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Tuesday, November 19, 2019

The Differential Digital to Analog Converter


In a previous post I showed some of the latest DA-Converters I built. Among those a fully differential all silver version. In this post I will present that one in more detail.

While previous versions of the DAC used a single ended output stage with ELROG ER801A directly heated triodes, this one doubles up the number of tubes for fully differential operation.

The output stage follows a similar circuit topology as my differential 10Y line preamplifier.

It follows the same approach as the other DACs with separate power supplies for the output stage and digital sections.

In addition it uses an even higher precision clock, which comes from a separate 'SuperClock' module from eXD. Here we see some construction steps. The Clock module is mounted directly on the underside of the top metal plate.

Then silver output transformers and silver plate chokes on the next level. And finally the DAC boards:

There are two DAC boards mounted on top of each other one for each channel for  fully differential operation. The finished DAC:

The receiver board is mounted to the back plate and receives two clock signal from the Clock module through coaxial cables.

The High voltage and filament supply are similar to that of the differential line preamplifier.

The classic full wave bridge configuration with 4 TV damper tubes.

The digital supply chassis:

This now houses 3 independent power supplies, one each for the receiver, Clock Module and DAC sections.

The power supplies can be placed separately.

Or stacked.

The digital supply receives a switched mains voltage from the analog supply which acts as the master.

Digital and Analog supply voltage are fed via separate umbilicals to the DAC chassis.

Which has both XLR and RCA outputs. Inputs are Coax or USB, selectable via the switch at the front.

The DAC with the tubes lit up.

This DAC will be on demo at next years High End show in Munich.

Best regards


Sunday, November 10, 2019

Tube of the Month : The 6DA4 / 6DM4


Since we did not have any TV damper tube yet this year, I dug up another type from my stock: The 6DA4 / 6DM4.

Regular readers of my blog know how fond I am of TV damper tubes. They play a vital part in my amplifiers since almost all my power supplies use them as rectifiers.

The 6DA4 and 6DM4 are very close in their electrical parameters so are interchangeable. Many different tube designations have been registered for TV damper tubes even when there virtually was no difference between them. Probably to avoid patent or licensing fees. It was common practice later on to print several designations on tubes. This was especially often done for the 6DA4 and 6DM4 so I cover both types in one post. They are actually also very close to the 6AX4 which is the TV damper I use most often. But I also started to equip my power supplies with 6DA4 or 6DM4 in the recent years. I wrote about the technical background of TV damper tubes in other posts, especially in the 6AU4 Tube of the Month post, so I won't go into those details again. For complete specs of the 6DA4 / 6DM4 please refer to the General Electric data sheet. In this post I will show many different 6DA4 and 6DM4 from various manufacturers. If you are bored by TV damper tube porn, now is the time to move on, if you are into this stuff, be prepared to gaze at tons of tube photos.

Starting with the RCA 6DA4.

This one comes in a tall bottle and has a massive, reassuring plate structure:

Some details:


Looks very similar.

A RCA with both tube designations:

A 'coin base' type.

More RCAs:

General Electric of course also made many different 6DA4 / 6DM4 tubes.

They also made them with the coin  base:

Yet another GE:

Next we have a Tung-Sol 6DA4.

Zentih 6DA4A / 6DM4A:

Another Zenith:


It was quite common that the glass would not sit straight in the tube base. After all these tubes disappeared inside a TV set so this was not of concern.  Very different now when I use them on top of a power supply chassis where they want to be looked at.


Magnavox was an equipment manufacturer who branded tubes from other tube companies.

More rebranders:

RTT, Lindal:




Next we have various tubes made by Raytheon:


And lastly another of the big players from the vacuum tube era, Sylvania:

Like most TV damper tubes these were made in a huge variety of styles. Also a lot of rebranding and crossbranding was going on as can be seen in the construction details shared across the various brands and makers. I hope you enjoyed this photo series.

Best regards