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Old 24th April 2013, 01:13 PM   #1981
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Today I spent some time and did some simulations on the power amps I have built and in the process of rebuilding. The ISP and VAS stages use current sources and cascodes as usual, and the OPS stage uses L-MOSFETs source follower.

The following are the result:

IPS + VAS PSRR

20Hz -> -69dB
50Hz -> -68dB
100Hz -> -67dB
200Hz -> -66dB
500Hz -> -63dB
1,000Hz -> -59dB
2,000Hz -> -56dB
5,000Hz -> -45dB
10,000Hz -> -39dB
20,000Hz -> -33dB
50,000Hz -> -25dB
100,000Hz -> -19dB


OPS PSRR

200Hz -> (unable to measure)
500Hz -> -101dB
2,000Hz -> -78dB
20,000Hz -> -57dB


I am not sure how accurate the models are as I got them from a friend. The figures are a bit rough but are indicative.

With the above simulation results, I have come up with a passive LCR filter to be installed for the front end (i.e. IPS and VAS) rails, which will achieve -44dB at 20Hz and much better higher up. This will make the front end to have better than -110dB PSRR in the entire audio band.

For the OPS rail, in order to achieve -110dB PSRR I need the power supply to provide -53dB. In other words, the power supply impedance at 20kHz must be 0.00224R or lower. This will be very difficult to achieve. Using 14 x Rubycon ZLJ 680uF/100V close to the output L-MOSFETs on a power and ground plane may help achieve this figure. But I think that the two layer PCB is very difficult to design. I don't think I can make that number. If I can manage to have the impedance seen by the L-MOSFETs to be 0.01R then I would be achieving only -93dB at 20kHz.

But before we go any further, we must first establish what total PSRR (20Hz to 20kHz) of an amplifier is necessary for good sound. I have had no ideas at all and came up with only an arbitary number of -110dB. -140dB was previously suggested by others.

I hope you guys can continue the discussions and come up with something. I will be away overseas from tomorrow and will be back in May.

Regards,
Bill

Last edited by HiFiNutNut; 24th April 2013 at 01:15 PM.
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Old 24th April 2013, 02:35 PM   #1982
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This post'll be buried by May, but the main piece you're missing is the control loop will attenuate output stage errors by its excess loop gain. Provided control loop is implemented with sufficient PSRR that it's own "noise" floor with respect to ripple on the rails is low enough for it to see those errors. In a traditional discrete power amp with Miller compensation and ~25dB gain achieving 50+dB excess loop gain at 20kHz is not trivial. This is part of the reason why people find audible differences in current feedback amps and feedforward compensation. A simpler solution's to use an IC with good GBP; 50MHz parts like the LME49710 and LME49811 have about 70dB loop gain at 20kHz.

The other piece is the analysis I linked some pages back in this thread, which yields required PSRR as a function of bypass capacitance. For practical reservoir sizes, typical amp quiescent currents, and typical signal levels the supply ripple will be around 30dB larger than the signal. So upwards of 90dB PSRR in the control loop is wanted through some combination of filtering, regulation (usually more attractive than filters), and the control devices' own PSRR. 110dB is not a bad default as 90dB is arguably rather minimal. 140dB is overkill but trivial to achieve on paper when using IC regulators and control devices. In practice, laying out a PCB to hit an O(100nV) noise floor is not so easy and quiet control devices in a loop with low noise gain are needed (LME49990 or AD797 at unity gain, for example).

Last edited by twest820; 24th April 2013 at 02:38 PM.
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Old 24th April 2013, 10:30 PM   #1983
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Twest820, Thanks for your post. I will fly tonight but before that I will come back here when I can.

My amp uses two pole Miller compensation. The amp has a close loop gain of just over 30dB. OLG is 87dB at 100Hz and falls down to 70dB at 20kHz. So it has 40dB excess loop gain at 20kHz. I don't know if that is good enough. I don't have the knowledge and skills for amplifier design and I only play with other stuff like power supply, etc. Is the amp good enough? or poor?

I derived the above listed figures in close loop so I think they should include everything, such as feedback correction, etc. I created two perfect DC sources, one for the front end and the other for the backend, and created a voltage source of 8V peak to peak in varying frequencies and put it directly on the rail, separated by small R and H from the original perfect DC sources. I then injected the maximum allowable signal before clipping (1.8V) at 10Hz at the input of the amplifier and then checked the ripples on the output of the amplifier. The noise at the output then compared to the rail noise of 8V.

Last edited by HiFiNutNut; 24th April 2013 at 10:35 PM.
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Old 24th April 2013, 11:59 PM   #1984
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Quote:
Originally Posted by twest820 View Post
Passively filtering mains ripple off the supply only plays nicely with class A amplifiers. For class B type loads it generally makes the problem worse, not better.

twest820, how right you are!!!!!!!


I have just tested my passive filter for the front end. While it does a great job of eliminating ripples from the rails, it does not work well because of the loads and the filter's impedance generating its own ripples.

So now I will need to look hard to find a 3 terminal regulator that does +/-90VDC.
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Old 25th April 2013, 01:10 AM   #1985
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Quote:
Originally Posted by gootee View Post
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.
Yes - I was inspired by that work of Terry's when he posted it - I could not though duplicate his lack of series inductance with my setup. I was using an LCR meter to measure though and he used a network analyser. Frank (fas42) has since pointed me to a couple of Terry's posts on my blog. I find myself in agreement with Terry - why don't amplifier manufacturers adopt this approach? For myself I'm keen to explore 3D capacitor wiring solutions before I settle on a 2D one like his.

Quote:
The total wire length in your setup looks to be getting too long. i.e. Lots of inductance.
Yes - I have some (quite a few!) very close-in SMT ceramics soldered on to the pins of the chipamp, so inductance isn't too much of a worry. If inductance was a problem I reckon the FFT of the PSU noise would show bumps at higher freqs - but in fact its decreasing monotonically as freq rises. With the X5Rs in circuit the challenges are getting the mid-band and LF PSU noise down.

Quote:
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.
Yep its a great scheme and I built one. I may end up building more, we shall see...

@HiFiN - not to rain on your parade but why terminate your PSRR analysis at 20kHz? Since the rail currents are haversines for a classAB amp, there are going to be plenty of harmonics above 20kHz on the supply. I reckon those get inside the control loop when PSRR is inadequate and will introduce IMD, greying out the perceived sound
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Last edited by abraxalito; 25th April 2013 at 01:14 AM.
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Old 25th April 2013, 01:29 AM   #1986
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Quote:
Originally Posted by abraxalito View Post
HiFiN - not to rain on your parade but why terminate your PSRR analysis at 20kHz? Since the rail currents are haversines for a classAB amp, there are going to be plenty of harmonics above 20kHz on the supply. I reckon those get inside the control loop when PSRR is inadequate and will introduce IMD, greying out the perceived sound
I look at it until about 100MHz when looking at power supply line filtering, and that is why I am always interested in using some RF chokes as they may be more effective than capacitors. But those are for filtering the line, rail / switching noises or EMI pick up. I guess that amplifier signals would not get higher than 40kHz so the load generated noises on the rails would not be much higher in frequencies. Here I guess that low impedance is what is needed. In terms of low impedance, modern electrolytic capacitors have impedance quite flat from about 20kHz to 200kHz. So if the impedance is low enough for 20kHz, it is low enough fro 200kHz. Above that frequency, perhaps low impedance is less important because there is no signal generated noise, but filtering still is. Filtering high frequencies is easy. Providing a low impedance at high frequency is not. I think they are two separate things. So in a word, for filtering noises, I look into the MHz region. For providing a low impedance to the load, perhaps 20kHz is sufficient because in that case up to 200kHz is automatic and even 1MHz is still pretty good.

Of course, I may be wrong here because perhaps the low impedance is needed until the unity gain point of the amplifier?

Last edited by HiFiNutNut; 25th April 2013 at 01:36 AM.
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Old 25th April 2013, 01:33 AM   #1987
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Are there any 3 terminal regs supporting +/-90VDC rails at low current?
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Old 25th April 2013, 01:36 AM   #1988
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Quote:
Originally Posted by HiFiNutNut View Post
So it has 40dB excess loop gain at 20kHz. I don't know if that is good enough.
The folks on the solid state forum could probably give a better answer than I could even if I had the schematic and loop gain plot, but it's reasonable. Your own math says you want 55dB, so presumably it's not good enough.

Quote:
Originally Posted by HiFiNutNut View Post
So now I will need to look hard to find a 3 terminal regulator that does +/-90VDC.
Well, one way to solve that would be to ask if you really need a 500W/channel amp and, if not, drop the rails to what you do need. If that gets you under 40V at the high mains corner, well, problem solved. If not, here would be a good starting point for Maida options. I'm not aware of any negative builds but the main difficulty in flipping the floating topology to the negative rail is finding a suitable PNP pass device.

For the positive rail there's also the TL783. And the dual positive topology if you've a winding available.

Quote:
Originally Posted by abraxalito View Post
Since the rail currents are haversines for a classAB amp, there are going to be plenty of harmonics above 20kHz on the supply. I reckon those get inside the control loop when PSRR is inadequate and will introduce IMD, greying out the perceived sound
The DAC probably doesn't stop at 20kHz either---if it's a slow rolloff type it'll go to 35kHz or so with redbook---but generally if something's well designed at 20kHz it holds up OK quite a ways higher. This is one the reasons a 90dB PSRR is a minimal target; if you design to, say, 110dB at 20kHz then usually the minimum PSRR requirement is met out to 100-200kHz.
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Old 25th April 2013, 01:36 AM   #1989
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Personally, as a rough guide I would want to see good behaviour of the power supply up to 200kHz, do everything in your power to minimise the supply's impedance as seen by the appropriate parts leads up to at least that frequency. Remember, audio amplifiers start to fall apart above the audio band, but that's exactly where the FB mechanisms need to function correctly, and that's not going to be helped by the voltage rails glitching at those critical frequencies ...
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Old 25th April 2013, 01:53 AM   #1990
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I want to run on +/-85VDC rails for the sub (20Hz -50Hz) and woofers (50Hz - 180Hz twin 10 inch drivers 3.5R). I will be running 4 way active speakers that require 4 pairs of power amplifiers.

The Maida does not seem to be the best fit for me.

The TL783 is interesting. I don't know if it would and how well it would work with negative rail. I prefer the standard instead of LDO, as the output of the LDO is pretty much affected by capacitance.

Any other options?
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