Electron Engine ™
Printed Circuit Boards by Emissionlabs
Output Impedance vs input Impedance - Some Notes.
there is belief, transformers have an impedance. This is not so, because they have a wide range of impedances they can work with, and transform within that range any impedance from the one to the other side.
So if you connect an 8 Ohms resistor to a 1:1 transformer, guess what you see at the other side? It is 8 Ohms. So do we have an 8 Ohms transformer then? Well connect 4 Ohms to the one side, and you'll see 4 Ohms. Changing nothing to the transformer can not make it an 4 Ohms and am 8Ohms transformer at the same time.
Ohm's law applies just normally. So when using a 5:1 step down transformer, suppose we connect 5 Ohms to the output. Assuming we generate 10 Volt across it, the current will be 2 Ampere, and the power is 20 Watt. The transformer transforms voltage and current with a factor 5. So at the primary we have 50 Volts, 400mA. No surprise, that is also 20 Watt. But what is the impedance of 50V, 400mA? Just use Ohm's law, and you get 125 Ohms. So now knowing the windings ratio, we can still specify this transformer as 125 Ohms primary, if it is intended for 5 Ohms speakers. But .... connect a 6 Ohms speaker, and you will see 150 Ohms primary impedance.
To avoid this game with Ohm's law, we can just say the impedance is transferred with the square of the windings ratio. So 5x5=25.
The switch boards we offer here, change the SECONDARY windings ratio, but serializing the secondary windings in another way. This changes the windings ratio of the transformer.
Now, assuming we specify the same PRIMARY impedance, the change in windings ratio, will REQUIRE another load, in order to get this primary impedance.
OR.... and this is important to understand: Assuming we do NOT change the speaker impedance, in that case the change in windings ratio will result into a change of primary impedance. This is how the system works.
Lets make an example for LL1623 with 3k primary, and the EE16 board gives 4Ohms or 8 Ohms at the output. So flip the switch, and swap the 4Ohms speaker for an 8 Ohms speaker. This is what is says in line #6 above here, in the table. But what happens if you leave the speaker of 8 Ohms connected, and flip the switch to 4Ohms anyway? Many things! The transformer will now change from a load of 3k, to a load of 6k. The output tube can now much easier drive this load and sound quality will improve. What will not improve is loudness, but if you have more than enough loudness anyway, the better sound quality becomes very audible. Moreover, the microphonics of the whole amplifier will be reduced by a factor two, which becomes EXTREMELY helpful, in such cases where there is more then enough output signal, and the amplifier becomes (of course...!) hum sensitive and microphonic.
Conclusion:
After reading this, hopefully you are not more confused as before. It is just so, you can never say in advance, an 8 Ohms speaker sounds only good at an 8Ohms output and not at 4Ohms or 16 Ohms. Some speakers have 8 Ohms written on them, and are in reality 6 Ohms, or 10 Ohms. Also we need to mention the extreme, sometimes absurd impedance changes some speaker manufacturers allow their products to have in the low frequency range. These PCBs, EE16 and EE18, allow dual impedance. So of your 8Ohms speaker sounds best at the 8 Ohms Output, that is like expected, and should be so in many cases. But.... just not always. So if by any chance, you find it sounds better at the 4 Ohms Output, just by flipping the impedance selector switch, you will be glad you purchased those PCBs.