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F5 TURBO V2
One of the things holding back the amount of current we can deliver is the
power supply and the Source resistors we use to ballast the output devices.
The output devices themselves are not usually the culprits, having been
designed to switch rather high currents. The Fairchild devices originally
specified in this design can do 22 amps each. You might therefore expect a
pair of them to peak out at 44 amps, but from V1 you see that with 0.5 ohms
of Source resistance they only give you about 23 clean amps.
What if you remove the Source resistance altogether?
First off, you get the 44 amps. In addition, with Fets you get an output stage
that will deliver more Class A power at a given bias figure due to the square
law character of the Fets. Unfortunately you also tend to get a thermally
unstable circuit that is prone to bias hogging. This is true of Vertical Mosfets
and even more so with Bipolar transistors. There are a couple of examples of
high end amplifiers on the market which boast no ballast resistance, but the
praise for their sound is accompanied by rumors about reliability.
As an alternative, you can consider carefully matched Lateral Mosfets which
have a declining temperature coefficient. Unfortunately they tend to have
lower current ratings, and so are not suited to this particular goal.
Fearless Amplifier Builders (FAB) might just go ahead and try output stages
without Source or Emitter resistors. For the rest of you, here is the V2:
It looks just like V1 except that there are some diodes in parallel with the
Source resistors of the output devices. These are some of those fancy newfangled
high speed power diodes from Vishay, MUR3020W, rated at 400A
peak and having an equivalent resistance of about .03 ohms per parallel pair.
Inserting these in parallel with the 0.5 ohm Source resistance gives us an
impedance which is very low at high currents but devolves to the 0.5 ohm
value at less than an amp.
The result is that the amplifier can now deliver 38 amp peaks:
Well, that was easy. The distortion character at ordinary power levels is
pretty much the same, and ditto for bandwidth and such. Actually it's pretty
much the same until you get a couple amps going to the load, and then the
F5 Turbo starts flexing its newfound muscles.
You do have to watch a couple of things. Here is a curve from Vishay's spec
sheet that gives the voltage drop versus current for different temperatures.
We see that these devices will slowly start conducting at voltages just above
the idle voltage across the 0.5 ohm Source resistors. It will be necessary to
heat sink these diodes, remembering that their case is electrically connected.
The point at which the diodes conduct is temperature dependent, so you will
want to set the bias so that it makes a nice transition above the bias point and
doesn't run away when the amplifier gets hot.
If you are competent, fearless and also own a fire extinguisher, you can find
this point. Just run the amplifier into a reasonably low impedance until it gets
good and hot – as hot as you plan to let it get - ever.
Then adjust the bias to a point below where the idle current starts to really take off. You should find
that this point is around 0.4 volts across the 1 ohm resistors.
If you are a fraidy-cat, then just set it at 0.3 volts, and conservatively fuse the AC line.
A number of DIYers have speculated on or even tried the Toshiba 2SK1530
and 2SJ201 Mosfets.
These are really nice (discontinued) audio transistors
and are very fine for F5's, but keep in mind that with a 12 amp rating they
have a smaller maximum current than the Fairchild parts.
This will not
prevent them from sounding very good, however, and I do recommend them
in general.
Of course you are going to need a beefier power supply to do deliver all this
additional power. Here is an example of such a power supply.
Power supplies like this have been described elsewhere, and there is nothing
magic about this one – we are just scaling things up. The same Vishay
rectifiers are used here, and you see that we are using lots more capacitors.
The CRC filtering is still there to keep the supply noise lower, but the
resistance has been lowered to 0.067 ohms to ensure low losses. You want
CLC? Go for it.
The thermistor in series with the primary is familiar, and I
recommend the CL30 type. Of course this shows the 120V AC version. For
240V the primary coils are wired in series and the fuse value is halved.
The power transformer needs to be VA rated at least twice the actual
dissipation of the amplifier. The secondary AC voltages can be approximated
by multiplying the DC rail voltages by 0.75, for example, for 48 volt DC rails,
each secondary is going to be about 36 volts. Your actual results will vary a
bit with different transformer manufacturers.
As commonly seen in my previous projects, you will also see a thermistor /
diode bridge combination used to provide ground loop isolation between the
chassis, which is hardwired to the AC outlet ground, and the circuit ground.
An ordinary CL60 thermistor and a 35 amp rectifier bridge should suffice.
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