Pre-amplifier designs | Power amplifier designs | Speaker designs | Mains-borne interference | Radio interference

Some constructional tips;

Anything directly connected to the mains is potentially lethal and must be considered so. Handling of live components is not an option and care to insulate exposed connections within the case must be made. At all times, before attaching instruments and handling components, first switch off the unit and discharge the electrolytics in the supply via a 15R, 5W resistor. Take care to discharge across the capacitor's terminals, and not to chassis.

All tools should be insulated and care taken when moving tools around inside an amplifier; the metal shaft say of a screw-driver falling on a live supply and power stage can result in much consternation and expense, and in a very short space of time. One chap, who did this after nonchalantly waving a screwdriver over a brand new unit, opened out of idle curiosity, was upset to find that human-induced faults are generally far more complex and costly than 'natural' ones.

Never attempt construction or repair unless you know how to use a soldering iron and multimeter. Damage and frustration, or worse, will be the only result otherwise. Building from scratch with no prior experience can be done, but with patience.

Never attempt construction or repair when intoxicated. One former 'engineer' friend simply left a trail of destruction in his wake which helped nobody, even those in wheelchairs. A classic case of 'what one believes is more than one knows', keep the intoxication for the listening. Taking something apart is easy. Putting it back together again so that it works, perhaps even better, is something else and may not suit everybody.

If you start making mistakes, stop.

Two design papers that give useful advice are those by Linsley Hood ('Class A/AB mosfet power amplifier', E&WW March '89) and that by Mark Alexander ('The Alexander Current-Feedback Audio Power Amplifier', Analog Devices Application Note AN-211, Audio Products 4-57/71) which contain much detail including PCB, component and wiring layouts. Douglas Self's "Audio Power Amplifier Design Handbook" and "Solid-state guitar amplifiers" by Teemu Kyttala are excellent sources. "High Performance Audio Power Amplifiers for music performance and reproduction" by Ben Duncan covers a wide range.

Pre-amplifier designs

A small selection of designs from the 60's and 70's and a fair range of RIAA stages.

J L Hood; Many of these designs were available from Hart Electronic Kits Ltd, Penylan Mill, Oswestry, Shrops, SY10 9AF, UK, the top-line RIAA preamp being the K1450AS. A simpler version can be found in his modular preamplifier covered by Wireless World, October 1982, an excellent source of food for thought.

Designs published in Elektor by T Giesberts (see below) are recommended also, although their reliance, in some instances, on selected components may deter some.

See also datasheets for low-noise opamps like the AD797, TLE2071, LT1028, OP-47, NE5534, LM833A, SL561 and ZN459 linear ICs, which can usefully reduce the component count.

Some purists, especially moving-coil owners, maintain that a direct connection to the input transistor is best. Others, that a transformer's presence is distinct. Some form of LF blocking is considered desirable to attenuate the components imposed by the turntable which can result in alarming speaker cone excursions resulting from modulation of the desired signal. JLH mentions modification of the RIAA curve in the WW article mentioned above.

When testing quantities of components for matching, selection, etc, obviously a large batch will yield a larger number of close matches. However, say merely doubling the small-signal transistor count can yield useful results.

An interesting high-end pre-amplifier design (low-noise, wide-band, no eq/filtering) by Giesberts ('Battery-operated pre-amplifier', elektor, 1/97 p14 and 2/97 p36) offers interesting solutions. Match this with the 'AF input module' (elektor, 7-8/97 p80) and 'AF input selection' (p103).

Power amplifier designs

The high current slewing conditions when transient signals are fed into difficult loads can produce higher order harmonics as some output stages 'catch-up' with the demands made. These tend to 'harden' the overall sound. The gradual non-linearities in class A designs where the output devices continuously operate in the linear collector region are held to produce lower order (2nd and 3rd) harmonics commonly found in musical instruments. Because of the high quiescent currents involved, quite substantial hardware arrangements are required for outputs of 10W or more, compared to class B operation.

J L Hood; "80-100W MOSFET audio amplifier" Wireless World, August 1982 (and corrections WW, Sept '82, p63).
"80W MOSFET audio amplifier" ETI, May 1989 (note missing feedback resistor), sold by Hart as the K1100 series.
See other designs by this author.

T Giesberts; "Active 3-way loudspeaker system", Elektor, July/August 1993 (PCB No 930016).
"Medium power HEXFET amplifier", Elektor, December 1993 (PCB No 930102).
Recommended: "Compact power amplifier (fast high-end design)", Elektor, May 1997 (PCB No 970043-1).
"High power power amplifier", Elektor, February 1999 (Diag No 990001-11).
See other designs by this author.

Recommended: An interesting and adaptable fast amplifier design covering a useful range of output powers appears in the datasheet for the SSM-2131 opamp, by Precision Monolithics Inc.

Another, more complex, fast design by Giovanni Stochino appears in Electronics World, August 1998. Some constructors have reported difficulty in obtaining 1N4448 diodes for this design. This arises from a typo in the text accompanying the circuit diagram. The diagram proper shows 1N4148s, the high voltage (HV) types being selected 'by applying a reverse voltage of 130V via a 10k resistor and measuring the current, which has to be less than 10mA. The yield is normally higher than 50%'.

A selection of compact and inexpensive designs that have drawn favourable comment can be found here.

If pressed to name power amplifiers that have 'delivered', the RSD 800b (400+400) used at the test venue must figure largely, since this is a very class act to follow. Indestructible, this one amplifier alone for years, driving four 18" scoops, must rank amongst one of the world's most powerful aphrodisiacs, being no doubt responsible for innumerable pregnancies, and much else besides.

Next must come the very compact Technics SE-9060, an array of 4 of which certainly deserved a VC for the punishment and abuse it suffered. Together with it's companion SU-9070 stereo flat preamplifier, SH-9010 equaliser (a beauty), SH-9020 meter unit, RS-1500US open reel tape deck and RS-9900US cassette system, which all behaved faultlessly, these can truly be described as decent bits of kit, if now appearing much dated. This rig drove 2 w-bins and 4 portable flown clusters + satellites via a discrete custom active crossover, a combination that proved punchy and eminently mobile.

If something less complex is sought it is felt that, in the author's and other's experience, special mention must be made of an Hitachi FET application that is very definitely worthy of examination.

Having being pressed repeatedly for views of the most impressive hi-fi one is surprised by how one is drawn to the external design. In no particular order, one thinks of the Cambridge P series, the Armstrong 600 series, the Lecson AC1 and AP series and late 70's B&O. Of this era, groundbreaking Nordic 'minimalist' and British 'technical' were superceded by American 'aerospace' and Japanese 'efficient'.

"Old School"

Much can be learned from early designs, again the higher end of the markets exhibiting better standards of workmanship. Design philosophies and priorities are then seen to vary over time. Since 'rule-books' had still to be written, as it were, new approaches cropped up, whether it was valve, germanium, silicon, hybrid, Darlington, diode-switching, paralleled output pairs or splitting the RIAA eq between three stages.

Interestingly, it would appear that the manner and forms in which amplification and equalisation can be applied are as diverse as language.

The grand-daddy of all integrated solid-state stereo amplifier designs has to be the very instructional Dinsdale MkII that appeared in Wireless World (1965). A diverse range of other publications encouraged practical innovation, including Practical Electronics, Practical Wireless, ETI, etc. The '70s are generally seen as the 'era' of amplifier design, many famous designs and much important work being done in this decade.

Gibb's and Shaw's excellent Gemini (PE, '70/1) offered much in this context, compared to the Quad 303 of the same date. The lower end of the market supported a lot of kits, customers being attracted by cost savings, and even simple designs became popular. A fair number of the small and very popular Texans (PW, '72), whose approach was copied extensively, put in an appearance, a modifed example giving many years of robust service. Kits and designs by people like John Linsley Hood and Stan Curtis became available. Modules by Crimson, ILP, Maplin and others eased construction, many building their own simply because what they wanted could not be readily obtained.

Methods used for output protection can be interesting. Notable is the Lecson AP3 (inverting) with power transistors in series (a layout expressed in Darlingtons by B&O in their 4400), the output stage then being capable of safely dissipating all of the energy available from the power supply. Another Stan Curtis design, the ETI System A (inverting, August '81) has a very 'meaty' output for domestic purposes (90A, short-term duration). Then there's the Cambridge P60/P80 series with FET limiting, many lessons about internal dissipation being learned from the P50.

The AP3, together with it's AC1 preamp ('74), and now very rare FM tuner, made for a very striking appearance and performance, for instance, employing FET signal switching. Mechanical switch contacts became the bane of an enthusiasts' life as did the DIN connector convention.

Quad made a name for themselves with some very striking cabinet designs that still prove popular, although their electrolytics were prone to failure and the predilection to 'modify' (extensively!) a design disappointed some customers. Sometimes described as 'the best amplifiers around', detractors abound. For example, although the 34 preamp was nice to look at, one never quite understood why the mono switch, despite convention and previous designs, was moved to the middle of both slope filters, after the volume, tone, balance controls and filters. Again, criticism was possible in respect of the quality of the components used. The matching 405 power amp earned the nickname 'the pig in the poke' in some circles using a surprisingly 'old' opamp (LM301A) for it's input and 'slow' output transistors (BDY77), although Bernd Ludwig's excellent mods (1999) are definitely worth a look, as are those of others motivated by the need to improve these designs. A good web-site for data can be found here.

An excellent site carrying PDFs of the service data of powerful 'studio' power amplifiers can be found here.

At the other end of the scale, one recalls a range of (comparatively) tiny amplifiers where in order to remove the induced hum from the mains transformer, AC was fed back into the signal path cancelling the induced noise. Not a purist approach perhaps, but certainly simple, innovative and effective. Then there's the so simple 3 transistor germanium jobby that continues to give more than 40 years of virtually constant use. I've even seen a Russian 2-transistor class A!

A popular, inexpensive and rugged small design, the Edwin amplifier, appeared in the 1975 Elektor Book 75 (ISBN 0 905705 00 9). Fed by another Elektor design, the Preco preamp (May '76, featuring a wired remote control facility and variable width control), this was a combination that seemed to appeal to many, especially if equipment was to be hidden or not readily accessible.

At about the same time, the A&R Cambridge (or Arcam) A60 appeared and proved popular to the extent that one user offers advice to upgrade it. This, as a commercial design, differed from the norm with substantial smoothing, switch-on muting and a current-limited and clamped DC output. However, as with Quad and the Edwin, the use of quasi-complementary output pairs could contribute to low-level distortion.

A good spread of robust bipolar designs appears in the RCA Solid State '74 Databook Series; Power Transistors and Power Hybrid Circuits (SSD-204B), adaptations being found in many brands of combos, hi-fis, etc. A sturdy, and inexpensive, 70W version (to suit a 'youth' environment) appears on page 600 and a more sophisticated 120W version on page 618. These designs feature excellent protection and quite conservative distortion specs, however, as with all data, read the circuit diagram carefully first. The main negative smoothing capacitor for one design (on page 611) is shown reversed. Wiring as per diagram would probably result in a loud bang and not much else.

At the same time, a web-page shows a 30W FET JLH design whose speaker out is taken before the output capacitor. Such an arrangement is not good. It would probably sound crap, and cook the speaker. Another JLH design appeared in ETI (see above) with no feedback resistor, making a pretty complex, but hefty, buffer! Naturally, I am not innocent of this sin, so beware.

Although 'headroom' is nice to have, many enthusiasts often like low power designs (<20W) whether they be bipolar, valve, FET, class A, etc. This doesn't surprise since even an output of some 3W can have neighbours rushing to complain, hearing being logarithmic in respect of volume. This means that to make a system sound twice as loud, you need ten times the power. The modified Texan, mentioned above, though tiny, drove two immense speakers, and did very well. At the test venue mentioned above, the VU meters on the power amplifiers rarely, if ever, registered above -10dB (10% of available output), even when 'the joint was jumping', -15dB (3%) being normal.

From the eighties, three designs are memorable. The NAD 3020 (1981), though popular with 'audiophiles', possibly because of it's 19" rack mount case and LED output indicator, relied on a common and slow output pair (2N3055 & 2N2955) and made interesting claims in it's brochure that could make one smile. For example, the supply (±28V) is described as 'high-voltage and high-current', although being unexceptional and normal for a low power amplifier.

The perhaps simpler Sugden A48 mkII (1983) was noted for it's 'heavy-duty' retro approach paralleling meaty TOP3 devices (TIP35/6s) and reverting to AC output coupling, this capacitor being double the value of the main smoother. Drawbacks, apart from the inclusion of high-voltage high-value electrolytics, included the transformer having two secondaries (one for each amplifier) which, on occasion, proved problematic when seeking a replacement, and the Zobel / Boucherot network being placed after the output relay.

The Cyrus 1 (1988), although a more complex, high quality build, the emphasis on compactness and 'modernity' were felt to detract from the function. Virtually all components fitting on a single PCB, this used high voltage fast switching devices (BUV28s) in it's output and an interesting 'soft-clip' feature. The RIAA stage differed interestingly from the norm using low-noise NE55** ICs, as did the innovative case, the plastic top half of which was considered flimsy. Heat-sinking was limited and a steel case would have helped with screening.

Armstrong made some excellent designs that proved to be reliable, such as the 600 series. Later designs like the 700 series, though rare, are certainly worth a look.

Interestingly, and if memory serves correctly, none of the amplifiers mentioned above employed either mains filtering or soft-starts, and notably (even when readily available) no V-FETs.

The use of amplifiers that operate in bridge mode (effectively doubling, at least, the power output) can result in catastrophic confusion for the inexperienced, especially when multiple arrays of speakers, and their attendant leads, are involved. Simplicity is recommended.

(thermionic valves)

Based on specifications alone, it can be seen that some valve designs offer a greater dynamic range than some semiconductor designs. However, experience has shown that the view that valve is superior to semiconductor has arisen principally from guitarists who enjoy the distortion of 'soft-clipping' given by over-driven valves, compared to the unforgiving and 'harsh' clipping of a semiconductor-based power amplifier design. This is made obvious when compared with the demands of, say, a keyboard, strings or sax player. Many poor guitarists will attempt to hide limited technique with a vast selection of pedals and effects, most good guitarists having at least some classical training or acoustic experience.

Some custom valve designs seen (especially for reggae afficionados) employing multiple arrays of KT88s, can be very impressive to look at. Especially when they dim street-lamps for miles around when switched on. However, it is felt that the 'good' attributes applied to the sound produced are due more to the speakers used, rather than the amplifier.

The much discussed Williamson (1950) design, it is felt, really only had the Partridge transformer going for it. Manufacture at this time was very limited, and the significance of individual efforts was therefore more profound. It is doubted whether Williamson himself even built a prototype. If he had, he would have discovered the very nasty instability problems built into the power supply alone. Identifying feedback (new for the day) and bandwidth of stages as important, it was a good start, but was very seriously flawed. Those who maintain that the approach is correct if followed exactly usually admit to succeeding only after great expense. Try it for yourself and you'll see why.

Having been the long-term owner of a Roger's Cadet and other 'nice' output stages using EL34s, 6CA7s, 6L6WXGs, 5881s, KT66s and KT88s, the precision, power, reliability and delivery of the bipolar Technics SU-V6, and many others, is found to be far more preferable whether the programme material is classical, reggae, house, trance, funk, jazz, punk or ballroom. The inclusion, then, of what amounts to a small electric cooker in the heart of what is supposed to be an amplifier appears nowadays to be unnecessary. Unless, of course, one aspires to be a rock guitarist.

If a commission arose for a PA system to be built from scratch the first choice would probably be a semiconductor symmetrical design such as that used for the SSM-2131 opamp, mentioned above, or perhaps something by Giesberts, Hitachi, or even a class D, thus negating the 'microphonics', inefficiency, hum, weight and more, inherent in many valve designs. The poor quality of build of some commercial valve designs has reinforced this view. For another opinion, click here.

Mains-borne interference

As with logic hardware some designs of sound gear are more prone to mains-borne interference than others. Even some good quality JLH kits have been shown to be sensitive, despite extensive disassembly and modification. The worst offenders appear to be central heating, motors, welding kits and flourescent lighting, principally because of the wideband noise burst associated with arcing being carried by radiation or the building's wiring. Lightning strikes can figure also.

One recalcitrant gas boiler was finally stilled by fitting a varistor and snubber network across the offending contacts, within the boiler's steel casing. Don't attempt this if you don't know what you're doing.

The inclusion of a mains filter makes eminent sense when using high-efficiency mains transformers and the use of varistors, or voltage-dependent resistors can improve immensely. However, many 'quality' designs may include a HF capacitor across their secondaries only.

Dual bidirectional thyristor overvoltage protectors like the Bourns TISP3600F3, or Littelfuse SIDACtor devices, can provide transient protection to signal and telecoms lines and so protect their insulation.

One situation, at an engineering shop where spikes of some 750V were crashing the firm's computers, was resolved by taking the mains feed from the supply at the fuse-board and using a succession of varistors (320V high energy, 275V and 250V) and a coil (2 x 4mm solid core, bifilar wound) on a old AM ferrite rod. Screening of these and a spiked earth helped reduce interference on the new supply line, but were unable to cope with 'brown-outs' caused by the main load dragging the supply down because of the inadequate feed to the premises. In order to overcome 'skin-effect', earth runs should NOT be via single solid-core cables but by a thick multi-strand, the thinner individual strands the better. High-voltage ceramics between cases and ground will further reduce the HF impedance.

Some projection TVs seen have 3 such coils, usually toroids, in their mains inputs. Large value mains polypropylene motor-run or oil capacitors can be wired across the mains after the coil. Some discontinued GPO PSUs I used to buy in for their big electrolytics had a magnetic trap wound on the transformer feeding a 36µF capacitor that was virtually the same size as the transformer.

Lightning is another matter all together. Notwithstanding aerials, mains feeds should run underground in areas affected, and those points above ground should have adequate arrestors at each end of the feed. In severe storms isolation of the supply might be considered, but by then, a separate means of generation might have been found necessary.

What has to be remembered is the sheer brute force of the energy involved. Whilst manufacturers can offer testing to say 8kV, this compares to nought when one sees a monitor screen exploded, the glass melted outwards giving an impression of something out of 'Alien'. It can also leave interesting intricate fern-like patterns all over the place, whilst shorting, if not exploding, everything in sight.

Equipment near a lightning strike may still function but over time will experience a number of failures of components then stressed. An economic judgement may have to be made to scrap these items, rather than continually repair them.

Radio interference

Some of the best RF protection techniques seen have been found in cheaper Japanese makes, which probably have greater chance of operating in close proximity to a transmitter. For new builds combined mains socket / RF filters are readily available; a steel case is best for screening. The addition of small caps (100pF-1nF ceramic) between the case and input earths at point of entry can help, although severe cases may require ferrite beads or small chokes in series with the inputs. Combined 'three terminal capacitors' are available. These combine two coils in series with the signal between which, usually, a ceramic capacitor is grounded. Useful in communications and on computer boards, for audio use a high value, say 10nF. Check any RC output loading for open circuits. Earthing, if available, should be as substantial as possible.

One client lived so close to a powerful transmitter that the surfaces in contact between two connectors acted as a detector, with consequent garbling of the desired signal.

One household had a son who was 'into' CB. For unbroken hours at a stretch this individual would commit his thoughts to the air-waves, nobody apparently replying. His use of a speech-shifter, thus garbling but leaving the speech intelligible as such, was obvious to all living in the area since his signal broke-through into everything. TVs, small radios, stereos, baby alarms, intercoms, even keyboards were all affected to the point where they could not be used. Pater clearly couldn't give a hoot, and the authorities were slow.

Such mismanagement of emissions can interfere with other far more critical external systems and loss or injury could occur, not to mention the large number of people who are thoroughly imposed upon by such crass callousness. An easy way to temper the problem is to drive a nail through the antenna feed. Done with care, this can take out a number of RF power amps before being detected. However, one morning their front garden was found to be miraculously devoid of it's proud, immense and imposing crop of pampas grass heads, all of which had suddenly disappeared completely without trace. The interference, which had lasted months, just as miraculously ceased the same day.

The use of similarly annoying mobile car-mounted rigs and sound systems can be reduced by the discreet introduction of a whole mackerel into the outlet hose of the offending vehicle's heater system. A 'wobbulator' built into a portable radio decimated the AM reception on a quiet beach, operators of radios being forced to turn them off because of the racket.

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