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Separate LC legs LR Preamp Supply

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Posted on September 14, 2014 at 14:17:39
chrismercurio
Audiophile

Posts: 155
Joined: August 4, 2008
 Hi,

This winter I'm rebuilding my preamp supply which takes some planning. Outside of cash outlay and chassis real estate, what are the differences between going:

LCLC then splitting at the last leg with separate LC legs for left and right vs independent LCLCLC supplies for left and right given a single chassis and power transformer?

Best,

Chris

 

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RE: Separate LC legs LR Preamp Supply , posted on September 14, 2014 at 15:16:42
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 They are both equally overkill.

 

Less is better than more, posted on September 14, 2014 at 17:22:11
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012
 Shoot for a ripple number, not part count. More parts in the power supply than what are needed sound worse. You can create parasitic oscillations and tank resonances if you don't know what you are doing. Chokes need a certain level of current to function. Make sure your chokes are designed to function with your current draw. All chokes are electromagnetic radiators as well.

 

RE: Separate LC legs LR Preamp Supply , posted on September 14, 2014 at 18:34:07
chrismercurio
Audiophile

Posts: 155
Joined: August 4, 2008
 Thanks guys.

I've used PSUD . The 3 stage gets the best noise figures, and the the two stage that I'm using currently doesn't cut it in the real world when it comes to noise. I was really asking about whether shared supply in a preamp is okay where it splits at the final leg or if shared supply is fine? What kind of performance drawbacks are their with shared supply in a low current preamp?

 

RE: Separate LC legs LR Preamp Supply , posted on September 14, 2014 at 22:21:19
twystd
Audiophile

Posts: 2723
Location: Austin,Texas
Joined: December 9, 2000
 I have a PDM (pseudo dual mono) power supply that is overkill on a line stage, even with 101dB and more speakers. Mine has a common input choke, with more than critical inductance. After the input choke, it splits into two caps (L and R), then goes to two chokes, left and right, then the two L and R final caps. So it's a LCLC filter, that goes PDM after the input choke.

My theoretical advantage is that, by splitting earlier, you can get better separation of the dynamic demands of current fluctuations, from channel to channel. Also, by using two chokes in parallel, you can reduce the overall impedance of the supply. IMO, always a good thing if you can. The downside is that it takes an extra choke and cap.

The economics of chokes can sometimes allow you to buy two smaller chokes, rated for one half the ma., with less DCR, for the same or less you'd pay for one double the ma., at a given DCR. So you may maximize your L/DCR price ratio.

The third possible advantage is, you can physically mount these two chokes in parallel, in relatively close proximity, orientated in opposite directions, which should help cancel radiated electromagnetic fields. I do this in hopes of minimizing any effect the devices would have on surrounding components, hopefully helping to lower the noise floor. I'm trying to emulate some of the advantage a dual C core choke has, using more conventional ones.

I couch this last statement in conjecture, because I haven't tried to experimentally confirm it, due to the lack of the proper instruments, and the expertise to use them. This has been a common construction technique that I use, whether it's really effective or not, I don't know.

Whether you go down any of this route or not, I agree with others, who feel that the amount of stages you propose is excessive, even for a phono stage.

twystd

 

Then you need to adjust your parts values, posted on September 15, 2014 at 05:18:58
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012
 I can sim a LCLC in PSUII that has virtually no ripple. Once you get to beyond 0.001 vAC ripple you are chasing software ghosts, not sound quality. And no, trying to pull 10mA out of a separate LCLCLCLCLCCLCLCLCLC leg does not work very well.

 

RE: Separate LC legs LR Preamp Supply , posted on September 15, 2014 at 07:25:04
Thomas Mayer
Manufacturer

Posts: 602
Joined: May 8, 2001
 Hi!

There is no general answer to this. It depends largely on your circuit and specifically on the PSRR (power supply rejection ratio) of your stages. With high PSRR little is gained by separate LC per channel. If it is low it might be mandatory

Also more important than separating left and right channel is how you separate first and second (or third) stage as there might be more interaction between these

best regards

Thomas

 

RE: Then you need to adjust your parts values, posted on September 15, 2014 at 09:07:54
mobiasloop
Audiophile

Posts: 100
Location: SW Missouri
Joined: August 14, 2014
 In the Westrex days in Hollywood as well as RCA, Mixer preamps were made that supplied 6-12 or more two stage preamps from one power supply. But the supply had one 30H choke and 7 tubes alone and very well regulated. It's just a question of how well the power supply is designed and how much ripple/current draw it serves. Moe Bias
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare

 

RE: Then you need to adjust your parts values, posted on September 15, 2014 at 12:34:48
chrismercurio
Audiophile

Posts: 155
Joined: August 4, 2008
 Then, do any of you ever use a plate choke in the power supply if the current demands are really low? (Massive Henries, high DCR, low current)

 

RE: Then you need to adjust your parts values, posted on September 15, 2014 at 16:15:44
mobiasloop
Audiophile

Posts: 100
Location: SW Missouri
Joined: August 14, 2014
 Counterpoint I believe did it in one of their more expensive preamps. Shure had a vacumn tube phono preamp (almost identical to a Dyna PAS3X except it had transformer coupled balanced outputs) that used a choke in a seperate chassis power supply. So yes it's just the designer's choice and how well the parts are employed. I see little need for separate power supplys/chokes for each channel. You just need to choose circuits with very good PSRR and choose a well designed power supply.I looked up the Westrex RA1479 power supply used in a recording mixer and it's ripple content at a DC output of 275v B+ does not exceed .001v at it's specified current draw up to .055A . Moe Bias
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare

 

RE: Then you need to adjust your parts values, posted on September 15, 2014 at 16:59:18
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 "do any of you ever use ... Massive Henries, high DCR, low current."

Large inductance, yes. Low DCR isn't beneficial in a preamp if all the circuitry is Class A.

 

RE: Then you need to adjust your parts values, posted on September 15, 2014 at 19:51:16
chrismercurio
Audiophile

Posts: 155
Joined: August 4, 2008
 It's a single stage DHT preamp. Class A.

 

PS Design, posted on September 15, 2014 at 22:29:02
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 Class A always draws the same average current, so RMS regulation from the perspective of loading is not an issue. Also, the instantaneous values within the average should be supplied by the last capacitor in the chain feeding each stage. The primary goals for the PS in a low-current Class A preamp should be ripple reduction and isolation. The latter includes isolation between the channels as well as stage to stage (if more than one) within each channel. Low DCR is not necessarily beneficial to these functions, and many multi-voltage preamp supplies use resistors instead. Personally, I would consider it a waste of resources to use high inductance, low DCR chokes for this purpose.


 

RE: Then you need to adjust your parts values, posted on September 16, 2014 at 00:31:06
Thomas Mayer
Manufacturer

Posts: 602
Joined: May 8, 2001
 Yes, I do that often in preamps

Thomas

 

RE: Then you need to adjust your parts values, posted on September 17, 2014 at 10:56:24
chrismercurio
Audiophile

Posts: 155
Joined: August 4, 2008
 Thomas,

I was planning on using Lundahl chokes. I asked Per about it and he indicated, "Today all our chokes for power supplies are wound using paper insulation, as we have experienced that this makes the chokes more quiet. Technically, yes you can use the LL1668 in a power supply. However, they MIGHT not be as quiet as our dedicated supply chokes." Have you had any issues or noticed this? Part of the reason I was looking at their products was for ease of placement and the other was the lower inherent noise and radiated fields. With 50-100H chokes available, it's very easy to get a low noise supply with small value caps.

Do you think it's worthwhile to make separate legs for left and right? Or it's not important especially if it's all in a single chassis?

Kindest regards,
Chris

 

RE: Then you need to adjust your parts values, posted on September 18, 2014 at 03:04:42
Thomas Mayer
Manufacturer

Posts: 602
Joined: May 8, 2001
 Hi!

For preamp voltages you will not have issues with buzz from the plate chokes.

>>> Do you think it's worthwhile to make separate legs for left and right? Or it's not important especially if it's all in a single chassis? <<<<

See my post above, this heavily depends on your circuit (PSRR).
I usually feed each channel and each stage with a separate RC or LC leg

Thomas

 

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