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RJM Audio Sapphire Desktop Headphone Amplifier
RJM Audio Sapphire Desktop Headphone Amplifier
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Old 11th February 2019, 12:31 AM   #1471
itsikhefez is offline itsikhefez  United States
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Quote:
Originally Posted by rjm View Post
14 ohms is quite punishing, even if the efficiency is reasonable. You can see from the attached image that the output power of the Sapphire in the standard 37.5 mA bias configuration drops off considerably below 60 ohms.

There is nothing about the circuit that makes it incapable of driving these loads with low distortion however, it's just that raising the bias current would be optimal.
Thanks for the info.
What changes need to be made to increase the bias current? (I believe mine was a 4.1t version).
How much higher would you go? 75, 150?
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Old 11th February 2019, 03:38 AM   #1472
itsikhefez is offline itsikhefez  United States
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Quote:
Originally Posted by rjm View Post
14 ohms is quite punishing, even if the efficiency is reasonable. You can see from the attached image that the output power of the Sapphire in the standard 37.5 mA bias configuration drops off considerably below 60 ohms.

There is nothing about the circuit that makes it incapable of driving these loads with low distortion however, it's just that raising the bias current would be optimal.
Quote:
Originally Posted by itsikhefez View Post
Thanks for the info.
What changes need to be made to increase the bias current? (I believe mine was a 4.1t version).
How much higher would you go? 75, 150?
OK I took a look at the notes in the spreadsheet and seems like I need to change R11, R12.

A few questions:
1) Based on your graph, 75mA seems insufficient still. Since it's hard to see where 14ohm lands on the graph, what would be the optimal value? I assume somewhere between 100-150 ?
2) I assume that even if I go for 150mA, the default R17/R18 of 4.75 should remain?
3) Is the gain calculation the same after changing R11/R12?

EDIT:
Just stumbled upon #5443967 and saw the values in the updated BOM.
I will follow that for 150mA...would appreciate any other thoughts worth considering.

Thanks!

Last edited by itsikhefez; 11th February 2019 at 03:47 AM.
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Old 11th February 2019, 06:29 AM   #1473
itsikhefez is offline itsikhefez  United States
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To my understanding these headphones require 200mW @ 16ohms. Based on the graph, the 75mA is not sufficient but 150mA seems overkill.
Given the large caps required, perhaps 100mA or 125mA would be better?
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Old 11th February 2019, 11:05 AM   #1474
rjm is offline rjm  Japan
Richard Murdey
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Quote:
Originally Posted by itsikhefez View Post
To my understanding these headphones require 200mW @ 16ohms. Based on the graph, the 75mA is not sufficient but 150mA seems overkill.
Given the large caps required, perhaps 100mA or 125mA would be better?
My answer I tried to post earlier got zapped as diyaudio went offline for a while.

Anyway ... the latest BOM you can download from my website gives more info on the bias current mods. There is no change between it and the 4.1t BOM, other than the electrolytic capacitors have been reduced in number and increased in size.

But, no, no one "needs" more than a couple of mW of headphone output power. Much like solid state power amplifiers, its a question of headroom and dynamic reserve rather than clipping distortion. So it's a matter of will it sound better, which in turn is a question of whether the additional distortion and noise from running at higher bias current will or will not balance the reduction in distortion achieved by the increased headroom.

My recommendation stands: for 60-300 ohms, use the standard configuration. for 16*-60 ohms, use 75 mA.

*14 or 15 ohms, also. Close enough.
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Old 13th February 2019, 07:38 AM   #1475
rjm is offline rjm  Japan
Richard Murdey
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pcb-sapphire-42b2-bom

New BOM for review. I've changed the Power Options sheet to remove the 150 mA version, keeping the 37.5 mA and 75 mA version.

I've heard the story that the burger restaurants offer a choice of three sizes as a psychological ploy in order to upsell customers to at least buying the medium-size burger.

For the Sapphire amp, the same psychology was operating in a negative sense. The additional choice was causing indecision and confusion. It would be better to settle on just "regular" and "large" options, rather than "small" "medium" and "large". That way most people would go with the normal bias mode, and anyone who didn't would have a clear single choice remaining.

Also, the scaling was wrong anyway. As usual I was overthinking the problem. And 12 W heat dissipation for the stereo circuit at 150 mA is kinda nuts. 75 mA provides plenty of power into low impedance loads. Let's leave it at that.
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Old 15th February 2019, 09:01 AM   #1476
itsikhefez is offline itsikhefez  United States
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Thanks for the update Richard.. the latest BOM is clearer.

Reading the power graph posted earlier, I was under the impression that both 37.5 and 75mA version would perform the same way for loads above 60ohm, thus the 75mA would be a more versatile choice for any headphone from 16-300ohms.

Quote:
Originally Posted by rjm View Post
So it's a matter of will it sound better, which in turn is a question of whether the additional distortion and noise from running at higher bias current will or will not balance the reduction in distortion achieved by the increased headroom
Does the 37.5mA perform "better" for high impedance cans because the extra headroom is not needed and distortion is added?
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Old 16th February 2019, 07:08 AM   #1477
itsikhefez is offline itsikhefez  United States
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Silly question but which caps are considered in the power supply and which signal path ?
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Old 17th February 2019, 12:01 AM   #1478
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@itsikhefez

1. That's a good way of looking at it: increasing the bias current widens the "useable" range of headphone impedance to include smaller values. While above 100 ohms or so the clipping level is unchanged with bias current, the distortion levels at lower output power does change with bias current. The relationship between distortion and current and load is complicated enough that I don't feel confident in trying to map it out, but essentially the optimal point is moved to larger output power as the bias current increases.

2. Formally only in the input coupling cap, C1, is in the signal path. I don't count signal return currents for this, so that's not to say that the others are unimportant ...
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Old 2nd March 2019, 03:13 PM   #1479
ustas is offline ustas  Israel
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Have a little question why trafos? Somebody use with RJM switching power supply 2A 12V?
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Old 2nd March 2019, 09:26 PM   #1480
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Have a little question why trafos? Somebody use with RJM switching power supply 2A 12V?
Linear supplies are much less problematic re. RFI and sound good. I wouldn't recommend switching supplies, but you can do it if you want. Just remove the Z-reg section and use the main amplifier circuit powered directly from +/-12 V rails.
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