Newbie question on suitability of A40 with Shackman electrostatic HF panels - diyAudio
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Old 16th July 2009, 11:03 PM   #1
njgale is offline njgale  England
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Default Newbie question on suitability of A40 with Shackman electrostatic HF panels

Hi All

I've been reading for a while, but as I didn't manage to spot a thread that helped with the question I'm trying to answer, I guess it's time for my first post.

Bit of background first so you can see where I'm coming from, please bear with me for a moment....

I have a small (2' x 1' x 9") pair of Dynastatic speakers, designed in the UK by an ex-BBC engineer, Alex Shackman, in the '70s, that have 6" cone bass drivers and his 8"x6" electrostatic HF panels. Initially I ran them passive as designed, and they do sound really gorgeous compared to my old setup, but then, because the HF panels are known to be hard to drive, I decided to play about with an active configuration. This works much better and sounds really nice to my ears.

But I do still detect the NAD 3150 power amps that drive the HF units can struggle sometimes; I believe the panels dip below 1ohm at high frequencies. Typically I believe they were normally recommended to be driven by Quad II valve amps, and I could go down the valve route, but then I started thinking about building a pair of amps and was looking at the A40.

Unfortunately there is little in the way of specs available for the Shackman panels, apart from they run at a comparatively low HT voltage of around 400v and have a *very* small stator gap - they use a cotton-like thread to space the membrane from the stators, and were designed in the era of low powered amps.

What I do know is that they sound really special, so I'd like to get them paired up with amps that don't mind an awkward load

So my questions are
a/ how do the A40's like driving very low impedence loads?
b/ is there anything else I should consider when thinking of using them for driving small electrostatics (crossover frequency ~ 600hz)?

(Oh, and the reason for building amps is that the "ultimate plan" is to build the amps and panels into transmission lines based on this interpretation of the Radford TL http://homepage.mac.com/tlinespeaker...visited-WD.pdf but that's another story)

Anyhow, thanks for reading this far, and would very much appreciate any thoughts and suggestions.

cheers
Nick
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Old 17th July 2009, 06:44 PM   #2
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The A40 will work, but is not the best choice. I would consider
a stock or scaled-down version of the A75 with the folded cascode
front end.

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Old 17th July 2009, 10:14 PM   #3
njgale is offline njgale  England
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Thankyou That's really useful. Looks like I have some more reading to do over the weekend.

cheers
Nick
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Old 23rd July 2009, 01:17 AM   #4
njgale is offline njgale  England
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Ok, so having done *some* reading (so many threads, so little time ;-) I also came across those for the recent F5 amplifier, and was wondering if that would be a suitable but slightly less scary alternative for a first build, especially as I read that "it will drive a 2 ohm load without burping, and 1 ohm without misbehaving" in the Op/Service Manual. I think that gets pretty close to what I need for the Shackman panels...

The sheer scale of the A75 seems a bit daunting to me (and SWMBO did go slightly pale when she looked over my shoulder at pics in the A75 gallery and ask "some pointed questions" about my sanity )

cheers
Nick
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Old 2nd September 2009, 12:15 AM   #5
njgale is offline njgale  England
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Join Date: Feb 2009
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<muzak>
Nearly there on buying all the parts, just the xformers now and then the soldering can begin...
</muzak>

And I've only reached page 201 of the "F5 power amplifier" thread; it seems to be growing faster than I have time to read it
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Old 1st July 2010, 07:53 PM   #6
njgale is offline njgale  England
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Join Date: Feb 2009
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I've finally got a full stereo F5 going (joined the "fry a channel" club and had to do a rebuild after a small error with putting the mosfets the wrong way round) and biassed it all up (easy with 3 meters clipped to the right places ). I also built a B1 to drive it.

I was planning to build two full F5's and keep the active setup, but thought I would revert the speakers to passive for a bit, and the F5/B1 combination is more than capable of controlling them, going louder than I need by some way, and sounding really sweet and smooth.

So can i say a huge "thankyou" Nelson. I've had a ton of fun getting back to my soldering iron after a 25 year absence, and learned a whole load of unexpected things about how amplifier layout works (or, mostly in my case, doesn't!). I still have more to do (repackage the whole lot in a wife-acceptable format, solve the transformer buzz etc) plus another amp to build so I can experiment with an active setup, and some later Shackman crossover/power supplies to create new boards for and fix up..

But first I'm going to listen to some music

cheers from UK
Nick
(will post some pics when i figure out how)
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Old 1st July 2010, 08:20 PM   #7
njgale is offline njgale  England
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Join Date: Feb 2009
Location: Reading
<remembered>

I was worried about biassing, then came up with this, soldering short leads to the resistor legs, which made it a doddle
Click the image to open in full size.
The meters all attach to a block just out of the bottom of the shot.

And when it was all together, I did a hi tech measurement of heat sink temp
Click the image to open in full size.
which is slightly high at 59C but seems to sit like that all day so I won't add a fan
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