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Apart from a high ouput impedance via the OPT

Posted by Timbo in Oz (A) on October 29, 2014 at 18:33:04

In Reply to: Where is the magic in tubes? posted by airtime on October 23, 2014 at 08:06:57:

As BoldEagle says - "because there are other reasons besides the voltage divider effect why amps sound different. "

IME? I have a lot more NFB around the OPT stages of my two rebuilt LEAK Stereo 20s than in the OEM state, the output impedance is quite a bit lower, getting above 1 ohm only below 100 Hz.

And my speakers don't drop below 8 ohms anywhere except in the saddle of the Rb tuning's twin peaks, in a QB3 'critically damped' alignment.

I am unlikely to get any major FR effects from the speaker / amp interactions. This was the case with the loaner Radford amps I'd had before, too. High NFB was a USP for the STA25, and the 15 had also been modified to match. There was very little tonal difference from the SS Luxman integrated I had.

I think most of the magic that I get comes from two things;

i) The PSU energy storage that valve amps have even vintage restored to original or current MFR. And now that we can use modern high-voltage filter caps of much higher uf, it gets even better.

Storage in a PSU is half the capacitance times the voltage squared. eg An SS amp with +/- 50 volts has 2,500 to multiply into half the capacitance. my LEAKs have 340 volts giving a multiplier of 13,600, so more than 5 times ahead.

The effect on the sound is much the same as that from a BIG SS power amp whose PSU and power headroom means it is idling most of the time. Bass from a big valve PSU can combine the warmth and nuance of valves with the slam and dynamics of a big PSU.

ii) the linearity of triode or UL connection for the treble. In a semi-active bi-amped system with the mid-bass driven by a pentode mode amp - with NFB.

In rebuilt OEM form the ST20s were very nice classic-era valve amps, like a good pair of restored QUADIIs but sweeter and a bit brighter. The modified amps are a whole other thing.

Look in reviews here at AA.

A CCS on the splitter/driver stages is a very good idea.





Warmest

Tim Bailey

Skeptical Measurer & Audio Scrounger

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