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Old 13th December 2012, 10:39 AM   #7811
KSTR is offline KSTR  Germany
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Quote:
Originally Posted by a.wayne View Post
Interesting 100:1 , what about placing additional caps at output , i plan on using a cap per output right at the output stage , seperate from the main cap bank of 160k / ch ....

How to calculate the appropriate value ...?
When ESL1, ESR1 are the parasitics of the bigger cap,
and ESR2 is the parasitic of the smaller one,
the optimum capacitance is (smallest one that will have good effect) :
C = 4*ESL1 / (ESR1+ESR2)^2.

It's also clear that ultra low ESR of the main cap is not a good idea if you want to add smaller ones (and bypasses) remotely.

For a true flat impedance profile the bypass ESR should equal the main cap's one, then the bypass circuit is the complex conjugate of the main cap. Of course at HF the bypass' ESL dominates the picture again.


LTspice is very helpful to illustrate the effect if there is severe mismatch (as mentioned by Frank)
Find attached a sim file for it (plot V(out), or if you want correct units in linear view, plot V(out)/1A). Rename to *.asc before opening.
Attached Files
File Type: txt parallel_caps_ind_cancel.txt (1.4 KB, 15 views)

Last edited by KSTR; 13th December 2012 at 10:42 AM.
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Old 13th December 2012, 11:25 AM   #7812
dvv is offline dvv  Serbia
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Oh man, this is becoming wild.

I do not doubt KSTR's words, but please consider - first just about everybody drills you that you need low ESR capacitors for best performance, and now KSTR tells us that you should not use low ESR caps if you want to bypass them?

KSTR, let me ask you outright - if you have two say 10,000 uF/63V caps in parallel per supply line feeding your power amp, how would YOU bypass them? If at all?

10,000 uF/63V caps by Fisher & Tausche.
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Old 13th December 2012, 12:49 PM   #7813
AndrewT is offline AndrewT  Scotland
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the 10m+10m are separated from the bypass (local decoupling) caps by some milliohms of cable and trace resistance. That helps attenuate the tendency to ring.

The decoupling ceramic also needs some damping, this implies that a hiQ cap will be underdamped. So we choose a NON C0G/NP0 cap. The X7R, etc. have a lower Q due to the higher esr.
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Old 13th December 2012, 01:02 PM   #7814
KSTR is offline KSTR  Germany
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Quote:
Originally Posted by dvv View Post
KSTR, let me ask you outright - if you have two say 10,000 uF/63V caps in parallel per supply line feeding your power amp, how would YOU bypass them? If at all?

10,000 uF/63V caps by Fisher & Tausche.
Those are good ones. But they will go inductive in the lower kHz range (eg 5Khz for 100nH), no way around it.

For DIY (for commercial use it would infringe patents) I'd try to use real inductance cancellation (google for Timothy Neugebauer papers about that) and no bypass, then feed the supply via coax or stripline to the point of load (will be OK for some 10cm). Then check experimentally if there is ringing and try install snubbers if needed. Amps are current sinks and DON'T damp the supply, the transformer side also does not damp most of the time (when diodes don't conduct) and shock-exites any tank-circuit at the diode on/off times.
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Old 13th December 2012, 01:39 PM   #7815
dvv is offline dvv  Serbia
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Originally Posted by KSTR View Post
Those are good ones. But they will go inductive in the lower kHz range (eg 5Khz for 100nH), no way around it.

For DIY (for commercial use it would infringe patents) I'd try to use real inductance cancellation (google for Timothy Neugebauer papers about that) and no bypass, then feed the supply via coax or stripline to the point of load (will be OK for some 10cm). Then check experimentally if there is ringing and try install snubbers if needed. Amps are current sinks and DON'T damp the supply, the transformer side also does not damp most of the time (when diodes don't conduct) and shock-exites any tank-circuit at the diode on/off times.
That's what I keep saying, F & T are my standard, but I'll gladly take BC Components as well. I have 20+ on stock at all times.

On the power amp side, I tend to use 100 uF in parallel with 2.2 uF Wima metalalized, and lastly 1 Ohm+470 nF Wima in series to the ground. That takes care of most residual capacitor inductance, but 470 nF is a tentative value only, it could be anything from 680 nF to 220 nF. No way of knowing, you just have to try and see.

Anyway, thank you for responding.
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Old 13th December 2012, 02:56 PM   #7816
a.wayne is offline a.wayne  United States
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This is going deep , xtra oxygen applied for ..

K,

This is going to be difficult to match ESR For all the caps , i was hoping to use what was already in Inventory .
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Old 13th December 2012, 03:18 PM   #7817
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All the electronics in the World will never be able to measure the difference between an A Class amp and a Class A amplifier....

What you need is a simple tape measure.

Wallet size x Ego size x Beat the Jones size = Best Amplifier.
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Old 13th December 2012, 03:19 PM   #7818
bcarso is offline bcarso  United States
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@Brad

I understand your musings and, make no mistake, I am definitely not opposing your views.

On the other hand, like Krell sound or not, no-one can possibly say that Dan d'Agostino doesn't know his trade too well. Meaning, that if he left it as is, there is very probably some reason for that. Possibly he thinks this is low enough to be inoffensive, or solving that problem may cause problems appearing elsewhere, I don't know.

Let me remind you of what I think could be an amp designer's worst nightmare, he has no idea what his amp might be connected to, and people have been noted to come up with the strangest of ideas in time.

I've noted a few oddities like that in my time as well. In 2001 or 2002, I was desperately trying to make Graham Slee from UK to dump his NE based headphone amp for an AD 826. Eventually I did it, and he did admit it sound quite a bit better with the AD, but he also noted that it had an unexplained offset. Didn't bother him, he has a cap in series with the output, but it sure bothered me, this 0.6V offset.

Solving the problem was no bid deal, just a simple 1N4148 diode from output to the minus Vcc line, but that offset shouldn't have been there in the first place, it flew against their own Data Sheet.

I sent them a mail outlining the problem, with a schematic I was using. Got no reply, BUT the next batch of these op amps, purchased about 4 or 5 months later on had the problem resolved. It was almost pointless using a servo, it was that good.

These things happen. Just looking at the internal topology of modern op amps it's hard to imagine that a mistake should never somehow creep in, after so many checking and rechecking.

In 1990 or 1991, a friend and I were doing a text for a local PC magazine related to numeric coprocessor (remember those?) effciency and speed. The notm was, of course, Intel, but another company was making a lot of waves with their take on the matter, for the life of me I can't remember their name now. Anyway, I supplied the hardware, and he supplied the software, which was a massive architectural program he wrote, based on MS Fortran (among other tests).

During the test, we discovered that the alternative had serious problems understanding numbers 0 and 1 - believe it or not. Consequently, it kept churning out wrong results on Fortran based programs (and I hasten to add, in those days, MS Fortran was THE standard against which all others were compared).

We got in touch with the company via a BBS board (all the rave in those days) and explained the problem. They were quick to answer, tahnking us for bringing their attention to a problem, and they would get back to us. And they did, about two months later, sending us a new version of the chip, with a request that we try it out. We did, and it worked just fine.

It happens. No more, no less.
We are also supposing that the HFN/RR measurements are accurate, and that the amplifier was not oscillating into some strange load, or as the result of some bogus connections. Note that the only inputs are balanced, and perhaps the output is also. No mention is made in the sidebar of the test equipment used, although as I'm only a sporadic reader of the magazine maybe they disclose this sometimes, and it is always the same. Again, although Stereophile isn't perfect, it does make me appreciate them by comparison.

I haven't been a close reader of Krell specs in previous reviews, so I have no idea how they tended to run in terms of signal-to-noise. I only picked up anecdotally that they are powerhouses in terms of high current --- in fact weren't they initially developed to support some very-low-Z and inefficient ribbon speakers --- were they ones from Apogee?

Actually I find a lot of highly-touted equipment to have disgracefully high noise, so I am not singling out this latest effort from D. d'A. When six of us were still cooperating in a speculative class D amp effort and about to go to a tradeshow, it was impossible in the short term to find a digital source low-enough-noise to have the power amplifier noise dominate. People remarked as to how the noise at the fairly-efficient speakers was virtually inaudible, even with the ear pressed up against, and I reminded one of the consortium that it was strongly dominated by the Bel Canto DAC, loaned to us along with the big speakers by Andrew Jones. That itself had replaced the (iirc) Simaudio piece, which was much worse for noise.

As I say with respect to the reviewed amp, I'm just puzzled and curious. I am even tempted to speculate about the conscious use of stochastic resonance, although the effectiveness of that will be very loudspeaker-efficiency-dependent.
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Old 13th December 2012, 05:07 PM   #7819
a.wayne is offline a.wayne  United States
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Krell ......!

Krell Full Power Balanced 350mc monoblock amplifier Measurements | Stereophile.com


This is a very quiet amplifier, with an A-weighted S/N ratio of more than 95dB. This is referenced to 2.83V output; this is equivalent of more than 130dB referenced to the amplifier's clipping point into 8 ohms! Even without the A-weighting network and extending the measurement bandwidth to 500kHz only worsened the S/N ratio to 84dB (again re. 2.83V).

Levels of harmonic distortion were also very low, particularly into higher impedance loads. The bottom trace in fig,3, for example, was taken at 2.83V into 8 ohms, and doesn't vary from 0.005% across the band. Reducing the load to 4 ohms increased the THD figure slightly to 0.007%, though into 2 ohms, the distortion went up to 0.03%.
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Old 13th December 2012, 05:29 PM   #7820
DF96 is offline DF96  England
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Quote:
Originally Posted by dvv
Oh man, this is becoming wild.

I do not doubt KSTR's words, but please consider - first just about everybody drills you that you need low ESR capacitors for best performance, and now KSTR tells us that you should not use low ESR caps if you want to bypass them?
Yes, this is where 'everybody knows' perhaps clashes with physical reality. Unbypassed caps need low ESR for low total impedance. Bypassed caps need somewhat higher ESR for low total impedance. Even simple circuit theory can be counter-intuitive.
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