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A Tube amp without coupling capacitors? Possible?
A Tube amp without coupling capacitors? Possible?
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Old 14th August 2019, 06:37 PM   #101
mashaffer is offline mashaffer  United States
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Quote:
Originally Posted by kodabmx View Post
Interesting scope trace for 100kHz through an IT.

Here's my scope trace of 100kHz through a Carli 1uF X2 MKP safety capacitor which I use for coupling.

Also included is a trace at 1MHz.

Not bad for a 50 cent capacitor...
I have not heard of anyone using a safety cap as a coupling cap. Can you elaborate on benefits aand drawbacks and how you decided to try them in the first place?
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Old 14th August 2019, 09:18 PM   #102
smoking-amp is offline smoking-amp  United States
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Quote:
Originally Posted by 50AE View Post
No, it is not. Can't believe people still believe in that. It is only CAPACITIVE of you use it like a capacitor.

Transformer wired, the potentials at the secondary will be the same as the ones primary and no capacitive currents flow.
You are correct in that no "common mode" current flows thru the bifilar capacitance, but differential voltage most certainly does couple capacitively along with the distributed L component, which is what counts. You can pull the magnetic core out and it will still work for MF to HF.

Putting thicker insulation on the bifilar wires will also degrade the performance. The bifilar configuration does greatly improve (reduce) the leakage inductance, for good HF performance. The 1:1 case keeps the voltage across the distributed bifilar capacitance constant if the core is present and well coupled. So in that ideal case the C component would not conduct AC current. Never the less, the C and L coupling components are in parallel. One could just as easily say the C component does all the coupling and the L component sits idly by as all the load current goes thru the C.

Finite wire diameter causes the distributed L coupling (which is in air or dielectric) to have finite coupling impedance, instead of zero, so some voltage differential developed across the distr C is inevitable. You simply have two low impedance couplings in parallel, and the lowest reactance path will take the bulk of the transmitted current.

Ideally the bifilar L would prevent current thru the distributed bifilar capacitance from occurring, but practically, the outer parts of the winding will have leakage L relative to the core, allowing some divergence of voltage between the wires, which the distr. capacitance will continue to keep in lock fortunately. Just as in the no core case. Wire resistance inevitably causes some voltage divergence as well if there is current. (always some magnetizing current + any load current)

Last edited by smoking-amp; 14th August 2019 at 09:43 PM.
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Old 15th August 2019, 01:07 AM   #103
TonyTecson is offline TonyTecson  Philippines
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A Tube amp without coupling capacitors? Possible?
Quote:
Originally Posted by 45 View Post
You have to pay me! Do you want to know my fee mr talker?

It is possible to get wide bandwidth even without bifilar. Actually Pieter T shared some of his experience on this as he was asked to do something with as close as possible FR but also withstanding high primary-to-secondary DC voltage differential. The latter sometimes being a limiting factor for bifilar IT's. Always a matter of requirements and trade-offs....nothing new.

OT's do not need bifilar at all for large bandwidth. It's just how much effort you want to put in it or if you buy how much money.

this is non bifilar for example:
http://www.monolithmagnetics.com/sit...20rev%2001.pdf
it is all about the money then....
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Old 15th August 2019, 04:48 AM   #104
Wavebourn is offline Wavebourn  United States
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A Tube amp without coupling capacitors? Possible?
Quote:
Originally Posted by mashaffer View Post
I have not heard of anyone using a safety cap as a coupling cap.
I use them routinely. Great caps.
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Old 15th August 2019, 04:55 AM   #105
TonyTecson is offline TonyTecson  Philippines
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A Tube amp without coupling capacitors? Possible?
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Originally Posted by Wavebourn View Post
I use them routinely. Great caps.
sssshhh..... that is supposed to be a secret.... now the prices of those things will rise......
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Old 15th August 2019, 04:57 AM   #106
Wavebourn is offline Wavebourn  United States
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A Tube amp without coupling capacitors? Possible?
Quote:
Originally Posted by TonyTecson View Post
sssshhh..... that is supposed to be a secret.... now the prices of those things will rise......
Let it be, I already have several bags.
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Old 15th August 2019, 05:10 AM   #107
TonyTecson is offline TonyTecson  Philippines
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A Tube amp without coupling capacitors? Possible?
with metalized polypropylene used, who can complain?
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Old 15th August 2019, 05:30 AM   #108
jjasniew is offline jjasniew  United States
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I've looked at "safety caps" for years as coupling caps, have a pair in my Sophia amp now, because I ruined the Sophia branded capacitors by soldering near them... They are only 250V, so I have them on the cathode side of the cathodyne phase splitter. The two remaining good Sophia caps on the plates.

These caps were used in the AC input filters of switching power supplies to minimize conducted emissions. They're called "safety" because they typically have markings reflecting various world wide safety agency approvals for direct connection to AC mains use. These agencies are concerned with things like leakage current out the ground prong and since these caps as used in the filter topologies determine that more or less, the safety agancy wants a damn good cap - reliability wise - in that spot. Otherwise, they're just a good quality cap, many are polypropylene, so it's understandable they'd make a good coupling cap for audio.

My take on a previous question about why ceramics could sound as good, is that they probably have a lot of DC across them. Caps generally are more linear with significant DC bias. Other than that, I have no explanation how ceramic caps used in an audio stage coupling circuit could sound as good.
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Old 15th August 2019, 05:47 AM   #109
Wavebourn is offline Wavebourn  United States
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A Tube amp without coupling capacitors? Possible?
Quote:
Originally Posted by TonyTecson View Post
with metalized polypropylene used, who can complain?
Who? Unhappy boutique dealers.
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Old 15th August 2019, 07:27 AM   #110
baudouin0 is offline baudouin0
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Just because an amp has low distortion does not mean you will like the sound. You may prefer the amp with distortion. That's what guitar amps are all about. On a well designed amp ceramics may be fine, or they well be producing some 2rd harmonic which makes the amp sound 'sweet'.

Maybe you should swap then out for polypropylene, the distortion will be lower but that nice sound may vanish.

In my books a hi-fi amp should be blameless in that the output should be a copy of the input. However that's quite tricky with valve amps, and we would all be buying modern amps with .0002% distortion.

However its a challenge to make a valve amp with low distortion which what makes it interesting. The technology is from a age when I was a kid. Plus they glow, look like a beast and there's a real sense of danger building them.
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