amplifier bias drifting

[Craig405]

Thanks for replies,

I like the simplicity of your soft start richie00boy, dont need to worry about conecting the secondary to run low voltage circuits.
I think a 1.3KVA toroid should have a soft start 😀

Interestingly this amp has +/- 76V rails and as mentioned earlier in this thread to get 200W into 8 ohm you'll only need 56V peak so maybe this amp's rails are supposed to sag??

Thats somthing else to consider hmm..

 

[AndrewT]

..........Interestingly this amp has +/- 76V rails and as mentioned earlier in this thread to get 200W into 8 ohm you'll only need 56V peak so maybe this amp's rails are supposed to sag??

I know of a 2x 400W into 4 ohm amp that sounds astonishing in the bass with just that amount of capacitance.

I have a set of Sugden P128 that are designed like this.
Low capacitance (9400uF to each rail) and high transformer regulation 175VA EI to each rail. It makes for pretty good 8ohm drive from it's +-72Vdc supplies achieving 195W and fair low bass extension.
It is specified as a 4ohm monoblock amplifier and achieves only 300W into 4r0 and bass performance is poor compared to other wideband amps driving 40hm speakers.
I suspect the reason for adopting low smoothing capacitance and high transformer regulation is due to the deliberate rail sag on sustained bass tones to protect the output stage. The rails sag to about +-56Vdc. (a cheaper way to design an amp when they could have done so much better).

I have tried this amp on +-25mF and from a decent toroid using a 4pair output stage (in lieu of the standard 2pair) and the performance is transformed for the better. But the toroid cannot fit the space vacated by the two EIs.

 

[Craig405]

Well now with this in mind im tempted to go for smaller cheaper 33Kuf caps that will fit into my case ( is was going to wire a bank into case with heavy gauge leads)

ill be driving 8 ohms 90% of the time and the transformer probably has decent regulation.

Many thanks for the help all, i think i know what must be done 🙂 .