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Monday, August 25, 2014

Differential 10Y Line Preamp, Part 3


After living with this new line preamplifier for almost a month, it is time to write a bit about the sound impressions.

When I was confronted with the challenge to find ways to improve upon my single ended line stage, that was a tough one. I am very happy with that preamp and have not changed the design in many years. There are users of this line stage around the globe who gave the most positive feedback about it's sound. There was also a test recently in a webzine called hifistatement. Click here to read that report, which is now also available in english.

Many years ago I experimented with differential designs. Mostly in phono stages. The results were extremely good and the total lack of capacitor coloration has some intriguing qualities. But those phono stages were very big, heavy and costly. So after going commercial with audio I decided to scale back a little and stay with the single ended designs.

I always wanted to try the same differential approach which I applied in said phono stages to a line preamp with directly heated triodes. So this was the opportunity to revive this concept.

Is it worth the extra effort and cost? Yes, the difference is clearly audible. But not huge. Don't expect big steps in sound quality from such a high level. It is more of an evolutionary step than a revolution. And honestly I am glad that the basic sound character remains very similar, despite the different circuit topology. It just improves on the strengths of the single ended concept. Resolution and tone colours get further enhanced. These are already extreme strengths of the single ended line stage. Yet the differential one can improve on them without negative side effects which often occur with higher resolution (artificial, antiseptic sound). Especially the tone colours seem truer to the original. This is due to the lack of capacitors in the signal path. Even the best caps add their bit of coloration. It also adds upon the 'lifeliness' of the presentation. It gives an even better feeling of 'being there'.

This line stage becomes the top level of my offering. It will now be built in a very exclusive version using transformers which are silver wire on finemet cores. Based on the positive results with tho line stage the differential phono stages will also get revived as an alternative series. I am also thinking about power amps to complete this line. Stay tuned!

Best regards


Saturday, August 23, 2014

One Million Page Views


Today the page view counter hit the 1.000.000 mark!

Thanks to all readers of my blog for your interest and support!

Best regards


Sunday, August 17, 2014

Tube of the Month : The 1LE3


Last month we had a directly heated tube with the unusual Acorn shape and base. This month I'd like to present another tube with a base which I have not shown before, a Loktal tube, The 1LE3.

The Loktal base (sometimes also written 'Loctal') was introduced in the 1930ies by Sylvania to compete with RCA's metal tubes which were on Octal bases.

The Loktal tubes had center shaft with key for correct orientation and an indent which locked the tube safely into the socket. Hence the name which indicates this Lock-In feature. These tubes were especially targeted at military and portable applications which require the tubes to sit tightly in their sockets. But they were not very successful and not many manufacturers besides Sylvania made these tube types. The Loktals were actually all glass types and used the wires which extend through the glass as base pins. This way they had low pin capacitances and were especially suitable for RF use. The metal ring around the bottom left the area around the pins exposed and was probably there only for shielding.
These bases and also the pins of Loktal tubes are a bit probe to corrosion. On the base that is merely a cosmetic issue but tube pins usually need some cleaning after longer term storage. This was probably also a reason why they never really found wide spread use. There had been many equivalents to popular Octal tubes. these are still available at good prices due to the uncommon base. Some of these will be covered in future Tube of the Month articles. Smart amplifier builders resort to such ignored types instead of hunting down expensive Octal types. With some cleaning of the pins these are well usable and worth considering for audio projects. But let's get back to the 1LE3. I have chosen this one for this month since it is also a directly heated triode for small signal applications. It's filament needs only 1.4V at a minuscule 50mA. So this tube would be quite suitable for using filament bias. The complete technical data can be found here. The plate curves in the data sheet look quite promising:

As the curves indicate the 1LE3 is meant to work well into Class A2 territory with the grid going positive and it stays fairly linear there. As always, let's see how an actual tube holds up to this on the curve tracer:

Let's have a look at some 1LE3 tubes. As mentioned most of the Loktals had ben manufactured by Sylvania. All 1LE3s I have come from this manufacturer.

Different style boxes:

Branded Philco, but made by Sylvania:

1LE3 produced for the military and also labelled VT-239, which was the military designation of this tube:

Let's have a closer look at the base:

On this tube we see the corrosion which is quite typical for Loktals:

Some close up shots:

I have not used this tube yet in an audio circuit, but find it very interesting. So many tubes and so little time...

Best regards


Tuesday, August 12, 2014

D3a LCR Phono Stage Cost Down - Phase 1


So far the D3a LCR phono stage is the best I have to offer for vinyl playback, except for the similar preamp equipped with Telefunken EC8020 tubes or the exotic DHT phono stages. The circuit is built without compromises an this comes at considerable cost and requires also a lot of space. Since that price is not affordable for many who inquire about such a phono stage, I developed a cost down version of it. At the same time also the size has been reduced which is also an issue for many.

The goal of this phase 1 of the cost down was to down size the circuit with no or only minimal impact on the sound quality. Phase 1 indicates that this version will be followed by yet another further cost optimised version. More about that later.

An obvious possibility for cost and size optimisation are the many chokes used in the original circuit and the oil capacitors. The chokes have been minimised to just 2 in the power supply. All oil caps which are not directly in the signal path got replaced by electrolytics. The rest is kept the same. 2 stage circuit, both stages transformer coupled for low output impedance and 600 Ohm LCR RIAA EQ.

The reduction of number of chokes and oil caps allowed to mount everything inside the chassis which halves the volume. Everything is mounted on sub modules in sandwich fashion. The vibration damped mounting of the tube sockets as seen in the original design is kept, although now hidden inside the chassis:

The inside of the finished signal section:

The power supply is constructed in a similar fashion with everything inside the chassis. Due to the restricted size, the all tube rectifier bridge had to be replaced by a single rectifier, a 6AX5 augmented by silicon diodes to form a hybrid rectifier bridge. Both the high voltage and heater voltage maintain the choke input supply theme. All the caps in the PSU are electrolytics now. Power transformers for high voltage and heater supply are kept separately:

The finished prototype of phase 1 of the cost down in operation:

Although the chassis look nice arranged like that or stacked, they are supposed to be kept further apart from each other. Although usable as above a slight hum got induced into one channel (the one closer to the PSU). But separating the units by just 10-15cm already made the hum disappear.

How big a toll did the down sizing take on the sound? Actually surprisingly little. This is due to the fact that only parts not directly in the signal path got replaced. All the strengths of 600 Ohm LCR equalisation are maintained: Very natural and detailed representation wish beautiful tone colours and very clear articulation of voices. Maybe resolution is a tad lower compared to the big preamp.

A down side for some might be that the tubes are not visible anymore. Therefore these units are much easier to dust off.

If you had contacted me in the past for a LCR phono stage and the price was too high, check again. These might fit the bill now. The phono stage can be built for MM or MC sensitivity with built in MC step up transformers. MM gain is 42dB at a very low output impedance. Gain can be increased by 6dB with a 4 fold increase in output impedance. Gain could also be reduced down to 36dB if needed. MC gain depends on the step up ratio of the MC transformer whig best selected.

The first listening results were so encouraging that I decided to follow this one by a phase 2 of a yet further cost reduced version. That version will let go of the transformer coupling in the output stage and will do without chokes in the power supply. Stay tuned for an update when phase 2 gets finished.

Best regards


Friday, August 8, 2014

10Y Differential Line Preamp, Part 2


In the previous post about this differential line stage I showed some of the initial chassis construction. In the meantime the preamplifier is finished and the concept is proven.

After the subassemblies with the tube sockets had been mounted, part of the many chokes used in the circuit got attached on the top side.

These will later be hidden under some beautiful metallic bordeaux red covers. The bottom side of the plate with some wiring:

The main part of the circuit is in the inside of the chassis:

The transformer volume controls can be seen on the left side. Each input goes through an input transformer which splits the signal to drive the two tubes per channel. These are mounted in a vertical row behind the volume controls. Each set of input transformers is wired for different step up, 0dB, +6dB and +12dB. Behind the input transformers are line outputs which drive the volume control autoformers and many more chokes.

As can be seen in the photo no capacitor whatsoever in the signal section. A truly capacitor less signal path!

The finished preamp:

Here with it's external power supply:

The power supply is basically the same as that of the single ended version, just adapted for the increased current.

The backside shows the three inputs and pair of outputs:

Side view:

The four 10Y tubes which amplify the signal:

The ground configuration needed a bit of tweaking. Now the prototype is playing and is very quiet. First listening tests immediately showed the qualities of this concept. Eliminating the last capacitor from the signal path is certainly worth the effort. While the single ended 10Y line stage is a brilliant performer with gobs of resolution and tone, the differential line stage adds even more naturalness and body.

Stay tuned for more detailed listening comparisons between the differential and single ended line stages.

Best regards