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Fixing The Echo Mona Preamp Gain Controls

By Sid of Stone Marmot

April 19, 2009

The Echo Mona is a digital recording studio computer input/output board. It had four A/D converter inputs, each with a preamp, and six D/A converter outputs, all with maximum 24 bit resolution and up to 96 kHz sampling rate. It is a fairly decent unit that had gotten good reviews when it first came out. I've been happy with mine except for one thing: The preamp gain controls are unreliable.

The gain controls started getting hard to set about a year after buying the unit. A small change in a gain control would give little gain change and then suddenly result in a big change in gain. The gain would also jump you removed your hand from the control. Over the years these controls have gotten so bad that it is virtually impossible to set any reasonable gain levels. The problem is really apparent when I am recording a stereo input and I'm trying to match the channel levels for this stereo input.

Usually this is an indication that the control is dirty. Ordinarily you could squirt some contact cleaner into the control, turn it from end to end rapidly a dozen or so times, and the problem would be fixed. But these controls are the small plastic type with no obvious opening to squirt contact cleaner into.

Figure 1 shows the top of the Echo Mona circuit board. To get at this board, I had to remove virtually every mechanical fastener in the unit. Not very service friendly! The controls of concern are the small black and silver rectangular objects with shafts sticking out of them just to the right of the big black connectors. The fifth control at the far right of the board is the headphone level control. I've never had any problems with this control. All of these controls are mounted in the unit strictly by soldering the leads to the printed circuit board. This is not a very secure mounting technique. Fortunately, I haven't had any problems yet with broken leads or solder joints.

Figure 1 - Echo Mona circuit board top

Figure 1 - Echo Mona circuit board top

Figure 2 is a close-up of the gain control for channel 4. There is no obvious way to squirt electronic contact cleaner into the control. I tried squirting cleaner onto the control outside, hoping enough cleaner would seep through any material interfaces to get inside the control. But this did not seem to work.

Figure 2 - Echo Mona preamp gain control

Figure 2 - Echo Mona preamp gain control

The control is a 1.5 kohm reverse taper control. Two of the leads are tied together so that the control is acting as a variable resistor, not a potentiometer. The taper has a rather extreme variation, as the control changes from 1500 ohms to 200 ohms in less than a quarter turn, to about 50 ohms at half turned, and changing the remaining 50 ohms over the remaining half of the control shaft rotation. When adjusting, I saw the control jump 5 to 10 ohms for small rotations of the shaft. The resistance would also jump 5 to 10 ohms when I removed my fingers from the shaft. These kind of changes are significant when operating in the upper half of the control range.

I never use the controls in the first quarter of a turn area. Since I usually feed synthesizers, which all have their own output volume controls, into channels 3 and 4, I really don't need the gains of these channels to be variable. This would also be true if you normally feed external preamps into these channels and just want a reliable line input. So I could replace the channel 3 and 4 preamp gain controls each with a fixed value resistor. So that is what I effectively did: I installed a 61.9 ohm resistor across the terminals of each control for channels 3 and 4. If these controls are turned full counterclockwise (minimum gain), the result is the the control's full 1.5 kohm resistance is in parallel with the 61.9 ohm resistor for about 59.4 ohms equivalent resistance. This is about the same as if the control where normally turned up about half way. Even if the control were measuring about 1300 ohms, 1300 ohms in parallel with 61.9 ohms is about 59.0 ohms, less than a 0.4 ohm, or less than 1 %, change. This is acceptable.

The gain can still be adjusted a little. The minimum gain for these channels is now what used to be the midpoint gain before the resistors were added. 5 to 10 ohm jumps in the control now have a less significant impact on the channel gain for most of its range. Only at the extremely high gains, which I rarely use, do the control gain jumps become a problem again.

Figure 3 shows the underside of the Echo Mona circuit board with these resistors soldered across the terminals of the preamp gain controls. Figure 4 shows a close-up of one of the resistors. The top row of three pins on the control are for mounting. The bottom row of pins are the active resistance, with the two leftmost pins being tied together. So you can solder the resistor from the rightmost pin to either of the other two pins.
Figure 3 - Echo Mona circuit board bottom

Figure 3 - Echo Mona circuit board bottom


Figure 4 - Echo Mona with resistor installed across gain control

Figure 4 - Echo Mona with resistor installed across gain control

To modify your unit, adjust the control to about where you usually set it. Then measure the control's resistance. Choose a good quality resistor (preferably metal film) of approximately this measured value and solder across the control's terminals as shown in Figure 4. The resistor must be small as it needs to fit between the bottom of the circuit board and the chassis with some clearance when the unit is reassembled. Be sure to clean off all the residual solder flux so you don't have future noise or corrosion problems. Also, make sure there are no accidental solder bridges to or between the pins in the top row.

For the other two channels, I paralleled a 301 ohm resistor across each control. This gives about 244 ohms when the control is set for minimum gain, which is about the same as if the control were turned up about a quarter of the way before installing the resistor. Since this resistor has a much larger value than the ones installed in channels 3 and 4, channels 1 and 2 will be more sensitive to control variations than channels 3 and 4. But this is still much better than without the resistors, especially since I usually set these controls between 30 to 60 % of the way up before adding the resistors, so they were less sensitive to 5 to 10 ohm shifts in the control anyway.

This is not a complete fix. But it is a vast improvement and makes the Echo Mona very usable again. If you have decent soldering skills and the choice is either to do this fix or spend $500 to $1000 for a new computer audio interface, you may want to try this. It could save you a lot of money.

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