R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1]   Go Down

Author Topic: U47 B+ voltage effect on sound  (Read 1079 times)

afterlifestudios

  • Full Member
  • ***
  • Offline Offline
  • Posts: 160
  • Real Full Name: John Raham
U47 B+ voltage effect on sound
« on: September 07, 2020, 11:55:02 am »

Is there anything wrong with connecting my NG PSU to a Variac and adjusting the voltage to experience the differences in tone?  Then adjusting dropping resistors to hit the measured target? 

My B+ on an average day here (mains voltage 115VAC) is about 100VDC.  I thought this might be an easy way of seeing if it’s worth changing the dropping resistors to get it up to 105VDC?  Or just do it?
Logged

uwe ret

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 72
  • Real Full Name: Uwe Sattler
Re: U47 B+ voltage effect on sound
« Reply #1 on: September 07, 2020, 04:33:25 pm »

There will be no harm as long as you will remain within around 10% of the nominal 110V AC. However, a 5 V difference in the B+ will have negligible, if any, audible effect on the acoustic properties of the microphone. I suspect the rectifier in your NG power supply may aged and developed higher than nominal internal resistance. Be careful when attempting to replace it with a modern silicon diode bridge, its much lower resistance may jeopardize the mains transformer by excessive in-rush current upon power-up. A suitable resistor, or better yet, a NTC thermistor in series should be used.
Logged

RuudNL

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 63
  • Real Full Name: Ruud van Steenis
Re: U47 B+ voltage effect on sound
« Reply #2 on: September 08, 2020, 09:24:54 am »

The topic starter mentioned that his mains voltage is 100 Volts, instead of 115 Volts AC.
That is 87% of the voltage the power supply was made for.
I think it is obvious that the DC output voltage of the power supply will now be lower too.
If DC output was 105 Volts at 115 Volts AC input, one could expect something in the region of 91 Volts DC now.
IMHO the exact voltage is not very critical.
As uwe ret mentioned already, a voltage of + of - 10% is accepatble in most cases.
I have an original Neumann U47 power supply here, that delivers 110 Volts DC. (Under load.)
Since the filament of the tube is underheated, I do not worry about this.
The reduction of the polarisation voltage and plate voltage on the tube in your case can be neglected in my opinion.
Logged

uwe ret

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 72
  • Real Full Name: Uwe Sattler
Re: U47 B+ voltage effect on sound
« Reply #3 on: September 08, 2020, 11:24:04 am »

The OP stated that his PS output is 100VDC at a mains of 115VAC. That makes for a rather inconsequential 5% difference to the nominal 105VDC.
Logged

RuudNL

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 63
  • Real Full Name: Ruud van Steenis
Re: U47 B+ voltage effect on sound
« Reply #4 on: September 08, 2020, 12:03:11 pm »

My misinterpretation. But anyway: no problem!
Logged

afterlifestudios

  • Full Member
  • ***
  • Offline Offline
  • Posts: 160
  • Real Full Name: John Raham
Re: U47 B+ voltage effect on sound
« Reply #5 on: September 09, 2020, 12:28:02 am »

Thank you, gentlemen.  Yes, that correct.  100VDC B+ on my NG (loaded) with mains at 115 VAC. 
I’m more curious about the sonic effect of the resulting change in capsule polarization voltage, than the VF14.
I’ll give it a try, keeping the Variac so that the measured B+ is within 10% of 105VDC.
Thanks again for your thoughts.
Logged

RuudNL

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 63
  • Real Full Name: Ruud van Steenis
Re: U47 B+ voltage effect on sound
« Reply #6 on: September 12, 2020, 04:00:33 pm »

I am interested in your findings!
I don't suppose + or - 10% will make an audible difference, but I never tried...

By the way: I measured the unloaded voltage of the original Neumann U47 power supply.
The unloaded DC voltage of my power supply is 378 Volts!
So it is very important to connect the cable and microphone first before switching on the power supply!
The capacitors in the power supply will hold the voltage for a long time.
And I am sure the VF14 wouldn't like a burst of 378 Volts! (Which means theoretically a filament voltage of ~126 Volts...)
Logged

afterlifestudios

  • Full Member
  • ***
  • Offline Offline
  • Posts: 160
  • Real Full Name: John Raham
Re: U47 B+ voltage effect on sound
« Reply #7 on: September 12, 2020, 09:38:01 pm »

Yes indeed.  No “hot swapping” here.  If one my NG PSU’s gets turned on unloaded, I drain the caps with a big resistor and measure for any stored voltage before connecting the mic. (Either that or wait 20 mins, as those caps can hold a charge for quite a while.)
Logged

Jim Williams

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 585
Re: U47 B+ voltage effect on sound
« Reply #8 on: September 13, 2020, 12:20:38 pm »

Place a 1 meg 1/2 watt resistor on the output of the psu caps to ground to drain them when shut off. It also provides a load if the mic is not connected.
Logged

RuudNL

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 63
  • Real Full Name: Ruud van Steenis
Re: U47 B+ voltage effect on sound
« Reply #9 on: September 13, 2020, 02:42:39 pm »

I am going to install a 10 Watt/120 Volts zener across the output.
This will reduce the output voltage to a safe value, even with no load.
When loaded with the microphone, the voltage will be lower than 120 Volts anyway.
(So the zener doesn't 'exist' with the microphone connected.)
And even when the microphone gets connected with capacitors charged to 120 Volts, this won't hurt the microphone.
Logged

Jim Williams

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 585
Re: U47 B+ voltage effect on sound
« Reply #10 on: September 14, 2020, 12:15:51 pm »

Zener diodes create a lot of rf hash and noise. Place a fast .1 uf film cap across it to filter it.
Logged

RuudNL

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 63
  • Real Full Name: Ruud van Steenis
Re: U47 B+ voltage effect on sound
« Reply #11 on: September 14, 2020, 12:31:28 pm »

True, when they are 'zenering'...
But under the zener voltage a zener diode is virtually non existent.
(A .1 uF capacitor won't help very much to reduce zener noise in the audio frequency band.)
Logged

Jim Williams

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 585
Re: U47 B+ voltage effect on sound
« Reply #12 on: September 15, 2020, 11:21:29 am »

Most diode noise is ultrasonic.
Logged
Pages: [1]   Go Up