GoodSoundClub - Romy the Cat's Audio Site

In the Forum: Melquiades Amplifier
In the Thread: A different breed of 6C33C amplifier.

Post Subject: The Sound of 6C33C preamp.Posted by KOTriode on: 10/25/2011

Romy, No, the DAC was not set at maximum level (for best sound) while Cyrille listening to the system. It was set at normal volume for the whole system.

When Cyrille dropped by, I was showing him the Field Coil open baffle speaker system which consist of a 12in Supravox woofer, 2 Supravox 215 Field coil and 8 ribbon tweeters per side ,the system efficiency is about 100 db/w. The power amp used was a 20W 845 for the Supravox, an 8W 845 SE is used for the ribbons. This is normally a tri-amp system, but on that day, it was setup as a bi-amp system to test the 6C33C amp, since them 6C33C amp is not yet broken in, I decided to use the 20W 845 SE. The source was a DIY wireless DAC with tube output (and pretty low output impedance). I played quite a few different tracks to show him what the system can do.

Just before he left for New York, I want him to hear the new 6C33C preamp (not fully break-in with just 5 hrs on the clock). What I did was inserting the preamp between the DAC (without changing the DAC level) and amplifiers and then set the preamp so the volume is about the same (the preamp act more like a buffer now).I choose three tracks that we played earlier, Temptation by Krall, La Boheme live by Aznavour, and the Firebird by Eiji Oue Reference Recording. We listened to the tracks, remove the preamp and re-inserted it about 3 times.

What we both discover is that with the preamp the soundstage is much larger (by about 25%) and extend way beyond the speaker edges, there is also more depth when listening to Aznavour live track. The tonal balance is not that much different except that Tempation track, the bass was deeper, the midrange tonal balance remain the same, but there is so much more detail and speed, the high was smooth and much more extended through the ribbons.

This is for me an unexpected result, since I was expecting tonal balance to change with this 6C33C preamp or any additional amplification stage, it wasn't in this case.

The preamp was not without fault. On this 100db system with 8ft away from the speaker there is a faint 60Hz hum, on the bench the hum was measured at about -58db which is quite high. My guess is that the hum came from the transformer and was picked up from the chassis, since like most power tube when using in a preamp, there is much higher microphonic than small 9 pin or octal tube. There is more work to be done to eliminate the hum. I will probably try to mount the tube sockets on some kind of rubber minimize vibration from transformers.

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