F5 power amplifier

Alright, don't gang up on the urban industrial dweller:p Maybe you can help me learn something instead!

I understand how to calculate the 3db point of a low or high pass filter F= 1/2pi*RC. The examples I can find just have a series resistor and capacitor to ground.
But in the case of input to the F5, or many other amps for that matter, there is also the shunt resistor to take into account. For the "R" in the equation above, how does the 100K resistor fit in?
Would it be 1/2pi*101,000*C? This does not seem right to me.
I know this may be an elementary question, but could not find it here or elsewhere.
 
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Joined 2002
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My understanding is that as long as the 'load' has a much higher impedance than the filter its shunt resistance does not affect the functioning of the filter. In the case of input filters, the series element is usually selected to be quite a bit lower than the load impedance.

In your case, 100k shunt element is not factored into the equation.

Also, a lower shunt element will only affect attenuation, not rolloff (at least talking about ideal components),
 
My new dual mono F5

hi

I decided to do a little change my F5 ,to the turbo version



http://www.hostingpics.net/viewer.php?id=173070DSC0860.jpg

because I want cascoding the jfet, doubling the output stage, bigger power supply (2 X 500VA), fast and low loss rectifier ...... :)

The beginning of the change is the case because it will bring two transformer and 4X 100 000 uF capacitor


http://www.hostingpics.net/viewer.php?id=177677DSC0845.jpg



http://www.hostingpics.net/viewer.php?id=511264DSC0859.jpg



http://www.hostingpics.net/viewer.php?id=787696DSC0854.jpg

the pcb



http://www.hostingpics.net/viewer.php?id=410844DSC0855.jpg

the DC protection for speaker



http://www.hostingpics.net/viewer.php?id=618157DSC0862.jpg

And wire it all, the concern is to find the time because of work



http://www.hostingpics.net/viewer.php?id=857931DSC0863.jpg

and .....



http://www.hostingpics.net/viewer.php?id=924531IMAG0004.jpg



http://www.hostingpics.net/viewer.php?id=999979IMAG0003.jpg

Not really finished Yet

it sound beautiful, with better bass
The bias is 2A for each channel and the power supply 2X28V DC
Big thanks to Mr . PASS for the design and for the member of the forum for the help

Excuse-me for my poor english
 
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Hi all, could anyone clarify the procedure for setting the bias on the F5 for me.
As I understand it you have to set the two pots that are in parallel with the 2.2K resistors that supply the input FET'S 's to minimum resistance and then gradualy increase them in turn until the desired bias is reached, also checking the offset voltage.
Is this correct, or have I misunderstood? At initial start up the input FET's will be connected directly to the +- supply rails with no source resistance? Sorry to be so dumb
Alan.
 

6L6

Moderator
Joined 2010
Paid Member
Start with resistance turned to minimum position.

In a perfect world you have 3 meters. Set all to DC volts.

Place meter across R11, another across R12, (the source resistors on the outputs) and one on the speaker outputs.

Turn pots (plural) up until you show 0.1V across the source resistors. Once they show some bias, now by adjusting one of the pots, set any DC offset you have to zero.

Once the Offset is zeroed, increase the bias again, to .2 or so, and then remove the offset.

This is a series of steps that will increase the bias, and probably remove some of it when the offset is zeroed. Two steps forward, one step back. This is normal.

Continue to increase bias in .2V steps or so and then zeroing offset.

Once you get it to 0.55V across both resistors, zero the offset again, and let the amp sit for 1/2hr. Lay the lid of the chassis on top, to simulate normal airflow.

After it's all warm and stable, tweak the bias one last time to 0.6V and zero offset. If it makes it all the way to normal bias, button it up and enjoy! Check again in a week.

If it doesn't, (with BL-grade Jfets you likely won't have any problem...) please report back with the maximum amount of bias you can achieve with zero offset.

As for the Jfets being attached to the rails with no source resistance, yes, that's correct. With no resistance there will be no current drawn that way, and they won't turn on.
 
Official Court Jester
Joined 2003
Paid Member
alibear meant on drain resistance ( with pots zeroed)

input jfets will draw current , set with their own source resistors (roughly) while their dissipation will be certainly higher than in fully biased amp state

however , that is not dangerous

just to mention - using just two DMMs is enough - one for voltage across (any of two ) output source resistor and one for offset

series bulb thingie for first power up is wise to have
 

6L6

Moderator
Joined 2010
Paid Member
:up: I was referring to supplier or vendor envy/competition :mad: not forum admin intent.;)

Not anything I worry about.

The most important thing in a good and complete BOM is seeing an example of the component to buy... Sometimes you don't have any info on, say, lead spacing, until you see a part listed and then you can see that, as well as dimensions, etc...

BOM are always useful to the hobby builder.