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RJM Audio Sapphire Desktop Headphone Amplifier
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Old 27th November 2017, 11:22 AM   #1331
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Richard Murdey
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RJM Audio Sapphire Desktop Headphone Amplifier
Default updated Sapphire BOM

The mouser part no. for the pin header has been corrected/updated, also the mouser part nos for the optional screw terminals mentioned on the notes page have been corrected/updated. No other changes.
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Old 29th November 2017, 01:35 AM   #1332
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How to use connect a rotary switch to the Sapphire 41m boards to enable front panel gain selection.

Experimental. The inductance of long lead wires may cause stability problems, especially with the closed loop configuration.
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Old 1st December 2017, 02:26 AM   #1333
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Default Sapphire 41p preview

Looking at 41m today there isn't much I would update, but recently I have moved away from poly fill for the power traces so I thought I should bring the Sapphire 4 to this new design convention.

The component layout is unchanged. Minor edits to the metal and silkscreen layers only.
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Old 1st December 2017, 04:22 AM   #1334
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41p gerber file attached
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Old 6th December 2017, 01:43 AM   #1335
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The next production Sapphire board revision is 4.1t.

There are no substantive changes from 4.1m. I needed to reorder the boards and decided to edit the trace and metal layers for consistency with my most recent designs at this opportunity. The circuit and component layout is unchanged.

Minor edit from 4.1p above.

Basic changes/improvements since 4.1m:

- power V++, V--, V+, V- poly fill replaced by 60 mil traces. Keeps the layout cleaner and should be easier to solder to.

- graphics and RJM Audio branding updated to current logo etc. I've dropped the "4" from both the Sapphire and Phonoclone names.

- Transistor outlines labelled with a dot for NPN, dash for PNP. I think this makes it a lot easier to populate the boards.

- zero potential pads marked by rectangle outline, non-zero / signal+ by beveled outline.

- C1 alternate pad lead spacing shortened.

- The spacing of the "heat sink sandwich" Q13-16 has been reduced to align the transistor pad spacing of the back-to-back packages. (It was fractionally too wide before, but didn't affect mounting at all.)
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File Type: jpg pcb-sapphire-41t-sch.jpg (564.5 KB, 434 views)
File Type: png pcb-sapphire-41t-pcb.png (57.6 KB, 180 views)
File Type: png pcb-sapphire-41t.png (55.9 KB, 158 views)
File Type: png sapphire-41t.png (194.9 KB, 153 views)
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File Type: zip pcb-sapphire-41t-bom.zip (424.4 KB, 33 views)
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Last edited by rjm; 6th December 2017 at 01:49 AM.
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Old 7th December 2017, 11:35 AM   #1336
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Quote:
Originally Posted by rjm View Post
How to use connect a rotary switch to the Sapphire 41m boards to enable front panel gain selection.

Experimental. The inductance of long lead wires may cause stability problems, especially with the closed loop configuration.
Front panel gain rotary switch tested and works fine, so if that's a convenience feature you want to add go right ahead.
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Old 16th December 2017, 11:46 AM   #1337
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I have a ton of audio-related items to wade through this weekend, let me deal with this one first, it's the most straightforward.

I have two Sapphire (4) builds at home, one is configured as a headphone amplifier, the other as a line stage. The headphone amp is set up closed loop, the line stage open loop. The linestage gain is switchable 3-6-9 dB. Today I converted the line stage to run closed loop, keeping the gain setting 3-6-9 dB as before. R3X = 2.21k, R2A = 4.75k, R2B = 4.75k, R2C = 1.5k.

When I first build the linestage I reported I couldn't really hear it in my system so I might as well keep it open loop. Recently I was comparing (phono + line stage) vs (phono stage direct) into my Sapphire headphone amplifier, and I could detect a small loss of transparency with the line stage present.

The open loop Sapphire line stage induces a slight "thickening" of the music signal, adding warmth and smoothness and the expense of resolution. LTSpice says the open loop has about 10 dB higher 2nd and 3rd harmonic distortion than the closed loop at 0 dB signal output, 6 dB gain. We are talking about the difference between -90 dB 2nd harmonic distortion and -100 dB. The distortion is barely perceptible, but it is perceptible.

On the principle that the best line stage is the least audible, I switched to the closed loop. The sound is clearer and cleaner, more solid.

I see no downside with going closed loop in this circuit. Even at low 6 dB gain where phase margin is supposedly marginal, I encountered no hint of any nervousness. It sounds almost exactly as before, just slightly cleaner. In comparison the open loop version has a slight vintage feel to it. I prefer the closed loop R3X version hands down.

I've added some line stage gain configuration suggestions to the attached BOM 4.1t2.
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File Type: png sapphire line comparison.png (102.7 KB, 137 views)
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File Type: zip pcb-sapphire-41t2-bom.zip (426.5 KB, 31 views)
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Old 17th December 2017, 10:31 PM   #1338
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Default sapphire-41t1 70x90 fixed short

Sapphire board is standard Eurocard, 80x100 mm.

I had a request for a fractionally smaller one, so the attached Gerber is 79.8x99.8 mm. Apparently the fabrication house was adding 0.05 mm to the size automatically which pushed the unit cost into the next price bracket.
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Old 9th January 2018, 07:20 PM   #1339
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Default Finished Rack Mount V4

Completed this project around Christmas but had to wait to source and modify one of the knobs before finishing.

As noted earlier, I built the v3, liked the sound, but had a couple of issues. In the small chassis, I had no place to keep it except on top of the m/c phono stage and that proximity caused some interference. Second, the small size of the chassis made separation of signal and power cabling difficult.

I decided to build the v4 in a rack mountable chassis, killing two birds with one stone. However, with a kind of permanent rack chassis the impedance jumper system would have been impracticable to use. So Richard was kind enough to provide a solution for a panel mount gain switch which he posted a few weeks ago. That worked out fine.
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Old 9th January 2018, 10:21 PM   #1340
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An excellent demonstration of the advantages to using an oversize enclosure.

In addition to providing plenty of separation between the power supply and input signals, all the hardware can be placed in the ideal position without compromise.

It's the most formal component layout and frankly still the best: viewed from the backplate, input signal far left, power supply on the far right, input to output sections moving left to right.
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