If the amp is getting low Z load, for example 3Ohm, the momentary impulse power via FET may reach 200W (100W rms). It might be enough to fry it.
Ok I read through again.
From post #2 it looks like your intention was to use current limiting circuitry.
So you have made a mistake. You should have wires running from r17 and r18 to the source lead of the mosfets.
From post #2 it looks like your intention was to use current limiting circuitry.
So you have made a mistake. You should have wires running from r17 and r18 to the source lead of the mosfets.
I think there are two most likely possibilities:
1) The amp was oscillating in the system.
2) The amp was inserted in the system without proper
burn-in and adjustment, and when it drifted up to 150 mV
offset, this was corrected with an increase in bias of the
opposite side, and then it drifted even higher until the
amp broke.
😎
1) The amp was oscillating in the system.
2) The amp was inserted in the system without proper
burn-in and adjustment, and when it drifted up to 150 mV
offset, this was corrected with an increase in bias of the
opposite side, and then it drifted even higher until the
amp broke.
😎
Yes.
I can't remember now, but I think bias current increases when you put the lid on.
If your case is not well ventillated and your parts are not matched (you are using paralleled outputs from memory) then your mosfets could fry.
A picture of the amp would also help.
I can't remember now, but I think bias current increases when you put the lid on.
If your case is not well ventillated and your parts are not matched (you are using paralleled outputs from memory) then your mosfets could fry.
A picture of the amp would also help.
So, if I read the schematic right, if I want to include the additional mosfets in the protection circuitry I also need to add the 1k resistors (R17, R18) to the sources - correct?
lets call them R117 and R118...leading from pin 2 of the ZTX's...
Carl
lets call them R117 and R118...leading from pin 2 of the ZTX's...
Carl
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yes. I am assuming you have these resistors mounted on the source leads of the mosfets or on a separate output pcb.
I think you could also probably use the one resistor on the PCB and run separate wires for each mosfet. Maybe Nelson can confirm the better approach.
I would also consider, not using the current limiting circuit as another option. So if you don't use it pull it all out - I have no idea how the current limiting circuit might react with the amp circuit if active components are left on the pcb that are not properly connected as intended (could cause oscillation etc).
I think you could also probably use the one resistor on the PCB and run separate wires for each mosfet. Maybe Nelson can confirm the better approach.
I would also consider, not using the current limiting circuit as another option. So if you don't use it pull it all out - I have no idea how the current limiting circuit might react with the amp circuit if active components are left on the pcb that are not properly connected as intended (could cause oscillation etc).
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Thank you all for the replies...
I have an order in for 10 pairs of s2k170s and 2sj74s and 2 new CHR70s....
I believe that I have a few spare 1k film resistors around. Once the JFETs arrive I will perform the surgery...
Thanks for the advice and suggestions.
I have an order in for 10 pairs of s2k170s and 2sj74s and 2 new CHR70s....
I believe that I have a few spare 1k film resistors around. Once the JFETs arrive I will perform the surgery...
Thanks for the advice and suggestions.
check your mosFETs and reduce back to the standard F5 to check your circuit is OK.
Run it for a while, repeatedly re-checking the output bias and the output offset.
When you recognise the pattern that it follows from cold through warm up to hot, then add the extra pair. But first you must select a matched pair for the Ns & the Ps
Match Vgs @ your intended Id (Ibias)
Run it for a while, repeatedly re-checking the output bias and the output offset.
When you recognise the pattern that it follows from cold through warm up to hot, then add the extra pair. But first you must select a matched pair for the Ns & the Ps
Match Vgs @ your intended Id (Ibias)
I agree 100% with Andrew.
Start off with the original circuit first and then add the additional mosfets once you have sorted out this problem.
I also hope you have the required heatsinking for this amp.
If you can manage it, a picture might help.
Start off with the original circuit first and then add the additional mosfets once you have sorted out this problem.
I also hope you have the required heatsinking for this amp.
If you can manage it, a picture might help.
I have been running a stock F5 since february and only recently decided to try the additional mosfet pair...really just because.
It is all packed in a Hifi2000 4U case with heatsinks rated at 0.31C/W.
It is all packed in a Hifi2000 4U case with heatsinks rated at 0.31C/W.
Hi,
is the 50W (4@24V*0.525A) dissipation all through one 0.31C/W heatsink?
or
have you split the Ns from the Ps?
is the 50W (4@24V*0.525A) dissipation all through one 0.31C/W heatsink?
or
have you split the Ns from the Ps?
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If you have been using single output with 1.3A bias then I would think a parrallel pair at 0.525A each should be ok.
Build it again and make sure you add current limiting to both output pairs, or completely remove it.
Build it again and make sure you add current limiting to both output pairs, or completely remove it.
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Yes, one heatsink. I was running 1.7A (.800V)per device before - rather hot.
When I dialed it down to .525V it gave 1.12A per device which is a little hotter in total. When it was running the sinks were certainly hot but I could keep my hand on them for a few seconds.
It is placed in a well ventilated spot.
When I dialed it down to .525V it gave 1.12A per device which is a little hotter in total. When it was running the sinks were certainly hot but I could keep my hand on them for a few seconds.
It is placed in a well ventilated spot.
You are trying to dissipate a bit over 100W.
Initially I would drop your bias by 20% (ie drop dissipation to about 80W)
Also try building it without current limiting circuit.
Initially I would drop your bias by 20% (ie drop dissipation to about 80W)
Also try building it without current limiting circuit.
Well I added the 1K resistors to complete the protection circuit and it has been idling steady for 2 hours now. .525v and <4mV DC at the outputs - nominal drift.
I will let it run for a couple days and check periodically. I thought when I lost the CHR70s I lost at least one 2sk170 but that is not the case...at least I'll have some spares now.
I would prefer to keep the protection circuit in place for now until I have run it with speakers for a while...
The sinks are hot but I can touch them for about 8 seconds before it starts hurting.
My rail voltage is a confirmed 22.2V so yes I am dissipating 99.1 watts per channel...I may very well dial the bias down to as close to 1W per fet...
I will let it run for a couple days and check periodically. I thought when I lost the CHR70s I lost at least one 2sk170 but that is not the case...at least I'll have some spares now.
I would prefer to keep the protection circuit in place for now until I have run it with speakers for a while...
The sinks are hot but I can touch them for about 8 seconds before it starts hurting.
My rail voltage is a confirmed 22.2V so yes I am dissipating 99.1 watts per channel...I may very well dial the bias down to as close to 1W per fet...
Provided you have the necessary heat sinking anything under 40W per fet should be fine. 20W to 30W per device will be more than safe.
You probably don't want to exceed 100W total dissipation per channel with those heatsinks.
If you have a thermocouple you could also measure the temperature of the devices compared to the heatsink temp.
You probably don't want to exceed 100W total dissipation per channel with those heatsinks.
If you have a thermocouple you could also measure the temperature of the devices compared to the heatsink temp.
How can we help you when you feed us conflicting data?
Can you confirm that the Ns and the Ps are on the same 0.31C/W heatsink?
Are the two Ns matched?
Are the two Ps matched?
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Sorry...you are 100% correct...it is .525V NOT 525mA...
Yes the N's and P's are on the same heatsink.
The mosfets are not matched - the jfets are...I was under the impression that matching jfets are more critical.
Yes the N's and P's are on the same heatsink.
The mosfets are not matched - the jfets are...I was under the impression that matching jfets are more critical.
there's the problem.
One or more of the mosFETs is hotter than the remainder. Overstress the FETs by fitting too small a heatsink and then running one of them at a higher bias current and your amplifier tells you it does not like what you ask it to do.
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