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Old 25th April 2013, 02:57 AM   #1991
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DigiKey has a nice parts search engine. Good luck. You'll need it.
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Old 25th April 2013, 03:08 AM   #1992
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You can use the usual suspect (LM317T) to give you (say) 90V output voltage, just you have to take great care not to short the output. That's because this family of regs is floating. You probably can provide some sort of short circuit protection with judicious use of zeners.

In any case I'd suggest if you really, really need 500W then you look at delivering it with a bridged topology. This at a stroke halves your PSU impedance, nearly halves your total required PSU volts and also makes much better use of the available capacitors - akin to comparing half-wave with full-wave rectification from a PSU pov. Running bridged then allows you to use only one regulator, rather than needing two.

@twest - does anyone use those slow roll-off filters? Can't think of any advantage in selecting them myself My own DACs have a 50+dB brick wall filter around 17-18kHz
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Last edited by abraxalito; 25th April 2013 at 03:15 AM.
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Old 25th April 2013, 03:22 AM   #1993
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My experience is 317 based Maidas seem to have a tendency to hit SOA trouble, which is one of the reasons I didn't suggest a 337 based Maida for the negative rail. A pass device intended for HV use and suitable soft start addresses that. Though at only 90V SOA might not be too much of an issue, particularly with a 317HV---the blown 317 Maidas I'm aware of were on tube B+ supplies in the 300-500V range.

Bridging on 45V rails allows direct use of a 317HV but if you allow 15-25% margin for mains overvoltage events there's no turnkey integrated negative regulator I'm aware of for rails over 32-35V. Single supply off a regulator like 738 isn't a bad idea but makes managing startup and shutdown transients more involved. If a Maida's too much for NutNut I wouldn't particularly suggest going there. (Single supply also yields asymmetric headroom as the dropout's with respect to the positive rail which, if one's trying to maximize swing off the rails, isn't usually where one would want to have it.)

I think I may be a bit confused as to why one would care about a sub amp's performance at 20kHz, though.

Last edited by twest820; 25th April 2013 at 03:35 AM.
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Old 25th April 2013, 04:13 AM   #1994
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I was too quick to say that Maida did not seem like the one for me - I had a very quick look earlier at the schematic and I thought it was using an opamp, which would require a power supply for its own, hence would be a "NO" for me. From your later post I knew I was wrong. I had a second look and it seems that perhaps a 317/337 based maidas is an option.

But this can get very complicated because I will need to consider when the amplifier is turned on and off, each separate PSU (front end Maida and backend large capacitors) has different speed of charging and discharging which may cause problems to the amplifier.
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Old 25th April 2013, 04:26 AM   #1995
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Look more closely at how the op amp is powered when you have time. It's not any different than the error amp inside of a 317 or 337. Well, other than the complexity's more oviously exposed, but you'll have to deal with that and a variety of related sequencing problems one way or another to ensure SOA is not violated (welcome to power supply design ). You might also want to read the discussion of protection diodes in National's (er, TI's) LM317 datasheet.
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Old 25th April 2013, 04:32 AM   #1996
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Quote:
Originally Posted by twest820 View Post
I think I may be a bit confused as to why one would care about a sub amp's performance at 20kHz, though.
I have non-sub amps to build, but they still run on rail voltages of +/-56VDC.

For the sub and woofer amps, they will handle frequencies below 200Hz. But if they are noisy amps, would they pollute the higher frequency regions?
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Old 25th April 2013, 04:46 AM   #1997
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Quote:
Originally Posted by twest820 View Post
Look more closely at how the op amp is powered when you have time. It's not any different than the error amp inside of a 317 or 337. Well, other than the complexity's more oviously exposed, but you'll have to deal with that and a variety of related sequencing problems one way or another to ensure SOA is not violated (welcome to power supply design ). You might also want to read the discussion of protection diodes in National's (er, TI's) LM317 datasheet.
I have built the lm317/337 regs many times in line level applications. The best sound is to use them as pre-regs then small LCR to get rid of the noise from the lm317/337 then CFP capacitor multiplier with huge, low impedance caps following it as in that case I could get electrolytic caps of any size and any impedance directly to the opamp pins without causing any LCR resonances whatsoever. If using lm317/337 alone then their output inductance of 400nH to 2uH would cause impedance peaks / ringings with low impedance caps, not the best possible sound with opamp circuits.
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Old 25th April 2013, 04:54 AM   #1998
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Could you measure a difference on the line outs between the above and a more typical output cap arrangement?

Quote:
Originally Posted by HiFiNutNut View Post
For the sub and woofer amps, they will handle frequencies below 200Hz. But if they are noisy amps, would they pollute the higher frequency regions?
Possibly. Measure the sub's SPL and the amp's output noise and then you'll know.
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Old 25th April 2013, 08:28 AM   #1999
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Thanks, guys. I am going to the airport now. See you again here in a couple of weeks time.
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Old 7th May 2013, 05:29 AM   #2000
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I have returned from my holiday, during which I was thinking about the PSU.

I previously thought that passive LCR filter for the front end was not good enough hence was thinking about using a high voltage regulator instead. That was because in my standalone simulation of a front end LCR filter, using reasonable, practical value of capacitance the low freqency impedance 4 x 10R and 8 x 680uF = 5,440uF was at 8dB at 20Hz with a voltage drop of no more than 1.5V. In that simulation, the LCR filter makes ripples worse below around 55Hz, and is bearly useful below a few hundred Hz.

My power amp IPS and VAS draws 25mA. With 40R in series in the LCR this translates to 0.025 * 40 = 1V at DC.

However, due to the challenges I was facing to implement a good sounding high voltage regulator, I revisited the LCR filter approach. I was surprised to discover that the LCR filter is not as bad as I originally simulated.

When a diode is used in the front of the LCR filter to prevent "reverse" current and the LCR filter plugged into the real amplifier circuit with ripples on the rails (instead of a standslone LCR circuit with perfect voltage source), the diode works wonders! It buffers the voltage to prevent excessitve voltage drops and in addition smoothens the ripples by quite a lot!

The following are the simulation result of the PSRR of the front end LCR filter of D-C-RC-RC-RC-RCCCC (D=1N5819, R=10R, C=680uF):

20Hz: -26dB
120Hz: -38dB
2000Hz: -52dB

The rail PSU PSSR before the front end LCR filter:

20Hz: -11dB
120Hz: -24dB
2000Hz: -46dB

The IPS and VAS stages use CCS and cascodes which provide the following PSRR:

20Hz: -69dB
120Hz: -66dB
2000Hz: -56dB

Below are the combined PSRR:

20Hz: -107dB
120Hz: -128dB
2000Hz: -154dB

At higher frequencies up to 40,000kHz PSRR would not be much worse than the above figures, but the simulation took too long to complete and the ripples were too small for the simulation to be accurate therefore was not completed.

The price to pay are the bulk of the D-C-RC-RC-RC-RCCCC (D=1N5819, R=10R, C=680uF), as well as 1.5V voltage drop hence the loss of some power. The gain is the achievement of -107dB worst case PSRR.

The risk is free comparing to an active regulator.

Of course, a regulator will provide even much better PSRR but the risk of having possible higher fault rate (exceeding SOA of active devices, inrush current, turn on/off thumbs, reverse current, amplifier clipping under heavy load condition, etc) as well as the complexity do make me think trice before heading this path.

What do you think?
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