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GML 8304

4-channel Reference Quality Mic Preamp By Hugh Robjohns
Published October 1999

GLM 8304

This state‑of‑the‑art multi‑channel mic preamp, designed by the celebrated George Massenburg, is light on frills but heavy on quality.

George Massenburg has an impressive reputation which reaches into virtually every part of the audio industry. His work as an internationally renowned producer and recording engineer is well known, of course, but so too is his status as a designer of some of the finest audio equipment and associated systems available. He is even credited with the invention of the parametric equaliser!

Massenburg's designs are manufactured and marketed through GML (George Massenburg Laboratories), a company he established in 1982. Products available from GML include the reference‑standard 8200 parametric equaliser, the 8300 microphone preamp and the 8900 dynamic gain controller. The mastering fraternity is catered for with the 9300 A‑D converter and 9500 parametric mastering equaliser, whilst the studio market is offered GML mixing automation and moving‑fader systems, the Master Recording Console and the High Resolution Topology line‑level mixing console. Sony have even licensed software modelling of the GML parametric equaliser for their top‑end OXF R3 'Oxford' digital recording console, and other companies are starting to license similar products, opening a whole new avenue of development for GML.

The subject of this review is the 8300‑series mic preamp mentioned above. It's a 'back‑to‑basics' design derived from the GML Master Recording Console, and comes in two stand‑alone versions — the 8304 4‑channel model reviewed here and a field‑upgradable 2‑channel version, the 8302.

The Package

The 8304 is a two‑box affair consisting of the preamp proper and a separate power‑supply module. The preamp is a neat, simple 1U rackmounting box, finished in black and boasting just four knobs and five LEDs on its front panel. The rear panel is no more complex, featuring pairs of XLR sockets for the audio and power connections, plus toggle switches to activate phantom power on each channel, and an earthing connection strip. The overall impression is of solid and reliable construction with no money wasted on fancy design frills or unnecessary features.

The front‑panel controls are 12‑way GrayHill rotary switches which set gain in 5dB steps from 15 to 70dB (with a specified +/‑ 0.25dB tolerance). There is no provision for stereo‑linking adjacent channels, but the gain calibration is exemplary and equal settings on individual channels appear to provide identically matched gains, with no discernible shift in stereo imaging at all. The unit has an extremely wide frequency response without switchable LF filtering, so care should be taken to avoid losing headroom through rumbles or mechanical vibrations from connected mics.

Setting input gains with the 8304 requires some experience and common sense, as there is no level‑metering as such — the metering on the recording system must be relied upon — although each channel has its own red overload LED which illuminates at +24dBu (3dB below output clipping). The only other indicator is a single green LED which lights when power is connected.

Each channel is provided with a Switchcraft female XLR on the rear panel, for the mic input, and a male XLR for the output. An adjacent toggle switch enables phantom power, which is sourced through 6.81kΩ resistors and can provide up to 40mA. A male 5‑pin XLR accommodates the external DC power input and a loop‑through on a female 5‑pin XLR allows daisy‑chaining of up to three preamps or other GML products from the one external power supply. A terminal strip provides the connection between signal and chassis earths and can be broken if required. All wiring conventions are clearly printed on the rear of the unit.

The 8304's inputs are transformerless and electronically balanced to an extremely accurate standard, but the outputs are unbalanced (although effectively earth‑compensated). However, there are dire warnings, both in the accompanying literature and on the unit itself, about not shorting the output of the unit (ie. connecting pin two to ground), as permanent damage can result, so some care must be taken when interconnecting the 8300.

GLM 8304 rear panel connections.GLM 8304 rear panel connections.

Interior Design

Much as I detest external power supplies (particularly those of the wall‑wart variety), it has to be said that the 8355 power unit supplied with the 8304 is impressive. It's surprisingly heavy for its diminutive size (approximately 150 x 102 x 145mm) and contains a hefty mains transformer, substantial smoothing capacitors, and over‑rated TO3 cased power regulators to stabilise its +/‑ 28V output. Mains input is via a fused IEC socket with line voltage selector, but no power switch is provided.

The only real drawback with this power‑supply unit is that it is not rackmountable and so must reside in the bottom of a bay or flightcase. However, the metalwork of the case is easily substantial enough to allow it to be bolted down, after a few minor DIY modifications, if required.

The preamplifier itself is beautifully engineered, and is obviously hand‑built with high‑quality components throughout and a lot of attention to detail. The 4‑channel model contains four identical circuit boards, mounted side‑by‑side, running front to back, and covering the whole of the floor area in the case. The 2‑channel version is obviously only fitted with a pair of input cards, but can be upgraded to four channels later if required.

The preamplifier itself is beautifully engineered, and is obviously hand‑built with high‑quality components throughout and a lot of attention to detail.

The front‑panel LEDs and rotary switches mount directly onto the circuit board, but the rear‑panel switches and XLRs are fixed to the chassis and linked via good‑quality screened wiring. The circuitry is apparently derived from the input module of the GML Master Recording Console (as is the 8500 parametric equaliser) and uses a discrete class‑A bipolar (transistor) topology for the active gain stages. Although there are no ICs in the signal path, there are a number of ICs on the circuit card, which serve as special Massenburg‑designed active servo controls, to provide DC stabilisation.

Two small daughter‑cards are supported above the main channel PCB. One contains a combination of six thermally bonded transistors and some surface‑mount components, and is presumably the front end of the preamp. The other card is an extremely densely packed board which appears to be the main gain stage. There are no component markings anywhere on any of the PCBs — not even for the functions of three sealed trimmers or two wire links. In the case of the latter, the notes supplied with the machine suggest that it is possible to reset the output wiring so that pin 3 is hot, and presumably one (or both) of these wire links is the means to do that.

Listening Tests

I used the GML with a variety of high‑end mics including Neumann KM100s, KM84s, U87s and a TLM170, AKG C414s, an Earthworks SR71, a Soundfield ST250, and a CAD VSM1. In all cases I felt I was hearing just the microphone — the preamp did not seem to add any obvious sonic signature of its own. However, the bottom end always seemed more realistic and natural through the GML than through any other mic preamp I tried, which included the front ends of the Mackie 1604 VLZ Pro and Yamaha 03D mixers, and the MindPrint Envoice voice channel.

Setting up the 8304 is very straightforward, although I found myself testing the output leads with a cable checker before connecting anything, as I didn't want a silly wiring fault to wreck the output devices! The unit supplied phantom power with no problems and easily accommodated even the semi‑pro line‑level output from the Soundfield mic. When I first used it as a dual‑stereo preamp I religiously went through the channel gain‑matching process every time I needed to alter the channel gains, but I quickly discovered that this was totally unnecessary, such is the accuracy of the 8304's internal gain matching.

I also experimented with using the GML at opposite ends of a 100m multicore, first as a line driver at the microphone end (running the multicore at high level), and then as a conventional mic preamp with 100m of cable between it and the microphone. It performed extremely well in both cases and, given its unbalanced output, far better than I expected as a line driver (see 'Line Driving' box for more information). I could not discern any audible difference between the sound of a mic connected with a 5m lead straight into the preamp and the same mic after 105m, at the other end of a star‑quad cable multicore. Neither the GML nor the electronically balanced line inputs on a Yamaha 03D complained in any way about this use of the preamp as a line driver and, again, I could not discern any significant changes in quality when it was used in this way. Clearly, the GML has an extremely well‑engineered front end, combined with a very powerful and stable output driver.

The overall sound quality of this preamp is simply sublime. I found that the most harmonically complex instruments, such as 12‑string guitars and harpsichords, sailed through it marvellously, and transient‑rich percussion was handled with equal aplomb. Voices sounded utterly natural and transparent during my testing (especially so with the fabulous Soundfield mic). The essential quality of the GML 8300 models is that they provide gain with nothing added or taken away — as the perfect preamplifier should.


Although it's obviously not cheap, the GML 8304 does represent good value for money in terms of its capabilities, build quality and overall sound character — or lack of one! There are a number of loosely comparable 4‑channel mic amps on the market, the most obvious competition being the Focusrite Red 1 quad mic amp. This features transformer balancing, polarity switching and individual VU metering in a stylish 2U case, and costs around 20 per cent less than the GML. However, while other products may appear to share similar technical specifications and may seem financially more attractive, I think perhaps that the 'Massenburg ear' has been used to create something rather special here, which justifies its premium.

Spec Check

At this level of performance, specifications rarely mean very much, as the technical aspects of the system, which engineers like to measure, often don't reflect the subtle sonic differences and qualities which will persuade a purchaser to go for one system over another. However, just for the record, the 8304 provides a very impressive 80dB of common‑mode rejection at 100Hz and 10kHz, and accepts +12.4dBu maximum input level before clipping.

With a +20dBu output level, distortion is 0.0015 percent at 30dB gain, rising to 0.007 percent with the maximum 70dB gain. Maximum output level is +27.4dBu, and effective input noise is ‑126.5 dBu with a 150Ω source. The theoretical limit of ‑129.5dBu is reached with a 0Ω source. Frequency response is within 0.3dB between 1.7Hz and 260kHz, and phase shift reaches 22 degrees at these extremes.

Line Driving

Microphone signals are small, weedy things, often down in the hundreds of microvolts region. Sending a signal that delicate down a few hundred metres of microphone cable (as might be required at a major outdoor gig, for example) is possibly not the greatest idea! For a start, the mic signal is already very prone to degradation from interference. In addition, the output buffer of most microphones is not usually designed to cope with the kind of difficult load a long cable presents. The end result is that sometimes the mic signal that falls out at the end of the wire can bear little resemblance to what went in, and once the subtle details have been lost there is no way of bringing them back.

One way around this kind of problem is to use a line driver — a device which raises the signal to a sensible line level, and then drives the cable with a properly designed buffer circuit to cope with the excesses of capacitance, inductance and other aspects of physics that make cables difficult things to squeeze signals through. Big PA companies might use dedicated multi‑channel line‑driving systems to pass the microphone signals from the stage to the front‑of‑house console, but some standard mic preamps can also perform very well in this application (as does the GML 8304), with useful benefits when the cable run is longer than a few tens of metres.


  • Totally neutral sound character.
  • Unusually high levels of gain available.
  • Superb specifications.
  • Good line‑driving ability.


  • Lumpy power supply.
  • Unbalanced output.


A no‑frills 4‑channel microphone preamp with a sound quality and standard of construction which easily justifies its high price.