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1970s Design Indulgence

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 20 May 2019 at 1:48pm
The bathtub curve:



Elcap, Jens Both [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]

The curve for 2200uF can be imagined to be just left of the 1000uF curve.

A 2200uF electrolytic used to reduce the emitter resistance of T2 to much below its intrinsic value (circa 4 ohms) introduces some non-linearities after around 1kHz.

The resistance is only small at less than 0.1 ohms where it transitions from a straight line, but it is 2.5% of the 4 ohms intrinsic resistance. We will lose 2.5% of the required open loop gain, and because of the change from capacitive towards inductive, it will hold implications for phase.

To maintain the 4 ohms intrinsic resistance as an impedance, such that it is 4 ohms at all frequencies - or at least those which give us the sibilance trouble - a bypass capacitor should be calculated to be better than 0.1 ohms at the harmonics we wish to control.

If that were 9kHz we find we need nearly 180uF, which would also be an electrolytic, so isn't going to help.

But if we think of it as paralleling resistors, then a smaller value might help. Trying 4.7uF in simulation showed a problem of paralleling which results in negative impedance due to the formation of a tank circuit with stray inductances.
emitter bypass capacitors


We find that 1uF works well in simulation, and its resonant frequency (dip) is around 2MHz, which we don't really want, but a value of 100nF simulates too high in frequency. The resonance might however be damped by the 1k (total) emitter resistance and the parallel 2200uF capacitor's remaining impedance.

So we'll give 1uF a try.


Edited by Graham Slee - 20 May 2019 at 1:49pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 20 May 2019 at 3:51pm
There is another way to make the VAS a bit more linear and that is by placing a small resistor in its emitter, making say (big) Re 10 ohms in addition to (little) re 4 ohms.

This however, causes peaking using the 1uF from above. We can use 100n instead which doesn't work as good but is better than nothing.

Overall this might result in a more relaxed sibilance band sound? Possibly, but burn-in is full of little surprises...


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 hours 41 minutes ago at 7:19pm
Just read: "...power transformers do not have a sound. They are not in the signal path!"

The law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be conserved over time. This law means that energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.

Current is the flow of charges around a circuit.

Kirchhoff's Current Law says that the sum of all currents flowing into a node equals the sum of currents flowing out of the node.

So the sum of all currents flowing out of a node must have flowed into it.

Signal current is supplied by the power supply; therefore signal current must flow back to it. Current flow is from positive to negative (electron flow is opposite).

Then what? Does it just stop there? How can it? It would constitute the destruction of energy, which cannot happen. Perhaps it's dissipated as heat? Sorry, no!

Signal current flows back up the power supply!

Unless the reservoir capacitors are absolutely huge in a power amp, there is only one route back for low frequencies, and that is via the rectifiers and transformer secondary('s). It will contain crossover distortion of the bridge rectifier, but any harmonics high enough in frequency will "reflow" via the reservoir capacitors.

And this is why if there is sufficient ripple because the reservoir capacitors are too small, you get hum.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 4 hours 3 minutes ago at 10:57am
Q: Is isolating the transformer and associated power supply in a separate case 'all that it is cracked up to be'? Is the effect of the power supply 'components' only connected by the so-called "umbilical cord" to the amplifier going to alleviate distortion/magnetic fields IN the amplifier signal itself?

A: The thing about circulating currents is something I always check, but we only need to go back to schoolboy physics and the battery, bulb, switch and some of that cotton covered wire, to get the picture.

Current flows from battery to bulb (with switch closed) and then back to battery. Current flows in battery and if we make the bulb a heavier load, battery gets warm proving the point.

So simply adding more wire just adds resistance, or in ac terms impedance. There is also inductance and capacitance in the umbilical, but not a substantial amount, but then again it just depends on how sensitive circuitry is to the not substantial amount. Here I'm thinking voltage regulators.

If using a toroidal transformer amongst a sensitive circuit and the toroidal transformer is made lossy enough (less efficient), it won't vibrate so much due to saturation forces, and won't put out much of a field to be picked up by circuit wiring/PCB tracks.

But using a frame transformer (such as an EI) the radiated field is stronger, and even though I/you/we can obtain a specially made low magnetising-current transformer, there will be a signal level where it's bound to be picked up. Even a PCB track is inductive enough.

In that scenario it is best to make the supply remote because radiated energy decays with distance. For example I use a plastic cased PSU1 and tell people to mount it off the rack, which gives distance and also isolates mechanical mains frequency vibration from getting to the turntable arm and cartridge.

But, the current has to flow from, and therefore back to, the power supply, and where's it going to go inside it?

If the load current is small enough and we make the reservoir capacitors large enough, the entire signal current will flow in the reservoir capacitors because at the signal current the capacitor impedance will be smaller than the transformer secondary impedance. Caveat: the high frequencies might not flow too well in a large electrolytic, requiring some film capacitor bypass.

Even if the PSU output is a voltage regulator (a series type such as a 78 series, or an adjustable), it has to flow back to the starting point, which is the positive end of the caps. It cannot flow back up the voltage regulator sense pin. So, wherever we put it, the power supply is part of the amplifier.

Crap in the secondary either becomes part of the signal current, or debilitates the reservoir capacitors in some way if they don't work well with the type of crap. Either way they are now part of the signal.

Measuring it can be frustrating. FFT (spectrum analyser) displays on the test gear flicker all over the place because of LF noise anyway. The 1 kHz distortion test always looks the same. It would be nice to know a measurement which would reveal it. Measurements only get put on analysers by popular demand...

A 50/60 Hz transformer has to work on 60Hz, and therefore on 50Hz - a lower frequency - you will get greater saturation than at 60Hz. The trouble with toroids is they're more "tangential" at this increase.

If you're only drawing a little signal current the saturation is at its greatest, and the crap is at its greatest. Solid-state always sounds great loud, but might not be so good in the first watt. In home listening we rarely escape that first watt.

Then, if there is some household appliance going, it puts its rubbish on the supply, even if it's next door (like my workshop is off a separate supply to my house). The voltage offset (some wrongly call it DC) puts more voltage on one side of the ac swing than the other. The transformer asymmetrically saturates.

Music also contains asymmetric waveforms. The result is distortion at low or first watt, listening levels.

An EI transformer generally doesn't saturate as much, and when it does it is not as "tangential", reducing the chance of placing it in the signal current, and reducing its chance of debilitating the reservoir capacitors. The problem is then how to prevent hum being induced in adjacent circuitry.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 2 hours 20 minutes ago at 12:40pm
[QUOTE=Graham Slee]Q:
'So simply adding more wire just adds resistance, or in ac terms impedance. There is also inductance and capacitance in the umbilical, but not a substantial amount, but then again it just depends on how sensitive circuitry is to the not substantial amount'.
This is very true and whilst I am unsure how much 'measured' performance of the umbilical would show up there will be audible differences between different cable materials, constructions etc (and other connectors), so another variable in the system....

The Source turntable, Audiomods V micrometer, Dynavector XX2-2, Accession M, Elevator, Leema Antila 2Seco CD player, TDL studio 1.
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