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woodrough 27th April 2013 02:17 PM

Ribbon Microphone Preamp
Hi, Perhaps this may be a quick question:

So I am in the process of designing a ribbon microphone. I am getting alot of my info from "" and was wondering what a suitable preamp would be for a 4mm wide 2.5u thick ribbon in between two neodymium magnets (2"x1/4"x1/4").

I prefer a simple and minimal component count preamp design and I think I landed on one. Check out the image below. A single pentode wired like a triode? Whatever it may be, I enjoy the concept of single stage amps.

Being that I didnt fabricate the microphone yet, I dont know the impedance of the ribbon. But when I do measure the impedance of the microphone, what is the process for setting the proper resistor value at the input to match the impedance?

I also heard somebody mutter the idea of using a potentiometer to accommodate a wide range of impedances. But my question as well is how do you tell by ear that the microphone is loaded properly?

Some Preamps have input attenuation and output (gain) control. Should I consider all that? This one schematic I supplied doesn't feature any of those.

SY 27th April 2013 02:24 PM

This will be extremely noisy. A ribbon impedance is likely to be an ohm or less, and the equivalent noise resistance of a pentode will be 1k or more. If you use a step up transformer (30:1 is typical), the source impedance seen by the tube is going to be several hundred ohms, which is at least in the ballpark for triodes- then you have to fight the effect of the input capacitance on the transformer... Tube preamps for low output, low source impedances are NOT trivial to design!

woodrough 27th April 2013 02:32 PM

Oh wait, I dont know if this adds to your statement, but on the microphone, there will be a step up transformer inside from Edcor.

This transformer is 1:37. The output DCR of the transformer is 87 Ohms. Will this make it more apropriate for a tube preamp to function less obnoxiously?

woodrough 27th April 2013 02:36 PM

Also on a second note, I am not married to the concept of using a 6AU6 pentode. I have some nice luxurious D3A tubes but that may require me to seriously reconsider every component surrounding it.

SY 27th April 2013 02:47 PM

D3a is a MUCH better choice. This still won't be ultra quiet, but it will be a phuc-ton quieter than a 6AU6!

You'll want a much quieter supply (DC on heaters, elevated heater potential, common mode filtering, snubbered HT secondary), since a pentode will not have good power supply rejection.

woodrough 27th April 2013 07:46 PM

If I were to use a D3A in this schematic, should I go about using this similar layout, (no cathode resister) etc or should I do cathode bypass like this:

How would you recommend going about redesigning for the D3A? What about impedance matching? attenuators for load matching?

The power supply I'm not too worried about, I have some chokes I can use etc.

Eli Duttman 28th April 2013 01:10 AM

RCA was/is famous for their "velocity", AKA ribbon, microphones. They used a SUT into a 6SL7. Other than RIAA equalization, this situation is quite analogous to that found in handling a LOMC phono cartridge.

If you're willing to "wrestle an alligator", consider the Russian 6C45П (6s45p), as the 1st tube the signal "hits". That tube, like the 5842, comes perilously close to oscillating, while being stored in its cardboard box. EXTREME measures that suppress parasitic oscillation are very much in order. Place a 5.6 KOhm carbon composition "stopper" on both grid connection points. Place a 100 Ω carbon film "stopper" on the plate connection point. Finally, place ferrite beads on the heater power wires, as close to the socket as you can get.

woodrough 28th April 2013 01:31 AM

Oh man, I never heard about this Russian magic before,
DIY 6c45 Amplifier
sounds like a cult classic. It probably isnt realistic to have a 6s45p be OTL right?

In fact, is there a OTL preamp thats minimal like this? Regardless, Im realy starting to dig this mini magic trick

Chris Hornbeck 28th April 2013 03:20 AM

You might end up going through *two* input transformers to get things acceptably quiet, one in the mic and one in the preamp. Ribbon mic output levels are measured in mouse farts per furlong, and that's on a good day. Sounds like a fun project, fersure. What are you using for magnets?

All good fortune,

woodrough 28th April 2013 04:42 PM

I figured a output transformer on the preamp would be necessary,

The two magnets would be 2" x 1/4" x 1/4" neodymium bars.

I figure Edcor would be a healthy solution for all transformers required

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