All Grown Up
What a metamorphosis this Magnavox amplifier has gone through! Performance wise, there's simply no comparison to the original design -- and yet, the basic topology of the original design remains in tact, albeit with a few enhancements. But before I get into the performance results, let me stress that one of the most important things about the amplifier remains unchanged, which is what so many folks love about these amplifiers in the first place: The sound. Oh, its enhanced alright! But rather than considering it enhanced by way of a change in sonic character, consider it enhanced by a shot of steroids.
The upgraded version has significantly improved performance in virtually all areas but one, with that one area intentionally left at the value of the original design. Consider:
1. POWER OUTPUT: This is now 14.0 Watts RMS continuous power output per channel with both channels driven, which is slightly down from the 15.5 watts using the original OPTs of the non-upgraded modified design under the same conditions. This loss is insignificant, and due to the slightly higher primary impedance of the Z-565 transformers, versus that of the original devices. However, the upgraded Maggy will now supply this power level -- continuously in both channels -- from 20 Hz to 20 kHz! This is a performance level that the original OPTs simply cannot touch. And, it is high quality power output too:
2. DISTORTION: Within 1 db of 14.0 watts X2 (both channels driven), THD is < 1% from 25 Hz to 20 kHz. Even at 20 Hz under the same conditions, distortion is ~ 2.5%, which is still amazing performance for the upgraded design.
3. FREQUENCY RESPONSE: This is dead flat (+/- 0 db) from 20 Hz to 20 kHz, with a smooth roll-off on either side of this band. At 40 kHz, response is down just .5 db.
4. STABILITY: This is now Absolute, with no value of capacitance only loading causing any tendency towards oscillation. 10 kHz square waves are presented with a flat top and no ringing, while on the low end, pulse conditions result in very rapid settling -- even with no load of any kind attached.
This all represents very high performance, meaning that within its power class, the so modified and upgraded 9300 takes a back seat to very few comers, regardless of manufacture, and hangs in there with some of the very best 6BQ5 offerings out there, and surpasses most. In one way however, the upgraded version (intentionally) retains an important performance trait of the original design:
5. INTERNAL OUTPUT IMPEDANCE: With the original design, this was clocked at 1.59 Ohms, referenced to the original OPT having a 4 Ohm output winding. With the modified amplifier, this did not materially change, coming in at 1.52 Ohms, again referencing a 4 Ohm output. With the upgraded design, the internal output impedance again remains basically unchanged at 1.68 Ohms on the 4 Ohm output tap, and 3.27 Ohms referencing the 8 Ohm output tap. As a performance specification, this is hardly "high fidelity" like, with the best designs showing less than 1 Ohm internal output impedance, in reference to a 16 Ohm output tap. So what gives?
In an amplifier using the topology of the basic Magnavox design, the internal output impedance (that is, its ability to hold a steady output voltage in the face of a varying load impedance) is based almost entirely on the amount of NFB used. The more feedback used, the lower the internal impedance becomes. A low internal impedance means that an amplifier having that trait can exercise great control over the bass driver cone, to eliminate any "boominess" in the reproduction of bass frequencies. Another term for an amplifier's internal output impedance is its Damping Factor, with a low internal output impedance equating to a high Damping Factor. Sonically however, a very low internal impedance amplifier can tend to suck the life right out of the presentation, leaving the reproduced sound as having a "dry", un-engaging quality. On the other hand, a very high internal impedance amplifier will cause such boominess as to make the bass response sound artificial and bloated, without any definition at all. As with so many things in life then, the answer lies in the middle.
It is the somewhat elevated internal output impedance then that gives the Magnavox amplifiers their endearing listening quality that so many enjoy. With a little bit of internal output impedance, the bass will still be defined well enough, but there was also be a base that the sound builds on or a fullness that makes the sound for many so engaging to listen to. Because this IS a Magnavox amplifier, retaining this electrical quality is the best way to retain the sound that endears so many to it. With that inherent quality retained then, it is the improvement in the other specifications with the upgraded design that gives this version it's "on steroids" performance. On the other hand, if a high damping factor is what your speakers sound best with, then the original Dynaco SCA/ST-35 designs using the Z-565 might be better suited for you.
So, for those who think that the modified and upgraded Magnavox amplifier now has nothing left of its original build but its chassis, choke, and power transformer, understand that throughout all of the modification and upgrade efforts, they all still seek to retain the original essence of its sonic signature, allowing it to remain true to its heritage as a Magnavox console amplifier.
Finally, also note that the output tubes now idle at just 20 mA total cathode current each. This is largely due to the use of EFB for controlling the output stage operating point, but was also helped slightly by the new Z-565 OPTs as well. At this level of current draw, each output tube is dissipating a ridiculously low 6.4 watts at the plate, which amounts to just 53% of the conservative Design Center rating system. With such low power consumption, the power transformer remains just warm after several hours of operation. Tube and component life will be extended immeasurably.
Oh, two other housekeeping notes:
1. Using a good bottom plate will ensure the lowest hum and noise levels, and should be part of any quality rebuild. In the pics, I should a dirt cheap easy way to facilitate one.
2. It won't be long before one of the AK Magnavox fanatics wonders about hooking up the screen taps on the Z-565 to engage UL operation. Go for it, and knock yourself out. I didn't do it, and don't have any real plans to do it. If that is what you want, Dynaco already did that in their models with this transformer. This is a Magnavox thread, and in trying to stay as true to the original topology (and sonic signature) as possible, I have left the the output stage in it's original form, save the application of EFB. I mean no disrespect to those who wish to try it -- but there's only so many hours in the day, so I'll leave that to other inquisitive experimenters.
In the listening room, this amplifier has all the enjoyable listening qualities that define it's humble beginnings. But it can make itself known real quick, handling even the most challenging material with relative ease. I never ever say never, but it would be hard to imagine squeezing more from these little guys while still retaining the essence of what they are. For those who are so inclined to get the last ounce of performance from their Maggy, this one will make your friends stand up and take notice!
Dave
Pics include:
Below: Near textbook displays of 10 kHz square waves: Left on top, Right on the bottom.
Below: 20 Hz at 1 db below 14.0 watts RMS in both channels. Distortion is starting to rise as discussed, but this is still excellent performance for this power level, that the vast majority of 6BQ5 offerings cannot touch.
Below: At 25 Hz however, distortion at the same power level has dropped to well under 1% (about 0.65% avg).
Below: Same power level at 20 kHz. This is exceptionally clean performance for an amplifier of this power class.
Below: An easy bottom plate to minimize hum and noise: 3/8 inch plywood cut to size, with aluminum duct sealing tape lining the top, and four rubber feet underneath.
Below: A completed underside shot.
Below: Those bad boys behind the Magnavox don't scare it at all. Within its power capabilities, it can run with them just fine.
Below: The schematic of the modified, upgraded version. The power supply page for the modified version remains the same for the upgraded version, except that the 340 volt supply point now becomes a 320 volt point.