My take on a discrete shunt voltage regulator

[Salas]

I think you need a photo of his total thing to replicate in case you wanna do checks on the HP anyway.

[HiFiNutNut]

Guys, don't worry about that LTSpice thing. It is not related to your regulators. I was giving a generic example to argue the case why Overm found the Rubycon ZA sounded a whole lot better than a film cap and to answer for Telstar anf fff0's curiosities.

[fff0]

Errm.. in conclusion , is the below statement correct?

All film cap only local decoupling is no good and its ok to have electrolytic cap + film cap (so the impedance is not too low that affects the DF)

[HiFiNutNut]

Film caps have much better DF / DA figures therefore should have lower distortions. MKP is better than MKT in this regard, which again betters electrolytic.

However, my discussions with the LTSpice model did not consider any other factors of capacitors other than capacitance, ESR and ESL.

Generally, bypassing electrolytic capaicitors with film capacitors is for the reason that the ESL of an electrolytic capacitor is high at higher frequencies, making it ineffective, while a film capacitor has much lower ESL so it works well at high frequencies.

When inductance (L), capacitance (C) and resistance (R) are met they form a resonant circuit and will ALWAYS resonate unless there is sufficent R there to damp it. The problem with bypassing is this: the output of a regulator may be inductive, wires have inductance, they interact with the capacitors and resonate, unless you have sufficient resistance to damp it.

The reason film capacitors can easily cause resonance is because their ESR is very low. On the other hand, non-low-ESR type of electrolytic capacitors usually have enough ESR to damp the resonance.

But then if your PSU has extremely low inductance and you use ground plain, etc, the circuit may have low enough inductance that a film cap alone may work very well.

With regards to "All film cap only local decoupling is no good and its ok to have electrolytic cap + film cap ", whether the statement is correct or not really depends. Under unusual conditions when the electrolytic capacitor is VERY VERY LARGE comparing to the film cap and it has the right amount of ESR then it can damp the circuit. You need to use some formulae to calculate it or model it with Spice. It is usually not practical.

[Overm]

put a film cap 0.1uF or 0.01uF at the opamp supply pin directly and connect the other end to ground

Just curious, did you try just a Cap over the supply pins on the opamp, so not to the ground?

[qusp]

Originally Posted by ikoflexer

Oh I see, you were talking about another cap locally on the load. I'll test that tomorrow. Thanks Bill!

We better find a recipe to make sure people without an oscilloscope at home can build it and be sure it works well. This is the problem when we're after the last drop of performance. It becomes harder to build right.

surely 5cm-7.5mm max (shorter in many cases) with remote sensing, 4.7uf PPS and local 100pf/10pf PPS or PET bypassing of the load should have me sorted?

[HiFiNutNut]

Originally Posted by Overm

Just curious, did you try just a Cap over the supply pins on the opamp, so not to the ground?

I tried it long time ago. At the time I had no equipment to measure it. So unfortunately I can't give you an answer.

[ikoflexer]

Originally Posted by qusp

surely 5cm-7.5mm max (shorter in many cases) with remote sensing, 4.7uf PPS and local 100pf/10pf PPS or PET bypassing of the load should have me sorted?

Test it and please let us know how it went.

[HiFiNutNut]

Added Rubycon ZL 47uF / 25V to the Vref this morning.

Negative rail was found not regulating. The 17V/5A protection Zener ran HOT!!! I got -18.6V on the negative rail, exceeding the maximum rating of the opa627 ($50) of -18V.

Spent some time debugging it. Suspected that the J201 on the negative has gone to heaven. Took it out, measured it and confirmed the case. Replaced it with a new one. The negative rail was regulated again. As expected, the opa627 survived. I remember for once I had 23V on the pos rail for half a minute or so and the opa627 survived. I think I will get a 15V zener instead of the 17V. Even with 5A rating, I touched it and almost got my finger tip burnt! The max current should not exceed the CCS current of 150mA and dissipation should be (26-18)V x 0.15A = 1.2W. Theoretically the zener should be fine. In reality, I don't know if the zener can survive continuously with such temporature.

I found no reason that the J201 could be burnt, even if the 47uF was dead short. So I guess it might be that when I soldered the sense wire I over heated the J201.

Anyway, adding the 47uF to the Vref does not upset the sound for sure. It is not run-in yet. I also changed the reg output electrolytic cap from 100uF ZA to 47uF ZL. Waiting for them to run-in before making any subjective accessment.

[HiFiNutNut]

47uF on Vref it took very little time to reach the voltage. Next time I may try 100uF.

The caps may have been run-in. They were used a few months ago.

The sound seems to be even a little more cleaner. Note that it was already very clean.