If the diodes that bypass the source resistors turn on, even just a little, during your bias setting adjustments, you lose the stabilising effect that the Source Resistor provides.
The bias becomes unstable, rather than stable, and potentially could run away when you change temperature slightly.
Remember, the bias voltage is applied to the series combination of Vgs and Vrs.
If Ibias tries to increase then Vgs must reduce and thus Ibias tends to reduce. This is a stable feedback effect.
Replace Vrs with Vf of a passing diode and the Vbias= Vgs+Vf equation loses the stablising effect of Rs. Take care !
Setting up an F5 is not the same as setting up an F5t
The bias becomes unstable, rather than stable, and potentially could run away when you change temperature slightly.
Remember, the bias voltage is applied to the series combination of Vgs and Vrs.
If Ibias tries to increase then Vgs must reduce and thus Ibias tends to reduce. This is a stable feedback effect.
Replace Vrs with Vf of a passing diode and the Vbias= Vgs+Vf equation loses the stablising effect of Rs. Take care !
Setting up an F5 is not the same as setting up an F5t
So what part of my setup suggestion was risky to follow? You are correct the diodes increase the possibilities of instability, but taken slowly and according to what I suggested, its a non issue. The F5, and with it, the f5T, remain the easiest amp i have encountered in terms of build/bias difficulty.
first you need to get som other heatsinks. horisontal fins is a very bad idea for a amp like this
whats the rail voltage and bias? is it with or without diodes. if you have diodes, how are they mounted?
and how many output pairs?
The voltage would be more or less what it is in the schematic I think it was a couple of volts higher, the bias when the right channels fets blew was about 3/4 of the total recommended power (not sure without looking at the manual) . its using the diodes. I am no expert and built the electronics for this for a friend not as a job!
There were hum problems original so I redesigned the power supply 3 times and the 3rd using tea bags boards and worked well with no hum, this amp I spent hours and hours on over a course of 3 months or more without payment for my time, mainly because it was being built for a friend! I could have done other things with my life in that time.... in this world you try to do something nice and you sometimes just end up with unhappy people all round
id rather not take the money if it was offered anyway to keep the owner not so unhappy with me. with my previous experience of building a couple of F5's I decided to build this F5t but knew at the time it was a bit out of my league.
I presume this amp has some instability but to me the problem is unlikely to lie with the temp of the heatsink or fets.
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This is true and I realised this mistake a little too late, was thinking of turning them round or mounting on another sink. when the amp blew it was nothing to do with heat, this happened when I turned the volume up on my PC... strange sound then 2x mosfet started burning. I have no idea what that is all about but when my PC volume was low there was no problem.
Temporary solution the connections were strong just messy, no hum on speakers etc. not everyone makes everything perfect when prototyping ;-)
Thanks
it was never a job in the first place just meant to be a friend wanting another friend to hear a good amp, at-least I learnt my lesson (stick to music and don't tackle things out of my depth!)Indeed originaly set the bias a bit too high and thought my god that's a hot heatsink, sorted that out fast and had the amp working for some days.
after that I never set the bias to full power again as I was afraid to up it Until i was sure the amp was stable, so kept it around about the 1/2 mark.
with still a lovely sound.
I used 2 cheap and one expensive multimeter.
its interesting how doing something with a good heart and intention can be totally the wrong thing to do and lead to unhappy people lol
gabsta. yes i do live in norway. right smack in the middle of the country
a cheap multimeter(or 2 of them) is all one need.
when the sinks are mounted this way, they are also more prong to changes in ambient temp. i still think that the sink is atleast partly the couse of the blown outputs and fire.
also i think the bias was set to high for the sink temp/Source resistance. and the fact that the outputs are mounted high on the sinks do not make things better.
Yeah id definitely swap them round or another solution. The top of the sink was hotter.
Will share but it's a long term project i'm afraid
Iíjscobar,
what type of matched mosfet do you get from Spencer?
To be honest, I don't recall. I don't have the kit nearby, as I said: this is a long term project. But I guess he delivered the parts mentioned in the BOM
http://www.fetaudio.com/wp-content/uploads/2012/11/F5T-BOM-Cost2.pdf
http://www.fetaudio.com/wp-content/uploads/2012/11/F5T-BOM-Cost2.pdf
now that we have JFETS in the store, could someone comment on what grades would work best? I think "Papa" mentioned "B and C" but aren't sure I remember correctly.
http://www.diyaudio.com/store/parts/lsk170-to-92-jfet.html
http://www.diyaudio.com/store/parts/lsj74-to-92-jfet.html
There are links to their spec sheets
http://www.diyaudio.com/store/parts/lsk170-to-92-jfet.html
http://www.diyaudio.com/store/parts/lsj74-to-92-jfet.html
There are links to their spec sheets
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Folks:
Your advice on the positioning of the toroid and power supply pcb would be appreciated. I'm building several F5T amps with friends. We've purchased 5U/400mm "big amp" enclosures from the Store, 1000 VA SumR toroids and are using large power supply pcbs supplied by Buzzforb. My question: is it better to (a) center the toroid in the chassis and mount the power supply pcb on standoffs directly above the toroid or (b) follow the more traditional approach of mounting the toroid towards the front of the enclosure and the power supply pcb on the floor towards the rear? It seems to me that (a) minimizes any blocking of the vents on the bottom plate and would maximize airflow through the chassis around the toroid/ps pcb stack, whereas (b) maximizes vertical space above both the toroid and ps pcb at the possible expense of impeding air flow from below.
As newbies, we're open to either approach and assume that someone has exhaustively assessed both options. And if not, does anyone have a considered opinion?
Regards,
Scott
Your advice on the positioning of the toroid and power supply pcb would be appreciated. I'm building several F5T amps with friends. We've purchased 5U/400mm "big amp" enclosures from the Store, 1000 VA SumR toroids and are using large power supply pcbs supplied by Buzzforb. My question: is it better to (a) center the toroid in the chassis and mount the power supply pcb on standoffs directly above the toroid or (b) follow the more traditional approach of mounting the toroid towards the front of the enclosure and the power supply pcb on the floor towards the rear? It seems to me that (a) minimizes any blocking of the vents on the bottom plate and would maximize airflow through the chassis around the toroid/ps pcb stack, whereas (b) maximizes vertical space above both the toroid and ps pcb at the possible expense of impeding air flow from below.
As newbies, we're open to either approach and assume that someone has exhaustively assessed both options. And if not, does anyone have a considered opinion?
Regards,
Scott
Scott - I would do whatever will place the PSU board closest to the amp boards. In the 5U, that will likely mean the toroid towards the front and PSU towards the rear, however you have lots of flexibility in heighth because the 5U is so tall.
It's going to come down to wether or not you can mount the PSU and toroid on the floor of the amp, in a way you like.
It's going to come down to wether or not you can mount the PSU and toroid on the floor of the amp, in a way you like.
While on the topic of the 5U case (which I am also using) and its floor space for power supply doo dads, I have a question. My previous F-5 build has a floor pan made from 1/8 inch 6061 aluminum, and I mounted the POWER SUPPLY diodes (MUR) to the floor pan of the amp as heatsinking. With the 5U case, this would mean attaching them to that perforated metal floor....is this sufficient heatsink for those MUR PS diodes? (As supplied from Buzzs group buy kit)
Thanks,
Russellc
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