Tube Ribbon Mic Schematic

[Marik]

Wouldn't this just be simpler to build a tube preamp optimized for ribbon mics and use your mic in front of it?

The main purposes of making ribbon mics active are
1. To ensure an optimized load on the ribbon, effectively decoupling it from the preamp input.
2. Decouple long runs of the mic cable, so its LCR doesn't affect the ribbon performance.
3. Put much less stress on the 'high quiet gain' of the preamp requirement.

Best, M

 

[Marik]

After a little more thought, I think that speaks against my idea. You can't beat a 1:35 transformer in terms of noise. No matter how good the tube is.

Many years ago I made LOTS of experiments of different configurations, including transformerless, tube and solid state topologies, ground grid (including with low ratio transformers), cathode follower, etc. The best compromise in terms of sound quality vs noise performance I found is making entire gain in a very high ratio transformer, followed by a unit gain buffer--that's what we use in our active mics.

Best, M

 

[rock soderstrom]

The main purposes of making ribbon mics active are
1. To ensure an optimized load on the ribbon, effectively decoupling it from the preamp input.
2. Decouple long runs of the mic cable, so its LCR doesn't affect the ribbon performance.
3. Put much less stress on the 'high quiet gain' of the preamp requirement.

That sums it up pretty well.

I'll allow myself to copy a few other Marik quotes into this thread, which helped me as a ribbon newbie to the subject:

In a ribbon microphone the load through the transformer comes back to the ribbon and effectively controls its motional impedance, restricting its movement. Cable capacitance, resistance, and inductance make this load frequency dependent. To more accurately calculate it you need to take the equivalent schematic of the cable and calculate it as a LCR (including loading impedance) filter.

In order to smooth out resonances and get a flat response the ribbon must be loaded both acoustically and electronically. It is important to note both of those have similar effect. In a well designed ribbon microphone acoustical load is calculated so that electronic one is about 10 times of the ribbons impedance (after translated through transformer ratio).

Say, the ribbon impedance is 0.15 Ω, so with a 1:36 transformer it is about (considering the losses) 200 Ω output impedance. Say, with a typical 1,5kΩ-3kΩ input impedance of modern pres the ribbon sees a good and comfortable x10. If we load the transformer with 250 Ω then in parallel with (say) 3kΩ it's already some 230 Ω. Then the ribbon sees it as 230/36^2=0.18 Ω and its motion becomes quite restricted severely affecting sonics and performance.

For long runs of cables we recommend only active ribbons.

 

[rock soderstrom]

Many years ago I made LOTS of experiments of different configurations, including transformerless, tube and solid state topologies, ground grid (including with low ratio transformers), cathode follower, etc. The best compromise in terms of sound quality vs noise performance I found is making entire gain in a very high ratio transformer, followed by a unit gain buffer--that's what we use in our active mics.

Thank you for sharing your experience with us! I've just posted some other quotes from you here to complete the picture. Thanks again.

 

[Tubetec]

Im seeing a similar effect in a reverb recovery transducer , I thought Id throw my thoughts about it in here as there are many parralels to ribbon mics.

The effect of load and cable capacitance changes the way the springs responds to stimulus , its clearly visible in REW . As Ive done stuff to reduce cable and transducer capacitance the movement of the spring opens up at higher frequencies , its like the effect of extra capaciatance is its trying to choke the movement.

I checked the transducer on the DMM /LCR , came out at
370 ohms dc
380@100hz
470@1000hz
[email protected]

So the inductor has an impedence that rises with frequency , typically you might see this transducer loaded with 220k -1Meg ohms and around 1meter of low grade co-ax , it more or less presents a dead short to HF , no wonder it sounds bad too .

I have few Reslosoound ribbons with hi/lo-z output ,
The idea of a cable with an inline cathode follower or plugin Fet module could perhaps give new life to these type of mics ,
With the off the shelf mic fet boosters your stuck with around 20kohms at the input , bridging off a hiz stepup transformer you might want higher, a lot higher.

Thanks again Marik ,
sums up the situation very nicely .

 

[Marik]

Im seeing a similar effect in a reverb recovery transducer , I thought Id throw my thoughts about it in here as there are many parralels to ribbon mics.

The effect of load and cable capacitance changes the way the springs responds to stimulus , its clearly visible in REW . As Ive done stuff to reduce cable and transducer capacitance the movement of the spring opens up at higher frequencies , its like the effect of extra capaciatance is its trying to choke the movement.

I checked the transducer on the DMM /LCR , came out at
370 ohms dc
380@100hz
470@1000hz
[email protected]

So the inductor has an impedence that rises with frequency , typically you might see this transducer loaded with 220k -1Meg ohms and around 1meter of low grade co-ax , it more or less presents a dead short to HF , no wonder it sounds bad too .

I have few Reslosoound ribbons with hi/lo-z output ,
The idea of a cable with an inline cathode follower or plugin Fet module could perhaps give new life to these type of mics ,
With the off the shelf mic fet boosters your stuck with around 20kohms at the input , bridging off a hiz stepup transformer you might want higher, a lot higher.

Thanks again Marik ,
sums up the situation very nicely .

That makes perfect sense. It looks the raise is pretty steep at 7,8kHz. Our ribbons stay under 1kOhm up to 100kHz. But it is much easier task to make a smooth and well behaved system when dealing with such a low source impedance as a ribbon coupled to very high permeability of torroid transformer amorphous core we use. For measurements we use HP 4800A vector impedance meter and HP 4274A LCR meter. Both are awesome and show an angle, as well.

Best, M

 

[commanderberti]

next step! VRM-26 ribbon... + 1:2 transformer (from GAP73),1uf C8 and 120k grip to ground! sounds very good! S/N ratio ist 8 db better than with direkt GAP73 pre.. !

here a 2 samples VRM26

 

[Khron]

next step! VRM-26 ribbon... + 1:2 transformer (from GAP73),1uf C8 and 120k grip to ground! sounds very good! S/N ratio ist 8 db better than with direkt GAP73 pre.. !

here a 2 samples VRM26

What about without that GAP73 transformer? Isn't there already one inside the VRM26?

 

[commanderberti]

What about without that GAP73 transformer? Isn't there already one inside the VRM26?

yes vrm26 has a tranformer inside, but levels&sound in tube "mode" ar better with a 1:2 transformer behind the vrm-26 transformer to push the grid level..