211 Class A2 Amplifier
211 A2 Amplifier211 tube amplifiers have always delivered a nominal 12 to 20 watts in Class A operation and gained acceptance for this service. As a means to deliver more power from the 211 tube, we at Electra-Print Audio have updated the traditional design using Class A2 operation delivering easily over 40 watts due to higher swing high voltage music power. The purpose of the design is to operate a less expensive, easily obtained popular tube, in a practical and acceptable high powered circuit.
Driver StageTo start with, the driver must be capable of many watts of pure clean power easily converted into high voltage grid drive without compromising bias. A special driver transformer was developed for this purpose with common mode termination capability, the A2LD-7 and its updated versions.
Unfortunately no tubes can offer this drive capability as the operational chip power amp. The distortion, even at high drive levels, is not measurable.
Problems to be overcome were hum, oscillation and possible chip failure for the reason that the chip does not like to be in the same chassis with a high voltage operated output tube.
Our circuit shows a means to stabilize the input of the chip amp from its own common output. This generates a virtual ground to use as an input balanced reference point. This virtual ground is isolated from all grounds of the power output tube. No tube operational currents should be in shared with this ground.
Stage CouplingThe interstage transformer is built as single low impedance to step-up, into common mode loading and will assure, looking back into the chip amp, no power supply switching noise and ripple will appear.
Output TubeThis common mode drive when applied to the 211 grid will offer very high levels of power at a constant impedance. The grid will see a low DCR for its circulating currents. This is necessary for high power levels.
One new and interesting feature is the DC filaments of the 211 have now been balanced into the tube’s filament to grid electron field, by common mode cathode resistors. This provides a very large decrease of hum level at the power tube output. The cause of the hum is due to the power transformer filament winding capacity riding on top of the filament power supply and filter. The noise floor from zero reference is about -60 db which is very quiet.
Output TransformerFinally the output transformer features the use of electrostatic shields on its secondaries to reduce interwinding capacity. The very high voltage swing of the 211 without this shield (the high voltage near 20 kHz) would be show up on the secondary then shunted to ground causing distortion due to it loading back the 211 at high frequencies.
Specifications211 A2 Amp specifications using current manufactured Chinese 211 tubes:
Bandwidth, @1 watt, -1 db 17 Hz to 25 kHz
Power output @ 2 kHz at 5% THD is 39 watts into 8 ohms load
Power output @ 2 kHz to moderate clipping is 43 watts into 8 ohms load
|Distortion||1 W||5 W||10 W|
These figures may vary with different 211 tubes used. The compromise is of music delivered within these specs, the sound is very life like to the many that have listened to these prototypes.
Resulting operational features:
a] No feedback on a very high voltage swing results in extremely good amplitude linearity. No global feedback giving greater dynamics to the music.
b] High frequency has low distortion due to electrostatic shielded partial silver secondaries.
c] Headroom over 10 watt listening level is over 400%.
d] Not a feature but note worthy, is the 20 Hz higher distortion due to output transformer core mass limit to practicality. This will exist on all SE amplifiers to some degree as core mass used.
e] Our Electra-Print amplifier design emphasizes the most important things found in recorded music including, detail, dynamics and presence.
Last updated: Monday June 15th, 2009 at 02:36:53 PM