GoodSoundClub - Romy the Cat's Audio Site

In the Forum: Audio Discussions
In the Thread: It’s mad, mad, mad... electricity.

Post Subject: Follow up on the core issues on tube SMPSPosted by JOHNR on: 7/14/2016

I did not follow up on the issues relating to improved performance on small smps when used on filament and DAC power supplies.

Here goes:

The first is that the source impedance of this type of supply is very small e.g. less than one ohm. The speed of the output is 500 kilohertz and the energy storage is held in a very small inductor.Because this technology creates noise spikes between five and twelve megahertz, the output requires a number of common mode chokes. Capacitors do not help because of their self inductance. My circuit uses three cm chokes to absorb this noise and they serve a dual purpose in that these also keep other sources of radio frequency noise out of the load.

Surprisingly, the removal of such noise from a tube filament gives a very large reduction in background noise and gives the impression that the tube is sonically much quieter and faster.

Technically, common mode noise has no real power since it is only a pulse however, it becomes mixed with wanted signals and disrupts the sonics. The only way to control it is with a ferrite based inductor arranged in series with both legs of the power feed pathway.
The absolute perfect power system would use one choke per power supply to each active device but except for small circuits, this is not practical.

When we come back to the importance of the 500 kHz operating frequency, small polymer capacitors are used to control the residual noise so that we can obtain an almost perfect d.c. supply. These have very limited ESR and ESL which is quite important.

Whilst developing this circuit, I found that any common mode noise fed to a filament will appear on the tube output amplified by a factor of 1 (tube physics). By using a single supply on the small signal tubes, sonic improvements can be obtained.
This also applies to the 45, 2A3 and 300B power tubes as well.

In the case of a dac using either one supply (ESS series) or dual supplies (1702 series), the sonics show a very large improvement. Background noise is no longer an issue giving much improved silence between notes.
Hope this helps.

WRT smps based B+ supplies, the same rules apply but the circuit is more complicated so I will deal wth that some time in the near future.

In the past, when discussing the use of common mode chokes, it has been stated that one choke kills the bass. I counted the chokes in my personal system and it comes to 54. I guess it has something to do with how they are used.

JohnR

Rerurn to Romy the Cat's Site