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Old 1st December 2012, 10:41 PM   #11
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Default Mysterious SE OTL headphone amp from China?

Quote:
Originally Posted by piniu29 View Post
If I did not know that manufacturer of the amplifier does custom version called (modded / upgraded or reference) which exactly use the GEC tube instant of pentode I would thought that it have to be like this.
La Figaro 332c perhaps?

jeff
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Old 2nd December 2012, 06:16 PM   #12
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Quote:
Originally Posted by piniu29 View Post
Just simply converted the wiring as shown in the attached schematic, the amp with the new input tube is running fine but the sound is not even good, lack of speed, flat, with less dynamic and floppy bass......

......Guys I need your help to adjust the amp and get it sound as good as the previous version with the rear triode grounded plate mode.
Every clues are welcome.
Thx.
I would analyze it's performance with some measurements.
What is the frequency response and distortion with different loads ?
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Old 2nd December 2012, 08:22 PM   #13
piniu29 is offline piniu29  Ireland
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Quote:
Originally Posted by DF96 View Post
If you want it to sound like the original circuit then why not go back to the original circuit? You have completely changed the input stage. There is no overall feedback, so you are likely to hear any change.
I should better clarify my question English is not my native language sorry about that.
The designer took the stock version of this amp and tweak it to use with 6j5,cv1067 instead of pentode 6ж4,6sj7.
I wondered if the "customized" version of this amp is a step up from the original design so why not try it same in my amplifier too?
The input stage wiring his "customized" amp is exactly like on attached schematic as you see there is no overall feedback, from what I know all of his amps were built without feedback.
In his "customized" amp there is some components I don't know the real value, one of them is the R5 cathode resistor for the V2 and R7 actually in place of R7 hi put some "mysterious" white plastic box from which are led out two wires directly soldered to the (8) socket of the V1 and other end to the ground. I tried figure out, what the "white box" is and what is inside?
If there is only the R7 bias resistor so why is hidden in box for what reason? What I think hi could bypass the R7 with capacitors or used different way to bias the tube like battery bias or so? also the power transformer are slightly different can provide 140V from secondary my one is 135V and filament can handle more amperes but I don't know if the few volts of difference could change the operations points of the tubes and have significant effect on sound quality?

PS.
Just yesterday I tried to bypass the R7(1K) resistor with some electrolytic caps and noticed significant improvement.
so maybe it is the bias issue with the CV1067?
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Old 2nd December 2012, 08:27 PM   #14
piniu29 is offline piniu29  Ireland
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Quote:
Originally Posted by vinylkid58 View Post
La Figaro 332c perhaps?

jeff
That was close, it is 339
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Old 2nd December 2012, 09:46 PM   #15
piniu29 is offline piniu29  Ireland
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Quote:
Originally Posted by artosalo View Post
I would analyze it's performance with some measurements.
What is the frequency response and distortion with different loads ?
Unfortunately my measurements eq are limited I am just amateur hobbyist but i planning to purchase one of the 16 Bits Picoscope I add it to my long wish list for Christmas
For now I used the computer with the ARTA software.
I attached few screen shot of my measurement , my 2 old multimeters are not true RMS so I cant proper calibrated the levels in/out. The volume was set up at normal for me listening level .
The measurement are for my modded amp with the input stage not changed yet (grounded plate) and output coupling capacitors 265uf.
I'll try to do some measurement with converted input stage to GEC 1067 later and come back to u..in meantime.
Attached Images
File Type: png FR-3.png (12.2 KB, 167 views)
File Type: png SA DK339 5998+5693 50 ohm.png (70.3 KB, 162 views)
File Type: png DK339 5998+5693 300 Ohm.png (73.3 KB, 157 views)
File Type: png FR - DK339 7236+5693 300 Ohm.png (77.2 KB, 153 views)
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Old 2nd December 2012, 10:22 PM   #16
DF96 is offline DF96  England
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The mystery white box could contain a resistor, a bypassed CCS, an LED, a rechargeable cell or some combination.
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Old 5th December 2012, 09:18 PM   #17
piniu29 is offline piniu29  Ireland
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Quote:
Originally Posted by artosalo View Post
I would analyze it's performance with some measurements.
What is the frequency response and distortion with different loads ?
Finally I found some time to measure it. External loopback (line-out - line-in) Arta software,three different dummy load resistors 33, 50 and 300 Ohm.
I hope it would help?
Attached Images
File Type: png FR 5998 CV1067 33 ohm.png (78.1 KB, 129 views)
File Type: png FR 5998 CV1067 300 ohm.png (74.5 KB, 16 views)
File Type: png FR 5998CV1067 50 ohm.png (75.6 KB, 17 views)
File Type: png SA 59981067 33 ohm.png (85.7 KB, 19 views)
File Type: png SA 5998CV1067 50 ohm.png (79.1 KB, 17 views)
File Type: png SA 5998cv1067 300 ohm.png (80.3 KB, 19 views)
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Old 9th December 2012, 09:43 PM   #18
piniu29 is offline piniu29  Ireland
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I was very surprised with the effect / result I got by bypassing R7 with capacitor, after reading about different types of bias here I decided to try it in my amp. According to Morgan Jones in his book, cheap 3mm red LED's are best and have one of the lowest impedances and low noise.
Wow the small red LED(1.82V bias) was an instant, huge improvement in the bass and dynamics, it seams that my headphone amp with the input GEC1067 back to life!
Another way of biasing the (V1) I've tried was using a rechargeable battery(Ni-ZN & Ni-MH) instead of a LED to bias the input stage. I'm still impressed with the sound of my LED biased cathodes but none have come close to the difference of putting a battery for cathode bias. Battery bias kept the dramatic improvement in the bass and improve the presentation in the mid and upper range. Seems to me that the issue with the flat sound, loose and flabby bass has permanently gone.

I have question to you guys.
The amplifier is very minimalistic if you look at the schematic I drew, there is no decoupling capacitor between driver and output stage.
I do not know whether it is good or bad idea to have capacitor in this place? if yes what value and type will suit best? also there is no a grid stopper resistor do I have add one at input?

Any suggestion are welcome
Thank you.
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