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RJM Audio Sapphire Desktop Headphone Amplifier
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Old 11th August 2016, 06:34 AM   #981
rjm is offline rjm  Japan
Richard Murdey
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I would counsel against putting the IEC jack on the board like that if the boards are to be fixed to the bottom plate of the chassis rather than the back panel. As it is now, the back cutout must be perfectly aligned or you'll have to shim the standoffs. Not fun.

I don't feel there is particular need for additional filter capacitance on the power supply board like that, but it is up to you if you want to include it.

Surface mount diodes are fine is someone else is installing them, but I think through hole would be a better choice for diy.
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Old 18th August 2016, 01:47 AM   #982
gtose is offline gtose  United States
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Originally Posted by rjm View Post
I would counsel against putting the IEC jack on the board like that if the boards are to be fixed to the bottom plate of the chassis rather than the back panel. As it is now, the back cutout must be perfectly aligned or you'll have to shim the standoffs. Not fun.

I don't feel there is particular need for additional filter capacitance on the power supply board like that, but it is up to you if you want to include it.

Surface mount diodes are fine is someone else is installing them, but I think through hole would be a better choice for diy.
rjm, thanks for the feedback!

I luckily have access to a CNC machine I'll be using to for the rear panel cutouts, so hopefully alignment won't be an issue. Also, I just built a reflow oven, so I'm using this as a chance to give SMDs a try.

I've added the extra filter caps because this will be powering an additional circuit per channel, a relay switched passive attenuator when I get time to design it.

Thanks again for your comments, I just got my DigiKey order in with the rest of the amp components, can't wait to get it built!
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Old 19th August 2016, 05:31 AM   #983
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Richard Murdey
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I'm certainly looking forward to seeing the final result.
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Old 21st August 2016, 01:52 AM   #984
gtose is offline gtose  United States
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I'm certainly looking forward to seeing the final result.
Thanks! I'm looking forward to finally listening to this amp after sitting on it for a few months (almost a year?!).

I spent some time last night matching the transistors on hfe. If anyone is interested, I did the matching using one of the Atmega328 transistor tester kits available on eBay and elsewhere. Initially, I was skeptical of its accuracy so I validated some tests using the circuit outlined at http://www.geofex.com/article_folders/ffselect.htm. Overall, they agreed and made testing 200 transistors much easier!

RJM, my results in the grouping of transistors were very similar to your observations in #438. The NPNs were very tightly grouped for about 80/100. The PNPs were much more widely distributed. I ended up selecting the tightest group of 18 PNPs as near to the commonly occurring hfe for the NPNs (288).

I am just about ready to sit down and start soldering, but before I proceed a quick question:

The metal film resistors I am using (Takman REY25) are not available in 1 ohm. Instead, I have 2.2 ohm resistors to use for R19, 20, and 21. How important is it for these to be 1 ohm? I must say I don't understand the circuit enough to understand their purpose or the impact of changing this value. I do have 1 ohm resistors available but it will clash with my resistor color scheme .
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Old 21st August 2016, 02:40 PM   #985
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Richard Murdey
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The change to R21 won't do anything but increase the output impedance by an ohm. Changing R19 and R20 will reduce the output bias current from 32 mA to about half that.

If you are really worried about your paint job use 2x 2.2 resistors in parallel, the second one can be soldered to the underside of the board.
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Old 24th August 2016, 06:03 PM   #986
ammel68 is offline ammel68  United States
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If anyone is interested, I did the matching using one of the Atmega328 transistor tester kits available on eBay and elsewhere.
I have several variations of these Chinese component testers.

When checking the Hfe of transistors, I notice I can get a different reading every time I push the "test" button.
Hfe can vary as much 10 or 12 for a given transistor.

I noticed nobody else has mentioned this about these testers.

So are you supposed to take an average of several readings, or what?
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Old 24th August 2016, 08:26 PM   #987
Stan63605 is offline Stan63605  Canada
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When checking the Hfe of transistors, I notice I can get a different reading every time I push the "test" button.
Hfe can vary as much 10 or 12 for a given transistor.
The Atlas tester I use is much more consistent. I think it's an Atlas DCA55. I can recommend it.
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Old 25th August 2016, 12:38 AM   #988
mlackey is offline mlackey  United States
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The Atlas tester I use is much more consistent. I think it's an Atlas DCA55. I can recommend it.
Agree. I have noticed that parts from a single run (i.e. parts from same reel or with same date code) are more consistent than parts from different runs.

In a PM with another forum member, it was suggested that I use the tester to find the "outliers", or the grossly dissimilar parts, and exclude them from the selection. In a decent circuit the hFE and/or Vbe values should not dominate the results. I'd offer that a 10-20% variance is pretty close, but as always YMMV.
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Old 27th August 2016, 11:33 AM   #989
lung is offline lung  Germany
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Default 3.0 Boards ordered

Hi Richard,
sent you mail for 3.0 boards.
Greetings Rolf
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Old 27th August 2016, 08:03 PM   #990
ammel68 is offline ammel68  United States
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The Atlas tester I use is much more consistent. I think it's an Atlas DCA55. I can recommend it.
Thank you for the recommendation. Hopefully I can get one in the future.
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