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HH V200 MOSFET Amp

Amplifier


As of early January, HH passed into the Receiver's hands for what seems to be the last time, by all accounts. This amplifier dates from 1979, so why are we reviewing it? 3 good reasons:


1) V200s will be available (and cheaply too, one hopes) for some time to come, as distributors sell off their stocks.

2) The inherent longevity of MOSFET amplifiers means secondhand amps will be around for some time to come.

3) And last, the HH 'V' series MOSFETs have been very popular in recording studios since their introduction.

Design History



The early HH amps used ordinary Bi-polar transistors like everyone else. The TPA series, introduced in the early 70s were reliable workhorses, but didn't exactly excel in respect of sound quality. In 1977, their first high-power 'super-amp', the S-500D appeared. In practice, the early versions (1977-78) failed monotonously, and often put 60 volts DC across the speakers at the same time. By 1979, these problems appeared to have been sorted out, but by now, audiograde MOSFET power transistors were readily available from Hitachi, and so HH launched their 'V' series, using these new devices.

Let's just refresh ourselves on the advantages of building amplifiers with MOSFET transistors. MOSFETs are largely self-protecting, and this in theory, at least, makes a more rugged, simpler and altogether more reliable power amplifier. Of course, V200s in studios don't as a rule suffer the abuse meted out on stages, but if you've ever had an amp 'go DC' spontaneously, and take out an expensive pair of monitors, any amplifier which is less likely than most to commit Hari-Kari is welcome. And MOSFET amplifiers are, as a rule, particularly unlikely to apply a high DC voltage to your speakers if anything fails inside.

And if the amplifier designer is good, it's also possible to produce a better sounding amplifier with MOSFET devices. Essentially, the top-end should be cleaner and more accurate. The absence of protection circuitry helps too: in conventional Bipolar amplifiers, protection against low impedances and short circuits was often responsible for all manner of nastiness in the sound, especially if your speaker's impedance dips too low. In this instance, the protection circuitry would jump in to limit the current/voltage to safe levels. If this gross limiting effect occurred repetitively, it could make specific instruments or vocals sound wrong, or at least, very distorted - which is scarcely helpful in the context of studio monitoring.

Facilities



At first sight, the V200 has perhaps the optimum combination of features for studio work, taking into the account the disproportionate extra cost of amplifiers with full metering facilities and the like. The gain controls are stepped types, but instead of the twenty 1dB increments (21 individual 1dB steps), the scaling is given in decibels, and is irregular like a conventional rotary pot. Turning down the gain from maximum, the initial steps are in 2dB increments, which is too course for interchannel balancing (stereo balance).

In addition, there was a cumulative error of 0.6dB, viz: both channels are inherently out-of-balance. This need not make a difference, but it certainly doesn't help matters. At lower settings, the increments become bigger still; the penultimate step is 10dB, from - 50 to -60. For the purpose of balancing and setting up the gain structure (sensitivity) of the monitoring system, a 20dB gain swing is the most you'll need, so more than half the control range on this amplifier is wasted, unless you're running a cassette machine (or some other source without its own gain controls) directly into the amp. Put simply, these controls are inferior to the stepped attenuators we've looked at in previous reviews.

Clip LED's marked 'peak' to tell you that you're overdriving the amplifier are sited above their respective channel gain control. These were found to be accurate to within 1dB when driving 8 ohm (or higher impedance) loads, but into 4 ohms loads, clipping is indicated by the LED 1.5dB after it's already happened, and the indication is over 6dB in error on a 2.5 ohm load. The significance of this is that many speakers rated at 4 ohms impedance exhibit impedance dips down to 2.5 ohms or lower at certain frequencies. And under these circumstances, the 'peak' LEDs will fail to indicate the resulting overload. You shouldn't, therefore, rely on them; no indication doesn't necessarily guarantee that the V200 is being driven within its capabilities.

Also on the front panel is an LED to indicate operation of an internal thermal cut-out. MOSFET output transistors don't suffer greatly from the effects of excessive temperatures as do ordinary Bi-polar devices, but high temperatures are, in general, bad news for any electronic equipment, so it's a useful safety feature, particularly as excess temperatures usually signify some form of abuse.

The V200 can operate in bridge-mode by operating an internal slide switch. An LED on the front panel lights to confirm bridge-mode operation. This raises power output into 8 ohms from 100 watts stereo to 200 watts mono. Running into impedances below 8 ohms in the bridge-mode is not recommended, but is feasible: on all Bipolar amplifiers, protection circuitry wil greatly reduce the power available in the bridgemode if you attempt to drive into illicitly low impedances, ie. there is no benefit and quite possibly severe distortion.

This being a MOSFET amplifier, however, there is no protection circuitry, so abuse is possible - and the amplifier's heatsinking will become very hot. The thermal cutout, then, acts as a last ditch form of protection - only one which doesn't mangle the music, for it switches off the mains power outright.


Bountiful



The components on the back come close to minimalist perfection. The inputs are clearly marked, and are truly universal, for both jack and XLR sockets are provided, and either may be used with either balanced or unbalanced sources. For balanced operation, you will also need to add the (optional) plug-in transformers, and use an 'A' gauge stereo plug if you're using jacks. On the XLR, pin 2 is hot, pin 3 cold. For unbalanced operation, a mono jack plug or a standard (pin to pin) XLR cable suffices; pin 3 is already grounded internally.

The output sockets are also a joy to behold - other manufacturers please note. 4mm terminals suit banana and bunch-pin plugs, or bared wires. Getting a good grip upon the terminals for secure clamping is easy, and neat wire dressing isn't so important as with some oriental 4mm terminals. In addition, the legend reminds us that for bridge-mode only, you (a) will be using the 4mm sockets and (b), the speaker(s) should be connected across the two red (+) terminals; the 0 volt (black) terminals aren't used. Parallel XLRs are also provided, so the tastes of most readers will be catered for.

Next, there's a groundlift option, by means of a removable link. This enables us to separate signal ground (0V) from the chassis, which is, in turn, connected to mains earth. Thus, with the link removed, the exposed metalwork can be safely earthed without inciting incidental earth loop hums and buzzes. If in doubt, it is best to 'lift' the ground in this manner. One niggle: the tiny link bar and its fiddly screws are easily lost. Perhaps the best safekeeping is to put them in a plastic bag and then securely gaffer the bag to the inside of the amp, away from anything that becomes hot, ie. behind the input sockets.

Mains voltage selection is of the basic 240/120 volt variety. Finer variations for 210V (or whatever) are largely superfluous when normal variations in the mains supply voltage are taken into account. The switch is accessible from the rear panel, but cannot be operated accidentally - you will need a screwdriver or coin. The mains input is via the normal IEC (Euroconnector) plug, and the clearly marked fuse values for 250V and 117V supplies respectively. These voltages differ from the 120V and 240 volts given on the voltage selection, but for all practical purposes, you can take 117V to mean 120 volts = North America, whilst 240V = Europe. Lastly, a pair of rear handles/supports make life easier if your V200 isn't rack-mounted.

Exterior



The case is strong, cogent and aesthetically very tasty - one of HH's higher achievements. The V200's basic size is 2U (3½") high but sadly it's essential to have a gap of ½" or more above and below the amplifier for ventilation, with the heatsinks being inside. So the actual rack size taken up will be at least 4½ inches, if not 3U (5¼"). From time to time, you may have to remove the V200's covers, to switch over to bridgemode, for instance, but unlike Japanese amplifiers, it's not necessary to undo 26 fixing screws to gain access to the guts.

One other neat point: the covers have a felt lining around their edges to prevent audible hum in the quiet (?) studio and/or domestic environment. This effect is particularly noticeable on amplifiers without felt if the lid isn't screwed down firmly. Essentially the powerful 100Hz or 130Hz magnetic field coming from the mains transformer can cause adjacent areas of steel to vibrate in sympathy.

The V200 is an easier amplifier to service than most of those reviewed so far, if not the easiest. The MOSFET power transistors are mounted in sockets, and can be replaced in a matter of minutes with the aid of a screwdriver. The main PCB - which, incidentally, holds the drive circuitry for both channels - is less accessible, but still easy enough to work on. Besides, in practice, failures in MOSFET amplifiers are usually limited to the output devices; components on the PCB are unlikely to fail (see 'Repairing MOSFET Power Amplifiers', E&MM, June 1983). Better still, all components are standard types. So, servicing in the future should present no difficulties. Even the MOSFETs, marked with HH numbers are standard Hitachi devices, and are available from power FET specialists like Pantechnic.

Looking now at the power supply side of the circuitry, most of the mains connections are sleeved, and those which aren't would not be readily brushed against by accident. As is (was) usual with HH, wiring is immaculately loomed, but I do not feel the layout contributes to sound quality: high current pulses in the supply rails are able to crosstalk into the signal ground. The DC supply rails are fused, which is bad practice in Bi-polar amplifiers, but not on the V200, for full rail voltage is unlikely to appear across the speakers (ie. a DC fault) if one of the fuses falls out, or fails spontaneously.

Falling out is not as unlikely as it sounds, as the fuseholders do not offer a secure grip. It's also likely that the contacts will oxidise in time, which will not improve performance. As these fuses should only fail if something drastic arises, soldering them in place is advised; a small dab of solder will ensure a secure connection. Don't forget to switch off before attempting this.

For balanced mode operation, a pair of valve-type sockets inside - one for each channel - suit a plug-in transformer. Otherwise, a 'jumper' plug links pin 3 of the XLR (etc) to ground for unbalanced operation. The amp comes set up in this latter condition. Two transformers were available: the 600 ohm model is only needed in the obsolete power matched systems used in the USA. For most purposes, the 10k 'bridging' model should be used, so your console 'sees' a high load impedance.

Performance



As with any competently designed MOSFET amplifier, the V200 outclasses most Bi-polar designs at the top end. Listening on Kord and Electro-Voice speakers, treble is smoother and more detailed. At the same time, the early versions of the 'V' series were criticised, along with all the other original MOSFET amplifiers (c.1979/80) for an atrocious bass sound. Partly, this was due to a poor power supply design - MOSFET amplifiers show up deficiencies in this region very easily. And some of it was undoubtedly psychological: MOSFET amplifiers are potentially very clean performers, and the improved accuracy in reproduction can actually be perceived as wrong because it's unfamiliar.

Having said that, I felt the bass on the review model seemed thin compared to other MOSFET amplifiers, and whilst I am still apt to believe it's all in the mind, two experienced drummer friends did disagree. Two things are certain though: there's no measurable roll-off in the audible bass regions, and in the top-end or midrange of a Bi- or Tri-amped system, the V200 is one of the best choices one could make.


The power output and sensitivity just before clipping, versus impedance figures are as follows:

700mV (just below 0dBu) gives nearly 50 watts into 15 ohms 650mV (-2dBu) gives 78 watts into 8 ohms

480mV (around -5dBu) gives 120 watts into 4 ohms

Thus you should calibrate your console metering to read 0dB at 650mV - or just below - if you're driving 8 ohm speakers, for example.

Frequency response is -3dB at 90kHz, which is the optimum, as it results in a less than 0.3dB loss at the highest audible frequency (20kHz), whilst at the same time, RF interference will be adequately attenuated in most instances. If not, opting for balanced inputs should help.

Conclusions



The V200 comes close to being a very good design indeed. It's small (unlike the Yamaha PC1002), it rackmounts (unlike the Quad 405), and it's also one of the easiest of amplifiers to service and maintain. And all the basic requirements are satisfied without incurring excessive cost. It has faults, of course, but given the basically competent design, this amplifier is the one that's worth upgrading. So, for readers with some DIY aptitude, here are 3 modifications well worth making:

1) Change the attenuators for 1dB stepped types, or a Bourns Conductive-plastic pot. This makes interchannel balancing possible.

2) Enlarge the power supply so that each channel is fed its own, separate bridge rectifier and reservoir capacitors. This will enhance stereo performance, especially at the bass end.

3) Modify the 'peak' LED circuitry so that it reads as a true clipping indicator. With this mod, there's less likelihood of unknowingly driving the amplifier into clipping and blowing speakers as a result. This is particularly relevant on speakers with a 5 ohm or lower impedance.

These relatively simple modifications will make the V200 the best all-round amp we have reviewed in HSR to date, even given reservations about the bottom-end performance. As the V200 is a very popular-monitoring amplifier, we may publish some upgrade notes if the idea seems popular; we await your letters please.

Pantechnic are at 17a Woolton Street, Liverpool, L25 5NH. Tel. 051-428-8485.

Price of the V200 is £413.54 inc. VAT, although bargains should be available if you shop around.

(Contact Details)



Previous Article in this issue

B&W DM110/DM220

Next article in this issue

Isotrack Signex CP44 Patchbay


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

 

Home & Studio Recording - Mar 1984

Gear in this article:

Amplifier > H/H > V200

Review by Ben Duncan

Previous article in this issue:

> B&W DM110/DM220

Next article in this issue:

> Isotrack Signex CP44 Patchba...


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