Manley Labs Reference 240 monoblock power amplifier
Giving someone a present is usually intended to place the giver in a favorable position vis-à-vis the receiver, n'est-ce pas? But giving someone a gift that works badly might instead produce a rather negative reaction in the recipient. So I'd call it a wash: Giving someone a watch that breaks is the same as giving them no watch at all.
The Manley Reference 240 is the little brother of the Manley Labs Reference 440 monoblock reviewed by Jonathan Scull last December (Vol.18 No.12, p.155). At $9000/pair the 240 is not inexpensive, but it is certainly less dear than its big brother. While it is not the sort of contraption you'll find in the home of your average grade-school teacher, the Reference 240 might appeal to an audiophile with a penchant for tubes and some sense of fiscal propriety.
I'm not a tube-swapping kind of guy (I've learned that fooling around with hot tubes can burn ya), or even a tubeophile (life is too short to spend half my waking hours curled around a hot component trying to achieve optimum output-tube bias). I prefer to spend my reviewing hours judging a component's neutrality, transparency, and fidelity rather than its ability to make me wax lyrical over Frank Sinatra. However, my time with the Manley 240s reaffirmed my faith that some tube amplifiers are for manly men.
Tubes by design
There are a lot of tube amplifiers on the market, but David Manley's designs are unique in several exceedingly important ways. Manley Labs makes all of its audio transformers in-house, employing several proprietary design techniques, including twin-coil output transformers. In a twin-coil transformer, each leg of the push-pull topology is identical because it involves an identical number of turns in the transformer, guaranteeing that DC resistance is the same for each leg. Moreover, since there is a separate and physically identical coil for each phase half, everything elseleakage inductance, primary inductance, DC resistance, impedance, and stray capacitancewill also be identical.
This transformer design, coupled with Manley's use of a double driver tube for each phase of the push-pull circuit, makes the Reference 240 a true differential push-pull amplifier. David Manley is adamant that the only proper way to make a push-pull design is with a fully balanced differential circuit. Many of the problems with crossover distortion that are blamed on push-pull designs are a result of improperly balanced legs, he feels. (Think of a chair with one support 2" shorter than the other and you'll get the idea.) Manley designs may be among the only tube amps on the market that fulfill the potential of the push-pull design philosophy.
While the Reference 240 has both balanced XLR and single-ended RCA inputs, it is not a fully balanced differential design from input to output. The XLR input signal goes to a Manley-built transformer, which converts it into a conventional single-ended signal to be fed to the amplifier's input stage. The single-ended RCA input goes directly to this same input stage, which consists of a 12AT7 twin-triode tube, with its halves in parallel. This input tube's output is sent to the splitter stage, which consists of one double-driver, a 6414 tube. Each half of the push-pull circuit uses a separate 6414, the outputs of which drive the EL34 output tubes. There is nothing radical about the circuit topology. The goal is to take a tried-and-true design and execute it as well as is humanly possible.
The Russian-made EL34 Tubes used in the Manley 240 are matched using a proprietary computer program. In theory, since each output tube has its own individual bias adjustment, matching should not be necessary, but Manley has found that far better performance can be achieved if the tubes are critically matched. My experience with the Reference 240 has shown the output tubes to be phenomenally stable. I checked the bias several times during the two months that I had the amplifiers in-house, and I never had to make more than minuscule adjustments to any bias setting. This remarkable stability may be disappointing to those tweaks who live to check tube bias. (They could use the time saved to listen to music.)
The Manley 240 has four separate power supplies, one for the AC heaters, one for the B+ voltage rail, one for the screen grids, and one for the bias circuit. The very stiff, 600V B+ supply has a capacity of 2600µF. The power transformer has four primaries so that the Manley 240 can have a "Soft-Start/Ever-Warm" idling capability. In this mode, only 5% of the voltage and 5% of the current are fed to the amplifier circuits. This is quite different from most "standby" switch positions, which merely remove or reduce the B+ rail, something that can actually shorten tube life. The Soft-Start setting on the Manley 240 extends tube and component life by allowing current and voltage to pass constantly through all components so they are never fully turned off.
Footnote 1: Yes, I know, in the age of Entertainment Tonight, Inside Edition, and Geraldo, this has become an oxymoron right up there with "military intelligence."