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In the Forum: Audio Discussions
In the Thread: Devid Berning amplifiers: the anti-trnsformers frenzy?
Post Subject: Saturation contd.Posted by morricab on: 10/2/2007
What I find interesting though, Romy, is that many of the SET measurements show gross distortion even when putting out very low power, which suggests that they are saturated from the high static DC current running through them as they must for a Class A SET. You are also right that to get everything you want from an output transformer requires a VERY expensive construction and top materials. However; even an expensive transformer is no guarantee of success. Look at the stereophile website (see you thought there was nothing useful there but in this case the measurements tell a lot!) at the WAVAC amp. This crazy priced SET has very bad distortion in the bass. I am sure the OPT is very expensive in this amp but the manufacturer apparently never bothered to check if the OPT they chose was appropriate to the situation! Of course it will sound big and wooly and bloated with 10% or more distortion.
http://stereophile.com/tubepoweramps/704wavac/index5.html
Overall SET amps like Lamm and KR audio sound cleaner and more correct AND they also measure cleaner and more correct...with little or no corrective feedback (Lamm uses a bit I believe and KR audio uses none). I think that in fact measurements DO mean something when the amp is not using negative feedback, which seems to mask the true behavior of the amplifier (ie. open loop linearity). The more linear an amp is open loop (ie. without negative feedback) I think overall the better it will sound. This includes errors from the power supply, circuit layout (eg. hum and noise), OPT, thermal issues etc. All of these things will have a profound affect on the open loop linearity and many designers cover up these issues (but do not cure them) with negative feedback.
I mentioned the dynamic bias issue with PP amps. It is more a problem with OTL but it is also a problem with transformer coupled PP amps and is partially why they don't achieve the overall purity of a SET. When you bias an amp statically, you can match them pretty closely. However; put a demand on the tubes and watch them diverge! I know of only one amp on the market that addresses this issue with a kind of dynamic biasing scheme (not disclosed) and that comes from VacuumState. It is an 18 watt 300B PP amp (the designer refers to it as differential as it has no phase splitter and is fully differential) that is the first I have heard to get the low level resolution and tone color of a SET. Also bass is quite controlled and natural. It is better than most SETs I have heard and as it uses a very good OPT, my guess is that it has minimal distortion in the bass as well in addition to being zero negative feedback. www.vacuumstate.com Rerurn to Romy the Cat's Site