Retro Hardware: Neve Class-A Channel Amplifiers, Part 1

As far back as the late 1980s recording and mixing engineers in the know began acquiring bits of select, older gear for their own use to give their tracks some rich, engaging character that was missing from the newer IC-based mixing consoles and digital records that had become the norm throughout the decade.  Discrete, Neve console channel amplifiers from the 1970s were among the first such devices to be used thus, not only because of their availability with so many older Neve consoles being replaced by newer IC-based Neves and SSL's, but also because of their modular format that makes racking up a few channels a rather simple affair needing only a power source and I/O connectors to function fully.  Most other desirable consoles of the 70's weren't so simple to rack, often requiring extensive metalwork, cutting of pcb's, etc. to get the job done. 

Of course it didn't hurt their popularity that these old Neve channel amps sounded amazing, and over the last two decades stuff with the lightning "N" on it has become stupidly expensive, even to the point of absurdity in some cases (routing modules that have had all useful actives and transformers removed still with asking prices of over $1000 each.... c'mon... really??)

This post is the first in a series that I'm writing about old Neve gear, specifically 80-series (recording) channel amps, 53-series (broadcast) channel amps, germanium modules, dynamics units, class-A circuits, class-AB circuits, and some later IC-based designs, as well as various random modules and any other tidbits I find interesting.  While I cannot say these will be fully comprehensive discussions, I will try to provide far more detail than is typically found in such discussions.  I am a rather rabid collector of information and have amassed a great deal of it over the last dozen or so years, and as a lover of all things vintage Neve I am writing this hoping that others with a similar passion will find it helpful, informative, and even fun to read.  So let's' get rolling.  First up:  class-A channel amplifiers.

Since this post deals specifically with class-A Neve channel amps, perhaps I should briefly explain what class-A means (as opposed to, say, class-AB as used for the output amplifiers in the 1081 module and its derivatives and other modules including all the discrete broadcast modules).   "Class-A" refers to the type of amplifier blocks used in the modules.  A class-A amplifier conducts during the entire 360-degrees of a waveform cycle (visualize a complete cycle of a sine wave), meaning it never switches off.  For the sake of comparison, class-B amplifiers (not used in discrete Neve designs) have two active portions each of which amplify 180 degrees of the waveform (think a sine wave divided into 'upper' and 'lower' halves) in what is called a "push pull" configuration.  These two partials, when combined to produce the complete output signal, may not always match each other at the crossover point between amplifier halves resulting in a rather unpleasant sounding phenomenon called 'crossover distortion'.

Class-AB amplifiers are something of a combination of class-A and class-B in that there are still two active portions doing the conducting, but instead of each taking an equal 180-degrees of the waveform they each amplify a bit more than half the waveform and and essentially crossfade between each other.  In practice this eliminates crossover distortion, but does offer a different, and (to my ear) more dynamically responsive characteristic with a touch of sheen compared to class-A (at least in the old Neve stuff) with the class-A circuits having a bit more sluggish, pillowy dynamic character with a lot of weight and authority.  The Neve class-AB gain blocks also run more efficiently than their class-A cousins and generate less heat.

The earliest solid state Neve circuits were all class-A throughout and use germanium transistor amplifiers.  These used -24vdc power, or a 'positive ground' supply.  I will do a future post to discuss these lesser known, but still very appreciated discrete germanium transistor Neve modules, but for now let's stick with the far more familiar, silicon transistor family of modules that run on +24vdc "negative ground" supply.

The first of these silicon transistor Neve channel amps was the 1063. 

Neve 1063 Channel Amplifiers

Designed in 1969, the 1063 was the first Neve channel amp to use the beloved red/grey/blue Marconi knobs against the RAF Blue/Grey front panel paint with white screening in capital letters.  This aesthetic is the most well-known and appreciated of all vintage Neve color schemes (as evidenced by the fact that today AMS-Neve uses this appearance for most of their outboard gear, regardless of whether or not the circuits inside bear any actual similarity to the classic circuits the units are made to resemble.  But I digress...).

The 1063 offered a single balanced input via large, round, canned version of Neve's 10468 mic input transformer presenting a 1200 ohm load to both mic and line sources.  This input transformer (soon to be wearing a far more well known rectangular housing in subsequent Neve modules) was actually a stock Marinair design used by other manufacturers including Calrec, Audix and Cadac.  The "10468" designation is an internal Neve part number. 

Neve 1063 Guts (note the single, large, round mic input transformer).

The 1063's equalizer is a simple 3-band affair consisting of high and low shelves with a peaking mid band.  The shelves use a Baxandall type design providing a high shelf at 10kHz and a low shelf selectable between 35, 60, 100, and 220Hz.  These shelves provide a cut/boost range of +/- 16dB each.  The mid band is an inductor-based design allowing choices of 700Hz, 1.2, 2.4, 3.8, and 7kHz with cut/boost range of +/- 18dB. 

The variable bands featured a dual-concentric control set with an outer ring, made of aluminum, selecting the frequency and the inner plastic knob controlling the amount of cut or boost.

Rounding off the facilities of this channel amp is an inductor-based high-pass filter with a -18dB/octave slope and selections of 50, 80, 160, and 300Hz.  There are also two pushbutton switches at the base of the front panel, one to engage or bypass the EQ and the other to invert the channel's polarity 180 degrees.

The mic preamp design in the silicon transistor Neve class-A channel amplifiers is a 3-gain stage affair with two preamp stages and an output stage.  At gains of 50dB and lower only one preamp stage and the output stage are used.  At 55dB and above all three stages are used.  Between 50dB and 55dB gain positions is an "OFF" position to eliminate pops and such from all of the signal switching going on between these two positions.  (Actually, the 1290 mic preamp module, the only true, 3-stage, class-A, Neve mic preamp that didn't also have an accompanying equalizer (which I will discuss in a future post) had no such "OFF" position, but more on that another time). 

The gain switch on most of these modules is a somewhat involved affair using a 3 pole, 24 position switch that serves several purposes:  it selects mic or line input source (not on the 1063, which has only one input source, but on most other models), it operates a switchable pad between the input transformer secondaries and the first active gain stage, it switches between 2 and 3 total gain stage configurations, it operates another set of pads between the various active stages, and it alters the actual gain of one of the two preamp gain blocks.  All of these various combinations result in the total gain for each setting.  This differs considerably from more modern methods of varying feedback around a single gain stage (or two) to adjust gain, and it results in varying timbres with different settings.  This varying tonality characteristic (relative to gain selection) has become part of the charm of these vintage devices.

Like all class-A Neve modules the 1063 uses the Marinair-built LO1166 output transformer that has a gapped lamination stack designed to pass DC through the windings without saturating the core.  The Neve class-A output stage pulls current through these windings which provides for a rather efficient class-A output amplifier. 

The basic functionality of the 1063 module set a precedent for all future class-A Neve channel amps in that most subsequent models included a mic/line preamp, 3 band EQ and filter(s).  Also like most class-A units to follow it measures 8.75" high x 1.8" wide x approx. 10" deep.  Unlike the 1063, most (but not all) other channel amps provided separate mic and line inputs featuring their own input transformer, the Neve part # 10468 for the mic input and the # 31267 for the line input.  (In a subsequent post I will discuss at length the various transformers used by Neve during this era, their origins, and the unique designs and qualities of each, so stay tuned for that).

The 1063 was quickly followed by the 1064, which was similar in functionality, though the 1064 provides separate mic and line inputs each with their own input transformer (10468 for mic and 31267 for line).  The 1064 was a physically larger module, however, measuring 12" high x 1.8" wide x 12" deep.  The larger front panel real estate provided room for separate controls for all functions rather than the dual-concentric controls of the two lower bands on the 1063's EQ section. 

The mic preamp still used the same design as the 1063, though the separate line input functionality was added to the gain switch assembly. 

The three-band EQ featured the same Baxandall high/low shelves and inductor-based peaking midrange design as used on the 1063, though frequencies selection was a bit altered:  10kHz on the top, 700Hz, 1k2, 2k4, 3k6, and 7kHz in the mids and 35, 60, 100, and 220Hz on the low shelf.  Intead of pots for the cut/boost controls the 1064 used rotary switches for stepped (and recallable) functionality. 

HPF was switchable at 45, 70, 160, and 360Hz. 

And, of course, EQ in/bypass and phase switches at the bottom.

The 1064 is the same size as the well-known and revered 1081 channel amp (which will be discussed in a future post), and the two can be used interchangeably in their respective consoles.

That's all for this installment, but there's plenty more to come (some might think too much! :)  So stay tuned....

-JC