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Old 10th August 2013, 06:50 PM   #1
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Default Power Supply for Tube Preamp

Hello again, I was wondering if anyone has ever made a quiet tube rectified power supply for a Tube preamp?

I'm looking to provide 300 VDC, at 50 milliamps. I already have an Edcor Transformer, 5AR4, 22uf 500 volt, and then Hammond 10H 100m choke, that I will feed several PI filters to bring the rippled and voltage down.

However, whenever "I" build a supply like this, I still have a decent amount of noise present. This could be ME< or it could be the supply- I don't know.

But I'm thinking that regulating that 300 VDC is the way to go, perhaps with a chip?

That's what I'm looking to you folks for advice for.

Any ideas welcomed:-)

Aric
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Old 11th August 2013, 06:54 PM   #2
DF96 is offline DF96  England
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Do you mean noise or hum/ripple? Most people routinely get the grounding wrong when building a PSU. Adding extra smoothing sections then makes no difference, apart from the cost and size.
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Old 11th August 2013, 07:23 PM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by aricaudio View Post
22uf 500 volt, and then Hammond 10H 100m choke, that I will feed several PI filters to bring the rippled and voltage down.
22µF looks a bit flimsy for the task of smoothing 50mA, but anyway, it doesn't make much difference: even if you increased it to 100µF, it would only bring a ~12dB improvement, which is sizeable but not enough to gain complete quietness.
This means that DF96's explanation is probably right: there must be something wrong in your wiring. Probably the ground, but there may be other possibilities too.
If you post your exact circuit, it can be simulated to evaluate how much ripple you'll really incur (with a perfect wiring).
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Old 11th August 2013, 07:36 PM   #4
RJM1 is offline RJM1  United States
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You could use something like this.
MOSFET Regulator
But You would have to change the mpas92,42 to mpsa94,44 and the feedback resistor R9 to about 68K.
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Old 11th August 2013, 10:19 PM   #5
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Check out the 'Maida regulator'. It's a LM317 regulator used in a configuration to allow for high voltages. Good performance, low parts count and a proven design. I've build several with great results. My current preamp (2x 12ax7) uses one at 325V. I don't have the exact numbers at hand, but the ripple is in the few milivolt range. Morgan Jones (I own the 3rd edition) has a great section on the design, but lot's can be found here on the forum and elsewhere online.
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Old 12th August 2013, 08:05 AM   #6
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My advice diy Salas Simplistic HV version 2: Simplistic Mosfet HV Shunt Regs
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Old 12th August 2013, 01:45 PM   #7
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pics and a schematic would help with troubleshooting your problem. Regards
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Old 14th August 2013, 12:21 PM   #8
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Default Schematic Image

Thanks for the replies and sorry for the delay. I'm going to try the PSU in a seperate chassis, with an umbilical cable to the preamp, with conductors of 14/4.

The plan is to send 12 VDC down two wires of the cable, and the 300 volts down the other two.

The preamp has 4 separate channels, 2 are 12ax7 phono stage left and right, and the other 2 are 12ax7 line level preamp channels.

This is a continuation of a project I was building a few weeks back which I decided to scrap.


I am laying out this one from scratch, and really want to get the noise floor down as much as possible.

Please ignore the SS rectifier supply in the drawing I will not be using that, but rather will be using the Tube Rectified version above it.

The second picture is an RIAA stage for the phono that I am implementing into the desgin.
Attached Images
File Type: jpg img273.jpg (385.9 KB, 285 views)
File Type: gif riaa_eq_amp2.GIF (20.6 KB, 265 views)
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Old 14th August 2013, 01:00 PM   #9
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Quote:
Originally Posted by aricaudio View Post
Thanks for the replies and sorry for the delay. I'm going to try the PSU in a seperate chassis, with an umbilical cable to the preamp, with conductors of 14/4.

The plan is to send 12 VDC down two wires of the cable, and the 300 volts down the other two.

The preamp has 4 separate channels, 2 are 12ax7 phono stage left and right, and the other 2 are 12ax7 line level preamp channels.

This is a continuation of a project I was building a few weeks back which I decided to scrap.


I am laying out this one from scratch, and really want to get the noise floor down as much as possible.

Please ignore the SS rectifier supply in the drawing I will not be using that, but rather will be using the Tube Rectified version above it.

The second picture is an RIAA stage for the phono that I am implementing into the desgin.
I would use the choke with SS as well as the tube rectifier . That would let you increase the cap value without increasing the peak load on the transformer average would be about he same.
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Old 14th August 2013, 01:59 PM   #10
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by aricaudio View Post
I am laying out this one from scratch, and really want to get the noise floor down as much as possible.
With such a filter, the noise will be vanishingly small: it is below the default resolution of LTspice.
Since the p-p ripple on C3 is ~215µV and the final RC brings an additional 60dB, the result should be 215nV. Unlikely to cause problems. If you still get hum, that simply means there is something wrong in the wiring.

Note that the supply will be capable of providing only 2.3mA for a 300V output.

From a purely academic perspective, the circuit is improvable: if you make R4 and R5 approximately equal (15K + 18K for example), you will gain a few tens of dB quietness, should that matter...
Attached Images
File Type: png 300Vsupp1.png (86.1 KB, 267 views)
File Type: png 300Vsupp2.png (99.7 KB, 255 views)
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