History

 

As a Teenager I used to tinker with tubes of old radios. From this emerged different examples of short waves and radio receivers, audio amplifiers and other devices, such as a UHF Converter for Television from West Germany. Even though semiconductors had been already developed, their real success in form of transistors and integrated circuits came only some years later. And with this tubes became "out of fashion" for most application areas.

But today tube technolgy is again a growing market. .Since the historical low point in the 80´s the production output is continuously rising with first and foremost easterne-europe and American companies establishing. The reason for this upgrowth is for one that electronic audio devices built with tubes obtain better sound results, although they come off badly in most measuring parameters compared to devices with semiconductors. Why is that?

 

 

 

Background

 

Every electronic amplifier should as possible reflect onlythe signals which are generated at his inlet and this namely unaltered.

Typical measuring parameters for verifying electronic amplifiers in general are the distortion factor and frequency response. Such measurements always refer only to a single pure tone (sinus). 

But in fact every acoustic moment consists of an huge amount of simultaneous single tones. If several single tones reach the amplifier inlet simultaneously, there will always occur unwanted mixed results which will also be measurable at the outlet besides all wanted signals.

 

The verifying of electronic amplifiers regarding such mixed results occurs therefore in measurands like intermodulation and difference tone factor.

It showed that a distortion factor in a range of 1% is not sensed a much as disturbing if apart from that only occur little mixed results. In contrast to that a difference tone factor of 0,1% causes the music to be sensed as flat and without lifelines.

 

Tube amplifiers are characteristic for its extreme few mixed results. The cause for that is to find in its mostly linear characteristic curve of its tubes and as an outcome of that an remote inevitable circuitry-wise degenerative feedback. With Electric amplifiers with semiconductors in signal paths, comparable or even better results are only achieved with a great effort.

 

 

Let´s get started!

 

By chance I had the opportunity to listen to an industrial manufactured tube amplifier. Despite its simple built-up electric circuit (as it showed at the low prize), I still was deeply impressed by its pleasant warm and clear sound. Can you build something like this yourself?

So what better to do than to start researching and looking for an old tube project where you can combine old knowledge and experiences with the new possibilities since the fall of communism?

Although there were enough construction kits on the market to built an tube amplifier yourself I finally was convinced of a project of a company located in Berlin. Within the framework of seminar papers and study dissertations they analyzed electric circuits of over 100 different industrial manufactured tube amplifiers, and initially developed an universal configurable amplifier, with whom they could rebuilt all these different circuits. After evaluation of an extensive field-test, in which the tested persons had to rate the individual electric circuits, they started in a second step to optimize the circuit. They worked in an exact and well-grounded manner on this topic and even analyzed the suitability of equal tube types but with different producers.

As an result stood out the "Black Cat 2", whose documents are available for download and provided for private use only. You will find more information and links to the project on the homepage of TU Berlin and also on „Jogis-Röhrenbude“.

 

 

Circuit charactericstics

 

  • continous conception as differential amplifier over all units
  • no semiconductors in signal path
  • choke (10H) in power supply for anode circuits, therefore low noise level and high signal-to-noise-ratio
  • separate anode voltage control for both channels
  • separate grid voltage control for both channels
  • separate heating voltage control for each of 4 preamplifier tubes
  • separate warm-up curve (e-function) for each preamplifier tubes to prevend heating wire breaking during cold start
  • biasing of final stage tubes from outside without opening the amplifier - setting and monitoring (by LED) at rear side
  • channel muting during start-up time (approx. 20 seconds)
  • power supply for optional phono preamplifier is already integrated
  • consequent using of industrial plug-in connectors (easy to service)

 

 

 

In the end I transformed the "BlackCat" into a "WhiteCat" in giving the amplifier a stainless-steel housing. All necessary construction plans and engineering data I developed myself with a 3D CAD program.

 

 

 

 

 

Technical Data

output power

2 x 41 Watts (RMS)

amplitude curve

10 Hz … 40 kHz between +/-0,1 dB (!)

phase curve

-12 … +12 degrees between 10 Hz … 40 kHz

distortion factor

0,033 %

difference tone factor

0,002 %

signal-to-noise-ratio

>100 dB

 

 

 

Conclusion

 

As we all know, there is no accounting for taste. It´s everyone’s own choice if you like my construction or not. But I am convinced that there won’t be any diverse opinions when it comes to evaluating the sound performance, once you listened to a BC 2. Everyone who knows the goose bump feeling, knows what I mean.

 

Meanwhile I started running several examples of this tube amplifier. Every single one of these self-made devices stunned me with their clear and pure signal. The sampling and localization for example of instruments is excellent. I caught myself listening more often to certain CDs because now they “found” the right amplifier. But it also works the other way around: A bad recording or mixing will not get any better with the BC 2.

 

At first the realization of this tube amplifier project seems like a lot of effort. But regarding the prizes on the market for comparable amplifiers a self-construction is absolutely worthwhile.

 

 

 

Phono amplifier

 

Tube amplifiers in general are built to connect one or more signal sources between which you can switch (Like the BC2). Typical signal sources are CD-Player, Tuner or various DAT systems, Mini disk, but also record players.

For physical reasons I won’t go in to detail here, records have a standardized but nonlinear frequency-response-curve, which has to be corrected while playing. This is the task of phono amplifiers as they have a frequency-response-curve called RIAA and in the same time they amplify the mostly weak signal of the records.

But of course many ways lead to Rome. This is why there exist several circuits, who all the more and the less offer the required equalizer curve.

All these circuits were tested in the earlier mentioned dissertation. They all got evaluated in terms of sound quality in a test run similar to “Black Cat”. As a result they finally developed a phono amplifier. You can find the datas and information free for Download (for private use only) on the already mentioned websites.

 

I adjusted the design of this phono amplifier to the outlook “White Cat”. Again, all necessary construction plans I developed myself with a 3D-CAD-program. As already mentioned the power supply of the phono amplifier is carried out by the BC2. Both devices are connected through a multipolar wire lead which holds the power supply wires, as well as the wire for both audio signals.

 

 

 

 

My first audio impression did not disappoint me, even though I only used an old GDR-record player with a tangential arm. I guess this player could not fully max out the phono amplifier. Later I tested the same with two good middle-class turntables from Pioneer© and DUAL© with magnetic systems. The result: A Powerful bass and a good ranging of the heights confirmed the exact course of the characteristic curve distortion for the phono amplifier in both devices.

 

For all who don’t know: Old records need to be cleaned from time to time. This is not only necessary to preserve the vinyl and the scanning system, but it also reduces the so typical spatter and sizzling noise so you sometimes have to look twice to really be sure what signal source is actually connected to the amplifier. Some people even have special equipment and a scrubbing solution for cleaning their records. But this is everyone’s own choice.

 

But even with records it is still the same: A bad mixing or pressing is an almost impossible task even for a phono amplifier.