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1970s Design Indulgence |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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I've found a transformer manufacturer to do the reverse engineering required to make the RS 160VA transformer suitable for voltages other than 250V. Reading the intro of Morgan Jones Valve Amps vol.4 - he had the help of a real Sowter! Sowter Transformers still exist but I doubt there'd be that level of help today, as the ownership of the company has changed. I remember engaging with Sowters' around 34 years ago. The request was for a 0.6 ohm output impedance output transformer. It was for an amplifier to drive all the little speakers in the intercoms in a sheltered housing complex - all at the same time! All that wiring capacitance and inductance eh? Quite complex and no site exactly the same! My client was part of the Cable & Wireless group, and one of their engineers put me onto a guy called Phil at a company called Transtronic - who made transformers. The reason was cost, and the Sowter transformer priced the job too high. Otherwise I'd have been with Sowter instead of Transtronic. All told, I built 120 of the amplifiers, having to work day and night to complete the order, otherwise C&W would be into liquidated damages. Edited by Graham Slee - 19 Oct 2021 at 7:59am |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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By kicking everything into China we have no need for so many engineers, and those this country has will never gain any experience, and so will have zero interest in bothering. I recently had to explain grain oriented electric steel to a technical sales lad working for a boastful transformer manufacturer supposedly into aerospace - God help us!
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BackinBlack
Senior Member Joined: 05 Feb 2012 Location: Hinton, N'hants Status: Offline Points: 2020 |
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I'd forgotten that chapter . When you consider the windings on a toroidal Transformer there will always be an uneven nature in their distribution around the core as inevitably they are more densely packed in the centre than the outside of the toroid. This must lead to "gaps" in the windings at the periphery. I suppose the answer would be a larger diameter core to reduce the close packing in the centre, but nothing comes for free. I wonder if the so called R core transformer seeks to address this?
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Just listen, if it sounds good to you, enjoy it.
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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Voltage regulator oscillation Tools required 1. Download a copy of "Jones - Valve amplifiers 4e.PDF" and refer to pages 416 and 417. Enter the values by Jones into the Okawa calculator and you should get the same or similar value as Jones' equation top of P417. This assumes zero input impedance to the regulator. In truth, there is about 10 inches of wiring at the bottom end of the adjacent decoupling capacitor, and at 20nF per inch you can add 200nF to the regulator inductance. As Jones explains, "A perfect Thévenin source in series with an inductor would look identical" in describing regulator output impedance (P412). Without some additional resistance the voltage regulator (here it's a TL783) can oscillate if the output is capacitive loaded (which it must be). The input should be decoupled by a capacitor to prevent this. However, as discussed regarding dirty and clean grounds, we cannot do any better than the HT decoupler we already have, but that has lots of series inductance to power ground. If we add resistance to the Jones values in the Okawa calculator, we can see the Q fall. Eventually the result changes to "The system does not oscillate". This resistance is placed in series with the regulator's output capacitor. Often, the value found will be less than one ohm, and so a one ohm resistor will suffice. Having used up all my one ohm resistors, I am using 2R2, and that means that at around 3kHz the A section of the circuit sees a 2R2 source impedance. There will be less than 0.1V "wiggle" to the A section supply at 3kHz upwards. This should not lead to a worse sound. However, if we look at page 424, we can read about "regulator sound." Although Joes' book is about valve amplifiers, the same applies to solid-state.
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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I think it may be of more interest to read this example, on page 451 of Jones' book... "The DC resistance from 0 V to each 260 V pin was measured. One was 99 Ω and the other 89 Ω, and this disparity is common in layer-wound transformers because it is cheaper to layer-wind a centre-tapped transformer with one half of the winding on top of the other, so the average diameter of the outer winding is a little larger than that of the inner winding, resulting in a slightly higher copper resistance. Unless balanced by adding an external resistance to the inner winding, a ripple component at mains frequency appears at the output of the rectifier, which is not particularly well attenuated by an LC filter. Thus, we should add a 10 Ω resistor in series with the 89 Ω winding to make both 99 Ω." Now, consider what I was saying about using two secondaries in parallel, and therefore there will be circulating current in one of the secondaries ("a ripple component at mains frequency"), and as we don't use an LC filter, it won't be attenuated at all. Once the smoothing capacitors "burn-in" to the latest "components" or artefacts, which might take several hours, or days, they make themselves known through what's heard. |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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Although I downloaded free Morgan Jones' book, and most of it is about valves, I highly value the power supply section which has been very helpful to date. I have therefore bought the paperback version by way of thanks to the author. It's the right thing to do.
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BAK
Senior Member Joined: 14 Mar 2010 Location: Kentucky, USA Status: Offline Points: 1744 |
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The center-tapped transformer secondary is shown below in a full-wave rectifier valve circuit. Picture is from: RC-22_RCA_Receiving_Tube_Manual_Jul63 . This is the same circuit for solid state using silicon diodes (without the small filament winding shown in the top of the drawing). The DC smoothing filter would be connected after this. The larger center-tapped secondary will have unequal winding copper resistances when one half is wound on top of the other. This is the same problem with 2 separate secondaries of equal voltage... one sec will have more copper resistance than the other sec when one is wound on top of the other. BUT, adding a small resistor to "balance" the 2 secondaries adds a new problem to the rectified DC delivered to the smoothers in an unbalanced voltage. The winding without added resistance now delivers more voltage than the other due to the series resistance voltage dropped. The only answer is to make the transformer have the 2 secondaries exactly equal in # of turns and in copper resistance, as in side-by-side and not one on top of the other, OR use a single winding and a full-wave bridge rectifier.
Edited by BAK - 21 Oct 2021 at 11:54am |
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Bruce
AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Proprius & Reflex M or C, Enjoy Life your way! |
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