# Outside The Box

#### woody

In this vidio of the 2021 BAF Nelson makes a little talk starting at 12:40 called Outside The Box where the main
discusion was about injecting some extra curent into the output stage of an amplifer. But at 20:10 Nelson pipes up and says
" But wait there's more " at this point he describes having some DC bias voltage in the speaker voice coil to lower speaker
distortion. He makes a point that about 1V of DC in the voice coil shouldn't harm most speakers Oh I know this sounds
like herisy but after all the title of the talk was Outside The Box. So this makes me wonder if some of us building a MOFO
using the 193T inductor or an inductor with a lower DCR do we realy need that output cap after all.

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#### Ben Mah

Paid Member
That would be a cool experiment to try with a junk speaker. The DC resistance of the speaker would want to be reasonably high since it would be in parallel with the inductor. The speaker DCR parallel with the inductor DCR would reduce the amount of resistance that the mosfet source would see, so that would change the biasing of the mosfet. Also if the speaker DCR is low, a substantial amount of DC current would flow through it. So voltages and currents would be affected. The amount of change would be much less if the speaker DCR is high.

#### woody

Did you look at the vidio in it Nelson was using this with some expensive drivers even one of his Lowthers.

#### Ben Mah

Paid Member
I did say that it would be a cool experiment. I am pointing out that it would be best to try it with a speaker that has high DCR in the case where a Mofo is used for the experiment. You will need to monitor the current through the mosfet if you are using a speaker with low DCR, the mosfet bias voltage would be affected as the current will change. You may then want to adjust the bias voltage for a reasonable current through the mosfet. If the speaker DCR is low, a substantial current will pass through it.

I believe Lowthers have reasonably high DCR, with 8 Ohm drivers having a DCR of about 7 Ohms. That is reasonable high compared to the 0.7 Ohm of the 193T, so the Lowther will not substantially affect the bias voltage of the mosfet or the current through the mosfet, and the dc current through the speaker would be low.

#### planet10

Paid Member
I suspect along the same lines of single-ended OPTs or DC biasing crossover caps.

On first blush it would also push th edriver out of its most linear range. Maybe even beyond xMax on some fihg sensitivity FR drivers.

dave