Differential Parallel Feed

We all start somewhere
Post Reply
User avatar
Thermionic Idler
Old Hand
Posts: 1018
Joined: Sun Apr 22, 2012 8:22 pm
Location: Southsea

#1 Differential Parallel Feed

Post by Thermionic Idler »

A question to the floor if I may...

I was reading a transcript of a talk that Lynn Olson gave to the ETF in 2004 - here: http://www.nutshellhifi.com/library/ETF.html

I found it very helpful in terms of understanding signal paths as current loops, and how best to optimise them.

However, there is one circuit at the bottom of the page that I'm having trouble getting my head around, namely this one:

Image

It looks like a good way to split the current load across two CCS's if you have a lot of current to deal with. It also looks to me like it would balance the operating points if the valves drifted away from a perfect match.

My question is around the 2-10uF capacitor connected between the two halves of the primary. The article doesn't explain its presence. I assume its purpose is to stop the two CCS's from interacting with each other, but still allow an AC connection. What I don't understand is how the value of 2-10uF was arrived at. Obviously that's desirable in terms of it not needing to be an electrolytic, but would it not also act as a filter with a value like that?
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
User avatar
Nick
Site Admin
Posts: 15694
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#2 Re: Differential Parallel Feed

Post by Nick »

The explanation for the cap is in the title "parallel feed". Its main job in a parafeed connection is to prevent DC from flowing in the output transformer.

Its valve can be calculated by the the inductance of the load using the normal LC equation and the target -3dB point.

If it was electrolytic, it would be a bad place for one, as it would either need two back to back, or a bipolar one.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
User avatar
andrew Ivimey
Social Sevices have been notified
Posts: 8305
Joined: Mon Jun 11, 2007 8:33 am
Location: Bedford

#3 Re: Differential Parallel Feed

Post by andrew Ivimey »

Parafeed yes to kill dc on the interstage transformers and didn't Mr olsen say the make of cap here was crucial to the sound quality not the amount of mfd per se. if I remember I settled on solen with a Russian Teflon in parallel. ( In 'Amity' this was after the input driver triodes and the interstages before the output stage pairs)
Philosophers have only interpreted the world - the point, however, is to change it. No it isn't ... maybe we should leave it alone for a while.
User avatar
Mike H
Amstrad Tower of Power
Posts: 20156
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#4 Re: Differential Parallel Feed

Post by Mike H »

^ Wot they said ~ personally I would try a good audio grade polyprop for the C.

Also not absolutely necessary that the transformer has two primaries, the C will work exactly the same if positioned at one end or the other.

Unless anyone knows differently.....

:D
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
User avatar
andrew Ivimey
Social Sevices have been notified
Posts: 8305
Joined: Mon Jun 11, 2007 8:33 am
Location: Bedford

#5 Re: Differential Parallel Feed

Post by andrew Ivimey »

Wot he said...
Philosophers have only interpreted the world - the point, however, is to change it. No it isn't ... maybe we should leave it alone for a while.
User avatar
Nick
Site Admin
Posts: 15694
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#6 Re: Differential Parallel Feed

Post by Nick »

Mike H wrote:^ Wot they said ~ personally I would try a good audio grade polyprop for the C.

Also not absolutely necessary that the transformer has two primaries, the C will work exactly the same if positioned at one end or the other.

Unless anyone knows differently.....

:D
Though if the transformer is wound so that the geometry is balanced, then adding the leakage inductance caps to the drawing will show that a symmetric result layout can in some situations be better.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
User avatar
andrew Ivimey
Social Sevices have been notified
Posts: 8305
Joined: Mon Jun 11, 2007 8:33 am
Location: Bedford

#7 Re: Differential Parallel Feed

Post by andrew Ivimey »

I'm sure that symmetry, in this situation, is vital.
Philosophers have only interpreted the world - the point, however, is to change it. No it isn't ... maybe we should leave it alone for a while.
User avatar
pre65
Amstrad Tower of Power
Posts: 21367
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#8 Re: Differential Parallel Feed

Post by pre65 »

As I may have said somewhere on here before, 4uf comes up as a common size for parafeed capacitors.

Last time I tried parafeed I used a 4uf Russian cap.(a brown one with 2 feet)
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
Thermionic Idler
Old Hand
Posts: 1018
Joined: Sun Apr 22, 2012 8:22 pm
Location: Southsea

#9 Re: Differential Parallel Feed

Post by Thermionic Idler »

Thanks for the help everyone, appreciated. :) I think I understand now.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
Post Reply