RT-RMP01 Internal 30dB Gain Stage for Ribbon Microphones
P48 phantom powered gain stage.
This design is a low-noise internal gain stage for lower-output ribbon microphones which require an amplification of 60dB or more.
It reduces the gain required from the preamp or mixer, which may be less satisfactory at gains much over 40dB.
The circuit is intended to be fitted in the microphone housing near the transformer. It is possible to build it in an external metal box, in which case the connection to the microphone should be kept short. This will be satisfactory if done well, with all metalwork and connector shells firmly connected to circuit ground.
The circuit needs standard P48 phantom power and gives an accurately impedance-balanced output. It includes a position for an indicator LED which may be included if useful and also has positions for optional components to load the transformer secondary.
Dimensions 49 x 28 mm
Board constructed using LM394 transistor pair.
click image for pdf
Rebuilding a T-bone RB-500 with RT-RMP01 and Lundahl transformer.
The RB-500 has a well designed ribbon motor with a long fat ribbon, capable of response at much higher frequencies than most, and perfectly symmetrical so it's good for MS recording technique.
The rest of it is less good, there is a lot of metal grill stuff which needs throwing away and a transformer which won't win any prizes. The quality of the metalwork is adequate at best. At least it is big with a lot of room to spare in the base.
Lundahl LL2915 ribbon mic transformer fitted in base. Normally it would be mounted at the base of the ribbon assembly, but putting it here reduces reflections and improves symmetry.
New cable gland, also screw for attaching ground connections.
Thick plastic foam disk in base.
The PCB is connected to a short length of the original starquad. The LED is extended on some wires to go through a hole on the front of the mic.
Board in place with secondary connected.
More foam with primary wires sticking out. This type of stiff packing foam has low sound reflection properties and improves the sound of the microphone. Before modification the base was mostly empty.
The head basket and motor are connected to the primary.
Ready for testing.
Some out of tune noodling. This is a small 20W Fender practice amp, with built-in reverb and fuzz and rather a shrill sound. Here you can hear the pronounced prox effect of ribbon mics, and gentle fall-off at the high frequencies. The mic is six inches from the front of the amp, on centre.
This microphone has now been sold, ones like it are for sale occasionally or can be commissioned, please contact me by email if you are interested. Price range starting from around £300.
Frequently Asked Questions
1) That transistor pair is obsolete. (Stephen, Cambridge)
(written before I changed the spec to MAT12)
Yes it is. I designed this back when it wasn't, and the LM394 was still fresh
and minty. Use something else. I have good results with a couple of BC549C.
Also the MAT12 is interesting. The two transistors don't need to be matched.
2) I don beleeve it work u dont put no transistor lik dat an get teh low
niose amlifier innit? (Stan, Colchester)
Well yes actually, if you choose a suitable low noise transistor and set the
current in the correct area for the source impedance, you get close
to optimum results. If you resort to more expense and complication
you might get a little better, but I don't think anyone would notice.
3) You obviously don't know that the polarity of the output will depend
on how the transformer is connected, as you don't have any indication
of how it should be connected. (Henry, Oxford)
How clever of you to notice that. Have a gold star.
4) It is too big to go in the microphone. (Stephen, Cambridge)
So put it in a diecast box with a couple of XLR connectors. Then you can
put your name and logo on it, call it a "Cloudraiser" or "Shirtlifter"
or something.
5) it got a led in it i don fink u shud ave a led in it cuz leds is
niosy an that innit? (Stan, Colchester)
No it doesn't cause a problem. Replace with a wire link if you don't
like it.
6) i fink u shud av afet in it cuz fets got no gate curant an tranisters
put curant thruogh transformer an mak it magnet so it dont work proper
innit? (Stan, Colchester)
No, you will find that is not a problem.
7) I don't think you have studied the literature. Are you aware of the
amplifier in the National Semiconductor Application Note AN-222? A much
more elegant and sophisticated design with higher gain and
overall negative feedback. (Henry, Oxford)
Yeah... good luck with that.
8) I put it together correctly and it doesn't work. (Timothy, Newcastle)
No you didn't.
