| Choke cpaazitance, round 2 [message #31847] |
Tue, 28 March 2006 04:41  |
PakProtector
Messages: 935
Registered: May 2009
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Illuminati (2nd Degree) |
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Hy-hy!!!,
I did a frequency sweep like I did for the MQ items. These have a large C-core, and dual sectioned coils, one on each leg. The core is gapped with some poly film. L at 6o cps is ~650 Hy. The gap took a bit of L out, but past expeience shows that they sound better if the L is more disconected from signal level.Anyway...these both resonated at a few cps past 1k Hz. As predicted by theory, the impedance goes through the roof, and the starts falling. This behaviour leaves me wondering about using Iron like this with high output Z drivers in Zero-nfb amps. Good thing that for open loop amps, one does not usually deal with high source Z on purpose...:) I am modifying the amp so that the slightly taller chokes can be installed, and then the Brown will hit the fan( no slight to the package mis-handlers at UPS intended ).
cheers,
Douglas
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| Re: Choke capazitance, round 2 [message #31848 is a reply to message #31847] |
Tue, 28 March 2006 05:42  |
Damir
Messages: 1005
Registered: May 2009
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Illuminati (2nd Degree) |
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If I understand you correctly, with self resonance f~1kHz and L~650H, we can expect: Cw = 1 / (4Pi^2 * f^2 * L) ~ 39 pF Can you explain your results / findings in more details?
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| Re: Choke capazitance, round 2 [message #31852 is a reply to message #31851] |
Tue, 28 March 2006 14:06 |
PakProtector
Messages: 935
Registered: May 2009
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Illuminati (2nd Degree) |
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Hey-Hey!!!,
The choke in question is a CT grid choke. On PS inductor resonance, it is not a bad practice. One LC stage to remove nearly everything but a 120 cps ripple, then a second LC with a tuning C shunted across the L to establish its resonance at 120 cps. Nearly pure DC is the result. The numeric analysis would seem to idicate such a widely varying load on the driver stage would be bad news...these are ov course the same methods that would have us using SS for amplification...:)
cheers,
Douglas
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| round 3....heh-heh-heh [message #31853 is a reply to message #31848] |
Tue, 28 March 2006 18:06 |
PakProtector
Messages: 935
Registered: May 2009
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Illuminati (2nd Degree) |
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From the Honourable Damir:
If I understand you correctly, with self resonance f~1kHz and L~650H, we can expect: Cw = 1 / (4Pi^2 * f^2 * L) ~ 39 pF Can you explain your results / findings in more details? ********************* It's too bad that half the windings don't have a quarter of the capacitance! Half the coil has half the L, unless it's by itself...then ov course it has a quarter of the end to end L.
cheers,
Douglas
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| Re: Choke capazitance, round 2 [message #31855 is a reply to message #31852] |
Wed, 29 March 2006 05:01 |
Damir
Messages: 1005
Registered: May 2009
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Illuminati (2nd Degree) |
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I posted about it in the "Tubes" section some time ago. If we have pentode or cascode stage with, say 15k load and internal resistance of 100k, then "effective" load is 15k//100k = 13k.
If our driver has, say S=4mA/V and grid choke as a load, then amplification on some higher frequencies, say 1kHz is A= S*Ra = 13*4 = 52 times. Choke impedance on 1kHz is theoretically Z = 2Pi*f*L = 2Pi*1000*650 = 4Meg >> 13kOhms.
But, on low frequencies, say 40Hz, our choke impedance is only 163,3k and our amplification is now: A = S * (13//163,3) = 4*12 = 48 times.
Although it is only 0,7dB of difference, I`m pretty sure that it is audible. Plus falling of highest frequencies `cos of low pass filter formed with Rout of the driver and Cw.
What you think about actual measurements of frequency response in real amp with real components (grid chokes) ? 
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| Re: Choke capazitance, round 2 [message #31857 is a reply to message #31855] |
Wed, 29 March 2006 10:08 |
PakProtector
Messages: 935
Registered: May 2009
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Illuminati (2nd Degree) |
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Hy-Hy!!!,
I think it is time to measure tha amp again. It has gotten a bit on the heavy side...but that may have improved its power delivery.The E-Linear plate-to-grid NFB will do a bit to level the response. The 20-20 lineage of the output TX should grant it full power down to its advertised low frequency limit...:) I will measure it soon. Must get proper load that won't overheat.
cheers,
Douglas
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