Alternatively just remove the current limiters altogether.
I personally never bother with them.
If I want to take out the current limiting can I simply cut the a resitor lead from r21/r22? My f5 is built with current limiting and its much easier to just snip a resistor lead while it's in the amp instead of taking about the boards and removing the resitors and ZTX devices.
Attachments
Total nube rube here... I've got sitting on my table a completed F5 but I've got a problem. I've powered the thing up and tried to set the bias.... and nothing. I can crank the pots to their max and absolutely nothing across the resistors.
A little info about this specific build:
I've opted to run 32V rails. I used boards from DIYaudiostore which are laid out according to the F5 Turbo article. I've built the boards to the design except that I've omitted R15-R18. R19-R20 remain. All the transistors are Linear Systems parts.
In probing around I'm getting power to the boards. I've attached heatsinks to the JFETs and they get slightly warm. I have modest amounts of offset at the outputs (15-22mV+) which also doesn't change regardless of how much a turn pots P1 and P2. I'm thinking based on what I've read that R5 and R6 are too low in value and may need to be increased. Would this thinking be correct? Could R19 and R20 be allowing the current limiting to kick in with the higher voltage? This is my first amp and I'm really a bit lost.
A little info about this specific build:
I've opted to run 32V rails. I used boards from DIYaudiostore which are laid out according to the F5 Turbo article. I've built the boards to the design except that I've omitted R15-R18. R19-R20 remain. All the transistors are Linear Systems parts.
In probing around I'm getting power to the boards. I've attached heatsinks to the JFETs and they get slightly warm. I have modest amounts of offset at the outputs (15-22mV+) which also doesn't change regardless of how much a turn pots P1 and P2. I'm thinking based on what I've read that R5 and R6 are too low in value and may need to be increased. Would this thinking be correct? Could R19 and R20 be allowing the current limiting to kick in with the higher voltage? This is my first amp and I'm really a bit lost.
It's not an F5T. It's still got some current limiting and it's only got one pair of MOSFETs. I have no idea what measurements you'd like to see or how to take them properly. The schematic is the orginal F5 as laid out in it's revised form with P3 in the F5T article. I'm feeling like you think I know a lot more than I probably do.
Affirmative! Biased it up and gave it a listen. It sounds very good. I've still got thermal room to bias it higher and I'll be playing with that a bit more tonight. It does have a tiny bit of hum from the power supply, but it can barely be heard over the ambient noises outside. I LOVE the sound of it! The sound is delicate and detailed, but muscular, fast, and physical. It's exactly what I hope it would be. I'm extremely happy I managed to build this thing and that it worked! Thank you for tempting guys like me to dig into this stuff!
I agree!
So far the amp sounds excellent! Couldn't be happier with it! I've got it biased up to .515V across the source resistors, which if my math is right, is about 1.1 amp on each side with rails that measure an actual 34.5V. Sinks are running at about 55C at their hottest spots and the MOSFET cases measure at about 78C. No magic smoke! I installed a big copper heatsink between the JFETs and I'm glad I did since those are running about 53C with the case closed. I really couldn't be more pleased with it and I've got all of you to thank. Without Nelson Pass's eloquent descriptions and explanations in the F5 and F5T articles, as well as his Burning Amp 2016 lecture, and the valuable information I poured over in this thread and several others, there's no way I could have built this thing or troubleshot the issue I had. I'm very thankful for all of you.
So far the amp sounds excellent! Couldn't be happier with it! I've got it biased up to .515V across the source resistors, which if my math is right, is about 1.1 amp on each side with rails that measure an actual 34.5V. Sinks are running at about 55C at their hottest spots and the MOSFET cases measure at about 78C. No magic smoke! I installed a big copper heatsink between the JFETs and I'm glad I did since those are running about 53C with the case closed. I really couldn't be more pleased with it and I've got all of you to thank. Without Nelson Pass's eloquent descriptions and explanations in the F5 and F5T articles, as well as his Burning Amp 2016 lecture, and the valuable information I poured over in this thread and several others, there's no way I could have built this thing or troubleshot the issue I had. I'm very thankful for all of you.
DC offset weirdness...
This is a 3.0 version of the F5 built recently with matched Jfets from a member here. Offset is stuck at 170mv and is not sensitive at all to changes in P1 and P2. The voltages you see here are a couple minutes after startup, they were set to .6v yesterday after a couple hour warmup.
The amp sounds great and is not tripping the speaker protector but I'm concerned about this behavior. Any ideas why I can no longer adjust offset to near zero?
This is a 3.0 version of the F5 built recently with matched Jfets from a member here. Offset is stuck at 170mv and is not sensitive at all to changes in P1 and P2. The voltages you see here are a couple minutes after startup, they were set to .6v yesterday after a couple hour warmup.
The amp sounds great and is not tripping the speaker protector but I'm concerned about this behavior. Any ideas why I can no longer adjust offset to near zero?
