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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: 15 Oct 2021 at 7:49am
So, looking at the plot above, we see the noise floor on the FFT (unfortunately, the frequency axis scale is cut off due to not having enough screen).

The HF stuff was also measured on a stray piece of wire on the bench with no amplifier in the room, so you can ignore the Himalayas above 50kHz, and the first spike at 20kHz (switcheroo noise).

From about 50kHz upwards, you're in emitter follower oscillation territory, but if this were as such, the spikes would not be -90dB - more like off scale!

You might be able to see the blue and brown traces in the dip, which are in the audio midrange band. As they drop through -110dB you can see them separately. This is where the transformer rotation had effect. In its starting position, it was -90dB with no valley, and so the third harmonic was seen bobbing up and down like a buoy in the sea.

By adding the GOSS band, some of the stray field was mutually induced into it, and by being absorbed, provided sufficient attenuation so as not to couple it by mutual inductance to the nearby circuitry.

However, some leaked through, and by rotating the transformer a little here and a little there, the "hot spots" were positioned so as to interfere less.

My bet is that if the transformer was evenly wound as specified by Terry, the interference would be more uniform, and even if it needed the GOSS band, it would not have to be rotated, and as such, I'd be happy that a DIY made unit would be on spec.



Edited by Graham Slee - 15 Oct 2021 at 7:55am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Oct 2021 at 8:41am
I've invited Terry to read my last three posts in the hope he can offer some kind of answer. Until then, I think we're stuffed (and this being the 21st century...Wink)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2021 at 8:10pm
Now for the results after refitting the RS trafo. It was rotated to get the lowest mid frequency noise, but with no GOSS band available for its size - quite impressive.

noise

The 1kHz harmonics remain unchanged - Vq at 24mV E-E (one watt per channel continuous).

harmonics

Onset of clipping remains at 44 watts, showing a wide harmonic range but at less than 0.1% THD. In its proper use there will be no signal this high, unless pushed to do a disco party (not recommended).

clip level harmonics

For noise comparison, this is a plot of no amp! Open probes with no amp in the room.

noise of open probes


Edited by Graham Slee - 17 Oct 2021 at 8:11pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 18 Oct 2021 at 8:40am
This topic will be mothballed until I find a transformer manufacturer who is able to comprehend physics.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 18 Oct 2021 at 10:07am
Just out of curiosity, how might it perform with an SMPS? Not in the spirit of everyday 1970s design I know.

Ian
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: 18 Oct 2021 at 6:06pm
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.
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Post Options Post Options   Thanks (2) Thanks(2)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 18 Oct 2021 at 6:30pm
One noticeable effect in trying different transformers is the click and pop reduction or exaggeration when playing vinyl.

Clicks are accentuated by slewing complications, and is relatively easy to get right in a phono stage, of the right sort of circuitry.

What tends to be forgotten, is that the phono stage cannot correct slewing problems of subsequent stages.

The reason for this is - contrary to the beliefs of some - our phono stages do not have any click removal, just that the circuit propagation doesn't make a hash of it.

The same high speed rising edges of clicks continue to the amp, and the amp can ruin the job through circuit propagation difficulties. The description "circuit propagation difficulties" covers a multitude of sins.

The recently tested transformers using vinyl as source material were rated as follows

250V primary high-end design, 225VA: largest click annoyance

A 230V primary design, 225VA: not quite as bad

The RS 230V primary, 160VA: what clicks? (OK, they're there if you listen for them)

Some of you remember how mucky my vinyl isEmbarrassed
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