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Custom Audio Filter designs for Ham AM - Dynamic Bandwidth Control




 
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Author Topic: Custom Audio Filter designs for Ham AM - Dynamic Bandwidth Control  (Read 5411 times)
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K1JJ
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« Reply #50 on: February 17, 2020, 12:30:53 PM »

Don, thanks for the sibilance control info.   I am still experimenting and will try it soon.

I removed my PMC 300A limiter and found a cleaner, more natural sound, as expected. Even the extreme highs seemed 1 KHz narrower and cleaner on sss's. Not sure what that is all about - maybe some IMD from the limiter doing its job. The positive peaks are also about 10% higher than before.  

So right now I am using just the Symetrix 528E -   the Berhinger Ultragraph 31 band EQ side-patched in, then 3 DB of 528E compression... that's it.  I really like the natural line up of positive peaks phasing based on my own voice. The A/B mic phase switch is dramatic. If this station were used by other people it would be a different story.  Maybe just a small touch of LP filtering will do the trick when the parts arrive.

Bob, could you post the QIX limiter URL or schematic?  Is that his 5-band homebrew version?

Someone mentioned to me last night that Steve has one of his low freq audio bands phase-flipped to add in better to the rest of the lineup. That sounds like an intereting idea to tailor ones own audio phase when one of the bands is 180 degrees out, etc.  I can imagine a 5 band processor with a seperate phase toggle switch when entering each band - rather than a single mic switch covering all bands. Customized to one voice.


Just thinking out loud...   Wink

T
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« Reply #51 on: February 18, 2020, 01:19:34 PM »

Bob, could you post the QIX limiter URL or schematic?  Is that his 5-band homebrew version?

Just thinking out loud...   Wink

T


I printed the schematic out at home. I recall Steve posting it within a post about something totally different. I did a quick search but could not find it. Perhaps Steve can repost it?
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« Reply #52 on: February 18, 2020, 07:21:14 PM »

Well, I received the parts for the 5.5 KHz LP filter and built it on a terminal strip prototype style.  I measured each part for capacitance and inductance and found them off a little. I padded or subtracted small values to get them as close as possible. The filter is not perfect but still built reasonably close to design.

On the bench, I ran the tones thru and was very pleased at the roll-off.    With a full scope screen of signal there was no amplitude change when going from DC up to about 4.7 KHz. At this point going higher in freq made the amplitude decline very slightly until about 5.0 KHz. Then it dropped like a rock after 5.5 KHz and continued down to a faint signal past 6.5 KHz and was gone at 7.5 KHz.  


I didn't make any phase measurements  yet, but at least the filter design worked very FB.

This is exactly the rolloff I would want if I had a wish list... :-)

So now the question is how much it will affect the transmitter being such a bad-ass filter of 10th order?  I'm hoping that if it works reasonably and has a roll-off like this, I could use it when condix get crowded and leave it off when the band is quiet.    Having a 4.0 KHz and 7.5 KHz version of this would round out the harem.


I'll try it on the rig later tonight and see...  I remain 50-50 optimistic.

T


* Filter 5.5.png (10.69 KB, 535x187 - viewed 52 times.)
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« Reply #53 on: February 18, 2020, 07:30:44 PM »

Get the QIX schematic and I will take a look.
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« Reply #54 on: February 18, 2020, 11:46:36 PM »

Now it's Schaefer time!

(please excuse the rough camera to screen shots for now)

I installed the new passive L/C 10th order Butterworth LP filter into the last position of the audio chain before it goes to any transmitter or modulator.  With it bypassed I set up the audio for the cleanest sound and normal  high fidelity with no high cuts or any other type of roll off. Just the 528E mic preamp driving the 31 band EQ and then 3 DB of compressor action...   The first picture below is the spectrum of the audio (NO FILTER) when socking yallos, chhhhh's and ssss's. IE, whatever I could do to talk it up and make some splatter, though I did not overmodulate.


As you can see, this is why I was concerned about bandwidth. The highest highs were extending out to 10-12 KHz.  Sounds clean in the monitor and the IMD tests out good. This is simply what is going into the mic without restrictions.  The waterfall is abominable.

Now, the next two pictures show exactly the same types of modulation activity except the filter is in-line. The filter is the last thing in line in the audio chain before it goes to the modulator, etc. Notice the bandwidth cuts off like a haircut at about 6 KHz. The waterfall is sharp and clean. The spectrum shows a beautiful roll off slope.

The good part: When A/Bing the filter in and out,  I could not see any meaningful difference in the positive peaks. If any, maybe 5% less but hard to see with voice modulation. I am still able to get the near-sharkfin audio pattern I like to see and the peaks bounce up to positive 150% with -100% negative. These pics are not a good representation of the true extent of the positive peaks because I am doing a camera to screen shot with poor control of everyting.

Bottom line is I think this is what I have been striving for. The filter is rolling the transmitter off sharply just as it did on the bench WITHOUT adversely affecting the positive peaks. With the filter in, both the scope and the REA mod monitor show the peaks hitting similar levels as without the filter.

Needless to say I am estatic about these results and will now give it some time on the air doing A/B tests. Once finished, I will see if John/JSW still wants to make up some filter boards. I definately want to make a 4 KHz and 7.5 KHz board too; maybe all three on one board in a single box.  I like the idea of switching in a brick wall filter number and living with it rather than turning a knob. It's a man thang.

BTW, notice in the last two pics the roll off on the spectrum scope. I like the drop off a lot. I'm not sure why the phase shift (not measured yet) is not adversley affecting the peaks, but that's OK by me. Using the REA mod monitor I often see 140% positive peaks with -85% negative. Some may not agree with this approach, but this is exactly what I have been trying to achieve.... big positive peaks and controlled narrow bandwidth when desired.

SLAP ME FIVE!

T


Actual RF transmitter shots taken while modulating 1500w pep into a dummyload...

Pic 1 is bad, no filter:  10-12 KHz highs spreading out from 3760 to 3749 area


The only change is the filter....

Pic 2 is good, with 5.5 KHz filter:   6 Khz highs haircut shown on waterfall, from 3760 to 3754 area

Pic 3 is good, with 5.5 KHz filter:    6 KHz haircut on waterfall












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« Reply #55 on: February 19, 2020, 12:15:48 AM »

To better show the roll off on the spectrum scope, I injected a hard series of ssss's simulating white noise. Notice it brought the bandwidth out one more KHz. Normally these should not show up in normal speech too often. Notice the positive peaks are pinned while the negative are under -100%.  This is a sign that for my voice, anyway, the extreme highs are in phase.

A word of warning for those building a filter... during sweeping I did notice a slight hesitation pause in the roll off at about 6.7 KHz. All other freqs were smooth. I think it may be due to part tolerances.  I also noticed that the inductors should be at right angles to one another or they couple magnetically and effectively shunt themselves. I saw an inductor change from 25 mH to 28mH just by moving it in alignment to another.

Notes:

*** BTW, my mic polarity switch stays in the same polarity whether the filter is in or not, further indicating there is little phase shift happening, at least to frequencies that favor my own voice.

Also, this filter was populated with undesirable ceramic caps. I will use film-poly caps next time which are better for audio. Can it get any better?

** The roll off shows a -45DB drop starting at 5.5 KHz and ending at about 7.2 KHz.   IE, if someone were +40 DB over S9, their side products at 7.2 KHz up the band would be down to about S8, near the average 75M noise floor.  The proposed 4 KHz filter will be even tighter.

**  The audio quality of 5.5 KHz bandwidth sounds really FB to me in the monitor.  It's really more like 6KHz before the filter really kicks in. That is plenty of fidelity for average band activity 75M voice operation.

*All tests were made at about 250 watts carrier or 1500 watts pep on highest peaks showing the amplifier(s) IMD is good and we are dealing mainly with low-level audio bandwidth control.

** The passive filter insertion loss is only about 3 DB or so. I will measure more accurately later.  The L/C inductors keep the feedthrough resistance low compared to R/C resistors.

T


Below:   ssss's white noise with filter in   -  Check out the spectrum roll off at 6 KHz and above:


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« Reply #56 on: February 19, 2020, 06:19:38 AM »

Nice. A passive Courtesy Filter. Every man should own one!
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« Reply #57 on: February 19, 2020, 08:26:59 AM »

A good source of broadband test noise is from an FM broadcast receiver with no antenna and tuned to the end of the dial where there are no signals. It's close to white noise, but technically pink noise due to the 75uS de-emphasis in these receivers.

Nice work on the filter, Tom!  It will be interesting to read more about the tests and hear it on the air.
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« Reply #58 on: February 19, 2020, 01:00:28 PM »

Thanks for the comments guys. I just knew it could be pulled off in the end... :-)

A "Passive Courtesy Filter"   (PCF) I like it!   Grin

I'm building a small aluminum box for the filter with toggles to do fast A/B tests. I'm curious what a trained ear like the Tron has to say about its insertion invisibility.

I'll take a picture of the unit, but it is just a mass of parts on a terminal strip.  John/JSW and I are talking about a PC board at the moment.


Here's some more notes about the filter PCB layout regarding actual exact parts availability:

When two inductors (like the barrel style I am using) are put in series or parallel, they need to be physically separated or they will couple together. Also previous stages using one inductor need to be away from each other. They use magnetic cores here, so are like mod transformers for audio. This applies to all inductors in the circuit…. Keep them separated by an inch or more if possible.  One way to isolate them is to keep them apart at least a few  diameters from each other or place them at 90 degree angles so that their poles are opposing coupling.

I was thinking that every part should have extra pads for each lead to allow for series or parallel insertion in case we need to adjust values or use parts that need parallel or series to help availability.  It’s a simple circuit but the parts availability makes it tough. I had to struggle getting everything close to the exact values in the design.

There’s no problem making the board bigger to allow for good filter isolation since we are building three filters on one board. IE, not a time to make the board tiny.   Unwanted poles coupling will degrade the roll off skirts and probably create unwanted ringing or ripple effects.  

T
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« Reply #59 on: February 19, 2020, 01:29:50 PM »

Tom,

Is the plan to make separate filters and switch them in/out, or to connect narrower sections in series with wider sections to narrow the bandpass as new sections are added? You may be able to reduce the part-count by adding sections in series that way. Just wondering.


Don
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« Reply #60 on: February 19, 2020, 01:43:24 PM »

Tom,

Is the plan to make separate filters and switch them in/out, or to connect narrower sections in series with wider sections to narrow the bandpass as new sections are added? You may be able to reduce the part-count by adding sections in series that way. Just wondering.

Don


Hi Don,

Good pondering...

I don't know enuff about filter design to answer that with any expertise, but according to Frank/GFZ, the filter should be used in its design entirety. To tap off the middle to simulate a smaller filter will upset the design. IE, use it from beginning to end with 600 ohm loads. I use 600 ohm resistors at the in and out right now. The insertion point will add some more loading, but I did not optimize that yet.

As for putting them in series/ cascade, I think the same problem occurs since the overall filter doesn't adhere to a "systematic" design from a single job optimization. I believe all parts of the filter work together and do their jobs no matter where in the circuit they are. But you could be right about making a 5th order set of smaller filters (each terminated) and putting them in series, why not?


That said, I will surely try your idea with the finished filters to see what happens when it goes into a 7.5 KHz filter and then enters a 5.5 KHz filter. Will the rolloff be cleaner? I dunno yet. How about two 5.5 KHz filters in cascade? Shud be an interesting test.  My goal would be to approach DSP skirts until the audio showed signs of artifacts or any kind of modification in the lower freqs below cutoff.

For now, I plan to keep it simple and bullet proof - make three separate filters, all optimized via the computer and selecting parts to exact values, and either use relays or toggle switches at each end to completely disconnect one filter and add in the next one.  I want poles and input/outputs well isolated from each other, just like we do with RF amplifier design.  

Based on this 5.5 KHz working filter, I know the 4 KHz and 7.5 KHz version will work for sure now.  


Last night I tried it on the air for a minute and had to smile as I watched the skirts cut off at +- 6 Khz. Before this I would cringe and worry who was nearby and talk more softly just in case...  Grin  Just look at my first pic again (showing 10-12 KHz  pre-filter bandwidth)  a few posts back to appreciate what I mean. And that was using the same clean transmitter as shown in the next pics with the filter in line.


T
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« Reply #61 on: February 19, 2020, 05:46:56 PM »

...
Before this I would cringe and worry who was nearby and talk more softly just in case...  Grin  Just look at my first pic again (showing 10-12 KHz  pre-filter bandwidth)  a few posts back to appreciate what I mean

T
That's what motivated me, too. I always got glowing audio reports using my QIX designed class E but then I started using the KFS web SDR to listen to hard-to-copy guys. Holey moley I was as wide as the Montana sky. Hence, my SCAF box.

I'm curious, though. Why put the bandwidth limiter at the end of the audio chain? I put mine near the beginning on the naive assumption that it made no sense to do compression and equalization on frequencies that I planned to chop off later. Maybe it makes no difference.

Your results are darned impressive.

Jon
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« Reply #62 on: February 19, 2020, 07:57:18 PM »

I'm curious, though. Why put the bandwidth limiter at the end of the audio chain? I put mine near the beginning on the naive assumption that it made no sense to do compression and equalization on frequencies that I planned to chop off later. Maybe it makes no difference.

Your results are darned impressive.
Jon


Thanks very much Jon!

That's a good question that I haven't thought out yet.  Where to place it in line???
I put it at the end thinking that if there were a chance of ANYTHING generating frequencies above 5.5 KHz, I wanted them cut. It won't help IMD generated by the amplifiers later on, but I wonder if there are frequencies generated within the audio gear due to mistakes such as overdriving, the EQ too hot or whatever, that this would be the ultimate audio trap.  I'd like to hear other opinions on this. Where do they usually place high cut filters in commercial gear?  It would be easy for me to move it.

Tnx for the question.

T


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« Reply #63 on: February 19, 2020, 08:02:27 PM »

And I finally put it in a beat up, used mini-box. This is a prototype AKA very JS, but solid and works FB.

I used a toggle switch at each end of the box for in/out.  This will be in line for daily use until I build a PCB version for three filters and put them in a nice box.  This unit can also be used later to test the effects of putting two filters in cascade, etc.


The terminal strip is actually mounted upside down so most capacitors are hidden. The electrons don't seem to mind.


Additional testing notes:

** After testing the 5.5 KHz filter more with fast A/B in-out switching I have determined there is no effect on the positive peaks or asymmetrical character of the audio.  In addition, I am beginning to think that a 4.2 or 4.5 KHz filter would do a better job at guarding the "sacred" broadcast +- 5KHz bandwidth guideline.   Each filter starts really working a little past its design rolloff freq and the 5.5 filter is more of a guardian of 6.5KHz bandwidth.  So a 4.2 might be a good guard of 5 KHz during busy times and still give 5KHz of fidelity.


T

On Pic 3 of the station:  Notice I am running out of room to sit. My legs go where the plastic trash can is now.   I do NOT want any more rigs in the shack...  Grin


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« Reply #64 on: February 19, 2020, 09:37:01 PM »

I'm curious, though. Why put the bandwidth limiter at the end of the audio chain? I put mine near the beginning on the naive assumption that it made no sense to do compression and equalization on frequencies that I planned to chop off later. Maybe it makes no difference.

Your results are darned impressive.
Jon


Thanks very much Jon!

That's a good question that I haven't thought out yet.  Where to place it in line???
I put it at the end thinking that if there were a chance of ANYTHING generating frequencies above 5.5 KHz, I wanted them cut. It won't help IMD generated by the amplifiers later on, but I wonder if there are frequencies generated within the audio gear due to mistakes such as overdriving, the EQ too hot or whatever, that this would be the ultimate audio trap.  I'd like to hear other opinions on this. Where do they usually place high cut filters in commercial gear?  It would be easy for me to move it.

Tnx for the question.

T



I EQ ahead of the compressor for that same reason. Although I use some pre-emphasis in the EQ, bandwidth limiting is the primary reason for my EQing. A passive LPF, as is being designed here, would be very useful as a first stage after the mic, or maybe better yet, just after the mic preamp. I'd try those two places first.

One of the problems encountered with a passive LPF is differing I/O impedances in the audio chain. Typically, modern audio processing gear have high input impedances and low output impedances. This is by design, and is called impedance bridging. There's no standard (that I'm aware of) so a filter may yield different performance when used with different pieces of equipment. This might negate the additional effort put into "tuning" the filter by careful selection of component value tolerances. I suppose that loading the filter with a terminating resistor would mitigate the problem by stabilizing the response into different loads, but at a cost of additional loss and reduced S/N ratio. This may not be as much a problem in radio communication as it would be in a studio recording environment. In the end, close enough is probably close enough.
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« Reply #65 on: February 20, 2020, 12:03:07 AM »

One of the problems encountered with a passive LPF is differing I/O impedances in the audio chain. Typically, modern audio processing gear have high input impedances and low output impedances. This is by design, and is called impedance bridging. There's no standard (that I'm aware of) so a filter may yield different performance when used with different pieces of equipment. This might negate the additional effort put into "tuning" the filter by careful selection of component value tolerances. I suppose that loading the filter with a terminating resistor would mitigate the problem by stabilizing the response into different loads, but at a cost of additional loss and reduced S/N ratio. This may not be as much a problem in radio communication as it would be in a studio recording environment. In the end, close enough is probably close enough.


Hi Don,

I changed the filter from the last position in the audio chain -  to right after the preamp feeding the EQ.   For whatever reason the bandwidth was about 1 KHz wider under normal speech and sss's. It may be because of a poorer impedance match for the filter, I dunno.   I have the EQ peaking out at 5 KHz and rolling off after that.   I am using 600 ohm terminating/loading resistors on the input and output of the filter.

I put the filter back at the end of the chain and like what I see again.   So I will continue testing with this lashup. I plan to fire up the 4X1 plate modulated rig and see how well behaved that is next.

T

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« Reply #66 on: February 20, 2020, 06:43:11 AM »

One of the problems encountered with a passive LPF is differing I/O impedances in the audio chain. Typically, modern audio processing gear have high input impedances and low output impedances. This is by design, and is called impedance bridging. There's no standard (that I'm aware of) so a filter may yield different performance when used with different pieces of equipment. This might negate the additional effort put into "tuning" the filter by careful selection of component value tolerances. I suppose that loading the filter with a terminating resistor would mitigate the problem by stabilizing the response into different loads, but at a cost of additional loss and reduced S/N ratio. This may not be as much a problem in radio communication as it would be in a studio recording environment. In the end, close enough is probably close enough.


Hi Don,

I changed the filter from the last position in the audio chain -  to right after the preamp feeding the EQ.   For whatever reason the bandwidth was about 1 KHz wider under normal speech and sss's. It may be because of a poorer impedance match for the filter, I dunno.   I have the EQ peaking out at 5 KHz and rolling off after that.   I am using 600 ohm terminating/loading resistors on the input and output of the filter.

I put the filter back at the end of the chain and like what I see again.   So I will continue testing with this lashup. I plan to fire up the 4X1 plate modulated rig and see how well behaved that is next.

T


Yeah, 1 kHz is a lot. That's more than I had expected.

If the cause is impedance source/load related it makes it difficult to produce a design that will work across the board. If you built ten of them and sent them to ten friends, they'd all have different results when used in their setups. This may be the bug that can't be built-out of a passive design.
As a test, you could put attenuators (perhaps 10dB) fore and aft of the filter, to reduce the loading effects and try it in different positions. I think that would confirm the theory. As a custom design, it works like a charm. Mass produced, maybe not so much.

If your equipment is generating harmonics that are being caught by the filter (with it in the last position) that's a whole 'nother story.


Don
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« Reply #67 on: February 20, 2020, 12:31:25 PM »

Hi Don,

That's good info, thanks.

I can probably improve on this design by adding buffers, like emitter followers to the input and output easily enuff.  I'll ask Frank/GFZ if he can come up with a simple design in his software. I have room and terminals to add something.

In the meantime I'll make some measurements to see what the impedances are where it works best and doesn't.

My gut feeling is that the EQ is generating the sss's that need to be tamed. The filter works well after the EQ, at the end of the chain, but we'll see.   I like the idea of isolating this very effective passive filter anyway. Maybe as simple as a 2N2222  in and out.


Another add-on is I want to design and implement a filter for ssb DXing too. My main ssb exciter is about 3 KHz wide due to wider filters I added in the I.F.  The norm is more like 2.4 KHz when the band gets crowed, so I could use a 2.0 KHz filter or so to roll it off at about 2.4 KHz.   When the 75 DX window is busy and the noise floor is low this little difference of 600 Hz can make a big difference, especially when everyone is clustered close together and running a combination of sdr and 2.4 KHz rigs. Some even run 2.1 Collins type mechanical filters or equivalent.  When in Rome, use Roman filters... :-)


T
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« Reply #68 on: February 20, 2020, 12:55:01 PM »

Hi Tom,

You may be out of room but you have double plus good transmitters.

I Like those filters very much, very nice job. I'm one who appreciates the results of straightforward analog circuits.

Have you found that the high resistance of the coils (e.g. the 33mH ones are 90 Ohms) reduces the sharpness/Q of the filter sections?

I ask because I'm curious about the response and the effect of inductor resistance on that kind of filter. I'm not sure how to interpret the waterfall displays dB-wise. Do you have spice curves on them with the chosen inductors?

Some time ago, I built a 500 Ohm in&out passive one for 3.5KHz. It's based on the "Bonadio" article circuits but with standard L values. It worked very well.

Low resistance coils (6.9 Ohms for 30mH) were used to get the sharpest cutoff diverse situations like handling up to 5 Watts input, and limit insertion loss to 6dB which was a bonus, and also sharpness of cutoff.

Models were also made in LTspice for -3dB@ these other frequencies of 2.9, 4.0, 4.45, 5.0, and 5.25KHz.
Slope is 3dB/58Hz on the 3.5KHz one and 3dB/110Hz on the 5.25KHz one, so it is comparable to the ones you designed (3dB/113Hz).

I built only the 3.5KHz one. It was put in the 500 Ohm line right before the input to the driver of the modulator. There was no effect on clarity, but I have not tested with any pre-emphasis either. Termination in and out seemed important.
3.5 KHz LPF: http://amfone.net/Amforum/index.php?topic=43634

PJ
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« Reply #69 on: February 20, 2020, 01:31:43 PM »

Tom,

Yes, buffers! Maybe op amps at each end set to unity gain? They could be powered by +48V phantom voltage allowing placement just after the microphone. A nice simple desktop accessory. Perhaps a characteristic impedance for the filter could be selected that makes component selection easier too. This just keeps getting better...


Don
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« Reply #70 on: February 20, 2020, 01:55:02 PM »

Some good ideas!

Pat:  That Bonadio filter using inductors down into the 10 ohm range sounds appealing for better Q and selectivity.  Yes, I am dealing wth 90 ohms + on some of these inductors. I didn't consider more than insertion loss at the time.  I read thru the thread and very interested.

Don:  Yes, an op amp at unity gain might work well too.  I notice Rick/KHK and Steve/QIX both have used them to isolate their audio filters.  

Well, my bandwidth problem is solved for now - I can run this passive filter in the meantime and relax in the catbird seat while we put together the next improved version.  I am very happy with the present rolloff and invisibility of it now - so it can only get better.  Tonight I try it on the 4-1000A plate modulated rig. The only difference in the set up is it will be driving Frank/GFZ's MOSFET driver board with a 2.6K input load. Probably no problem at all being a terminated 600 ohm filter.

My farther out goal is to get an analog filter to work "almost" as well as a DSP filter. And clone that design for the many bandwidth jobs I have.

Circuits welcome for evaluation...   

BTW, I will be testing out the SCAF portion of Rick/KHK's project soon. I have not closed the door on using this switched capacitor  approach and have verified my passive filter has no meaningful effect on the positive peaks, assymetry, etc.  It may just be a myth that these filters affect the freq below the cutoff adversely.

T
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"You know my name"  https://youtu.be/noGjJyEDm5s?t=135

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« Reply #71 on: February 20, 2020, 03:33:27 PM »

Don, thanks for the sibilance control info.   I am still experimenting and will try it soon.

Bob, could you post the QIX limiter URL or schematic?  Is that his 5-band homebrew version?

Just thinking out loud...   Wink

T

Here you go, third post down the list
http://amfone.net/Amforum/index.php?topic=42552.0
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« Reply #72 on: February 20, 2020, 04:27:06 PM »

Here you go, third post down the list
http://amfone.net/Amforum/index.php?topic=42552.0


Don, 

I read over the thread.  Looks like you were the lead horse building the NPL / filter.  Are you still using it and did it eliminate the wavelet foldback problem?  I had interaction between the wavelets and NPL limiting in my trial using the older PDM board section - kinda a crossover distortion.   The wavelets appeared mixed with the NPL line... strange.   

So I now live with not naturally going over -99% negative. There is still plenty of pos peaks doing so. (+150%)       Maybe I shud try the latest version.    Did anyone make a board or wanna sell a working point to point?

T
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« Reply #73 on: February 20, 2020, 05:37:49 PM »


Tom,

Per my on-the-air comment... the input of the filter "looks back" at the source. If that
is the output of the 528 compressor box, it's likely a 5532 or similar opamp at the output.
Nominally it is designed to "drive" a 600 ohm load rail-rail. So whatever that output Z
works out to be is like a shunt to ground. If you put a 600ohm resistor to ground at the
input, then the filter "sees" the output Z of the opamp + the 600ohm in parallel as the
input Z. Aka much lower than "600 ohms."

Probably won't change a Butterworth filter very much. Would change an eliptic filter.

That's why I said 600ohm resistor in series. This assumes ur driving it with a source that
is actually substantially lower Z, as is usually the case.

On the output side, just an emitter follower running enough current to drive the next
stage is all that is needed. The base or gate would get a 600ohm to ground, to give
the filter the requisite design load.

If there is some insertion loss, either an opamp or a transistor at the input or output
configured for a little gain is ideal.

Btw, it sounds really good with the filter...

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« Reply #74 on: February 20, 2020, 06:35:43 PM »

Well, tonight I met up with a big group on 3885 AM and asked five of the experienced "refined ear" AMers what they thought about the 5.5 KHz filter performance.  (Tim/HLR, Bear/GCR, Tina/IA, Ron/ RON, Bob/KBW.... did I miss someone? Most of these guys have SDR spectrum scopes.

I did a transmission testing the filter in and out. As expected, without the filter I was 10-12 KHz wide per sideband. With the filter in all of the guys were reporting a well-defined 6.5KHz bandwidth. I could hear the excitement in their voices telling me it was working very FB.

The surprising subjective comments were that it sounded BETTER with the filter in. Some described it as sounding smoother and more focused in the frequencies that counted, rather than spreading out the extreme highs.  Not one had a negative comment with all very positive. Tron added that with his narrower receiver he could hear no difference in audio at all.. a good thing.

Ron/RON sent me a recording off his SDR. I could hear a slight difference in extreme highs.... they sounded basically good with the filter in or out. There WAS some more sparkle in the 'T' sounds with the filter out as expected.   So this is also good news because the highs were being chopped off nicely looking on the spec scope.   There is not a lot more voice intelligence or ambience above 6-7 KHz anyway but on quiet days, why not?.


I'd say the "Courtesy Filter" is a success.  (Thanks Don)     I will switch it into a different load tonight; into the GFZ MOSFET audio board to drive the 4-1000A plate modulated rig.
  

Bear: Thanks for taking the time today to run the tests on the air.  OK on the filter loading, resistor input in series and other comments. I will try some of the things you mentioned.

*** The more I listen to Ron's recording in +- 10 KHz bandwidth, the more I can hear the high cut when going from 11 KHz bandwith to 5.5 -6.5 KHz.  There is definitely some ambience gained without the filter. That is why there is a place for the 7.5 KHz filter on quiet days.


T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz,  +-6.0 KHz or  +-8.0 KHz when needed. 

"You know my name"  https://youtu.be/noGjJyEDm5s?t=135

There's nothing like an old dog... a puppy... a dog in its prime... or ANY dog!
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