators the other way around, does that do anything bad or reduce its efficacy?
The U attenuator only works one way.

Also, by convention, the signal flow in XLR connectors is out of the male, into the female. You won’t be able to use your pad cable backwards.
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Old 6th June 2020, 04:30 AM   #5
Javs is offline Javs
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Join Date: May 2015
Just an update, I made a 10kohm pad and it worked perfect. All the hiss is gone, just about exactly 20db, made 9 of them and they are all perfectly matched, no measurable detrimental affect to sound response at all.

Used two 4.7kohm and 1kohm for the bridge.
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Old 19th February 2021, 01:30 PM   #6
lizhuoyin is offline lizhuoyin
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Join Date: Feb 2021
Default Is it 1K instead of 10K?

Is it 1K instead of 10K?
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Old 19th February 2021, 10:10 PM   #7
Rick PA Stadel is offline Rick PA Stadel  United States
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Join Date: Apr 2017
Location: greater Kansas City
No. Usually we speak of the load it presents to the driving/source device -- in this case about 10,4k ohms. Since the driven device typically has 5x to 25x higher impedance than the 1k ohm 'bridge' resistor, its effect is generally ignored.

Attenuation for this set (again, ignoring the load) is 20,34dBV.

Concerning 'U' versus 'H' pads -- the only way you could need to flip it ('run it the other way'), is if you have equipment that has wrong gender/signal-flow XLRs. I recommend fixing/correcting that equipment, rather than expecting pads and adapters to accomodate

One other thing I'd like to point out: If you build an attenuator into a male XLR, use something besides felt-tip or MagicMarker to identify it! After many years, and lots of wear and tear, you can have a real head-scratcher trying to sort why that dirty-rotten-cable seems to have attenuation!!

Cheers
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Old 19th February 2021, 10:15 PM   #8
lizhuoyin is offline lizhuoyin
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Join Date: Feb 2021
Thanks. Sorry about the confusion. I was talking about 10kohm OP talked in his post. It looks like he's talking about the shunt resistor and it's 1kohm.
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