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2x100W HMOSFET POWER AMPLIFIER
The Ambit HMOSFET Mk III amplifier The HMOSFET amplifier is based on the design published in ETI - and generally conceived to provide an alternative to those DIY enthusiasts who find other kit amplifiers to be unecessarily stark and generally 'unprofessional' in terms of design and finish. It is not cheap, for it is designed to last - with large reserve margins where appropriate, and fully tried and tested performance, into a .variery of load conditions ranging from laboratory resistors, to real loudspcakcrs ... .. and ears. For it is not very difficult these days to produce amplifier _designs with excellent measured performance, but there is still a discernible difference between those that sound 'alive' and those suffering from a variety of minor dynamic failings that add up to something not quite satisfying. But since it is not possible to make an amplifier that sounds good without also producing good test results, here are the HMOS p'arameters: Output per channel into 80hms
into 40hms 0.1% THD 20Hz to 20kHz both channels driven Distortion from 1 OW-100W output into 8 ohms
IMD and products
Slew rate Voltage gain Input for 100W output
63Hz 1kHz 10kHz 20kHz 1kHz+10kHz 10kHz+11kHz 19kHz+20kHz
100W/80hms
Residual hum/noise into 8 ohms (unweighted) Offset voltage
123W 156W
0.03% 0.03% 0.03% 0.04% 0.04% 0.05% 0.06%
40v per uSec 22x 1.4v 230uV 3mV
All the above values are RMS - not many people still try to confuse the issue with peak powers - or worse. Having established that the amplifier competes with the best in the field as far as the 'numbers' are concerned, it is very important to establish the dynamic behaviour beyond the limited insight afforded by IMD measurements. The tone burst test is quite revealing in many cases - showing up bad
PS U design as much as anything. The illustration below is taken using the Mark III preamp, and switching the input selector line with the squarewave shown on the bonom trace. The input to the PA is the top trace, and the output is the middle trace. You will see that there is no discernible difference - with even the minute glitches being faithfully reproduced. (It also serves to demonstrate the click-free nature of the preamplifier input switching circuitry.)
With PSUs rated to more than twice the nominal output, overload recovery is instantaneous - which combined with the basic amplifier capacity of 100W+ makes all domestic listening completely strain free. There is no substitute for capaciry - as with car engine design - and although smaller and more frantic amplifier designs can be made to 'sound' big - it's still like trying to squeeze another drop from the sponge rather than just pouring from a jug.
To keep everything going without fuss, a relay is used to connect the loudspeakers a short while after switch on -and also to disconnect the loudspeakers if any fault condition occurs that disturbs the DC offset voltage on the speakers. The pilot lamp flashes during the time the relay is open, to provide visual indication of the contact state - should the power to the protection unit fail, the relay will automatically fail safe.
--------------------------------~~--------------------------------------------------------------ambit international 200 North Service Road, Brentwood. Essex. CM14 4SG (0277) 230909 Eiil
m Catalogue part 4 Amhit International
Two sets of loudspeaker terminals are provided on the rear panel - with switching via heavy duty mains switches using solid silver contacts. The relay terminals are gold plated, since there is a greater danger of arcing occuring .
Facilities existing for both AC and DC input coupling, also 600 ohm input may be used via a Cannon socket (extra) instead of the standard phono inputs.
Further information available on request - inc. the construction manual if you want to check out the skills needed to construct. (£2.50 - refundable on purchase of complete kits.)
100W into 80hms at 1kHz 100W in 8 ohms at 10kHz
The reliabilit y/performance t radeoff The ultra low impedance of the bipolar power transistor is both its strongest and weakest point. Good because low output impedance in audio design means that the loudspeaker load impedance curve is flattened out and thus presents a far more constant response with regard to frequency - but bad because the device can be blown instantaneously under adverse conditions is the device is turned on 'actoss the rails' by HF transients or instability. For a high powered bipolar amplifier design to be reliable, there is inevitably some comrpomise in the form of output current limiting which inevitably means additional resistance in the output. Without additional resistance, the only way to provide protection is either to grossly overrate the output devices - or to limit the PSU capacity. Limiting the PSU capacity is also going to have unwanted results in transient handl ing - and so attempts have been made to try to get at the output protection through other means. None have been able to work fast enough to save the output devices (or the output devices are themselves too slow !!)
The resid ual dist ortio n
Sho uld you attempt to make one ?? One of the most frequently asked questions concerning Ambit DIY Hi Fi concerns the degree of expertise needed to make it work. The HMOSFET amplifier is suited to construction by the reasonably experienced - and is as much concerned with being abl e screw the chass is and hardware together correctly as being abl e to sold er. If in doubt, get the instruction book first. We will endeavou r to sort out any problems that may arise - but provided reasonable care is taken to be as accurate as possib le, then no specia l equipment or skill is needed. Amplifier adjustment is very straightforward thanks to the simple and repeatable design now proven in nearly 1000 of the PA 101 B amplifier modules.
10dB per division vertically, 5kHz per horizontal division. Not the most revealing test - but one that will certainly embarrass a large number of designs. Much of the art of producing good THD at high frequencies lies in the correct routing of PC layouts as much as the basic circuit configuration. A lot of the skills and tricks of bipolar design have been evolved to overcome the basic problem of power bipolar devices - namely that they are rather slow, and thus require careful frequency compensation networks in the negative feedback loop. The inherent speed of HMOS means that most of these techniques are now unecessary, and that the design can revert to a more simple and easily understood configuration.
The HMOSFET possess inherent current limiting and thermal stabilization - which has allowed this amplifier design to provide direct connection of the sources of the output FETs to the load (via the relay). If you want the lowest possible impedance connection, then you may wire directly from the amplifier modules to the output terminals - although the dangers of DC offsets and switching transients make this policy very preacrious for the loudspeakers. The HMOSFET amplifier will drive two pairs of loudspeaker.; in parallel if required - and if we want to demonstrate the output reliability by drawing sparks from a short circuit, the gound connection needs to very good before the output from the speaker concerned becomes noticeably distorted . For very vigourous applications, PA modules with four output devices are also available - but we strongly suggest that you will be hard pressed to use this additional capacity in domestic applications (150W into 8 ohms with existing PSUs)
One of the most revealing pictures of all is what's left after the fundamental sinewave has been notched out. However, since until some of you lot buy an HMOS amp we cannot afford a new low distortion oscillator - the measurements are somewhat limited by the residual distortion of the test oscillator. Disregarding the vertical gain shifts between the two traces, you will see that the output distortion closely follows the characteristics of the input distortion. There is no evidence of crossover spikes frequently claimed endemic in MOSFET design.
Residual distortion of audio oscillator (0.03"10) Residual distortion 10W into 80hms at 10kHz
m ambit international 200 North Service Road, Brentwood, Essex. CM14 4SG (0277) 230909
AMBYTE - Ambit 's microprocessor consultancy group When calling/writing. please address enquiries to Jonathan Burchell c/o AMBiVTE division.
Introduction Ambyte is the addition of new digital and 'processing ' skills to Ambit's already well established analogue and RF expertise. This combination of disciplines means that as a design group, we are uniquely qualified to develop processor based products that interface with the outside world of RF. The problems of RFI from a processor controlling functio ns within an RF sensitive enviroment (synthesisers etc) are not at all trivial - and it is unlikely that such designs can be undertaken by anyone who did not possess sufficient skills ' in-house' to design, develop and manufacture the complete system in-house. Coupled with this speciality, we feel that this approach sets us apart from other consultancy groups which are largely set up as the front door to large system 'houses' - or distributors of specific manufacturers products. Being so closely associated with Ambit, prototype developments can be produced from existing stock parts at great speed. And the general commercial awareness imparted through this association means that designs will include readily available and cost-effect parts, hopefully preventing the spawning of DoDos. A good example is the range of keyboard parts that we stock -enabling fast design capability for any type of keyboard - with off-the-shelf parts and key tops. And as distributors of components for keyboards, we are mindful of market developments and trends when making our selections.
Sloo hard disk interface Enables any S100 user to attach the versatile Micropolis Winchester disc system - turning a microcomputer system into a serious tool for both commercia1 and industria1 applications.
interfaces for oeher forms of databus - complete with canridge b~ckup and operating system development are available to suit your custom requirements
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What can AMBYTE do for you 717
Essentially. AMBVTE is an OEM and industrial facility intended to support the expanding range of Ambit digital products - such as the frequency synthesisers, digital plotter systems, serial data LeD decoder and display modules etc. But we also offer the range of Periflex 5100 Meus with data canridge backup, hard disk capability and a wide variety of 'serious' small computer peripherals. Software skills are available to support the hardware capability, and these include programming in assembler and high level languages for most of the available MPUs (8bit), including' Z80, 8080, 2650, 6800, 8048 etc. The software skills are essential fo r the correct interface of various hardware configurations· and projects undertaken include disk operating systems, peripheral interface drivers, plotter control - and the inclusion of different drivers into operating systems such as CP/M and CDOS.
The increasing importance of hard/fixedlWinchester disk technology in the microcomputer is one of Ambyte's specialities. Cartridge backup, and bus interface controllers are available for a variety of systems and applications.
Disk systems As well as the hard disk systems mentioned above, we offer standard 8" and 5.25" floppy systems. The Micropolis hard disk system is probably one of the most reliable available, as well as offering a host of carefully considered features and facilities - including: MPU based controller, fully asynch. buffered data transfers from host to controller - with automatic error recovery procedures, variable sectoring and sector size, the ability to transfer whole tracks at a time. Spare sectors are provided to cope with media defects. The interface is via an 8 bit bidirectional data bus and 9 control linesa coupler for the S100 bus is available for the drives (which contain from 7.1M to 35.69M bytes of information - formatted)
Micropotis 1220 series Up to 35 .69 Megabyte of formatted storage with Micropolis reliability - average access time 34mSec., transfer rate 933kByte per second.
High resolution (24 bit graphics/machine contn
A most comprehensive solution to rOb! machine tool and plotter positioning.
Fibre optic 200 and 500MHz data links An ideal combination of digital and analogue expertise for monitoring remote data aquisition systems under conditions of severe(up to 100kV/m pulsed RF) EM!. Optional IEEE 488 bus for computer/ instrument interface. The remote rechargable NiCad powered transmitter is fully shielded -and operates up to 500m away from ehe receiver.
rn ambit Internat ional 200 North Service Road, Brentwood, Essex. CM14 4SG (0277) 230909
