F5 Turbo Builders Thread

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nashbap

Member
Joined 2012
WalterW said:
Ah, I always use this great post from Jacco to calculate F5 gain:
Click to expand...


Yes, this is a great post. Thanks Jacco.

With 45 V rails I am planning on increasing the gain to 26db using 380 ohm 5 W resistors for R7 thru R10 using the formula above. Is this right? Just wondering why Papa hasn't mentioned increasing the gain for the F5TurboV3 since he did so when going from the F5 to F5Turbo V1 wherein he specifically states "If we are going to put out more power it is appropriate to have some more gain....."

Also, with 45V rails what JFET voltage should I calculate R27 and R28 for? I see at least 1/3 rail voltage being mentioned.

Thanks. Nash
 
BobEllis

BobEllis

Member
Joined 2003
nashbap said:
...Just wondering why Papa hasn't mentioned increasing the gain for the F5TurboV3 since he did so when going from the F5 to F5Turbo V1 wherein he specifically states "If we are going to put out more power it is appropriate to have some more gain....."
Click to expand...

Papa likes to leave something for us to do. Our education isn't complete without a little DIY calculation. :cool:
 
D

dazed2

Member
Joined 2010
nashbap said:
Yes, this is a great post. Thanks Jacco.

With 45 V rails I am planning on increasing the gain to 26db using 380 ohm 5 W resistors for R7 thru R10 using the formula above. Is this right? Just wondering why Papa hasn't mentioned increasing the gain for the F5TurboV3 since he did so when going from the F5 to F5Turbo V1 wherein he specifically states "If we are going to put out more power it is appropriate to have some more gain....."

Also, with 45V rails what JFET voltage should I calculate R27 and R28 for? I see at least 1/3 rail voltage being mentioned.

Thanks. Nash
Click to expand...


Yes Thanks Walter and Jacco for that too!

I don't know how you guys find these past posts so quickly, search function here is tedious. Anyways, wasn't there a post somewhere on this thread that pretty much already calculated that at +-50Vdc the current spec of R27 and R28 would give about 16V to the Jfets? Sorry I can't remember the formula....
 
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nashbap

Member
Joined 2012
buzzforb said:
12-16v for the cascode is a nice range.
Click to expand...


Thanks Buzz.

So I will go with 392ohm resistors(closest to 380ohms commonly available) for R7 thru R8 for a approx 26db gain and with 45V rails. Not sure whether these should be 3W or 5W resistors. Can someone please advice.

Also, should the source resistors be 3W or 5W since I am using 45V rails? This for F5TV3.

Thanks.
 
roger57

roger57

Member
Joined 2006
LSJ74 GRade Selection

Has anyone on this thread used the Grade A LSJ74 parts? Since there's no B grade available, just wondering if it's better to go lower...or higher.

If higher, has anyone used the Grade C parts at the minimum Idss of 10mA?

(This post is also on another thread...)
 
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Daniel(Del)

Member
Joined 2005
BobEllis said:
You may also want to increase the values of R7/8 to increase the gain so that you can take advantage of those higher rails.

Also, watch the values (and dissipation) of R25-28, adjust them to keep the voltage across the jfets reasonable. I'm thinking of using 20V zeners in place of R27/28. You can increase the values to reduce dissipation.
Click to expand...

Hi, thank you for attention!

When increasing the value of the feedback resistors to increase gain, feedback will be reduced. Can't this be a problem? Especially because with high voltage rails, bias needs to be reduced, to keep dissipation within reasonable limits.

Also, can R25 be changed without affecting the cascode operating point?

Thanks
Daniel
 
BobEllis

BobEllis

Member
Joined 2003
Daniel,

Yes, increasing the gain you can eventually run out of feedback. Decreasing the feedback can alter the character of the amp, but we are only talking about a relatively minor changes. No need to worry about it.

Changing R25 changes the cascode operating voltage. R25 and R27 form a voltage divider. The cascode voltage is Vrail * R27/(R25+R27) With the values in the article the cascode voltage is roughly 1/3 of the rail voltage. Increasing R25 without changing R27 will reduce the cascode voltage and vice versa.

Replacing R27 with a 10-20V zener diode would allow adjusting R25 without affecting the cascode operating point. Pick R25 to allow at least 5 mA through the zener to give it enough current to work with.
 
BobEllis

BobEllis

Member
Joined 2003
AudioSan said:
acctualy. there is no need for parallel source resistors if the voltage drop is lets say 400mV.
0.4V/0.47ohm=0.85A
0.85A*0.4V=0.34W. a roule of thumb regarding resistors are 2X. so a single 1W will suffice. so a single 0.47ohm 3W is more then you'll ever need:)
Click to expand...

That makes sense for idle conditions, but what about sustained high power operation? Just 64W into 4 ohms is 4A. Divide that into the 2 output devices per rail and you still have 2A per Source resistor (or pair), giving you 2W. A pair of 3W resistors still seems more than you need, but have you considered derating for the elevated temperature inside the case and that the resistor(s) is(are) not mounted in free space?

I know, we aren't likely to actually operate at that kind of power, but shouldn't we be able to operate continuously at rated power? In the big scheme of things, the additional cost of a pair of 3W resistors over a 1W resistor is pretty small. Not to mention that this is DIY where we tend to overdo everything. ;)
 
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