John Curl's Blowtorch preamplifier part II

[Rick Miller]

CG,

Going back to the RC on the speaker end of the cable, I have tried this with .01uf and 10 ohms and it does work. It improves things across the board.

A product called Walker HD links is the same thing and the cost is around $350 USD. The more I do to reduce RFI in my SS equipment the more tube like it sounds.

Something else to think about is that some folks say that speakers act like antennas to the AM radio band. I know that the voice coil is sheilded, but AM radio antennas are a coil wound around a ferrite rod, and speakers are a coil around a magnet.
I am going to connect a driver to my Tektronix 7L12 spectrum analyzer and see what I get.. Also how about the air wound coils in the crossover acting as antennas?

[PMA]

Originally posted by john curl
Having fun? At least it got you thinking,

I have been measuring these interference voltages in audio for at least 6 years. In my professional field of work, I work with this regularly.

Do you have any papers on intereference?

[scott wurcer]

Originally posted by scott wurcer
I lot of shields leave much to be desired. A single twisted pair of #10 Romex in a length of 1/2" schedule 40 pipe might be made to work out well with some RF damping components at either end. The raw parts are readily available.

Been away from DIY home too long, I meant schedule K copper pipe as a shield.

There are some international standards for EMI robustness around, getting xDSL to work in the same box as 802.11 can be difficult since the xDSL signal is sensitive to the same IMD issues. There are some recommended tests mentioned too.

[CG]

Originally posted by Rick Miller
Something else to think about is that some folks say that speakers act like antennas to the AM radio band. I know that the voice coil is sheilded, but AM radio antennas are a coil wound around a ferrite rod, and speakers are a coil around a magnet.
I am going to connect a driver to my Tektronix 7L12 spectrum analyzer and see what I get.. Also how about the air wound coils in the crossover acting as antennas?

Those AM antennas have to be resonant in the AM band in order to have any significant efficiency. Once upon a time when tuning caps were used in radios, there was usually a separate section just to tune the antenna to resonance. It sure would be bad luck if your speaker cross-over coil just happened to resonate at the frequency of the local AM station. In any case, I'd think that the pseudo-Zobels you placed at your speakers would kill the efficiency, but who knows?

The good news is that those types of loop antennas are pretty direction and have deep nulls. That doesn't help with speaker placement, but it may be useful as an analytical tool.

I guess one solution if you have that problem might be to couple another coil to the field of the cross-over coil, and load it heavily. That might kill or at least dampen any RF resonance without screwing up the audio.

I've read about those Walker parts. From what I can tell, they're just what you built. There's some other guy who sells a less fancy version with good quality parts for much less.

This is a different problem than the common mode resonance of the cable working on the amplifier output.

[john curl]

Will you folks get back to the BLOWTORCH? What are we doing here? Rick, you too!
(Rick Miller and I are personal friends, have been for decades)

[Rick Miller]

John, were you concerned about RF entering the Blowtorch thru the output? If you were, how would you recommend reducing the RF? I believe that the Blowtorch does not have a global negative feedback loop so in that case it would not be subject to RF from the output coming back to the input. Perhaps this is another advantage of a non feedback design. Being all Fet places it at a great advantage to start with as Fets are far less affected by RF that bipolars.

It’s a great design John!

[PMA]

It can easily get in through input - connectors insulated, along the wires.

[Rick Miller]

PMA, when insulated connectors are used I have seen a small value (.01-.001uf) non inductive cap between the shield of the connector and the metal chassis. Have you used this technique, and if so how successful is it in reducing RF and does this cap have any audible impact on the sound of the equipment?

[AndrewT]

Originally posted by Rick Miller
I have seen a small value (.01-.001uf) non inductive cap between the shield of the connector and the metal chassis............................... does this cap have any audible impact on the sound of the equipment?

of course it has an effect on the sound. That's the primary purpose of RF and VHF interference suppression.

Reduce the effect of the interference and we EXPECT to hear the difference.

[john curl]

Hi Rick, to be honest, some of the solutions suggested by both you and PMA remind me of 'throwing the baby out to change the bath water'. If you have that much RFI, something is wrong with where you live. However, if you were located at a broadcast station, I can see where this might come up. Do you have problems like this, Rick, at Comcast?
What concerns me is that super RF proofing, using ceramic caps, shielded wire on the inside of the shielded box, etc. , will change the quality of the sound we are trying to preserve. Bear's silver wire has no shield, and 'we don't need no stinkin' shielded wire inside our box'.