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previous: K Bull Randy, The rant was well worth it. ... -- 8/15/2000 3:08 AM View Thread

Re: November vs. Invader

8/15/2000 4:18 PM
Randall Aiken
Re: November vs. Invader
No, I haven't ever tried a toroidal OT. I've been scared off by the necessity to use an automatic bias servo to keep the DC imbalance at zero to avoid core saturation. Those toroids reportedly can't handle very much offset DC. I have a great design for a bias servo, but it takes a bunch of solid-state stuff that I just can't bear to stick in my amps. :)  
I may give one a try some day, just to see how bad the saturation problem really is. It could be that it is acceptable for the range of bias current mismatch you get in normal guitar amp operation, but not acceptable for hi-fi. The other problem is that they don't come in 4,8, and 16 ohms, just a single output impedance, so I'd have to get a custom-wound toroid.  
Magnetics, Inc., makes some nice "Kool-Mu" toroids I've used in switchmode power supply designs. They have a special distributed air gap that allows them to handle large DC offsets without saturating. Unfortunately, they don't make a core large enough for an OT. I don't know what core material the Van Der Veen trannies are made of.  
I think it would be a good thing to use toroidal PT and OT's, as it would keep the weight down, along with the radiated magnetic fields. As for the tone, who knows? It might be great!  
Randall Aiken