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RJM Audio Sapphire Desktop Headphone Amplifier
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Old 10th February 2017, 11:47 AM   #1101
rjm is offline rjm  Japan
Richard Murdey
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RJM Audio Sapphire Desktop Headphone Amplifier
@ammel It was 2k originally before I changed the BOM based on Stan's experience.

@everyone Attached is an image of the bottom metal layer of the Sapphire 4.0b board. As that middle-management type in Star Wars says "We've analyzed their attack, sir, and there is a danger" ...

The red traces are the input to Q1,Q2 and the near-zero potentials on either side going into the trim pot. This is all high impedance area, biased to about zero volts by the voltage divider R4,5 between the power planes V+ and V-, colored green and purple.

You can see that these power planes go right up to the red traces. This presents a risk, as any accidental resistance from V+ or V- and those red traces will act in parallel with R4 or R5, pulling the bias voltage up or down. Since R4,5 are 100k, a few megaohms would be enough to move the bias value several hundreds of millivolts.

While it's unlikely to be a practical issue please be fastidious about not getting flux or grease or miscellaneous glop spread all over the area around the trim pot.

In future iterations I will address this by adding greater isolation or possibly a shield around the sensitive traces.

note: no I don't think this is what caused Stan's problems~
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File Type: png pcb-sapp-40b.png (82.0 KB, 415 views)
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Last edited by rjm; 10th February 2017 at 12:04 PM.
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Old 10th February 2017, 05:44 PM   #1102
Stan63605 is offline Stan63605  Canada
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Originally Posted by rjm View Post
you wrote millivolts earlier though

"OL, says Fluke; the other side measures 271mv"
Sorry, my bad.
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Old 10th February 2017, 05:50 PM   #1103
Stan63605 is offline Stan63605  Canada
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Quote:
Originally Posted by rjm View Post
... power planes go right up to the red traces. This presents a risk, as any accidental resistance from V+ or V- and those red traces will act in parallel with R4 or R5, pulling the bias voltage up or down. Since R4,5 are 100k, a few megaohms would be enough to move the bias value several hundreds of millivolts.

While it's unlikely to be a practical issue please be fastidious about not getting flux or grease or miscellaneous glop spread all over the area around the trim pot.

note: no I don't think this is what caused Stan's problems~
I usually wash down a pcb with methyl hydrate or contact cleaner to take away all the excess flux and glop that can be there after working on a board. This allows me to see if there are any solder bridges or such. Sapphire 4 was no exception, I'm quite certain this was not an issue. But I see how easily it could be.
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Old 10th February 2017, 11:13 PM   #1104
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Richard Murdey
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Since you confirmed the resistance of R4,5 in situ to be 100k exactly, we know for sure it wasn't an issue. And anyway I knew without asking you'd have been very careful. I just put it out there because it came up as I was reviewing possible causes.
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Old 10th February 2017, 11:42 PM   #1105
Stan63605 is offline Stan63605  Canada
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Default I like Sapphire 4

Today I listened to some favourite test tracks on Sapphire 4.0b, then put my Sapphire 3.0f boards back into the box as quickly as I could (sure am glad I used terminal blocks!) and listened again, then switched to 4 again as quickly as possible.

Conclusions: In 4, there is definitely more "air" in a more cohesive, natural soundstage than 3.0f. There is also a more dynamic sense of transients, such that the music has more snap to it, more vitality. The low frequency floor that I sense in 3.0f seems to have fallen away in 4: 4 is open and airy and natural, but with maybe a hint of warmth. 4 gives me the best sense yet of recording studio space.

If fact, I was kind of shocked that when listening through my 3.0f boards after hearing 4 just minutes previously, the music just kind of laid there, by comparison. In 4, my headphone listening has reached a new standard.

A new recording in my collection just a week or so ago is Bill Frisell's "When You Wish Upon A Star"; I love his cover of "You Only Live Twice", and I'm already well familiar with it - it's the kind of track you want to hear again, immediately. It showed the comparisons I mention most obviously. I could feel the recording space.

Another great recording that shows these contrasts is "Muddy Waters Folk Singer". This is another well-engineered recording that plays so well.

Woodsman's "All Tangled Up" is a dense sonic texture that recedes a bit into itself through 3.0f; Sapphire 4 restores the percussive snap that makes it work.

JJ Cale's "To Tulsa And Back" is a nice recording that also shows these contrasts: I've been focusing on "Chains of Love". I could go on...

No question in my mind: 4.0b has left Sapphire 3.0f behind.

My hat is off to you, Richard!


Note: my Sapphire 3.0f is running an SMC-mount AD8610 op amp on a Brown Dog adapter. I preferred the 8610 to the OPA134. I have not heard the later version of 3 that accommodates designer op amps, so take my comparison notes with that knowledge in hand.
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Old 11th February 2017, 01:35 AM   #1106
mlackey is offline mlackey  United States
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Quote:
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I just put it out there because it came up as I was reviewing possible causes.
RJ,

And it is good of you to do so.

The following link is to an article written long long when when super-regs were state of the art. But the subplot is interesting: some things never change.

http://waltjung.org/PDFs/Improved_PN_Regs.pdf
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Old 11th February 2017, 02:23 AM   #1107
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Richard Murdey
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bench notes:

I tried the feedback modification where R3 is connected to the output rather than the current source node R11-R12. R2 was 100 ohms, R3 was 2k21. Test load 47 ohms, 60 mV input, 50 kHz square wave. Works as advertised with C2 = 0. The expected bandwidth is about 2 MHz.

*****

BOM 40b9 attached. R6 changed to Bourns type, and recommended values for R2 R3 with the mod have been changed as I realized the gain formula is different in this configuration. It's R3/R2+1, not 0.5 R3/R2+1 as it is with the normal connection.
Attached Images
File Type: jpg DSC_1102s.jpg (359.8 KB, 415 views)
File Type: jpg DSC_1106s.jpg (340.2 KB, 400 views)
File Type: png pcb-sapphire-40b4-R3mod.png (55.3 KB, 382 views)
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File Type: zip pcb-sapphire-40b9-bom.zip (228.2 KB, 13 views)
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Last edited by rjm; 11th February 2017 at 02:30 AM.
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Old 11th February 2017, 02:36 AM   #1108
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Richard Murdey
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Quote:
Originally Posted by Stan63605 View Post

No question in my mind: 4.0b has left Sapphire 3.0f behind.

My hat is off to you, Richard!
Thanks Stan for taking the time to do a comparative review! Much obliged.
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Last edited by rjm; 11th February 2017 at 02:53 AM.
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Old 11th February 2017, 04:10 PM   #1109
Stan63605 is offline Stan63605  Canada
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bench notes:

I tried the feedback modification where R3 is connected to the output rather than the current source node R11-R12. ...
Listening test? Have you tried this or are your 4.0b boards currently farmed out to another builder? You wrote that 4.0b, compared to your last 3 build, showed some recession of treble intensity and transient response. I didn't get that from my listening and that leaves me wondering what else I may have missed in skipping the last iterations of 3.

Back to my board inequity. I'm tempted to try another set of zeners. Except that it's working and sounds so lovely I'm reluctant to take it apart again. That's how I got stuck on AD8610's. My experiment with designer op amps many posts back was a crashing failure, so I didn't try any others.

The evidence suggests that I follow your lead, but something tells me I need to find the bug first.
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Old 11th February 2017, 10:06 PM   #1110
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I have the boards still, just not cased up. I checked the mod to make sure it passed square waves without flying off its rails, that's it so far.

With a resistive load it works as simulated, but the real world influence of the headphone inductance for example is not considered.
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