Re: Re: measurement summary
The PSU was actually charging and discharging between 20.2 and 25.2 V. I think the explanation is in the transformers. I am using 120VA EI transformers. The resistance of the secondaries must be limiting the charging rate and the load regulation may be rather poor for these transformers, since they were running very near the VA rating . The trace on the scope looked more like a triangle than a sawtooth. A toroidal transformer could probably charge at a higher rate. I also have a 1 meter run of 18 gauge wire from the PSU to the amplifier. It shouldn't be too hard for me to calculate the resistance of the wire and determine if there was a significant voltage drop across it.
Jeremy
Pedja said:
The PSU was actually charging and discharging between 20.2 and 25.2 V. I think the explanation is in the transformers. I am using 120VA EI transformers. The resistance of the secondaries must be limiting the charging rate and the load regulation may be rather poor for these transformers, since they were running very near the VA rating . The trace on the scope looked more like a triangle than a sawtooth. A toroidal transformer could probably charge at a higher rate. I also have a 1 meter run of 18 gauge wire from the PSU to the amplifier. It shouldn't be too hard for me to calculate the resistance of the wire and determine if there was a significant voltage drop across it.
Jeremy
please use "monstersize" trafo when performing those tests - so you are absolutely certan that there is no limitations.....ei regulates better than torroid btw......but anyway, very interesting test - thanks.
Wouldn't a film cap in parallel with the main psu capacitors shunt alot of the high-frequency content to ground?
I know alot of people don't like bypass caps, but here sounds like somewhere they might come in handy.
I know alot of people don't like bypass caps, but here sounds like somewhere they might come in handy.
such is reccomend on the datasheet, but not located near the main caps but rather near the chip itself.
JoeBob said:
Not really. The cap is the CAUSE of the hf content. If you read the earlier posts, you will see that increasing the cap generally increases the hf content, because the current pulse gets narrower/sharper and that means a higher hf spectrum.
It would be different if the hf signal was fed to the cap via an impedance, then you would have the normal low pass filtering action.
Jan Didden
JoeBob said:
I agree with Janneman, it might actually make it worse. A classic Dr Jeckyll vs Mr Hyde. It may increase the peak current at HF and be worse, on the other hand it may present a Lower Z at those freqs to the subsequent load. Which is preferred? It may be a 'horses for courses' situ. Almost near unpredictable. This is why you cannot really design overall 'by the book' and why a combination of gut feeling and thorough experimentation cannot be substituted. What comes to mind is a time (and I suspect that it still does happen) when there were those who said that all amps sounded the same. Yet Matti Otala (the TID guy) said there are thousands of reasons why they sound different. I have never heard two amps sounding the same. That's the honest truth. So power supplies make a difference, that we can all agree? So why be surprised if some use it to shape the kind of sound they like? Provided the end result works, right?
Joe R.
Joe Rasmussen said:
there is no question that power supplies make a difference. But how many of us can hear it at -50db over 1Khz, on an amp with over 40-50db psrr?
I cannot.
Materiality is the key, everyone.
Guys, I don't want to start a discussion here, we're just debating this, and this is an interesting one.
But it seams to me that what is good for digital is not good for analog.
Would you put a single electrolythic cap on the PSU pins of a DAC?
I would bypass it with a small ceramic/polyester cap near the chip's pins.
Is this bad for a GC?
Wouldn't a PSU be better if it has low impedance at high frequencies?
But it seams to me that what is good for digital is not good for analog.
Would you put a single electrolythic cap on the PSU pins of a DAC?
I would bypass it with a small ceramic/polyester cap near the chip's pins.
Is this bad for a GC?
Wouldn't a PSU be better if it has low impedance at high frequencies?
Beside the content and level of the HF noise generated by the larger cap value, the problem with larger_main+small_bypass_cap is the way in which these two caps are integrated. The larger cap has inductive rise at some point (which is for us too low). So we put one small cap that can keep the impedance low further above. So we’ll indeed have further above that our wanted low impedance, but the meeting point of two caps might be one new resonant frequency.
As Carlos pointed out, in the digital (and high speed opamp) circuits it is usually beneficial to put small bypass close to the chip, because here you have more HF problems than in classic slow analog circuits. Bypass in the analog circuits can make more problems than it solves.
Pedja
As Carlos pointed out, in the digital (and high speed opamp) circuits it is usually beneficial to put small bypass close to the chip, because here you have more HF problems than in classic slow analog circuits. Bypass in the analog circuits can make more problems than it solves.
Pedja
Pedja said:
OK for power op-amps.
Modern signal op-amps are very fast, and do benefit from bypassing.
Like the LM6171.😱
Excellent, BTW.😉
And my beloved OPA627.😀
I took some steps toward the real world. One result of it is attached. Supply is made of 400W, 2x22V toroidal transformer, double bridge formed by SB560 and 2x4700uF per voltage, then I have the regulation. Drawn current is 80mA. Absolute level is not scaled either to DC or to 1V. I think it is anyway good for better understanding of what comprises the waveform regularly called a 100Hz ripple.
(Interesting thing: the higher noise floor you see below 300Hz went down for 10-15dB when I added 1 Ohm series resistor after the rectifier. So, the RC filter on work or something else…)
Short experiments with some parameters told me that the situation might not change exactly as predicted in the simulations but the simulations were far from being totally wrong either. Once again, take them as the orientation of how some parameters influence the things but remember that the transformer and diodes actually used as well as the parasitic properties will determine the final results.
I might do one day more extensive and systematic measurements of this kind, varying as much of parameters as possible, but it could last. So, no comparison from me for now but someone else might send a classic 1000uF GC supply FFTed (at the moment I don’t have any such GC at hand and I could only improvise) so we can compare it (of course “generally” and not “directly”) with the graph above.
Pedja
An externally hosted image should be here but it was not working when we last tested it.
(Interesting thing: the higher noise floor you see below 300Hz went down for 10-15dB when I added 1 Ohm series resistor after the rectifier. So, the RC filter on work or something else…)
Short experiments with some parameters told me that the situation might not change exactly as predicted in the simulations but the simulations were far from being totally wrong either. Once again, take them as the orientation of how some parameters influence the things but remember that the transformer and diodes actually used as well as the parasitic properties will determine the final results.
I might do one day more extensive and systematic measurements of this kind, varying as much of parameters as possible, but it could last. So, no comparison from me for now but someone else might send a classic 1000uF GC supply FFTed (at the moment I don’t have any such GC at hand and I could only improvise) so we can compare it (of course “generally” and not “directly”) with the graph above.
Pedja
I think my take away from this is that 1) there is little high frequency content in voltage. and 2) paired that with an amp having decent psrr, PS ripple shouldn't be a major problem.
However, i would add that when large caps are used, current going through the diodes / bridge may carry a lot of HF content. Thankfully, that gets BETTER under heavier load.
However, i would add that when large caps are used, current going through the diodes / bridge may carry a lot of HF content. Thankfully, that gets BETTER under heavier load.
carlosfm said:
a 50w rms / 8ohm amp can drop upwards of 3amps. For a 1-ohm resistor, that means a drop of 3v peak per rail.
I am not sure sure if that is worth a 10 - 15db drop in the <300hz range, where psrr usually is pretty good.
millwood said:
Yes, I agree, 1 ohm is too much, but 0.1~0.22 should be OK.
I was talking PSUs generically.
As for op-amps (even power op-amps in GCs) I always put a 0.1 cap between + and - pins on the chip.
That makes PSRR at high frequencies a little better.
carlosfm said:
It doesn't do diddly for PSRR. PSRR is an attribute of the amplifier itself.
It MAY do a little to reduce hf ripple, but with .1uF it will be minimal.
Jan Didden
FFT MEASUREMENT - not simulated
Hi Guys
Pedja sent me an email earlier in the week. When I mentioned that I had done FFT measurements of power supplies, he asked, as I recall it, if I could supply one of the JLTi or gainclone, using 1000uF caps etc.
Can't supply exactly that at the moment, but will as soon as possible and post it here.
What I will furnish is an FFT measurement of a hybrid amp (tube front end & Mosfet outputs) capable of 115W into 8 Ohm and 180W into 4 Ohm. This is an amp called the JR-100 that I designed for a company called Audio Fidelity about 12 years ago.
A few details so that the measurement can be seen in correct light. Power supply is +&- 50V DC, from 500VA tx, uses standard 35A bridge (nothing special) and 15.000uF/63V electros. The bias current is 300mA. The measurement was taken across the plus 50V rail, so it is single ended (I could do it across both rails - but needs to configure for balanced input on my equipment - trickier to wire up - but could be illuminating to do).
A 'true' AC measurement shows that total ripple is 55mV. Because it is a 'true' measurement, this includes the 100 Hertz ripple and all harmonics in total.
The measurement is calibrated. The 0dBu line at the top is 0.775V (following the normal convention). That means we can roughly calculate the AC value of every harmonic. For example, the 9th harmonic at 900 Hertz is -60dB, that means we can calculate its AC value as -60dB relative to 0.775V RMS. Your calculator will show that to be 0.775/1000 = 0.775mV out of 55mV total.
Please feel free to make your own comments and observations. Certainly there are similarities with Pedja's, but I don't think his is calibrated in msg #131?
Joe R.
Hi Guys
Pedja sent me an email earlier in the week. When I mentioned that I had done FFT measurements of power supplies, he asked, as I recall it, if I could supply one of the JLTi or gainclone, using 1000uF caps etc.
Can't supply exactly that at the moment, but will as soon as possible and post it here.
What I will furnish is an FFT measurement of a hybrid amp (tube front end & Mosfet outputs) capable of 115W into 8 Ohm and 180W into 4 Ohm. This is an amp called the JR-100 that I designed for a company called Audio Fidelity about 12 years ago.
A few details so that the measurement can be seen in correct light. Power supply is +&- 50V DC, from 500VA tx, uses standard 35A bridge (nothing special) and 15.000uF/63V electros. The bias current is 300mA. The measurement was taken across the plus 50V rail, so it is single ended (I could do it across both rails - but needs to configure for balanced input on my equipment - trickier to wire up - but could be illuminating to do).
A 'true' AC measurement shows that total ripple is 55mV. Because it is a 'true' measurement, this includes the 100 Hertz ripple and all harmonics in total.
The measurement is calibrated. The 0dBu line at the top is 0.775V (following the normal convention). That means we can roughly calculate the AC value of every harmonic. For example, the 9th harmonic at 900 Hertz is -60dB, that means we can calculate its AC value as -60dB relative to 0.775V RMS. Your calculator will show that to be 0.775/1000 = 0.775mV out of 55mV total.
Please feel free to make your own comments and observations. Certainly there are similarities with Pedja's, but I don't think his is calibrated in msg #131?
Joe R.
An externally hosted image should be here but it was not working when we last tested it.
Joe,
This is no load, just the 300mA bias?
Did you do any measurements under (sine or music) load?
(I know, you give us the world, and immediately we want another one. Sorry)
Jan Didden
This is no load, just the 300mA bias?
Did you do any measurements under (sine or music) load?
(I know, you give us the world, and immediately we want another one. Sorry)
Jan Didden
janneman said:
Hi Jan,
Yes, you're right, it reduces ripple, nothing to do with the chip by itself.
Anyway, what value would you suggest?
Thorsten suggests 1uf, but that's a big cap to put under a circuit between + and - pins of an op-amp most of the times (at least if it's a polyester cap).
janneman said:
If I was to do that, there would be a near infinite set of parameters to define. If under load, what fequency or frequencies, single or multiple? What ouput level? 1W? 10W? 100W? What load, 8 Ohm, 4 Ohm or some reactive load simulating a speaker As for using music, how do you do FFT measurments under music/dynamic conditions when what you see above is averaged out over 1000 samples? The very nature, and shortcoming (?), of FFT measurements precludes it.
I do believe, at least hope, that one day a meaningful standardised (or set of) dynamic tests can be devised. With the increase of computing power, such type of tests may become a reality? That will get away from the severe limitations of static measurements.
That may be the 'another world' you wanted? Make that both of us!
Joe R.
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