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Old 25th October 2012, 02:50 AM   #11
Carlp is offline Carlp  United States
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OK, Yooper, please remember I'm a newbie to PS (so far I've mostly just built what designers spec). But I'm trying to learn. I'd consider a CLC (from what I've read, it seems like a good route in many cases) but first I wanted to see if there's any noticeable benefit to going this PS route using parts on hand. And, as twest says, it's Class A so the burning doesn't seem to matter from an energy standpoint... (Oh, and I didn't know that about Fluke meters. I know the 12B is designed for electricity not electronics per se).

I did at one point try further filtration of the SMPS (again, just a CRC) but I only had a 100R resistor at the time and it burned WAY too much voltage. I'll look into sizing the inductor for a CLC but frankly I don't know anything about the quality of the noise on the SMPS. I've got a boat anchor scope I'm trying to learn how to use but I'm slow to figure out how to measure such things.

Quote:
Depends on the PSRR of the amp's control loop and your desired SNR on the output(s).
Twest, I don't know a control loop from a SMPS noise signal (see above, but I'll read up on it) so don't know how to make the tradeoff you mention. Also, I'm not sure just now how to decide on appropriate SNR as it seems to depend on so many factors and I'm someone who LOVES to swap gear to try different combos. Where I have a choice, I gravitate toward less noise is better. But then I'm a card-carrying cheapskate and try to get by with scavenged parts whenever possible.

BTW, my goal of 24v at the PS output is only b/c that's what the designer built. I'm reading up on if/how to push that to closer to 30v (with ~15v at the drain) and may go that route eventually. I'll look at your iron, twest.
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Old 25th October 2012, 02:56 AM   #12
agdr is offline agdr  United States
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Quote:
Originally Posted by Carlp View Post
Would this schematic on it's own provide a pretty ripple-free output...
Not too bad... red is output. Adding C2 and D6 recommended.
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Old 25th October 2012, 03:38 AM   #13
agdr is offline agdr  United States
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...also consider using the LM317HV instead, 60V input vs. 40V, so you don't pop it with line voltage fluctuations like twest820 mentioned.

LM317HVT/NOPB National Semiconductor | LM317HVT-ND | DigiKey
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Old 25th October 2012, 03:40 AM   #14
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Originally Posted by Carlp View Post
Also, I'm not sure just now how to decide on appropriate SNR as it seems to depend on so many factors.
The limiting factor in the linearity of audio systems that's hardest to get away from is the drivers' phase modulation distortion. Usually that's around -65dB. If you want the rest of the system to reveal that floor then the accumulated distortion from the source to the power amp should be around -80dB or lower. From there it's just a matter of counting up all your stages, adding up their distortions, and seasonng to taste. A good rule of thumb is THD+N on any individual stage should be -100dB or lower.

For example, say you have a power amp outputting a fairly typical home audio level of 100mV RMS that has 1V of ripple. For the control loop to hit -100dB on the signal that means 100dB + 1V/100mV = 120dB of PSRR is needed. Typically the long tailed pair and VAS stages in a discrete amp manage around 70dB PSRR, meaning you need another 120dB - 70dB = 50dB PSRR from somewhere. Most regulators provide around 60dB. So you get 70dB + 60dB = >120dB PSRR and life is good.

Looking back at the topology you're using here I see it'll have rather less than 70dB PSRR, though. The easiest way to address that is to change the amp topology, but that's probably a different project and a topic for the solid state forum. You could throw more engineering at the supply here but past the first regulator you toss in it'll be subject to diminishing returns. If you like tweaking that's probably fine though; class A has the worst ratio of quiescent power (ripple) to output power (signal) but it's kinda fun as all sorts of supply lowpassing that doesn't work so hot with class B/D/G/H operation is admissable for class A.

As an aside, be careful with the structure of the ground star as the 0dB CMRR of the unbalanced input isn't something that improvements in the supply's circuit topology can mitigate---it's a layout thing, both in managing where the currents go supply and in feeding the FET's gate.
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Old 25th October 2012, 04:23 AM   #15
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Quote:
Originally Posted by Carlp View Post
I did at one point try further filtration of the SMPS (again, just a CRC) but I only had a 100R resistor at the time and it burned WAY too much voltage. I'll look into sizing the inductor for a CLC but frankly I don't know anything about the quality of the noise on the SMPS. I've got a boat anchor scope I'm trying to learn how to use but I'm slow to figure out how to measure such things.
In an example application, I had 28V with 200 mV p-p ripple, filtered this with a 1mH common mode choke followed by 2000uF of SMPS RB caps folowed by a 38uH toroidal choke then 8000uF RB caps. Twisted pair cable used thoughout single point earthing at the filtered output etc.
Ripple was unmeasurable with a CRO, a spectrum analyser showed a 30dB drop in the class A amplifier noise floor. This type of filter is not so effective on noise less than ikHz so the SMPS needs to have good line rejection. For this application I planned on using a linear reg to remove the sub 1 kHz noise but the customer was happy with filtered result.
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Old 25th October 2012, 09:02 AM   #16
AndrewT is offline AndrewT  Scotland
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Originally Posted by Carlp View Post
So AndrewT, are you saying I should aim to drop the PS voltage from 30v (what's coming out of the bridge despite the roughly 35v unloaded transformer voltage) to 29v? If so, I'd probably aim for 15v at the drain. I think the 2sk1058 can handle those conditions (the gate to source v might be close to the max) but I'm not sure and I'm new to transistors. I'd be burning 15w on the 15R load resistor, but it's 50w and heat sinked, so shouldn't be a problem.
where did that come from?
I never mentioned 30Vdc nor 29Vdc.
I said
Quote:
Expect the R to drop <<10% of the total available voltage. A target value for reasonable filtering may be around 1V of Vdrop for a 35Vdc single polarity supply
Vdrop across the R of the CRC should be a lot less than 3.5V for a 35Vdc supply.
I showed an example where 0r68 drops ~1V
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Old 25th October 2012, 09:07 AM   #17
AndrewT is offline AndrewT  Scotland
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don't use a regulator to feed a ClassA power amplifier.
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Old 25th October 2012, 11:58 AM   #18
Carlp is offline Carlp  United States
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Quote:
where did that come from?
I never mentioned 30Vdc nor 29Vdc.
My mistake, AndrewT. I read your post as saying I should aim for a 1v drop, and since my loaded iron was putting out about 30v rectified, not 35v, I figured you meant I should aim for 29v out of the PS. And that's why I asked you - I wasn't sure I got it right. Thanks for clarifying.

Also, can you elaborate on why not to use a regulator on a Class A amp?
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Old 25th October 2012, 12:14 PM   #19
Carlp is offline Carlp  United States
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Originally Posted by agdr View Post
Not too bad... red is output. Adding C2 and D6 recommended.
Thanks, and thanks for the LM317HV. I didn't know about that item. I'm also going to look at the LM338. Ever seen a LM338HV? Mouser doesn't show one. Anyway, our power is pretty stable - I've never seen it go over 122v, and after rectifier losses and resistor drops, I don't see the LM317 ever seeing over about 32-35v. But that is fairly close to the 40v max.

Quote:
For example, say you have a power amp outputting a fairly typical home audio level of 100mV RMS that has 1V of ripple. For the control loop to hit -100dB on the signal that means 100dB + 1V/100mV = 120dB of PSRR is needed. Typically the long tailed pair and VAS stages in a discrete amp manage around 70dB PSRR, meaning you need another 120dB - 70dB = 50dB PSRR from somewhere. Most regulators provide around 60dB. So you get 70dB + 60dB = >120dB PSRR and life is good.
Twest, thanks for fleshing this out with numbers (though I'm confused about a 100mV signal output with 1V ripple). The numbers help me visualize what's going on. And yes, a different topology is for another project (I'm mostly a tube builder, but interested in learning about transistors). Also grounding (I do use star grounding) is certainly a big issue for me. I'll be reading more on that to learn where problems arise.

Carl
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Old 25th October 2012, 10:25 PM   #20
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Originally Posted by AndrewT View Post
don't use a regulator to feed a ClassA power amplifier.
Is that personal experience? If so some circuit details would be of interest. My experience class A amplifiers and regulated supplies has been good, it is pretty tough to get power supply noise below 60 dBc with passive filters. Attention to the noise specification and the transient response
of the regulator circuit is a must.
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