A minutes is a long time for the soft start, I have found that the first few seconds is enough if the thermistor is between 10 and 20 ohms (120v or 240v respectively).
A few seconds gets you past the point of taking out the circuit breaker and fuse if it is properly sized.
A few seconds gets you past the point of taking out the circuit breaker and fuse if it is properly sized.
It sounds like your thermistor has too much resistance to work as it should in that circuit. It should have a hot resistance low enough to charge the capacitors to 90% or more of the final voltage in a few seconds. I have over 300Kuf in my F5V3 monoblocks and they have the thermistors in the circuit for less than 10 seconds and do not blow fuses when the thermistors are shorted out by a relay.
Hello all !!
I'm building F5 amp and after the first bias adjustment, in about 5 seconds I have to swich of because the R 11,12,18 and 19 are too hot..... and in the other chanel, the R13 is burned.... I'm not sure about why.... somebody can help me ?
I'm building F5 amp and after the first bias adjustment, in about 5 seconds I have to swich of because the R 11,12,18 and 19 are too hot..... and in the other chanel, the R13 is burned.... I'm not sure about why.... somebody can help me ?
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Perhaps post in this thread for more exposure, but also check Q6 orientation (and perhaps Q5) depending on what parts you used.
Q6 looks toasty, and if it's the same parts I used, it looks to be in backwards.
An illustrated guide to building an F5
Q6 looks toasty, and if it's the same parts I used, it looks to be in backwards.
An illustrated guide to building an F5
I use the f5 v3 guide
Firstwatt F5 amplifier v3 - diyAudio Guides
I believe are well mounted... really ??
Firstwatt F5 amplifier v3 - diyAudio Guides
I believe are well mounted... really ??
I can't see any standoffs supporting the pcb. Are there any shorts?
Also, probably better moving it to the F5 thread as ItsAllInMyHead suggested
since you are building the standard F5.
Also, probably better moving it to the F5 thread as ItsAllInMyHead suggested
since you are building the standard F5.
Some time ago i bought used F-5T amplifier which was never work as it should. The previous owner had problems with hum on the left chanel and he was unable to adjust bias on the right chanel. I notice that Q4 on that chanel is running much hotter than other transistors.
Amplifier is build in 4H/400 Modushop chassis and use 2x30V 600VA transformer. So i have to small chassis and to high voltage out of the PSU.
What would be the best way to resolve this? Buying new transformer or lowering the bias?
I would like to resolve this before finding the hum and bias problems.
Any help appreciated.
Amplifier is build in 4H/400 Modushop chassis and use 2x30V 600VA transformer. So i have to small chassis and to high voltage out of the PSU.
What would be the best way to resolve this? Buying new transformer or lowering the bias?
I would like to resolve this before finding the hum and bias problems.
Any help appreciated.
As i see it, you have 2 choices. new transformer with lower voltage. Or lower bias current.
in any case. you should cascode the J-fets.
2x30V transformer chould give you around +/-40Vdc. with 1.8A bias it sums up to around 150W dissipation pr ch. your chassis should handle that.
in any case. you should cascode the J-fets.
2x30V transformer chould give you around +/-40Vdc. with 1.8A bias it sums up to around 150W dissipation pr ch. your chassis should handle that.
oh yes. that chassis is good for 300W pr chassis.
Well-. As log as not the ambient is 30 celsius.
Well-. As log as not the ambient is 30 celsius.
I agree. What I would do in your place is to lower bias and then sort out the issues with the amp before spending money on different transformers. Once you have it running reliably then you can look at the cost and benefits of purchasing new transformers.
I took some measurements on left channel ( the one with no bias adjustment problem) and i have:
2x 41.5V from PSU.
0.365V across the 1 ohm resistors.
5mV offset on output
The heatsink temperature is around 54 deg C. which is 32 deg C. above room temperature.
2x 41.5V from PSU.
0.365V across the 1 ohm resistors.
5mV offset on output
The heatsink temperature is around 54 deg C. which is 32 deg C. above room temperature.
Well. That is spot on my ballpark guess. About 1.5A bias and 54 celsius. You can crank it even higher. But be aware of the diodes. Make sure they dont start conducting current witin the bias range.
Or go to something with deeper fins.
I got one from China that has 70mm deep heatsinks, but the quality left a little to be desired. (I spent hours cleaning up the backs of the heatsinks to make them flat.)
If it possible i would like to keep the Modushop chassis as is.
I tried to measure ( on the other channel) voltage across 1 ohm resistors but Q4 gets crazy hot after a minute.
I get between 0.29V to 0.35V across them on cold heatsink.
Amplifier plays music normaly.
I tried to measure ( on the other channel) voltage across 1 ohm resistors but Q4 gets crazy hot after a minute.
I get between 0.29V to 0.35V across them on cold heatsink.
Amplifier plays music normaly.
F5T not quite my cup of tea. But this could be a number of things, no? Poor matching of output fets, wrong value source resistor, or insufficient heat conduction to sink comes to mind. But surely there are other possibilities
1: Have you checked Q4s source resistor (ohmmeter with amp turned off?)
2: Have you checked if q4 is properly matched, or did the previous owner provide you any info on where he bought them/how they are matched? What is the current through source resistors when Q4 gets «crazy hot»?
3: have you tried retightening q4 to the sink?
Without cascode on those voltages, you may have damaged your FETs, but dunno if that explains this.
You could also have an oscillation problem, but strange it applies only to one fet. Are optipnal C’s on the OS installed?
It is strange if you get temp increase if there is no current increase at the same time.
Doubt you’ll get anywhere with these tips, but hope you solve this!
Btw: 5U 400mm takes about 250w dissipation MAX in my room. 400 iow is not good to 300watts, at least not if your temp limit is 55 degree sinks.
I too bought a failed F5T project. I took the easy way ought and ordered new boards, and postponed the project until my skills improve.
Regards,
Andreas
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