Insight into "U47-inspired" with EF12 tube?

[deuce42]

At the risk of adding my unhelpful/ unsolicited opinion to this thread -  Soapfoot, enjoy experimenting and learning about the different permutations of your EF12 circuit, but for the sake of stating the obvious, do not underestimate the impact of the capsule as some earlier posters have noted.

In my experiments*, the differences in capsules used on exactly the same circuit was by far more drastic than using the same capsule with different tubes or different circuits. At least to me anyway.

Experiment with the EF12 for your own learning curve and the enjoyment in doing so, but don't become too caught up in it at the risk of neglecting really thorough capsule research.



*(granted these experiments weren't undertaken by someone working on a daily basis with Neumans for 40 years and whom has achieved "web golden ear deity status" but I did learn a thing or two)

 

[soapfoot]

There is a BOM, and the transformer can be bought here:
http://www.edcorusa.com/default.aspx

This was their reply to my e-mail inquiry:

That is a PWRC100-1 cost is $23.03 each.  You can call in at 800-854-0259 to place your order as this item is not on the website.

Phyllis Weston
EDCOR Electronics Corporation
7130 National Parks Hwy.
Carlsbad NM 88220
Phone (575) 887-6790
Fax (575) 887-6880
www.edcorusa.com

Thanks for this.  Do you happen to know what the specs are on this transformer?  Secondary voltage? Multiple secondaries?  Thanks!!

 

[soapfoot]

for the sake of stating the obvious, do not underestimate the impact of the capsule as some earlier posters have noted.

Noted.  I was planning on either a Sneesby K7 or a Thiersch Blue Line, depending on budget.  Thanks.

 

[0dbfs]

The best way to find it it to experiment with it, and what easier way to experiment than using a trim pot like Rodabod advised?

Axel

Chameleon Labs makes a LDC tube mic with a pot in the PSU that adjusts the heater bias. It is labeled as COLD/WARM. We have a bunch of them here at our studios.

Cheers,
j

 

[boogietube]

Yes, the transformer specs are exactly like the schematic. In fact, they were specially made for this psu. Someone here absorbed the setup cost so now that they have it on file, it is a minimal cost to buy one.
Sean

120V 60HZ primary
Dual secondary
160VAC @10MA
12VAC @500MA

 

[MagnetoSound]

The best way to find it it to experiment with it, and what easier way to experiment than using a trim pot like Rodabod advised?

Axel

Chameleon Labs makes a LDC tube mic with a pot in the PSU that adjusts the heater bias. It is labeled as COLD/WARM. We have a bunch of them here at our studios.

Two different things. Roddy was talking about the cathode bias. This is arguably a matter of sonic preference.

Heater voltage (not bias) should ideally be set at the optimum point for lowest noise and tube longevity and is not so much an 'adjust to taste' parameter.

 

[0dbfs]

The best way to find it it to experiment with it, and what easier way to experiment than using a trim pot like Rodabod advised?

Axel

Chameleon Labs makes a LDC tube mic with a pot in the PSU that adjusts the heater bias. It is labeled as COLD/WARM. We have a bunch of them here at our studios.

Two different things. Roddy was talking about the cathode bias. This is arguably a matter of sonic preference.

Heater voltage (not bias) should ideally be set at the optimum point for lowest noise and tube longevity, although some might choose not to.  :

Oops! Helps to read and connect words-to-brain.

Anyway, I was thinking of how the u47 fixed bias (and ioaudio's mk7) is tapped off of the heater and somehow connected this information with the mic I mentioned (which is probably cathode biased). Although the heater in the u47 is tapped off B+ so there would not be a dedicated heater line from the PSU.

details, details...

Best,
jonathan

 

[boogietube]

So, does that mean that the MK7 PSU can't be used? I wouldn't think so, but I'm a fresh newbie.
Sean

 

[soapfoot]

well, for one thing the voltages are way too high.  I need to build a supply that has 105v B+ and about 5-6v for filament/bias.

 

[zebra50]

Stewart, designer of...

...nothing really - it's all copied from standard circuits.

So, does that mean that the MK7 PSU can't be used? I wouldn't think so, but I'm a fresh newbie.
Sean

Yes, it can. 

Make sure you have a beefy enough transformer for the extra heater current, and use ohm's law for B+. That's it really.

 

[boogietube]

I thought ohm's law could be applied here to drop some b+ voltage and the variable resistor on the heater can be used to adjust the voltage. I'm working on an EF14 mic and that's what I understood from the replies to my posts.

Sean

 

[zebra50]

Looks like EF12 and EF86 have the same 200 mA current requirements, so no problems.

EF14 is 450 mA!

 

[soapfoot]

This is all interesting.

So, apologies for the "noob" questions, but:

1) do you see any disadvantage in having an unregulated supply for B+ and a regulated supply for the filament?

2) When working toward a target voltage, would this math be right?  Say I wanted 105 VDC output.  To find the desired VAC input, is the following equation right?

(V -1.4) * √2 = 105V.  Solve for V. 

In this case, V = about 75.6 VAC.

Is that totally wrong?  My reasoning is that 1.4v is dropped in the bridge rectifier, and after smoothing the DC voltage will be at or very near the peak AC voltage, hence the multiplication by the square root of 2.

Also, how do I account for the load resistance in the microphone itself... i.e. absent a regulator, how do I know the correct voltage without the microphone connected?  Thanks for your patience.  I have 2 music degrees but was never better than a "B" student in math.

 

[zebra50]

1) do you see any disadvantage in having an unregulated supply for B+ and a regulated supply for the filament?

Cross posting - See the answers to the same question on Klaus's (and my comment on this above - I'm now reading 5 threads about your design across two forums).

2)...Is that totally wrong?

Not wrong, but that would get you 105V at the output of the bridge rectifier.
But of course you need filtering.
See the MK7 or G7 PSU
You don't have the experience to design one of these from scratch, so pick a known design and adapt.

 

[soapfoot]

So wait... I thought I had accounted for the filtering by multiplying (v - 1.4) times the square root of 2?  What am I missing here?

You don't have the experience to design one of these from scratch

You can say that again

, so pick a known design and adapt.

The problem is finding the right one to adapt, and knowing how to adapt.  Evidently I'm missing some things.

 

[zebra50]

So wait... I thought I had accounted for the filtering by multiplying (v - 1.4) times the square root of 2?  What am I missing here?

That's the voltage relationship due to the rectifier.

http://en.wikipedia.org/wiki/Rectifier#Full-wave_rectification

But then you need to get that bumpy supply as smooth as possible - so you need caps and resistors/ inductors to make it smooth, just like in a guitar amp.

The problem is finding the right one to adapt

It doesn't matter.
Pick something, preferably something simple that is known to work.
Build it.
Debug it by taking measurements and asking when you get stuck.
Then start again and make something even better.

 

[soapfoot]

That's the voltage relationship due to the rectifier.

http://en.wikipedia.org/wiki/Rectifier#Full-wave_rectification

But then you need to get that bumpy supply as smooth as possible - so you need caps and resistors/ inductors to make it smooth, just like in a guitar amp.

Right, this much I understand.

Oh!  right.  The voltage drop across the resistors in the Pi filter sections.  I forgot to account for that.

 

[Gus]

zebra50

You still have the microphone with the switchable heater voltage power supply?

For the new to building a condenser microphone

A hint

The power supply is a big part of the tube microphone.  The details count.

 

[soapfoot]

Right.  This is actually not my first tube microphone, but I believe you.  My experience with audio circuits (mostly guitar and hi fi amps, a couple microphones, a couple mic preamps) has ALWAYS been that the power supply is anything but an afterthought.

In a tube mic PSU, what are some of the "critical details" in your opinion, Gus?

 

[zebra50]

You still have the microphone with the switchable heater voltage power supply?

Hi Gus,

Yes, I have a home built supply with variable tube, capsule and B+ voltages. It gets used a lot for experiments & development.