What are the advantages to a Class A amp & what are the trade offs?

jspohn - I own an EAR 890 70w Class A zero feedback amp.  It does not have sufficient power for my Legacy Focus speakers which has a tough impedance and 6-12" woofers.  My highly modified Dynaco ST 70 with 30w Class A/B kills it on the focuses (tremendous bass control and dynamics).   When playing both amps on my Legacy Signature IIIs with an easier impedance and 6-10" woofers, the two amps sound very, very similar.  From this experience, I found that a Class A, very hot amp does not deliver into low impedance and/or big driver cones despite a smaller amps Class A/B design (not ultralinear but voltage regulated transformer tap design).  

I have several Yamaha CR620 35 watt Class A/B 1978 receivers that sound excellent and can drive the Focuses just fine (Tom Port of Better Records recommends this pairing).  I use them on smaller smaller speakers for my two video set-ups.

That's where you are 100% incorrect.  That Dynaco is NOT a typical unit. It has been modified to have superior control of deep bass.  My friend Robert Pincus and Audiogon poster Oregonpapa know how powerful my little Dynaco amp is.  It is not as refined as my 130 watt monoblock Class A/B tube amps are, but the voltage regulation, non-ultralinear type is based on the same design concept.  You have never heard a Dynaco ST70 like mine.  The EAR Class A amp has a very poor damping factor and cannot tolerate my Legacy Focus speakers.  Both my Dynaco and mono-blocks have feedback, variable feedback for the mono-blocks although the designer said it was about 6 to 8 db global type at its minimum which we like best.

Very interesting thread. All I can say is I recently bought a pair of Class A 45w SET monoblocks that greatly improved my system. Built by Vlad of Audio Mirror. I never had monoblock amps or an SET amp so I took a chance. I did not get them because they were Class A design, but because they were SET and I have always been very curious about SET amps. They have a very uncolored, neutral but natural sound. Cymbals sound like..well, real cymbals. They are open, clear space between instruments and voices, yet very organic, dynamic and sound very effortless. When I listen to music I hardly adjust the volume. You hear detail, background at whatever volume is present. They run very hot but my room is very cool in summer so it’s not an issue. I could care less about the power bill...you only live once and can’t take it with you! They seem a perfect match for my Egglesston  Works Fontaine Signatures. Thx again for this thread. Always something to learn here on A’gon!