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|6th October 2009, 08:12 PM||#1|
Join Date: Oct 2002
Xenyx 802, removing the tone controls.
I decided to post this here, as I reckon most people who bought one of these did so to make measurements with the ECM8000.
Setting the tone controls to 'flat' (the mid position that the pots 'click' into), isn't bad, but it isn't flat. How far away from flat yours might be is entirely dependant on component tolerances, so it would make sense to bypass them entirely.
Removal of the side and back panels is easy enough. Once done you have access to the back of the main board. This lets you see almost all of the active devices - if you wanted to replace any of the opamps, this is all you'd have to do to get access. There are a couple of things on the top side of the board, but nothing to get excited over.
The opamps are duals from Cool Audio and if you desired would be fairly easy to replace. However I can't see any real reason for doing so, any noise or distortion is swamped by the mic.
If you want to get access to the top side of the board, you've got a task ahead of you. There's nothing hard about it, it's just a lot of work.
The first picture shows you what needs to be removed to allow the top of the chassis to come loose. This boils down to all the knobs and all the nuts holding the jacks in place. The screws with red arrows pointing to them need to be removed. The screws with the yellow arrows do not, leave these alone. They hold separators, that the backside of the PCB screws into.
Now to what you see once you've removed the side and back panels - the second attached picture.
The six screws that you can see are what screw into the separators. If you've removed all the knobs, screws and nuts from the top side and want to free the PCB, you'll need to remove these. However it is not needed if all you're interested in is bypassing the tone controls.
The area inside the yellow oval is the area we are interested in.
The third picture shows that area but enlarged some (and I discovered paints arrow function)
Lets start off with the opamp on the top right hand side, the one with the arrow pointing at it.
The balanced output from the mic pre feeds into pins 5 and 6, where the opamp converts it into single ended. Pin 7 is the output and is what we are interested in.
The other side of the opamp, pins 1,2 and 3, work in inverting configuration (to allow for negative gain) and are what works the tone controls.
Pin 4 is the negative supply, pin 8 is the positive supply.
The trick is to find where the output of the tone control section ends and the next stage begins.
Pin 1 of the opamp is one side of the output, the other side, after all the gubbins in the feedback, is the black arrow with the yellow boarder. This is the point where the tone control section ends, ready to drive the next section.
As can be seen there is a pad at this point, and directly above it, there is another one - the yellow arrow with the white centre pointing at it.
On the other side of the board you will find a cap at this point - a 10uf 25v polarised lytic acting as a hi pass to block any DC. If you remove this cap, you also break the connection from the tone controls to the next stage.
The easy way to bypass the tone controls is to cut the trace directly to the right of the pad that the black arrow is pointing at, thus cutting off the signal going from the tone controls and into the cap. Now all you need to do is feed the clean signal (before the tone controls) into the point where the black arrow points towards.
This means you need to solder a small wire from the pad the black arrow points at. Then you solder the other end of the wire onto pin 7 of the opamp, where the red arrow points.
If you don't want to hack into the PCB, follow the above to remove the top of the chassis and de-solder the capacitor.
Then resolder the cap on the bottom side of the board, with one leg soldered to the pad that the white arrow points towards. Then solder a wire from the other leg on the cap to pin 7 (red arrow) of the opamp.
You could always leave the cap out entirely, but it was obviously placed there for a reason and I'd rather leave it in.
If done as is detailed here you should find that the tone controls in channel 2, the second mic input, no longer function. I decided to leave channel 1 alone, so I'd have both with and without. If you want to alter channel 1 instead, it's identical to channel 2. I'm sure you can see that the circuit directly below the one with all the arrows is exactly the same.
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Last edited by 5th element; 6th October 2009 at 08:15 PM.
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