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Wednesday, August 21, 2019

The SplitPath EQ Phono Stage


Since a few years I had an idea how to implement the LCR EQ in a different way, but kept it on the back burner due to other projects. Now I finally took a shot at what I call the SplitPath EQ.

This concept drastically reduces the signal path for part of the frequency spectrum. In my quest to improve sound quality keeping the signal path as short and direct as possible always was a key factor. But at a certain point there is a limit reached since total gain needs to stay within practical limits and I always want to keep output impedances low. I always feared that this new approach would run into various problems in an actual implementation but I am happy to present the first working prototype which was implemented with D3a tubes.

Above the phono section which like all my preamps has an outboard power supply. While the phono section received a new layout to adapt to the requirements of the new circuit, the power supply remained the same.

Some technical background about this idea. One of the main tasks of a phono stage (besides providing a lot of gain) is the EQ of the signal. When vinyl is cut, the bass frequencies need to be attenuated and the highs boosted. This is to avoid that the grooves would need to be cut with excessively large amplitudes for each bass frequency hit, while the higher frequencies usually are not having very big amplitudes. Below a simplified diagram which shows how the signal is attenuated relative to 1kHz.

Below 500Hz the amplitude is dropped off by 6dB/octave down two 50Hz below which the attenuation is stopped. At 2122Hz there is another corner above which the amplitude is boosted by 6dB/octave. In the phono stage this needs to be reversed. In a passive EQ this is done with a filter which attenuates the various parts of the spectrum accordingly. That means that the highest frequencies are attenuated by 40dB more than the lowest. This always bothered me since the signal needs to be amplified accordingly only to be attenuated again in the EQ. Now my thought was why not handle the part above 1kHz separately from the bass EQ. In simple terms what I did was to let the higher frequencies bypass an entire amplification stage and recombine them with the rest after the bass EQ.

So the frequencies above 1kHz pass through only a single gain stage in the phono, while the bass frequencies pass through 2 stages as in my previous phono circuits. The actual EQ is done with LCR networks. The challenge was to recombine the frequency parts with the right levels to have correct overall frequency response and phase behaviour.

Please understand that I do not disclose more details or schematic diagrams. This circuit contains all my knowledge and experience from decades of working on phono circuits.

Here we see the construction steps of the prototype.

Everything is placed such to have minimal signal path length and well defined and short ground paths.

Lots of trials, measurements and simulations were required to get things right. Here the completed phono:

How does it sound? Well I got what I was hoping for. Yet further enhanced resolution and transparency in the frequency range above e 1kHz which results in extremely natural voices and beautiful reproduction of ambience. Also the noise floor dropped even further.

As with all things in life, there is no free lunch. The overall gain is quite a bit lower compared to my current phono. So attention is necessary to integrate this phono into an existing system. In my system I can use it without issues with Lyra cartridges and the 10Y linestage. However I am at the upper limit of volume control range. To address this I am about to develop a new linestage which will have a dedicated input for this phono to provide a bit more gain.

This new phono stage will not obsolete my current one which has more gain and is more universally useable. Both versions reside alongside in my portfolio with this new one being the higher end alternative. Next step will be to implement this EQ approach also in the differential phono. Stay tuned.

Best regards


Wednesday, August 7, 2019

Deutsche Elektronenröhren Manufaktur GmbH Anniversary


Today it is exactly three years since I announced the founding of Deutsche Elektronenröhren Manufaktur GmbH and the continuation of the manufacturing of ELROG tubes after the former ELROG GmbH Elektronenröhren & Co KG went bankrupt.

A lot has happened in those 3 years. Not only did we completely redesign every tube type to increase reliability and sound quality, we also developed new tube types.

The ER845 and ER211 were the first 2 types made by the former ELROG GmbH, which was joined by the ER300B in 2014. The most notable changes were done to those two, by inverting the structure to raise the heat generating plate higher up in the bulb as done in other 845 and 211 tubes. This eliminated heat problems in amplifiers with poor air ventilation around the tubes.

The ER300B was joined by two other UX4 based directly heated triodes, the ER50 and ER801A, the latter not only meant as replacement for the original types 801, 801A and VT62 but also the 10Y and VT25. The ER300B got completely overhauled with a more ruggedised filament and new plate structure. The thoriated tungsten filament was kept as this is the main reason for the sound qualities of our ER300B which is highly regarded as among the very best on the market and preferred by many over any oxide filament type.

And of course we also decided to make rectifiers in form of the ER274B and ER274A. These rectifiers surpass other 274B and 274A types with higher voltage rating. Many of our customers replace a 5U4 with the ER274B and it can also work in applications designed for the 5R4.

The ER284 and ER242 are premium variants of the 845 and 211 and are electrically compatible with those. They use molybdenum plates instead of graphite.

The latest addition are the ER887 and ER888. These are new tube types which do not have old production equivalents. These are driver tubes specifically designed to provide the large grid voltage swing to drive 845 output tubes in the best possible manner and in a 2 stage amplifier. Both types are electrically identical. The ER888 uses a molybdenum plate while the ER887 has a graphite plate. With these we now have a total of 11 tube types in the portfolio. Development will not end here and more new types will follow.

I would like to thank our customers and the entire tubephile community for the continuous support and encouragement. And of course a big thank you to the hard working staff at Deutsche Elektronenröhren Manufaktur GmbH, producer of ELROG vacuum tubes made in Germany.

Best regards


Wednesday, July 31, 2019

Tube of the Month : The 6BL7


This month we take a look at a double triode. Meet the 6BL7.

The 6BL7 was developed for TV vertical deflection oscillator and amplifier applications. It is similar in structure and pinout to the 6SN7 and 6BX7 but they have different amplification factors and other parameters.

The octal base pinout is shown on the left. The 6BL7 has an amplification factor of 15 which classifies it as medium mu triode. It has a hefty 60mA maximum cathode current and each of the two triode systems can dissipate 10W. Albeit not both at the same time. Total maximum dissipation for both triodes together is limited to 12W due to the compact size and small bottle. Still impressive and both systems connected in parallel would be able to work in an output stage of a small power amp or headphone amp. Capable to deliver about 2W in single ended operation. The plate resistance is about 2k per system so 1k when operated in parallel which is in the territory of typical output tubes. Such parameters require a rather high filament current of 1.5A. It would also make a great driver for large output tubes since it can deliver quite impressive voltage swings. For complete technical specs refer to the General Electric data sheet. Lets have a look at the plate curves:

And some curves taken from a tube on the tracer:

An interesting tube which gets little attention nowadays.

I haven't really used this tube myself and the small heap of 6BL7GTA above is all I got.

So lets have a closer look. Here is the tube in all its glory:

The rather large plate structures (for such a small tube) are immediately apparent.

Being designed for TV deflection service the tube had to be quite rugged.

The tube in operation:

A nice little gem!

Best regards


Friday, July 12, 2019

The Digital to Analog Converter


Here some photos of the first customer ordered DAC to be shipped soon.

Besides the black color for the transformer covers and capacitors this unit has an upgrade to the high voltage power supply compared to the prototype.

The PSU uses an ELROG ER274A rectifier in a hybrid bridge together with 2 TV dampers.

For some thoriated tungsten glow from both the main unit and the HV PSU.

The digital power supply is again housed separately.

A peek into the digital power supply shows why, separate choke filtered supplies for the receiver and DAC boards.

The DAC unit:

The DAC will be shipped with ELROG ER801A output tubes. Old production 801A or 10Y can be used as well.

And also an inside view showing the DAC and receiver boards.

Some more infos about this DAC here.

Drop me an email if you are interested in such a DAC for yourself

Best regards