Another small note: conventional tube preamps can be dangerous for transistor amps unless they have an automatic muting relay with a timer circuit. A conventional tube preamp with cathode follower output typically has the output node (the cathode) somewhere between 150 to 200 volts, with a 1~3 uF capacitor keeping that DC potential out of the output of the preamp (a blocking cap). Unfortunately, as the preamp powers up or is turned off, large transients that are a significant fraction of that 150 volts can appear on the RCA output, which is why a muting relay (with timer delay) is required for safety.
This was harmless during the days of power amps using nothing but tubes, in the Fifties and early Sixties. The power amp would be warming up and unable to transmit the preamp transient to the speaker, and in addition, vacuum tubes are not harmed by high voltages ... they can accept hundreds of volts without risk of arc-over.
Transistor power amps are completely different. The input differential pair, either bipolar or FET, can be destroyed or degraded by pulses larger than 10~20 volts, as well as passing on speaker-breaking pulses since transistor amps turn on instantly. The damage to the input pair can happen even when the power amp is turned off, unless the power amp has input muting relays (the relays are usually on the output side).
The classic tube preamps of the Fifties and Sixties never had muting relays because they weren’t needed ... the tube power amps didn’t mind, and took a while for them to warm up and amplify. Muting relays appeared on high-end gear in the Seventies and later, because users were losing amps and speakers because of these transients. The catch with muting relays is they can introduce a new source of distortion from slow development of corrosion on the relay contacts.
Transformer coupling has the charm that it can never pass DC, under any circumstances, and unlike a capacitor, does not store energy. In addition, the circuit itself is balanced, and is only slightly out of balance as it warms up. Although we do not recommend it, you might only hear a minor "click" when the preamp is turned on with a "live" power amplifier. Doing this is dangerous for your system and we do not recommend it; always turn on preamps before power amps, and always turn off power amps first (and wait a little while) before turning off source components.
Being careless with a conventional tube preamp is quite different. The survival of your system depends on the reliable operation of the muting relay ... for example, if you have a power outage, or a power interruption, does the muting relay instantly activate? It must, if your transistor power amp is going to survive. A tube power amp would give a helluva bang (tough on your speakers) but it would survive just fine.
The output transformer sidesteps all that. No energy storage, and does not pass DC, ever. (There is no metallic connection between primary and secondary, only magnetic.) No requirement for a muting relay. No timing circuits that require a separate 5VDC supply. Worst case scenario, you might hear an annoying click, but no amps are harmed, or speakers destroyed.
