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In Reply to: RE: Resonance posted by PakProtector on May 16, 2017 at 09:38:27
Yes, SRF should create a peak. Maybe I would have seen that if I'd swept higher than 100kHz. The dip is clearly a different form of resonance, but like I say, it was too small to be of concern.
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Buy Chinese. Bury freedom.
Follow Ups:
It is the first Parallel resonance you are looking for. Look lower...:) It should be above 120 cps, and is *VERY* unlikely to be above 2,000 cps.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
The dip would seem to be series resonance. In series LC circuits, X L and X C cancel, at the resonant frequency. That leaves only DCR to impede current flow.
Eli D.
You're correct, the dip would be a series resonance. Sorry, typing too fast, had a brain lapse.
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Buy Chinese. Bury freedom.
It is the first Parallel resonance you are looking for. Look lower...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
There are no discernible peaks in the voltage that appears across the choke when I sweep it. That only happens when an external cap is attached.
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Buy Chinese. Bury freedom.
What Douglas is talking about indeed happens midband typically around 1Khz and will not show up in a typical frequency sweep because it is at the self resonant point where the impedance is the highest. Other interesting things to note about this is that at the self resonant frequency the impedance is purely resistive and most LCR meters sample at 1Khz and spit out incorrect results for audio chokes since they are sampling near the SRF. I think this behavior was the origin of the myth that some amount of DC is required to "turn a choke on" and the lack of DC current (in a gapped device) is the reason a handheld LCR meter gives a reading quite different for what is printed on the can. One other reason handheld meters typically spit out seemingly incorrect numbers is the AC excitation level. The norm of a handheld meter is usually 100mv @ 1Khz and even the meters that can sample at 120hz will give low readings for power transformer primaries. This isn't because the meter is incorrect, it is just measuring the wrong thing. In order to know the correct inductance of a power transformer you need to measure it at 120 / 240V... anyways sorry for the slight tangent and back to the regularly scheduled discussion
In order to see the SRF you simply put a large valued resistor in series with the choke and a scope across the choke and sweep frequency for the highest value of AC across the choke. Again this does not show up in a traditional sweep because for our uses the value of the choke is 10X+ the value of the source giving flat response throughout the area where the self resonance occurs.
dave
"put a large valued resistor in series with the choke and a scope across the choke and sweep frequency for the highest value of AC across the choke."
I appreciate your input, but I described this exact process in my first reply above to PakProtector:
"I searched for resonance by driving the chokes with signal through a 100K resistor. Monitoring with a scope, there's a very slight dip in amplitude just above 20kHz using that method. I assume that's the self-resonant frequency. Driving the chokes with 10K ohms, the dip is almost impossible to detect, and it disappears with a 1K source. FWIW, I also measured inductance and resonance with 30 mADC flowing, and there was no noticeable difference. Based on all this, I expect these chokes to provide a frequency response of at least 10Hz to 50kHz (+/- 0.5 dB) in cathode follower service."
Funny, I never thought this would devolve into a discussion about the characteristics of the chokes. If anything, I expected at least a few inmates to tell me how awful a CF will sound. :)
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Buy Chinese. Bury freedom.
I know that using a choke in this application is "cool", but I wonder why not use a CCS, instead? In my experience, a CF with a CCS sounds way better than a CCS with a cathode resistor (never tried a choke), and a CF with a CCS and a constant voltage strap (the SLCF, a la Allen Wright) sounds better yet. In fact, the latter reaches a state of transparency, in my opinion. AW knew his stuff.
It has been some time since I tried this...but I recall picking a resistor that left 80% of the signal voltage across the choke at its self resonant point. This of course being the parallel LC point that delivers a peak...
I did not find one I was not disappointed in...LOL
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
"I did not find one I was not disappointed in...LOL"Do you mean choke performance or the amplifier using a choke?
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Buy Chinese. Bury freedom.
Edits: 05/17/17
The choke's low resonant frequency. That applied to so-called 'vintage' units I measured. It left me directing my winder to use a few of the capacitance reduction techniques which yielded not-so-subtle improvements in the SRF...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
We must be missing something here in the discussion. With your test setup you should see both attenuation at low frequency (from the finite inductacne) and at high frequency (from the shunt capacitance) The dip you are seeing above 20Khz is not the behavior of the SRF and at high frequencies you should be able to track down the point where the capacitance dominates. (typical -3dB point with a 6dB slope just like happens at LF.)
Now in between those two points you will see a peak in amplitude across the choke and that happens at the SRF point. If the peak is not showing up then it may be hidden in the "passband" of the lowpass and highpass filters at which point you need to increase the source impedance in order to "flush it out"
I just swept a choke with 100K and 500K source Z and the 100K SRF was harder to find and with the 500K the SRF peak was clearly there.
I do understand your meaning.
"With your test setup you should see both attenuation at low frequency (from the finite inductacne) and at high frequency (from the shunt capacitance)"
Yes, that's how it behaves.
"you should be able to track down the point where the capacitance dominates. (typical -3dB point with a 6dB slope just like happens at LF.)"
I monitored the rolloff while sweeping by hand. It becomes evident above 20kHz (just above that very slight dip I mentioned), so I didn't quantify it.
"Now in between those two points you will see a peak in amplitude across the choke and that happens at the SRF point."
I've already repacked these for temporary storage, but maybe I'll break one out and test again. 500K or 1 Meg might reveal the self-resonant frequency. As you said earlier, none of this will create performance issues when the choke is in a circuit with an operating impedance of less than 500 ohms.
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Buy Chinese. Bury freedom.
Agreed on the performance thing... Its a wonder chokes find any use in audio given the far superior measured results from a CCS ;-)
This discussion seems to come up frequently. For me, it's an easy decision. Hate semiconductors, love stored energy. Voila!
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Buy Chinese. Bury freedom.
but you do need a negative supply voltage, for sure.
The stored energy in a choke allows the cathode to swing positive and negative equally, even without a negative supply. This creates a huge dynamic range, and it eliminates the deficiencies designers often unknowingly introduce into followers. Bottom line - it's neither necessary nor desirable to contaminate your music with silicon.
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Buy Chinese. Bury freedom.
Thx again.
you've taught me something valuable, and for that I thank you, but I am talking about a tube CCS, with two haves of a high Gm triode in cascode. (I use ECC99 most often.) No silicon need apply. But again, I realize you do need a negative supply for said tube CCS.
I am wondering why is it that I have not read more about using a choke in a CF before this, either here or on the various relevant other websites? And doesn't a robust CCS achieve the same goal of allowing the cathode to swing with equal amplitude in both directions? Albeit, the parts count is much higher.
can't resist diverting things a bit. I find that a triode on the bottom, and a full pentode on top does quite well. Given the *GENERAL* characteristics of a pentode approximate a cascade, it is not going too far IMO to call this one a triple...LOL They do eat up some voltage, that is for sure, but with the right bottom triode selected, can do a pretty fine job driving a 1000R load( If you want to stack the deck in a linestage shoot out listening session, bring an amp with a nutty low input grid resistor ). Tell them at some point what happened...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Thx
Not just a pentode, but a pentode stacked on a triode to form a composite; a cascode. It is all about creating some floating voltage references. You'll want a large current set resistor for the lower triode, and you need one for the upper pentode's grid so the lower element has enough voltage to work under. A CCS driving a resistance works well, and floats adequately. And then there is the pentode's screen supply; dropping R from B+ and a cap bypass works well. Be careful of cap-to-case capacitance; no need to shunt stuff to ground...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Probably why I did not get your point the first time.
Honestly, I've never built and auditioned a high-voltage vacuum tube CCS. Whether it would be the equal of a choke, sonically speaking, I don't know. I do think that simpler is usually better, and the choke certainly wins that contest.Regarding the lack of literature on this subject, I can think of a few possible reasons. First, cathode followers aren't too popular these days. That's due in large part to a legacy of poor designs. It's much more difficult to create a good CF than a standard, common cathode stage. If they're not done right, they sound awful. Then there's the fact that a CF has no voltage gain. That often relegates it to secondary functionality such as tape out jacks and signal loops in guitar amps, applications in which sonic purity generally takes a back seat to practicality. Finally, most good DC-capable audio chokes are produced for use at the anode. Circuit impedances are higher there, so the requirements for the choke are more stringent and more costly. A decent pair of plate chokes can easily run $200-300, and that puts them out of range for many builders.
I paid less than $25 each for the chokes I intend to use in this project. Except for the high-ish DCR, they're probably as good as anything out there for this purpose. They could also be used as plate chokes, assuming the tube has an appropriate Rp. Keep an eye on eBay if you want to give something like this a try on the cheap. :)
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Buy Chinese. Bury freedom.
Edits: 05/19/17
"Bottom line - it's neither necessary nor desirable to contaminate your music with silicon."
My apologies, and I do hope I don't start start a big interruption to the flow of this thread, but I am always so puzzled when I read statements like this one. Unless your chosen sound source is old gramophone records, you are surely listening to music that has already been "contaminated" by a huge amount of silicon? Does one more silicon item, in itself, have that much effect one way or the other?
Another possibility is that the vacuum tube stage is adding in something to the sound, in which case indeed, one might find it doesn't do that as well if silicon is included. I find that a perfectly good reason for preferring to use a tube amplifier, though I'm not sure all tube lovers would accept this premise.
Chris
"you are surely listening to music that has already been "contaminated" by a huge amount of silicon? Does one more silicon item, in itself, have that much effect one way or the other?"
If that's the criteria you're using to qualify the value of audio equipment, I'd say you might be in the wrong forum. There's a lot more research and online activity in the solid-state genre, and it's much simpler to build. I build with tubes; you can use whatever parts make you happy. :)
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Buy Chinese. Bury freedom.
"If that's the criteria you're using to qualify the value of audio equipment, I'd say you might be in the wrong forum. There's a lot more research and online activity in the solid-state genre, and it's much simpler to build. I build with tubes; you can use whatever parts make you happy. :)"
I'm not disagreeing with the idea of choosing to build with tubes; that's what I do also, in general. The only thing I'm puzzling about is the notion that silicon devices in some way "contaminate" the music. My point, in this regard, is that the sound source one is commonly using (a CD) has been produced using silicon all the way from the microphones to the output of the CD player, and so why should one additional silicon device make any appreciable difference to the level of "contamination" of the music?
Perhaps I am taking words like yours, or other similar ones I have seen elsewhere, a bit too literally. As I say, I have no problem at all with understanding the concept that one wants to build using just tubes, and no silicon. But I was puzzled by the "contamination" justification, which seemed to overlook the vast number of silicon devices that the music signal had already passed through.
Anyway, carry on with the thread, which I find quite interesting, and don't let me divert it down these side alleys!
Chris
LOL...I just giggle when I hear that, 'contaminate with silicon' idea. I have built a tube CCS a few times, and while some of the performance parameters are not as good as the SS implementations, the critical ones certainly are. Things like a lack of voltage variable capacitance for one.
Also, to say that a choke is a better load because it stores energy and can operate with less voltage across it only matters if delivering an adequate amount of voltage is expensive; either to the design implementation as a whole, or outright Cash outlay...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I'm with you on the stored energy thing but from a purely objective viewpoint, when I look at the impedance curve of a choke I quickly start twitching in fear. It wasn't until I became comfortable that a large value but varying load isn't remotely close to being a problem in the circuits we are dealing with. Then learning to ignore the flat earthers that pound their chest about constant impedance loading finally allowed me to sleep at night after a long day of enjoying music.
dave
as a certified flat earther, the worry about changing loading only extends to the discontinuity the grid exhibits when going positive. The change is not small...LOL Cathode drive, here I come.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
When a pentode or FET voltage follower is the object of the exercise, I'll take a choke over a constant current sink, "in a NY minute". I'm not saying a CCS can't work, but "head butting" between entities, each of which is "constant" current, is something (IMO) to avoid.
Eli D.
Must beg to differ...a cathode follower( or source follower, be it single element or cascode) is only a constant current if its lower gate source voltage is fixed by a resistive element. When it is riding a solid sink, it is a variable current delivery device, and will settle into a constant gm operation due to it not caring about what the voltage across it is( the horizontal plate lines ).
Nothing to worry about IME...:)
cheers,
Douglas
and please consider 'cascade' a typo for 'cascode' when ever you see me writing it.
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
In fact, a cathode follower more closely approximates a constant voltage source. Using a constant current device (in my case, a choke) at the cathode is exactly the right thing to do.
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Buy Chinese. Bury freedom.
"If anything, I expected at least a few inmates to tell me how awful a CF will sound. :)"
Implementation details matter . Well executed voltage followers are among the most transparent active circuits extant. If somebody thinks otherwise, let them butt heads with SY over on DIY Audio.
FWIW, I'm partial to FET (source) followers, as high transconductance/ low O/P impedance is readily available.
Eli D.
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