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Noir, a two transistor headphone amp: class-A, single ended, 150mA bias
Noir, a two transistor headphone amp: class-A, single ended, 150mA bias
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Old 12th November 2019, 12:39 AM   #21
batty is offline batty  Australia
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Mouser Parts arrived yesterday less a couple of parts on back order.
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I have gone from 85 watts class AB amplification to an ACA.Gone on to the M2X.
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Old 12th November 2019, 07:29 PM   #22
nautibuoy is offline nautibuoy  United Kingdom
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I got a package from Mouser today and the PCB is on the way. In the process of sorting out a Galaxy too.
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Old 14th November 2019, 07:20 AM   #23
nautibuoy is offline nautibuoy  United Kingdom
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Is the Noir particularly prone to RFI pick-up? I ask because the build guide version utilises the steel panel version of the Galaxy enclosure and the kit front and rear panels are specifically grounded in the guide. I'm looking to use an all aluminium chassis. Thanks.
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Old 14th November 2019, 07:35 AM   #24
avtech23 is offline avtech23  Australia
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Noir, a two transistor headphone amp: class-A, single ended, 150mA bias
I used an all aluminium chassis and did not have any RFI issues with my build.
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Old 14th November 2019, 07:41 AM   #25
nautibuoy is offline nautibuoy  United Kingdom
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Quote:
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I used an all aluminium chassis and did not have any RFI issues with my build.
Cheers.
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Old 14th November 2019, 02:38 PM   #26
Mark Johnson is offline Mark Johnson  United States
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Noir, a two transistor headphone amp: class-A, single ended, 150mA bias
You do want to get pretty good electrical connection between circuit board ground, and the metal exoskeleton of the ALPS potentiometer. If the pot body is floating and not grounded, you might hear a bit of hum when you grasp the volume control knob. This is of course something you can experiment with on your own Noir unit.
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Old 14th November 2019, 07:51 PM   #27
nautibuoy is offline nautibuoy  United Kingdom
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Quote:
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You do want to get pretty good electrical connection between circuit board ground, and the metal exoskeleton of the ALPS potentiometer. If the pot body is floating and not grounded, you might hear a bit of hum when you grasp the volume control knob. This is of course something you can experiment with on your own Noir unit.
Thanks Mark. I'l probably be using a Noble pot but the principle is the same as they have a metal shell.
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Old 16th November 2019, 11:12 AM   #28
PjVervoorn is online now PjVervoorn  Thailand
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Hi,

What are the possible/advisable changes, if I want to drive a 250 ohm Beyerdynamic 880, preferably feeding the amp from batteries.

I guess the output cap can be replaced, by something in the range of 220uF to 1000uF?
Can I just lower the bias, by removing the 'parallel branches' of the power resistor, and what impact does that have (apart from potentially longer battery life and amp life span)?
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Old 16th November 2019, 02:52 PM   #29
Mark Johnson is offline Mark Johnson  United States
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Noir, a two transistor headphone amp: class-A, single ended, 150mA bias
PjVeroorn, I haven't tried the modifications you propose, and I don't think anybody else has either. You're a pioneer, venturing into unknown and unexplored territory. You're going to have a LOT of fun! Good on ya.

I don't see any advantage in reducing the size of the output capacitor, but it's your DIY project and you are in complete control. If you want to reduce that cap, go right ahead. The 3300uF capacitor called for by the Noir PCB, was chosen to give acceptable bass response when driving 16 ohm headphones. If you plan to drive a load impedance fifteen times larger, then perhaps there's little harm if { ... insert your opinions here ... } by a factor of fifteen.

Keep in mind that the Noir bias circuitry assumes the amp is being fed from a regulated supply. If instead you feed it from batteries, whose output voltage droops as they discharge, then the bias current in the amplifier will change. Since it's a single ended Class-A design, changing the bias current will change the perceived "sonic character" of the amp and, Murphy's Law predicts, not for the better. On the other hand if you recharge the batteries "early", perhaps when they've delivered, oh let's just say, (0.25 * RatedCapacity), then maybe you'll get an acceptably small voltage droop, which produces an acceptably small change in the amplifier's sonic character.

A stock Noir draws about 350 mA from a 24V supply. If you connected two (12V, 4 amp-hour) sealed lead acid batteries in series as a power supply, then it would take a stock Noir 170 minutes to draw (0.25 * RatedCapacity). Maybe that's acceptable to you. Recharge every three hours of operation.

Neither I nor anyone else has experimented with reducing the bias current in the output stage (or the input stage either). Feel free to try it and see if you are happy with the results you get.
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Old 16th November 2019, 03:11 PM   #30
nautibuoy is offline nautibuoy  United Kingdom
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My Noir PCB pack arrived today - nice quality.

Still have to be patient though as I'm waiting for the 'big' resistors (they were out of stock at Mouser) along with the Galaxy chassis I have on order from Modushop.

I'm also planning to use a lower value output cap as the minimum impedance headphones I'll be using Noir with is 32ohms. I've got one or two other variations from the standard build in mind too, though not to the amp circuit itself.
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