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Author Topic: common inductors for audio use  (Read 6526 times)
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Opcom
Patrick J. / KD5OEI
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« on: March 27, 2016, 11:09:33 AM »

I sent a friend off with the data for the good ol Khan Symmetra Peak among other things, and looking at the schematic from the patent it's simple, a collection of 7H chokes and 0.03uF capacitors. Inside the Kahn product, these things are hidden in a box that is potted so I don't know what kind of choke was really used. I do believe the values in the patent.

There are eight chokes and 8 capacitors, in between a pair of 600 Ohm to 15K Ohm transformers.

My question about this is whether the chokes are anything special or if they can be just the simple open frame EI lamination types without shielding. As long as coupling between them is avoided, I don't see why not.

Several small chokes rated 7H are available inexpensively, including some 25mA ones. Thre should not be a problem with small chokes as the unit is rated but +10dBm max.

My friend wanted to buy the Symmetra Peak, but it's needed for use here. There should be no reason why he could not build one. I think someone here already proved the values in the patent via LTspice.

The second question regards the two models -the SP-58-1A rated and SP-63-1A. The latter having a little better characteristics. I wonder what the difference is in the components that gives the '63' better response and lower distortion as well as lower loss?

SP-58-1A- THD 0.5% @ 40Hz, 0.2% from 100-15000Hz @ +10dBm. insertion loss 4dB.

SP-63-1A - THD 0.25% @ 15Hz, 0.1% from 40-20000Hz @ +10dBm. insertion loss 2dB.

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Radio Candelstein
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« Reply #1 on: March 28, 2016, 11:59:01 PM »

I don't know if this helps but I have a number of encapsulated audio inductors and they are marked CAC or Collins, and they are all toroids.

I would think they would have to have nickel-steel cores, so I doubt common filter chokes would cut it.

I would opt for an equivalent OpAmp circuit.

Phil - AC0OB



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« Reply #2 on: March 29, 2016, 06:52:14 PM »

Post the schematics of the two models, etc, and possibly I can shed some light?

IF the levels through the chokes are sufficiently low in value, and there is no DC on the chokes, I'd
not expect gapped cores... unless they skimped on the core size, OR extreme stability WRT inductance
value vs. signal level was necessary, in that case gapped.  But you can take an EI core that is ungapped,
pull the lams and gap it - but the resulting Hy value will be lower than the stock value... how much depends on the gap, which in turn should be set depending on the current/DC level or swing that will be present in the circuit.

Not sure why nickel core would be a benefit in an audio choke... could be, but not sure why it would be.

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« Reply #3 on: March 29, 2016, 07:58:13 PM »



...Not sure why nickel core would be a benefit in an audio choke... could be, but not sure why it would be.

                    _-_-

Nickel iron cores for audio inductors tend to have wider audio bandwidths, like from 75Hz to 15kHz.

May not be necessary if there is some pre-emphasis or tailoring going on.  Smiley

Phil - AC0OB
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« Reply #4 on: March 29, 2016, 08:20:42 PM »

Not sure I see that in inductors, Phil. They do better on HF in transformers, but at the expense of inductance, and so it is harder to get LF extension... for a choke, you want higher Z at higher freqs, not sure the standard core material, especially "hypersil" gives up soon enough to matter at all.

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Tom WA3KLR
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« Reply #5 on: March 29, 2016, 09:14:54 PM »


I would opt for an equivalent OpAmp circuit.

Phil - AC0OB

I think there was (and hopefully still are) op amp "phase rotator" boards for sale cheap.  I believe the seller was a W3xx.  This is much smaller and better than old schtuff.
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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
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« Reply #6 on: March 30, 2016, 01:21:13 AM »

The only schematic I have is derived from the patent. Someone made this for LTspice to include the transformers, rather special.
I have never seen the real schematics of either model. Here is the one from the patent.
The passive solution is preferred by some people rather than op-amps etc.


* symmetrapeak1.gif (7.92 KB, 580x300 - viewed 492 times.)

* kahn symmetra peak.png (40.16 KB, 919x782 - viewed 496 times.)
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Radio Candelstein
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« Reply #7 on: March 30, 2016, 07:40:53 AM »

"Several small chokes rated 7H are available inexpensively, including some 25mA ones."

They're not the right ones, they measure 7H with 25mA DC bias, but at 0DC the inductance is much higher.
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73 de Warren KB2VXA
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« Reply #8 on: March 30, 2016, 08:18:17 AM »

Which means they are not gapped.

But where does the DC bias come from?

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« Reply #9 on: March 30, 2016, 10:36:50 AM »

The choke is rated 7hy with 25ma flowing through it.  Most common chokes will have a higher than rated inductance without any DC current.  Problem with using common PS chokes in audio circuits is that they will pick up hum from other nearby transformers.  UTC made shielded audio chokes which don't pick up hum.

Fred
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« Reply #10 on: March 30, 2016, 01:58:58 PM »

Modern day version.   Works great,  I've built a few.   You can purchase,  built,  off eBay for about ten bucks a piece.


http://www.w3am.com/8poleapf.html

--Shane
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Opcom
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« Reply #11 on: March 31, 2016, 07:43:37 PM »

Which means they are not gapped.

But where does the DC bias come from?

                      _-_-

Those are intended for power supply use, the B+ current is the 25mA max. It was my mistake to cite them but was to suggest at suitable chokes would not be too expensive. So yes, the proper choke has to have the rated inductance at zero DC current because in the Symmetra Peak there is no DC.

The original instrument was built inside a steel box inside a steel chassis, though I forget if the front panel is ferrous or not. At 10dBm the threat from far away humming power inductors ought be minimal. It's all for the experimenter to try and to shield.

Since the LTspice model was kindly provided, it seems that a set of 'chokes on hand' could be used by changing the capacitor values. The difference would be the impedance  no longer being 15K in and out of that network.  In the patenty, it is mentioned that there is a certain phase delay or change at a certain frequency, and that is what is being sought for the application result. Certainly the cheap and easy off the shelf way is to use op amps, but the fun way is to dig into the whole thing. IIRC there is also an op amp circuit called a Gyrator, which simulates an inductance and can have a very high or low Q but that is also another topic.

there is also a change in some inductors dependent on frequency and it can't be ignored.
http://rsrelectronics.com/t_henry.htm
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Radio Candelstein
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« Reply #12 on: April 01, 2016, 11:30:23 AM »

Two earlier posts alluded to use of opamp phase shifters and availability of pc board assemblies. There is a finished and fully populated Allpass Phase Shifter PC board available on eBay for less than $20.00 with free shipping at:

http://www.ebay.com/itm/Audio-All-Pass-Filter-Phase-Rotator-Module-Board-/130681036896?hash=item1e6d326060:g:oiwAAOxy039TOhgl

The referenced site publishes the schematic, dimensioned board layouts, close-up photographs of a completed module, and performance specifications for this and most of their other modular products.

Yes, it's from China, however I have purchased several completely finished  modules from this source and all of them have been of high quality construction, performed to spec, and have been safely shipped in a uniquely-strong styrofoam box. Delivery to East Coast U.S. in less than two weeks, half of which is spent in USPS sorting facilities.

BTW: This site also refers to the W3AM web page describing Kahn's work and a modern solid state Allpass filter upon which their product is based:
http://www.w3am.com/8poleapf.html

Bob - NE
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