M49 Alternate schematic for cardioid only

[Khron]

there would be no issue

... Apart from the extra hassle, but hey, if people insist on going that extra distance for no good reason, who am i to say no?

Most U47 & M49 microphones I see have the backplate connected to a floating point. I figured there must be a benefit...

Most of those copy the original designs "just because". The benefit being ("cosmetic") look-alike-ness to the original designs. But then again, i'm just a self-confessed technological "heathen"

 

[elskardio]

So I got a working mic and it sounds already great.

I have a few questions about the M49 circuit:

In his schematic, Oliver omitted the C4 (8pf) plate to grid capacitor. Here's what I found so far.

C4 ("abgleichen") is used to adjust the output level (Higher value of C4 = lower output. It is a form of feedback.
I suppose you don't need a fixed mV/Pa. ratio. In that case simply omit C4.

C4 is a balancing capacitor for the low end and high end , it is usually 8 pf but can be tweaked it can range from 8-12 pf IIRC.

So I guess it both affect the mic's output level and frequency curve?
With no capacitor I find the mic sounding "harsh". I've tested different values and I ended up with 5pF.

Other question.
The grid to ground resistor is 100M on Oliver's schematic. The original use 150M. I've seen a lot of different value on various take of this circuit. Anyone can comment on how they selected the value here?

Cheers

 

[LevinGuitar]

So I guess it both affect the mic's output level and frequency curve?
With no capacitor I find the mic sounding "harsh". I've tested different values and I ended up with 5pF.

Other question.
The grid to ground resistor is 100M on Oliver's schematic. The original use 150M. I've seen a lot of different value on various take of this circuit. Anyone can comment on how they selected the value here?

Cheers

Take a look at the datasheet of the tube you are using to know the max grid resistor value you can use. 200M is pretty standard maximum value for any tube, use it if you don't want to cut your low end. The small anode-grid capacitor lower the gain stage, so reducing a bit the gain, your trasnformer don't "saturate?" so easy and you may find the response curve to be nicer.

 

[rock soderstrom]

Take a look at the datasheet of the tube you are using to know the max grid resistor value you can use. 200M is pretty standard maximum value for any tube, use it if you don't want to cut your low end.

The data sheets do not help much at this point. The maximum grid resistor is usually up to 1M, rarely up to 10M (EF86). This means that most tubes in microphones are operated outside their permissible specifications. I know of only one tube that really gives a high maximum value for Rg (180M!) in the datasheet. It is the Telefunken AC701, which was designed exclusively for microphones.

 

[elskardio]

your trasnformer don't "saturate?" so easy and you may find the response curve to be nicer.

That’s a solid point.
I ordered a bigger transformer from UTM and will run comparaison!
I’m currently using a 3U audio 9.5:1

 

[rock soderstrom]

I ordered a bigger transformer from UTM and will run comparaison!
I’m currently using a 3U audio 9.5:1

That's interesting, keep us posted about your comparison results.

 

[LevinGuitar]

That’s a solid point.
I ordered a bigger transformer from UTM and will run comparaison!
I’m currently using a 3U audio 9.5:1

But the point here is the ratio I guess. You'll have bigger and cleaner low end with bigger laminations but the unpleasant high end will be "tamed" only reducing the gain or increasing the transformer ratio.

Neumann used huge transformers and still had that caps there

 

[elskardio]

Neumann used huge transformers and still had that caps there

Yes I ordered multiple values of Mica capacitor and once I installed the new transformer I will run some tests

 

[elskardio]

I finally finished the first mic yesterday and I'm pleased with the result!

The UTM0549 clearly adds some extra low end... but the 3U GZT-87performs really well and I wouldn't hesitate to use it in an M49 style microphone.

Some notes on the 6S6B. The -V (military) version is definitely less microphonic and has slightly lower noise. But the regular 6S6B still outperforms most 5840 I tested.

I'm using Arienne Flat 47 in this one. Great capsule!

Cheers

 

[Migs 31]

@Khron :”Most of those copy the original designs "just because". The benefit being ("cosmetic") look-alike-ness to the original designs. “

Nostalgia I know, I know.
But a Yoke on the M49 looks good.
Darn !!!
M