Thrax Audio introduction & Tech pt.4

POWER SUPPLY

Now we at the stage where we have to power our silent transformer coupled gain stage. The power supply should not disturb the ground plane that the gain stage is using for its reference.

Passive filtration as used in 99% of tube amplifier provides great results mainly due to the low currents involved in tube electronics. Chokes are the absolute greatest tool for power supply filtration and they are very effective at stopping EMI/RFI.

Regulated power supplies with tube electronics are bulky hot and complex. Requiring few tubes and usually end up being way more complicated than the amplifier they power.

Solid state regulators just can’t deal with the voltages involved, the integrated circuits give up around 100V and discrete regulators are as complicated as the ones made with tubes but a lot more fragile.

As a result people assume passive filtering is all that can be done and never looked at the issue again.

The development of electric cars brought to market new solid state devices for power electronics capable of working at 100’s or even 1000’s of volts, so we immediately took advantage and designed a regulated power supply for our tube electronics, but here comes the difference.

A major issue with all power supplies is the varying load current demand. Playing music forces the whole music envelope to go through the power supply creating an abundance of inter modulation artifacts that no matter how much you filter, always get through to the output. The regulator will keep the voltage stable no matter the current consumed forcing current peaks not normally associated with the circuit operation. Most regulators revolve around an error amplifier that compensates for deviation. The fact that you need an error to compensate says all that's needed.

Facing this challenge we took a different approach. Instead of filtering and fitting the current variations of the amplifier stage, we prevent their formation in the first place. Wait, how?

We decided to make the gain stages to appear as resistors to the power supply. No variation of current at all. With no modulation, then there is no need for filtering!

How do we do it? A shunt regulator is a two terminal device with a fixed working voltage. When wired parallel to the load it passes as much current as it can or none at all to compensate for any movement relative to the reference voltage on it end. Maintaining that voltage constant. Well how is this any better than the regular ones? In the particular case when power to those regulators is supplied by constant current sources, meaning the current is kept constant at all voltages regardless. You end up with a very interesting solution. No AC current flows outside the loop between the amplifier and the shunt element.

The power supply completely detached itself form the gain stage as the current is constant maintained by the constant current regulator and the voltage is maintained constant by the shunt element. Another Eureka moment in the design of the Dionysos. Actually this has been used in measurement gear for some time :-)

This topology is known as current fed shunt regulators and at the voltages involved I think ours was among the first commercial implementations.

This way the power supply has no AC current going trough the ground plane and it makes a whole lot of difference. And as I mentioned earlier I love chokes, so to make the operation of the regulators easier we inserted a choke to filter the noise from the mains transformer and rectifiers stopping also all the RFI.

As you see our simple idea for the Dionysos started to get complicated. To manage this complication we went a step further. We decided to hand over all the controls and monitoring to a microprocessor to make sense of it all translating all the functions to the simple familiar controls we all are accustomed to. What else and how the Dionysos does it is coming up in the next post.