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 The Currawong - Silicon Chip's hybrid amplifier
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 Return to top of page · Post #: 1 · Written at 2:25:38 PM on 25 September 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
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In the October issue of Silicon Chip, there's a teaser article by Leo Simpson and Nicholas Vinen about a class AB ultralinear hybrid valve / solid state stereo amplifier prototype they have been working on for 6 months. Valves used are 12AX7s and 6L6s.

They haven't published the circuit as yet, but the article discusses the design in some detail and is accompanied by photos.

Key specs are: 10 watts per channel into 8 ohms; harmonic distortion 0.2%

I'm somewhat surprised to see Leo Simpson's name associated with this project as his skeptical/antagonistic views on the value of valve amplifiers are pretty well known.

I am keen to see what sort of SC readership interest there is for the Currawong project.


 
 Return to top of page · Post #: 2 · Written at 2:52:32 PM on 25 September 2014.
Brad's avatar
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The subject field was blank for this thread so I've added one. Feel free to change it to what was originally intended. Smile

I can't wait to see this. I've wanted Silicon Chip to do a valve amplifier for years now (as have a lot of other people) though a project was judged as unviable at the time it was brought up. I am sure the result will be good. Leo Simpson worked on the latter versions of the legendary Playmaster solid state amplifiers described by Electronics Australia back in the 1980s.


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A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 3 · Written at 3:03:11 PM on 25 September 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
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My post did have a title: "The Currawong" - Silicon Chip's hybrid amplifier.

Seems there's a bug associated with inverted commas in a title?

Sure, Leo Simpson has been around for decades, but I've read him poo-poohing those who feel that the sound of valve amplifiers is noticeably better than solid state amps. Some of his attitude probably comes from the silly money that some audiophiles are willing to pay for valve amps today. I believe that he has also consistently resisted suggestions/requests from readers to do a valve amp project.


 
 Return to top of page · Post #: 4 · Written at 3:15:44 PM on 25 September 2014.
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Seems there's a bug associated with inverted commas in a title?

Quite right. I thought I patched this but it looks like I only did that for the 'edit post' function. I'll patch the 'start thread' function in the next hour or so. Problem was that inverted comma is a special character in the Microsoft environment and its use can lead to issues when not handled correctly. To cure the issue elsewhere I convert a " to a '. It gives the members less to worry about.

Some of his attitude probably comes from the silly money that some audiophiles are willing to pay for valve amps today.

Some of the pricing is a bit over the top. But it's like buying an S-class Mercedes over a Holden Caprice. Both cars can be driven at the legal limit and provide the driver with superior comfort. The Merc has a few extra fancy features and the Holden's cabin is so big you can play a game of tennis in there, especially in the back seat. The cost difference is close to $200,000.00 but some people just want the three-pointed star on the bonnet instead of a lion mostly because of that little bit of extra exclusivity, even though Mercs aren't exactly rare these days.

I believe that he has also consistently resisted suggestions/request from readers to do a valve amp project.

An accurate appraisal of the situation I think. Every time he tips the mail bag onto his desk he'd be cursing such correspondence due to the amount of it. I hope the project succeeds and I'd definitely be putting a kit on the shopping list if it was within my budget. Let's hope so. I just hope they leave out the blue LEDs they included with the nixie valve clock kit. I want to experience the real visual warmth of the valves along with the tone they produce.


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A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 5 · Written at 3:57:12 PM on 25 September 2014.
GTC's avatar
 GTC
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I just hope they leave out the blue LEDs

Sorry to report that it comes with blue LEDs -- purely cosmetic "to illuminate the output transformers". Beats me what the attraction is these days with the blue LED. Anyway, they can be omitted.


 
 Return to top of page · Post #: 6 · Written at 4:02:53 PM on 25 September 2014.
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Bugger, though you are right, it's a simple matter of not installing them. I am still looking forward to having a look. If it wasn't raining at the moment I'd go and get my copy of the magazine. In the clock project the LEDs shine up through the guts of the nixie valves if I recall correctly. It's no biggie but every time I see a blue LED used that way it reminds me of cars with blue instrument panel lighting which I can't see correctly because the blue light makes things go blurry. Whereas I can see the old fashioned colour scheme that most cars used to have quite sharply. Orange needles on the instruments, white numbers and green calibration marks on a matte black background.


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A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 7 · Written at 9:45:46 PM on 25 September 2014.
DJ Oz's avatar
 Location: Central Coast, NSW
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Blue led these days... yeah I remember when they were Expensive and a new colour Smile
but that was in the 80's

Yes he seems very anti Valve lest wise in the respect to purity of sound

Thanks GTC I'll make sure I grab Oct Edition


 
 Return to top of page · Post #: 8 · Written at 9:54:04 PM on 25 September 2014.
GTC's avatar
 GTC
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Thanks GTC I'll make sure I grab Oct Edition

Just repeating that the October issue contains a teaser article only. They haven't stated when the full article will appear, only this: "until then, stay tuned".


 
 Return to top of page · Post #: 9 · Written at 11:51:37 PM on 25 September 2014.
Brad's avatar
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I've had a looksee at the article and overall it seems easy enough to build. They are looking at longevity by leaving the electrolytic condensers out of it. Should be fairly easy to build and then house in a cabinet with a pressed aluminium base and a wire grille over the top.


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A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 10 · Written at 7:16:06 AM on 26 September 2014.
Wa2ise's avatar
 Location: Oradell, US
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If you use an output transformer in the collector of a transistor, the collector curves look a lot like plate curves in a pentode tube amp. Only on the input side substitute base current for grid voltage.
An example of a transistor audio amp using an output transformer, here in an AM table radio:

Such an amp -might- have some "tube sound"...

Just thinking here, maybe a PNP transistor's collector connected to a tube plate, and that connection also goes to a speaker, and the other end of the speaker going to ground. The tube cathode biased at say -150V and the transistor at some positive voltage enough to have the same output impedance. And a feedback loop at DC to keep DC out of the speaker. As I said, just an unbaked idea... Smile


 
 Return to top of page · Post #: 11 · Written at 12:53:03 AM on 27 September 2014.
DJ Oz's avatar
 Location: Central Coast, NSW
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Emphasis on" teaser" Thanks GTC ... I did get that but yes an important fact to note
Cheers Smile


 
 Return to top of page · Post #: 12 · Written at 1:07:10 PM on 18 October 2014.
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 Art
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They are still charging $6.95 for a single digit of blue 7 segment LED at Jaycar...
really funny stuff for something so old hat, where I work it takes white seven segment displays to impress.


 
 Return to top of page · Post #: 13 · Written at 5:09:16 PM on 30 October 2014.
Brad's avatar
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Part 1 of the Currawong is in the November edition. Looking forward to building this for Xmas. Smile


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A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 14 · Written at 4:02:05 PM on 2 November 2014.
Brad's avatar
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The description looks like it'll be over three editions, so perhaps a new year project...


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A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 15 · Written at 7:55:45 PM on 3 November 2014.
DJ Oz's avatar
 Location: Central Coast, NSW
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I just found this on YT ... Dave's EEblog with Silicon Chips Nicholas Vinen (end of Sept 14 it went up) some discussion on the The Currawong amongst other things.

https://www.youtube.com/watch?v=adSk6suM29E.

Would have been useful addition before I suppose but the Circuit Descriptions out in the Nov 14 issue as posted above by Brad.

Dont know how much interest there will be in this one but it will be interesting to see.

If a kit happens it will be new production tubes and no doubt pricey too thought from the design I guess they tried to minimise that but I am sure a lot will use what they have valve wise (that could invoke some interesting Discussions).

The PSU is a Solid State Design with Slow Warm up for HT...its a little unique in its make up due to other limitations, not just your straight... lets replace a Tube Rectifier with silicon idea.

Its using off the shelf power transformers from memory and as output transformers PA line Transformer in a strange way ..but considering these things are killers here price wise I guess its "do as you must".

Even if your not intending to build it its probably worth exploring the circuit for some of the ideas they put in to over come the limitations of getting other parts normally associated with Valve Circuits.

Anyway just thought I add the link thats all really.


 
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