48v Power supply for McCurdy Phono Pre - diyAudio
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Old 15th September 2013, 08:42 PM   #1
ArchMus is offline ArchMus  Canada
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Default 48v Power supply for McCurdy Phono Pre

Hi All.

I have a McCurdy Phono Pre (the unit is from the late 70' or early 80's.) that initially had a lot of noise and AC oscillation in the audio path. I did a complete cap change (both phono cards and power supply) and that sorted things out…mostly.

This unit has a 48v power supply. Recently, As an experiment, a friend and I disconnected the original power supply and connected a 48v wall wart power supply instead. Well, the noise floor dropped about 20db.

As things stand now with the original power supply, there is still a little AC in the 48v DC coming out of the power supply. But, more worrisome is an ultra low frequency (below 20hz) that modulates the DC and the extra AC component. I can eliminate the ultra low frequency with a high pass filter but, of course, it shouldn't be there in the first place.

Is it worth replacing components with higher grade/newer ones or is there a "silver bullet" component that can be replaced?

The supply is small enough (and there is enough room in the case) to consider replacing it completely too.

The noise floor with the original power supply floats between -57db and -67db.
The noise floor with the 48v wall wart supply is about -80db.
and, for comparison, my 90$ Project phono pre has a noise floor of -90db.

Also, I've attached schematic that I created. I hope that it makes sense.

Parts

D1, D2 = 060
D3 = 1N4757
R1, R2 = 910Ohm
R3 = 100Ohm
R4 = 47kOhm
R5 = 10kOhm
Q1 = 2N3054
Q2 = 2N2102
C1 = 47uf 63v
C2 = (Film)? 0.01M 630v
C3 = 100 uf 63v
C4 = 220uf 63v
C5 = 1000uf 63v
C6 = (film?) 0.1M 250v
C7 = (Ceramic?) 01u

Thanks for your time,

Mitch
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Old 16th September 2013, 05:42 AM   #2
ArchMus is offline ArchMus  Canada
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Sorry. Here's the Schematic.
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File Type: png McCurdy Power Supply Schematic.png (28.6 KB, 152 views)
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Old 16th September 2013, 09:39 AM   #3
alayn91 is offline alayn91  France
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Hello,

What is the use of R1, C1, R2 and C4 ??

Zener D3 has no limitation current resistor ??
Between Q1 emitter and D3 cathod, for ex.

Are you sure Q2 is a NPN ?
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Old 16th September 2013, 12:52 PM   #4
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This is one possibility....
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Old 17th September 2013, 03:47 AM   #5
ArchMus is offline ArchMus  Canada
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Hi Rolf and Alayn91.

A schematic for the power supply was not available so I created it based on the circuit board. Here is the layout of the circuit board.

Q1 has "2N3054 RCA CH 7629" printed on it.
Q2 has "2N2102 612" printed on it. Any search results return an NPN Bipolar transistor.
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File Type: png McCurdy Power Supply Layout.png (55.8 KB, 126 views)
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Old 17th September 2013, 01:16 PM   #6
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Very,very strange.The power supply will not work as it is.There are so many faults that I wonder who on Earth managed to make up this mess...

I've drawn another way it could be designed.Note that this second drawing is NOT complete-just an illustration of the principle(how the two transistors could be connected together)
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File Type: jpg 0fel var2.jpg (55.8 KB, 113 views)
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Old 18th September 2013, 04:14 AM   #7
ArchMus is offline ArchMus  Canada
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Hey Rolf.

I went back to the circuit board and verified that it was me who made the mess. I missed a trace and the 2N2102 flipped. Hopefully the revised schematic makes more sense.
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File Type: png McCurdy Power Supply Schematic Revised.png (30.6 KB, 96 views)
File Type: png McCurdy Power Supply Layout Revised.png (57.3 KB, 35 views)
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Old 18th September 2013, 11:22 AM   #8
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Well done!
Now it looks 100% OK.
(don't worry about making mistakes-we all do and hopefully we learn something from it).
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Old 18th September 2013, 12:13 PM   #9
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If you changed all the electrolytics, have you got them the right way round ?

I would start by testing D1 and D2.
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Old 25th September 2013, 05:25 AM   #10
dangus is offline dangus  Canada
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It's not really regulated: there's no feedback, so it's more of an amplified zener arrangement. Perhaps there's some interaction between the preamp and power supply that results in "motorboating".

Any 3-terminal regulator should provide much better regulation. The LM317HV might be the best suited.
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