Jun 11, 2021

41MXP SE amplifier Part 2


Choke input is the way to go for most SE A1 amplifier with moderate current (below 100mA). Two good reasons:

1- Ripple is very low behind a choke, equal to 

V Ripple = Vout / 6 π2 f2LC√2

almost a perfect sin wave with very little transients in power transformer unlike input capacitor filtering.

2- Rectifier choice is greatly facilitated as we don’t have to deal with complex calculations mandatory for operation with capacitor input supply. Usually, tubes makers provide all needed information for choke input in their datasheet. You will always be safe if you stay well within the max peak inverse voltage and peak plate current.

For basic ripple and time constant calculation refer to LCR Phono preamplifier Part 3


Step by step construction

CNC aluminum top plate, epoxy coating, teflon soldering standoff, silvered solid wire and Belling Lee vintage sockets
Holco, Kiwame and Allen Bradley resistors. Aerovox V161 coupling capacitors (IMHO best for the price with ITT PMT/2R when you are looking for a neutral well balanced sound).
Sub chassis holding high voltage capacitors and second filtering choke
I was willing to build an all-British valve amp. Chosen rectifier is another staunch workhorse: the venerable UU5/VR39, this is a full wave rectifier found under so many code numbers (1821, 441U, APV4, CV1039, CV3760, DW4/350, MU12, UU120/350) and used for decades by the British services. Plenty of good ones still available. These were the ancestors of the world famous 5Z4G, GZ30 and GZ32.
Also tried a Philips AZ50, fine rectifier with ample sound-stage but slightly more obscure and a Cossor 43IU, in my opinion the very best on voices.
On the bench for a few tests

100Hz & 10KHz squares

VU39 versus 43IU

Listening report

Setup: Garrard 301, FR24/Entré EC30, UA7082/AT33 Mono, C3g phono preamp & 6J5 line preamp

For me this amplifier combines the R120 clarity and the PX4 warmth, with more punch. Low end is ample and well defined, but a little rearward. For sure the Klipsch do not help, but I am not frustrated at all and can listen to cello or piano solo without the feeling of instruments lacking of body. Mids/heights are detailed, subtle, refined, never harsh or tiring. Cymbals are just like in the real life and trumpet rendered with excellent articulation and correctness. Jazz voices are just perfect, nothing else to say. From that point of view it is the most natural and sensual of all the amps I have. The ML4/41MXP is a combination I can listen effortless for hours even at high-level.

I built several amps until today and I can affirm that this one is a top contender along with the Philips MC1/60 and the Visseaux A710

Vinyls, vynils & vynils encore