Neumann / RFT MV 101 PSU / Preamp Question

[smilan]

Hi, I bought a Neumann / rft mv 101 measurement microphone without PSU.
I can buy the original PSU and a couple of 00 011 preamps (with high impedance outputs) for $260 or to build an alternative PSU with an output transformer for the mic for much less money.
If I buy the original preamp and would like to mod it with blanched output, all I need to do is to add an output transformer to the output?
Do you think those preamps will sound good enough to invest in them?
Or would it be wiser to go with the alternative PSU?

This is the original 00 011 schematic:



This is the alternative PSU:

 

[jokeramik]

This is not an Neumann Preamp..)
This unit was developed and manufactured in Dresden by VEB Messelektronik "Otto Schön".
I think this unit is a measuring preamp. You have the original documents?
Is it possible to send me a scan of the hole documents you have?
Then I can help you.

Be carefully, because there are some transistor arrays inside (B340 and B341). The arrays are out of production and no compatible product is available.

Best regards!

 

[smilan]

I think this unit is a measuring preamp. You have the original documents?
Is it possible to send me a scan of the hole documents you have?

I don't have the original documents.
I found this schematic online here:

 

[jokeramik]

I don't have the original documents.
I found this schematic online here:

I saw some papers in the pic...
Best regards!

 

[smilan]

One more question about this schematic:


In the upper (+240V) bridge rectifier, pin 4 of the power transformer is connected to the positive DC side of the rectifier via a 10nF cap and pin 6 of the power transformer is connected to the positive DC side of the +20V (lower) rectifier via a 10nF cap.
I never seen this kind of arrangement.
What is the purpose of those connections?

 

[rock soderstrom]

I never seen this kind of arrangement.
What is the purpose of those connections?

I think the designer of the circuit wants to filter out the switching noise of the diodes. That version is apparently the reduced version of this circuit, which is seen more often. How much it does, I can not tell you, maybe someone else can make a statement about it. I try to avoid the problem with better, faster diodes.

By the way, the 35V DC before the 7806 is borderline. The maximum input voltage of the 7806 (35V!) can be exceeded here by occurring mains fluctuations. Not to mention the unnecessary thermal load on the 7806.

 

[thor.zmt]

This is not an Neumann Preamp..)
This unit was developed and manufactured in Dresden by VEB Messelektronik "Otto Schön".

Yup, east german, not saying it's not good, just from there.

Be carefully, because there are some transistor arrays inside (B340 and B341). The arrays are out of production and no compatible product is available.

They can easily be replaced using single transistors, datasheets float around. I think BC550 or similar will do fine. There are also ton's of other decades obsolete and moreover obscure semiconductors.

I can buy the original PSU and a couple of 00 011 preamps (with high impedance outputs) for $260 or to build an alternative PSU with an output transformer for the mic for much less money.

Roll your own. It's much easier to do much better now.

But note that there 105V & 200V (as well as +/-21V and +12V) coming into this preamp, the 200V are RC filtered and pass to "Up" which I interpret as polarisation voltage.

There is an 80V regulator that likely powers a tube in the Microphone (Ua - Anode Voltage) and a high power regulator with a TO3 pass transistor providing a heater supply (Uh - Heater Voltage).

Do you think those preamps will sound good enough to invest in them?
Or would it be wiser to go with the alternative PSU?

It is a measurement Mic Pre, it should not "sound". It has up to 61dB gain.

This is the alternative PSU:
View attachment 105717

This seems to just power the microphone itself and the 100V should be 80V based on the schematics of the preamp. Not really consequential. Using 35V DC to make 6V DC seems also somewhat of a poor design, a lot of heat is going to be burned off.

Thor

 

[jokeramik]

They can easily be replaced using single transistors, datasheets float around. I think BC550 or similar will do fine. There are also ton's of other decades obsolete and moreover obscure semiconductors.

If you think. Did you read the datasheet?
The transistors are very well matched in h21E and in the difference in base emitter voltages for all individual transistor. The difference is = or smaller then 5mV and the ratio of h21E between any pairs of transistors is specified 0,8...1,25. And all transistors are on the same substrate. This ensures thermal synchronization. This is very important in this application in the differential amplifier.
Best regards!

 

[jokeramik]

Yup, east german, not saying it's not good, just from there.

A lot of users prefer the preamps made by Rundfunk- und Fernsehtechnisches Zentralamt der DDR located in Berlin and VEB Funkwerk Kölleda located in Kölleda more then the Neumann Pre´s.
Best regards!

 

[abbey road d enfer]

The transistors are very well matched in h21E and in the difference in base emitter voltages for all individual transistor. The difference is = or smaller then 5mV and the ratio of h21E between any pairs of transistors is specified 0,8...1,25.

A good designer should make sure their design is as independant of the element's characteristics as possible. Relying on such a specifically tight hfe is not right.
Vbe differences could matter for DC amplification, but here it's AC. I doubt variations of Vbe result in malfunction here.

And all transistors are on the same substrate. This ensures thermal synchronization. This is very important in this application in the differential amplifier.

It should not be important. With the relatively modest gain of each stage, what can a few mV of offset do?

 

[abbey road d enfer]

One more question about this schematic:

View attachment 106030

This circuit is based on the availability of a specific transformer. It's far from being perfect, but it is probably what the designer had and he designed the circuit around it.
As several have noted, starting with a 20V winding for producing 6V is not smart.
Transformers for valve products are becoming increasingly hard to purchase, which results in many DIY'ers using different xfmrs for HT and heaters. It offers much more flexibility. For example, you can easily find a xfmr with a single 9 or 12V output, which can be used for heaters, with just about the same circuit (although I would use a LM317 instead of the 7806).

Note that the markings of capacitors is quite misleading. M is for Mega; I doubt all these caps are Megafarads, very likely microFarads, which should be noted with a µ (character ALT+230) or more commonly a lower case u. Certainly not "M".
The capacitor noted 10 50V is some kind of oddity. 10uF is too small, 10F is too big, so I suspect it means 10 000uF or 10mF (note m lower case). It's quite big, I'm not sure it's that necessary.

 

[thor.zmt]

If you think. Did you read the datasheet?
The transistors are very well matched in h21E and in the difference in base emitter voltages for all individual transistor.

I am familiar with the transistor arrays. I am from former East Germany. I'd take the spec's "CVM GRANO SALIS MAGNITVDEM MONTIS" (with a grain of salt the size of a mountain). East German Semi manufacturing is an interesting story, but spec's should be seen less as garanteed and more as "we'd like it to do that.

Modern manufacture transistors (as opposed to 1980's) will easily match this AND even if the input pair is not perfectly matched, at most you get a trifle more H2. But really, modern transistors of the same reel easily match the stated matching, based on my own experience.

The difference is = or smaller then 5mV and the ratio of h21E between any pairs of transistors is specified 0,8...1,25. And all transistors are on the same substrate. This ensures thermal synchronization. This is very important in this application in the differential amplifier.

Yeah, you can use self adhesive copper tape to twin two TO-92 transistors. The circuit is AC coupled, so a little DC drift again doesn't matter.

Actually, replacing the two discrete Op-Amp's with a 5532 will likely improve performance.

Thor

 

[jokeramik]

Yeah, you can use self adhesive copper tape to twin two TO-92 transistors. The circuit is AC coupled, so a little DC drift again doesn't matter.

That isn´t a professional solution. You can do what you like.

 

[jokeramik]

Actually, replacing the two discrete Op-Amp's with a 5532 will likely improve performance.

Have you already done this?

 

[jokeramik]

I am familiar with the transistor arrays. I am from former East Germany. I'd take the spec's "CVM GRANO SALIS MAGNITVDEM MONTIS" (with a grain of salt the size of a mountain). East German Semi manufacturing is an interesting story, but spec's should be seen less as garanteed and more as "we'd like it to do that.

I´m also from the GDR. I know that the specs are guaranteed.
Best regards!

 

[jokeramik]

I am familiar with the transistor arrays. I am from former East Germany.

Why do you wrote East Germany? This a West German invention. We don't have to be ashamed of having lived and worked there..
Best regards!

 

[abbey road d enfer]

That isn´t a professional solution. You can do what you like.

In the last century, before the advent of multiple monolithic transistors, there were professional solutions, with dual heatsinks

 

[thor.zmt]

Why do you wrote East Germany? This a West German invention. We don't have to be ashamed of having lived and worked there..
Best regards!

The B340 used in the MV101 was made by RFT, East Germany.

Thor

 

[thor.zmt]

Have you already done this?

I don't have this preamp. But I can read a schematic.

Thor

 

[thor.zmt]

I´m also from the GDR. I know that the specs are guaranteed.

Sehr lustig. Sehr.

Thor