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Hello all,
I'm wondering the way that would affect the sound of my 300B SET amp, lowering the cathode capacitor value from 220uF/100v to 100uF/100v. As far as I know, the value is calculated in function of the cathode resistor value. Also, as far as I know, this cap is related with:
frequency filtering
noise reduction
gain
Here is the scheme and the values.
Thanks in advance
Follow Ups:
"As far as I know, the value is calculated in function of the cathode resistor value."The cathode resistor value is part of the calculation.
The cathode bypass cap has to bypass the impedance of the cathode of the tube (rk) in parallel with the cathode resistor value (RK), giving us (r).
If you don't take the impedance of the cathode itself into account the cap value you calculate will be way to small.
Here's a spreadsheet. Input the plate resistance of the tube (Rp), the total load impedance RL (in the case of an output tube, the output transformer's primary impedance), the value of the cathode resistor (RK), the mu of the tube and the desired -3db down point (f3).Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/09/15 08/09/15
Hi Tre,
ok, this make sense. I will try to fill the spreadsheet with all the data.
Besides the high pass filter function, what else is related with this cap that can vary if I will lower the value?
thanks for the calculator!
Regards
The plate resistance.When you bypass a cathode you prevent local current feedback.
Unlike voltage feedback, that lowers the plate resistance, current feedback increases the plate resistance.
When the output tube's cathode is not bypassed to the lowest frequency of interest, the tube's plate resistance increases right at the frequencies that you need it to be low, the low frequencies.
The higher the tube's plate resistance is, the lower the damping factor of the amplifier will be and the amplifier looses it's ability to control the motion of the woofer cone.
On the other hand, if you are not using the amplifier full range or if your speakers don't need a high damping factor then a smaller value cathode bypass cap might be better.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/09/15 08/09/15
Thanks Tre!
so the design of my amp (AN Kit One) use this high and not common value to elevate the damping factor and maybe and give a way to manage and tighten the bass response on wider kind of speakers. And the designer not only take in account the values involved in the formula. I will try to understand well this equation because I'm planning to build a high eff pair of speakers using a single full range drivers and may be benefited from a little tweak on this relation.
regards
"so the design of my amp (AN Kit One) use this high and not common value to elevate the damping factor and maybe and give a way to manage and tighten the bass response on wider kind of speakers. "
That value (220uf) might not be common in practice but the formula says it will bypass the cathode to 2Hz.
This keeps that filter from shifting the phase of 20Hz.
That's probably not needed but might have been part of their thinking for using that value cap.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
For what it's worth, the WE Model 91 used 16uF or 25uF at different times.
"The data sheet on the 91 claims a useable range of 50 to 8000 cycles...."Making some assumptions, a 16uf cap would give a -3db point of 25Hz. A little lower if the output transformer impedance is higher than 2500 ohms. Most of the lost amplitude is restored in one octave.
How low did theater speakers (or film sound tracks) go back in the day?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/09/15 08/10/15
Thanks Tre,
very helpful comments.
What is your opinion about using other kind of capacitors, non electrolytic?
Regards
I would use a film capacitor whenever possible.
It's easy for me because I don't play bass through my SET amplifiers.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"It's easy for me because I don't play bass through my SET amplifiers."
what do you mean? you use 2 or 3 amplifiers, one for each speaker driver?
and you you mean that film caps has an issue with bass response?
I don't get it
No, that's not what I meant.There's no reason to bypass the cathode lower than the lowest frequency the amplifier is going to have pass through it.
I bi-amp. I use a 200Hz low pass filter (1/2 of a electronic crossover) between my preamp and a SS amp that drives my JBL 15" woofers.
I use my SET amplifier from 200Hz on up. It has a cap in series with the input that creates a 6db per octave high pass filter at 200Hz so the output tube and the output transformer doesn't play much really low frequencies.
I therefore don't need to bypass the cathode of the output tube all the way down to the lowest frequencies the way I would have to if the amplifier was being used full range.
BTW IMO, film caps are better at all frequencies.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/09/15
TRE, I like the idea of using tuned amps for biamping. I've done this with a pair of Tympani 1D's at 6db/oct with very good results. Why did you choose to use an electronic xover for low pass? Was it for level adjustability?
I wanted a steeper slope.
My midrange speakers fall off all by themselves fairly rapidly below 200Hz but my woofers would play quite well above 200Hz.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
With the recent posts on musical loadlines I would almost think you would want a steeper slope on the midrange. A first order slope is only 18db down at 25hz. Is that enough? I'm speaking from the SET amp view, not the driver.
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Big speakers and little amps blew my mind!
You have a point but the cure might be worse than the illness.I don't want to go active on the high pass and I don't know where to get a really good 250Hy small signal filter choke (or any 250Hy small signal filter choke).
FYI, I use a 330k grid resistor on the input stage of my SET amp. (the value is large because I see no reason to "load down" what's driving it)
I have a small cap in series that creates the 6db high pass filter.
To make that a 12db high pass filter I would need a very large value choke.
Do you know of an easier way to do this passively?
Since I'm doing this at the input and it is a cap coupled 2 stage SET amp I guess I could create another 6db high pass filter by using the coupling cap.
Truth is, I intend to try this but just adding the 6db filter helped a lot. If there's more improvement to be had, I think I'll save it and savor it later. I'm enjoying what I have right now.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/12/15
Tre,
Wouldn't having R/C filters in series increase the slope without using a choke/inductor?
Yes. I have one at the input to the power amp and I could lower the value of the coupling cap between the driver stage and the output stage to create another.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Please do enjoy your system. It's just a question that's been in the back of my mind.
Phil Marchand uses a ferrite core (ferrite pot core/pot ferrite core?) coil in his PPXLOs that many of the knowledgeable guys at diyAudio swoon over, and would die to know where he sources them from. I don't think he sells them individually. His PPXLOs are second order, and they would be easier for me. Are they passive enough for you?
Why don't you want to use an active high pass? Just because it's active doesn't mean it has to add gain.
-------------------------------------------------------------
Big speakers and little amps blew my mind!
I can't find anything on line about the PPXLO.
The XM46 looks good by the impedance is too low (1k).
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Oh, and it says typical impedance is 1K. If you find it interesting, then you should send an inquiry to Mr. M. who says he can build anything you want. He is an extremely nice man.
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Big speakers and little amps blew my mind!
I'm sure he's a great guy but the higher the impedance the larger the inductors need to be and I think that will lead to higher and higher winding capacitance causing other problems.
LCR RIAA filters are available in 600 ohm and 10k but everyone seems to say the 600ohm sounds better.
But 10k is a lot easier to drive.
Every step of the way is a compromise of one form or another.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
> "Every step of the way is a compromise of one form or another."
You got that right. That's why there are 10,000 paths in this hobby.
Mr. M is a great guy. An old school Frenchman, and willing to talk to somebody on the phone for five minutes. You could ask him yourself tomorrow if you so choose.
Is this all an issue because you use a TVC or AVC? I think an active crossover would take care of that problem. Yet another compromise.
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Big speakers and little amps blew my mind!
"Is this all an issue because you use a TVC or AVC?"No, not at all. The AVC is a step down autoformer (unless you have it wide open) so the output impedance is lower than that of the source.
I'm just OCD and like to keep my load lines as horizontal as I can. Decreases the HD, ya know!
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/12/15
Just exploring the possibilities.Have a good one.
------------------------------------------------------
Big speakers and little amps blew my mind!
Edits: 08/12/15
.
Have Fun and Enjoy the Music
"Still Working the Problem"
My mistake. It's been a long time since I looked at his site.
-------------------------------------------------------
Big speakers and little amps blew my mind!
"I therefore don't need to bypass the cathode of the output tube all the way down to the lowest frequencies..."
And as a result, you can use smaller value caps, which makes film caps practical. [I think this might be the point that wasn't clear to the OP.]
--------------------------
Buy Chinese. Bury freedom.
nt
.
Have Fun and Enjoy the Music
"Still Working the Problem"
Most popular single driver, full range speakers roll off a lot in the treble and the response is very jagged as it goes down hill. Think about crossing over to a well-modded ribbon tweeter at the point where the roll-off starts. Look at the designs of Tonian acoustics.
Hello Frihed89,
yes, agree. That is exactly what I'm planning. Thanks for the advise. By the way, I would use the french driver LB12MKII(35Hz-15Khz @97.5dB) and I was thinkin in use some of their mid-tweeter....maybe also a tweeter
I will check Tonian.
Regards
Hello,
Bookmark the handy calculator, URL below, and it looks like yours computes to 0.8 HZ. It is NOT necessary to have such a low calculated value, so, the 220 uF value is way to big, NOT selected by anyone who ever LISTENS, just a theory - guesser.
It could be up to a factor of ten times smaller for the Rk bypass ( 22 uF is 8 HZ ) . THAT will allow you to use a film cap there, not an UGLY sounding electrolytic, as shown in the schematic !! I might try about 47 uF there, at the most. YOU need to LISTEN, AND DECIDE total value, trust your ears.
The Rk bypass cap is DIRECTLY in the SE audio circuit, and it needs to be carefully selected and implemented, and always HIGHER quality film if you want the best results.
It also needs to best be executed as multiple film caps, CAREFULLY chosen, because NO one value cap exists, that plays all the frequencies linearly.
In 2015, I personally would use a minimum of five, and up to seven different carefully selected film Rk bypass cap values, to get the audio band to play-back to me as sounding linear.
Have fun listening and learning !!
Jeff Medwin
nt
Hello Jeff,
thanks so much for your answer. I was trying to find such calculator, but I failed.
Well, the calculation was made by the engineer on Audio Note UK that supposedly heard beside calculate every passive component...so that's why I'm wonder if there is some other reason to such value besides the fq filtering. I'm was using a black gate non polar here and now an elna cerafine. So because this cap position has an enormous effect on sound, I was wondering what happens if I will lower the value. It will be a good thing though if I can use a film cap..... I was thinking on the new kaisei 100v caps that AN is producing now, but a film cap may be better.
Regards
Why get hung-up on buying audiophile "boutique priced" capacitors? What about generic caps, or industrial caps, film types ??The DIY trick and challenge is to "out-do" the boutique priced caps with generics, at 1/20th the price !! Maybe, in some cases, employ NOS generics, no longer manufactured !! Yes.
It will take more than one film cap ( bypassing ) intelligently, to have a shot at that. I just went through much of this, with my Type 45 SE amp build the last couple of months.
You can use this Forum's "search" engines to look up previous posts on film cap bypassing by the really good builders, such as Thorsten Loesch's in 1999-2000, and Dennis Fraker's, more recently. Its in here if you dig some. Then, build, listen, decide for yourself. Have fun.
Jeff Medwin
Edits: 08/10/15
The trick is to use methods that will best achieve the your desired outcomes for the build.I try to work from a well researched, solid base then voice as minimally as possible. Others work from (what I might think is) a less ideal, perhaps cheaper, base and voice heavily. Neither is right or wrong. What matters is that the approach satisfies the project objectives: practical, sonic, personal and otherwise.
Regards,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 08/11/15
Seems like you are justifying yourself. We each may not have the proper experience, to judge " what is a proper base ". Our thinking ( both of us ) could be delusional.Hence, subjective listening rules in my book. How it sounds, to the end-user, is what matters the most !!
Jeff Medwin
Edits: 08/13/15
Hello Jeff,
thanks for your advice.
Well, my amp was coming with the Elna Cerafine for the cathode bypass. I bought the BG when still were in production at a discount price. Sounds wonderful, very open and detailed. The Elna, a little more obscure, but maybe more "musical" and soft on the high frequency. I'm not too much friend in experiment with dozens of capacitors or swap a lot. This two caps was the only ones on more than ten years. One reason to this is that every cap or resistor that I bought for my amp, has to be imported , so the cost never is too cheap, so I have prefered to use, in the first place, the same than the manufacturer. Of course, there must be a bunch of extremely good and cheap caps out there and maybe now is the time to try some.
Regards
I have been experimenting with that recently. Contact me by email, address is above, and we can discuss inexpensive possibilities. No ONE cap will play all frequencies, and get all the " vibes" of the music, linearly. Hence, mutiple film bypassing of Rks.
Jeff
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