Can we estimate the running temperature of the feedback resistor when a continuous known dissipation is passing and we know maximum dissipation and maximum DeltaT?
If we now pass a transient, can we estimate the metal film peak temperature from these periodic transients?
Let's give some data to start with.
max Pd 600mW
max DeltaT 150Cdegrees
continuous operating dissipation 10mW.
Peak dissipation 100mW.
temp coef 50ppm.
If we now pass a transient, can we estimate the metal film peak temperature from these periodic transients?
Let's give some data to start with.
max Pd 600mW
max DeltaT 150Cdegrees
continuous operating dissipation 10mW.
Peak dissipation 100mW.
temp coef 50ppm.
I'm not worrying. I like to be informed.
I also like using 600mW resistors and not afraid to use two in parallel for 1.2W rating.
Where I'm going with this is to estimate the resistor value change with operating conditions and from that decide whether 1.2W is adequate in critical locations, like the feedback loop.
Looking at Lumba's post, I see nothing more than what has already been posted many times on this Forum.
I come here to learn, not for cheap jibes.
I also like using 600mW resistors and not afraid to use two in parallel for 1.2W rating.
Where I'm going with this is to estimate the resistor value change with operating conditions and from that decide whether 1.2W is adequate in critical locations, like the feedback loop.
Looking at Lumba's post, I see nothing more than what has already been posted many times on this Forum.
I come here to learn, not for cheap jibes.
Sure one can do this even so far as doing finite element analysis so that you even have a spatial pattern of thermal distribution 
Only question is, can you get somebody interested to do it?
As crude rule for worst case scenario you can multiply the temp coeff by 150°C to get maximum change under allowed conditions. And since that's still tiny and real world currents will lead to much smaller temperature elevations anyway I would not consider that a relevant factor.
Have fun, Hannes
Only question is, can you get somebody interested to do it?
As crude rule for worst case scenario you can multiply the temp coeff by 150°C to get maximum change under allowed conditions. And since that's still tiny and real world currents will lead to much smaller temperature elevations anyway I would not consider that a relevant factor.
Have fun, Hannes
Ha & Lumba,
your estimations are too simplistic.
A full temperature change due to a transient will lead to a 0.75% dynamic change in resistance for a 50ppm resistor. I cannot believe that leads to good reproduction.
Let's assume for a moment that the Rth r-a does double when continuous dissipation is 2% of rating.
That would give DeltaT ~ 250C/W * 12mW ~=3Cdegrees.
With 50ppm that is a DeltaR~=+0.015%. Not too bad from cold to operating temp.
Iif the ambient inside the amp has gone up from room temp by 10Cdegrees then DeltaT becomes 13Cdegrees and DeltaR is now 0.065%.
Now look at transient signals ~10dB up from continuous/average. Convert to peak value adds another 3dB.
We have dissipation ~54mW. If the transient Rth r-a were halved compared to the steady state the DeltaT becomes ~ 125C/W * 54mW ~=8Cdegrees. Leading to DeltaR ~0.034%. Add that to the previous operating temp DeltaR gives ~0.1% change in resistance.
Now what if the transient were 20dB above average.
The peak dissipation is up to around 380mW and DeltaT becomes +48Cdegrees. That leads to a total change in resistance of ~ 0.3%.
What does that do to output distortion?
It also confirms the advice given by many to over-rate critical resistors. I suspect it also explains the views of some that amps can sound different when fully warmed up compared to say 10minutes after switch on.
We could double the rating of the resistor, but is that enough?
I can guess just as well as anyone else.
I would like to know, rather than guesstimate. Even an estimate would be better. Informed intuition from experienced builders/designers are probably much better than my quesstimates.
your estimations are too simplistic.
A full temperature change due to a transient will lead to a 0.75% dynamic change in resistance for a 50ppm resistor. I cannot believe that leads to good reproduction.
Let's assume for a moment that the Rth r-a does double when continuous dissipation is 2% of rating.
That would give DeltaT ~ 250C/W * 12mW ~=3Cdegrees.
With 50ppm that is a DeltaR~=+0.015%. Not too bad from cold to operating temp.
Iif the ambient inside the amp has gone up from room temp by 10Cdegrees then DeltaT becomes 13Cdegrees and DeltaR is now 0.065%.
Now look at transient signals ~10dB up from continuous/average. Convert to peak value adds another 3dB.
We have dissipation ~54mW. If the transient Rth r-a were halved compared to the steady state the DeltaT becomes ~ 125C/W * 54mW ~=8Cdegrees. Leading to DeltaR ~0.034%. Add that to the previous operating temp DeltaR gives ~0.1% change in resistance.
Now what if the transient were 20dB above average.
The peak dissipation is up to around 380mW and DeltaT becomes +48Cdegrees. That leads to a total change in resistance of ~ 0.3%.
What does that do to output distortion?
It also confirms the advice given by many to over-rate critical resistors. I suspect it also explains the views of some that amps can sound different when fully warmed up compared to say 10minutes after switch on.
We could double the rating of the resistor, but is that enough?
I can guess just as well as anyone else.
I would like to know, rather than guesstimate. Even an estimate would be better. Informed intuition from experienced builders/designers are probably much better than my quesstimates.
Interesting thought Andrew, and similar to ones I've been thinking about recently.
Most series feedback loops however are ratiometric, a proportion tapped by two resistors configered as a voltage divider. Its the difference in the instantaneous values of the two resistors that matters.
Perhaps more complex but more accurate than simply using 'bigger parts ' would scale the resistors' tempco in inverse proportion to R^2; that way, the ratiometric change should be is zero over the range of allowable dissipation.
Most series feedback loops however are ratiometric, a proportion tapped by two resistors configered as a voltage divider. Its the difference in the instantaneous values of the two resistors that matters.
Perhaps more complex but more accurate than simply using 'bigger parts ' would scale the resistors' tempco in inverse proportion to R^2; that way, the ratiometric change should be is zero over the range of allowable dissipation.
Perhaps thermally strapping the two resistors in the feedback together like most do with ltp transistors. But what about thermal changes in the capacitor in series with the second resistor. This must also have some effect on the voltage divider ratio as well I would think. May need to include the whole network into your figures to get the most accurate results.
Andrew,
my answer was general, not based on calculation. Heat causes distortion in both active and passive components, the cooler devices the lower distortion. By common sense I meant awareness, having it constantly in mind. In the few critical cases, instead of difficult calculation, my solution is strong overrating, thus I feel confident (no worrying). Also, in resistors, good electrical and sonic properties rarely walk hand in hand.
my answer was general, not based on calculation. Heat causes distortion in both active and passive components, the cooler devices the lower distortion. By common sense I meant awareness, having it constantly in mind. In the few critical cases, instead of difficult calculation, my solution is strong overrating, thus I feel confident (no worrying). Also, in resistors, good electrical and sonic properties rarely walk hand in hand.
SY said:
This is correct. Two things, however. The distortion will be level-dependent, so you need to compare at different power levels. And IIRC, it shows up at lower frequencies rather than higher, and in this respect it doesn't behave like normal THD.
Also, the voltage fluctuation and voltage coefficient of the resistors should be considered as well as the temperature fluctuation and temperature coefficient. For example, you could connect two half-value resistors in series (which will reduce voltage fluctuation), also two double-value resistors in parallel (which will reduce temperature fluctuation), and compare these configurations against a single resistor.
At least in situations with sufficient signal swing and resistors having poor coefficients, you will be able to measure the effects as harmonic distortion.
regards, jonathan carr
That is what one would predict because of the thermal mass. Calculations based on transients are misleading- it takes time for a resistor to heat because of the mass and heat capacity of the resistive element, the thermal coupling to the body and leads (which have their own mass and heat capacity), and thermal transfer to the surroundings.
BTW, I would quibble with the notion that "normal" THD increases with higher frequency. This is certainly true of a solid state amp using feedback and compensation. But it's very normal for other devices (especially transformers and the tube amps that use them) to show rising THD with decreasing frequency.
I'd estimate resistor thermal time constants to be in the tens of seconds range or even longer. Any distortion would only show up at really low frequencies and also be increasing with decreasing frequency.
The heating will change output level though... But how much will a <0.1dB change be heard? Power compression in the speaker could easily be up to (or even exceeding) 3dB at high power!
The heating will change output level though... But how much will a <0.1dB change be heard? Power compression in the speaker could easily be up to (or even exceeding) 3dB at high power!
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