Are there any plans for a powered and remote controlled preamplifier?

Yes there are. We are currently working on a powered/controlled preamplifier to complement the SA400 power amplifier - see the In development page.

The Passive Preamplifier was produced to represent the absolute lowest distortion and noise that could be obtained. Clearly, if there are no electronics in the unit then there is nothing to distort the signal, no hum loops can be introduced, and noise will be at a minimum provided components have been selected for lowest resistance and highest quality, which of course they have.

Is your amplifier suitable for bi-amping my loudspeakers?

The SA400 is particularly suitable for bi-amping and as such is recommended. The reason being that each register (bass and treble for a two way system) is totally separated from each other and so Intermodulation of each register cannot occur.

It only has to be decided whether vertical or horizontal bi-amping is used. Vertical bi-amping would use one SA400 for the left channel, and one SA400 for the right channel, whereas horizontal bi-amping would use one amplifier for bass and one amplifier for treble. We have heard arguments for both configurations, but we prefer vertical bi-amping. The main reason for this is that both amplifiers (channels) will be used under identical conditions, which will ensure identical performance. Another reason for vertical bi-amping is that crosstalk will be minimised, hence stereo separation will be maximised. We would of course welcome your own findings.

The only disadvantage of bi-amping is that another SA400 amplifier will be needed, together with two "Y" input leads if the preamplifier only has one output.

My loudspeakers go down to 2 Ohms. Will your amplifier be suitable?

To the best of our knowledge there are no commercially available domestic high quality loudspeakers that could sustain the continuous power that the SA400 can provide. The reason for the high power capability is not to achieve high sound levels but to allow a wider than normal dynamic range together with an immediate transient response. We believe that this is a fundamental requirement of high quality audio equipment.

The acoustic power required to fill the average size room is much less than 1 watt. The most efficient loudspeakers are seldom greater than 1 per cent. And, the peak to mean ratio of even compressed music is in the order of 6dB or more.

So what does this all mean? Basically, the SA400 has been designed to provide effortless power delivery into even the most demanding of commercially available loudspeakers. Indeed, extreme care must be taken to ensure that the SA400 does not damage the loudspeakers or your hearing by playing at elevated sound levels. Please see the warning in the SA400 Manual.

At normal listening levels in the domestic environment the SA400 will be suitable to drive loudspeakers that go down to 2 Ohms. The Frequency response will be lowered but the -3dB point will still be over 30kHz.

Should I leave the amplifier in continuous standby when not in use?

Digital amplifiers with switching output stages are a totally different topology from analogue amplifiers, whatever the class.

Experience gained by our own use and other users of the SA400 has shown that it is not necessary to leave the amplifier in continuous standby when not in use. We would of course appreciate your own findings/views on this subject, as digital amplifiers are still a relatively new topology.

Please see the F.A.Q. concerning the burn-in time.

The SA400 has both balanced and phono Inputs. Which should I use?

If your source equipment has balanced (differential) outputs then use the balanced input connection to the SA400.

The real advantage of balanced inputs comes from the fact that two signals are provided that are both 180 degrees out of phase with each other, hence, any extraneous signal e.g. RF, hum, or noise will be cancelled out by the common mode rejection of the input stage. This is particularly Important with long (5 metres or more) input cable runs. If your source equipment is (as is normal) located close to the amplifier then the phono input will be totally adequate and will achieve the best performance.

What is "Bridged Output"?

The SA400 is a stereo amplifier, that is, it has two channels.

By throwing the Bridge switch on the rear panel the amplifier is converted to a single channel (mono) amplifier, which has the capability of approaching four times the power of one stereo channel.

Please see the operating Instructions for the specification in bridged mode and also which input to use and how to wire the loudspeaker connections.

What is the burn-in time for the SA400 amplifier?

It has been suggested in the hi-fi press and other sources that an amplifier benefits from a burn-in period ranging from some 20 to 100 hours. This may be true for conventional analogue amplifiers, but we have found that digital amplifiers like the SA400 do not require a burn-in over and above that provided by the 30-hour soak test done prior to the final test at the factory.

Again, digital amplifiers are relatively new; we would appreciate your own comments and views.

Which input and output interconnects would you recommend?

We will not recommend Interconnects for our equipment for several reasons:

1. We strive to make equipment that has a high specification and excellent sound quality, representing value for money. Some of the cables for sale today could easily cost more than our equipment. We cannot justify this situation.

2. To audition the various combinations of source equipment, loudspeakers and interconnects would be a never-ending task, as it is changing month by month. Whatever we recommend one month could change the next.

3. We do not know why Interconnects Improve the sound quality. We have studied the suggested reasons for any improvements, but as yet we are unable to scientifically quantify the results. We will leave that to the cable manufacturers. We do however, agree that Interconnects can "change" the sound.

Unfortunately this does not answer the question, and so we suggest you start with modestly priced cables and try them to establish whether you are happy with the sound of your complete system. Do not assume that because the cables are highly priced they will improve your system. The connectors on the other hand tell a different story. Always use good quality connectors that are gold- or silver-plated.

Why do you not manufacture a fully digital amplifier?

It has only been possible to design a high quality, high power 'digital' amplifier in the last 5 years or so, because before this the major components: - output devices, Magnetic materials and high frequency (greater than 500kHz) pulse width modulators, were not available.

Prior Art Class D designs were around, but the sound quality was just not acceptable. (Please see below for an explanation of Prior Art Class D Amplification.)

The output stage, and in this we include the power supply, is arguably the most critical stage in all amplifiers whether digital or analogue. A long time is needed to fully evaluate the output stage under the various load and programme material combinations, and to reach a conclusion as to whether a change of component or circuit does indeed improve the sound quality. All changes must be done one at a time in an ordered manor under different conditions, auditioned by many people. That's the only way we can make real progress.

So, the answer to the question is simply this:

We are now relatively happy that our output stage and the Tripath topology are achieving the best sound quality at this time. The next generation of amplifiers may well include a fully digital chain, but we will not release the amplifier if we cannot demonstrate a further Improvement in sound quality.

Prior Art Class D

'Digital' Amplifiers have been around since the middle 1950s. The class of amplification was then described as Class D. The "D" did not, as some are led to believe, refer to Digital but was simply the next letter after Classes A, AB, B, and C.

Amplifiers at that time were limited to very low powers in the order of 2 to 6 Watts. The Input signal was compared with a high frequency (approximately 100kHz) triangular waveform, which produced a pulse width modulated output signal with two discreet levels (on or off), which were then passed to the output stage. As these signals are either on or off the output stage dissipation is greatly reduced providing high efficiencies due to the much lower losses. The instantaneous sampling of the input signal was represented by the width of a pulse. To reconstruct the analogue of the digital signal an output filter was necessary. It was very difficult if not Impossible to Include feedback around this arrangement because to filter the 100kHz switching waveform to acceptable levels would necessitate the filter rolling off well into the audio band which would limit high frequency response, and also produce wide phase shifts with increasing frequency. Without feedback, distortion was destined to be very high. The early Class D amplifiers were only really suitable for low frequency, bass or sub-bass amplification at the higher powers (20 to 50 Watts), or car radio amplifiers at the lower powers (2 to 6 Watts).

It is interesting (for us anyway) to note that we did in fact make several Class D amplifiers in the mid 1960s for Mr Radford's car radio to replace his Motorola Valve Amplifier, from circuits published in Wireless World, with great success

Would I benefit from bi-wiring my loudspeakers to your amplifier?

Bi-wiring is a relatively cheap (depending on the cost of your selected Loudspeaker cables) way of 'possibly' Improving the sound quality of your system. The major reason for bi-wiring, is due to the fact that the 'power' of a music signal is contained in the lower, (bass) register. The power density then falls with increasing frequency.

If the cable lengths are relatively long, then the resistance of the cable will be relatively high. This means that there will be a voltage drop along the length the cable at the lower frequencies. The higher frequencies will also be subjected to these voltage drops, as the cable is common to all frequencies supplying the loudspeaker. Intermodulation of the high frequencies by the low frequencies will now result, giving a smeared or masked high frequency response.

If you are using high cost loudspeaker cables, then we would suggest that you try bi-wiring your loudspeakers before purchasing extra cables.

Would you recommend using your amplifiers in a fully active system?

Absolutely. While bi-amping separates each channel (left and right), and each register (bass and treble), a fully active system goes one step further by removing all of the frequency selective components (the network), inside the loudspeaker. Three major advantages result:

The "lossy" components of the network are removed, together with the wide tolerances that these large components can have.

The loudspeaker coil can now be driven directly from the very low output impedance of the amplifier, ensuring far better damping and motor control of the drive unit.

Individual level and frequency adjustments of the two registers are easier to achieve, allowing a much better tailoring of the particular room (probably the most important component in the audio chain), to the system.