F5 power amplifier

what happens when it fails

Well, if I may chime in -- the 10 ohms are now Panasonic 2W which are slightly larger than the Vishay PPC. You can sneak by with 1W here, but under some conditions 1W will fail. I used Dale 50R instead of the parallel 100R, but popular sentiment seemed to favor the former, so that's what is going out. The small wattage resistors are Mil-Spec RN55 or CMF.

I am disappointed that some folks didn't like the Ohmite 0.47 ohm TWW5 resistors -- these really get the heat off the board and are quite compact and low reactive impedance, and I used them as "one more worthy than I" had employed them in another amplifier article. In this application I have used Mills, Halco, Caddocks and the Ohmites. Horses for courses I guess.

The F5's I have in use have Caddocks in the 10R, 50R and 0.47R positions. These are attached to the heat sink as they won't dissipate much more than 1W on their own, but I burned my own PCBs for the application. I also pimped the input circuit a bit so that the two halves would behave a bit better at ham radio frequencies! This is ridiculous for audio...but my square wave has a faster rise time than your square wave!

If there was a figure of merit for elegance and simplicity vs sound quality, this amplifier would win hands down.

Hi,
I've just put together a PTP using .33watt resistors for R1&R2. Peter Daniels says he uses 1/4watt here with no problem. What I'd like to know is, if it should fail what is the consequence? Would it just stop working or would it blow up parts? If it can blow my speaker I will revise with safer margin of error :eek:
Thanks for any comments.

Garrett
 
There is a trick worth knowing - You can use a potiometer
to reference to any point between Source pin and Ground,
tweaking for the sweet spot.

:cool:

Hi Mr. Pass,
Is there any problem replacing R1&R2 with a single 20ohm trimpot? I'd also like your opinion on using a 1.5K trimpot in place of the paralleled P1/R3 (5K/2.2K). Was this done for linearity or to increase the power handling? I'm looking to use vishay's 1280G .75watters without added parallel resistors if possible.:)
Thanks,
Garrett
 
That reminds me of another question I was meaning to ask.. Since trim pots aren't the most ideal resistive element sonically speaking, has anyone changed out the trim pot/resistor combo to a single resistor that equals the value of P1+R3 and P2+R4? Obviously you'd want to be certain to measure after the amp has settled in enough that you get the right value. It seems most people who have re-measured after a week or a month say things remain very stable so this doesn't seem too risky if done properly.
 
That reminds me of another question I was meaning to ask.. Since trim pots aren't the most ideal resistive element sonically speaking, has anyone changed out the trim pot/resistor combo to a single resistor that equals the value of P1+R3 and P2+R4? Obviously you'd want to be certain to measure after the amp has settled in enough that you get the right value. It seems most people who have re-measured after a week or a month say things remain very stable so this doesn't seem too risky if done properly.

Sounds OK to me.
The only problem is if you remove the pot from a PCB you might damage the tracks and cause a fault and possibly blow up the output transistors.
 
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Joined 2003
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Hi,
I've just put together a PTP using .33watt resistors for R1&R2. Peter Daniels says he uses 1/4watt here with no problem. What I'd like to know is, if it should fail what is the consequence? Would it just stop working or would it blow up parts? If it can blow my speaker I will revise with safer margin of error :eek:
Thanks for any comments.

Garrett

I inquired about this as well. My parts were one of the Tech-DIY kits, and the smaller Peter Daniels boards required standing up many or the resistors. R1 and R2 were 2 watt resistors that were so large I had a little difficulty getting them in.
In discussing this with Peter, he told me he had no problems with 1/4 watt in that position, and he should know.

Conversly Tech-DIY talks like 1 watt is pushing it. Original BOM lists R1 and R2 as .6 ohm. I havent heard anyone chime in or post they had problems with the lower value caps in this position. Since I had them, I used them. I'll post a pic of the stuffed board in case anyone else is using Peter's boards with the Tech-DIY parts kits. Perhaps those at Tech-DIY have, I'm sure they have had feedback from those whom used their kits.

Russellc
 
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There is a very good reason for 2W resistors in that position.
R1 and R2 are part of the feedback network and they see as much as one sixth (16,67%) of amp's output voltage which is 20V peak (max. value).
20V / 6 = 3.33V
Power dissipated on each of those resistors = (3.33 * 3.33) / 10 = 1.11 W (peak).
1/4W is OK only if you don't listen to your amp too loud. ;)
 
I inquired about this as well. My parts were one of the Tech-DIY kits, and the smaller Peter Daniels boards required standing up many or the resistors. R1 and R2 were 2 watt resistors that were so large I had a little difficulty getting them in.
In discussing this with Peter, he told me he had no problems with 1/4 watt in that position, and he should know.

Trace the current back -- all you will have is a bunch of burned out 1/4W resistors.
 
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Joined 2003
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There is a very good reason for 2W resistors in that position.
R1 and R2 are part of the feedback network and they see as much as one sixth (16,67%) of amp's output voltage which is 20V peak (max. value).
20V / 6 = 3.33V
Power dissipated on each of those resistors = (3.33 * 3.33) / 10 = 1.11 W (peak).
1/4W is OK only if you don't listen to your amp too loud. ;)

I installed the 2 watt blue resistors that came with the kit. Paul Daniels said he uses 1/4 watt in that position with no problems. Maybe just lucky, but as many boards as he's sold with that advice, you'd think he would have had at least one person tell him otherwise.

Anywho, I installed the 2 watt resistors that came with the kit.....

Russellc
 
I also used the 2 watt resistors in those positions. It seems like they can see a fair amount of current, and I tend to push things a bit beyond the design specs.





Question for anyone who has built both, how audible is the difference between the Fairchild and IRF P channel MOSFETs?

I saw somewhere a couple hundred pages earlier in this thread that someone was thinking of using the Renesas/Hitachi MOSFETs - did that ever get built?

I am curious with the fairchild parts going away if their is a better substitute for the output devices? I am curious to hear about tested replacements as opposed to simulated replacements.
 
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I also used the 2 watt resistors in those positions. It seems like they can see a fair amount of current, and I tend to push things a bit beyond the design specs.





Question for anyone who has built both, how audible is the difference between the Fairchild and IRF P channel MOSFETs?

I saw somewhere a couple hundred pages earlier in this thread that someone was thinking of using the Renesas/Hitachi MOSFETs - did that ever get built?

I am curious with the fairchild parts going away if their is a better substitute for the output devices? I am curious to hear about tested replacements as opposed to simulated replacements.

I bought both outputs, just in case. (Fairchild and IRF) It will be a while before I could give an opinion. I am going to build with the IRF devices. It is said they are slightly warmer, I'm betting the difference is trivial.

Russellc
 
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Joined 2005
Nelson mentioned a pot referencing source pin to ground, to adjust sweet spot
I take it "source pin" is referenced to Jfet

Is it something like this
 

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