The Machine


a high-fidelity headphones amplifier


a ultra-low-noise RIAA preamplifier.

Two phenomena of our hearing:

  1. Perfect octave: The frequency ratio of two tones of 1: 2 is a natural phenomenon that has been called the "fundamental miracle of music". Subjectively, an octave is perceived by the ear as a stable, continuous musical interval. That is the consonance (sounds harmonious).

  2. A harmonic series (overtone series) is a sequence of frequencies or musical tones in which each frequency is an integer multiple of a fundamental.
    The presence of even-numbered harmonics (overtones) in music is pleasing to our ears and enhances the sound. This is exactly what some tube amplifiers do: they enrich the spectrum with even-numbered harmonics.
    ( Please listen and give the price here: Project 7tubes )

What was the inspiration for this device to appear?

Why is the Harmotronix preferred?

  • Because this is finally the end of all arguments about the sound of the tube;
  • Because the main advantages of combining the Harmotronix with a solid state amplifier are:

Two basics of electronics:

  1. The basis of the basics is a triode in SE mode only: The transfer characteristics of a triode is approximately described by a second order function. This leads to a signal with an enrichment of the spectrum by even harmonics.

  2. Presence of overtones is proportional to the level output. So a typical tube sound only comes out at maximum volume.

Of course, money is also important to us.

A tube power amplifier requires an additional investment of several thousand US $ for a special speaker system: you cannot get a good sound for a tube amplifier with conventional acoustic speakers.
So the Harmotronix is the perfect combination of authentic tube sounds and your money.

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NOS tubes

Only vacuum triodes make a real tube sound. There is a theme for NOS tubes, however. New old stock (NOS), refers to merchandise being offered for sale which was manufactured long ago but that has never been sold at retail (from Wikipedia, the free encyclopedia).

There are still a few things to keep in mind for clever people.

During manufacture of a vacuum tube was created inside a vacuum, which deteriorates over time (over several decades!) for divers causes, for example:

Infiltration of gases (mainly helium) through metal-glass contacts, outgoing of gases from parts of the internal structure of the tube. This directly lowers the dielectric strength of the inter-electrode gaps.

During storage, however, there is also a chemical reaction between the cathode coating and these residual gases. And it's getting to a reduction in the emissivity of the cathodes. This process is known as "cathode poisoning".

Long-term storage of a vacuum tube therefore leads to bad effects: the noise heightens, the performance lower and the lifetime reduced radically.

Allegedly NOS-tubes should sound better than modern ones. Allegedly NOS-tubes cannot be compared to modern tubes. So far nobody has given a reason that prevents the production of an electronic vacuum tube on modern systems with modern technologies. Right, the quality of vacuum tubes and other parts is much better today than in the 50s!

Subject of a proper speaker

On my way to real tube sound, divers variants for building a tube amplifier were encountered. Yes of course, these should only be triodes. It is relatively easy to assemble an amplifier based on high performance 300B triodes. Of course, if you have output transformers from Japan or Sweden on hand. This is a solvable problem, even if it is quite expensive. Obviously the tube or amplifier doesn't sound by itself. The sound is produced by the speaker system together with the amplifier.

In acoustic systems for semiconductor amplifiers the sound of a vacuum tube becomes plop at low frequencies. The reason for this is that the tube amplifier does not strictly control the loudspeakers: Due to the weak coefficient of damping, the membrane at low frequencies flutters. If you can put the Thiel & Small parameters into practice, then of course you have the chance to find the right loudspeaker on the market, that has been specially designed for your tube amplifier. But the cost of doing this will exceed your bold vision.

Supply for a tube Amp

The most important task is to supply vacuum triodes with clean voltage. The right solution depends on the availability of a power transformer. All secondary voltages must be present. It can also be a combination of two transformers. The main condition is that you are only allowed to use toroidal transformers. Otherwise, good sound quality is just trash only.

There is a main function here: absorption of HF frequencies and power frequency up to the 20th harmonic. These disturbances are very low levels. Although they are barely audible to the human ear, they generate distortion, intermodulation with the useful signal, which is perceived as a kind of hard sound.

With regard to the high sensitivity of a vacuum tube to electromagnetic Field (from mains Power and from surrounding area, emission from switched-mode power supplies, mobile communications and the like), high-quality power supplies for electronic tubes are of crucial importance. It is required to supply an extremely clean voltage.

Another big challenge on the road to sound quality is protective grounding and the strict topology of the common line in the tube amplifier.

Supply is relevant to quality: there are also interferences from filament circuits. For example, two DC voltage supplies are used for the tubes filament  in the 7tubes machine: for upper tubes and for below tubes.  Accordingly, different values ​​of the bias are set in each circuit.

In addition, adjustment potentiometers (symmetry) are used here to  achieve the lowest interference level when a MM cartridge is already to the RIAA preamp connected.

Filament voltage

This is one of the important points for a high quality tube RIAA: Together with the value of the tube current, the cathode temperature determines the dimension of the flicker noise. Note that its value can exceed the useful signal of the MM cartridge ten times. Although the range of this main noise is of 0.5 to 10 Hz, it causes unpleasant modulation effects on the useful signal.

Therefore tubes should have a DC voltage of at least 6.4 volts at the current mains voltage  (the actual voltage in your power socket). It's perfectly fine: the manufacturer of the tubes specifies the maximum limit of 6.9 volts.

THD & frequency response

Sound quality parameters. This is not correct to measure only the output power and distortion of an amplifier, because total harmonic distortion (THD) and frequency response mean little to us hearing. You need a spectrum analyzer that can show us what is in the signal when it is excellent.

In these images of the fast Fourier transform to 7tubes project we can see the difference in the tube and transistor tone. Obviously only a low THD value does not make the sound smooth and natural.

The example images for 7tubes Headphones Amp shows that a tube amp can sound like a transistor amp, although the THD in this case is very low: -72dB (-52dB to level = 1).

By using a negative feedback it is easy to reduce the THD even more.

But then we hear the classic sound of a transistor amplifier with all the weak attributes of tube amplifiers. Namely: high production costs and high energy consumption, big dimensions. In addition the higher output impedance of the tube amplifier requires special loudspeakers.

The ratio of the 2nd to 3rd harmonics and the higher order harmonics is crucial for our hearing. The background of these higher order harmonics (up to the 20th!) Is mostly negative feedback.

So the negative feedback keep the 2nd and 3rd harmonics down, also making the lower THD, but leaving the higher harmonics, which are extremely uncomfortable for us, although they are very low. In general, if a amplifier has negative feedback, we need qualified preventive action against phase distortion. It's a very complex thing.

Therefore I find it attractive and productive to build an amplifier without negative coupling in a special design. In these special cases we want to hear a soft sound that is naturally neat for our ears.

It is possible to build a quality amplifier of high fidelity. This is relatively easy to create on modern solid-state chips. However, I want to have a warm, natural tube sound for my soul. That's why the discussion about minimal distortion is in principle not compatible with the tube sound.