.ed's System: MIL Modules

Having now consumed enough drugs of one type or another, I am now prepared to reveal the truth about my own system preferences. Obviously, I use a Scott tuner, but it may be attached to a receiver, especially as is the case with the modded R74S, which has a FM tuner that is superior in application to the 433, which I can only describe as inflexible concerning matters of multipath. Plus the R74S has a wonderful complementary output that I like to use to amplify the Gradients. Occasionally, I will drive the Quads with the 477's amplifier stage, thus supplying the Quad/Gradient combo with exactly matched amplifiers. I can do this trick with any number of Scott quasi comps, but I concluded a while ago that the later complementary designs are much better than their precedent design, such as that found in the 260. On occasion, I will use a pair of 342C's that have been adapted for use as power amplifiers in the manner described. As an endnote to this Scott nonsense, the 341 is an amazing little amp for the Quads, and the 357/367 series are even better bets. These later models use electrolytic capacitors that should last at least another 63 years, at which time the world will end, anyhow.

Amplifier

In actuality, I spend much of my time listening to the Quads without the Gradients hooked up. On these occasions, I use a MIL 257, which I constructed in 1994 specifically for the Quads. It can use several varieties of the 7868, including the oddball looking TV tubes that you can get for chump change. Nobody knows that they can be used in a 7868 socket. Talk about a lonely suitor.

As a habit, I use a matched quad of 7868's, and I have several spare matched sets. It's important that the tubes be closely matched, because the amplifier is cathode follower, and in fact, it uses a single cathode resistor/shunt capacitor for the cathode supply of all four output tubes. It's not possible to bias the amplifier, and unmatched tubes would cause no harm, as each tube would self-bias, but they would not match electrically and there's no way to compensate for differences between tubes, as there is in a fixed bias design, where bias and/or balance provisions are made. With fixed bias, it is possible to bias down a good tube to match the capabilities of a weaker tube.

 

The 257 uses 12AX7's as input amplifiers and phase splitters. My design employs a paraphase circuit which takes the plate and cathode follower outputs of the second plate of the 12AX7 and adds series resistance to the plate side to match the size of the wave that the cathode follower, with its 10% insertion-loss, is producing. But with 260v on the first plate of the 12AX7, there's ample signal, even with the insertion loss. The phase differential lines then hook up to a push pull amplifier at the first grids of the 7868 pair. These outputs pass through the output transformers, where the 450vdc on the plate line, get blocked, and the amplified ac signal is passed through three windings, 4, 8, and 16 ohms. These are amplitude selection points, since any mismatch that you may find with loudspeaker impedance won't be much noticed by the 257. Like any other output transformer connected amplifier, the working stage of the amplifier is protected by the output transformer, and any mild impedance mismatch on the secondary of the output transformer would produce a weak inductive response in the tube which would slightly alter the latent operating impedance of the tube, 4 to 8 k ohms. The 257 supplies 16 watts per channel to the Quads. I have used the amplifier with other loudspeaker systems, but since no other speaker sounds like the Quads do, I never bothered to form any useful impressions about it with other systems. The amplifier was made for the Quad.

Pre Amplifier

I began to construct commercial pre amplifiers in 1988, around the time when I first encountered another designer, Joe Rosen. We were introduced by Silas Woo, a guy from Hong Kong who took my preamplifiers and left me less interested in the end, in the business. But Joe was much more savvy with Silas, and I was always impressed by how well Joe did in his transactions. The guy could find gold. I recall him running out of Active Surplus with a handful of wire. He had bought the whole roll and was coming back with his car to pick it up. Joe wire. Later, Joe would order his wire to specification, and out of ego, had his company's name printed on it. It was good wire, but who cares. This is all about preamplifiers. Mind you, the one thing that a preamplifier has in abundance are wires. In fact, the wiring harness is by far the most difficult, error-prone task of the whole exercise of building a preamplifier.

The picture above shows the phono stage tubes on the right, and the line stage tubes on the left, beside the power supply. The preamplifier can use either 12.6 volts ac, or 6.3, as the filament supply, and in the case above, the power supply is oriented to 6.3 v. It is then doubled and filtered by 20,000 microfarads and regulated to produce between 12 and 13 volts d.c. to supply the filaments. This level of filtering ensures that the phono stage is completely free of power supply noise, and it makes it possible to use low output moving coil cartridges wthout a stepup transformer, with the gain set to maximum. This produces an adequately loud volume with no stepup transformer in the circuit. I occasionally set the system up like this, but since it demands a scrupulous adjustment between the levels of other input sources, I prefer to use a stepup transformer, since all of the inputs line up properly, and the phono system used in this manner sounds good enough to me. Noteworthy though, is that with a bit of extra gain, perhaps using a different design of phono (a single triode amplifier followed by passive eq, followed by a robust mu-follower -only one more tube required for both channels), I could balance the lines to match the slightly attenuated phono and have a transformerless phono stage with low impedance (5 ohms) feeding the first grid, and a low output impedance impervious to the impedance effect of the attenuator. With an average output of 5 k ohms, you could use just about any value of attenuator and the mu follower would easily supply full spectrum sound. My preamplifier uses dual 330k pots for level setting. This is about as low a value as you can go without encountering low frequency cutoff. The lowest value that you dare use in this instance is 100k, and in these cases, a bit of judicious series resistance has to be added to the phono circuit to maintain flat bass response.

The only other key factors to the phono stage that should be considered is that it is entirely ordinary in configuration. The RIAA/NAB eq fits into the negative feedback look, and its two most important values of capacitance were chosen to be 5-10% higher in value than would be typically used. I selected values of 158 and 562 pf, here, but the rest of the array uses the common values that are based around 10k, 100k, and 560k resistors. There are jumper points to permit a total bypass, or to convert the eq to NAB, for those who wish to run a tape head into the preamplifier. I have done so in studio settings, but never mentioned this provision to any of my clients of the preamplifier. I first used this design in a tape head drive that I assembled back in the mid 1980's, when I was recording music on a near-daily basis.

The coupling capacitors are the near mathematical equivalent of the values that I used in the line stage. That is, the phono stage, with its smaller signals, passes its signals through .02/.022/.056 coupling capacitors, where the line stage increases those values by ten, and I occasionally will use 1.0 uf coupling capacitors in the place of the .56 nominally used, because the latter value supports frequencies below 20 Hz (down to 7-10 Hz), while the .56 uf value will push the low frequency response to 20 Hz. This is not necessarily of any value to the average listener in employing a large value of output coupling capacitors, as most power amplifiers will block these low frequencies in their input stages or as a last resort, in their negative feedback loop. Look for a power amplifier with an adjustable damping factor to get the most out of the low frequencies.

Total storage at 375 vdc is 1100 uf. This storage is isolated for the last 160 uf that the plates are supplied with. A common supply feeds each envelope through a 270k resistor to deliver 121 volts to the plate. Both portions of the cascade circuit share this supply. Experiments with individual plate supplies within a cascade circuit have never gained anything from a qualitative standpoint, and I know of no designer who has managed to make any progress using this final stage of power supply separation. Mu follower circuits don't work well at all with such separation, but it has some sort of status as a holy grail for audio nirvana. Lots of addled, over drugged audio designers out there.

Back in the late 1980's I used a 12AU7 as my line amplifier, preferring its lower mu, and its greater ability to drive a low impedance circuit. But it wasn't long before I reverted to the 12AX7 as my tube of choice. My line circuit is close to a line circuit that I had constructed for a 'musical instrument,' which is to say, a device that altered sound, exactly what audio isn't supposed to be about. But its amplifier circuit was the same as any other cascade circuit, so I started to use in my preamplifiers. Like its precedent phono stage, it applies 121 volts to the plate, a nice intermediate value for a preamplifier tube. The 12AU7 had 90 volts on the plate. I would consider a voltage as low as 60 to be practicable, as these circuits have abundant gain, far more than an amplifier calls for, and lowering the voltage, obtaining a lower amplitude, etc., also yields you one of the wonderful side benefits of lower voltage: Lower noise.

Champ Amp...

The current preamplifier that I listen to uses the PC board shown above with a set of outputs attached directly to the dual attenuators. This permits the preamplifier to supply an attenuated line signal to an amplifier without being turned on. The 257 is connected to the powered line stage, and I turn on the preamplifier when using the 257 to drive the Quads, but I also use a separate champ amplifier to power a set of Linaeums. The champ amp in this case is the amplifier that comes in the cheap computer speakers that you get with your PC. It turns out that the amplifier, when driven with 12 volts, is capable of providing amazing sound levels as long as I stick to listening to Kraftwerk. Any venture into music with an abundance of midrange energy will drive this one watt amplifier into audible distortion, which I can't experience with the Quads without risking a fire at volume levels that, over 98 dB (which is about as much as I can get out of the Quads), will have my neighbours banging at my door.

I don't listen to the Quads as much as I should. The Champ/Linaeum combo is perfect for the background levels that I prefer with my sensitive hearing. But actually, I have a problem with the Quads that might surprise you: The bass that it produces tends to carry further than dynamic driver bass. The Quad will energize a room, and upon leaving my apartment and walking out into the hallway, I feel the accurate bass from my apartment some distance away, all the while believing that my listening levels were reasonably acceptable. "You called? I didn't hear the 'phone."

I have experimented with my Champ Amp at voltages varying from 3 to 12, and there is no discernible difference in quality until you push the unit, at which point, the larger wave size produces clearer sound. Since the amplifier can handle 16 volts before the capacitors leak or explode, using a box that comes with a 4.5 volt adapter with an adapter of 12 volts will do no harm. In any case, you can buy these types of speakers for about $10.00, so how bad can things get if it starts to melt after you hook it up to your Martin-Logans on a blind date. "You should have gotten some sound out of it.."

ed.