ZEN-like headphones amp

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#82
Great Sonics

Hi Tortello,

Just finished construction of your design and have been listening to LP's for a few days. The sound is simply incredible through my Sennheiser HD-600 headphones! No hum problems of any kind. Just great sonics. I am hearing all kinds of subtle details that I have never heard before! The turn-on thump using my HD-600's is a very low frequency, low level thud when I plug the amp into the wall socket. There is no turn-off thump. This is my first headphone project, so I cannot compare the sound to other stand alone headphone amps. I am using a MM phono cartridge (output approx. 2mV) and a 35 to 40 dB phono gain stage with outboard power supply (line stage disconnected, converted from the circuit published in Radio Electronics, Dec 1972, & Jan 1973). Since your headphone design has approximately 10dB of gain, I can drive it directly from my phono stage without the need for the line stage. I didn't plan to eliminate the line stage, it just happened that there was enough gain without it, so I dropped the line stage. When I get my Pearl boards done (40 dB gain), I should be able to send the signal straight from the Pearl to the headphone amp with a 20K pot in between.

I am using a separate power supply for each channel (two Radio Shack 25.2 VCT 2 amp transformers, #273-1512). After rectification and 3 stages of filtering, I have 32 volts DC at the top of each channel. In order to get the amp to balance with approximately 16 volts across the top and bottom sections, I found it necessary to trim R15 from 180K all the way back to 90.9K. Other resistor values I am using are as follows: R1=1K, R2=1K, R3=1K, R4=84.5K, R5=221, R6=221, R7=6.2, R8=2K, R9=221K, R10=2.2 (3watt), R11=2.2 (3 watt), R12=2.2 (3 watt), R13=20K, R14=33K, R15=90.9K, R16=475. For capacitors, I did some polypropylene bypassing on all electrolytics. For instance, for C5 I used two 470uF (Panasonic FC series from Digi-Key), bypassed with 1 uF and .47 uF Xicon metallized polypropylenes from Mouser.

For future servicing work, I did a comprehensive voltage point check on the circuit and obtained the following values. All voltages are with respect to ground and were taken after 1.5 hrs of operation with no connections to the input or output and with the power supply rails running at 31.9 and 32 VDC with respect to ground. All measured values are referenced to the original schematic that you attached toward the beginning of this thread:

C6-R14 node: 4.53 VDC right channel, 4.44 VDC left channel.
R6-R9-R15 node: 4.54 VDC right channel, 4.46 VDC left channel.
R12-Q2 node: .583 VDC right channel, .563 VDC left channel.
R6-Q2 node: 4.54 VDC right channel, 4.46 VDC left channel.
Q2-R9-R11-R10-Q3-C3 node: 15.6 VDC right channel, 15.3 VDC left channel
R11-C5-R8 node: 15.58 VDC right channel, 15.28 VDC left channel.
C5-R16 node: 0 VDC both channels.
R8-C4 node: 13.8 VDC right channel, 13.52 VDC left channel.
C4-Q3-R3-R4 node: 14.2 VDC right channel, 13.92 VDC left channel.
Q3-R2-R5 node: 20.1 VDC right channel, 19.74 VDC left channel
R1-R2-R4-C3 node: 26.2 VDC right channel, 26.0 VDC left channel
R10-Q1 node: 16.17 VDC right channel, 15.84 VDC left channel.
R5-Q1 node: 20.1 VDC right channel, 19.7 VDC left channel.
Q1-R1-C1-R7 node: 31.9 VDC right channel, 32.0 VDC left channel.

I hooked my sine wave generator to the input of both channels, and fed a low level 1 kHz sine wave into the amplifier. I then measured the following:

Left channel:
Voltage drop across R10: .563 volt with 1000Hz input; .561 with no input.
Voltage drop across R12: .577 volt with 1000Hz input; .575 with no input.
Right channel:
Voltage drop across R10: .549 volt with 1000Hz input; .549 with no input.
Voltage drop across R12: .556 volt with 1000Hz input; .555 with no input.


Thanks allot, Tortello, for sharing this great circuit!

Bill
 
#86
Bzzz....flower bag?
....... bzzz.....
 

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#90
Hello EJ,
in this application, you can employ almost any NPN device, with a reasonable gain - say hfe>100.
With the values as in the schematic diagram, Q3 collector current is about 3mA (!) and the Vce drop about 5V (!!)
I used ZTX450 type because I have some of these in my pocket...
Marcello
 
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