F5 Turbo V1, modified for more Class A power into 4 ohms - Page 2 - diyAudio
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Old 7th November 2012, 12:11 PM   #11
AndrewT is offline AndrewT  Scotland
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Cascode is generally to reduce dissipation in higher voltage versions of the F5t.
Low impedance drive capability generally does not suit high supply rails.

My conclusion from those two very general assumptions, is that cascodes are not for improving low impedance drive capability.

Back to your current vs passive crossover speakers.
The resistive low impedance dip is not the worst case for current demand. That is generally around 60% to 80% of the nominal speaker impedance, if the manufacturer has been honest in rating the speaker impedance.

The worst case is for fast starting or fast stopping transients.

These worst case music signals can result in speaker current demand approaching 5 (five) times that indicated by the nominal speaker impedance.
i.e. a 25W amplifier driving a 8ohms 2way, or more, passive crossover speaker can see a transient current demand of 12.5Apk.
If you substitute a 4ohms speaker then the transient demand could approach a worst case of 25Apk.

You cannot reasonably expect any ClassA amplifier to remain in ClassA at these enormous transient peak currents. The F5t will transition into ClassAB. The diodes bypassing the source resistors will help in trying to meet those transient peaks.
Multiple paralleling of the outputs will help in trying to meet those transient peaks.

I'd suggest you adopt both in a "normal" +-25Vdc PSU fed amplifier.
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Old 7th November 2012, 09:52 PM   #12
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everything is possible but it will demand some extreme cooling and very high bias
it is possible to bias the F5 so high, that it never leaves class A even at 2ohm load.
but if it is any point in doing so, thats something else
2.5A bias and +/-24V rails will let the standar F5 to stay in class A all the way at 4ohm load. so for the negative side. it will then consume 120W pr ch. instead of 65W.
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Old 8th November 2012, 08:33 AM   #13
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AudioSan View Post
.............2.5A bias and +/-24V rails will let the standar F5 to stay in class A all the way at 4ohm load...............
No.
Restated correctly:
2.5A bias and +/-24V rails will let the standard F5 to stay in class A all the way at 4r0 load.

The current demands of a 4ohms speaker will take the F5 amplifier into ClassAB when music peaks get to near clipping level.
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Old 12th December 2012, 03:23 AM   #14
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Thanks for all the helpful input,
so after reading more and thinking about it,
it looks like 2.5A bias and +/-24v rails seems a good option,

the cascode on the input stage seems optional (although many people prefer it even at 24v)

for the output stage, should I implement the source R diode bypass? i guess so.

if so to maintain the 0.3v across the 0.5R source R's i'd need 4 pairs of output devices?
have i got this correct?

if i want more Bias, would i have to add more output devices or just increase the source resistors so as to maintain the 0.3v across the resisors/diodes at idle.
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Old 12th December 2012, 04:46 AM   #15
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no need for cascoding.
if you going to use diodes with 2 output pairs and 2.5A bias, you need to lower the source resistance
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Old 17th December 2012, 07:34 AM   #16
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thinking this through a bit more,
If i build this as a stereo chassis, it's perfect if i decide to go balanced, I can convert each stereo amp into a balanced monoblock.
with the amps bridged/balanced they will effectively see 1/2 the speaker impedance so the higher bias /stock voltage seems to fit well?

does this chassis/heatsink seem enough for 2x175w dissipation?
Click the image to open in full size.
heatsink size 300x150 each side
Click the image to open in full size.

or would it need something as extreme as
Click the image to open in full size.
I can't get thermal specs on either of these chassis,
but the price is pretty reasonable.
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Old 17th December 2012, 09:39 AM   #17
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The first one is one heck of a nice chassis! Where do you get that from? If the heat sinking isn't enough, make two monoblocks!
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