Neumann M147 power supply (M147 schem added)

[Mailliw]

Morning,

I need to build a power supply for a Neumann M147. The only info I can find on the M149A power supply is from this page:
http://www.soundonsound.com/sos/dec00/articles/neumann.htm?print=yes

"... generates ±70V for the valve and capsule biasing as well as +32V and +5V supplies."

I just read Jakob's G7 page and I understand how his capsule is polarized. Would this mic work on the same principle? The front diaphragm at 0V, the center electrode and back diaphragm at say, +70V.

Thanks,
William

 

[Walrus]

The manual for the M147 gives the pinout info as follows...

Pin1 -70V
Pin2 +5V
Pin3 audio signal +phase
Pin4 +70V
Pin5 sensor line
Pin6 +32V
Pin8 audio signal -phase

Can't help re your other questions.

 

[Mailliw]

Thanks.

Hopefully when I get the M147 in my hands, I can eliminate some of the connections on the mic's end.

That sensor lines worries me...

 

[Mailliw]

I have a G7 PSU which I bought off a Lab member. I need to figure out how much current the M147 draws at +70V and at +32V and then adapt the G7 PSU.

M147 schematic

+32V line
The LT1124 data sheet says the supply current is ~2.5mA per amplifier (page 6).

2.5mA*2 = 5mA
Does it matter that the op amps are powered by a single, positive voltage? (+32V vs +/-16V)

Very roughly, R10 limits the current through paths T2 and T3: 32V/3.92k ~ 8mA

+70V line

Tube current using plate resistor: 70V/15k ~ 5mA

Does any of that make sense?

Thanks,
William

 

[burdij]

I think you are probably going to see 1 - 1.5 ma on the +70 line. The tube plates are driven by a current source. The typical tube preamp stage runs at about a mil. The second stage is an emitter follower. The total current is less than 5.5 ma. Again, there is a current source, so the current will probably be set less than that amount. So you are looking at less than 10ma total on the +32 line. The sense line on the filament input is for long cable runs. If you are using 25 -30 ft. of cable to the power supply, you can ignore the sense. Just make the filament supply variable and set the value to give +5.2 at terminal 2 of the mike connector. The +32v is referenced to ground (and the negative audio output pin). You would not be able to use a split supply unless you went rail to rail and didn't ground the center of the supply, effectively giving a +32 single output supply.

 

[ioaudio]

all the mess just to save on a proper output transformer...

 

[gyraf]

And then running a microphone signal unbalanced - it is not even impedance-balanced... :roll:

 

[Gus]

You might want to increase the fil voltage look around Klaus for some of Olivers posts on 147 and 149s. You don't need the sense BUT I would guess the neumann supply has a controlled start up both current and voltage. Why? The plate slow start and the sense line.

Make you cable, set the fil voltage to say 5.7VDC inside the power supply then measure the fil voltage in the microphone, meter connected at the sense line and adjust it for the voltage you want after the tube is warmed up.

The DMOS fet in the plate circuit also looks to be more of a soft start circuit than a constant current it has a Rdson of <10 ohms(from a fairchild PDF). 10 meg charging C5 (ramp)and being zener clamped to protect against gate breakdown. I have seen circuit like this used many years ago on hot swap cards. The power supply pins sometime were cut back a litle more of the supplies that should turn on last and some even had power mosfet circuits like this the gate would be powered by a higher voltage supply and a RC and zener clamp to ramp up and protect the gate.

Look back in the phantom soft start posts here I posted about using a circuit like this in slow start phantom supplies.

The output Z looks to be low enought and look at the added parts ARE there more parts inside the power supply before the 3 pin XLR on 2 and 3

Looks to be more of use a tube to call it a tube microphone circuit look at C17 and C2 and what they do.

 

[gevermil]

http://recforums.prosoundweb.com/index.php/mv/msg/11669/0/0/0/

 

[burdij]

You may want to take care not to reference the ground of the microphone to the shell of the mike and also to the "pin 1" or shield ground of the power supply. In order for this type of output to work correctly, the entire electronics, including the secondary side of the power supply has to have a floating ground reference. This is relatively easy to do with a switching power supply, which is what is used in the original unit. If the microphone output on either pin 2 or 3 is referenced to your shield ground, you will have unmanagable hum problems. The shell of the mike, the shield of the cable and the box of the power supply should be referenced to earth ground, of course.

 

[Mailliw]

Thanks for all the great info, guys.

[quote author="burdij"]The +32v is referenced to ground (and the negative audio output pin). You would not be able to use a split supply unless you went rail to rail and didn't ground the center of the supply, effectively giving a +32 single output supply.[/quote]

Sorry, I don't understand what you mean. What does a split supply refer to?

Here was my plan:
My G7 PSU gives ~180V. I drop that to ~90V using a voltage divider. The 90V feeds two TL783s, one for the +70V and one for the +32V.

 

[burdij]

You had asked about using a +-16V supply (a split supply) instead of a single 32V supply. Your current plan sounds like it will work.

 

[Mailliw]

Thanks buddy!

 

[Gus]

Looking at that schematic again I would think there is more between pins 3 and 8 from the microphone and 2 and 3 out at the power supply 3 pin XLR.

I would just build a supply from scratch.

Think about the possible power up. I would guess first the +32 then the plate supply(4 or 5 RC 10meg, .1uf) then the tube heater supply. Maybe this minimizes pops or noises at the output.

 

[ioaudio]

i am sorry, but you cannot make a psu for this mic. this is just not the way to go. i just cannot agree. :razz:
put in a nice tube and one of these (or similar):

 

[Gus]

Nice, homemade bobbin? wound like it should be?

 

[burdij]

Speaking of home-made transformers, have you seen this solution for making toroid power transformers with weird output voltage combinations. I purchased a couple of these for making power transformers for U47-type supplies were I need a high filament voltage to derive the grid bias for an EF86. Toroid supplies the pre-wound 115VAC winding and info on how to wind the custom secondaries. Fairly cheap at $30 each for the 80VA units.

http://www.toroid.com/standard_transformers/transformer_kits/transformer_kits.htm

They apparently don't supply the winding bobbin but I could make some up if there is any interest.

 

[ioaudio]

you can take any toroid and unwind the secondary und put on your own.
i dont understand this "kit".
in europe there are some nice sources for costum wound pwr toroids, even one-offs are possible for a ok price.

gus, yes, this is a bv8 copy on original laminations with costum made bobbins (i sourced out the milling of the plastic parts) and original winding scheme.
one is on it´s way to klaus h. for evaluation.

 

[volker]

Would somebody who saved the schematic be so kind to upload it somewhere?
The link from above isn't working anymore.

Thanks!