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 12th March 2012, 02:06 PM #1011 jacco vermeulen   diyAudio Member     Join Date: Oct 2004 Location: At the sea front, Rotterdam or Curaçao In case i talk mystery game again. Easy way out for high rail versions would be to scale the feedback resistor rating for peak output level. But in the case of a V3, that would lead to unpractical resistor sizes, or TO-220 types with heatsinks. Resistor rating is the continuous power they handle, for short periods they can cope with a multitude of that. (e.g., short term overload for a wirewound can be five to ten times higher, for up to a 5 second period) The resistors are heated by an average continuous load, on top of which come peak impulses. If the resistor rating is sized to max continuous level, one would have to check if the specific ohmic value can handle the occasional peak voltage burst, in the heated condition, requires a datasheet. (idle dissipation of the 10R is negligible, of course)
AndrewT
R.I.P.

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by AndrewT ................. Do the sums. eg Max V Output ~ 40Vpk (100W into 8r0 or 200W into 4r0 per half channel). total voltage across 10r + 220//220 = 40Vpk Ipk ~333mApk 10r resistor instantaneous peak dissipation ~ .333^2 * 10r = 1.1Wpk.............
Quote:
 Originally Posted by jacco vermeulen ...........idle dissipation of the 10R is negligible.........
1.1W instantaneous peak is not negligible, even for a big F5turbo.

Even more so when we are considering resistance stability of the NFB circuit.
__________________
regards Andrew T.

 13th March 2012, 01:04 AM #1013 a.wayne   Sin Bin   Join Date: May 2008 Location: Front Row Center So where are we with this ... 2 /3/5/ watter ...?
 13th March 2012, 07:30 AM #1014 UKToecutter   diyAudio Member   Join Date: Jan 2010 Location: Harrow, UK I'm changing the resistor pattern to accomodate the same resistor pad patterns as the other power resistors.
AndrewT
R.I.P.

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by a.wayne So where are we with this ... 2 /3/5/ watter ...?
do the sums for your amplifier.
__________________
regards Andrew T.

UKToecutter
diyAudio Member

Join Date: Jan 2010
Location: Harrow, UK
Quote:
 Originally Posted by AndrewT do the sums for your amplifier.
Andrew,

If the constant dissipation at 100W is 1.1W then would you agree that a 'safe' rule of thumb would be 1% of output power x2 for overhead or is the relationship non-linear?

If it is then;

1W for 50W
2W for 100W
3W for 150W etc etc.......

Andy

 13th March 2012, 02:16 PM #1017 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders No. For 40Vpk output signal, the maximum instantaneous peak dissipation is 1.1W To get 40Vpk from an F5T you would need supply rails ~+-45Vdc. That single amp would be capable of 100W into 8r0 and 200W into 4r0. If you bridge a pair to make a balanced F5T then you are in the 400W into 8r0 power range. You must do the sums for your supply rails. Post1004 shows that a bit of multiplying and dividing gets the answers you need. __________________ regards Andrew T.
UKToecutter
diyAudio Member

Join Date: Jan 2010
Location: Harrow, UK
Quote:
 Originally Posted by AndrewT No. For 40Vpk output signal, the maximum instantaneous peak dissipation is 1.1W To get 40Vpk from an F5T you would need supply rails ~+-45Vdc. That single amp would be capable of 100W into 8r0 and 200W into 4r0. If you bridge a pair to make a balanced F5T then you are in the 400W into 8r0 power range. You must do the sums for your supply rails. Post1004 shows that a bit of multiplying and dividing gets the answers you need.

Andrew,

thanks.
I've just gone back to 1004 and I understand.

I was working on the assumption that running in class A and having a set level of NFB (always the same proportion) would make the relationship a linear one.

Andy

 13th March 2012, 02:52 PM #1019 UKToecutter   diyAudio Member   Join Date: Jan 2010 Location: Harrow, UK OK, Using the formulae resistor dissipation would be as follows; Pk Voltage Calculated Value Value to use 25V 0.433W 1W 30V 0.625W 1W 35V 0.853W 2W 40V 1.11W 2W 45V 1.41W 3W 50V 1.74W 3W 55V 2.1W 4W 60V 2.5W 5W Last edited by UKToecutter; 13th March 2012 at 02:55 PM. Reason: data missed
 13th March 2012, 03:05 PM #1020 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders That seems like a reasonable table of values to minimise the resistance variation due to temperature induced effects when very high transient signals pass through. A normal F5 runs on +-25Vdc supply rails so the table could be completed by adding in a 20Vpk output and using a 600mW resistor. We are back to the original now. __________________ regards Andrew T.

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