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DIY Shure A15AS In Line Pad clone Recorders, Players & Tape Machines
Old 29th July 2010
  #1
Here for the gear
 

DIY Shure A15AS In Line Pad clone

After searching endlessly online to see if I can find the resistance values inside this device with no luck I finally decided to pull out my trusty meter and find out for myself. As I'm not so great with all these calculators I found I could just copy something that already works fine and that everyone here likes. Besides I could make one for about $13 as opposed to the $35 to $40 price tag for a brand new one. Although mine is not switchable like the Shure (15-20-25db) so I just made a fixed @ 20db that is just as good. This website explains how to make one very well and if your into all the calculations then you'll be right at home:

Uneeda Audio - Build your own attenuator pads

All you need is a Switchcraft S3-FM XLR tunnel and three resistors. Here ther the cheapest xlr tunnel I found online:

Switchcraft S3FM 3-Pin Female to 3-Pin Male Barrel Adapter: SWIT-S3FM Dale Pro Audio

As far as the resistors go all you need is two 1/4w 430 ohm resistors. These two resistors go one between pin 2 and the other between pin 3. The 3rd resistor is to be connected across (shunt) pins 2 & 3. The value of the shunt resistor depends on how much db cut is desired. The following is a guide according to what the Shure attenuator had inside:

15db= 180 ohm
20db= 95 ohm
25db= 54 ohm

You need to wire it according to the "Balanced H" configuration from the Uneeda audio site. It is the first diagram on the right.

Piece of cake

Hope this helps someone out.

Peace, Aljam
Old 31st July 2010
  #2
Lives for gear
 
emrr's Avatar
That may well be what Shure does with respect to values, but I don't like it. Their previous design was an O pad (1960's?), with shunts on both ends, which presented more of a matching impedance in both directions. I have taken several of them apart and converted them to U pads instead.

You want to use a U pad rather than an H pad; but that's what you've described (3 resistors) anyway. You want the shunt to be in the neighborhood of 150-250, and generally no lower. You may get a wildly varying freq. response with a 54 ohm shunt feeding a transformer coupled preamp. You vary the series resistances to get varying loss. 150-200 ohms is a fairly safe starting shunt range for most preamps; Jensen recommends 169 ohms in their plans. Remember that the additive value of the 3 resistors determines the load presented to the mic, which is why 20 dB is such a standard. With a range of series resistor values, you will find that some mics will change in response noticeably, and others won't. 20 dB generally gives the least change across the highest number of mic types.

Be sure to buy a longer barrel to give yourself room to fit the resistors in between the connectors. The cheapest barrels out there have nearly no room between. The switchcraft barrels are long enough.
Old 3rd August 2010
  #3
Here for the gear
 

Thanks Doug

Great info Doug thanks for adding some good constructive information to this post. I'm "Shure" others will find this very useful as I did.

Aljam
Old 4th March 2011
  #4
Here for the gear
I know this post is a little old now and probably not many people have the need or understand if they have the need for such a modification but i'd suggest to anyone if they have the need to add an attenuator pad to their setup they invest a little time doing the proper maths. It will make a huge difference and as "Emrr" pointed out, those values in the original post are probably too low for a shunt to match impedance with most mic gear.

Also just a note about the OPs measurements of the Shure resistor values. If the value of the resistors is measured IN CIRCUIT using a multimeter then its measurement will be affected by any other components in parallel with that component. You need to be sure nothing is sitting in parallel with the component being measured. This is where colour coding comes into play or desoldering the component to measure (which should only be done if you have the equipment and experience to desolder components without cooking your device)

The article posted in the OPs post has all the information needed and it will differ based on:

- The amount of -dB you require
- The impedance of the source/destination equipment you need to match to.

To provide a bit of an idea:

I needed to attenuate the output from an SD MixPre to the input of a Zoom H4n. The output and input are line level signals so this differs a lot from a mic signal due to the difference in impedance. The amount I needed to cut was -20dB. So where did I start? By looking up the input Impedance of the H4n. Most audio gear will provide the output/input impedance figures (unless it's really cheap gear) which you do need to match equipment effectively. The maths is pretty simple if you follow the examples they have and there is even a log table to help you out there if you don't know how to use a calculator (though even the windows scientific calculator will work fine).

Without rehashing whats on the page I know that "K" in my formula is 10 and the input impedance on the H4n (which is what I wish to match my pad to) is 1K Ohm. When padding Mics you probably want to match the impedance to the Mic but for line level signals the recommendation, according to the information at the site is you want to match the destinations impedance

The impedance that will be matched will be the value of the Rshunt resistor. Using the formula:

K = 1 + (Rseries/Rshunt)

We get:

10 = 1 + (Rseries/1000) Or to rearrange the formula to find Rseries:

Rseries = Rshunt x (K-1)

Since K-1 is 10-1 = 9

Rseries = 1000 x 9

Rseries = 9K Ohm

For a balanced XLR circuit we actually need 1/2 of Rseries (because there are 2 signal lines to deal with) so we simply halve the value and place one resistor on pin 2 and one on pin 3. Rshunt is then placed across pins 2 + 3 BEFORE the series resistors (refer to the images on the page).

1/2 of Rseries = 4k5

So finally I get:

Rseries = 4k5 (2 of these)
Rshunt = 1k (1 of these)

Which reduces my output signal by -20dB which then gives me a matched output level to the H4n Input allowing me to use the MixPre with limiters. Without this, the H4n will clip out way before the MixPre even at very low input volume.



A selectable pad switch could be included into this system by re-calculating the values for the Rseries resistor but you would need double pole switches to change the values.

It could be said that you "can" change the Rshunt value but this would then cause impedance mismatch and I am sure other issues would then occur.

Hopefully all this is correct and helps someone somehow. I do have an electronic engineering degree but haven't used much of this in many years.
Old 8th March 2011
  #5
Lives for gear
This is cool stuff, I'm wiring up some neotek strips and I need a way to wire up a switch that passes the signal fine, then when switched on it has a -20 db pad on it. I dont want to hijack the tread, but I think this goes hand in hand. does any one have a diagram of something like this? arty
Old 11th April 2018
  #6
Gear Nut
 

for this cable (Sescom LN2MIC-35DB-3 - Attenuator Cable for Tascam LN2MIC-35DB-3) from a Tascam dr100 to a canon 5d mk3, what are are the impedance of both the devices?
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