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1970s Design Indulgence

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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 Oct 2021 at 7:53am
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.

Contrast that "can do - will do" with today. Today such a project would take years (and so wouldn't happen), because there's either nobody with the experience, or if they have, they cannot be arsed (sorry, couldn't find a different way to put it).



Edited by Graham Slee - 19 Oct 2021 at 7:59am
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 Oct 2021 at 8:05am
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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Post Options Post Options   Thanks (1) Thanks(1)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 19 Oct 2021 at 8:34am
Originally posted by Graham Slee Graham Slee wrote:

Originally posted by BackinBlack BackinBlack wrote:

Just out of curiosity, how might it perform with an SMPS? Not in the spirit of everyday 1970s design I know.

Ian

We went there a while ago, but as 72V SMPS's are over a grand, I used two 36V in series. They sang to each other in the case, but it didn't interfere with the sound or measurements. I loaded the SMPS's by wasting power into a heatsinked resistor which quietened them a little, but nowhere quiet enough to go unnoticed. A single unit is quieter because the feedback circuit works as designed, but by putting two in series the feedback loops are confused with one output leaking to the other, and it's the instability in the SMPS feedback networks that makes them produce more noise. I was using SMPS's designed for series connection, but I guess the noise doesn't matter in other envisaged applications.

I'd forgotten that chapter Confused.
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?
Just listen, if it sounds good to you, enjoy it.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 Oct 2021 at 10:33am
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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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 Oct 2021 at 10:56am
Originally posted by BackinBlack BackinBlack wrote:

Originally posted by Graham Slee Graham Slee wrote:

Originally posted by BackinBlack BackinBlack wrote:

Just out of curiosity, how might it perform with an SMPS? Not in the spirit of everyday 1970s design I know.

Ian

We went there a while ago, but as 72V SMPS's are over a grand, I used two 36V in series. They sang to each other in the case, but it didn't interfere with the sound or measurements. I loaded the SMPS's by wasting power into a heatsinked resistor which quietened them a little, but nowhere quiet enough to go unnoticed. A single unit is quieter because the feedback circuit works as designed, but by putting two in series the feedback loops are confused with one output leaking to the other, and it's the instability in the SMPS feedback networks that makes them produce more noise. I was using SMPS's designed for series connection, but I guess the noise doesn't matter in other envisaged applications.

I'd forgotten that chapter Confused.
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?

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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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 hours 17 minutes ago at 2:47pm
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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