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dazed2 10th September 2012 04:04 AM

Lazy Man's Balanced F5?
Ok Forgive me if it is a little simplistic but my electrical theory is pretty non existant :confused:
I built the regular stereo F5 (with protection taken out) a year or so ago. Its biased a little higher, 0.75V for a bit more power.
The itch is now out for more power. After looking at the expense and massive weight it would be to build the Turbo version I'm concluding that building a second F5 may be a bit more cost effective.

Here is the idea.
Currently my pre-amp has balanced outputs. My thinking is that if I take my current stereo F5, and feed the +ve side of one balanced output channel to one channel and the -ve feed of that same balanced channel to the other channel I essentially would have a balanced mono F5. This should also mean that I would connect the one speaker to +ve and -ve side hot posts (red). This should also mean that it would essentially double the power of the F5 (25 -->50W) but will see the speaker at half the impedance.
Now all I have to do is build a second stereo F5 and I have two balanced monoblock F5's.

Does this make sense? Have I missed anything? Are there any downsides to this?

Zen Mod 10th September 2012 08:37 AM

good thinking

Melon Head 10th September 2012 10:27 AM

And on the seventh day God rested, and built a balanced amp

AndrewT 10th September 2012 02:32 PM

Have you read the F5x Thread?

dazed2 10th September 2012 04:37 PM

You mean the 78 page thread? Yeah sort of... last year :( and then stopped....
But thats the thing I didn't want to start mucking around a built PCB and stacking components on top of each other and cascoding etc.

Thats why the "lazy approach" :D take something thats already built and the layout is tested and see if a simple wiring change will suffice.

AudioSan 10th September 2012 04:51 PM

a think what andrew ment is: it may sound easyer then it acctualy is:)

vdi_nenna 10th September 2012 07:43 PM


You may be able to modify your existing F5 to a F5 Turbo v1 without building another case...

From Article:


Increasing the power supply voltage is the obvious way to get more power out of an F5. You can simply raise the supply rails to +/-32 Volts and get 50
watts into 8 ohms right away
without other modification. 24 V AC secondaries
on the power transformer will do it.
Don't forget to use higher voltage power supply capacitors. Probably you should also upgrade R9 through R12 to 5 watt resistors. Depending on your heat sinking, you will probably want to
adjust the bias so that the power transistors don't run too hot.
As a rule of thumb, the output devices should not be operated at more than
about half their maximum rating, and generally the case temperature needs to
be under 100 deg C. For most amplifiers this means a heat sink temperature
of about 50 to 55 deg C., which is the temperature that you can put your hand on for about 10 seconds.

No need for cascoding or bridging. You will need to add extra mosfets and resistors attached to the output stage. Your just tapping off connections that are already in place.
You will need to include the thermistors, but not the limiter. A bigger transformer will be necessary, but if you build a new amp, you'll need a bigger transfo, if yuo don't have one on hand.

IMO, this is easier and faster than building a new case and all that goes with it.

Hope this helps,


vdi_nenna 10th September 2012 08:29 PM


you'll need a bigger transfo
I meant to say "you'll need another transfo anyway..."

qusp 11th September 2012 09:48 AM

isnt this not balanced, but bridged?

bear 11th September 2012 01:09 PM

Yes, bridged.

The problem with bridged is that you may run out of output device current, and the unit will move over to class AB... so again you may want to add (parallel) output devices.

increasing the B+ on the stock F5 was discussed extensively in the original thread, iirc. I think what was said is that increasing the B+ was problematic and maybe not a good idea because of reaching the voltage limit on the input Jfets (saturation) and also the current capability of the output devices vs. load impedance is reduced. (the max current remains the same, while the power requirement goes up as the voltage increases)

If the F5 bias is made higher, the output devices are more in class A, dissipating more current and therefore power. Lower bias means less into Class A, more into AB. So for a given load Z, the amp will leave class A and go to AB sooner... it's ok to do this, but it's no longer "pure class A" and technically there is a minor glitch at the point where it leaves Class A and goes into AB or more precisely Class B for a short period of time and then comes back to Class A (on each alternating polarity of a given cycle).

But bridging by using the balanced output of a preamp is perfectly ok, just keep in mind the issues. (if you had say a 16 ohm impedance speaker it is a non-issue).


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