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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 7th October 2019, 02:14 AM   #1
Mark Johnson is offline Mark Johnson  United States
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Noir, a two transistor headphone amp: class-A, single ended, 150mA bias
Default Noir, a two transistor headphone amp: class-A, single ended, 150mA bias

This thread is for discussions about the Noir headphone amp, whose PCB is available in the diyAudio Store. Its features include:
  • Solid state, all discrete: no ICs, no vacuum tubes
  • Single ended, Class A circuit operating at 150 mA bias current (exceptionally high for a headphone amp)
  • No mains voltage inside the DIY chassis. Power is supplied by a commercial, safety rated, wall wart @ 24V DC
  • All thru-hole parts, mounted on a single PCB with generous spacing between components
  • Very easy to stuff and solder; suitable for first time DIYers. Component IDs and component values printed on PCB silkscreen.
  • Headphone output on front panel, linestage preamp output (RCA jacks) on rear panel
  • Pre-made front and rear panels, drilled and silkscreened, for DIYers who prefer not to do metalwork themselves. These are black PCBs, 2.0mm thick, cut to match the diyAudio Store "1U Galaxy" chassis front & rear panels.
  • Detailed, 37 page .pdf file named "Noir Build Instructions" walks you through the assembly process with plenty of photos. It's 4.7 megabytes long, over the size limit for attachments here on the Forum, but is available for download on the sales page of the Store.

AMPLIFIER CIRCUIT

The Noir amplifier is a two transistor circuit, which is an idea that has been around quite a long time. Figure 1 top shows a hybrid tube+transistor amp circuit, and Figure 1 bottom shows an all-BJT version (from D.Self's book), to name but two examples. Noir takes this basic circuit and simply scales up the second stage bias current to 150 mA, then employs high wattage power resistors as the "collector loads." Simple.

Fig 2 is the Noir schematic. Compared to Figure 1, the device polarities have been reversed. Now the input transistor is P-type and the output transistor is N. This is advantageous because N-channel MOSFETs have a gain-per-unit-silicon-area which is proportional to electron surface mobility. P-channels' gain is proportional to hole surface mobility, and is 2-3X less. All we need to do is flip the power supply upside down, and we get 2-3X higher gain per unit capacitance. An easy decision. Thus Noir uses a positive-ground, negative-supply arrangement, with the supply rail "NEGPWR" at about -22 volts. Just like germanium transistor radios in the late 1950s.

Transistor Q71 and "current regulating diode" D75 (a constant current source) form the input stage of the amp. They drive the gate of output transistor Q72, whose drain load is a 12 watt, 68 ohm resistor, implemented via a series-parallel arrangement of four 3W, 68R resistors. Each of these resistors dissipates less than 0.5 watts during operation, and even though they have a comfortable margin of safety (6X), they do get quite warm during operation. The midpoint tap of the resistors provides feedback from output stage to input stage. Thanks to equal resistor values on the top and bottom of the voltage divider, feedback is (1/2), thus gain is 2X (+6 dB). This choice of gain encourages the user to set the volume control potentiometer somewhere in the second half of its rotary travel, where channel-to-channel tracking is best.

Noir has a single ended power supply so it needs AC coupling at input and output; C72 and C76+77 provide these. As with other capacitor-coupled amps (notably the Pass ACA), Noir does produce an audible "thump" at turn on and turn off. If you find the thump objectionable, some elimination strategies include leaving the unit on at all times (as Pass suggests for the ACA), unplugging the headphones (or removing them) first, installing additional circuit boards/modifications inside the Noir chassis, et cetera.

POWER SUPPLY

Fig 3 is the supply schematic. The DC wall wart output is filtered by a common mode choke and applied to a bridge rectifier. This means Noir can accept both (positive center, negative sleeve) wall warts and also (negative center, positive sleeve) wall warts. An LC output filter is then applied for additional smoothing. Potentiometer R2 and resistor R7 let builders adjust the current flowing through the LED "pilot light" and thus, its brightness.

CHASSIS OPTIONS

Fig 4 shows Noir in a "1U Galaxy" chassis from the diyAudio Store, with the silkscreened PCB front panel installed. Fig 5 shows the same unit, but this time using the brushed aluminum front panel that ships with the chassis. (I drilled it myself, using a drill press). I don't own the necessary tools to engrave lettering on the panel, but perhaps that's not necessary. There are only three objects and the function of each one is so obvious that perhaps no lettering is needed. Headphone Jack, Volume Control, Power Switch. Simple.

Fig 6 shows the rear of the unit using the PCB back panel. Here it definitely IS helpful to have lettering, to help you remember which RCA jacks are inputs and which are outputs.

Fig 7 shows the stuffed and soldered PCB for Noir.

The final attachment is a Bill Of Materials in Excel Spreadsheet format for Mouser's BOM tool. The Noir sales page in the diyAudio Store has a link to a populated Mouser shopping cart, which might be more convenient when you're ready to order parts.

DIY BUILD COST

Now that the Store prices have been announced, we can estimate the cost of building a Noir at home: $70 for PCB + chassis, $85 for Mouser BOM (drops to $80 if you buy the ALPS Blue Velvet potentiometer at parts-express dot com instead of Mouser), $5 to $10 for fancy solid aluminum volume knob from eBay, $5 for misc nuts and bolts at your local hardware store. Total cost: approx $165. Of course you are free to purchase "Premium Parts" (exotic film capacitors, $100 potentiometers, silver wire, etc) if that's your pleasure.

HOW DOES NOIR SOUND?

Played into my Sennheiser HD 650s and also my A-T M50Xs, I would say that Noir is nuanced. Female vocals (Joan Diener, Marianne Faithfull, etc) were ethereal and otherworldly. And yet full orchestras sounded detailed and crisp, forceful with no detectable muddiness. "Mars" from Holst's "The Planets" suite was suitably terrifying. And ZZ Top kicked assz like only ZZ Top can do. Put on "Tres Hombres" , toss back a couple shots of Anejo , and daydream of Terlingua and Saragossa.

OLD DISCUSSION THREAD

Early prototypes of Noir were called by the code-name "T2". There was a forum thread for discussing T2, here, now closed, whose coverage of the early days might be of historical interest.

_
Attached Images
File Type: png Predecessor_Schematics.png (99.4 KB, 988 views)
File Type: png NOIR_Amp_schematic_09_Sept_2019.png (79.4 KB, 1059 views)
File Type: png NOIR_PSU_schematic_09_Sept_2019.png (41.9 KB, 1009 views)
File Type: jpg pinch_knob.JPG (704.4 KB, 980 views)
File Type: jpg Metal_Front_Panel.jpg (299.7 KB, 942 views)
File Type: jpg PCB_Back_Panel.jpg (167.5 KB, 246 views)
File Type: jpg Fully_soldered_PCB.jpg (331.0 KB, 485 views)
Attached Files
File Type: zip BOM_spreadsheet.zip (75.1 KB, 41 views)

Last edited by Mark Johnson; 7th October 2019 at 01:31 PM. Reason: add link to ALPS pot at parts express
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Old 7th October 2019, 06:46 AM   #2
wushuliu is offline wushuliu  United States
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Noir, a two transistor headphone amp: class-A, single ended, 150mA bias
Great name.
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Old 7th October 2019, 02:49 PM   #3
adydula is offline adydula  United States
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Great, congrats Mark...

I have been an enthusiastic builder and user of the "T2" now Noir for several months and its still a sonically excellent sounding amp for headphones.

I recently bought a set of Focal Clears and am using them right now with my T2 and its a very, very good pairing and the overall sound presentation is one of the best I have ever heard.

Listening to Wynton Marsalis Quartet, You and Me....the handclaps, high hat, perscussion are so very much life like...piano harmonics to die for good....with the great tonality
of the Focal Clears and this amp its really good.

This amp is not perfect but its really close when it comes to listening to well recorded music....a great DIY build...easy and quite affordable...

Alex
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Old 7th October 2019, 08:03 PM   #4
Mark Johnson is offline Mark Johnson  United States
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Noir, a two transistor headphone amp: class-A, single ended, 150mA bias
The solid aluminum volume knob on the Noir unit shown in the photo at the Store (the one with the pinchy squeezy shape), sells for $3.79 on eBay

1pcs, 28x28 SOLID SILVER STOVE TOP VOLUME ROTARY TURN KNOB FOR AMPLIFIER AUDIO 812854373090 | eBay
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Old 7th October 2019, 08:22 PM   #5
adydula is offline adydula  United States
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I like this one better than the pinchy one!!

:>)

Its solid aluminum, non pinchy!

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Alex
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Old 8th October 2019, 01:31 PM   #6
Mark Johnson is offline Mark Johnson  United States
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Noir, a two transistor headphone amp: class-A, single ended, 150mA bias
Is anyone able to offer this Noir builder some advice? If so please respond in that thread, where he's guaranteed to see it, and where there's no other topics under discussion.

Thank you very much, -MJ
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Old 11th October 2019, 11:07 AM   #7
JamieMcC is offline JamieMcC  United Kingdom
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Hi guys can I please have some guidance on installing the 3PDT switch not 100% on which terminals to connect to in order to best eliminate the turn on thump?

Electric Mini Toggle Switch 3PDT 2 Position ON/ON 9-PIN AC 250V/2A 125V/5A | eBay

Cheers
Jamie
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Old 11th October 2019, 12:46 PM   #8
adydula is offline adydula  United States
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Jamie,

Just took the top off and here are a few pix, sorry for the mis-focus on one.

I mounted the switch so the external toggle goes up and down. When its up the center pins and connected to the bottom pins. I did not use the top pins at all.
So when the switch is up or on the center pins take the L/R signals to the bottom pins and wired back to the L/R pins on the headphone jack.

The L/R outs from the pcb go to the center pins....I did not move or remove the other two wires on the jack socket, the ones next the front panel on the switch.

The power is on the third set of pins closest to the right side looking from the front. Again center and bottom pin used.

This is working for me well.

I did not put in the 68Uf caps for the turn on thump, I just wait 10-15 seconds before inserting headphones, issue solved for me.

Alex
Attached Images
File Type: jpg WP_20191011_08_30_07_Pro[9620].jpg (939.5 KB, 316 views)
File Type: jpg WP_20191011_08_30_16_Pro[9619].jpg (882.0 KB, 132 views)
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Old 11th October 2019, 02:00 PM   #9
JamieMcC is offline JamieMcC  United Kingdom
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Quote:
Originally Posted by adydula View Post
Jamie,

Just took the top off and here are a few pix, sorry for the mis-focus on one.

I mounted the switch so the external toggle goes up and down. When its up the center pins and connected to the bottom pins. I did not use the top pins at all.
So when the switch is up or on the center pins take the L/R signals to the bottom pins and wired back to the L/R pins on the headphone jack.

The L/R outs from the pcb go to the center pins....I did not move or remove the other two wires on the jack socket, the ones next the front panel on the switch.

The power is on the third set of pins closest to the right side looking from the front. Again center and bottom pin used.

This is working for me well.

I did not put in the 68Uf caps for the turn on thump, I just wait 10-15 seconds before inserting headphones, issue solved for me.

Alex
Thanks Alex will take a look inside and make sure I follow.
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Old 11th October 2019, 06:11 PM   #10
Mark Johnson is offline Mark Johnson  United States
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Noir, a two transistor headphone amp: class-A, single ended, 150mA bias
I've laid claim to post #10, and will set it aside as a repository of builder photos.
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