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RJM Audio Sapphire Desktop Headphone Amplifier
RJM Audio Sapphire Desktop Headphone Amplifier
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Old 29th June 2017, 03:32 AM   #1241
ammel68 is offline ammel68  United States
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RJM Audio Sapphire Desktop Headphone Amplifier
Quote:
Originally Posted by bczupa View Post
I already like the amp and I'm quite happy that I build it.
You stated in post 1236 that you have one channel closed loop and the other open loop.
Can you detect any differences in sound quality between closed and open loop?
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Old 29th June 2017, 08:50 AM   #1242
bczupa is offline bczupa  Poland
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Quote:
Originally Posted by ammel68 View Post
Can you detect any differences in sound quality between closed and open loop?
Well, rather not. I would need to spend more time testing it. With two different headphone sets, the results were different, but it is certainly not "quality" difference, just the different taste. With both options the sound is good - no distortions, ho hum, no noise.

I assembled both option at the start only for test purposes, just to measure things. For the little while I had set them differently I observed the difference of the sound level between the two channels so if was not an option.
When decided to actually listen to sound of the amp, I made both channels the same, and finally stayed with open loop and gain 26db (headphones 300ohm 94db SPL/mW)

Open loop seems to be more colorfull, more pleasant, adding more to the sound signature. Closed loop sounded more sterile, the dynamics was better but overall sound was rather harsh when compared to the open loop.

With high gain I would stay with open loop, but for low gain settings the closed loop option seems to be better.

I put pins in place of resistors and switched between a lot of different options, so I'm quite confused at the moment. I need to listen one fixed configuration little longer to get used to it and then after the change is made it will be easier to tell if the change is for better or not.
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Old 5th July 2017, 06:40 AM   #1243
ammel68 is offline ammel68  United States
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RJM Audio Sapphire Desktop Headphone Amplifier
Quote:
Originally Posted by bczupa View Post
With high gain I would stay with open loop, but for low gain settings the closed loop option seems to be better.
I'm not doing the high gain thing since I'm using this as a preamp and not as a HA.
The first boards I built were closed loop with 6dB of gain.

For the next boards I assemble, I'm thinking about trying the same 6dB gain and the open loop configuration.

Sounds like you may have confused yourself with not maintaining the same gain/loop configurations for both channels.
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Old 17th July 2017, 11:46 PM   #1244
withmatt is offline withmatt  South Korea
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Click the image to open in full size.
This is my build of sapphire4.
I just started populate, waiting for capacitors delivery.
I also made Power PCB to have transformer and bridge rectifier in one PCB.
following modifications are made on main PCB.
1. One Nichicon KG (20mm dia snap-in) instead of two parallel Nichicon KW 1000uF.
2. Around Zener Regulator I changed 4 Caps from Nichicon KW 100uF/25V (2.5mm ls, 6mm dia) to Nichicon FG 100uF/25V (3.5mm ls, 8mm dia)
3. Around Voltage Amplifier and Buffer stage, I changed 6 Nichicon KW to 6 Nichicon KZ 100uF/25V (5mm ls, 10mm dia)
4. changed 0.1uF ceramic capacitor footprint to 5.5mm/7.5mm FKP Print to use Vishay MKP1837
5. for gain switch, I used a two position DIP switch, instead of two jumper switch.
6. added 3mm dia LED (for each of + and - rail) to identify board is working or not.

Last edited by withmatt; 17th July 2017 at 11:50 PM.
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Old 18th July 2017, 12:28 AM   #1245
withmatt is offline withmatt  South Korea
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Photo here.
20170714_215127_HDR.jpg - Google Drive
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Old 18th July 2017, 09:17 AM   #1246
rjm is offline rjm  Japan
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I'm looking forward to see the final assembly.

One small thing: since you've used larger capacitors, it may not be possible to screw together the heatsinks and transistors once the capacitor C10,11 and C2,3,6,7 are in place. Clearance was already tight on the original layout. So you might want to fit the transistors/heatsinks first.
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Old 18th July 2017, 01:32 PM   #1247
bczupa is offline bczupa  Poland
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The very first modification I would do if I were making new PCB would be to move away C8, C9 so instead of separate heat sinks for transistors I would install one common heat sink vertically across PCB from top to bottom - a cheap bare piece of aluminium that assure that both channels have the same temperature, and you don't need extra place vertically anymore to accommodate typical to-220 heat sinks.

cheap, effective, and useful in term of temperature stabilization of both channels.

On the original PCB I mounted all transistors under the PCB, getting rid of heat sink, using aluminium enclosure case as large heat sink - easy to assemble but difficult to service and dis-assemble.
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Old 19th July 2017, 01:22 AM   #1248
rjm is offline rjm  Japan
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The heatsinks are barely needed except when bench-testing the full output power. Each transistor only dissipates 400 mW under normal operation.

I tried various layouts and heatsink arrangements, including oddly enough different iterations of the metal plate idea - my original plan actually. Not saying it couldn't be done, but the layout is fairly cramped and from my point of view the vertical heatsinks were the most space efficient option that was still convenient. Also, I had the heatsinks left over.
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Old 19th July 2017, 01:50 AM   #1249
withmatt is offline withmatt  South Korea
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I am using BC550C/ BC560C for Sapphire4 build because I have plenty of those left from previous project.
I found BOM error that BC550C is actually NPN and BC560 is PNP. So BC327's alternative should be BC560 and BC337's alternative should be BC550
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Old 19th July 2017, 02:48 AM   #1250
rjm is offline rjm  Japan
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Thanks. I've uploaded corrected BOMs to the website.
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