Yea, I think it is a peak current issue with the regular F5. But we are talking TURBO here and we expect current to be higher or why bother with TURBO?
I am out of my comfort zone making the next statement, but with V3 higher output power, all the feedback resistors should be looked at for more dissipation and maybe even higher wattage for R3 & R4 at some point.
Rush
changing the upper leg of the NFB from 100//100 to 220//220 reduces the proportion of the power delivered to the lower leg of the NFB.
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
2W resistors would do, but I would go a bit higher. try 3W to 5W
220r resistor instantaneous peak dissipation ~.333/2^2 * 220r = 6W.
This parallel pair would need to be mighty for a +-45Vdc turbo.
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
2W resistors would do, but I would go a bit higher. try 3W to 5W
220r resistor instantaneous peak dissipation ~.333/2^2 * 220r = 6W.
This parallel pair would need to be mighty for a +-45Vdc turbo.
instantaneous peak power will determine the maximum slope (slew rate) of temperature change in the resistor element (equivalent to change in junction temperature).
Since all normal Audio resistors have a non zero tempco, then the flatter the slope of the temperature then the better for the audio we are trying to pass.
Taken to the other extreme. Pass A constant DC current through the resistors. The slew rate of the temperature change is zero. Yes the resistor is at an elevated temperature but the resistance does not vary. That is the ideal that we are trying to achieve.
a quick reminder:
that example was for a 100W into 8r0 "half" turbo. That is a very big and powerful amplifier.
Since all normal Audio resistors have a non zero tempco, then the flatter the slope of the temperature then the better for the audio we are trying to pass.
Taken to the other extreme. Pass A constant DC current through the resistors. The slew rate of the temperature change is zero. Yes the resistor is at an elevated temperature but the resistance does not vary. That is the ideal that we are trying to achieve.
a quick reminder:
that example was for a 100W into 8r0 "half" turbo. That is a very big and powerful amplifier.
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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)
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)
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.
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
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.
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
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.
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.
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