U47-style bodies - who can make one?

[MagnetoSound]

Making that resistor in that fashion required substantial effort on their part at that time. There must have been a motivation.

It's a filament dropper.

1780 ohms is a pretty specific value. This could not have been found off the shelf.

It is flat so it can be fitted into a slot in the bellhousing for heatsinking purposes.

My second question is why do they put the cardboard resistor in the mic rather than the power supply box ?

It allows the use of a very simple power supply, outputting a single rail, lowering cost and increasing reliability. It also takes cable resistance out of the equation.

 

[Skylar]

MagnetoSound summed it up nicely.


It's all related to cost.

I believe the VF14 was chosen because they saw an opportunity to derive the heater voltage from the same rail as the B+...
lower part count, simpler PSU, one less conductor needed for the cable and connectors.

This is why the filament dropper is in the mic, not the PSU.

What other tube could they do this with? No magic or voodoo here.

Wirewound was probably a good choice for temperature stability, so I think riggler makes a good point there.

Again, no magic/voodoo.

I still feel the U47 designers knew physics and electronics intricacies on a level we are still trying to understand today.

They knew no more than what is known today.
Actually, in the fields of physics and electronics, we know more today by several orders of magnitude.
In fact, I am willing to bet that there are forum members here (or here at one time) who understand physics/electronics just as well or better than these designers did.
[I am definitely not one of these folks through!]


Ultimately, if you want to clone a U47 with a VF14, just use an off-the-shelf 1.8k wirewound or metal oxide and physically heatsink it to the body.
Problem solved!
This is a dropping resistor for the heater voltage afterall.

 

[Rossi]

I'm pretty sure, the VF14 was not chosen for lower cost. It was a special request tube that was used almost exclusively in the U47. I don't have an old price list but I'm sure it was much more expensive than an EF14.

There are some practical advantages to the solution chosen. One was that you have less voltage loss in the cable with the high voltage heater. Keep in mind that the U47 was designed for any kind of recording situation, including orchestra where you typically have long cable runs. Another advantage was the simple PSU. The U47 requires a well filtered filiament voltage and with just one supply voltage going to the mic, they could use the B+ which had to be well filtered anyway. Also, with a purely passive PSU, filtering a low voltage is less convenient than filtering a higher voltage - back then, lytics were not als volume efficient as they are now.

Also, the VF14 (even a lot of the non M-selected ones) is a very very low noise tube. Despite the relatively low grid resistor (60M), a well working U47 has about 15 dB-A self-noise in cardioid mode. That's better than most tube mics these days, and in fact better than the first generation of FET mics.

 

[desol]

I still feel the U47 designers knew physics and electronics intricacies on a level we are still trying to understand today.

They knew no more than what is known today.
Actually, in the fields of physics and electronics, we know more today by several orders of magnitude.
In fact, I am willing to bet that there are forum members here (or here at one time) who understand physics/electronics just as well or better than these designers did.
[I am definitely not one of these folks through!]

There was an interesting discussion at PSW about this topic. Klaus had a good point (i'll try to recall accurately) when he offered that Neumann's ability to make good sounding microphones not only came from their engineering prowess, but also the importance they placed on careful listening.

ie: a perfect marriage between engineering and listening.

 

[Skylar]

Good points Rossi,

I still think that cost was a big factor (in addition to the other factors you touched on like tube performance, longer cable runs, etc.).
Though the EF14 may have been a less expensive tube, how does the cost delta of EF14 vs. VF14 compare to the cost of extra parts needed for a separate, filtered heater rail that the EF14 would have required?
Do you know what I mean?
It may have still been cheaper to build a U47 with the possibly more expensive VF14 because of the simplified PSU.
And, of course, the other benefits you mentioned go along with it.

Also, I've seen the equivalent noise level reported as 24dB (see attached doc).
Granted it is a Telefunken document, but it's the only place I've ever seen an actual reported figure for noise on the U47.

 

[Skylar]

ie: a perfect marriage between engineering and listening.

I totally agree.

And I think that there are experts today with just as much talent as the Neumann masters.
But I'm not the Neumann fanboi that a lot of people are (not really on this forum, but GearSlutz & PSW...wow).

 

[desol]

I think their reputation is well deserved (the mics).

 

[synchroman]

Wow... What have I started... I am going to try it both ways... When the mic bodies are done...

I will try a modern resistor and a old style wirewound... I will post audio files and see if

there is a difference in sound.

 

[TheGuitarist]

So CAN'T WAIT FOR THE NEW PRODUCTION SAMPLES!!!!!

 

[Rossi]

It may have still been cheaper to build a U47 with the possibly more expensive VF14 because of the simplified PSU.
And, of course, the other benefits you mentioned go along with it.

Also, I've seen the equivalent noise level reported as 24dB (see attached doc).
Granted it is a Telefunken document, but it's the only place I've ever seen an actual reported figure for noise on the U47.

Of course, cost is always a factor. What I'm saying is: it wasn't the prime factor. The market then was different. Mics were bought by large institutions and firms (German broadcast was state operated until the 1980s). There weren't many mic manufacturers to begin with, and whoever needed a studio mic had to buy it for how much it cost. A few pennies more or less wasn't an issue. There weren't any comparable alternatives to a U47. What was an issue was the ingenuity and usability of the product. A mic that allows long cable runs was more desirable than a mic that didn't, even if it cost a bit less. Same with noise.

And I think that's pretty much the reason why modern mic design sometimes appears lame compared to those classic designs. These days a lot of effort has to go into cost effectiveness and achieving an attractive price vs. competitors.

re self noise: The measuring method may have been different then (I don't see A-weighting mentioned in the sheet you posted). Or they published very conservative specs - again there wasn't much competition. You'll find the 15 dB A spec in the later U47 documents at the Neumann site. Also, I measured mine, and it was in fact 15 dB A. The K47 has slightly higher output than the M7, so early U47 were probably around 17 dB A.

 

[Mike Cleaver]

A power supply is a power supply is a power supply.
A dropping resistor for the filament isn't going to make a difference to the sound.
If the filtering or regulation of a power supply is poor, then you will hear a difference.

 

[rodabod]

Although it's a simple circuit, there's quite a lot of things going on, all of which need to be balanced.

If you were going down the route of mimicing the original with a modern valve, then it's not quite as simple as just copying some of the parameters. For instance, you might want to measure the effects of NFB in the original. This is indirectly related to the heater voltage and therefore the dropping resistor which were also related to the original choice for the B+ Voltage..... and so on.

 

[peterh]

HI,

i think  what a lot of people who think about the U47 and its parts forget, is the fact that this Mic was deveolped right after the war, when Germany had nothing. So they had to use what they could get hold of. The VF14 Tube was a prewar design, it was produced, but had very few applications (products). So beside the fact of a simplified cable and powersupply, Neumann got these Tubes quite cheap (overstock). The V-Series Tubes were used in the "german people radio" (Volksempfaenger, cheap mass product), but not the VF14.
I think we shouldn't think too much of, why they choose a certain component other than cost and availableness.

Just my two cents.

Peter

 

[Rossi]

The VF14 is not a prewar design. The guy I bought my U47 from actually had a library of old technical magazines and bulletins. We talked about the tube, and actually had collection of announcements of newly available tubes of that time. The VF14 was announced in 1946 or 1947, I can't remember, but it was just prior to the appearance of the U47. If you then take into account the fact that Telefunken was involved with Neumann back then and also the fact that there are almost no devices other than the U47 that used that tube, it seems reasonable that the VF14 was  developed with the U47 in mind. The only other device known to have used the VF14 was a set top box for FM radio or something like that. And maybe that was only a way of getting rid of the tube rejects not suitable for microphone use. In any case the VF14 was definetly not an overstock prewar tube but a special request item.

 

[rodabod]

It is odd that they based it on a pentode design though. And effectively under-heated it according to Telefunken's specifications. Interesting.

 

[Rossi]

I think PRR once explained, that the industry concentrated on pentode development at a certain point. So if you wanted a cutting edge tube design,  pentodes were where you had to look.

As you know, microphone circuits are super high impedance and thus "far out" while datasheets are about what a device is surely capable of. Practically all tube microphones use their tubes out of spec: way too high grid resistors, often massively underheated. But you also know that you have to sort out tubes, sometimes at a high ratio, for mic applications. A tube manufacturer wouldn't want to deal with such high reject ratios.

 

[peterh]

The VF14 is not a prewar design. The guy I bought my U47 from actually had a library of old technical magazines and bulletins. We talked about the tube, and actually had collection of announcements of newly available tubes of that time. The VF14 was announced in 1946 or 1947, I can't remember, but it was just prior to the appearance of the U47. If you then take into account the fact that Telefunken was involved with Neumann back then and also the fact that there are almost no devices other than the U47 that used that tube, it seems reasonable that the VF14 was  developed with the U47 in mind. The only other device known to have used the VF14 was a set top box for FM radio or something like that. And maybe that was only a way of getting rid of the tube rejects not suitable for microphone use. In any case the VF14 was definetly not an overstock prewar tube but a special request item.

I must be completly wrong, but the VF14 is a EF14 with a 50mA Heater (V-Series). The EF14 is a prewar development, and so the VF14. Most of the 11 Series very at least planed to be available as a V-Type (see the thread: http://www.jogis-roehrenbude.de/forum/forum/forum_entry.php?id=47901), but because of the war only a few vere produced inhigh volume. Beause of several other reasons the industry used the U-Types instead. Both families were intended for AC/DC Radios, heaters connected in series.
You can easily exchange a VF14 by a UF14 and, if the UF14 is selected and you do a double blind A/B comparison, you will never notice in a U47. I have read a scan of a document (please dont ask me for the link, it is too long ago), were Telefunken offered the VF14, produced for the Wehrmacht, to Neumann for a very good price.
Exchangang by a EF14 is a different thing, you could but the resistor must "burn" a lot of energy. Using 4V DC would change the bias concept...

I know, mentioning those things take out a lot of magic and Vodoo regarding the U47...

Peter

 

[Rossi]

Well I never said the EF14 was postwar, but from the document's I've actually seen (in the original), the VF14 is. I've never seen a prewar VF14 nor has my seller, who is a broadcast technican and tube fanatic (hence a cellar ful of old bulletins).

Oliver Archut over at Klaus Heyne's forum holds that the VF14 was different from other VF types and not just a high voltage heater version of the EF14. Supposedly different materials were used that made the VF14 more tolerant for underheating and reduced grid leakage. If that's true or just voodoo, I cannot say. I've seen Ollie post BS, and I've seen him post very interesting stuff. Maybe the guys at Neumann know for sure, they have an opened VF14 on display at their headquarter.

I have heard EF14/14 circuits that were as low noise as a U47 (it takes a lot of tube selection, though), but as Roddy said, there are some things going on that are not quite as obvious. Certainly, you won't get the exactly same sound from an off-the shelf self-bias circuit (although it doesn't sound bad). Which is why I like Max' MK7 design: It takes available components to recreate a U47 kind of tonality. I've yet to build mine, though. I'm still waiting for Skylar's mic bodies.

 

[Skylar]

I received an email last night reporting that the next sample did not get completed this week (as promised last week).
Shipping for this new sample has been rescheduled for next week.

One thing's for sure: once this is all over, I'm throwing a massive party—everyone's invited!

 

[TheGuitarist]

I hope they give you some sort of discount for this whole fiasco.