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Old 12th December 2002, 04:29 PM   #21
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Default 6DJ8 in this HP amplifier

I tested a 6922 ( Sovtek ) with the supply rail running at 100 volts. The sound is good and I cannot make out any significant difference between this and the ECC82. The distortion was a little lower in the 6922 but this does not seem to be audible.
I find that I need at least 15 volts peak if I use a 600 ohm 92db per m/watt headphones ( Audio Technica ATH-911) . The Sennheiser HD-580 is 300 ohms and has a sensitivity of 97db/mW.
The gain of the circuit with 6922 is 8.0
I still see the 50Hz and 100Hz residuals on my analyser. They are about -65db this time. They were lower the last time I tested the system. Something must be wrong as I blew more power transistors in the power supply due to excessive voltage applied accidentally.
If you want a pcb layout let me know. Its worth trying. Sounds very nice.
Cheers.
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Old 30th December 2012, 05:17 PM   #22
roline is offline roline  United States
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Default Tube MosFet Hybrid Head-Phone Amp

This is the second hybrid head phone amp for me. The first was a stocking stuffer last year and the head elf liked it so much he started to make one for himself. I used Compactron tubes for 3 in one , first for gain stage and 2 for SRPP stage, with a MosFet A follower with CCS. I have used the triple triode, diode and sweep tube Compactron tubes in amplifier applications, all 12 pin tubes.
Simple topology, gain stage - tone section - buffer stage - output driver. The "GNFB" feedback is only between the SRPP and output stage, I did not want to run it back to the front end and include the tone section in the loop.
The power supplies are slow start to prevent turn-on noise and heavily filtered to make the phones "black hole quiet" with no input. I went from AC heaters to DC heaters to accomplish this.
Output voltage and quiescent current can be adjusted for application. I chose 12 V and 150ma for my phones. I cranked up the bias and drove 8 ohm efficient speakers to see if it could be done. My application is as a headphone and pre-amp for the tube power amp. I'm also including a RIAA preamp based on the groovewatt design.
The PCB is toner transfer, etched in a Ziplock bag to contain the mess. Brass eyelets are used to mount the big caps and for wire connections.
For the stocking stuffer I used mirror finish 304l stainless steel sheet as a heat shield, ground plane, reflector. The tubes are 6.3 volt in series, run on 12 volts so like old Christmas tree lights if one goes out, they both go out. The LED's under the tube are across the heaters so if one goes out, the LED goes bright to let you know which one to replace.
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File Type: jpg HEADAMP2.jpg (114.4 KB, 104 views)
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