• Fostex FTX 300 Amplifier
  • Fostex FTX 300 Amplifier

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Fostex FTX 300 Amplifier



The Fostex 300 is a high power stereo amp, with several facilities especially suited to control room use. With the exception of Yamaha, the Japanese have steered clear of the professional and even the semi-pro power amp market in the UK, a realm which is dominated by small American and British manufacturers. So the arrival of Fostex amps in the UK isn't simply a case of another oriental product butting in against many.

In the brochure the model 300 is rated at 100W into 8 ohms; into 4 ohm loads, it offers slightly more power - 150 watts. These figures are given for continuous sine waves (ie. oscillator signals) which are worst case test conditions. As a result, the figures are conservative, and on short term (5 second) sinewave tests, (which are more representative of musical conditions), we measured considerably higher powers, viz: 150 watts into 8 ohms, and 270 watts into 4 ohms, which is all to the good.

Homing in on the mechanics, the rackmount case has a low profile (2 rack units = 3½"), which is handy when you're pushed for rack space. At the same time, a low profile amp is inevitably going to be deeper than usual, and this one is no exception; so check your depth-wise rack space before contemplating purchase.

Panel Controls



On the front panel, there's LED metering, and a precision stepped attenuator for each channel, both useful features in the control room. The first five LEDs are scaled -40 to 0dB, and these refer to 775mV (also known as 0dBU and 0dBm). Because the scaling is relatively course, these LEDs are useful mainly for confirming that the amp is on, and for giving some idea of the instantaneous power level. For instance, a 10dB voltage change represents an approximate 10 fold change in power, so the -10dB LED indicates some 27 watts referred to full power into 4 ohms (270 watts). Checking the meters for accuracy, we found errors of between 1 and 3dB on the four lower LEDs, so don't try using these lower level indications for line-up.

By contrast, the 0dB and red 'error' (Overload) LED were found to be accurate within the vital fraction of one dB, and retained this accuracy relative to a variety of loads. This is significant because accurate overload detection can help save you a lot of blown speakers. It's also interesting to note that the clipping levels into 8 and 4 ohms are +2.5dBU (1.0V) and +2dBU (975mV) respectively. In other words, the 0dB LEDs light somewhat below full power, and the red error LED corresponds to the exact clipping point, which is normally taken to be the real zero level. This is a good feature, but it's one of several details that the instruction leaflet leaves you in the dark about. The one failing of the meter section is the location of the level presets. These are readily accessible with the lid removed, but this could prove very annoying with the unit rack mounted, as you'd have to remove it from the rack to re-align the meters. Ideally, Fostex should make these adjustments accessible from the front panel. Failing this, two holes neatly drilled at the front of the top cover plate would ensure easier access; you'd only need to withdraw the amp about ¾", instead of removing it outright.

Accuracy



The stepped attenuators are top-grade, custom-built units from Alps. Accuracy is their keynote, because regardless of setting, each channel is kept matched to within 0.25dB, this figure being confirmed by our measurements. Compare this to a standard stereo pot gain, which at low levels can fling one channel 6 or 7dB out-of-balance. Resolution is a sensible 1dB, over a 20dB range. This alone won't permit a dead accurate balance. However, once the balance has been set, and with a little practice, you'll be able to make speedy adjustments to levels with the Fostex, without having to reset the balance. A nice touch is the infinity position at the last step, below the -20dB setting. This is handy when you want to quickly, precisely and totally mute the input without turning the amp off. Unlike conventional pots, there's no feedthrough at the channel 1 signal to both outputs minimum setting; the input is physically disconnected. I'd also like to stress that these are real attenuators, and bear no relation to the 'click stop' pots (which are imitations) found on some music centres.

Mains switching is with a pushbutton, with a red LED to confirm 'power on'. Moving on to the rear, power enters on an IEC (Euroconnector) socket - which incidentally sports exposed terminals on the inside. The mains fuse is also inconveniently sited at the back. Of course, this particular fuse shouldn't blow unless there's a serious fault, but fuses do have a habit of breaking spontaneously after a while, and pulling heavy power amplifiers out of racks is no fun! Short of moving the fuse-holder to the front panel, you could wire the mains via subsidiary fusing on a readily accessible mains panel.

The audio terminations are relatively generous, facilities for both jacks and XLRs being available at the input. On the XLRs, pin 2 is wired hot to the IEC standard. Link-out connectors are provided as well, so the input signal can be daisy chained (linked from one amp to another), whether cables have jacks or XLRs. However, it seems unlikely that you'll want to do this in a control room; it's really a PA feature.

Next door, there's a mono/stereo switch. Again, this is a facility which should be accessible on the front panel. If for any reason regular mono/stereo switching is part of your routine, it'll be necessary to make other arrangements. Also, the instruction leaflet fails to describe the mono/stereo switch 'logic'. It doesn't power bridge, but overrides channel 2, sending the channel 1 signal to both outputs according solely to the channel 1 gain setting, ie. the other attenuator is overridden.

Lastly, the output connections are made via gold-plated binding posts. These terminals will also accept 4mm banana plugs, but the RS type usually used in the UK make less than perfect contact, so you may prefer to clamp down bared wires under the terminals. Parallel XLR outputs would certainly make life easier, in this instance, if it's necessary to be able to detach the amp quickly from its 'moorings'. Fortunately, there is an easy solution at hand, for assuming that you're unlikely to need the XLR link-outs (male connectors), it's easy enough to rewire these to their respective outputs.

Interior



Fostex distributors Bandive stress that this amplifier isn't intended for the rigours of use on the road, but insofar as the lack of machismo is mechanical, only the PCB supports are clearly not up to scratch.

Another area of more general, if mild, concern is the absence of insulation on the (mains) IEC connector, plus there are a number of other uninsulated terminals taking mains to the toroidal transformer and thermal breakers. It would be a good idea for Fostex to cover these with insulation boots, for whilst there's nothing necessarily illicit about bare live terminals inside equipment, some people do have to poke around inside when juice is present. Pending this, reviewers and readers with inquisitive fingers should beware!

Good Design



Despite this, the packaging and construction are excellent, and in line with 1983 design thinking in the UK; for instance, there's lots of evidence of attention to fine detail, something which is less prevalent in older designs.

The power transistors, for example, are mounted in sockets for ease of replacement, extensive use is made of Pozi-drive self-tapping screws, and the input cables are insulated with (heatproof) fibreglass sheathing as they pass alongside the heatsinks. None of these features are mere cosmetics - they're not even visible to the average onlooker; no, they're a sign of good design.

This is mirrored on the circuitry level. For instance, the critical high-current cabling is by and large loomed for minimum distortion, to audiophile standards, and although terminated with 'faston' lugs, the crimping is of top quality, and should be better than the average soldered joint. The PCB and power device sockets are wire-wrapped, and again the work is immaculate, although it can make repairs awkward, as wires can't be readily removed, nor snipped back. However, the main PCB is accessible from both sides, so unless it needs to be replaced outright, this particular problem shouldn't arise - we hope!

Power



Coming back to measurements briefly, the amp seemed well protected (long term reliability is another matter - that we can't test for). When driven at full level into a load significantly below 4 ohms, the red error LED comes on, and eventually, a relay disconnects the output (this same relay incidentally disconnects the speakers at switch-off, and delays reconnection at switch-on, for a few seconds, to allow the system to settle down, so avoiding bangs and clicks). Although specified for 2.5 ohm operation, the amp wouldn't drive a 2.5 ohm load at zero level (0dBU input) without the error LED and relay leaping in. There's nothing unusual in this though, and the amp is perfectly capable of giving its quoted power into 2.5 ohms; it's just that the input level needed to drive to full power at such a low impedance is now 10dB or so less in common with many other power amps. This does not necessarily mean you can drive 2.5 ohm speaker arrays with the Fostex though, for also in common with other power amplifiers, the ability to drive into 2.5 ohms without undue limiting distortion is best looked upon as a safety factor, to cover impedance dips in 4 ohm systems.

The frequency response was 0.25 dB down at 30Hz, and rolls off sensibly in the supersonic region, being 4dB down at 100kHz. However, the level is 0.4dB down at 20kHz, and accordingly the amp is marginally less bright than other, more accurate models. This error is only significant insofar as your system may already incorporate a top-end deficit, and with a good graphic equaliser you can apply the needed correction.

Despite a quoted spec, of -97dB, noise was measured at -75dBU over 10Hz ot 100kHz bandwidth, or -84dBU up to 10kHz. This discrepancy is probably allowed for on the basis of different noise measurement techniques. Besides, in practice the amp is quiet enough so that with the desk muted, hiss and hum are low enough not to be a nuisance unless (a) your control room is deadly quiet, and/or (b), your monitors are very sensitive, horn-loaded types.

Conclusion



Used within its limits, the Fostex FTX 300 compares favourably with other professional, yet Hi-Fi power amplifiers, and any audiophile-audible colourations it exhibits (other than a marginal dip at the top end) aren't gross enough to be characterised. This is often not the case with oriental amps, and Fostex are clearly into the same league of finesse at all levels as their USA and UK competitors. One snag is that this amp hasn't yet established a UK track record, although it has been around for some 18 months, in Japan, for example. And from the angle of workmanship rather than design, the best Japanese quality control is now good enough to be largely beyond reproach. No, the main concern relates strictly to repairs in a few years hence - there's no guarantee that parts will be available. This doesn't necessarily render the amp irrepairable, but it will push up repair costs, a point to be kept in mind whilst comparing prices. Although costing more than a similarly rated amp such as the Quad, take into consideration the controls, metering, multiple socketry, and error indication. None of these are vital facilities, but they do make life easier, prevent damage and allow you to monitor with greater finesse.

Recommended retail price of the FTX 300 is £458.85 including VAT. UK distribution is handled by Bandive, (Contact Details).



Previous Article in this issue

Pearl PM-66 Programmable Mixer

Next article in this issue

Home Recording With Digital


Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Dec 1983

Donated & scanned by: Mike Gorman

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Gear in this article:

Amplifier > Fostex > FTX 300

Review by Ben Duncan

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> Pearl PM-66 Programmable Mix...

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