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jackreynolds

Tin Man

Posts: 22 Member Since: 05/05/2012

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May 28 14 6:11 AM

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Hi Guys,

Just a quick question about the ELA m251 power supply.

There appears to be several variations of the power supply around, one of which uses a BJT, i would imagine as a current source.

DO you think the NPN transistor takes some strain off the transformer when large current spikes are needed for the charge phase of the first reservoir capacitor?

I also find it surprising that the B+ is only half wave rectified. I wonder why they chose this method.

Any thoughts on these power supply details?

Most other designs seem to just use pi RCRC filters and full wave rectification.

I only ask as i am building a 251 type mic and power supply and i'm trying to understand all the different options available and how they might affect the sound of the mic. 

Thanks in advance for any thoughts.

Cheers

Jack
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bockaudio

Platinum Blonde

Posts: 1,516 Member Since:02/02/2011

#1 [url]

May 28 14 4:55 PM

jackreynolds wrote:
Hi Guys,

Just a quick question about the ELA m251 power supply.

There appears to be several variations of the power supply around, one of which uses a BJT, i would imagine as a current source.

DO you think the NPN transistor takes some strain off the transformer when large current spikes are needed for the charge phase of the first reservoir capacitor?

I also find it surprising that the B+ is only half wave rectified. I wonder why they chose this method.

Any thoughts on these power supply details?

Most other designs seem to just use pi RCRC filters and full wave rectification.

I only ask as i am building a 251 type mic and power supply and i'm trying to understand all the different options available and how they might affect the sound of the mic. 

Thanks in advance for any thoughts.

Cheers

Jack

Which variations are you talking about. I've seen different cases containing basically the same thing.
The current source BJT is there for optimum heater noise. It's a big transformer and the inrush current is a non issue. Seems like you're looking backwards on this.
Great question about the haf wave, I have no clue as to why. I always use full wave.

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bockaudio

Platinum Blonde

Posts: 1,516 Member Since:02/02/2011

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May 30 14 10:14 AM

Sorry I realized the obvious answer for the half wave rect. The physical space and possibly cost of the selenium rectifiers available at the time. These parts had not yet been miniaturized into the convenient packages we've had for the last 40+ years.

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jackreynolds

Tin Man

Posts: 22 Member Since:05/05/2012

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May 26 16 2:22 PM

ELA M251schematic

Reviving an old thread, I have been looking for a higher resolution image of the ELAM 251 Power supply schematic (M950EC) but haven't had much luck yet. The 14th feb 1961 schematic floating around the internet is impossible to read. 

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