midwayfair
Well-known member
Hi, everyone. New here but not to DIY. (I build and design guitar stuff.)
I had some questions about the Scoeps circuit, Dorsey's variation of it, and a couple other ways to use it. I've read as many threads as I could find, but an awful lot of them seem to take some knowledge as read, which makes it hard to suss out a few things.
Here's the original circuit (large image):
http://www.sdiy.org/oid/mics/Schoeps.gif
Here's Dorsey's modified version, from Making Mics Better:
First, I have to say, I find this circuit incredibly elegant in the way it uses the output buffers to create the supply voltage for the input amplifier. I feel kind of silly looking for ways to change it, but some things are bugging me.
I'll run through my questions one at a time.
1) The charging voltage for the cap: Dorsey's circuit creates a 35V circuit, from which an incredibly tiny amount of current and voltage is applied to the center of the capsule. But in the original Schoeps circuit, one side of the capsule is held above ground by a 1M resistor (0v, but not GROUNDED grounded), which lets them switch in a cap for a pad (or boost?), and the center of the capsule goes directly to the gate of the FET. The center of the capsule in the original is biased with a tiny amount of current through the 1G gate resistor.
2) It seems like Dorsey's uses not only a few extra resistors to create the 35V supply voltage but that extra 1G resistor is quite expensive (I'm seeing them for $13 each at mouser). Was this likely so he didn't have to put any current/voltage on the FET's gate at all, or is there some sort of benefit to using a higher (35V) reference voltage in the Dorsey circuit?
3) The Schoeps circuit used a trimpot for the reference voltage on the gate/capsule -- could I do anything meaningful here when setting this using just my ears, or is there a nominal setting?
4) Can I use the Schoeps circuit with a ribbon mic with a transformer (either 1:35 or 1:110) as follows: Omit the 35V reference voltage; use a 1M for the gate resistor; ground one transformer lead and send the other to the input cap (to make sure there's never any DC on the ribbon). Is there any danger to the ribbon if I do this? Would there be there any benefit at all to doing that over, say, PRR's circuit here (which has enough gain for what I want and is super tiny):
http://groupdiy.com/index.php?topic=5743.80
[Tangent: Somewhere in that thread, though I couldn't find exactly where, it was suggested that the input caps could be left out of that circuit because the voltage should be identical on both sides of the . Is that safer than having them in, like if charge builds up on one side of them (in which case -- couldn't I just put a pull-down resistor from in1 and in2 to ground?
One last thing: Before someone just says, "just go experiment" -- which normally I'm happy to do -- I can't cut my own ribbons, and they're really frigging expensive and not the sort of thing I can replace at the drop of a hat if I blow one up while messing around. If there's a danger to the ribbon with something I've described above, I'd really appreciate it if someone just said so.]
Thanks for any help you can give.
Also, this forum's verification images are literally impossible to read.
I had some questions about the Scoeps circuit, Dorsey's variation of it, and a couple other ways to use it. I've read as many threads as I could find, but an awful lot of them seem to take some knowledge as read, which makes it hard to suss out a few things.
Here's the original circuit (large image):
http://www.sdiy.org/oid/mics/Schoeps.gif
Here's Dorsey's modified version, from Making Mics Better:
First, I have to say, I find this circuit incredibly elegant in the way it uses the output buffers to create the supply voltage for the input amplifier. I feel kind of silly looking for ways to change it, but some things are bugging me.
I'll run through my questions one at a time.
1) The charging voltage for the cap: Dorsey's circuit creates a 35V circuit, from which an incredibly tiny amount of current and voltage is applied to the center of the capsule. But in the original Schoeps circuit, one side of the capsule is held above ground by a 1M resistor (0v, but not GROUNDED grounded), which lets them switch in a cap for a pad (or boost?), and the center of the capsule goes directly to the gate of the FET. The center of the capsule in the original is biased with a tiny amount of current through the 1G gate resistor.
2) It seems like Dorsey's uses not only a few extra resistors to create the 35V supply voltage but that extra 1G resistor is quite expensive (I'm seeing them for $13 each at mouser). Was this likely so he didn't have to put any current/voltage on the FET's gate at all, or is there some sort of benefit to using a higher (35V) reference voltage in the Dorsey circuit?
3) The Schoeps circuit used a trimpot for the reference voltage on the gate/capsule -- could I do anything meaningful here when setting this using just my ears, or is there a nominal setting?
4) Can I use the Schoeps circuit with a ribbon mic with a transformer (either 1:35 or 1:110) as follows: Omit the 35V reference voltage; use a 1M for the gate resistor; ground one transformer lead and send the other to the input cap (to make sure there's never any DC on the ribbon). Is there any danger to the ribbon if I do this? Would there be there any benefit at all to doing that over, say, PRR's circuit here (which has enough gain for what I want and is super tiny):
http://groupdiy.com/index.php?topic=5743.80
[Tangent: Somewhere in that thread, though I couldn't find exactly where, it was suggested that the input caps could be left out of that circuit because the voltage should be identical on both sides of the . Is that safer than having them in, like if charge builds up on one side of them (in which case -- couldn't I just put a pull-down resistor from in1 and in2 to ground?
One last thing: Before someone just says, "just go experiment" -- which normally I'm happy to do -- I can't cut my own ribbons, and they're really frigging expensive and not the sort of thing I can replace at the drop of a hat if I blow one up while messing around. If there's a danger to the ribbon with something I've described above, I'd really appreciate it if someone just said so.]
Thanks for any help you can give.
Also, this forum's verification images are literally impossible to read.