Convert an ARTcessories ProMix 3 from line-level to mic-level output
Unfortunately, much of the advertising for the ProMix 3 (formerly the MQS-74) states that the output is
mic-level when in fact it is line-level.
So, if you bought one thinking it was a mic-level output and now you are disappointed fret not. There is a simple and quick fix (as long as you know how to wield a soldering iron with at least minimal skill). All we need to do is add a 30 dB pad to the XLR output to make the ProMix 3 output compatible with inputs expecting mic-level signals.
Crack open the case (standard disclaimers apply: you probably void your warranty by doing this, blah, blah, blah; I can't take responsibility if you undertake this activity). You will find that you need to remove the 4 screws on the side, and the 8 (or possibly 4, on the older models) screws going into the XLR connectors on the back. Note that you do NOT need to remove the screw above the power connector. You also need to remove the volume knobs (just pull them off) and then remove the nuts on the volume pots on the front of the unit.
You can now disassemble the case into two pieces. NOTE that that screw above the power connector that you didn't remove is connected to a case ground wire that goes to the center of the board. Make sure you don't rip that out of the board.
Inside you will find an extra trim pot between the headphone connector and the transformer that goes to the XLR output. This trim pot is inline with the XLR output. This is 1/2 of what you need to convert the line-level signal to mic-level.
You will need to add a 220 ohm resistor between the two input pins of the transformer. Do this on the back of the board. You can easily pick out the input pins of the transformer. Make sure that you lay the resistor down flat against the board as there are no standoffs between the bottom of the board and the case. You don't want your new resistor to short to the case. This picture shows how the resistor should look after it is soldered in place:
Next, you will need to adjust the existing trim pot to give a resistance of 6.73K ohms (6730 ohms). Connect a VOM (volt-ohm-meter) between the output (where you soldered the resistor) and the input of the trim pot (these are marked as "Test Point" in the picture above). While you have the VOM connected, turn the trim pot until the VOM reads 6.73K ohms.
That's it for the modification! Reassemble the case and start using your new mic-level output.
Bonus - The modification you just did does not change the level of the headphone connector. This is still a line-level output so you can still use your mixer for line-level applications by connecting to the headphone jack.
The combination of the existing trim pot and the 220 ohm resistor forms a voltage divider attenuator. See the Uneeda Audio - Build your own attenuator pads page for a discussion of how an attenuation pad works. Here is a schematic of what you are creating:
The exiting pot is the series resistor and the new 220 ohm resistor is the shunt. Together they form a 30 dB "L pad" (so called because the relative positions of the two resistors from an 'L'). That seems to be adequate to reduce the level to something near mic-level.
The Uneeda Audio page has a more detailed explanation of the theory, but let me go over the basics here. Note that I didn't create this, I just lifted it from the Uneeda Audio page and edited it a bit.
- Convert 30 dB to a ratio. The formula is: k = 10^(db/20). That's 10 to the power (db/20). The calculator says 31.6. 30 dB is a ratio of 31.6:1.
- To achieve this value of loss, the resistors that make up the voltage divider need to have values that fulfill this relationship: k = 1 + (Rseries/Rshunt). But where do you start? In this case we have some parameters that are determined by our existing hardware. The existing trim pot adjusts from 0 to about 8.7K ohms and we want Rseries to be much higher (a few orders of magnitude) than Rshunt so let's pick a few values for Rseries and calculate Rshunt from that.
- Using algebra, manipulate the formula to solve for Rseries when we know Rshunt and k.
Rseries = Rshunt * (k - 1).
Now repeat that process to solve for Rshunt:
Rshunt = Rseries / (k - 1).
Since k = 31.6, k-1 = 30.6.
- So, Rseries = Rshunt * 30.6 ohms. In other words, the series resistor has to be (roughly) 30 times larger than the shunt resistor.
- Likewise, Rshunt = Rseries / 30.6 ohms. Knowing the possible range for Rseries (0 - 8.7K) we can calculate the minimum and maximum values for Rshunt using the above formula:
Rshunt = 30.6 / 30.6 == 1 ohm (a bit to close to a short for my taste)
Rshunt = 8700 / 30.6 == 284.3 ohms
- Resistors only come in particular values, so there are only certain values that will work for Rshunt (without lots of extra effort).
- Let's try 100, 150, and 220 ohms for Rshunt.
Rseries = 100 * 30.6 == 3060 ohms
Rseries = 150 * 30.6 == 4590 ohms
Rseries = 220 * 30.6 == 6732 ohms
- Given that the max value of the pot (Rseries) is 8.7K, I wouldn't go higher than the 220 ohm value so we have some movement room.
- Now lets examine the reverse. If we have the 220 ohm resistor in for Rshunt, what values of attenuation can we expect to get by adjusting Rseries?
The steps we need are:
- Get the divider ratio by dividing the series leg value by the shunt leg value.
- Convert this to K by adding 1.
- Convert K to dB by calculating:
- dB = 20 * log(k) : read dB equals 20 times the log of K.
Let's try some values: k = 1 + (Rseries / Rshunt)
k = 1 + (1100 / 220) == 6
dB = 20 * log(6) == 15 dB pad
So, setting the pot to 1100 ohms creates a 15 dB pad.
k = 1 + (2200 / 220) == 11
dB = 20 * log(11) == 20.8 dB pad
k = 1 + (4400 / 220) == 21
dB = 20 * log(21) == 26.4 dB pad
k = 1 + (8700 / 220) == 40.5
dB = 20 * log(40.5) == 32.2 dB pad