sound practices reading club /22

Up to the next level. We are reading Euridice Preamplifier from sound practices issue 10, by Ciro Marzio, Cristiano Jelasi and Luca Chiomenti. They start off the article with some historical context for transformer coupling, then move on to their own reasons to experiment with transformer output line amps, culminating in the superior AC efficiency of transformer coupling. Then there is the catch, the transformers and their required quality. The next section is about construction details, after which the guys share ‘what’s it like.’ Finally they sent us off with ideas for trying out directly heated triodes and two experimental schematics.

teaser quote: ‘But it is astonishing that with modern capabilities, transformer coupling is so seldom used. After all, transformer coupled circuits were a mainstay of the highest quality professional studio gear through the 1960s. How did contemporary hi-fi design get so overspecialised and banal that a simple classic transformer-coupled circuit appears totally revolutionary?’

a gift from Joe Roberts, SP editor: from the sound practices CD, here is the original article.

my take

The power efficiency argument our trio of authors make is important, but not in the way they meant it. Class A single-ended is not where AC power / DC dissipation arguments go a long way. No, what matters is simply high AC (read: music) efficiency. Combining the experiences of other experimenters with my own, I formulated years ago that the biggest crime in audio is wasting AC power. When AC power is wasted (prime suspects: resistors seeing signal), the control that a tube (or fet/transistor) tries to exert over the next stage in the system simply dissipates in heat. In quite a few situations it also reduces how much a tube (etc.) gets informed about the demands of the next stage. With transformer coupling—power coupling—, the AC power wasting is very low and the driving tube is fully connected/informed with the demands of the next stage.

One thing I notice with the to schematics is that that the bottom (417A) one is true power coupled to the next device, whereas the top one (CV 5112) has a resistor across the secondary of the OPT and this put us straight back into AC power wastage land.

The authors are a bit fast with ‘avoiding the use of a very critical coupling capacitor in the signal path.’ In both schematics, both the 470µF at the end of the power supply and the 220µF at the kathode of the tube are fully in the signal path. All music flows through them. My immediate reaction is to get rid of them, bias the tube in another way and use a VR tube at the end of the PS. Then we are listening to the sound of no-caps, just the tube and the transformer.

Finally, I really like that the article ends with an invitation to ‘go even further,’ with ideas for tubes and two schematics of alternative topologies to get the creative juices flowing.

Now go and read the article, see you next week.