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 Elliott Sound Products

Frequently Asked Questions 

Updated January 2015

Over the past few years I have answered many, many e-mails.  Some ask questions of the circuits and articles presented here, but I also get a lot of general questions as well.  The biamp article still creates quite a few questions by itself, and along with the others, I have added this FAQ page.  Some of these FAQs used to be located in the Readers' Feedback page, and have been moved to make them more accessible.

Some of these questions are (almost) exactly as asked, while others are a 'composite' of many similar questions.  I shall leave it to you to decide which is which (assuming you care, that is :-) ).  A great many of the questions asked are already answered in the articles published on my website, so make sure that you look through the index of articles (and projects), because the info you need is probably already there.

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FAQ Index
Bi-amping / Tri-amping

Q:  Can I get a schematic for your phase-coherent crossover?

A:  Yes.  The Linkwitz-Riley versions (P09 or P125) are currently the favourites, and both can be found on the Projects Page

Q:  I am thinking of triamping.  What do I need to look out for?

A:  Be very careful of DC power-on transients, which will destroy tweeters.  A capacitor (not less than 20uF so as not to mess with phase relationships) in series will prevent damage, but preferably do not use an electrolytic (bi-polar or otherwise) unless you are willing to accept (possibly) compromised sound quality or (more commonly) relatively short life.  Note that many amps have a 'de-popping' circuit (generally a timed relay), which will eliminate the DC 'transient', but usually offers no protection against O/P device failure! (Read the updated section on triamping in the Biamp article) You might also want to have a look at the article on Class-A amps, which many feel are ideal for the top end.

It is also worth looking at a DC protection scheme, such as P33 or P111.

Q:  I want to biamp my system, but don't want to remove the internal crossovers.  Will this work ?

A:  Yes, but with some caveats.  You will not get the same power gains, since both amps will be reproducing the full frequency range, but you will get some degree of benefit in terms of intermodulation distortion, etc.  Overall, this is not the ideal solution, since it is almost the same as bi-wiring, but uses more amplifiers.  IMO it is a waste of time and money.

Q:  Can I biamp without an electronic crossover?

A:  No.  All you will be doing is 'active bi-wiring'.  See the previous Q&A

Q:  If I disconnect the internal crossovers in my speakers, what should I look out for?

A:  First, make sure that your electronic crossover is set for the same frequency.  Second, for a 3-way system with a 2-way electronic crossover, you have to keep the mid to high passive crossover section (unless you are going to triamp).  This will involve determining the circuit of the existing crossover, and only disconnect the low to mid+high section.  If you do not know how to do this, seek help from someone who does.  This is important!

Q:  My passive crossovers use impedance correction circuits.  How do I disconnect the crossovers and leave these in place?

A:  You don't.  These are needed so the passive crossover is not affected by the speaker impedance variation.  Electronic crossovers are not affected by impedance changes, so the correction circuits are redundant.  Although they waste energy and can be removed, leaving them in place will usually not hurt anything - just don't expect an improvement.

Q:  What about (notch) filter circuits? These are often used to remove objectionable tweeter resonances, so these still have to be there, right?

A:  Probably not.  If you are replacing a 6dB or 12dB passive crossover with a 24dB electronic circuit, you may well find that these are no longer needed.  Because of the original crossover slope and the impedance interactions, tweeter resonance effects are common.  They are all but eliminated by the steep slope of an electronic crossover.  If the tweeter resonance still causes a problem, it is better to remove the offending frequency electronically.  Passive notch filters can make the amplifier load 'difficult', and may cause instability in some amps.

Q:  In thinking about the sound difference between passive and active setups, I have come up with a question that perhaps you would give me your thoughts on.

I am wondering whether the main reason an active crossover sounds so good is the fact that you truly CAN have same-length speaker cables to each driver, whereas in a passive setup you don't?

Just as background, I have a stereo power amp sitting behind each speaker.  One channel drives the bass and the other drives the mid+high crossover.  With your active crossover in place, an interconnect feeds the power amp and then two, same-length speaker cables connect to the bass and mid drivers.

In the passive setup, the low-pass section for the bass driver has 2 coils in series - which are many metres of wire - whereas the 12dB high-pass slope of the mid-range driver has a cap in series.  Therefore the length of the cable carrying the signal from power amp to bass driver is many times the length of the mid-driver signal cable.  I understood the 'Audio Rule' was that you must have same-length speaker cables or you screw up the sound stage ... so, by definition, passive crossovers cannot obey this rule due to the coils that they use.

A:  Such a long question deserves a long answer (and needs it, too :-) ) The difference in path length with passive xovers is large, but the electrical delay is small (other than the phase delay that is common to all crossover networks).  I still think that the major benefits of an active crossover are the phase coherence, lack of impedance effects on the crossover, and the fact that each amp has a limited frequency range and you get extra headroom compared to a single amp.  The active version is completely unaffected by the driver characteristics (which change with frequency, power level, etc.), and in turn affect a passive crossover's performance.

The equal length cable theory is a bit of a myth really.  You can prove this to yourself by running a 10 metre and a 5 metre cable in parallel (or other numbers that remain passably sensible :-) )

If the 'rule' were true, then you would hear the difference, but the propagation delay is only nanoseconds, so you won't hear any change.  Electric current flows along cables at around 2.25E8 metres/second worst case, so if one had a 1 metre and a 100 metre cable in parallel, this represents a 440ns (nanosecond) delay between the two.  This would create a 1 degree phase shift (delay actually, but let's not split hairs :-) ) at a little over 6 kHz.  Even at over 20 kHz, the shift is only about 4 degrees - you get more than that acoustically by moving your head a few millimetres.

The cancellation caused by a 440 ns delay between parallel conductors is 0.08 dB at 100 kHz (which is negligible), so at normal audio frequencies it may be completely ignored.

Q:  I have heard that bipolar electrolytics are commonly used in passive crossover networks.  You and just about everyone else don't like them, so why are they still used?

A:  Price.  They are cheap and have high capacitance in a small volume.  However they are not reliable at high power, and will degrade in time.  It might be Ok to use them in impedance correction circuits, since they are not in the signal path (they are in parallel with the driver and amplifier when used like this).  However, because of the potentially high current they will be subjected to, their life may be reduced quite dramatically.


Q:  Can I increase the supply voltage on your amps to get more power?

A:  All power amplifiers have limitations on the maximum supply voltage and minimum load impedance.  Although you can use parallel output devices, the driver transistors, Class-A driver and input stages also have limitations.

For reasons that remain completely unclear to me, everyone wants to operate P3A (in particular) at above the design voltage rating, which is ±35V (or ±42V into 8 ohms only).  32VAC (for example) will give ±45V, and the amplifier will fail.  Maybe not today, nor tomorrow, but you will stress many transistors beyond their design limits, so eventual failure is guaranteed.

Power dissipation (and the likelihood of output device failure) is based on many things, and these were taken into account when the amp was designed.  As voltage increases, the probability of device failure rises exponentially.

The dissipation into a resistive load must not be used to determine safe operation, and the worst case load is a loudspeaker near resonance, where the voltage and current are 45° out of phase.  Instantaneous peak output transistor dissipation is doubled under these conditions, and if the transistors are hot and pushed to their limits, then failure is a certainty.

200W transistors will be driven into potential destructive operation with ±42V into a 4 ohm load (peak dissipation is in excess of 200W).  ±35V is a safe and 'transistor friendly' voltage, and the amp will give many years of faithful service at that voltage.  Even at ±35V, peak dissipation will reach about 150W into 4 ohm loads!

All of my designs are deliberately conservative, since many are built by novices, and it is obviously better if they work reliably.  All attempts to obtain more power by increasing supply voltage (and with little or no understanding of the stresses on the transistors) reduce reliability, and place the amp and speakers at risk.

For P3A, ±42V is the maximum (full power) voltage I recommend - not 45V, and not anything that is greater than 42V.  Period!

To use a higher voltage, the input transistors, current sink, Class-A driver, drivers and output devices must all be upgraded, then the input device bias will change, and performance may be reduced.

Amplifier design may appear to be 'trivial', but it is not, and any change to any operational parameters should only be done with a full understanding of the ramifications.

It also needs to be understood that seemingly large gains in power are not worth the decrease in reliability.  P3A at ±35V will provide an instantaneous peak power (before the supply voltage collapses) of just under 75W (remember too that the supply voltage will normally be higher than expected because of transformer regulation).  Also remember that the full +ve to -ve supply voltage can (and does) appear across the transistors, so with a ±42V supply, all driver and output transistors must be able to withstand 84V.

If the supply voltage is increased to ±42V, peak power increases to 110W.  It may look like a lot more power, but it is only 1.6dB - you need a 10dB increase for an amp to sound "twice as loud", and that requires going from 75W to 750W.  1.6dB is barely audible as an increase, and is not worth risking the reliability of the amplifier for.

Q:  Would you consider a design for a 1,000W amplifier?

A:  No, I would never consider a 1kW amp.  No loudspeaker can handle that much power, regardless of claims.  Look at a 1000W electric heater - feel how hot it gets and how quickly.  Note that the element is thick resistance wire on a ceramic former.  The resistance wire glows red hot after only a few seconds.

Since only about 1% of power is converted to sound, the voicecoil has to do the same work as the electric heater.  But look at a voicecoil - it is thin wire on a thin aluminium or Kapton former.  At 1kW it may last a few minutes if you are lucky.  At low frequencies, the voicecoil will leave the gap, the speaker will distort badly, and quickly fail.  In some cases, the spider or surround can be torn, or the voicecoil can suffer impact damage from colliding with the rear pole-piece

Unfortunately, the advertisements and even data sheets lead one to believe that many drivers will take that kind of power.  The vast majority will not.  I have tested a (claimed) 500W driver that distorted (and would have quickly overheated) with only 100W input.  It was a massive affair, having dual voicecoils, dual spiders, dual magnets and double the distortion.

Although it is useful to have headroom in an amp, it is far better to use smaller amps and an electronic crossover.  This makes the requirement for 1kW amps simply go away for all domestic hi-fi systems.  Having said all this, P117 has been published, but I hope no-one is silly enough to build it :-)

Q:  What is power bandwidth and what relationship does it have to overall amplifier sound quality? Does it have some relationship to transient intermodulation distortion, even if indirectly?

A:  This remains a contentious question.  Power bandwidth is (in amplifier terms) the bandwidth that the amp can provide full power (usually measured at the -3dB frequency).  It is closely related to the slew rate of an amp.  If any signal is faster than the amplifier can handle, then intermodulation products are generated (TIM).  Few modern amplifiers suffer this problem - even if the amp can only provide full power up to 10kHz, this will not cause slew rate limiting, as the signal levels are so low.  There is some evidence that such a limited bandwidth creates other audible effects, possibly due to the much lower gain at higher frequencies, meaning less feedback and higher distortion.  Just about every designer on the planet will give you a different answer .

Q:  When an amplifier gets a complex audio source (such as an orchestra soundtrack) does this create greater load conditions at the speaker end and adversely affect damping ratios determined in the normal manner.

A:  An amplifier does not care one iota about the complexity of the signal.  A single frequency or multiple frequencies all at once make no difference.  At any point in time, there is only one voltage present, and the amp will amplify it.  This of course is only true if there are no frequencies that cause a rate of change of the signal that is outside the bandwidth of the amplifier.  There are two distinct (and separate) things to deal with - the instantaneous value of voltage, and the rate of change of this voltage.  If either is outside the amp's capabilities, you will get distortion (of one form or another).  Output impedance is (to some extent) frequency dependent, and varies with the feedback ratio - this in turn is reliant on the open loop bandwidth of the amp.  After this, it gets complex !

Q:  What I mean is does an amplifier's output impedance actually rise during complex musical production or is this all nonsense?

A:  This is nonsense.  The amplifier's impedance is affected by frequency (so will be different at different frequencies), but the complexity of the music has no direct effect on the impedance.

Q:  Does any of the above have anything to do with why valve and solid state amplifiers can sound different?

A:  It might.  Valve amps usually have a power bandwidth that is almost independent of feedback, and is limited mainly by the valve electrode capacitance(s) and the output transformer construction.  Have a read of the article about valve and transistor amps "Valve amps - do they really sound different".  There are quite a few other factors (output impedance especially) that have a greater influence on the perceived sound quality.

Q:  I suspect that many or all of the above potential problems are greatly reduced by bi-amping or tri-amping.

A:  There is no doubt on this score.  Anything that reduces the demands on an amp helps, but it helps even more when the speakers are driven from a defined and constant impedance - this does not happen with most passive crossovers.  Biamping is more about the speakers than the amps.  Very few (if any) modern amplifiers are stressed by the full range signal, although impedance dips caused by passive crossovers may be responsible for some subtle (or not so subtle) effects on the sound.  This has not been proven (to my knowledge).

Q:  Can I operate the 60W amplifier (or any of the others) into a 4 ohm or 2 ohm load for more power?

A:  Four ohm loads will usually be acceptable, but I strongly recommend using paralleled (or more powerful) output transistors.  It may also be necessary to use a small heatsink on the drivers, as their dissipation is usually more than doubled.  Two ohm loads are very difficult for any amp not designed specifically for this impedance.  I do not recommend using any of the published circuits for 2 ohm loads.

Q:  Is is better to use transistors or op-amps for amplifiers built today?

A:  It depends on what you want to achieve - a well designed discrete power amp still outperforms any power opamp, but good opamps outperform (technically, at least) any discrete circuit for preamps.  However, the P37 (DoZ preamp) circuit is a good indicator of what can be done with only a few parts, and that measures well and sounds very good.

There is no simple answer.

Q:  I don't have an oscilloscope, so how can I know if my amp has crossover distortion?

A:  A reader sent me his idea for this, and it is both simple and ingenious.  You need a very clean sinewave source of between 100 and 150Hz.  Set up the amplifier providing some power into a resistor load - about 1V RMS into 8 ohms is usually enough.  A cheap piezo tweeter connected across the load will make harsh clicking noises if crossover distortion is present.  Adjust the level to zero to make sure that it is not spurious noise.

If you are adjusting the quiescent current on an unknown amplifier, set it so that the clicking noise just goes away (or stops decreasing).  You must check that all transistors remain at a sensible temperature.  (Thanks to Raymond Quan for the idea.)

Q:  As far as I can tell, there is next to nothing on PWM amplifiers at your site, despite the fact that recently there seems to have been some dramatic improvements in chip(set)s that enable truly amazing compact (size of a credit card) amplifiers.  I find it curious that you don't seem to have addressed PWM amplification at all.

A: You are (almost) correct - there is not a lot, but there is a very comprehensive article that discusses PWM/ Class-D amps.

There are many problems with DIY Class-D amps, and the possible need for surface mount components is only a small part of it.  Making/ debugging something like that requires considerable expertise in digital signal (or switchmode) analysis, and absolutely requires the use of an oscilloscope.  Many of the required parts are also hard to get and relatively expensive, especially in small quantities.

There is nothing curious about it - many people have problems getting a simple discrete amplifier to work, and I don't even want to contemplate the questions I'd get if I had a PWM amp project.  I agree that it is interesting, and in a few years most high power amps will be PWM, but at this stage I would be a fool to develop and publish one !

For more information on the design of amplifiers, see the various articles (and references) I have published.

Opamps & Sound Quality

Q:  Which opamp sounds the best? I hear a lot of conflicting stories about the sound of various opamps and wondered if you can help.

A:  This is a very common question, and it's one that I get up to several times a week.  In simple terms, there is usually no difference whatsoever, but that needs qualification.  Most of the opamps that I recommend in various circuits will produce measured results that are virtually identical, but some low-cost opamps may be noisier than more expensive devices that are optimised for low noise.

Many people claim that a particular opamp may have 'better bass' or 'smoother treble' than others, but this is never confirmed with a proper double blind test.  If the two are measured, the response will be found to be identical, and usually (but not always) distortion will be well below the limits of audibility with any decent opamp.  Despite claims to the contrary, test instruments can detect frequency response variations and distortion products far better than anyone's ears.

Most claims of 'superiority' of one device over another are wishful thinking.  The only meaningful comparison is double-blind (ABX or similar), and any test where the listener knows which device is being used is fatally flawed.  There can be differences, but they are rarely audible and often only detectable with specialised test equipment.

Q:  Why do I hear a slight hiss when the volume on my preamp is advanced?  It's especially noticeable with phono preamps.

A:  This is another very common question.  All electronic circuitry makes noise, even resistors.  Gain stages using opamps or discrete parts generate their own noise, as well as amplify any noise from resistors and/ or preceding gain stages.  High impedances are worse, and high voltages are worse again.  See Noise in Audio Amplifiers for more information.

Mobile (Car) Audio

Q:  i have a 1200watt car amp and 2 600watt car speakers with no car.  witch brings me to the problem how do i hook it all up in the house.  if you have any advice e-mail me at  tanks!

A:  In case you were wondering, that was an actual e-mail I received (some are a lot worse!).  You will need a big power supply, probably with a 12V car battery (situated outside the house, please!) for backup and to provide for surge currents.  A battery charger will not be helpful, because there will be excessive noise on the 13.8V supply (that's the actual charge voltage for a 12V battery, and most car amps are power rated at 13.8V, some at 14.4V).

I get quite a few of these questions, and quite frankly, I'm sick of them.  This is not a trivial undertaking - one person even suggested that he already had a wall transformer, and wondered if that would work - it wouldn't.  To those who may be thinking of asking ... don't.  There are many forum sites that deal exclusively with car audio systems, and you should ask them, not me.

Q:  Do you have a design for a car power amplifier.  I want to use your 60W amp from the 12V supply, what do I need to do?

A:  For useful power from conventional hi-fi amplifiers, you need to increase the supply voltage.  This requires a switch mode power supply.

Q:  Are you planning to publish a suitable switching supply suitable for any of your amplifier designs?

A:  Yes, a 300W supply design is available on the Projects Pages.  However ... there is a fundamental problem with switch mode supplies - the ferrite transformer core.  Unless you can get one that is quite similar (and achieve the same primary inductance), the supply will just blow up.  Unfortunately, these cores are not readily available to hobbyists, but they can be obtained.  In addition you need high speed (Fast or Ultra-Fast) diodes, but these are now readily available.  The layout and construction of switching supplies is such that they are difficult for the home constructor inexperienced in building them.

Q:  About the Project 89 Switchmode power supply for car audio ... Can it be modified to work (step-down of course) from 220VAC (120, 240, etc.) rectified to 300VDC? If not, where can I find a design for a +30/-30 SMPS rated at 500W or more?

A:  The SMPS is not suitable for 220V (or any other mains voltage) operation without major re-design.  It is an entirely different matter to make a high voltage step-down system, and apart from PC type supplies, there are thankfully few DIY designs on the web for off-line switchers.  As a DIY project they become rather daunting (and dangerous), and it is highly unlikely that I will ever attempt a design for publication.

Q:  Can I use your preamp or crossover in my car system?  I am especially interested in the Linkwitz-Riley crossover / sub-woofer controller / quasi parametric equaliser.

A:  Yes you can.  You need to establish an 'artificial' earth (ground), at 1/2 the supply voltage, and input / output capacitors must be used (make sure you check the polarity of electrolytics).  There is a project for a simple preamp and artificial earth in theProject Pages.  This can be adapted to your needs.  Alternatively, you can use Project 69 (Switchmode power supply for preamps) to obtain ±12V at up to 45mA

Printed Circuit Boards

Q:  How come most of your PCBs are single-sided?

A:  Good question.  Since most of my readers are hobbyists or beginners, double-sided boards are difficult to re-work.  Through-hole plating means that it is difficult to remove parts without cutting off the legs/pins, and even then, a very good solder-sucker is needed to clear the hole again.  Since it is not uncommon for professionals (with expensive equipment) to damage double-sided boards, beginners and other non-professionals will simply be paying more for a board that is easily damaged.

However, there are now quite a few projects that do use double-sided PCBs, but they remain a problem if the constructor makes a mistake.

Q:  Can I get a copy of the PCB artwork for any of the projects?

A:  No.  I do not supply artwork to anyone.  The PCB layouts take considerable time to develop, and I must not only recoup the time spent, but I also need to be able to pay the costs of operating the website.  This is my primary source of income, so I don't think it is reasonable to expect me to give away my intellectual property.  No-one else who sells products will give you their design information either.

Q:  With most of your boards, I have to run wires for the pots.  This is a real pain.  Why don't you use PCB mounting pots?

A:  I dislike running wires as much (or more) than anyone, but if I have the pots on the board, you are limited to using the same type of pot (some are not readily available in other parts of the world).  In addition, you would be limited to using the same layout as I designed for - the pots would be the distance apart that I designed the board for, and the PCBs would be considerably larger (and therefore more expensive).  You would be much more restricted, and would not be able to use the layout you want.

Q:  I sent a faxed order, but I didn't receive any confirmation.  The first I knew that it was received correctly was when the boards arrived.  Why didn't you let me know?

A:  I normally only contact you if there is a problem with the card details, if I am temporarily out of stock (rare, but it has happened), or if I can't determine what you want.  If the fax is clear, and I can validate it properly, then the order is processed and despatched, usually within 2 (±2) days of receipt (i.e. it may be immediate, or could take up to four days, depending on my workload).  If you want a reply, send an e-mail (preferably with your address as well so I can cut and paste it).  NOTE: Faxes are no longer supported.

Q:  Can you give me an estimate on the cost of assembling Project (insert any project number)

A:  No.  The cost of parts varies from one supplier to another, and even more energetically between different countries.  It is simply not possible for me to try to maintain any sort of price list, as there are too many projects, too many suppliers, and too many countries and currencies.  This is your job, mine is to produce the projects :-)

For power amplifiers, a reasonable estimate can be made by pricing the power transistors and doubling it to cover the cost of the smaller devices and passive components.  The heatsink, power transformer, filter capacitors and case will normally exceed the cost of the amplifier itself by a wide margin.  The availability (and cost) of these varies widely from one country/ supplier to the next.

Q:  Do you have power supply boards for any power amplifiers ?

A:  No.  The power supply for power amps consists of a transformer (too large to mount on a PCB) a bridge rectifier (which needs to be chassis mounted for cooling) and filter capacitors.  If I designed a board for filter caps, it would cost a ridiculous amount of money because of its size, and you would have to use caps of the same physical size as I designed for.  This would be extremely restricting, and would make component selection much harder than it should be.  Power supplies are best hard wired using thick wiring and you have far greater flexibility for mounting.

Q:  How can I add a volume control to your Project 'XX' power amplifier? (XX is any amp project number)

A:  This is a very common question, and is fully explained in the article about potentiometers (pots).  However, I will also cover it here, as I might save some e-mails.  The basic schematic for a volume control and a drawing of a pot are shown below ...

Typical Pot and Amplifier Connections

For more information, see the article Beginners Guide to Potentiometers.  Generally, a value of between 10k and 25k (or thereabouts) is fine, and log pots are 'traditional'.  The above article shows how to make a linear pot behave more logarithmically than a 'real' log pot, and this is usually very much better than cheap to medium priced log alternatives.  Expect to pay fairly serious money for well matched conductive plastic log pots, or get the same performance from a budget carbon pot suitably modified.


Q:  How do you do your schematics? They are always very clear and seem to be able to fit a large diagram into a small area.

A:  The original drawing comes either from Protel (professional schematic/PCB software) or SIMetrix (simulator program available from SIMetrix.  While both do a reasonable job in their own right, neither (nor anything else I've tried) produces a schematic with the specific attributes I like to use.  Some are done using templates I've created.

So, silly though it might sound, I use one of the most under-rated programs that Microsoft produces - Paint.  This allows me to get diagrams literally pixel perfect, and also lets me move components into positions that schematic capture programs won't allow.  Once the diagram is the way that I want it to be, it gets a final processing using a very old copy of Photo-Impact.  This lets me remove all redundant colours to minimise file size.

I probably spend a lot longer on my drawings than most, but IMO it's worth the effort.  There's really no point having a good article, and then producing a drawing that either won't fit on the page, is difficult to read, or just plain messy.

For what it's worth, I use the same reasoning for web pages ... all are written in HTML without the aid of any specific software.  I can write in Notepad just as well as CoffeeCup (which I use because it has a spell checker).


Q:  I have been doing some research into what my next hi-fi upgrade might be and i have been reading about how vast am improvement can be made with the filtering of AC power.

According to 'everyone' such a device can lower noise floors on a system so that all you hear is the music and no longer the system.  It makes sense that the cleaner the AC signal the cleaner the output signal will be although i have been unable to find any really hard evidence that this is the case (in terms of graphs or whatever).

What do you know or think about this? I have heard of several types of filtering methods, ranging from some product called a ***** to battery banks to run amps from cleaner DC.

A:  First, with no music, place an ear next to the speaker drivers.  If the noise you hear is just a gentle hiss, then you are already at the system noise floor, and there is not much you can do to improve this.  Hums and buzzes are probably due to wiring, and re-routing the leads keeping power separate from interconnects (etc) may improve matters.

Unless you have recognisable evidence of mains interference, external filters and conditioners serve no purpose.  I'd steer well clear of anything that claims to be a 'magic bullet', and batteries may be noisier than a good mains supply in some cases (the chemical reactions create electrical noise).  Mains filtering is only effective and useful if the system's background noise is audible from the listening position, and is the result of mains interference.

If there is no interference and the system noise is inaudible from the listening position, then nothing needs to be done, as you do not seem to have a problem.

Q:  Can we obtain tremendously high voltage gain using a FET or valve (vacuum tube) with a current source load or bootstrap circuit

A:  Using a current source (or bootstrap) load with valves or FETs will make them more linear, but does not increase the voltage gain by anywhere near as much as with bipolar transistors.  The voltage gain is almost an illusion, largely because the bipolar transistor is a current device (output current is determined by input current, not voltage).

Q:  What is the reason that bipolar transistors have such a high voltage gain when used with a current source load versus valves or FETs

A:  Transistors are current controlled current sources, and the others are voltage controlled current sources.  Because the current is controlled by a voltage, valves and FETs have a much lower voltage gain, and high impedance loading (however applied) will not cause a huge change in the transfer characteristics.  (This is the simple explanation, by the way :-) )

Q:  What is your opinion on using an 'off-line' power supply, using a capacitor to drop the mains voltage, then regulating using zener diodes.  Will it affect the sound? For better or worse?

A:  DON'T DO IT !   These supplies are extremely dangerous, and are intended only for equipment that is not connected to anything else (for example, stand-alone mains appliances).  This method must never be used for audio equipment, since there are always connections to the outside world.

Q:  Will an amp that is built with film caps be better than with ceramic caps?  Can you advise any sources on the Net about how caps affect sound and what ones are better for using in audio projects?

A:  Film caps versus ceramics is (mostly) no contest.  Multilayer (aka monolithic) ceramics are not particularly stable, and their capacitance varies with voltage, temperature and whim.  They are by far the best for bypass applications, but should never be used for coupling or in filter circuits and the like.  For coupling and any filter application, use film or 'metallised' types for values of 1nF or more, or C0G/ NP0 ceramics for values less than 1nF.  These are extremely stable and have no 'bad habits'.

There are countless claims that some film caps sound better than others, but I know of no definitive test that has ever proven this in listening tests.  Many measurements will show differences, but these have never been proven to be significant in a properly conducted blind test.

Q:  I am thinking of using Brand X amplifiers, and Brand Y cables.  What do you think?

A:  I will (generally) not endorse any brand of amp,cable or other product in these pages, although the occasional comment may be in order.  Basically, my response is that you should try it and listen to how it sounds.

Q:  I want to build one of your published projects.  Are there kits or circuit boards available?

A:  I currently have PCBs available for quite a few of the more popular projects, and currently offer a complete module for P27A (guitar power amp).  Kits are not provided because of the cost of purchasing large quantities of expensive devices.  Check the Purchase PCBs page for the latest information.

Q:  I am confused about transformer voltages and VA ratings, and I want to build a power supply for an amp I am building.  What do I need?

A:  This is a very common question, and the answer is different in nearly every case.  The article on Amplifier Design has a few details, but there are extensive answers in the Linear Power Supply article, as well as the beginners' articles on transformers.

The VA rating is simply the product of volts and amps (hence VA) - it is not called 'Watts' since the VA rating and Wattage are very different due to the rectifier and filter caps.  A rule of thumb for Class-AB amps is that the transformer should be rated at a minimum of 1.5 times the maximum power from the amplifier(s).  In some cases you may use a larger or smaller transformer than suggested, but I recommend that you read the articles on power supply design first.  For Class-A, the ideal is that the transformer is rated at a minimum of 4 or 5 times the RMS amplifier power rating.  It must be remembered that the load in Class-A is continuous, and a transformer that is too small will overheat and be ruined.

Q:  Which of the amplifiers in your projects pages sounds the best?

A:  Perhaps surprisingly (perhaps ??), I don't recommend any one of them over any other.  The Audio Pages are about experimenting, learning and building.  All the amps (and preamps) work well and sound good, and it is up to you to decide which one you want to build.

Q:  Do you do custom design work?

A:  Yes.  Send me an e-mail, and I will see what can be done.  I do not have the time (and you don't have the money) for development of projects for personal use.  My time is very limited most of the time, so I may not be able to help at all.

Q:  Do you plan to produce a project for an AM/FM hi-fi tuner?

A:  No.  These are quite reasonably priced as commercial offerings, and are too hard (i.e. almost impossible) to align for the home constructor with no RF test equipment and specific skills in this area.

Q:  What does 4k7 (100R, 2n2, etc) mean? Does this mean 47k (100 ohms, 2.2nF, etc)?

A:  4k7 means 4.7k - this is basically a European standard and is used to ensure that the decimal point is not missed.  100R means 100 ohms, and is used so the omega symbol does not have to be inserted all the time.  2n2 (and similar) means 2.2nF, and the same logic applies as with resistors.  It is now fairly common in most countries, although usage is mixed.

Q:  What does 0R47 mean?

A:  0R47 means 0.47 Ohms - another variation to the European standard and again used to ensure that the decimal point is not missed.

Q:  What does 2.2nF mean? What is 'nF'?

A: A nanofarad is 1E-9 Farad, and is equal to 0.001µF or 1,000pF.  This is the preferred nomenclature for capacitors between 1000pF (1nF or 0.001uF) and 1uF (1000nF).

Q:  Do you have time to even answer one of these pain in the butt questions?

A:  It would seem so :-)

Q:  On a different subject altogether, I heard that you helped set up the SAE (School of Audio Engineering).  Is this true?

A:  Yes.  Along with John Burnett and Tom Misner.  I was only there for a short while, until I became aware that the operation was not (IMO) above board.  Without going into detail, it transpired that I couldn't work with Tom Misner for a number of reasons, not the least being his (self acknowledged) lack of ethics.  As a result, I quit on the spot, and John (Burnett) followed a few months later.

For more information on the topic, have a look at John's education FAQ page - Lenard Audio FAQ.


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Copyright Notice.This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2000.  Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws.  The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference.  Commercial use of the material herein is prohibited without express written authorisation from Rod Elliott.