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That was very interesting post, here are some thoughts of my about it.

KOTriode wrote:

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.

Ok, it was not at it optimum sonic level but at least it was at identical level of digital degradation for no preamp test and with preamp in the signal path. If so then it was fine.

KOTriode wrote:

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.

From what you describe I can conclude that your DAC is not good. You said you use DIY wireless DAC with tube output. I do not know what wireless mean in this context, but it is not important. I also know do not like any tube output stage in DACs, I have written about it extensively. Anyhow, my rational to blame your DAC is following: the sonic changes you describe shall not take place if your DAC is able to drive your power amp properly. It is not only about output impedance. Something in that DAC’s out stage does not work properly as a good preamp much not make a difference as you witnessed. With very good preamp you might have a little better dynamics and slightly better bass but it shall be all. If you witnessed so dramatic soundstage change, change in detail and speed and higher extension of HF then you are just way beyond of definition of preamp transparency. Are you sure that what you observe was not change in cables or something like this?

KOTriode wrote:

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.

As I said above, if I got the result you did then I would revise what my front end outputs. Find another DAC, of phonostage of CD player with DC-coupled SS output stage, it shall not be any good devise but any consumer unit would do. Do the same experiment with insertion of your preamp. You most like will not has the same difference.

KOTriode wrote:

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.

Oh, this is nothing. I am sure you will find out the problem and address the hum. The hum must not be there, the preamp shall be dead silent and with indirect heated 6C33C it is very simple to do, particularly with regulated circuitry. Beside the standard ground loop procedures and dealing with ripples you might want do not let the undirected heaters to hang without reference to gerund…

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