Reply to thread

Message: <blockquote data-quote="GordonW" data-source="post: 2288780" data-attributes="member: 568981">Actually, if you get down to the most basic fundamentals, tubes are more linear devices than even the most modern semiconductors.Tubes and transistors have a property called transconductance... this describes the "gain" available. Put a small voltage in, and it gets amplified to a bigger voltage... in a nutshell, that's transconductance.However, NO device keeps the EXACT SAME gain over the entire voltage range. To describe the changes in gian vs. input voltages, engineers use what's called a "transconductance curve"... a graph of input voltage vs. output voltage. All devices (tubes and transistors) have the general property that it takes a bit of input to make them turn on at all... they do NOT immediately start to conduct output until there's a small amount of voltage. Hence, there's a "flat spot" on the curve, at the bottom, where things are NOT linear. Also at the top of the curves (at high voltage) all devices start to go non-linear... the output voltage cannot "keep up" with the input anymore.Between these two extremes, there is a relatively "flat" area, where the output is just a simple multiple of the input. This is called the "linear region", and is where all amplifying devices SHOULD be used.To avoid the bottom region, most amps use "bias", which consists of applying just a little bit of static voltage to the amplifiying device, to 'prime' it so it's ready to turn on with the least bit of additional input. This avoids the "flat spot"... otherwise known as "crossover distortion", near zero-level.However, the UPPER limit, is a bigger problem. The more dB (ie, higher range of gain) available between the bottom problem area and the top problem area, the better the device as an audio amp. Smaller linear regions == less available dynamic range and less resolution at lower signal levels.Now, given that, it's interesting to find that tubes have, in almost every case, a BIGGER linear region (ie, they can go from LOWER to HIGHER output levels than transistors) than any semiconductors. Even the most modern MOSFET devices, STILL can't match the linearity of something like a 6SN7 or 6SL7 tube...not only does the tube have more distance between the lower and higher problem areas, it's FLATTER (ie, the transconcuctance curve is more a straight line) in the MIDDLE of the range than ANY semiconductor, built to date!To compensate for this solid state amps have to use MUCH MORE feedback than tube amps, which can EXACERBATE time-delay related distortions. Phase shift from feedback, is a KNOWN cause for mis-behaviour in amps... the less needed, the better...SO, from a fundamental standpoint, tubes still have better inherent behaviour as audio amps than transistors. Until you start getting into pretty exotic solid state designs (such as the pulse-width-modulated output coupled with a "gain cell" variable gain input, as used in the new PS Audio amps, which seem to be the MOST LINEAR solid state amps EVER, but which are UNLIKE ANY other solid state amps on the market) which find innovative, non-standard ways to completely side-step the whole gain/linearity issue, you're stuck with the shortcomings of the transistors in most solid-state amps...Regards.Gordon.</blockquote>

Verification

Top

What's new

Forum list