Placid HD Version 2.1 - A significant step forward. - diyAudio
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Old 7th November 2014, 07:33 PM   #1
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Default Placid HD Version 2.1 - A significant step forward.

A few years ago I undertook the design of a shunt regulator to add an alternative to ordinary series regulators to our kit line up. That effort was realized in the form of the "Placid".

The placid has always come in two forms - single supply, and bipolar (with -BP suffix).


The original placid was very successful, but not very flexible - it was not really suitable for heavy loads. Some time later I was designing Legato - which uses a lot of current - and this led me to design a new version with some substantial changes. First - this version had no opamp - it was all discrete. Second it could handle much more current - so I called it Placid HD. Placid HD met my design goals, but it's limited bandwidth error amplifier meant that it was not ideal in terms of PSRR nor line/load regulation. Still - it was a good design in terms of output impedance and current capability. We (myself and many in the community) learned a lot from that supply - and it could be modded to be truly exceptional.

Last year some forum members (especially AVR300) presented me with a challenge I could not turn down. They explored with me changes to the circuit which made it substantially better then it's stock form. I published advice on how to implement changes to the circuit which fundamentally changed the way the error amp worked. basically the changes revolved around the way the error amp itself was supplied. I implemented a bootstrapping technique and powered the error amp from a voltage derived directly from the clean regulated output. This resulted in drastically better PSRR. This was the genesis of big things to come for Placid.

Based on what I had learned from my friends and from 3 years experience with the Placid I undertook a radical redesign.

The design change can be summarized in these goals I hoped to achieve:
  1. Stiffen the CCS by changing the VREF scheme and increasing the gain of the pass element. This I accomplished by using CFP where the power element was a Darlington. This drastically increased the output impedance of the current source - which is a very good thing.
  2. Create predictable filtered VREFs so that people can more easily create fixed voltages and current outputs. This I did by scrapping the LED VREFs and using filtered 2.5V references which are temperature stable.
  3. Create a faster shunt element with very high input impedance. This was accomplished using the same CFP/Darlington scheme as the CCS.
  4. Create an error amp with VREF that are only referenced to GND and the clean output - they are bootstrapped. To make this practical I substituted the long tailed pair in the Placid HD with a near rail-to-rail input/output opamp. This is the single biggest change (for the better) to the circuit.

All of these changes together make up the "Placid HD 2.1" power supply.

The result of months of back and forth and myriads of tests and prototype revisions have resulted in what I am confident in my best power supply to date.

I will let others comment on it's effects - but I can tell you it's output is flat out quiet - and it is completely stable and rock solid. It has all the benefits of a shunt regulator but with much better PSRR than most. It's load regulation is superb - and it is stable into capacitive loads.

I find myself going back over old designs with new enthusiasm because I don't have to worry as much about circuits with less than ideal PSRR.

I wanted to take the time in this post to thank you all for your support and testing and contribution to the success of this remarkable power supply - but I can't remember you all so I will thank a few.

AVR300 - thank you for being critical and friendly at the same time!
Terry Aben - thank you for inspiring some of my early work.
Brian Donegan - thanks for making awesome kits.
Leon Van Bommel - thanks for taking time to document so that others can understand and build these cool things we enjoy.

There are many others - so if I missed you please don't be offended. I appreciate all of you.

Build one - you will love it.

Cheers!
Russ White
Attached Files
File Type: pdf placid_hd_bp_2.1_schematic.pdf (50.5 KB, 154 views)
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Last edited by Russ White; 8th November 2014 at 02:10 PM.
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Old 9th November 2014, 10:00 AM   #2
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Bought both Placids 2.1 already a while ago. From what I read they should be a very nice improvement from the previous version. Suprised to read almost nothing about them on DIYAudio... Hope to find some time somewhere in the near future to build them...
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Old 9th November 2014, 04:39 PM   #3
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Thanks, Russ. I'll order a pair for BIIISE to compare to the prior Placid HD. Is the set-up and V/I adjustment the same as before? If not, is a new manual forthcoming?

Will lessons learned here propagate a new Trident?
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Old 9th November 2014, 05:49 PM   #4
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A guide for V2.1 can be found in the documents section.
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Old 9th November 2014, 07:48 PM   #5
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Quote:
Originally Posted by David Garretson View Post
Thanks, Russ. I'll order a pair for BIIISE to compare to the prior Placid HD. Is the set-up and V/I adjustment the same as before? If not, is a new manual forthcoming?

Will lessons learned here propagate a new Trident?
They really are not equivalent circuits - but yes - trident was already redesigned in much the same way as Placid was.

There is a new version of AVCC on the way (possibly available now - or being manufactured now - Brian can chime in) - on the latest AVCC we utilized an ultra low noise series VREF and filter to gain the same temperature stability we wanted on the Placid. This too was a nice change for the better.

Cheers!
Russ
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Old 10th November 2014, 01:19 PM   #6
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Quote:
Originally Posted by Russ White View Post
There is a new version of AVCC on the way (possibly available now - or being manufactured now - Brian can chime in) - on the latest AVCC we utilized an ultra low noise series VREF and filter to gain the same temperature stability we wanted on the Placid. This too was a nice change for the better.
I have been shipping this version for several weeks now. I have been building them myself until now, however. I will soon get a larger mfg run done.
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Old 10th November 2014, 01:28 PM   #7
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Just to be shure: this AVCC is a drop in replacement fot the one I got wit my BIIISE?
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Old 10th November 2014, 01:34 PM   #8
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Quote:
Originally Posted by Leeuwarden View Post
Just to be shure: this AVCC is a drop in replacement fot the one I got wit my BIIISE?
Yes it is.
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Old 10th November 2014, 01:40 PM   #9
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Thanks, I'll order one

Edit: I just did!

Last edited by Leeuwarden; 10th November 2014 at 01:49 PM.
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Old 10th November 2014, 01:59 PM   #10
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Let me know what you think.
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