Noir, a two transistor headphone amp: class-A, single ended, 150mA bias

Mark Johnson

Member
Paid Member
2011-05-27 3:27 pm
Silicon Valley
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.

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adydula

Member
2010-08-10 5:46 pm
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
 

adydula

Member
2010-08-10 5:46 pm
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
 

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adydula

Member
2010-08-10 5:46 pm
Been comparing the Noir amp to Schitt Magni3 and Pete Millets Bute amp, which I just reduced the gain from 11x to 5.7 and now to "2" and changed its 552 to a OPA551PA lower gain design op amp of the same family from TI.

All are great amps and perform admirably, but sonically they "sound" different.

The Noir amp with 55 ohm Focal Clears is one of the best combinations I have heard..its a three dimensional holographic soundstage that really surprise me each time I use this amp.

Songs that are recorded live or is a "hot" studio...ie lots of reverberations its easy to hear these and feel what it was like in the studio where the song was recorded.

I think this is one of the nicest traits this amp has going for it...

Again a well done DIY project Mark!

Alex
 

Mark Johnson

Member
Paid Member
2011-05-27 3:27 pm
Silicon Valley
Yes, it'll be glorious and happy with those.

I own and use several different headphones with my Noir, ranging from 25 ohms (Fostex) to 250 ohms (Beyerdynamic) to 300 ohms (Sennheiser). It drives them all and makes beautiful sound IMO. 600 ohm cans will be an easy load.

Here's a plot of measured data from 84 different headphones, taken from this spreadsheet on head-fi. I overlaid measured data from Noir in blue; it's the max available RMS output current (without clipping) vs RMS output voltage. Take a look: the Beyerdynamic 600 ohm headphones' requirements (line 21 of the head-fi spreadsheet) are comfortably below Noir's maximum output curve.

Do keep in mind that Noir's voltage gain is "only" +6dB; if you want ear-splitting volume, don't connect it to a very low output source.

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