Neumann M49 PSU and Neumann U67 PSU

[distorter]

Hi guys, what is the difference between Neumann M49 PSU and Neumann U67 PSU?
I know the filament voltage is 4v and 6.3v. However, when I measure the M49 psu, I am reading 35v unloaded and 4.5v loaded. When I measure the U67 psu, I am reading 6.3v unloaded.
Why the PSU for M49(ac701k) measures differently when loaded and unloaded? Shouldn’t it be constant 4v when measure unloaded?

 

[Moby]

Hi guys, what is the difference between Neumann M49 PSU and Neumann U67 PSU?
I know the filament voltage is 4v and 6.3v. However, when I measure the M49 psu, I am reading 35v unloaded and 4.5v loaded. When I measure the U67 psu, I am reading 6.3v unloaded.
Why the PSU for M49(ac701k) measures differently when loaded and unloaded? Shouldn’t it be constant 4v when measure unloaded?
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View attachment 99088

You should use Ohm's law to calculate the voltage drop across the series resistors.
Also, Nu67 should give you -15v unloaded. Unless you have one of the DIY fully regulated heater PSU

 

[Tubetec]

Remember the NN48a originally came with Nicad stabiliser cells , later these were deleted and replaced with a transistorised regulator by Neumann . The older supplies use a large resistor, 115 ohms to drop the right voltage on the heater , in the event the rail is unloaded and without the stablyte cells 35 vdc from a 25 ac supply sounds about right .

 

[distorter]

You should use Ohm's law to calculate the voltage drop across the series resistors.
Also, Nu67 should give you -15v unloaded. Unless you have one of the DIY fully regulated heater PSU

Thank you Moby! Yes, my U67 PSU is a regulated one from Andreas Grosser. Is it safe to switch my NN 48u to a fully regulated heater psu for my M49?

 

[distorter]

Remember the NN48a originally came with Nicad stabiliser cells , later these were deleted and replaced with a transistorised regulator by Neumann . The older supplies use a large resistor, 115 ohms to drop the right voltage on the heater , in the event the rail is unloaded and without the stablyte cells 35 vdc from a 25 ac supply sounds about right .

Thank you Tubetec! May I ask what is the right way measuring my NN48u when loaded in order to make sure the voltage feeding my ac701k is 4v? Is it a better choice to switch to a modern fully regulated PSU for my M49?

 

[Moby]

Thank you Moby! Yes, my U67 PSU is a regulated one from Andreas Grosser. Is it safe to switch my NN 48u to a fully regulated heater psu for my M49?

So, if the U67 PSU is regulated, heater should be 6.3-6.5v. Since it's regulated.
Of course it's safe to switch to the regulated PSU but you should consider the possible sound diference. It's subtle but existing.
I prefer R/C ones, possibly with zener "preregulation" as NU67, but it's up to you.

 

[distorter]

So, if the U67 PSU is regulated, heater should be 6.3-6.5v. Since it's regulated.
Of course it's safe to switch to the regulated PSU but you should consider the possible sound diference. It's subtle but existing.
I prefer R/C ones, possibly with zener "preregulation" as NU67, but it's up to you.

Very useful information to me. Thanks a lot!

 

[radardoug]

Come on Moby, 4 volts d.c. is 4 volts d.c. Not going to affect the sound AT ALL.

 

[Tubetec]

Its been said already , you really want to measure across the tube socket to get the true
heater voltage , the cable itself will drop a small voltage , so if you measure at the PSU end with the mic connected you might see +0.25v above what you expect .

Ive certainly heard a few very knowledgeable people say passive RC filters sound different to regulated supplies in tube mics and pre's . All the mics I have are regulated heater supply and its not easy to make that into an RC filter because of the voltage drop needed to make it quiet . The older Neumann PSU also uses a 1HY choke in the heater supply . Im planning on making a battery based tube mic supply soon , no regulators , 4 Nimh cells for 6 volt heaters ,3 cells for 4v heaters . NICAD cells could give slightly different voltages depending on if you want your heaters underrun . The one worry I have is heater inrush current from batteries at switch on ,owing to the low internal resistance .

 

[Moby]

Come on Moby, 4 volts d.c. is 4 volts d.c. Not going to affect the sound AT ALL.

I'd rather not discuss that topic. Just stating my experience.

 

[tomas.borgstrom]

I'd rather not discuss that topic. Just stating my experience.

My experience is the same as yours

 

[Marc HENRY]

Compared to CRCRC or batteries, ordinary Regulated PSU for the heater in mics sounds bad (sometimes VERY bad) when this voltage is used for the fixed bias of the tube. ( U67, M49 prior M49c, C12a, U47 vf14, ... )

 

[Khron]

I'm quite disappointed noone seems to bother going for capacitance multipliers, for de-noising, say, regulated voltages. It was good enough to "even" include in the circuitry of the U87 and U47fet...