Manley Labs Manley Massive Passive Stereo Tube Equalizer Why Tube Gain Stages?, Phase Shift?

Phase Shift? Why Tube Gain Stages?

Deadly topic. This is probably the most misunderstood term floating about in the mixing community. Lots of people blame or name phase shift for just about any audio problem that doesn't sound like typical distortion. We ask that you try to approach this subject with an open mind and forget what you may have heard about phase for now.This is not to be confused with "time alignment" as used in speakers, or the "phase" buttons on the console and multi-mic problems.

First - all analog EQs have phase shift and that the amount is directly related to the "shape" of the EQ curve. Most digital EQs too. In fact, one could have 3 analog EQs, 3 digital EQs, and an "acoustic equivilant", and a passive EQ, each with the same EQ shape, and ALL will have the same phase shift characteristics. This is a law, a fact and not really a problem. The two exceptions are: digital EQs with additional algorithms designed to "restore" the phase, and a rare family of digital EQs called FIR filters based on FFT techniques.

Second - Opinions abound that an EQ's phase shift should fall within certain simple parameters particularly by engineers who have designed unpopular EQs. The Massive Passive has more phase shift than most in the filters and shelfs and leans towards less in the bells. Does this correspond to an inferior EQ? Judge for yourself.

Third - Many people use the word "phase shift" to describe a nasty quality that some old EQs have and also blame inductors for this. Its not phase shift. Some inductor based EQs use inductors that are too small, tend to saturate way too easily, and create an unpleasant distortion. The Massivo (of course) uses massive inductors (compared to the typical type) which were chosen through listening tests. In fact we use several different sizes in different parts of the circuit based on experiments as to which size combined the right electrical characteristics and "sounded best". The other very audible quality people confuse with phase shift is "ringing". Ringing is just a few steps under oscillating and is mostly related to narrow Qs. It is more accurately described as a time based problem than phase shift and is far easier to hear than phase shift. For our purposes, in this circuit, these inductors have no more phase shift or ringing than a capacitor.

Fourth - A given EQ "shape" should have a given phase shift, group delay and impulse response. There also exist easy circuits that produce phase shift without a significant change in frequency response. These are generally called "all-pass networks" and are usually difficult to hear by themselves. You may have experienced a worse case scenario if you have ever listened to a "phase-shifter" with the "blend" set to 100% (so that none of the source was mixed in) and the modulation to zero. Sounded un-effected, didn't it, and that may have been over 1000 degrees of phase shift. Group delay and impulse response describe the signal in time rather than frequency and are just different ways of describing phase shift. Some research shows these effects are audible and some not. The Massive Passive tends to show that group delay in the mids is more audible than towards the edges of the spectrum and there may be interesting exceptions to generalities and conventional wisdoms. The audible differences between EQs seems to have more to do with Q, distortions, headroom and topology than with phase shift.

Fifth - Phase Shift is not as important as functionality. For example, we chose very steep slopes for some of the filters because we strongly believe the "job" of a filter is to remove garbage while minimally affecting the desired signal. A gentler slope would have brought less phase shift but would not have removed as much crap.

The stupid answer is the name on the unit is "Manley Labs" and that is what we do. Unlike a current trend, we do not use tubes for THD, clipping character, cool marketing buzz-words, or plagiarism. We began building tube gear because we preferred the sound when it was un-fashionable and re-introduced these glowing gain bottles to both the hi-fi and studio communities when there was virtually no fresh tube designs available. We also stress that, its not just the tubes, but the way they are used. The sound of a piece of gear is due to many design details and many of the components - always has.

In the Massive Passive, the tube gain stages are new designs developed for this unit. We try to use minimalist techniques where ever possible and use the appropriate technology for the purpose. Simple vacuum tube circuits can excel for medium gain voltage amplifiers and high headroom output stages. A simple tube stage offers better linearity than an equally simple transistor circuit. Transistors have a logarithmic transfer curve and are essentially current devices. Transistor circuits are typically built with huge excess gain which is used for more feedback in order to tame the linearity (THD) but this feedback seems to cause audible transient problems and is directly responsible for the harsh clipping character. Op-amps, which can be less noisy and lower THD, are complex circuits which force music through many transistors and may also bring crossover distortion artifacts and headroom issues into play. The only alternative would have been for the Massivo to use FET / MOSFET high voltage discrete circuits. Someday, we may introduce a version like this but don't phone us up every month asking if we are working on it. We'll let you know.

We use an exceptional op-amp/discrete circuit for the input buffer in order to drive the 150 ohm (worst case) EQ circuit. Not an appropriate place for tubes. The first design used a transformer (3:1) for impedance conversion but it had a 10 dB voltage drop and thus 10 dB more noise. The new input circuit isolates input loading and allows the tube circuits to be better optimised. We use two similar all-tube gain stages per channel for interstage and output line drivers which together cancels some distortion. The output is capable of driving up to +37 dBu! This stage also uses a separate winding on the output transformer (also custom designed) for a little negative feedback to allow lower output impedances and minimal transformer coloration. In other words, because we expect some engineers wanting to boost 20 dB at 100 Hz occasionally, the circuits had to be capable of cleanly delivering it (regardless if the next piece in the chain can deal with it). We used tubes for more headroom (300 volt power supply) rather than more clipping. Generally too, tube circuits clip a little smoother than mega-negative feedback IC circuits.

Some may question "tube reliability" but most major studios have many 30 or 40 year old tube compressors and EQs running every day and some with the original tubes. Not many 15 or 20 year old transistor units are still working or wanted. Tubes will eventually burn out (so do transistors), however, usually you can easily get the type of tube used 30 years ago and you won't need a soldering iron, schematic or technician. Your parents probably used to "fix" the old TV. The bottom line is, good gear tends to be more reliable, and if a problem develops, is both easy to fix, and carries great factory support. We understand that it sometimes involves your professional livelihood and this is indeed often serious and you depend on it. If this is the case, consider getting a few spare tubes which covers 90%+ of emergencies with immediate fixes. Our service department has a great reputation with phone support and fast turn-arounds too.

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