DIY Tube Mic project

[Emmathom]

Well back to 6AK5 main thread...
I assembled all the components of the microphone.
The GE tube is a bit higher than the Philips I used for my tests but it fits, although it touches its top.
So I decided not to block the circuit with the screws knowing that the body + grid wrapping the microphone will ensure good mechanical support.
The Neutrik NTE10/3 is glued with a drop of neoprene.
Space is limited but everything fits.

Edit : NTE10/3 center tap is connected to star ground thru 100Ω + 1nF //

Next I will do some voltages tests and then audio tests (as soon as I receive my 7 pins connectors and after a 10 days of vacations )

 

[Emmathom]

I propose a classic Decca tree equivalent with a "small" directional array up front (ORTF & Blumlein are the posterchildren for that) instead of a mono microphone and suitable microphones at the rear. No need to obsess about "M50".

I propose to create a means for a time delay correction for all microphones used, to allow seamless mixing.

And I propose to use microphones with excellent low frequency characteristics (anything but omnis sounds weedy in the bass and EQ don't quite fixes it) for low frequencies with an arrangement that gives the best stereo localisation for low frequencies and microphones with excellent high frequency characteristics (e.g. 1/4" Cardioid Condensor or ribbons) with an arrangement that gives the best stereo localisation for medium and high frequencies.

I then propose to create a crossover between the two sets of microphones at around 700Hz. In the "old days" I did that with RL circuits and used allpass circuits (front array delayed with RL ladder) to compensate the 1.5m distance between front array and rear microphones.

I did a plan of a setup according to your proposal.

* an ORTF center 3m from front scene
* an AB cardio couple 1m behind / 2m apart + high-pass from ~700Hz & -2,94ms delay
* an omni couple 1,5m behind / 2m apart with low-pass to ~700Hz & -7,35ms delay

 

[thor.zmt]

I did a plan of a setup according to your proposal.

No, you did not.

* an ORTF center 3m from front scene
* an AB cardio couple 1m behind / 2m apart + high-pass from ~700Hz & -2,94ms delay
* an omni couple 1,5m behind / 2m apart with low-pass to ~700Hz & -7,35ms delay

No idea what that is supposed to do?

I prose something VERY different. Only 4 microphones (of which two may be integrated into a single body).

* a minimal "point" array, [e.g. Blumlein crossed 8, XY, M/S, ORTF, DIN, NOS - according to taste] set to have the correct recording angle to capture the ensemble being recorded. Microphones should have excellent HF response and consistent directivity

* a spaced pair of microphones, usually large diaphragm variable set to "omni" (which actually become directional at high frequencies).

Exact spacing between microphones subject to adjustment, start at 1.5m between the spaced pair and front 1m forward.

Front minimum array is delayed to give a coherent wavefront with the rear pair for a signal at center stage.

The front array output is split into two sets of signals, one ~ 700Hz first order highpass, the other unequalised and no highpass.

The rear spaced pair output is split into two sets of signals, one via adjustable first order lowpass (normal setting 700Hz), the other unequalised and no highpass.

The "default" setting is to have the low-passed rear spaced pair level reference and to raise gain for the front to balance the tonality.

We effectively get a system that at 700Hz has an equal amount of time difference stereo and amplitude difference stereo (whereas a minimal array only is 100% amplitude difference stereo)..

Below 700Hz the "stereo" system progressively slides towards time difference stereo and at 175Hz it is effectively purely time difference stereo.

Above 700hz the "stereo" system progressively slides towards amplitude difference stereo and at 2800Hz it is effectively purely amplitude difference stereo.

The extra signals from front and back microphones allows extra "dryness" or "wettness" to be dialled in.

And there is a operation mode where the minimal array is run without high pass and the "high pass" is left to the natural ~250Hz corner far-field roll-off caused by the cardioid operation of a small capsule and the rear spaced pair operates below ~250Hz to fill in. It usually "gels" less well than a higher crossover but this is not categorical.

For larger ensembles I preferred 88 or XY in front which allow the widest recording angles.

If we record front & back digitally into 4 tracks at 24/192 (which in 2024 I can see no reason why not) we can do everything in POST. Exact crossover frequency and how much extra "dry" or "wet" we dial in is then easily controlled and remixed until just so.

As I only ever recorded to two tracks on tape, such luxury was naturally unavailable to me and it was critical to set "the box" with my DIY Mic-Pre's and the mixer etc. up right, by ear and fairly fast, then ask for a quick rehearsal of an ffff passage of the programme and then of one pppp one before starting the recording and hopefully not being the reason why a take had to be repeated.

Thor

 

[Emmathom]

Ok :

* A cardio SDC pair (in XY for example) is placed at center
* An omni LDC pair spaced by 1,5m-2m is placed at 1m-1,5m behind cardio SDC pair
Distances have to be adjusted...
So 4 tracks are recorded (or 2 stereo ones if you prefer)

In mixing :
* The cardio pair (dry) is duplicated and a high-pass filter around 700Hz@-6dB/octave is added (= wet)
* The omni pair (dry) is also duplicated and a low-pass filter around 700Hz@-6db/octave is added (= wet)
Now we've got 4 stereo tracks to deal with between front & rear / dry & wet
The front XY pair is delayed according to the distance with the rear pair...

 

[k brown]

Ok :

* A cardio SDC pair (in XY for example) is placed at center
* An omni LDC pair spaced by 1,5m-2m is placed at 1m-1,5m behind cardio SDC pair
Distances have to be adjusted...
So 4 tracks are recorded (or 2 stereo ones if you prefer)

In mixing :
* The cardio pair (dry) is duplicated and a high-pass filter around 700Hz@-6dB/octave is added (= wet)
* The omni pair (dry) is also duplicated and a low-pass filter around 700Hz@-6db/octave is added (= wet)
Now we've got 4 stereo tracks to deal with between front & rear / dry & wet
The front XY pair is delayed according to the distance with the rear pair...

Beautiful recordings of Classical ensembles do not require such complications.

 

[Emmathom]

Beautiful recordings of Classical ensembles do not require such complications.

I post an excerpt of a classical ensemble done with an ORTF pair + my 6AK5 tube mic on the harpiscord and I believe it sounds great...

But @thor.zmt approach may be interesting to try...


final concert 16-44

 

[thor.zmt]

But @thor.zmt approach may be interesting to try...

I am not sure is it is "interesting".

It is more complex than either ORTF (et al) or Decca, but in my view not unduly so. We often use additional "ambiance" mic's, in my case they simply have more clearly defined job description.

If you listen to Decca and Deutsche Gramophone classical recordings they are almost the opposite, spatially. And neither is all that realistic compared to a first third center seat in (say) the Barbican. Each gets some aspects rights, at the expense of others.

Decca is spacious, realistically so, but the ensemble is amorphous, almost more so than from the boxes in the back.

DG is extremely clear and instrument placement is precise, but mostly en miniature, it sounds small, it fit's into my 300 sqft front room in a semi-terraced victorian house in London..

What I was seeking was to marry these two, to have realistic spaciousness and a feeling "they just removed my walls and transported me into a concert hall" that Decca excels on but with a highly resolved and instrument specific sound stage like DG.

Hence this combination of minimal directional stereo center and spaced omni in the back.

As super bonus, it's all completely mono compatible, which was something that mattered a LOT back in the 80's of the last century of the last millennium.

Thor