80dB now but still needs improvement at 1KHz

It's interesting that the classic filters (Butterworth, Chebychev, Cauer) are based on mathematical approximations that pre-date electronics. They were used to compose tables for artillery or something.

But filters don't have to implement that math. One can Spice a filter and tweak it (say, to use available parts) to do whetever works for you.

An LC or active filter from the Williams or Lancaster books can be a starting point.

We discussed non-reflective LC filters here some years ago, and they were mostly hacked by simulation. The new Mini-Circuits absorptive filters have more theory behind them.

Chebychef did his calculations on the mechancal lnkages used to operate the valves of steam locomotives, generating linear motion and sharp movement from the superimposed effects of rotary motions.

Twin tee is what I was thinking of, indeed. Of course, it's essential to use carefully matched components of good quality. It would be fiddly, yes.

In some distant past, I've used a box we called a 'Null Detector' which did just that. You'd null out the fundamental, and what was left was the distortion. I don't recall if it was good enough for

-80dB though.

Jeroen Belleman

Did he indeed? Interesting. Do you perchance have a reference?

Thanks, Jeroen Belleman

I think the general idea is that if you truncate a power series, here is a better set of coefficients than just chopping off the infinite set.

That mattered when math tables were computed by hand. Nowadays, a computer can just use the first 150 terms.

One could use a good resistive divider to halve the signal amplitude. If the filter is linear, the THD shouldn't change.

Or add a bit of DC bias.

Ceramic caps can be awful.

"Selective Voltmeters" were cool. I think I still have an old HP.

That's a Russian (brute force) solution. Hamming windows are computationally cheaper.

Did someone ruffle your feathers?

I remembered reading about his work on steam engines several decades ago, but can't give you a reference. Apparently the maths he developed for sharp steam valve movement was later found to be applicable to electronic filter cutoffs as well. The linkages for linear motion were something I didn't know about until I looked it up on Wikipedia.

You needed a lot of space and good room acoustics to get the best out of them.

Bass-reflex looks good on paper but sounds horrible. The human ear hears the 6dB/octave roll off as 'natural' but with an unnatural 'honk' superimpose on it. Bass transients are coloured by the honk and give hang-over effects.

It may not matter for synthetic pop music, but bass-reflex speakers can give misleading results if they are used for monitoring acoustic music recordings.

Thanks. Mechanical linkages are interesting and fun and obviously useful. Have you heard of Theo Jansen's 'strandbeest'? Plastic pipe contraptions that 'walk' on the beach powered by wind. They are eerily life-like.

Jeroen Belleman

They can, but don't do that. Wasting computer time for using a lot of terms is just wasteful. Minimax polynomials and the likes (rational approximations) are used all over the place, thanks Tsjebisjof.

Groetjes Albert

Calculating a sine or a log to 15 significant digits wastes milliwatts. AI wastes gigawatts.

Your posts are getting ever more annoying and unreadable.

At 1 Kc/s, a distortion figure of 90dB represents one cycle in 10^9, so you would have to run it for 1000 seconds before the startup transient became insignificant.

"Liz Tuddenham" snipped-for-privacy@poppyrecords.invalid.invalid> wrote in message news:1r37pyc.1bodve2fz5t4wN% snipped-for-privacy@poppyrecords.invalid.invalid...

The startup transient was not included in the FFT but in any case the circuit has other issues so here is a circuit which works as intended. Harmonics are approaching 100dB down and 1k c/s is below 90dB. Simulate overnight then stop. It will be at about 250 seconds. Select roughly the last 50 seconds and FFT on current zoom extent. Number of data points may need increasing.

What reality would say is anybody's guess and it has been pointed out that real measurements 100dB down may not be meaningful.

We could probably have a discussion here about why c/s is better than Hz

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InstName C8 SYMATTR Value 15n SYMATTR SpiceLine V=16 Irms=271m Rser=0.594318 Lser=0 mfg="KEMET" pn="C0603C153K4RAC" type="X7R" SYMBOL res 240 -144 R90 WINDOW 0 -1 46 VBottom 2 WINDOW 3 35 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 6.34k SYMBOL res 448 -144 R90 WINDOW 0 -4 61 VBottom 2 WINDOW 3 39 55 VTop 2 SYMATTR InstName R8 SYMATTR Value 13k SYMBOL njf -128 -176 R90 WINDOW 0 -37 23 VRight 2 WINDOW 3 -9 -3 VRight 2 SYMATTR InstName J1 SYMATTR Value J112 SYMBOL voltage -352 400 R0 WINDOW 123 0 0 Left 0 WINDOW 39 10 135 Left 2 WINDOW 0 10 0 Left 2 WINDOW 3 15 104 Left 2 SYMATTR SpiceLine Rser=0.01 SYMATTR InstName V2 SYMATTR Value 6 SYMBOL res 48 272 R0 WINDOW 3 36 65 Left 2 SYMATTR Value 10.5k SYMATTR InstName R10 SYMBOL schottky 496 -288 R0 WINDOW 3 -17 -26 VRight 2 SYMATTR Value BAS40HY SYMATTR InstName D1 SYMATTR Description Diode SYMATTR Type diode SYMBOL res -112 -64 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 82 SYMBOL OpAmps\\LT1994 176 128 R0 WINDOW 3 10 -65 Left 2 WINDOW 0 11 -95 Left 2 SYMATTR InstName U1 SYMBOL schottky 528 448 M0 WINDOW 3 52 -26 VRight 2 WINDOW 0 -18 3 Left 2 SYMATTR Value BAS40HY SYMATTR InstName D3 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 640 448 M0 WINDOW 3 49 -26 VRight 2 WINDOW 0 -21 4 Left 2 SYMATTR Value BAS40HY SYMATTR InstName D4 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 528 608 M0 WINDOW 3 25 87 Left 2 SYMATTR Value 220k SYMATTR InstName R16 SYMBOL res 720 608 M0 WINDOW 3 27 87 Left 2 SYMATTR Value 220k SYMATTR InstName R17 SYMBOL res -112 -256 R0 WINDOW 0 38 42 Left 2 WINDOW 3 36 66 Left 2 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL res 320 -384 R0 WINDOW 0 -50 69 Left 2 WINDOW 3 -54 98 Left 2 SYMATTR InstName R13 SYMATTR Value 220k SYMBOL res 448 -32 R90 WINDOW 0 -4 61 VBottom 2 WINDOW 3 39 55 VTop 2 SYMATTR InstName R9 SYMATTR Value 1.07Meg SYMBOL res -80 -288 R180 WINDOW 0 40 70 Left 2 WINDOW 3 45 42 Left 2 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL res -80 432 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R5 SYMATTR Value 33k SYMBOL res 176 416 M90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R15 SYMATTR Value 220 SYMBOL res 16 560 M90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R14 SYMATTR Value 10k SYMBOL cap 176 352 R180 WINDOW 0 -37 41 Left 2 WINDOW 3 -50 13 Left 2 SYMATTR InstName C7 SYMATTR Value 15n SYMATTR SpiceLine V=16 Irms=271m Rser=0.594318 Lser=0 mfg="KEMET" pn="C0603C153K4RAC" type="X7R" SYMBOL cap 720 128 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C13 SYMATTR Value 100000n SYMATTR SpiceLine V=6.3 Irms=0 Rser=0.002 Lser=0 mfg="TDK" pn="C575OX5ROJI07M" type="X5R" SYMBOL res 736 160 R0 SYMATTR InstName R18 SYMATTR Value 600 SYMBOL res 192 -400 R270 WINDOW 0 35 55 VTop 2 WINDOW 3 -6 55 VBottom 2 SYMATTR InstName R12 SYMATTR Value 220k SYMBOL cap 176 -352 M0 WINDOW 0 -22 9 Left 2 WINDOW 3 -75 48 Left 2 SYMATTR InstName C6 SYMATTR Value 10000n SYMATTR SpiceLine V=6.3 Irms=0 Rser=0 Lser=0 mfg="Murata" pn="GRM155R60J106ME15" type="X5R" SYMBOL cap 416 -352 M0 WINDOW 0 -32 51 Left 2 WINDOW 3 -72 7 Left 2 SYMATTR InstName C10 SYMATTR Value 10000n SYMATTR SpiceLine V=6.3 Irms=0 Rser=0 Lser=0 mfg="Murata" pn="GRM155R60J106ME15" type="X5R" SYMBOL cap 368 448 M270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName C9 SYMATTR Value 10000n SYMATTR SpiceLine V=6.3 Irms=0 Rser=0 Lser=0 mfg="Murata" pn="GRM155R60J106ME15" type="X5R" SYMBOL cap 416 640 R0 WINDOW 0 -37 6 Left 2 WINDOW 3 -77 53 Left 2 SYMATTR InstName C11 SYMATTR Value 10000n SYMATTR SpiceLine V=6.3 Irms=0 Rser=0 Lser=0 mfg="Murata" pn="GRM155R60J106ME15" type="X5R" SYMBOL cap 608 640 R0 WINDOW 0 -33 9 Left 2 WINDOW 3 -72 51 Left 2 SYMATTR InstName C12 SYMATTR Value 10000n SYMATTR SpiceLine V=6.3 Irms=0 Rser=0 Lser=0 mfg="Murata" pn="GRM155R60J106ME15" type="X5R" SYMBOL cap -224 160 R0 WINDOW 0 -22 9 Left 2 WINDOW 3 -79 48 Left 2 SYMATTR InstName C1 SYMATTR Value 10000n SYMATTR SpiceLine V=6.3 Irms=0 Rser=0 Lser=0 mfg="Murata" pn="GRM155R60J106ME15" type="X5R" SYMBOL cap -224 432 R0 WINDOW 0 -22 9 Left 2 WINDOW 3 -75 50 Left 2 SYMATTR InstName C2 SYMATTR Value 10000n SYMATTR SpiceLine V=6.3 Irms=0 Rser=0 Lser=0 mfg="Murata" pn="GRM155R60J106ME15" type="X5R" SYMBOL cap -16 224 R180 WINDOW 0 23 78 Left 2 WINDOW 3 -74 78 Left 2 SYMATTR InstName C4 SYMATTR Value 10000n SYMATTR SpiceLine V=6.3 Irms=0 Rser=0 Lser=0 mfg="Murata" pn="GRM155R60J106ME15" type="X5R" SYMBOL diode -80 688 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D5 SYMATTR Value 1N4148 SYMBOL cap -64 688 M180 WINDOW 0 24 56 Left 2 WINDOW 3 24 8 Left 2 SYMATTR InstName C5 SYMATTR Value 1000n SYMATTR SpiceLine V=6.3 Irms=0 Rser=0 Lser=0 mfg="Murata" pn="GRM032R60J105ME05" type="X5R" SYMBOL OpAmps\\LT1678 -32 -432 M0 WINDOW 0 -57 50 Left 2 WINDOW 3 -47 89 Left 2 SYMATTR InstName U2A SYMBOL diode 192 -544 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D6 SYMATTR Value 1N4148 SYMBOL diode 272 -544 R90 WINDOW 0 58 31 VBottom 2 WINDOW 3 -28 34 VTop 2 SYMATTR InstName D7 SYMATTR Value 1N4148 SYMBOL diode 352 -544 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D8 SYMATTR Value 1N4148 SYMBOL diode 432 -544 R90 WINDOW 0 58 27 VBottom 2 WINDOW 3 -27 31 VTop 2 SYMATTR InstName D9 SYMATTR Value 1N4148 SYMBOL diode 512 -544 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D10 SYMATTR Value 1N4148 SYMBOL schottky 608 -288 R0 WINDOW 3 -16 -25 VRight 2 SYMATTR Value BAS40HY SYMATTR InstName D2 SYMATTR Description Diode SYMATTR Type diode SYMBOL res -80 -560 R270 WINDOW 0 33 55 VTop 2 WINDOW 3 -3 55 VBottom 2 SYMATTR InstName R4 SYMATTR Value 10k SYMBOL res 32 -384 R0 SYMATTR InstName R6 SYMATTR Value 10k SYMBOL diode 592 -544 R90 WINDOW 0 59 29 VBottom 2 WINDOW 3 -25 30 VTop 2 SYMATTR InstName D11 SYMATTR Value 1N4148 SYMBOL res 912 304 R0 SYMATTR InstName R19 SYMATTR Value 600 SYMBOL OpAmps\\LT1678 288 576 M0 WINDOW 0 21 -42 Left 2 WINDOW 3 25 24 Left 2 SYMATTR InstName U2B TEXT -448 632 Left 2 !.tran 0 1050s 50s startup

The startup transient of the FFT would have to be included, so that may set a limit to the accuracy of the simulation measurements. Even if you start on a zero-axis crossing, you will get spurious results because of the finite length of the sample.

This can be demonstrated by doing FFT on samples of fewer and fewer cycles from the middle of an apparently pure sinewave. As the sample length decreases, spurious harmonics begin to appear, even though they don't exist in reality.

I don't know if it is 'better' but it suits me - and nobody seems to have any difficulty understanding it. By using a descriptive term, rather than a commemorative one, beginners find it easier to understand as it sounds less like jargon intended to exclude the uninitiated.

(I have nothing against H.Hertz, it's just the principle that worries me).

I don't use Spice, so the rest doesn't convey anything to me.

Yes I understand that. It's actually a sinewave multiplied by a rectangular function. That's why I try to use a sample length of many tens of seconds. A window function might help but then I'd have to decide which window to use.

Yes that's why I sometimes prefer to use it.

I think it goes back to a time when some people thought they had more influence than they had, so we all had to use rectangular boxes for logic gates etc.

I don't currently have a lab to play in so LTSpice is useful but there's no knowing how close a model of reality it is.

An FFT implicitly connects the end of the recording back to the beginning. It's sufficient that the number of periods is integer.

Jeroen Belleman