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Started by changing the earthing arrangement in the nva based amps.
Went back to star earthing the 0V connection to the amp board and 0V connection psu on front end at the clean end of the output stage 0V . Speaker return to dirty end of 0V at rectifier end. Also put wires along side each other that were in the same current loops. Effect was bugger all. Still a moderately loud hum, no reduction at all.
Started messing about with the transformers whilst amp was switched on . I noticed the hum changing volume as I moved the little toroid around . Disconnected the psu's for the other adjacent mono bloc and tightly twisted the lead in and out wires on both the transformers still connected. This helped a lot and the hum reduced in volume considerably. Played around with position of both transformers and got them in a position where the hum is only just audible with my ear touching the mid driver.
Feel like I have got somewhere with this. I can still hear a noise through the speaker when I move a screwdriver along the faraday cage of the TIS cable. I am listening to one channel with all 3 drivers and the other with just the big 12" on the end of the Avondale amps. Sounds promising even though listening to the whole of the left channel and a third of the right.
Gonna play around and tidy up the arrangement in the pre amp next which looks like your usual diy work in progress (think spaghetti junction).
So does it still sound like I have an earthing problem or not ?
I haven't a clue whats going on either That is true to a certain extent and it is fun finding out. I do get a bit annoyed though when unexpected things happen and things aren't perfect first time.
Tried a few things and nothing worked - even a crocodile clip around TIS faraday cage to mains safety earth with system earth floating. No joy.
This morning I tried a cheapie phono cable between amp and pre and it still hummed. WTF. I only originally became aware of the hum when I introduced phono sockets and TIS between pre and power. So that didn't make sense to me.
Next step was to put the nva based amp back to single psu for the whole board. No hum with cheapie phono cable. Swapped cheapie for TIS and no hum. So somehow having separate psu's for front and back end was causing the problem. Well I never noticed it prior to getting the TIS.
Not sure exactly what the problem is
* interaction between transformers - doc does not have a problem
Not sure how it could be earthing as I did same as Avondale amps - one 0V link between output stage cap bank and input stage cap bank (straight after the last pair of caps on each )
Any ideas anyone ? Starting to go slightly mad here.
Musical Fidelity M1 DAC, AppleTV4 with Apple Music subscription, ripped CDs streamed via WD MyCloud Home Duo NAS, with PLEX media server.
Speakers - Large sealed cabs, with Fane 12-250-TC 12" full range drivers.
But with a stereo amp, they will always end up connected via the interconnects to the source component. There is never a perfect solution, just a good enough one.I would have thought that with a dual PSU, you would be going for complete isolation between the stages. Connecting them together at the final cap end of each seems counter intuitive.
I copied exactly how Les at Avondale does his voyager amps which also has separate psu for input & output stages. The only difference being my Avondale amps have a regulator on the input stages.
In the Avondale & nva I have one psu on output transistors & driver transistors and the other on LTP and VAS. I cut the middle of the 3 wire links on the nva board power rails.
I am going to draw a picture out of how I did it and post it up here.