I've been thinking about building an active ribbon microphone enclosed in a small sloped enclosure made by Hammond (roughly 5" cubed, sloped front). The idea is that I want to power it off of 2 9v batteries, and be able to have a 1/4" output and an XLR output to plug into various preamps. It will usually be used for sitting on the floor directly in front of a guitar amplifier, though it would be cool if it sounds good for vocals as well.
In any case, after a lot of reading and redditing, I've cobbled together a design for a preamp that I'm hoping will suit my needs. It should provide ~40dB fixed gain to the 1/4" jack and an additional 6dB gain to the XLR jack.
I'm not certain how quiet it's going to be, and the folks on Reddit had mixed opinions. Here is a link to the schematic. What you folks think? Keep in mind, the ribbon motor driver will be going through probably a 1:37 transformer before going into the preamp.
Thanks for any input, I really appreciate it.
In any case, after a lot of reading and redditing, I've cobbled together a design for a preamp that I'm hoping will suit my needs. It should provide ~40dB fixed gain to the 1/4" jack and an additional 6dB gain to the XLR jack.
I'm not certain how quiet it's going to be, and the folks on Reddit had mixed opinions. Here is a link to the schematic. What you folks think? Keep in mind, the ribbon motor driver will be going through probably a 1:37 transformer before going into the preamp.
Thanks for any input, I really appreciate it.
modified schematic
Many thanks to user "spicy_hallucination" on reddit for some help with this circuit, I've made some changes:
Here's the new schematic.
Many thanks to user "spicy_hallucination" on reddit for some help with this circuit, I've made some changes:
- Using INA111 now, so we don't have to worry about 0.1% resistors.
- Stacking the batteries and using a divider, so we don't have to worry about one rail draining faster than the other.
- Getting rid of an extraneous resistor on the input to the balanced line driver.
Here's the new schematic.
So what is the impedance of the ribbon/transformer combination?
If you have a transformer on the input why use a differential configuration?
Have you considered the correct termination network on the transformer output for best frequency response?
If you have a transformer on the input why use a differential configuration?
Have you considered the correct termination network on the transformer output for best frequency response?
I'm expecting the impedance to be 25k+ but won't know until the ribbon motor is built. I've decided to change those input resistors to be 10M instead of 1M.
There probably won't be a lot of common mode noise, but I'm going differential just because. Also with a few modifications (lowering gain from 100 to 1 or 2 and adding protection diodes for bumps & jars) this could be an offboard preamp for a piezo transducer, so the common mode noise rejection would be more important then, depending on lead length.
Finally, I'm adding some series 1% 390pF caps before those 10M resistors to make a high pass filter at ~40Hz. Should be good all the way down to 4-string bass guitar, but still eliminate some rumble.
I think I'll make a sort of shock mount for the ribbon motor inside the enclosure, using springs. And the enclosure will also have rubber feet. But that rumble filter could still be important for a microphone designed to sit on the floor in front of an amp.
There probably won't be a lot of common mode noise, but I'm going differential just because. Also with a few modifications (lowering gain from 100 to 1 or 2 and adding protection diodes for bumps & jars) this could be an offboard preamp for a piezo transducer, so the common mode noise rejection would be more important then, depending on lead length.
Finally, I'm adding some series 1% 390pF caps before those 10M resistors to make a high pass filter at ~40Hz. Should be good all the way down to 4-string bass guitar, but still eliminate some rumble.
I think I'll make a sort of shock mount for the ribbon motor inside the enclosure, using springs. And the enclosure will also have rubber feet. But that rumble filter could still be important for a microphone designed to sit on the floor in front of an amp.
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