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#1
Posted to rec.audio.tubes
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parallel chokes in a power supply
I need opinions on the use of two chokes in parallel in an otherwise
standard Pi type filter. c - L//L - c Assuming nearly identical chokes, ie, same part number and manufacturer, will there be any problems? I know the combined parallel inductance and resistance will be one half that of a single choke, and the current capacity will be doubled. Will one choke "take over" and burn out? I have "space in the chassis problems", so I can't use one big choke. -Don |
#2
Posted to rec.audio.tubes
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parallel chokes in a power supply
On Mar 18, 2:17*am, Don wrote:
I need opinions on the use of two chokes in parallel in an otherwise standard Pi type filter. *c - L//L - c Assuming nearly identical chokes, ie, same part number and manufacturer, will there be any problems? I know the combined parallel inductance and resistance will be one half that of a single choke, and the current capacity will be doubled. Will one choke "take over" and burn out? I have "space in the chassis problems", so I can't use one big choke. -Don Two parallel chokes in a CLC supply should have nearly equal winding resistance so the heat generated in each is the same. If they have equal inductance, the total L for 2 becomes half the value. Total L= 1 / ( 1/L1 + 1/L2 ) Would you still have enough L if you parallel 2 chokes? The Vac across the chokes causes negligible winding heat. Patrick Turner. |
#3
Posted to rec.audio.tubes
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parallel chokes in a power supply
On Mar 17, 11:17*am, Don wrote:
I need opinions on the use of two chokes in parallel in an otherwise standard Pi type filter. *c - L//L - c Assuming nearly identical chokes, ie, same part number and manufacturer, will there be any problems? I know the combined parallel inductance and resistance will be one half that of a single choke, and the current capacity will be doubled. Will one choke "take over" and burn out? I have "space in the chassis problems", so I can't use one big choke. -Don Analogue vacuum tube projects are very tolerant (in general) to small difference such as your project . I wouldn't worry about it . Unless it will power a precision sensitive circuit , such as a very stable RF source or a similar application . |
#4
Posted to rec.audio.tubes
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parallel chokes in a power supply
On Mar 18, 8:23*pm, wrote:
On Mar 17, 11:17*am, Don wrote: I need opinions on the use of two chokes in parallel in an otherwise standard Pi type filter. *c - L//L - c Assuming nearly identical chokes, ie, same part number and manufacturer, will there be any problems? I know the combined parallel inductance and resistance will be one half that of a single choke, and the current capacity will be doubled. Will one choke "take over" and burn out? I have "space in the chassis problems", so I can't use one big choke. -Don Analogue vacuum tube projects are very tolerant (in general) to small difference such as your project . I wouldn't worry about it . Unless it will power a precision sensitive circuit , such as a very *stable RF source or a similar application . With a CLC filter the LC section after the first C has a low frequency resonance which is calculated at Fo = 5,035 / sq.rt ( L x C ) where Fo in Hz, C in uF, and L in mH. This becomes critical in an SE amp audi amp which the OP may indeed have. Fo ideally should be less than 3Hz, but sometimes the required C2 after the L becomes too large a value in relation to C1 and L in order to get Fo low enough. Ideally, C1 should have reactance ( ZC1 ) at the rectifier ripple frequency of 0.1 x RL where RL is the output Vdc / Idc amplifier load. ZL should be 10 x ZC1 and C2 should be 0.1 x ZL. If there is a tube rectifier then C1 can only be a rather low C value, say 47uF. Suppose F Ripple = 100Hz, then XC = 34 ohms. Then XL = 340 ohms minimum so L = 0.5H. Then ZC2 = 34 ohms so C2 = 47uF. Ripple reduction factor = ZC2/ ZL = 34 / 340 = 0.1, and Fo between L and C2 = 33 Hz and the Q of the resonance curve is high. Usually the load of the amplifier is an R value way above what is required to damp the resonance. Suppose Vdc = 400V and Idc = 250mAdc, then RL = 1k6. At Fo = 33Hz, ZC2 = ZL = 103 ohms. Ripple reduction factor of only 0.1 is a poor quality outcome. To damp the Fo to stop the PSU bouncing up and down at 33 Hz the RL should be 102 ohms. But one could have a a series R of 102 ohms between L and C which would damp Fo OK but then there is an unwanted 25Vdc drop across the 102 ohms after the choke before C2. But in practice with a tube rectifier one wants ZL = 100 x ZC1, or 3,400 ohms in the example case. This means L = 5H, a rather large heavy item but its the right price for getting real Hi-Fi, OK. Then C2 still should be 0.1 x ZL which would give ZC2 = 340 ohms so C2 could be 4.7uF but this has a value of reactance at LF which fails to anchor the OPT take off at the wanted LF pole of the amp. ZC should be 0.01 x ZL so C2 = 47uF minimum. Then Fo L&C2 = 10.38Hz, and this is better than 33Hz but the outcome is still low quality and what is wanted is C2 = 470uF which gives Fo = 3.3Hz which is OK. The peak charge currents of the tube rectifier are mainly determined by C1 value and don't much increase because of following 5H and large value C2. At Fo = 3.3Hz, ZL = ZC = 102 ohms. To damp the Fo, 102 ohms is needed for RL or in series with C2 after L. A 5H choke might have winding R = 50 ohms which gives 3.125W heat at 250mAdc. The Rw is effectively in series with L value, so additional R could be 50 ohms for a flat response. Vdc drop across choke = 12.5Vdc and 12.5Vdc across the additional R. In practice the resonance peaking at 3.3Hz is not going to be a nuisance if the R is 50 ohms, mainly because 3.3 Hz is so far below the wanted AF band. So in fact additional R isn't needed. But its Best Practice to add the R. And then we would make things almost perfect by adding a second 470uF 450v rated C to make the filter C1 L C2 R C3, with the L = 5H and R = 50 ohms and make C2 = C3 = 470uF. Ripple voltage becomes extremely low, but that's what is needed for an SE amp. Its less important for a PP amp although the CLCRC filter is unsurpassable for where a well smoothed screen supply has more importance than a well smoothed B+ for the CT or OPT. I don't share your your slap-dash approach to audio amp design. For optimal outcomes we must worry about everything and spend a bit more on L and C. To avoid resonance effects at LF the alternative option is to dismiss the tube rectifier with its very poor peak current handling ability and use S1 diodes to feed whatever high C value we can afford. Peak charge current is then limited by winding resistance or HT and HT windings plus wall outlet resistance but to limit the high peak charge currents with say C1 = 470uF, a series R between HT winding and bridge should be used, say 10 x ZC at ripple F, so say 33 ohms. Instead of any L, we then may use 2 sections of RC filters using 33 ohms and 470uF which gives a ripple reduction factor = 0.01, and because Vr is so low at C1 the Vr at C3 is extremely low. The Vdrop across the total 100 ohms of added R is roughly about 25Vdc so the outcome is about as good as the CLC with 47uF, 5H, 470uF. But Vdc may be too high because the winding was designed for a tube rectifier where the Vdc might be 1.1 x Vrms of HT winding. With an Si rectifier we get Vdc = 1.35 Vrms, ( loaded ). OK, then the three R values are made larger to get the Vdc down to where we want it. There is increased heat production in the increased R which would otherwise be liberated as heat in the rectifier tube. The HT winding should have a number of taps to allow adjustments of B +. Factory made power trannies seldom have enough taps, and expect the user to work out carefully what he wants then buy accordingly. Many DIYers just don't make it through the design maze without ending up making a sub-optimal amplifier. There is always some damn price to be paid unless design is done well, and unless designers regard the exercise as critical. There is much design given in pages of RDH4 or at at my website pages listed at http://www.turneraudio.com.au/education+diy.htm Patrick Turner. |
#5
Posted to rec.audio.tubes
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parallel chokes in a power supply
My gut feelings on this are - Parallel L is not unlike series C and will likely have some of the same problems. Yes inductance and resistance will be halved but peak current may not, just as peak V in series C isn't, because real components aren't perfect. Series L would be better, two small chokes that can carry the current but with half the inductance each. How come two chokes are smaller than 1? It seems unlikely. |
#6
Posted to rec.audio.tubes
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parallel chokes in a power supply
On 03/21/11 10:29, mike s so witilly quipped:
How come two chokes are smaller than 1? It seems unlikely. the 2 chokes might be physically smaller due to lower current and therefore thinner wire. It probably has something to do with the inside diameter and wire thickness of the physically larger choke. I might also guess that the larger choke is rated WAY more than twice the current of the smaller ones. Or, it could have a potting can around it making the whole thing 'that much bigger'. Maybe it would work better to get the right choke for the job? /me is a fan of LC or LCLC to take full advantage of the input inductor's behavior of inducing voltage to maintain current flow. |
#7
Posted to rec.audio.tubes
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parallel chokes in a power supply
On Mar 17, 3:17*pm, Don wrote:
I need opinions on the use of two chokes in parallel in an otherwise standard Pi type filter. *c - L//L - c Assuming nearly identical chokes, ie, same part number and manufacturer, will there be any problems? I know the combined parallel inductance and resistance will be one half that of a single choke, and the current capacity will be doubled. Will one choke "take over" and burn out? I have "space in the chassis problems", so I can't use one big choke. -Don Some other matters to consider with various choke configurations: http://www.audio-talk.co.uk/fiultra/...dre%20Jute.htm Andre Jute Visit Jute on Amps at http://www.audio-talk.co.uk/fiultra/ "wonderfully well written and reasoned information for the tube audio constructor" John Broskie TubeCAD & GlassWare "an unbelievably comprehensive web site containing vital gems of wisdom" Stuart Perry Hi-Fi News & Record Review |
#8
Posted to rec.audio.tubes
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parallel chokes in a power supply
Thanks for all the help, everyone. I decided to use one big choke on the
top of the chassis, instead of two smaller chokes hidden away below deck. |
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