Bijou
https://web.archive.org/web/20110909022955/http://www.cavalliaudio.com:80/diy/bijou/main.php?page=ampschematic
A STREAMLINED FUTTERMAN DESIGN
The Futterman OTL design has been around for many years. It has principally been used for power amplifiers. In these arrangements, the output stage uses a bipolar power supply so that the output can be directly coupled to the speakers without a coupling capacitor or transformer.
The output is coupled to the headphones through both a large electrolytic capacitor (C5) and a smaller, 1μf film capacitor (C6) to get the best frequency response through the coupling capaciors.
An interesting application of the Futterman concept for a headphone amplifier is Rudy van Stratum's amplifier. This Futterman amplifier has some nice features, but as van Stratum noted in his article it was not able to hold a consistent 0VDC at the output, necessitating using a coupling capacitor anyway. Also the power supply arrangement, with so many separate supplies for the various bias voltages and bipolar supply, makes building the amp a fairly complex endeavor.
Nevertheless, as I was designing and building White Cathode Follower based headphone amplifiers, my thinking kept returning to the Futterman design. One negative feature of the WCF output stage is that its push-pull balance depends on the load. A properly designed Futterman does not lose push-pull balance for any load.
But, I kept getting stymied by the need for so many independent supply voltages and the attending complexity of the amplifier. Finally, I set a design goal to design a Futterman type headphone amplifier using just one high voltage secondary and one heater secondary. This would mean simplifying the typical Futterman supply from eight independent power supplies to just two. A second design goal was to use only current production tubes (all of my previous designs had use NOS tubes somewhere). At first the task seemed impossible, but as I played with various biasing schemes, it eventually became quite simple. And with that the Bijou Headphone amplifier was born.
Bijou Design
ONE CHANNEL SCHEMATIC
After introducing the ECC99s into the design and building my own Bijou, I made a few other changes. One change is to reduce the size of the coupling caps from 330n to 100n, making them smaller. This is compenated for by increasing the size of the grid leak resistors from 100k to 470k. A few other front end biasing changes were also made.
Several members of the Headize DIY community participated in a prototype evaluation of the amp and boards. As a result of this evaluation, it was decided to standardize the Bijou at 250V instead of the original 220V. Changes to the original Bijou are negligible. See the Tweaks section for 220V operation.
Here is the current Bijou schematic.
Bijou One Channel Schematic
Bijou One Channel Schematic
The output is coupled to the headphones through both a large electrolytic capacitor (C5) and a smaller, 1μf film capacitor (C6) to get the best frequency response through the coupling capacitors.
THE POWER SUPPLY
The power supply uses a single EZ80 rectifier and a mosfet series pass regulator. The standard 250V PS uses a Hammond 270DAX transformer. Using the 270DAX solves another problem with the original Bijou transformer the 270AX. With at total of 2.8A heater requirements and only a 2.5A secondary, the 270AX was slightly overloaded. The 270DAX has a 3.5A winding, easily supplying the full heater current.
Here is the PS schematic.
Bijou PS
Bijou 250V Regulated Power Supply
WARNING: This power supply uses potentially lethal HIGH VOLTAGES, more than 300V in certain places. Construction and testing of this PS should only be done by someone who is thoroughly familiar with safety precautions around high voltages.
WARNING: If you short the output of the regulator while it is under power, both mosfets will be destroyed and you will have to replace them.
If handled properly the PS will operate with stability for a long time. The one in my Bijou has been operating for almost two years without problems.
Bijou Parts Lists
Parts Lists for Amplifier Board
Kits (boards/parts) for the Bijou are available from Glass Jar Audio.
See the Tweaks section for 220V operation.
Part Quantity Description
Circuit Board
- 1 Bijou Amplifier Single Channel Board
Potentiometers
Volume 1 100kΩ Dual Audio Taper
NFB 1 250kΩ Dual Linear Taper
Resistors
R1, R12, R13 3 1/8W 1% Metal Film Resistor 10kΩ
R2 1 1/8W 1% Metal Film Resistor 2kΩ
R3 1 1/2W 1% Metal Film Resistor 100kΩ
R4, R8, R10 3 1/8W 1% Metal Film Resistor 301Ω
R5, R6, R7 3 1/8W 1% Metal Film Resistor 39.2kΩ
R9, R11 2 1/8W 1% Metal Film Resistor 475kΩ
Capacitors
C1 1 Aluminum Electrolytic 10u 250V
C2 1 Aluminum Electrolytic 1000u 16V
C3, C4 2 Metalized Polyester/Polypropylene Film 100n 400V
C5 1 Aluminum Electrolytic 470u 250V
C6 1 Metalized Polyester/Polypropylene Film 1u 250V
Diodes
D1 - D6 6 Rectifier Diode 1N4002
D7 1 Small Signal Diode - 1N4148
LED 1 3mm 1.2V - 5.2V LED
Vacuum Tubes
V1 1 Dual Triode 6922/6DJ8 and equivalents
V2 1 Dual Triode JJ Tesla ECC99 or Russian NOS 6N6Pi;
Miscellaneous
Sockets 2 9 Pin PCB Mount Tube Socket
Connector 3 3 Pin Phoenix 651-1729131
Connector 2 2 Pin Phoenix 651-1729128
Resistors
All resistors except R3 are 1/8W 1% Metal Film type - 10mm or less lead spacing.
R3 is 1/2W 1% Metal Film type - 12mm or less lead spacing.
RLED sets the current in the LED. Calculate RLED using the forumla RLED = ( 9 - VLED ) / ILED.
Capacitors
C1, C2 are Aluminum Electrolytic - 5mm lead spacing 10mm diameter.
C3,C4 are Polyester/Polypropylene Film - 15mm lead spacing, 18mm length, 5mm width.
C5 is Aluminum Electrolytic - 10mm lead spacing 22mm diameter.
C6 is Polyester/Polypropylene Film - 22.5mm lead spacing 27mm length 8mm width.
Diodes
D1-D6 are DO-41.
D7 is DO-35.
LED is 3mm.
Bijou Instructions
PS BOARD ASSEMBLY INSTRUCTIONS
Solder the components to the board from lowest profile to highest, roughly in this sequence:
Resistors that lay flat on the board and the zener.
The film capacitor (C2), the connectors, the trimpot (P1), and the resistors that stand vertically (R7 and R8).
The tube socket.
The mosfets and their heatsinks.
The electrolytic capacitors.
Insert the rectifier tube.
WARNING: Always use heatsink mounting kits for the mosfets and be absolutely sure that both devices are insulated from the heatsinks. Attaching the mosfets directly to the heatsinks will float the heatsinks at high voltage, significantly increasing your risk of shock and injury. Don't try to get away with this because you won't.
Make sure the the heatsinks are not touching any of the traces or other components. Attach the heatsinks to the board by soldering their through pins to the bottom-side pads.
AMPLIFIER BOARD ASSEMBLY INSTRUCTIONS
Populating the amp boards is straightforward. Follow the same instructions as the PS board leaving out the mosfet step.
Insert the tubes making sure you get them into the correct sockets.
Do not wire across J1 unless you are having hum or noise problems that cannot be solved by any other changes. See the Troubleshooting section for more information.
GENERAL
Clean up the solder flux residue from the board with isopropyl alcohol (or electronics flux remover) and a brush.
Inspect all solder connections carefully, using a magnifying glass, to make sure there are no solder bridges or cold solder joints. Use a multimeter in ohms scale to check for short circuits. Correct any mistakes before proceeding.
Bijou Wiring
BOARD WIRING FOR ECC99
The printed circuit boards are designed to fit into a standard star ground scheme. The two Amplifier Boards and one Power Supply Board should be wired like this (click to enlarge):
Bijou Board Wiring
How to Wire the ECC99 Bijou Amp and PS Boards
BOARD WIRING FOR 6N6PI
6N6Pi has a different heater arrangement from the ECC99. The 6N6Pi uses only pins 4 & 5. Wire as shown (click to enlarge):
Bijou Board Wiring
How to Wire the 6N6Pi Bijou Amp and PS Boards
JUMPER J1
The Bijou Amp boards have split ground traces, one trace for the input and phase splitter stages and the other trace for the ouput stage. This keeps the high current ground signal from the output stage out of the input ground currents. These two ground traces are wired separately to the star ground, as you can see in the wiring diagrams above.
Do not wire across J1 unless you are having hum or noise problems that cannot be solved by any other changes. See the Troubleshooting section for more information.
DOING THE ACTUAL WIRING
Because the heaters use AC, the heater wiring has been kept off-board to eliminate hum that might be caused by high heater currents running through board traces.
The star grounding arrangement shown here should minimize or eliminate hum and buzz.
Screw type board-mounted connectors have been used to make board-to-board wiring as simple and flexible as possible. However, it is not necessary to use the connectors. They can be left off the boards and the wires soldered directly to the through holes.
The Bijou boards are designed to be wired together without the need for additional terminal blocks. You can use additional blocks if you prefer the wiring pattern better, but you don't have to. Here's how to wire without extra blocks.
HEATER WIRING
Follow this procedure for heater wiring
Make two twisted pairs of 20awg wire long enough to go from the PS board to each individual amp board.
Connect these two pairs to the PS heater terminals, in parallel. Tighten down the connector screws.
Insert the other ends of the pairs into the ECC99 heater terminal block, one pair to each amplifier board.
Make two more twisted pairs that can reach from the ECC99 terminal block to the 6922 heater block. Be sure that these pairs are long enough to run around the outside of the board, away from the edges and close to the chassis. Do not run the heater wires under the board.
Attach each pair to the ECC99 block together with the pair from the PS. Tighen down the terminal screws.
Insert the short pairs into the 6922 heater block and tighten down the block screws.
The PS heater connections are automatically biased to 40VDC to reduce the HK voltage for the ECC99s. Nothing else needs to be done for a standard heater configuraion
B+ WIRING
Make two 22awg wires (with at least 600V insulation) that can reach from the 250V terminal on the PS to each amp board.
Insert the two ends of these into the 250V PS terminal block and tighen down the screw.
Run each wire separately to the 250V terminal on each board. Tighten down the screws.
GROUND WIRING
The star ground wiring can be done using a bolt that is first tightened down to the chassis with a lock washer and nut. You'll need solder lugs that will fit over the bolt.
Make each ground wire that will reach from the star ground terminals on the PS and Amp boards to the star bolt.
Solder a lug to one end of each of these wires.
Insert each wire into its respective star ground terminal on the boards.
Drop the lugs one at a time onto the bolt.
Tighten the lugs onto the bolt with another lock washer and nut.
You can use other techniques to star ground, including auxillary terminal blocks, but the lug method is about the easiest.
INPUT, OUTPUT, AND NFB WIRING
Make sure that you use insulated input and output jacks.
Input and NFB connections should be straighforward. Follow the wiring diagrams for running the wires from the jacks to the volume pot and from both pots to the Amp boards.
Run a wire from each output terminal to the headphone jack. Run the headphone jack ground to the star ground.
Make sure that left and right channels are consistent for the input and output connections.
NEGATIVE FEEDBACK POT WIRING
There are two possible ways to wire the NFB pot depending on what you want to show during the rotation. The fully counterclockwise position can represent either the Minimum NFB or it can show the Lowest Headphone Impedance. The clockwise rotation will then show the Maximum NFB or the Maximum Headphone Impedance respectively. This diagram illustrates how to wire the pot for either of these configurations.
NFB Pot Wiring
Two Ways to Wire the NFB Pot
Pay close attention to the way the wiper is wired for each configuration and remember that there are only TWO NFB connections on the Bijou amp board.
Bijou Setup
NITIAL SETUP
Follow this procedure to set up your Bijou for the first time. The first step is to ensure that the power supply is working properly, then the heaters, and then the full amplifier.
THE POWER SUPPLY
Connect the power supply board to the transformer. Double check the wiring to make sure that the HV and Heater secondaries are attached to the correct terminals on the board.
Connect the board to the star ground point.
Connect the ground for the line (mains) to the star ground point.
Be sure that you are powering the transformer through an appropriate power switch and fuse.
Attach a 100k/1W resistor to the 250V and Gnd output terminals.
Double check everything.
Attach a DVM set to at least 300VDC across the 100k test resistor.
Power up the PS module. The EZ80 should light up. If it does not, turn the power off immediately and recheck the transformer connections.
If the EZ80 lights ok check the voltage shown on the DVM. Adjust the trimpot, P1, until the output reads 250V.
WARNING: This power supply uses potentially lethal HIGH VOLTAGES, more than 300V in certain places. Construction and testing of this PS should only be done by someone who is thoroughly familiar with safety precautions around high voltages.
WARNING: If you short the output of the regulator while it is under power, both mosfets will be destroyed and you will have to replace them.
If the power supply comes up successfully, power off and wait until the output voltage drops below 10VDC before proceeding to the next step.
THE AMPLIFIER TUBE HEATERS
Before applying B+ to the amp you should make sure that your heaters are wired correctly. It is easy to make an error, particularly when wiring for either ECC99 and 6N6Pi.
Leave the 100k resistor and DVM connected to the PS.
Connect the heaters as shown on the wiring diagram in the Wiring section.
Power up the PS module again.
All four tube heaters should light up. If they don't power off immediately, check the tubes for open heaters, and double check the wiring. Make sure that pins 4,5, and 9 are wired correctly.
If all four tubes light, power down and wait until the B+ is below 10VDC.
FULL OPERATION
This is the final step in bringing up the amplifier.
Connect input and output jacks, the volume and feedback pots.
Remove the 100k resistor from the output of the PS.
Make all Star Ground connections as shown in the wiring diagram.
Connect the B+ from the PS to the Amp boards.
Connect the DVM from a B+ terminal to Gnd.
Double check everything!!!
Power up the amp. If you see any smoke, power down immediately.
If the tubes light properly and there is no obvious problem, read the B+ on the meter. It should be close to 250V.
Let the amp run for about an hour and then readjust the B+ to 250V.
Listen.
After about 50 hours of operation readjust the B+ again to 250V.
TROUBLESHOOTING
There isn't much that can go wrong with this amplifier, but just in case . . .
THE POWER SUPPLY
If anything smokes or burns up in the PS the likely cause is an incorrect value for one of the components, a component inserted incorrectly, or a short.
Check the orientation of Z1.
Check the orientation for all of the electrolytic caps.
Inspect the board again carefully for any shorts between traces. Insect the wiring for the same problem.
If the EZ80 doesn't light and the heater wiring is correct, then replace the tube
If the output voltage reads less than 200VDC and/or if adjusting the trimpot has little or no effect, the most likely cause is that the mosfets have been blown. Unfortunately, you'll have to replace them.
After replacing the mosfets, recheck all part values and orientations. If these are all correct and if both mosfets are good, the PS will come up to its specified voltage.
Typical internal PS voltages a shown on this diagram:
Bijou PS Voltages Bijou PS Voltage Diagram
THE AMPLIFIER
The amplifier circuit is simple and tubes are relatively indestructible. If voltages are off in the amplifier it is most likely caused by an incorrect component value or one or more of the diodes, D1-D6, installed backwards. The internal voltages of the amplifier are shown in the diagram below. It is not uncommon in tube circuits for some of these voltages to vary by 10% or so.
Bijou PS Voltages Bijou Amplifier Voltage Diagram
HUM AND BUZZ
If the star grounding technique is followed accurately, there should be no audible hum or buzz. If there is:
Turn the volume pot all the way down.
Check all of your ground connections to the star and at the points of origin on the boards.
Check that your power inlet ground is also connected to the chassis.
If there is buzz when you touch the volume pot, connect its case to the star by soldering a wire to it.
Make sure that the heater wiring is twisted from connection to connection, is well away from the boards and signal wiring, and lies close the metal chassis.
Make sure that the transformer and the rectifier tube are sufficiently far away from the signal wiring and amp boards.
JUMPER J1
If you've tried everything and still have problems, you can try changing the ground traces on the amp boards. To do this, follow these steps:
Solder a wire across J1.
Remove the star ground connection from the ECC99/Output connector.
Do not remove the star ground at the Input connector.
If strapping J1 does not improve or solve the problem, it is best to remove the J1 jumper and restore the star ground wire to the ECC99 terminal block.
Bijou Point to Point
HE ORIGINAL BIJOU
One other design goal, was to build the amplifier as compactly as possible. I decided to use the Bottlehead style for the layout with all of the controls and power sockets on the top plate because I wanted to have the plate made by Front Panel Express. I used their software to design the plate. I also used a Hammond 270AX power transformer that I had in my parts box. The transformer is actually slightly overloaded, because its 2.5A heater winding is running at 2.8A and its 50mA high voltage winding is running at about 50mA. This is pushing the transformer a little, and it does get warm, but these currents are not destructive to a 105C device.
After a few iterations designing the top plate, I settled on this 10" x 8" layout (click to enlarge):
Bijou Top Panel
The Original P2P Bijou Layout
Along with the Hammond transformer, I decided to use some of the Neutrik RCA and Headphone jacks that I had collected.
There was nothing special about building this amp. The wiring is straightforward. Heaters are AC so they are tightly wound and kept close to the chassis. There is a star ground in the center of the chassis plate. I built the mosfet regulator on a perf board which sits underneath the vent holes at the upper left and is attached using one of the screws that holds the power connector socket.
Here's a photo of the underside with PS, heater and ground wiring. The regulator is at the top right. Yes, I painted the perfboard black.
Bijou Partial Wiring
Heater and Power Supply Wiring
Here is the completed wiring. Notice the very large 330n Orange Drop film capacitors. The latest design has reduced these to 100n so they don't take up so much space.
Bijou Full Wiring
Completed Wiring
TURNING IT ON
When I first fired up the amp, there was a very small, but detectable buzz. I spent a lot of time trying to track this down, including rewiring some of the ground connections and fooling with the B+ wiring. Nothing, however, seemed to relieve the problem.
At one point during debugging, I connected a clip lead to one of the rectifier plates and let it drape onto the wiring. The buzz disappeared. There was clearly some switching transient radiation getting into the amplifier. And there was no way to rearrange the layout to move the rectifier farther from the amplifier.
After many more hours of debugging (and blowing up the mosfets by shorting the B+), I decided to take the easy way out. I connected a piece of wire to the rectifier plate where I had attached the clip lead and arranged this wire on the chassis as an antenna to cancel the buzz. In process of debugging I made a few other small changes before eventually restoring the amp to its original design. The amplifier is dead quiet now for any setting of the feedback pot and exhibits a tiny bit of noise only at full volume. And I'm not the first person to cancel noise by positioning wires.
If I were to build this amp again on this compact chassis, I would swap the location of the rectifier and the filter cap at the back of the chassis, moving the rectifier farther away from the body of the amp. I suspect that this would solve the buzz problem. It might also help to make a slightly bigger chassis to further separate the PS from the amplifier. This very small design was, again, an experiment to see what was possible.
PERFORMANCE OF THE P2P BIJOU
The only amplifier that I have to compare the Bijou to is my experimental Fender Tone Stack amplifier. To my ears, the Bijou sounds more open, with a better defined soundstage both left-right and front to back. There are no tone controls in the Bijou, so I can't crank up the bass like I can with the Fender amp, but the Bijou still delivers excellent bass. Its midrange and treble are crystal clear. Voices and instruments on good recordings stand out on the soundstage as clear as a bell.
This is an excellent headphone amplifier that should make most headphone addicts happy with what it can do. And it has enough power to drive my Sony's (45R) beyond earsplitting levels for me.
Bijou Tweaks
TUBE ROLLING
The Bijou is tightly designed around the operating points of the 6922 and ECC99 tubes. There is only one variety of ECC99 from JJ Tesla, but there are many varieties of 6922 including NOS 6DJ8s and 6922s as well as various current production tubes. Any of these should work.
It is also possible to roll other types of tubes as described below.
INPUT STAGE
Correct operating points are critical to proper operation of the amp. To get good results with other tubes you must change the value of the first stage's cathode resistor. The first stage is designed to have about 100V at its plate. Since the first and second stages are direct coupled, this voltage is also important to proper biasing of the phase splitter. To use other tubes, adjust the cathode resistor (R2) as shown. See the Amplifier Schematic and the Boards sections for the location of R2.
Cathode Resistor (R2)
Tube R2
6922 2kΩ
6N1P 1.5kΩ
6H30 5.1kΩ
WARNING: You can direct sub other tubes without making these changes, but you will certainly get bad results. Please do not blame the ampifier or its design for this result.
WARNING: Do not use tubes which have a heater tap on pin 9. This includes all of the popular 12XXX series tubes. The Bijou amplifier board has pin 9, the internal shield in the 6922, grounded to reduce noise. Using tubes with a heater tap at pin 9 will damage the heater biasing circuit and, possibly, cause other problems.
OUTPUT STAGE - 6N6PI
The ECC99 is based on the Russian 6N6Pi. The tubes are similar except that the 6N6Pi has sligtly lower plate resistance (higher current for a given operating point) and ECC99 uses pins 4,5 and 9 for its heater while the 6N6Pi uses only 4 and 5. Nevertheless, these tubes are close enough in performance to substitute the 6N6Pi for the ECC99. The Bijou amplifier board is equipped with configurable heater connections for the ECC99 socket so that 6N6Pi can be used.
To use the 6N6Pi, see the diagram in the Wiring and Ground section.
USING THE EZ81
Some of the Bijou builds that use 6N6P output tubes are drawing more current than was shown in the prototype process causing the input to regulator to drawn down below 250V and, thus, preventing the regulator from regulating to 250V. The easiest fix for this is to simply subsitute the EZ81/6CA4 rectifier for the EZ80. The EZ81 is pin for pin compatible. It draws 1A heater current instead of the 600mA drawn by the EZ80, but the Hammond 270DAX heater winding can support this additional current draw. A schematic is not really necessary, but here is one anyway:
EZ81 PS
EZ81 250V Regulated Supply
Regal's mods require the EZ81 Power Supply.
USING HAMMOND 370DAX
The Hammond 370DAX or 370DAZ have the correct secondary voltages but with a universal primary for operation worldwide. To use the 370DAX follow the wiring diagram below for the transformer connections. Note that the Violet and Orange wires are not used.
Bijou 370DAX Wiring
Hammond 370DAX Transformer Wiring
Wire colors are according to the Hammond website. See the Hammond website for wiring the primary for your line (mains) voltage.
220V OPERATION
The original Bijou was designed to operate with 220V B+ using the Hammond 270AX transformer. To run with 220V, make the following changes.
Amplifier - Change R5, R6, R7 to 33.2kΩ.
Power Supply - Change R2 to 34.8kΩ.
Use these links to get schemtics for amp and PS:
220V Bijou Amplifier Schematic
220V Bijou Power Supply Schematic
Follow the Instructions and Setup procedures. Set the B+ to 220V instead of 250V.
CHOKE POWER SUPPLIES
Using a choke in the PS was suggested by dsavitsk, a contributor to both Headwize and Head-Fi forums. Since others expressed interest in having a choke in the PS, here are two ways to do it.
250V Bijou Choke Power Supply Variation 1
Parts which change are shown in red. The 470u caps have been reduced to 47u. This keeps the ripple into the regulator to below 1mV. However, the regulator has a high ripple rejection (>100db at 120Hz). Thus, you can try making these values even smaller in order to use large film capacitors instead of electrolytics.
250V Bijou Choke Power Supply Variation 2
In this version the choke replaces both R7 and R8 and eliminates one filter cap. Because of this, the remaining filter cap can't be reduced in size.
REMOVING NFB FOR HIGH Z HEADPHONES
The Bijou is equipped with a variable NFB circuit. This helps the amplifier handle the range of headphone impedance from 32R to 300R. However, very little feedback is needed for high Z headphones. Some listeners may prefer no feedback at all.
If you are sure that you'll only be listening with high Z headphones, you can remove the feedback and improve the bandwidth of the amplifier by doing the followng:
Eliminate the 250kΩ feedback pot
Remove (or don't install) R12
Change R1 from 10kΩ to 300Ω
There are no harmful effects from this change. And you can always install the components needed to include the variable NFB capability.
REGAL'S MODS
AMPLIFIER
Regal, a member of both Headwize and Head-Fi forums modifed the input and phase splitter stages run at higher idle currents. The original Bijou was designed for a particular power budget. The current in both stages was kept to around 1mA. However, tubes generally sound better at higher currents. Regal's mod moves the operating point to about 5mA for both the input stage and the phase splitter.
“Broskie is quoted as saying that the 6DJ8 sounds better over 3 mA, Modjeski recommends 3-5 mA, Others on DIYAudio say 10 mA is good. So I came up with this biasing scheme for 5 mA, based on the fact that we’re almost all using a bigger transformer and better rectifier than the original design. With the new bias in place, I tested the voltage at the input to the phase splitter with brand new tubes and real old tube and got 105V (new) and 95V (old) , you can adjust to 100V by dialing B+ up or down.
The sound before was great but just a tad too smooth and polite. After the mod everything became fuller, punchier, and more detail lifelike. Big rich imagery is most important to me with an amp and the Bijou with the hotter input tubes really excel in this area. I do recommend 6DJ8 tubes vs 6922 tubes as they tend to deal with the low-ish (even at 5mA) bias better than their 6922 counterparts.
In comparison with my DV337 the Bijou is clearly superior with its lifelike imaging and conveyance of harmonics and overtones. Your milage may vary as my Bijou has been optimized for high ohm phones without using NFB and using far fewer electrolytics than typical. I really want to thank Alex for his time designing and supporting this amp.”
-Regal-
Parts changes are in red:
Regal Amp Mods
Bijou One Channel Schematic with Regal's Mods
Here are a few cathode resistors for rolling the most popular tube in with Regal's mods.
Cathode Resistor (R2)
Tube R2
6922 487Ω
6N1P 180Ω
6H30 1.3kΩ
Regal's mods requires using ECC99s (not 6N6Ps) and the EZ81 Power Supply.
AMPHEAD'S HEATER MODS
Amphead, a member of various forums, struggled with a build that had induced noise/buzz in the headphones. After many attemps with various changes the one that finally fixed the buzz is this one. It is to remove the heaters from the standard heater bias chain and ground the heater secondary of the transformer in the manner shown in this schematic:
Heater Mods
Bijou Power Supply Schematic with Amphead's Mods
This type of heater grounding is very common when the heater windings can be tied to ground. This balanced approach is better than tying just one side of the heater secondary to ground. 1/2W resistors are fine for this duty. They will only dissipate about 100mW.
If you are building from scratch and intend to use this mod, then leave out all of the components X'd out in red. Add the two 100Ω resistors tied to the star ground point.
If you have already built the PS you should remove R9 and C7.
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