Considering upgrading both electrolytic and film caps

arsun · 32834

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Offline arsun

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on: December 26, 2009, 08:29:33 AM
As the title says, I am thinking of upgrading both the film and electrolytic and film capacitors in the SEX amp. Having MQs and C4S boards on order, the film ones should be 0.1uF and 2uF each with at least 600V rating. I was thinking of Mundorf Silver/Gold in Oil capacitors, but they have a rating of 2.2uF instead of 2uF. Does this matter? Another candidate is 0.1uF TFTF Vcap and 2uF OIMP. I would love to go both with teflons but aura teflons are only ones with 2uF@600V rating and when I checked their prices, I gave up on that idea :) What do you think about those combinations? Can you recommend one over the other? I also would appreciate any other recommendations, preferably around the same cost, maybe a bit more, maybe a bit less...

For the electrolytic ones, I am thinking Elna Silmic II 1000uF 50V capacitors to replace 1000uF 35V ones. I read just a while ago that Grainger recommended 70uF@630V Obbligato Polypropylene Film in Oil caps instead of 47uF electrolytic ones, so I might as well get them, any other suggestions?

I appreciate your valuable comments. Thank you...
« Last Edit: December 26, 2009, 10:13:28 AM by arsun »



Offline arsun

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Reply #1 on: December 26, 2009, 08:37:00 AM
BTW, will 22uF 630V Obbligato caps will be a good replacement for 22uF 450V electrolytics, or is this a bad choice?



Offline arsun

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Reply #2 on: December 26, 2009, 01:43:40 PM
I was just goofing around and came across Mundorf TubeCaps. It seems like these can be used instead of Obbligato caps too. They have the values of 47uf @ 600V , 20uF and 30uF. There is also one 100uF @ 550V. 47uF seems to be a match, will there be a benefit to get 100uF? Also, is it better to get 20uF or 30uF to replace 22uF?

Thanks.



Offline Paul Joppa

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Reply #3 on: December 26, 2009, 04:13:19 PM
For the parafeed cap (2.0uF) the useful range is 1uF to 4uF - it does not need to be precision, except the two caps should be within say 10% of each other. Different values will likely sound a bit different in the deep bass from each other, but it also depends on the particular speaker so I wouldn't try to predict which value would sound best in your room, your system, and with your speakers.

Paul Joppa


Offline arsun

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Reply #4 on: December 26, 2009, 07:11:59 PM
Thank you for the reply, but I think there has been a misunderstanding. I am not trying mix and match different capacitors for the parafeed cap. I will either use Mundorf 2.2uF or Vcap OIMP 2uF. In case I will choose Mundorf, I will order Mundorf 0.1uF capacitors. If I use OIMP from VCAP, I will order VCAP TFTF 0.1uF ones. I hope that clarifies things a bit.

Also, do you recommend bypassing any of these capacitors with a lower valued one? I am still a bit vague about this bypassing issue. The total capacitance will the main one + the bypass cap right? So I am guessing the small-valued one might only affect by the order of magnitude it brings to the table. Am I thinking wrong here? I am sorry if this sounds funny or stupid, but I am really a newbie when it comes to these things. 



Offline Paul Joppa

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Reply #5 on: December 27, 2009, 08:32:29 AM
Here are my thoughts about bypassing capacitors.

Many capacitors, especially large value capacitors, have some kind of poor performance at high frequencies. This is especially true of traditional electrolytic capacitors which can become resistive or even inductive as low as 10kHz. In addition to not handling the high treble correctly, this can seriously degrade a circuit's immunity to RF noise, for example. A high-quality capacitor with a low-loss dielectric that maintains performance to much higher frequencies can "take over" the capacitor function by maintaining a more capacitive, lower impedance at these higher frequencies - effectively shorting out the badly behaving cap.

The technically-identifiable issue is that the combination now includes an inductor (the large cap which becomes inductive at high frequencies) plus the bypass capacitor, and they can create a resonance in the impedance function which, in severe cases could easily do more harm than good for the sound. I believe this is the reason some people do not like the sound of bypasses, while others do like it. A simple solution is to put a small resistor in series with the large capacitor so that the resonance is well damped, though almost nobody actually does that.

My own inclination is to use capacitors which have acceptable high frequency performance, and avoid using bypasses at all. But then, I design stuff and don't have to deal with getting the most out of a capacitor I already have - I can go get the one I want. For electrolytics, there are now many on the market that are rated for use in switching power supplies and remain capacitive well above 100kHz. Mylar has some problems at higher frequencies but polypropylene is nearly as cheap and maintains performance much higher.

There are several more subtle ways that capacitors can degrade sonics; microphonics and dielectric absorption are two well-known examples. Bypassing to improve those problems is not so well understood and capacitors are rarely specified for those issues, making it nearly impossible to analyze the situation. Experimenting is the only practical answer, and it requires substantial reserves of patience (and often money!). Experimenting with bypass capacitors is less likely to make a big difference here, compared with swapping the whole capacitor. But it's also less expensive, so the two may be equally cost effective.

Paul Joppa


Offline Grainger49

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Reply #6 on: December 27, 2009, 11:01:10 AM
My experience with my Paramours says that the Parafeed cap, the 2.2uF you are looking at, is the most important cap sonically.  PJ mentions that the value is not critical.  The 0.1uF is the interstage coupling cap.  I have swapped this cap in two pieces but I'm not sure it has the bang for the buck that improving the last stage power supply cap has.

In power supplies I put Polypropylene caps in my Paramours.  I have use PIO caps in my Foreplay and Seduction power supplies.   In your SEX amp that translates as the 47uF@450V caps.  That is the value that I used for my Paramours, IIRC.

Just my suggestions.



Offline arsun

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Reply #7 on: December 27, 2009, 06:31:01 PM
Thank you Paul and Grainger for your valuable comments. Greatly appreciated.

As far as I could get from the discussion, bypassing requires great luck to hit the best combination values at the first try, and I am not lucky when it comes to these things :). Also, it seems like the parafeed capacitor is the most important one to make a difference/upgrade in the sound, so that needs to be chosen carefully. I think I will pass this bypassing thing :). The best candidate seems to be the Mundorfs then. I would love to be able to stretch to teflons for this value, but they are really expensive...

Very small question about the resistors: I saw someone recommended to replace grid stoppers with 120 Ohm ones, can I ask the reason of such a decrease in value? I will keep most of the stock resistors as they are, maybe just upgrade the ones in the headphone section and get 120 Ohm tantalum ones. While at it, I might also order additional resistors, maybe one in place of 499 Ohm one in C4S board (found 500 Ohm ones, probably will get that) and the grid stoppers as recommended somewhere else around the forum. What do you think about this? I don't think resistors would matter that much, but I will place an order anyway, so adding a few more items, if they will indeed have any effect, won't matter for me.



Offline Paul Joppa

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Reply #8 on: December 27, 2009, 07:02:57 PM
Grid stopper resistors are not especially critical about value. The grid does not draw current normally except at very high frequency where the grid impedance is dominated by the electrode capacitances. So as long as they are smaller than the driver's plate resistance they will not affect the high frequency response. For the output tube they can limit the grid current on overload, which may soften the overload distortion character - but you should not be overloading the tube anyhow.

In the original Paramour we used 499 ohm metal film resistors which worked fine. When the Seduction came out I switched to 220 ohms because we used them as plate stoppers as well on the 6DJ8, which has a very low plate impedance, and I wanted the stopper to be insignificant with respect to that resistance. People have commonly used anything from 50 ohms to 10K ohms.

The gold standard is carbon composition for grid stoppers because they aer bulk carbon and have virtually no inductance. But in modest values (say, less than 1000 ohms) I am inclined to take the advice of VoltSecond. He has extensive professional experience which I respect, and says the metal film works equally well - the inductance is so small as to be inconsequential. I would expect carbon film to also work well.

Paul Joppa


Offline arsun

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Reply #9 on: December 28, 2009, 07:28:51 PM
Thank you very much for your valuable input. Is there any recommended type for the resistors in C4S boards (499 Ohm ones) ? Should I go with film, tantalum or carbon ones? I am probably getting tantalum for headphone 120R resistors. But I am wondering about the one in C4S boards... I promise, this will conclude my resistor questions :)

I will update here after completing the amp too. It won't be before the end of Jan or mid of Feb though. I still need to raise some funds...

Thanks again. I wish everyone here a happy new year...



Offline Paul Joppa

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Reply #10 on: December 28, 2009, 07:36:28 PM
For the current set resistors in the C4S, go for the best you can afford - i.e. tantalum bulk foil if you can. 5% is enough precision, it's all about low noise and linearity in that application.

Paul Joppa


Offline Grainger49

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Reply #11 on: December 29, 2009, 04:42:49 AM
Paul,

In the non-SEX applications, which resistor is the current set resistor?  I'm guessing the one attached to the base of the two transistors.

Of course I could be all wet and it could be the one in the emitter circuit.

You know I don't yet understand Current Sources or Shunt/Series Regulators.



Offline arsun

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Reply #12 on: December 29, 2009, 07:58:44 AM
For the current set resistors in the C4S, go for the best you can afford - i.e. tantalum bulk foil if you can. 5% is enough precision, it's all about low noise and linearity in that application.

Thank you for your reply. What is the watt rating for the 499R resistors on C4S boards? I cannot seem to find such an information. Also, is the value of 499-500 Ohm a must, or can you provide a range for it? I located some shinkoh tantalums at 500 Ohm rating, but there are two versions of it, one is rated at 1/2W, the other one is 1W. Audio Note tantalums are 475Ohm at 1W and 560Ohm at 1/2W. Then I also found some Caddock USF340 (0.33 watt, 1%, - radial lead) - Ultra Precision Tetrinox Film Resistors. All those three options are from the same web site and prices are close, so I can get any of them, I am more inclined to Shinkoh resistors according to the information you provided. However, just to be sure, among all those options, which one would you pick and with what watt :) rating?

Thanks again... I am decided more or less on every part for my upgrade path. I will let you know of the results. I really appreciate all the valuable input you provided... 



Offline Paul Joppa

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Reply #13 on: December 29, 2009, 09:17:28 AM
The current is set by R1, from the emitter of Q1 to the B+ supply - hope that answers Grainger's question! Current is approximately 0.855v divided by the resistance, so about 1.7mA. This matches the stock SEX amp. If you are using 6DN7s you can raise that current by using a smaller R1 resistor and it might even improve the sound. I would not go below 237ohms (about 3.8mA - this is the value used for Foreplay and the Paramount driver).

You can calculate the resistor dissipation, it's 0.855 volts times the current, or 0.0015 watts. Obviously pretty much any resistor will have plenty of power margin!

"Which would I pick?" - sorry, you know me, I don't do much experimenting with components. Here's what John "Buddha" Camille said in the original C4S manual:  "The value of R1 sets the limit current of the circuit and is critical to proper circuit operation. For best operation this resistor must be a low noise metal film or bulk foil precision resistor. ... "

Paul Joppa


Offline Grainger49

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Reply #14 on: December 29, 2009, 10:04:17 AM
Paul,

Yes, thanks.  That is exactly what I wondered about.  And the calculations will go some distance toward helping me understand these.