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RJM Audio Sapphire Desktop Headphone Amplifier
RJM Audio Sapphire Desktop Headphone Amplifier
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Old 11th October 2017, 06:23 AM   #1311
rjm is offline rjm  Japan
Richard Murdey
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RJM Audio Sapphire Desktop Headphone Amplifier
I've changed out the Sapphire 3 boards in my line preamplifier for this Sapphire 4 build.

For your reference, it is the standard "preamp" mod as described in the BOM, R15,16 1k, R17-20 47R. The gain configuration is open loop, R3 10k, R2A 10k, R2B 10k and R2C 3.32k for switchable 3-6-9 dB gain.

I chose these low gains as my amplifier was basically designed to use with a passive pre, and I listen at low volume anyway, so 3 or 6 dB gain is sufficient.

I chose the open loop option since at these low gains there is already plenty of feedback to lower the distortion of the voltage amplifier, there is arguably no particular need to temp fate and crank up the feedback by moving to the R3X option.

The BC327 and 337 were binned but I did not try to match Q1,2. Offset of one channel trimmed easily, the other was outside of the trim range of R6. The input offset is about 0.6 V ... almost a full diode drop, so something doesn't seem right. Investigating...
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Last edited by rjm; 11th October 2017 at 06:27 AM.
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Old 11th October 2017, 01:45 PM   #1312
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Richard Murdey
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The problem was traced to Q4 on the right channel, which I'd soldered in reversed 180 degrees, opposite to the silk screen - because. Because, already!

(I am still wrapping my head around the fact that reversing this transistor actually changed very little of the dynamic circuit performance, merely unbalancing the DC offset a little. It's like a 7 legged spider, still 90% combat effective.)

One of the nice things about working with the Sapphire at very low gain is the offsets are so much less, and super easy to trim. With 5k trimmer, there is no need to worry about measuring the transistors. Just stick 'em in and trim 'em out.

Just checked it all worked, with the Chromecast Audio as a source. That's the little puck on the left speaker. Amazing really...

Having a preamp with the correct gain is so nice. No extra noise, and fine adjustment with the volume in the sweet spot.
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Last edited by rjm; 11th October 2017 at 01:58 PM.
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Old 12th October 2017, 01:46 AM   #1313
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Richard Murdey
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Thoughts on using the Sapphire 4 as a preamp.

As my first major stab at a discrete transistor gain stage, I'm still surprised Sapphire 4 works. No hum, no heat, no noise, no distortion, no instability. Open loop or closed loop, high bias current or low, gain from 3 dB to 27 dB, the circuit is build-and-go, needing nothing more than a set-once-and-forget offset trim.

It's not too surprising that what works as a low noise headphone also works are a preamp though. The preamp mod bias current of 5 mA is ideal for loads 10k or above. For 600-10k range, the regular headphone amp configuration at 40 mA is preferred. That's the only difference between the two variants.

It might seem an awful lot of trouble to go through just for 3 dB voltage gain, but that gain also comes with an output impedance of 25 ohms to drive the amplifier and interconnects, as well as excellent PSRR and distortion characteristics. I've tried passive preamps and they just sound weak sauce. My power amplifier likes a low impedance drive, and the extra little bit of voltage gain does make a difference in how powerful the whole rig feels.

I won't make any substantial comment on the sound quality. A preamp is hard to evaluate at the best of times and I have no reference to A/B against. It sounds clean, and very very quiet. Upstream components are more clearly distinguished, and there is no tangible feel to the volume adjustment: loudness changes, but with no loss of resolution or clarity at low volume, and no sense of fade out or strain at higher volume. Definitely recommended, even if I can't say it sounds one way or another. I'm not convinced I can hear it but for it's absence.

P.S.

The Sapphire 3.1 I was originally considering to use as my next preamp is for sale. It's still set up as a headphone amp, 15/25 dB gain, with the 995 FET modules.
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Last edited by rjm; 12th October 2017 at 01:55 AM.
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Old 12th October 2017, 05:38 AM   #1314
ammel68 is offline ammel68  United States
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I haven't tried the open loop preamp configuration, only the closed loop with 6dB of gain.

So this circuit is stable with only 3dB of gain?
Reason I'm asking is because the BOM gain calculators(open and closed loop) only go down to 6dB. I was skeptical about going down to 3dB because some amplifiers oscillate if the gain is too low.

So for 3dB gain open loop, all I need to do is make R3 and R2A both 10k?

Thanks.
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Old 12th October 2017, 08:17 AM   #1315
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Hi ammel,

I left out that setting to keep the open and close gains consistent. I didn't think of anyone needing it, until I needed it myself! :P But, yes, it seems to work just fine at R3 10k and R2A 10k. The open loop config has excellent phase margin even at 3 dB, so stability is not a concern.
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Old 17th October 2017, 05:29 AM   #1316
ammel68 is offline ammel68  United States
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Hi Richard,

Having tried your Sapphire 4 as a 6dB gain preamp in both the open and closed loop configurations, I have to say I prefer the sound of the open loop configuration.
IMO the mids and highs seem to be a little more "present" than with the closed loop configuration, which sounded a little more "dark" and "closed in" for my taste.
Also, the bass response of the Sapphire 4 is excellent and notably better than any of the op-amp preamps(VGS + CFA) I've been using.

For the PS, I didn't even use separate transformers and rectifiers for each board. Just a simple and reliable Rat Shack 12V-0V-12V 3A transformer and a single bridge rectifier for both boards.

Input caps were inexpensive Dayton Audio 1uF 250V 1% tolerance caps:
Dayton Audio PMPC-1.0 1.0uF 250V Precision Audio Capacitor
Using better input caps may improve the sound even more...I don't know.

Perhaps more people on this site will build the Sapphire 4 and try it as a line stage if they're looking for a great sounding line stage that's easy to build AND still uses good old through-hole components instead of SMDs.

Last edited by ammel68; 17th October 2017 at 05:49 AM. Reason: Spelling
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Old 17th October 2017, 06:51 AM   #1317
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Richard Murdey
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Thanks for the report, both for the open/closed loop comparison and confirmation that it all works cleanly with a shared power supply.

The Sapphire 4 is a great diy preamp, but I sense that discrete solid-state line preamplifiers are one of the least popular things to build around here. It's just not something people tend to bother with. Let's just say I'm not anticipating a stampede of activity, even after your positive evaluation ...
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Old 17th October 2017, 07:44 AM   #1318
ammel68 is offline ammel68  United States
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Quote:
Originally Posted by rjm View Post
Thanks for the report, both for the open/closed loop comparison and confirmation that it all works cleanly with a shared power supply.

The Sapphire 4 is a great diy preamp, but I sense that discrete solid-state line preamplifiers are one of the least popular things to build around here. It's just not something people tend to bother with. Let's just say I'm not anticipating a stampede of activity, even after your positive evaluation ...
You're welcome. I hope to get more matched transistors and Zeners from you for future builds. I'll be sending my own Gerbers off in a few days to get some boards made.

If other people don't bother with it, then I feel that's their loss. Most passive preamps sound "sterile" and just don't do it for me.
Call me old school or whatever, but I like a low gain active line stage between my source and power amplifier to "flavor" the sound.
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Old 18th October 2017, 07:32 PM   #1319
Stan63605 is offline Stan63605  Canada
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Default Sapphire beyond the headphone amp?

In my experience, the preamplifier in a system is the toughest component to get right and, for me, it's most often the limiting factor in achieving satisfaction in overall sound - room effects notwithstanding.

Even though I've accumulated more caps, transistors, resistors, diodes, etc to build a second set of Sapphire boards to try the closed loop configuration, I'm mired in inertia because I am so pleased with my open loop boards. I really enjoy my Sapphire, but I have ample space for speakers and separate components and I do listen to them as much - even more - as I do my HA. So a Sapphire-based preamp is actually pretty appealing. It's the secondary aspects of a pre-amp that might hold me back - connectivity for other components - tuner, CD, turntable inputs, etc. Or dual subwoofers. Some will tell me to connect them via high-level speaker connections, but I'm not so sure.
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Old 19th October 2017, 03:22 AM   #1320
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Quote:
It's the secondary aspects of a pre-amp that might hold me back
Exactly. You, me, and everyone. The gain stage is the easy part. It's deciding what you want your preamplifier to do, and implementing that, which is hard. As well as input switching, there are mute, mono, and R/L invert switches, recording buffer, headphone output, phono preamp, as well as remote control functions to consider. It is quite dizzying.

Here's the blog entry for my preamplifier build, written back before I subbed in the Sapphire 4 boards. It just has a mute switch and two line inputs as "features". Working out the physical layout (which connectors and switches would be placed where, and how I would wire it all up) took far more time than putting together the boards.
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Last edited by rjm; 19th October 2017 at 03:25 AM.
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