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Valve Stereo Preamp -- Silicon Chip
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Return to top of page · Post #: 1 · Written at 10:29:00 PM on 24 December 2015.
	GTC Location: Sydney, NSW Member since 28 January 2011 Member #: 823 Postcount: 6803
In the January 2016 issue of SC, Nicholas Vinen publishes part 1 of a 2 part project to construct a single board stereo hybrid valve/solid state premp using a pair of 12AX7s. It is powered by a 15V DC 1A plug pack and has a switchmode supply on-board for the HT.

Return to top of page · Post #: 2 · Written at 6:41:12 AM on 25 December 2015.
	Wa2ise Location: Oradell, US Member since 2 April 2010 Member #: 643 Postcount: 833
If you use a wall wart (what we call 'em in the USA) that produces 50 or 60Hz 12VAC you could use a regular 320V to 12V AC transformer backwards to produce the B+. Use a bridge rectifier to avoid DC current in the transformer, and it's easier to filter twice the line frequency anyway. I'd use one of the tubes' centertapped heater to ground the heaters.

Return to top of page · Post #: 3 · Written at 8:09:52 AM on 25 December 2015.
	Brad Administrator Location: Naremburn, NSW Member since 15 November 2005 Member #: 1 Postcount: 7451
Sounds interesting. I can't see the January issue on the SC website yet though so you must be lucky enough to have got a hard copy from somewhere. This could be a companion for the Currawong and might be useful for the LP turntables that have come back to the market in recent times. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ A valve a day keeps the transistor away...

Return to top of page · Post #: 4 · Written at 8:11:41 AM on 25 December 2015.
	Robbbert Location: Hill Top, NSW Member since 18 September 2015 Member #: 1801 Postcount: 2116
Interesting... I'll have to try getting that edition. Thanks for the heads-up.

Return to top of page · Post #: 5 · Written at 4:16:12 PM on 25 December 2015.
	GTC Location: Sydney, NSW Member since 28 January 2011 Member #: 823 Postcount: 6803
you must be lucky enough to have got a hard copy from somewhere. I'm a subscriber, and I usually receive my copy some time after it hits the newsagents.

Return to top of page · Post #: 6 · Written at 4:29:20 PM on 25 December 2015.
	GTC Location: Sydney, NSW Member since 28 January 2011 Member #: 823 Postcount: 6803
you could use a regular 320V to 12V AC transformer backwards to produce the B+ This project's HT supply is transformerless. It uses an MC34063 switchmode regulator in step-up configuration and 100uH coil.

Return to top of page · Post #: 7 · Written at 4:36:49 PM on 25 December 2015.
	GTC Location: Sydney, NSW Member since 28 January 2011 Member #: 823 Postcount: 6803
This could be a companion for the Currawong Vinen says that this project is an update of mono 12AX7-based preamp designs published in Nov 2003 and Feb 2004, which were incorporated into the Currawong, and was triggered by a number of requests for a stereo preamp.

Return to top of page · Post #: 8 · Written at 6:30:17 PM on 25 December 2015.
	Brad Administrator Location: Naremburn, NSW Member since 15 November 2005 Member #: 1 Postcount: 7451
I subscribe but when I checked my GPO box on Wednesday it wasn't there. I wasn't really expecting it at the time though good to know it is on its way. I'll check again on Tuesday. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ A valve a day keeps the transistor away...

Return to top of page · Post #: 9 · Written at 5:05:13 PM on 27 December 2015.
	Art Location: Somewhere, USA Member since 22 October 2013 Member #: 1437 Postcount: 896
Why is it ok to use a switching supply? Do they explain anything in the article about making all the noise outside of audio frequencies? Most amps today are still using transformer supplies.

Return to top of page · Post #: 10 · Written at 6:03:12 PM on 27 December 2015.
	Art Location: Somewhere, USA Member since 22 October 2013 Member #: 1437 Postcount: 896
They don’t want to send mains to the project, and show that schematic for the public to build, so are able to supply the board overall with the DC supply.

Return to top of page · Post #: 11 · Written at 3:31:38 PM on 29 December 2015.
	Robbbert Location: Hill Top, NSW Member since 18 September 2015 Member #: 1801 Postcount: 2116
Purchased the issue from my local newsagent, so there should not be a problem for anyone else.

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