Audio – An interview with designer and founder of Terrastrato Amplifiers: Andreas Lohse
Why an audio system?
To simply listen to music, we do not need an extraordinary audio system. However, we do indeed require such a system in order to experience music, to witness it in the recording studio, at a chamber orchestra performance, at the jazz club – and this even though the musicians involved may have met decades ago. An excellent recording and a good playback system enable us to travel to the past or to another place, a soloist can be resurrected a few metres from where we are, as if her presence were palpable, as if one could imagine the stage floor underneath her feet or her taking a breath before starting her solo. This is a great promise that a good single-ended amplifier can fulfill interplaying with efficient loudspeakers, especially for recordings with a reduced recording technology, with only two condenser microphones in AB, Blumlein or OSS technology, the recording technique developed by Jürg Jecklin that separates two condenser microphone capsules at ear distance by a flat slightly absorbing disc in order to magic a three-dimensional sound stage into our studio or living room with only two loudspeakers. Often these are old recordings or recordings that we did not expect to spark such an incredible magic when we play them with an adequate playback system.
A playback system based on a technique of the 1920s-30s?
Yes, in the case of the amplifier, yes.
Why single-ended-triode?
An amplifier should be thought of as a link in a chain that embraces the physics of dynamic speakers, the propagation of sound in air and space, our ears and the signal processing of the brain. Only in this context will it be possible to explain why the performance of SET amplifiers is so often preferred in listening tests.
Unlike other amplifier designs, single-ended triode amplifiers are largely free of high order harmonic distortion and their intermodulation products, even without applying a negative feedback loop. Nevertheless, we measure without negative feedback high proportions of low order harmonic distortion, which, however, are not perceived as irritating or unnatural under real listening conditions and have far less detrimental effects on the imaging and the spatial representation – of “body”. I share the opinion that the less an audio amplifier is dependent upon negative feedback, the more natural it sounds. This places the focus on the triode as the center of a minimalist amplifier design, in which transient intermodulation distortion is also avoided as a matter of principle.
The disadvantage of SET amplifiers is their high heat dissipation, expensive components and a comparatively low power yield if the heat emission to the listening room is to be limited to a few hundred watts. Therefore, when selecting a speaker, speakers with higher efficiency are recommended.
What to start with?
With two efficient speakers, a 2-channel SET amplifier, a computer with a good DAC and an average equipped living room with pleasant acoustics and some freedom regarding the placement of the speakers.
If you could prescribe a media format for retail sales to the music industry…
Say: 1/4“ tape with 7 ½ or 15 ips playback speed in a colourfully printed cardboard box, always combined with a 48kHz FLAC download. That could be expensive, so we stay with the LP, combined with a FLAC download.
Why the need for a new amplifier?
There are 60-year-old studio monitors, professional cartridges, tonearms and turntables, tape recorders, 30-year-old DACs that are so good that no new development can drive them out of our studios and listening rooms. This does not really apply to old professional SET amplifiers, simply because they are so rare and they were already pushed from the market 70 years ago by more economical designs. Heavy SET amplifiers with large, directly heated triodes such as the Terrastrato Typ-1 depend on DC heaters, and these can only be generated using solid state rectifiers developed at a later stage.
How do I order?
With a little patience. Each Terrastrato amplifier is only assembled after ordering. An assembly time of up to 8 weeks must be expected. However, this also offers the possibility to respond to customer requests regarding interfaces and special components. But do not ponder too much on this, the selection and the location of copper wire, carbon film resistors, paper in oil, mylar and teflon capacitors have been made with the utmost consideration.
Printed circuit boards or free wiring?
I use 3mm thick boards made of phenol impregnated cotton for the mechanical construction. The boards have excellent dielectric and absorbing mechanical properties. They carry the solder posts on which all components are hand-wired and soldered. This construction is mechanically more resilient and safer than printed circuit boards.
Favourite speakers?
SET-amplifiers without global feedback become one with the loudspeakers to create a new sound image. In the case of efficient loudspeakers with controlled impedance characteristics, it can be assumed that a SET amplifier raises the loudspeakers to an unprecedented level of openness. Controlled impedance characteristics mean that the loudspeakers do not fall below the rated impedance by more than 25% at any given frequency – for example, drop to no less than 6 ohms at a specified 8 ohms. Loudspeaker manufacturers can provide information about this. This makes the choice of the “favourite loudspeaker” an interesting and individual matter of taste.
But surround sound?
A good 2-channel amplifier is capable of sharply reproducing a three-dimensional sound stage in the front half of the listening room – a good recording and good speakers are required. The standard digital surround sound systems do not offer an adequate alternative. A suitable stereo system is therefore the method of choice for the realistic reproduction of a sound stage. Ambisonics is still today the only free, open and multi-compatible multichannel system that allows sound field recordings with true-to-coordinate reproduction of sound source points in the entire listening room with a minimum of four channels – theoretically even completely analog. This cannot be achieved with quadrophony, Surround-Sound 5.1 and Atmos. I keep a close eye on developments in the field of Ambisonics. With two Terrastrato Typ-1 ampifiers and one computer, it would be possible to set up a perfect Ambisonics system, but I share the opinion that sound field recordings make sense only in non-aligned listening and watching – so in virtual reality productions.
Music all over the house?
Terrastrato is an advocate of a different kind of listening to music. The aim is a naturalistic, holographic representation of the original sound stage. A live musician, for example, has a “body” and particular place in this sound stage and is not to be found everywhere in the house.
And active speakers?
This term covers two procedures, which are often, but not necessarily, combined with each other: Firstly, the use of an active crossover network for tweeters and woofers in front of the power amplifiers (also known as bi-amping when using two amplifiers) and secondly, the placement of the power amplifier in the speaker cabinet. These “integrated” loudspeakers appear to be practical, but they also need at least one connected cable – for their power supply. As a rule, Class D amplifiers are used for reasons of thermal efficiency and their small size. If there is a choice between an integrated 2-way speaker box with two internal Class-D amplifiers and a discrete 2-way speaker system with at least one external Class-A amplifier, I recommend the latter.
Digital streaming and storage of music offers conveniences, analog power amplification free of digital artifacts, too. Wherever contemplative listening to recordings is of primary importance, we should not forego a discrete analog amplifier as the penultimate analog link in an otherwise digital playback chain. Digital sources and software (media devices, network players, digital signal processing) is constantly being further developed, quickly becomes obsolete and is then replaced. Good analog amplifiers and speakers, on the other hand, outlast generations as does a musical instrument.
Are cables crucial?
I assume that random changes in room acoustics – for example, due to a coat hung over the chair – could be more relevant than the differences between high-quality connection cables. This is not the case when connecting moving magnet cartridges to a phono preamplifier, because MM-cartridges react sensitively in the high frequency range to the capacitance of the connecting cable. And here we find a wide variety of cable capacitances due to different lengths, inner cross sections and the dielectric of the connecting cable. As a rule, the ideal total capacitance of the preamplifier input, of the connecting cable and the inner wiring of the tonearm lies between 250pF to 500pF. On the Terrastrato Typ-R1 phono preamplifier, the total capacitance and the input impedance can be adjusted by plug-ins, where the input transformer for moving coil cartridgees can be plugged in. It is worth saying at this point that MC-cartridges are largely insensitive to different input capacitances in the pF range and therefore less sensitive to the selected connection cable. However, it is always correct to choose a coaxial, low frequency compatible connecting cable with a comfortable length, perhaps of one meter, and low capacitance and to supplement it with a plug-in to the desired total capacitance. A low capacitance shifts the resonance frequency of the MM-cartridge upwards, e. g. from 10kHz to 16kHz, which makes the system sound clearer, but it also increases the resonance frequency level and lowers the high-frequency presence range at approx. 7kHz, so that the system’s entire frequency spectrum no longer sounds balanced. By increasing the input impedance from 47kOhm to 100kOhm, the high-frequency presence range at 7kHz could be increased accordingly, but at the price of the resonance frequency level of approx. 16KHz being further increased. Cable parameters therefore play a major role when using MM-cartridges.