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Old 4th August 2017, 09:04 PM   #15631
McQuaide is offline McQuaide  United States
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Also, this was built from the F5 v3 schematic that goes with the F5 v3 boards. The turbo is not involved here. Thanks Kosst and peafarmer for your help!
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Old 4th August 2017, 09:33 PM   #15632
Kosst Amojan is offline Kosst Amojan  United States
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I didn't get any diagram with my boards. I went straight off the diagram for the F5 in the F5T article and made the deletions for the higher power output.
Not sure what to tell ya about the protection board. Bad ground?
I hope you get it sorted out. That thing will only sound better the more you use it!
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Old 7th August 2017, 01:06 PM   #15633
JeffYoung is offline JeffYoung  Ireland
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I was reading Nelson's article on finding the sweet spot, and wanted to add a P3 trimmer to my F5 to balance the JFETs.

I've already laid out my boards to the F5 "production" schematic, but I noticed that the "F5 w/P3" schematic includes a couple of changes to the temp compensation cct (thermistor connected to V+ vs. after the .47R) and the current limiting cct (Q5-base-to-V+ changed from 150R to 100R). I presume these have more to do with the MOSFET change (IRFP9240 -> FQA12P20) than the addition of the P3 trimmer?

Sorry for the noob questions; I'm still getting my head around this whole analog domain.

Thanks,
Jeff.
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Old 7th August 2017, 06:29 PM   #15634
Kosst Amojan is offline Kosst Amojan  United States
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I'm no expert, but I'm going to guess the placement of TH1 and TH2 after the source resistors allows for better tracking. The 100R and 150R values are strictly about current limiting. The values are contingent upon where you want Q5 and Q6 to open and limit output. Neither changes have anything to do with the choice of output MOSFETs.
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Old 7th August 2017, 09:12 PM   #15635
JeffYoung is offline JeffYoung  Ireland
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Hi Kosst,

I went back and read the section it the manual about the current limiting cct, and I agree with you on the resistor change: it looks like it raises the hard limit from 10A (150R) to 14A (100R). I don't plan on running the bias particularly hot, so I think I'll stick with 150R.

I had a look at my PCB design and it should be easy enough to move the thermistor, so I think I might make that change.

Cheers,
Jeff.
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Old 7th August 2017, 09:36 PM   #15636
Kosst Amojan is offline Kosst Amojan  United States
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The bias and current limiting don't have much interaction. The current limiting steps in as the amp transitions into class AB or if the outputs are shorted. The 10 amp number comes the current drawn by a load of about 2.5 ohm. It's a good number if your speakers have some mean impedance dip like mine do. You'll still want to bias as high as you feel safe in going because that's what dictates the class A envelope. Biasing it to the 25 wpc RMS is very safe assuming adequate heatsinking. I've biased mine up to 1.1 amp (.515V across the source resistors on 34.5V rails) and yeah, it runs piping hot, but it's stable and the MOSFETs never reach over 80C. It's doing a 75 watts peak in class A.
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Old 9th August 2017, 03:25 PM   #15637
JeffYoung is offline JeffYoung  Ireland
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I found a stash of NOS Harris (91 date code) IRFP9240s. I ordered 20 at $2.50ea, but I only need 10 or so. Give a holler if you've any interest in the other 10.

Cheers,
Jeff.

PS: if I get hit for the VAT coming into the EU, they'll be $3.12ea. Other folks in the EU will be used to that, but I'm not likely to do the paperwork to get it back if they go back to the US, so they'd still be $3.12. An Post sends me a VAT bill about 1/2 the time, so one never knows....

Last edited by JeffYoung; 9th August 2017 at 03:30 PM.
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Old 16th August 2017, 08:54 AM   #15638
phil0702 is offline phil0702  Austria
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Hey!

I am getting a little bit confused about the proper numbers concerning impedance and sensitivity. So, I have a speaker with 83 db (2.38V/1m) and 4 Ohm impedance. My preamp has an ouput voltage of 2.25V RMS.

Can I use this speaker with my F5?

I know that the F5 has 50 Watts at 4 Ohm Impedance, but I am not sure how this connects with the sensitivity.

Thanks a lot, Philip
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Old 16th August 2017, 09:26 AM   #15639
JeffYoung is offline JeffYoung  Ireland
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83dB is moderately loud (a bit louder than a garbage disposal). That figure is presumably @ 1w @ 1m.

Sound falls off by the square of the distance. So if you're sitting 10' (3m) away from the speaker, you'll need 9w to generate the same volume.

Every 3dB louder takes twice the power. Humans experience "twice as loud" as roughly 10dB. So you need roughly 10 times the wattage to go twice as loud. Since your F5 won't push 90w, you're only going to get to about half-again as loud (around 90dB at 10').

Only you can answer if that's loud enough. It's certainly not live rock concert levels (100+dB); on the other hand it would be loud enough for some.

Cheers,
Jeff.

(PS: this is how I understand it, but I'm no expert. Corrections welcome.)

(PPS: that's a pretty inefficient speaker. I think the majority of speakers would be in the mid- to upper-80's, while high-efficiency designs would be in the upper-90s into the 100s.)
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Old 16th August 2017, 10:19 AM   #15640
phil0702 is offline phil0702  Austria
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Hey!

Thanks for your nice answer! As I normally listen not that loud I hope that's ok!

The speaker itself, beside the low efficiency, i really really nice. See here: (in german but with diagrams)

DXT-MON

All the best, Philip
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