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Old 5th December 2004, 08:45 PM   #31
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I just added two PI filters on the filament supply of each tube. The filament supply was 6.7V and a bit too noisy at almost 300mV of ripple

So I added a 0.2 ohm resistor (nice power resistor that I had in stock) and a big 10,000uf cap for each tube. The voltage at the tube drops to a nice 6.3V and 60mv of ripple, much better
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Old 10th December 2004, 02:55 AM   #32
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Listening Session

My first lsitening was very interesting. The high were sweet and the bass was good too, but the medium had some coloration to it, in particular female voices. I was using a sole AuriCap coupling cap at the preamp output.

There was also the noise issue. There was two problems. I could here a rather loud buzz noise when activating the volume motor control. There was also a hiss sound in the background that reduce the dynamic of music.

I solved the two noise problems at the same time. I ground the case of the potentiometer/pot to the chassis ground. The hiss is just gone and the motor buzz can be eared only at high volume. I can live with that, no problem.

Then I installed the NOS Sprague coupling Caps instead of the AuriCap I had (Thanks Robert) and I can just say WOW!!!
The sound now is just great. Any coloration on voices is gone. The sound is just pure and liquid. The music timing just sing.

I couldn't believe the sound that can have just one cap. This NOS Sprague cap just blow the AuriCap to the sky

I listen to it all evening. Patricia Barber in SACD is cool. Return to Forever-Romantic Warrior punch and rock. Greats recordings just stand out. Chris Botti-Night Sessions is liquid and sexy.
Ayreon-Human equation bring down the house. My Naim CD Sampler with great acoustic recordings lovely. Steely Dan-Two Against Nature rocks, what a great pop reording. Diana Krall-Live in Paris swings. Gary Willis-No sweat, what a great bass player.
The voices are clear and well defined. Small details everywhere. It really sings.

Real music. This is the best preamp I ever built. I can wait to install CSS option and see what changes it will bring, for the better I hope

Bye...
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Old 11th December 2004, 01:31 AM   #33
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Here the complete circuit finished. Check the nice NOS Sprague coupling caps
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Old 11th December 2004, 06:49 PM   #34
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I did some measurements. Here the Unofficial Specs

Vin Max: 1.4Vrms
Vout Max: 14.6Vrms
Gain Voltage: 10 or 20dB

Zout: 600 ohms

BandWidth (-3dB, ref. 1Khz, 1V): 2Hz - 70Khz

Distorsion (in 10Kohms, 1V): 0.40% (1Khz), 0.38% (20Khz)

Noise (Input gnd): 0.2mv (min. Volume), 0.8mv (max. Vol.),

Crosstalk:
-between input channels (Ref 1V): -78dB (1Khz), -65dB (20Khz)
-between output channels: <70dB (noise floor, 1Khz), -62dB (20Khz)

Very good indeed
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Old 14th December 2005, 05:38 PM   #35
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Distorsion is probably lower than that because it seems that my distorsion analyzer got a problem. I need to re-test it using my Clio system. More on that later...
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