Presenting "The Leslieator" - a work in progress
Posted: Mon Jun 17, 2013 11:40 pm
On Sundays I mix sound for a church and, while my setup is quite functional, I'm always trying to do a little more with a little less.
Case in point: I currently have a stereo mic setup on a very nice Leslie 122 cabinet (driven, just as it is ordained in the scriptures, by a very nice Hammond B3.) The resources I use are: three mics, three mixer channels and one mix subgroup. Too much for one instrument, as lovely as it is. I have used small electret condenser mics in my day job and there are many circuits that show how to use these mic elements for various purposes, so I started developing this pre-mixer to economize on mixer resources.
My goal with this "Leslieator" is to pre-mix a "Left" pair of electrets, crossed over at 800 Hz, and also to do the same with a "Right" pair. Each of the pairs will have one mic for the top half of the Leslie cabinet and one for the bottom half. Another goal, equally as important, is to learn how to design and build such a thing. The blend and EQ are all just preset with equal gain top and bottom. The 800 Hz second-order crossover point matches the crossover frequency used inside the Leslie to split the signal before feeding the split into the upper and lower rotating drivers. (The drivers don't actually rotate, but you know what I mean.) (Possible future features: variable top/bottom balance, also experimenting with other crossover filter choices.)
So, a simple 4-into-2 mixer, with preset EQ and level. Two channels into my main mix, and no need for further separate signal paths for the top and bottom of the Leslie. Full stereo swirl on both top and bottom. (Possible future feature: mono blend for the two bottom mics.)
The story so far: it's built and almost working, using 4558's on Radio Shack proto board in a small project box.
By "almost," I mean that all 4 mics are showing up where they should in the mix, but there is also an oscillation present in both channels, even when the project box is closed up, shielding the circuit.
In response to this problem I added the 230 pF and 560 pF caps seen in the feedback loops of the two op-amp stages configured for gain. This did not kill the oscillation. The 1000pF caps shown in the mic detail are soldered right to the pins on the electret capsule. The electret capsule is connected to the box by two-conductor, shielded cable, and the capsules are tucked into the covers from 1/4" connectors which make for a shielded "body."
Power supply bypass is made from V+ to V- directly under each 4558, 10 nF ceramics soldered to the bottom of the board. Layout is not too ugly as far as I can tell. I will post a build shot soon.
Do the mic cables need to be shielded for the last few inches (inside the box)? At the moment the cable shields end at a small sub-board that carries the mic wiring, and the shields all connect to audio ground there. Then the individual mic signals travel a couple of inches to their op-amp +ve inputs on a single conductor apiece.
Any suggestions on how to stabilize this circuit? I'm SOOO close!
(By the way, ignore the cap and resistor values as shown around the filter sections, they are set up for 1KHz as shown, but I've recalculated them for 800Hz. I recently learned that this is the correct crossover frequency used in the Leslie.)
EDIT: Schematic has correct filter components for 800 Hz and I have removed the R's shunting the virtual earth nodes to ground per JR's correction.
Thanks,
-j
Case in point: I currently have a stereo mic setup on a very nice Leslie 122 cabinet (driven, just as it is ordained in the scriptures, by a very nice Hammond B3.) The resources I use are: three mics, three mixer channels and one mix subgroup. Too much for one instrument, as lovely as it is. I have used small electret condenser mics in my day job and there are many circuits that show how to use these mic elements for various purposes, so I started developing this pre-mixer to economize on mixer resources.
My goal with this "Leslieator" is to pre-mix a "Left" pair of electrets, crossed over at 800 Hz, and also to do the same with a "Right" pair. Each of the pairs will have one mic for the top half of the Leslie cabinet and one for the bottom half. Another goal, equally as important, is to learn how to design and build such a thing. The blend and EQ are all just preset with equal gain top and bottom. The 800 Hz second-order crossover point matches the crossover frequency used inside the Leslie to split the signal before feeding the split into the upper and lower rotating drivers. (The drivers don't actually rotate, but you know what I mean.) (Possible future features: variable top/bottom balance, also experimenting with other crossover filter choices.)
So, a simple 4-into-2 mixer, with preset EQ and level. Two channels into my main mix, and no need for further separate signal paths for the top and bottom of the Leslie. Full stereo swirl on both top and bottom. (Possible future feature: mono blend for the two bottom mics.)
The story so far: it's built and almost working, using 4558's on Radio Shack proto board in a small project box.
By "almost," I mean that all 4 mics are showing up where they should in the mix, but there is also an oscillation present in both channels, even when the project box is closed up, shielding the circuit.
In response to this problem I added the 230 pF and 560 pF caps seen in the feedback loops of the two op-amp stages configured for gain. This did not kill the oscillation. The 1000pF caps shown in the mic detail are soldered right to the pins on the electret capsule. The electret capsule is connected to the box by two-conductor, shielded cable, and the capsules are tucked into the covers from 1/4" connectors which make for a shielded "body."
Power supply bypass is made from V+ to V- directly under each 4558, 10 nF ceramics soldered to the bottom of the board. Layout is not too ugly as far as I can tell. I will post a build shot soon.
Do the mic cables need to be shielded for the last few inches (inside the box)? At the moment the cable shields end at a small sub-board that carries the mic wiring, and the shields all connect to audio ground there. Then the individual mic signals travel a couple of inches to their op-amp +ve inputs on a single conductor apiece.
Any suggestions on how to stabilize this circuit? I'm SOOO close!
(By the way, ignore the cap and resistor values as shown around the filter sections, they are set up for 1KHz as shown, but I've recalculated them for 800Hz. I recently learned that this is the correct crossover frequency used in the Leslie.)
EDIT: Schematic has correct filter components for 800 Hz and I have removed the R's shunting the virtual earth nodes to ground per JR's correction.
Thanks,
-j