Building a 250W class A amplifier
Everybody wants more power.SE amplifiers are unmatched in sonic quality,however,not everyone owns a high performance highly sensitive speaker.
Design parameters for our task is sticking to Class A,and the need for a simple tube design delivering 250W per channel.Difficult task indeed.
Lets start with the output tube.I never liked the KT-XX family and the EL-XX close relatives.I always prefer DHT for our SE amplifiers and preamplifiers.However,i wanted to keep the step down transformer as low ratio as possible.Can we use a 2~4:1 ratio transformer and keep parasitic capacitance and inductance as low as possible?
Well,you need very high current/low resistance tubes to keep it in True Classs A mode with such a low ratio step down tranny .My favorite non DHT tube is the Sveltava 6C19p , a very high current ,very low resistance tube.It's internal resistance is about 9 times lower than a 845 tube.
Push pull designs...are not my favorite.So how can we get 250W in SE Class A topology.We use two parafeed SE OTL(!!) amplifiers per channel in balanced mode.This is not push pull as one might say,but two separate SE OTL amplifers ,one of which is inverted.
Each SE OTL parafeed bank is biased at 2.5~3Amperes,so 5~6Amperes in total!!
Driver stage uses my all time favorite E280F,high tranconductance tube,in my opinion the best mid-signal tube out there.We use a proprietary topology,which uses a combination of direct coupling and inductive coupling,for uncompromised performance.Driver stage can swing more than 100Vrms with bandwidth of DC~250Khz!
So,two SE OTL parafeed amplifiers in balanced mode driving an ac coupled 4:1 transformer can deliver 250W in Class A?You bet.
The reflected output resistance is 0.35 Ohms without the use of global or local reedback,and the max current swing is a record breaking 24 Amps!
The 120kg monoblocks will be launched soon.Stay tuned.