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RJM Audio Sapphire Desktop Headphone Amplifier
RJM Audio Sapphire Desktop Headphone Amplifier
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Old 28th July 2019, 11:28 PM   #1501
rjm is offline rjm  Japan
Richard Murdey
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RJM Audio Sapphire Desktop Headphone Amplifier
1. The input DC blocking capacitor is required to isolate the output offset adjust biasing network from the volume control.
2. Yes, the circuit will drive small speakers. Some kind of thermal protection circuit and short circuit protection on the output is suggested, as well as bigger heatsinks.
3. RJM Audio
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Old 19th August 2019, 04:43 AM   #1502
DRONE7 is offline DRONE7  New Zealand
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About to commence build using recently arrived 4.2b boards.

Will be using closed loop configuration and driving AKG K240's (1970's originals @600 ohm)

Any pointers to optimise for these headphones appreciated.

One question....I read of the tight thermal bonding for the transistors Q13/16 and Q15/14...
The bom makes no mention of any isolation pads for these transistors... is that an assumed given or do I just bolt them all together with their heatsinks as one big thermal mass without any isolation??
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Old 19th August 2019, 09:16 AM   #1503
blackdod is offline blackdod  Europe
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This transistors has a plastic coating. They do not need to isolate. You can screw them to the heatsink directly.
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Old 20th August 2019, 04:49 AM   #1504
DRONE7 is offline DRONE7  New Zealand
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Yes !, of course..:-) I see now...
Did you use any thermal compound between all the components ?
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Old 20th August 2019, 05:35 AM   #1505
blackdod is offline blackdod  Europe
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Not necessary. This transistors runs with very low temperature.
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Old 21st August 2019, 10:48 AM   #1506
Mixi is offline Mixi  Europe
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Richard, I really like your designs and have spent some time reading about them on your website. Now I have one question regarding the Sapphire: Are any modifications necessary to use it as a line stage amp instead of using it as a headphone amp?

Has anyone used the sapphire together with one of your phono stages to build a preamp including phono stage?
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Old 21st August 2019, 11:26 AM   #1507
blackdod is offline blackdod  Europe
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I have used my Sapphire with Richard's VSPS and I think that as a preamp this combination would work perfectly.
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Old 21st August 2019, 02:02 PM   #1508
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Richard Murdey
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RJM Audio Sapphire Desktop Headphone Amplifier
Quote:
Originally Posted by Mixi View Post
Are any modifications necessary to use it as a line stage amp instead of using it as a headphone amp?
The optional changes are given in the Sapphire BOM file. You can reduce the bias current and change the gain to optimize it as a line stage, but you don't have to do either if you don't want to.
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Old 21st August 2019, 04:05 PM   #1509
Mixi is offline Mixi  Europe
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ok, I should have looked at the BOM sooner. I thought it would just be a list of parts but this is more like a design manual. very impressive! thanks!

might sound like a strange question but is the Sapphire equally well suited to be a line stage as well as headphone amp or would it be a better headphone amp than line stage (or the other way round)?
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Old 21st August 2019, 11:24 PM   #1510
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Richard Murdey
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RJM Audio Sapphire Desktop Headphone Amplifier
Quote:
Originally Posted by Mixi View Post
ok, I should have looked at the BOM sooner. I thought it would just be a list of parts but this is more like a design manual. very impressive! thanks!

might sound like a strange question but is the Sapphire equally well suited to be a line stage as well as headphone amp or would it be a better headphone amp than line stage (or the other way round)?
Hmm, calling it a "Design Manual" is a nice suggestion, thanks! I'll look into making that change.

I use Sapphire 4 for both my preamp and my headphone amp.

The preamp build is described in a old blog post, here. I've replaced the Sapphire 3 boards shown with the updated rev 4 circuit, but the physical layout is unchanged.
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