Can't believe it's been four months since i updated this thread. Been all nva based amplification since September when i swapped out the avondale boards on bass duties.
I am listening to my speakers quite a lot at the moment with as much different music as i can throw at them so decided to do something whilst listening. I need to get my last two nva monblocs up and running to make up the six pack that i always promised myself
Two on bass drivers, two on mid bass and two on the tweeter family
. I could go to eight (separate out the two bass drivers in the isobaric pair) but that would be just greedy and it would take up half the room
These last two amps are going to be on tweeter duties because i am running out of big transformers. What i do have are two 200VA 28-0-28 V ac single centre tapped secondary transformers that used to live in my Naim Nap140 amplifiers (long pulled to bits and abandoned
) I am going to use them for the output stages of these two amps.
For the input and driver stages i have four rather lovely audio grade 100VA EI transformers with twin centre tapped secondary windings of 26-0-26 V ac. I am going to use two of these for the input stages, the other two are going to be used elsewhere (phono stage). I have used these before on tweeter duties but with one winding on the output stage and one on the input stage. I do love what a good EI transformer does (not the stray field though
The plan here is to use one of the secondary windings for the positive side of the psu and one for the negative side. Yes it can be done, i am using figure two.
It helps make use of the whole of the transformer rather than just one winding. I have one powered up and it gives 39-0-39 V dc on the output of the kendeil smoothing capacitors. A slightly blurry photo.
I am using Avondale Audio minicap boards for this(had them for ten years) which have rectifier diodes on and then six 6,800 uf caps (three in parallel per polarity) with very small value chokes (12 uH) in between. These chokes filter out some high frequency crap. Not to be used on output stages though as we need to keep the psu impedance as low as possible.
There was a small snag. I couldn't use the psu boards as they were because the diodes share a common AC input trace and i needed them to separate in order to run each polarity of the psu from a separate transformer winding. So i had to lift one of the legs of two of the rectifier diodes (the negative side of the psu) to isolate them. Then i could wire in the two sets of transformer secondary wires. The transformer centre tap wires go to a common 0V point on the rectifier side of the board. Look away if you are from the internet diy electrical safety police
Just one more of those to do and the two output stage psu's. Waiting for some pads to electrically isolate the output transistors from the heat sink so i will get all the psu's ready and tested .
I have regulators on the front ends of my other four nva based amps. I had excess voltage from the four transformers so it was a good way to burn a few off for the input stage. In my opinion it does bring many benefits sound wise. Sorry Doc
I don't have any excess voltage to play with when using these transformers so no regulators on these last two.