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cost of passive voice LPFs not bad, not cheap tho'




 
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Author Topic: cost of passive voice LPFs not bad, not cheap tho'  (Read 5143 times)
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Patrick J. / KD5OEI
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« Reply #25 on: July 01, 2019, 09:39:52 PM »

Your softer, recent bypassed inductor design seems to be about equivalent to a simple RC low pass filter.
https://en.m.wikipedia.org/wiki/Low-pass_filter#/media/File%3AButterworth_response.svg

I disagree about equivalence to a simple RC low pass filter. You can check them together in LTspice.
If that RC curve was the same it would be -20dB @ about 1080Hz, not the 10KHz in wikipedia.

If anyone wants the ltspice and expresspcb files for the long board shown, they are here. Free for anyone to use or change.
Simple matter to change the freq., footprints, etc.
I have not had time to do anything, have boards made, etc. It might be nice paired with a pre-emphasis circuit.
bunkerofdoom.com/__scratch-monkey/3500Hz audio LPF LTspice and ExpressPCB.zip
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« Reply #26 on: July 02, 2019, 03:48:20 AM »

OSHPARK for PCBs chaps, they're in the USA so why look 'overseas'?

JB.
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w9jsw
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« Reply #27 on: July 02, 2019, 10:51:38 AM »

Pat,

Can't seem to access the files. Can you post the zip here?

I use EasyEDA and it works well with any of the PCB houses. ExpressPCB is a US based house with heavy setup fees and their software locks you into their house. When I did the 25 board run for the Mosfet driver board I used a US house and it was $100 for setup and then so much per board. China is much cheaper with no setup. $250 setup is robbery.

John
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« Reply #28 on: July 03, 2019, 02:27:29 PM »

OSHPARK has no setup fees.
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« Reply #29 on: July 04, 2019, 03:26:57 PM »

I just tested the file, it's OK. Perhaps something is improperly blocking it.
I would have put it here in the first place but ZIP files are not allowed to be uploaded.

Most of what I did on this is a matter of personal preferences. design tools cost money, so there are may ways to go. Some people have access at work  others can buy a seat, others want a free program or a $30/month subscription.

I don't want to use EasyEDA because it is online / cloud based.
I would rather buy a month of Autodesk Eagle or something.
Want files and program on my computer and not forced to work online.


Here is why I chose what I did:

1.) Wanted a free CAD & turn key PCB.
2.) Considered a run of 20 pcs PCB to minimize the one-time costs.
3.) As much USA-made content as possible to support my countrymen first, not foreign interests.
4.) Able to get EMS sales engineer on the phone in clear English during my business hours.
5.) Per unit costs a reasonable $65, as complete kit.
5a.) BOM: ~$40 less PCB.
5b.) PCB shipped: $24.17 each total.
6.) Assembled/tested/verified version could have been offered as well.
7.) If I was going to have PCBs made, I want the best quality control over them.
8.) It's not a commodity product. For myself I don't care about $24 for the PCB.
9.) Nothing's stopping me from doing one piece for myself on a home made PCB or perf-board.
10.) Can this even be bought online? Passive 500 Ohms, not a board with ICs.

Hope that makes sense. Main goal is quality.

------------------------------------------------------------------------------------------------
Inductor selection:
One should use lowest DCR inductors on a 500 Ohm filter (2.3 Ohms per 10mH).
Even the $1-2 inductors (10 Ohms per 10mH) are poor performers.
Using cheap 10 cent (40 Ohms per 10mH) inductors is terrible.
(see plot to compare)
also .CSV as a text file for the BOM with part numbers.


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« Reply #30 on: July 04, 2019, 04:17:06 PM »

I used Precision Technologies for the driver board. US house based in Chicago. $100 setup and then a sliding scale based on board size and quantity. Express was considerably more.

I used to use KICAD which is free, but can be complex for the average user. Probably on par with eagle.

John
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« Reply #31 on: July 06, 2019, 03:05:46 PM »

In the days when I had an actual radio room, antennas, and iron, I had a UTC LML-4000 in the audio chain (pre-compression).  Still have it, if my inventory list is accurate  Shocked   Not quite as sharp rolloff but seemed to do its thing even with some Z mismatch.

https://drive.google.com/file/d/1ERNuEPvB3wKUfkAkTk_cuKl3XSdh_NCw/view?usp=sharing

Grant NQ5T
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« Reply #32 on: July 06, 2019, 08:27:03 PM »

Explain to me like I am the newbie I am --

Why do you want such a filter? It has tons of phase shift, time smear, etc., for the sake of a super deep fast cutoff, but why? I get that there's distortion and overload and other reasons to band-limit the audio, but  a roll-off of 12 or 18 dB is a huge voltage/power difference, and this thing is like 40+ dB down, even with the higher resistance inductors. Wouldn't a simpler filter have just as good real world results as far as modulators, transformers, etc., are concerned? Or just use a couple 12AX7s, and a handful of caps and resistors, as in the attached schematic?   Help me understand...

Ed


* active24dbBP.png (22.28 KB, 1661x753 - viewed 51 times.)
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« Reply #33 on: July 08, 2019, 08:25:12 AM »

Ed poses a good question (s)  ....  I am second guessing with this:  the sharp cutoff is an effort to keep preemp audio within a 10kHz rf frequency using am.   does this answer your question or does it need to be spelled out ?

Since you have loaded the params into Spice orone of its fellows, why not run the phase shift curves

My ancient filter training stated that an eliptical or Bessel response had the least phase shift.  I dont know about group delay.
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« Reply #34 on: July 09, 2019, 02:57:54 AM »

Hi Ed,

It's just a preference.

Here are my own reasons:
sharp cut-off.
analog
passive
>1dB insertion loss out to 3KHz.
500 Ohm impedance common in audio stuff.
Can pass a little audio power as well. 5W easily. Durable.


I'm not too concerned about phase shift. It's predictable in LTspice, but it's not for modern precision hi-fi use where minute frequency-based timing differences in harmonic sounds from some instruments would be more offensive than the delays caused by crossovers in speaker systems. It is more like mid-1950's hi-fi quality in regard to phase shift through the system including bass and treble controls and the like.

In fact, the Kahn Symmetra-Peak all-pass filter intentionally shifted the phase according to frequency and it was successully used in broadcast AM and FM to 'unstack' the high and low frequency elements that occur at the crest of the modulation cycle so that the average modulation is more symmetric and could be made higher.  There had not been complaints about phase shift from listeners as far as I know. The Symmetra-Peak is applicable to ham use since the offending stuff is well within the usual ham baseband. I have never compared the phase shift of the Symmetra-Peak to this simple filter since they are for different purposes.

A cursory look shows that both 'phase-smearing' devices are reasonably similar in that one respect:
In the LC filter there the phase slopes from 0 to 425 degrees at 0 to 3.7KHz.
The Kahn product shifts from about 490 degrees from 100 to 4Khz.
So there should not be an issue for ham radio.

What amount of phase smear would the string of RC filters in the cathode follower circuit have if designed for a 100-3500Hz +0/-3dB passband response and -20dB@4KHz? (more easy than the -43dB of the LC filter and a good question)
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« Reply #35 on: July 09, 2019, 10:45:45 AM »

Gentlemen,

On behalf of the OBC (Old Buzzards' Club) and the AOAL (Acronym Overload Action League), please provide the meaning of the following TLA's (Three Letter Acronyms):

LPF
SMD
TIM
DCR

HAND. YMMV. TIA.

73,

Bill, W4EWH
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« Reply #36 on: July 09, 2019, 04:39:50 PM »

LPF = low pass filter

SMD = surface mount device

TIM = Transient Intermodulation

DCR = DC resistance

Each of these may have several other meanings and must be taken in context.

Here is an article about inexpensive low pass filters that have a gentler slope.

* Real Audio Selectivity Using Standard Parts.pdf (699.5 KB - downloaded 28 times.)
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« Reply #37 on: July 10, 2019, 04:21:34 PM »


In my world 3.6khz rolloff with such a sharp slope is a bit "tight"... ymmv.

I designed a similar filter for a Globe Champ 350 - since it had an inductor there already.
Fewer stages... higher cutoff freq... I think it was ~5.5khz.

But I agree, one could put gentle first order or even second order rolloffs into successive
stages and obtain a satisfactory result as well... but that wouldn't work for a single "audio
chain" that was to feed multiple rigs.

Otoh, today one can buy rather inexpensive digitally based xovers (nominally for PS/SR)
and cascade two channels for impressive stop band action and be very "frequency agile".
But, it's not analog. Cheesy

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« Reply #38 on: July 14, 2019, 04:34:53 PM »

Do you have the filter design available?

Here is a nice Chinese pcb manufacturing video from PCBWay.
https://www.youtube.com/watch?v=_GVk_hEMjzs
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