To complete the PSU I need to buy a couple of transformers. You'd think this would be easy but I've spend a few nights considering my options.
The manual which comes with the kit (link above on an earlier post) reckons that 80VAC (or volt amp current) per channel should be more than adequate. So that would suggest two 80VAC transformers then? However, when you look around poeple seem to use a lot of different sizes.....arguing that bigger is better (soundwise).
There's also another complication of which I was largely unaware....the impedence of the speakers. OK, so I knew that one had to be careful when matching speakers to amps, particularly ones which are difficult to drive, but it seems this is far more important than I imagined.
Depending on the impedence of the speakers, different transformer voltages need to be selected. So for this particular chip, for 4 ohm speakers, you don't really want above 25VDC (volts direct current), 6 ohms around 28VDC and for 8 ohms around 35VDC. It's a bit of a pain that once a transformer has been selected that it limits current and future speaker selection.
I don't have any speakers for these amps yet. I need something small so was considering the Dali Lektor 1s (8 ohms). However, the Lektor 2s don't seem much bigger so I thought about these as a possibility (6 ohms). I was also considering using a couple of my Musos (4 ohms). Damn! I finally decided to go for 8 ohm speakers (not sure which ones), though I will return to this below.
So now I knew I wanted 35VDC. However, the rectifying bridge increases the AC voltage from the transformer by around 40%. So, to get 35VDC into your amp, you need a transformer with secondaries rated at 35/1.4 = 25V. OK, so 2 x 25V secondaries. I'm narrowing it down.
Back to the VAC. The volts x the amps required = VAC. How many amps are required? Hmmm, not sure. As I said earlier, people tend to go for monster transformers which is supposed to improve the sound. I decided to go for two 160VAC transformers. This equates to 160/25 = 6.4A of available current.
So, 160VAC transformer with 25V dual secondaries, i.e 2x25V. I didn't really consider any type other than toroidal but which one to get? I wanted quality. I wanted low noise. I wanted economy. In the end I went for two of these fully encapsulted jobbies from RS Components
here. Note: I believe that Nuvotem are a european version of Amveco.
Nuvotem fully encapsulated transformers.
So, back to the impedence issue. Although one could argue that an amp tuned to the attached speakers will be a better match and not suffer the consequences of commercial amps, which need to support all speaker impedences, it's a wee bit inconvenient.
Clearly, it'd be good if there was a way to accomodate any speaker. I have created a thread on diyaudio on this very topic
here.
From my generally ignorant view point, voltage regulation would appear to be the way to go....so that the power getting to the amp is reduced to match the attached speakers but it's never as simple as that. Any luck, I'll get a simple solution from the posters on diyaudio.com.
My useful(?) threads can be found
here.