AM508E Potentiometers
The AM-508E microphone from Adonis is in my opinion a great success. We can always criticize, of course, but strength to recognize its assets and the excellence of modulation reports obtained.
A user of my friends asked me one day if you could have the compression level adjustment as well as the output level in a more accessible way than by the bottom of the microphone.
In addition, in the low position, compression can be deemed insufficient while in Hi position, it is downright too much. Why not make this setting variable ?
It is all the more possible as the existing switching only connects C4 to the ground via a 560 ohms resistor in the Low position and a 10k in Hi position.
The idea is therefore to leave the Hi position with its 10K resistor (R8) in order to be able to switch to 45 dB if desired. And in the low position, instead of R7, put 270 ohms in series with a 4.7k potentiometer and thus allow a adjustment range between a few dB and about 30 dB.
As for the adjustable output level via a small variable resistor to which it is necessary to access with a screwdriver, this can be considered unsappointed depending on the use. Replacing it with a potentiometer accessible on top of the microphone is indicated.
The proposed modification
It consists of the addition of two miniature potentiometers on the adjoining flat part at the base of the swan neck. The available space is sufficient, both on it and inside, provided you stick to the 14 mm available under the battery pack.
The potentiometer on the left will be the one commanding the compression level and will be near R7 located on the edge of the PCB. The one on the right will settle the output level, also on the right side compared to the adjustable located on the PCB.
The most delicate operation, in fact, concerns the mechanical part. My project being to only exceed the end of the axis of each potentiometer to on the one hand allow the use of small button and on the other hand not have a visible nut.
Potentiometers
These are miniatures that are quite easily found on eBay (look for PMA4K7P, PMA1KP or RK097N). They have the interest of being very small, having a short axis and being able to be weld on a printed circuit.
Their support consists of three small tinned printed circuits. Two receive each potentiometer and the third links the other two and thus forms a small platform that will stick inside of the case. This allows only the axes to exceed.
Set up
The dimensions noted on the sketch opposite reflect the measured reality because I started by drilling the case before designing the rest.
Indeed, even by taking the greatest care to carry out the holes, adjustments are always necessary to obtain symmetry and this has an impact on the final difference between axes.
To allow an assembly so that the axes are perfectly parallel to each other and perpendicular to the support while respecting the gap between axes, I proceeded as :
A sheet of paper with a horizontal line surmounted by two perpendicular line spaced the calculated value so that once the printed circuits of the potentiometers are on these landmarks, the axes of potentiometers are at the right distance. Then a small double -sided tape glued on the intersections.
Each potentiometer is then glued vertically so that its printed circuit is in the right place, at the benchmark, while the potentiometer housing is aligned on the horizontal guide.
Then putting the printed circuit to connect the other two and fixing it by a weld point is a breeze.
The whole being now rigid sufficiently, it is held in a vice to finish the welding between the circuits. The longest circuit is then glued and the whole is set up and glued to the case.
It only remains to put the buttons and wiring.
Wiring
The old adjustable was previously removed and replaced by three wires, each end of which is welded on the 1K potentiometer.
Regarding the adjustment of the compression rate, R7 previously removed is replaced by a ground wire and a 270 ohms resistor on the other side. Their ends will weld on the second potentiometer (that of 4K7).
The connectors are put back in place and then the bottom plate.
Conclusion
I unfortunately cannot source the buttons used because I have had them for a long time, probably recovered in the past on a device dedicated to destruction. But this type of format is quite easily found for a 6mm striated axis. Prefer a button with a benchmark located on the top, where a hollow is present even leaving the possibility of filling it with paint of a color identical to screen printing, which will not fail to produce its Small "original" effect.
The tests were carried out with more than 150 watts and compression to the maximum without any HF return. It is always a risk of making changes to a microphone, especially a model with a plastic case completely devoid of shield like Adonis. On the other hand, the latter is on the one hand well designed with numerous RF decoupling, on the other hand the added links are sufficiently short.
In use, new settings have been much appreciated by its user who can now choose the compression level as well as the output level as a function of the transmitter/receiver, the mode used or the context more or less noisy ambient.
The buttons benchmarks have received a small touch of copper color paint which goes well with the serigraphy of the microphone.
