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Old 19th July 2008, 04:03 PM   #1421
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Juma,

I'm using using an 18 -0V dual seconday on my F5 and its DC output would only give 22v with the amp connected. Would your preamp still work good with 2v lower on each supply voltage rail? Or would it need to be regulated?
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Old 19th July 2008, 04:22 PM   #1422
juma is offline juma  Germany
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+- 22V DC is quite all right. I used 78L18 and 79L18 on PCB to regulate PS voltage to +-18 V from +-24 (standard F5 PS voltage), but 78L15 and 79L15 will do the job perfectly. You just have to make sure that input PS voltage must be at least 3 V higher than voltage you get from on-board regulators.
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Old 20th July 2008, 11:39 AM   #1423
panos29 is offline panos29  Greece
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Default F5 Critics anyone??

Allright, I have built and tested F5 for some 3 weeks now. Everything is in spec, fets are well matched, harmonics as they should, THD the same and DC offset less than 1 mV. Initial sound testing was done without thermistors and current limiting transistors and the outcome was very promising. On day 4, I added thermistors (EPCOS NTC 4k7 3%) and limiters and reajusted the bias. I took measurements and everything was spot on again. I used the amp for about 2 weeks driving my horn loaded Tannoy 385s but to my astonishment the bass was a lot less satisfying than before and the mid-highs were congested with no lifelike presentation, overall a very typical transistor mediocre sound. After two dissapointing weeks, I took off the thermistors again and readjusted the amps. The sound improved considerably! the difference is huge, the sound is alot more live, with excellent bass control, life like mid and vocals and excellent resolution and crystal clear extension in highs.
I really cannot explain this really huge difference with the thermistors in the circuit which is not measurable at all, but it is very much there in auditioning the amp. What is worth mentioning is that a friend of mine built exactly the same amps(using the same PCBs and components) and his findings are exactly the same!
Any ideas-explanations are more than welcomed!
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Old 20th July 2008, 12:52 PM   #1424
estman is offline estman  Estonia
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Default Re: F5 Critics anyone??

add thermistors and cl transistors separately, listening in the meantime.

oops! you removed only the thermistors, my mistake.
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Old 20th July 2008, 05:53 PM   #1425
KSTR is offline KSTR  Germany
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Panos,

very interesting findings! The only explanation I could think of, assuming that the thermistors are really the root cause (that is, nothing has been overlooked during experimentation) is that the specific EPCOS models you used are highly nonlinear, changing resistance with voltage and probably additionally having some memory (dynamic non-linear behaviour). One way to test this would be to build a simple resistor divider, 4k7-metallfilm + the 4k7-thermistor and then run distortion tests with that, applying about the same DC bias and the same AC voltage that we have in the circuit (~4V-DC and ~1V-AC or so). If this behaves well, as well as or only slightly worse than a divider with two 4k7 metall-film resistors used for comparison, then it must be witchcraft

- Klaus
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Old 20th July 2008, 07:58 PM   #1426
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Thermistor behavior is exponential.

Still, I have used both EPCOS and CanTherm and I can't say I hear a difference with either of them, although the measured THD% is lower with the thermistor and current limit resistors.

One more edit -- the thermistor should be as close as possible to the transistor it is measuring.
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Old 20th July 2008, 08:25 PM   #1427
panos29 is offline panos29  Greece
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Indeed the THD is slightly lower with the thermistors in circuit. The thermistors are attached to the plastic side of the body of the output mosfets and track the temperature very well. Still the sound is by far superior without these. The difference in sound quality is huge and its not a matter of subjective taste or anything.
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Old 21st July 2008, 09:38 AM   #1428
estman is offline estman  Estonia
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The thermistors are close to the tracks with high current. Could that be the reason?
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Old 21st July 2008, 10:32 AM   #1429
panos29 is offline panos29  Greece
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For reference, I am posting my pcbs, I know the mosfets are very close to each other but it was designed this way in order to be mounted on specific narrow heatsinks.
Attached Images
File Type: jpg f5a.jpg (40.3 KB, 1681 views)
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Old 21st July 2008, 10:45 AM   #1430
Magura is offline Magura  Denmark
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At the risk of opening a can of worms......

It is entirely possible that without the thermistors, the distortion specs. are better, and that it sounds worse to a lot of people.

I have made some blind tests with a number of audio nuts, and many preferred some odd harmonic distortion over the possible minimum, the same goes for that people seem to prefer odd over even harmonic distortion.

So, what I'm saying here, is that you just might prefer a little distortion, as it often makes the sound more forgiving and somewhat warmer.


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