DIY Strat (and other guitar & audio projects)

The Muddy Basin Ramblers (http://www.muddybasin.com / https://www.facebook.com/muddybasinramblers) recently recorded their new album at Soundkiss Studios (www.soundkiss.tw / www.facebook.com/soundkiss) here in Taipei. Part of their gear consists of a vintage ribbon microphone fed into an old tube pre-amp, and unfortunately this was proving to be a bit difficult to control.

According to Alex from Soundkiss, “That OP 6 is a beast of a pre amp- it is always eager to distort and get dirty sounding. Without the attenuation, I have had to ride the gain control between 0 & 1 on the dial to get a usable tone, and a fraction of an adjustment can make a huge difference.”

So today’s post is about how we made some attenuators to go between the mic and the pre-amp in order to calm things down a little bit.

Firstly a little definition may be in order. An attenuator is basically the opposite of an amplifier. Its purpose is to reduce the amplitude of an electronic signal, without distorting it. Since it is not adding any power to the signal, it is possible to make them without any power source. In fact the design we will use requires three resistors only. The trick is working out what values these resistors need to be.

Also note that in this post i will be using "impedance" and "resistance" pretty much interchangeably, even though there is technically a difference.

This is the type of attenuator we’ll go with (known as a U-pad attenuator):

And this is where we sourced most of our information from for making these attenuators: http://www.uneeda-audio.com/pads/

Before working out the required resistor values, we need to know the output resistance of the microphone and the recommended input resistance of the pre-amp, both of which, in this case, were 250Ω.

The three resistors, incidentally, have names. The two that are listed as R1/2 are known as series resistors and the other one (R2) is known as a shunt resistor. The series resistors’ job is to reduce the signal (since this is a balanced signal, these two resistors should be the same). The shunt resistor’s job is to make sure that the output impedance is returned to something close to the recommended input impedance for the pre-amp (250Ω in this case). Note that simply sticking a 250Ω resistor in as a shunt resistor is an approximation, as the resistances of the series resistors and the original output impedance of the microphone can affect the overall resistance by working in parallel with the shunt resistor. However, as long as the series resistors are of a high enough value, the effect will be small enough that it does not cause a significant problem.

Using the link mentioned above (http://www.uneeda-audio.com/pads/), I came up with the following list of values (we decided to try 10dB, 20dB, 30dB and 40dB only at first). Note that there are some compromises going on here, but since we weren’t sure exactly how much attenuation was required, it really didn’t matter. The main priority was to keep the output impedances about right and also to make sure that the resistors were well matched in order to reduce picking up noise, etc.

As an example, let’s say we want a 20dB drop. As previously stated, we would like an output impedance of 250Ω, so we’ll just go with a 250Ω resistor as the shunt resistor (R2). Again, this is an approximation, but it’s close enough in this case. Looking at the table at that link, we see that for a 20dB drop, the “k” value is 10. R1 is the shunt resistor value multiplied by the “k” value minus one, so in this case it would be 250*(10-1) = 2250Ω. Of course each series resistor is half of this, so we will require two 1125Ω resistors, or as close to this as we can find. Note that since the series resistors have to be as well-matched to each other as possible, it’s wise to buy 1% tolerance ones (the shunt resistor’s accuracy doesn’t matter as much).

All right, let’s make an attenuator (I think the following photos might be from the 40dB one).

First we locate one of these XLR Tubes:

Mine came already wired inside (I think its original purpose was to swap two of the signal paths):

We remove one of the screws and open one end:

And then we unsolder the wires:

Then we remove the screw at the other end and open that:

We unsolder two of the wires but can leave the third one in place, since we’ll be keeping that:

Then we join the three resistors like so (the shunt resistor is the one in the middle):

We add some solder:

And then we solder them to the two remaining legs of the XLR connector:

Next we reconnect the two wires that we previously unsoldered to the ends of the series resistors (having slightly shortened them first):

Add some heat shrink to minimize the risk of short-circuits:

Twist the two wires from the series resistors together:

Feed the sleeve back over the wires and secure with a screw:

Solder the wires to the connector at the other end of the XLR tube (note that these wires did not go back in the same position that they were originally in, since, as previously mentioned, this XRL tube’s original purpose was to swap two of the signal paths):