V76s build - work in progress

[Ilya]

Why are the transformers not rotated 90 degrees to each other and/or encapsulated with mu-metal?

What transformers are you talking about?
Input transformer is PCB mounted and is mu-metal shielded. Output transformer does not have to be mu-metal shielded because the signal level is quite high.

90 degrees rotation is used between power transformer and input or output transformers to minimize the induced mains frequency. This is more relevant to the EI core transformers. My power transformer is toroidal. And it is mounted in such a way that minimizes induction to the sensitive input networks and input transformer.

 

[jokeramik]

What transformers are you talking about?
Input transformer is PCB mounted and is mu-metal shielded. Output transformer does not have to be mu-metal shielded because the signal level is quite high.

90 degrees rotation is used between power transformer and input or output transformers to minimize the induced mains frequency. This is more relevant to the EI core transformers. My power transformer is toroidal. And it is mounted in such a way that minimizes induction to the sensitive input networks and input transformer.

I mean the two transformers in the back left corner.
From my point of view and taking into account the known physical properties of magnetic fields, there is interference between these two transformers. Due the higher level, the degree of interference is high, since the magnetic field also increases. You can certainly do that with a decent audio analyzer meter if you fully control one channel. Then there will be crosstalk, which is never desired. I would always pack the parts in appropriate shields if possible. If not available, I would definitely rotate these 90 degrees. It is of course your project and your decision.
Sorry for my english, I´m not a native speaker.
Best regards!

 

[Ilya]

Well, this is a single channel preamp. One of those transformers is actually not a transformer, but the anode load choke for the output stage. So there is no need to rotate the choke which "sees" almost the same signal as the output tx.

 

[C12VR]

Ilya, this just occurred to me; did you test the Cinemag with a secondary load? Cinemag recommends 500k; maybe this could help FR?

 

[Ilya]

Ilya, this just occurred to me; did you test the Cinemag with a secondary load? Cinemag recommends 500k; maybe this could help FR?

No, I didn't. But Cinemag is reasonably flat, it's Sowter that falls off too fast.
I'll see if I can run a quick test with 500k load on the secondaries and if it makes any difference.

 

[C12VR]

No, I didn't. But Cinemag is reasonably flat, it's Sowter that falls off too fast.
I'll see if I can run a quick test with 500k load on the secondaries and if it makes any difference.

Thanks. Maybe also try reversing secondaries. Apparently the capacitance to ground can change?

 

[Ilya]

Thanks. Maybe also try reversing secondaries. Apparently the capacitance to ground can change?

I've loaded secondaries with 470k and it did not change anything.
I did a secondaries swap with Sowter and it did improved things there. But frankly, I ran out of steam to do anything else with the unit. As it is now, it's completely within specs and flat across the whole audio spectrum. The worst is -3dB @20k at some pad settings or with LoZ engaged.
I'm putting the lid on this and moving on.

 

[C12VR]

I've loaded secondaries with 470k and it did not change anything.
I did a secondaries swap with Sowter and it did improved things there. But frankly, I ran out of steam to do anything else with the unit. As it is now, it's completely within specs and flat across the whole audio spectrum. The worst is -3dB @20k at some pad settings or with LoZ engaged.
I'm putting the lid on this and moving on.

Ah. Too bad. Thanks for reporting.