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Old 17th September 2008, 07:46 PM   #2221
PWatts is offline PWatts  South Africa
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I've measured KSA's at varying bias levels and found that above a fairly low bias the distortion stays relatively unchanged. However, distortion CONSISTENCY did improve with bias i.e. the THD vs. power graph was quite flat, much flatter than most class-AB and class-D amps. So even though the absolute THD percentages aren't too impressive and actually higher in areas than at lower bias levels, the key is that the figure doesn't change much as the power changes, which is IMO more important than varying distortion levels that may be lower at some power levels. Amps exhibiting this behaviour usually have great transient bass response ("grip/slam"), and this amp is no exception.

It should be noted that the power supply needs to be up for the task since increased bias will cause sag and increase 50Hz harmonics, thereby increasing THD. Especially at lower (load) power levels high bias therefore typically results in higher THD due to the large 50Hz components. Separating the output stage power supplies will help with this though since ripple on the input & driver stages affect total distortion far more than just on the output.

Moral of the story: push the bias as high as the power supply and heatsinks will allow. Even though it's always a good idea, if the supply is on the smallish side, get a separate supply for the PCB's. The quality of the power supply is paramount to get proper performance at high bias levels. Note I say quality; a cheap 1kVA toroid and large capacitor bank will not help much if the transformer has poor load regulation and the caps high ESR.
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Old 17th September 2008, 07:57 PM   #2222
K-amps is offline K-amps  United States
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How much should one be concerned if the center taps of the large/ little transformers do not show 0v potential difference... would this not induce hum into the amp or maybe DC offset?

I man should one be concerned or just go implement the dual trafo and be able to trim off the differences??
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Old 17th September 2008, 08:02 PM   #2223
PWatts is offline PWatts  South Africa
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Well the centre taps of two separate transformers will naturally not be at the same potential to each other, and if you left them floating it would be a problem, but since you tie them together it becomes 0V after all.
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Old 17th September 2008, 09:02 PM   #2224
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my friend who made krell ksa100 with 2pairs of mjl21193/21194 wants to have the technical datasheets of krell ksa100 and krell ksa50..SNR BW and THD..do you know them?..thanks
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Old 17th September 2008, 09:17 PM   #2225
PWatts is offline PWatts  South Africa
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I've posted measured spectra and simulation graphs somewhere in the thread, check around October 2006. Otherwise I can see if I can dig them up somewhere. The power supply and transistors used are likely to lead to different performance though.
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Old 17th September 2008, 09:51 PM   #2226
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oww i had a look at some of them.they are too long..i dont have much time...are you a krell expert ? what do you think about the feedback capacitor of krell ksa100? is it very important ?i have bipolar Frolyt 6,3volts 470uf.and 10volts 470uf..i dont know they are quality enough.. i think i will use Blackgate 16 volts 470uf..my friend is gonna use two capacitors as a feedback cap..he says its original and better..i dont agree with him :S
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Old 17th September 2008, 10:02 PM   #2227
PWatts is offline PWatts  South Africa
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A good quality cap is recommended, at the very least something from a reputable manufacturer and not a generic thing. Unless you go Black Gate I'd say rather use two standard electros than a bipolar.

The voltage is very low; you can use 4V too if you like.

I would recommend a single 470uF 6.3V Black Gate NX. If you look at the literature of Black Gate N/NX series it explains that they actually are 2 series caps inside the same package to reduce ESL and space compared to two discrete caps. So using two of them is not only unnecessary but also counter-productive since you're lengthening the signal path and adding extra inductance. Also important if you use one of these BG caps is that the 100nF bypass cap (C4) should be omitted.
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Old 17th September 2008, 10:15 PM   #2228
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i didnt understand..is it better to use 2 good caps than using normal quality bipolars?
i can use nichicon muse bipolar if cant get blackgate..is it not enough? thanks
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Old 17th September 2008, 10:20 PM   #2229
PWatts is offline PWatts  South Africa
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Generally speaking you can get better quality polar caps than non-polar ones and therefore you're better off with two quality non-polars. A bipolar Muse should work well though. Two Elna Cerafines would be better, and a BG NX the ultimate.
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Old 17th September 2008, 10:35 PM   #2230
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ow i see..i think its about the inductance? connecting two inductance in series makes a half inductance right? its the best to use two high quality bipolar caps in series connection right?
and question two..is there two normal caps in series inside the bipolar caps?what is the real purpose of bipolar caps?someone says its for not having phase errors..i opened the caps but didnt understand..thanks
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