F5 Turbo V1, modified for more Class A power into 4 ohms

[gbullimore]

so the F5T looks great, especially the V3 version,
but i realized this would be overkill for my needs,
what i would ideally like, is decent power in to lower impedance,
as you know most 8 ohm speakers dip to ~4 ohms through part of their frequency range,
many 4 ohm speakers dip to 2 ohms or less due to the passive crossover.

so while the F5T V1 has enough power, i'd like to maintain the power into low impedance,
is it as simple as paralleling more output devices, and increasing Bias?
i'll try to attach a schematic of my suggestion.

[gbullimore]

well, actually the Bias voltage would stay the same but total bias current increases,
is the the right approach, or would i be better to base it on V3 with reduced supply voltage ??

[gbullimore]

actually i guess it would be fairly easy to have a switch for higher / lower impedance speakers.
switching out the extra output FET's

[Zen Mod]

that one will need to be one Big MF switch

[gbullimore]

true, but a 16A relay should do?
one set of contacts per FET,
should only need to handle a couple of amps.

but it would certainly be a switch over when powered off,
maybe even a plug in connector to connect the other 1/2 output devices.

is the idea generally sound though for the 4 ohm F5T, or is it better to base it of the V3 with diodes bypassing the 1R's

is it recommended to cascode the input devices or not with 32v supplies?

sorry for all the questions.

in return feel free to ask away on the fine detail of driver/transducer design!

[Zen Mod]

if you need more current , just go with normal rails but with diodes parallel to Rs

3 pairs of output , biased appropriately , will cover all sane needs

anyway - I can't imagine that you're going to use one day easier , another day harder speakers , to drive with same amp .

[gbullimore]

ok,
so based on V3 but lower voltage rails about +/- 32v.
keep the input cascode?

my speakers change fairly frequently, I design drivers for a living!
but yes, not every day :-)

it would be good to built it with perhaps 4 pairs, and have pairs 3 and 4 (relatively) easy to disconnect,
that way i'm not filling the room with heat if i have 10-16 ohm speakers,
but i'm still ok if i have a 4ohm speaker the next month.

[Zen Mod]

if you design drivers for living , it's not insane to have 2 different amps

whatever - if you are not in mood for that , idea with additional output pairs is doable , but you must arrange those additional ones from same matched groups as stationary ones

and be careful to observe potential difference in output offset , between just stationary vs. stationary + additional

input cascode is , for my experience , beneficiary even for regular version

[gbullimore]

ok, thanks very much for the feedback,
if the amp needs re-adjusting then this would be annoying (although not impossible)

as for multiple amps, i would love to.... but as it is i get enough distortion in my ear because i change speakers a lot,
having spare amplifiers would equal much more distortion from an already very tolerant girlfriend

[Zen Mod]

Originally Posted by gbullimore

.....
if the amp needs re-adjusting then this would be annoying (although not impossible)

......

if you arrange matched outputs as I already wrote , that doesn't need to be major issue , or issue at all