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Old 20th April 2013, 02:41 PM   #1961
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I have a thread going over here: Possibly the most frugal high-end sounding amp? - Page 3
where I've shown a recording of the PSU noise compared to the output signal. My amp gives about 60dB PSRR (with respect to the output) and the PSU noise is about -50dB relative to the output signal. My target is to get this below -60dB wrt the output signal.

Note if you use a more 'mainstream' chipamp (LM3886 being the canonical example), they give PSRR plots relative to the input in the DSs.
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Old 21st April 2013, 12:56 AM   #1962
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I am thinking about having a C-Multiplier for the input stage and the VAS stage. In this way, I should be able to achieve a total of at least -125dB and likely -140dB PSSR from 20Hz up. This shouldn't be too difficult to achieve because the input and VAS stages require only very low current. I think the output stage has inherantly higher PSSR and does not require regulation, not to say that regulation is extremely difficult if possible. (Therefore, it makes no sense to regulate the entire power supply of a class AB amp).

As a matter of fact, I have the above in my mind before I jumped in this thread, as I thought that by "regulating" the front end stages, perhaps having super large capacitance for power amplifiers is unnecessary, because the larger ripples / higher impedance may not cause much harm to the amplifier provided that the PSSR of the front end stages is very high at -140dB. (It will still require low impedance caps to be placed very close to the output device pins, but this is for higher frequencies, not the lower frequencies, therefore the capacitance value need not to be very high).

What do you think? I can be very wrong hence am seeking your ideas.

abraxalito: your "Leaning Tower of Song" is truly amazing.

Last edited by HiFiNutNut; 21st April 2013 at 01:04 AM.
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Old 21st April 2013, 01:10 AM   #1963
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I've been wondering about output stage PSRR for a while now - on my blog comments I discussed this I think. I tried to simulate a simple emitter follower OPS in LTSpice but found the transistor models provided suck, so gave up with that. Then I went to look at an emitter follower output buffer - LME49600. This shows about 75dB of PSRR but only up to a handful of kHz, its down to 65dB by 20kHz and continues to fall beyond that, faster than 20dB/decade. I'm curious though whether your theory about not needing regulation for the OPS is actually correct - I guess the proof is building both and listening to see if any improvements are evident when regulation is used. What kind of output power are you aiming for? Are you intending a bridged amp?
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Old 21st April 2013, 02:32 AM   #1964
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In my simulation I got possibly 30dB higher PSRR on the output stage comparing to the input and VAS stages hence my above assuptions. But then you reminded me that I cannot be sure about the LMOSFET models I used neither (I guess they might have come from Bob Cordell's models?).

If I assume that the output stage has 65dB PSRR at 20kHz (that is your figure), checking my linear power supply now on +/-85V rails with 10,000uF + 10uH + .022 + 10,000uF + 10uH + .022 + 20,000uF, it has about 40dB PSRR from 1kHz to 100kHz. So the combined PSRR is 105dB at 20Hz. I think that is not bad at all.

Note that this 105dB is at the output after amplification, unlike in the front end before amplification of 30 something dB. So if your 140dB requirement is applied to the front end then the back end only needs about 110dB, and 105dB comes close, depending on the gain of your amplifier.

But then at lower frequencies such as at 20 to 100Hz, the PSRR is worse from perhaps 80dB to 100dB. But I think our ears can accept much higher distortions in these frequencies, so in real life it may not matter.

I noticed that for the back end, for every additional 10,000uF I throw in, I can achieve about 3dB PSSR improvement from 1kHz to 100kHz. Is it worthwhile? I am not so sure.

Last edited by HiFiNutNut; 21st April 2013 at 02:38 AM.
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Old 21st April 2013, 02:47 AM   #1965
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I have realised that 40,000uF (I planed for my new amp while my existing amp has only 30,000) per rail per channel comparing to many DIY amps is perhaps very little, but comparing to most commercial amps including the HiEnd amps is perhaps quite large.
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Old 22nd April 2013, 03:17 AM   #1966
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Quote:
Originally Posted by HiFiNutNut View Post
I am thinking about having a C-Multiplier for the input stage and the VAS stage. In this way, I should be able to achieve a total of at least -125dB and likely -140dB PSSR from 20Hz up. This shouldn't be too difficult to achieve because the input and VAS stages require only very low current. I think the output stage has inherantly higher PSSR and does not require regulation, not to say that regulation is extremely difficult if possible. (Therefore, it makes no sense to regulate the entire power supply of a class AB amp).

As a matter of fact, I have the above in my mind before I jumped in this thread, as I thought that by "regulating" the front end stages, perhaps having super large capacitance for power amplifiers is unnecessary, because the larger ripples / higher impedance may not cause much harm to the amplifier provided that the PSSR of the front end stages is very high at -140dB. (It will still require low impedance caps to be placed very close to the output device pins, but this is for higher frequencies, not the lower frequencies, therefore the capacitance value need not to be very high).

What do you think? I can be very wrong hence am seeking your ideas.

abraxalito: your "Leaning Tower of Song" is truly amazing.
PSRR or not, the primary function of the PSU and decoupling capacitors is to supply the CURRENT demanded, exactly when needed. The current IS the music signal.

There is a minimum total reservoir plus decoupling capacitance that you can not go below, if you expect enough current to be available.

This has been discussed at great length, in this thread, and quantified. The total capacitance will directly affect the maximum output power rating that the amplifier can be claimed to have. And that can also be dependent on the minimum frequency that has been specified.

Furthermore, there is the matter of overcoming the inductances of the supply and ground rails, which could degrade the transient response (not to mention the feedbak loop's quelling of higher-frequency harmonics' distortion) if there is not enough decoupling capacitance close-enough to the output devices. The minimum for the decoupling capacitance might be relatively small, maybe only hundreds of uF, but it is amazing how many designs omit it entirely.

Too little decoupling capacitance, or too much distance between the capacitance and the output device, can also cause spikes of the rail voltage, due to the changing current trying to pass through the power and ground rails' inductances, and those voltage spikes are separate from the rail voltage changes caused by the capacitor discharges.

In general, too, it is best to keep all current-flow loops as small as possible, to minimize radiated electromagnetic fields, and closely-located decoupling capacitors are a solution for that, as well.

Last edited by gootee; 22nd April 2013 at 03:20 AM.
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Old 22nd April 2013, 03:44 AM   #1967
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Quote:
Originally Posted by abraxalito View Post
I have a thread going over here: Possibly the most frugal high-end sounding amp? - Page 3
where I've shown a recording of the PSU noise compared to the output signal. My amp gives about 60dB PSRR (with respect to the output) and the PSU noise is about -50dB relative to the output signal. My target is to get this below -60dB wrt the output signal.

Note if you use a more 'mainstream' chipamp (LM3886 being the canonical example), they give PSRR plots relative to the input in the DSs.
Very interesting link! I noticed that you were wondering about how to connect more capacitance. You "might" want to check out the good idea that Terry Given posted, in this thread, involving the use of a blank double-sided PCB for each rail's power and ground. The total wire length in your setup looks to be getting too long. i.e. Lots of inductance. For Terry's cap array boards, you only need to drill one hole for each cap (and remove a little copper around each hole edge, on the top side). Then don't remove any other copper from either side of the PCB and just put one lead of each cap through the hole, bend 1/2-inch or so of both leads against the copper, and solder. My plan was to then put the actual output device(s) on a daughterboard that would be a millimiter or two above where two such arrays met, so that the power and ground connections could be as short as possible. At the other sides of the arrays would be the rectifiers.

I collected the links to the relevant posts, and put them at the bottom of the post at the following link, which also has some other excellent information:

LM3886 component selection

Last edited by gootee; 22nd April 2013 at 03:47 AM.
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Old 22nd April 2013, 04:09 AM   #1968
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I think that once the minimal amount of capacitance is met, which is usually not a lot, the capacitance is more related to the question of how low we want the rail ripples to be, which affect THD-N depending on the PSRR of the amplifier, than the question of how much cucrrent (hence the power) the amplifier can deliver.

Adding more capacitance makes only a tiny difference in the supply current or the power of the amplifier.

It is not the capacitance but the transformer voltage and transformer regulation that determines the rail voltage. For a given load, the current hence the power is determined by the rail voltage. The capacitance does not alter the rail voltage by much (except for the lowered ESR hence the impedance of the power supply which slightly alters the voltage and current, but comparing to the load resistance this can safely be ignored).

So I think the amount of capacitance required is more or less a question of how low the ripples we require for a given distortion requirement of the amplifier, which strongly depends on the PSRR of the amplifier.

I love the idea of placing hundreds of uF close to the output devices. This is necessary to lower the impedance at higher frequencies. Even one inch distance can affect the THD-N at 20kHz, which can clearly be revealed in simulations.
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Old 22nd April 2013, 04:20 AM   #1969
AJT is offline AJT  Philippines
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Quote:
Originally Posted by HiFiNutNut View Post
I think I will give the rail fuse a miss. I have burnt the rail fuses a couple of times when I was poking around the circuit boards while testing. Under normal use, I have not had a fuse burnt for the past 10 years.

Thanks for your information.

shouldn't you be thankful you got fuses in there? what could have happened other wise?
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Old 22nd April 2013, 05:15 AM   #1970
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Quote:
Originally Posted by HiFiNutNut View Post
I think that once the minimal amount of capacitance is met, which is usually not a lot, the capacitance is more related to the question of how low we want the rail ripples to be, which affect THD-N depending on the PSRR of the amplifier, than the question of how much cucrrent (hence the power) the amplifier can deliver.

Adding more capacitance makes only a tiny difference in the supply current or the power of the amplifier.

It is not the capacitance but the transformer voltage and transformer regulation that determines the rail voltage. For a given load, the current hence the power is determined by the rail voltage. The capacitance does not alter the rail voltage by much (except for the lowered ESR hence the impedance of the power supply which slightly alters the voltage and current, but comparing to the load resistance this can safely be ignored).

So I think the amount of capacitance required is more or less a question of how low the ripples we require for a given distortion requirement of the amplifier, which strongly depends on the PSRR of the amplifier.

I love the idea of placing hundreds of uF close to the output devices. This is necessary to lower the impedance at higher frequencies. Even one inch distance can affect the THD-N at 20kHz, which can clearly be revealed in simulations.
That too.

But as you said, the capacitance determines how low the rail voltage will dip for a given RMS load current. So for a given rail voltage and speaker impedance, the capacitance determines the maximum output signal amplitude (and thus the max output power rating) that won't make the amplifier clip.

That is very important, if you are wanting to minimize the capacitance.

Many use the RMS current for a sine signal at max rated power to calculate the minimum capacitance needed. But it's not bullet-proof unless you assume DC at the PEAK level of a max-rated-power sine. Also, don't forget to subtract the amplifier dropout voltage from the rail voltage, first, in addition to the desired ripple amplitude, since the "load" is the amplifier output stage in series with the speaker impedance, not just the speaker impedance.

See my spreadsheet a few pages back.
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