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    Coupling capacitor values for MK-IV build.

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    Stevedel

    Posts: 5
    Join date: 2012-02-15

    Coupling capacitor values for MK-IV build.

    Post by Stevedel on Wed Aug 08, 2012 11:13 am

    Greetings, all.
    This is my first post to this forum but I have been following it with great interest.I have a MK-IV kit, on order from Dynakitparts, which will power a JBL 2245H 18" subwoofer.
    I have just completed a Dynaclone ST-35 with Mundorf Silver-in-oil coupling caps and am very pleased with the result. I am going to use the same output caps on the MK-IV. I have read, however, that the MK-IV bass response (same as ST-70 ) can be improved by using .47 MFD caps instead of the .1 MFD specified. Anyone experienced with this modification? Can it effect other performance parameters?
    Great forum you folks have here.
    Thanks so much.
    Stevedel

    Bob Latino
    Admin

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    Location: Massachusetts

    Re: Coupling capacitor values for MK-IV build.

    Post by Bob Latino on Wed Aug 08, 2012 1:32 pm

    Hi,

    I have altered the value of the main coupling capacitors in the VTA ST-70 and ST-120 amp kits and found no perceived change in the sound of the amp. I have used .10 uF, .22 uF and .33 uF caps at various times. In checking with Roy Mottram, the designer of the VTA driver board, he did mention that larger caps will (very slightly) drop the frequency of low frequency roll off point. At least on the VTA driver board, Roy figured maybe a .10 uF coupling cap would give a roll off point @ 17 Hz, .22 @ about 13 Hz and .33 uF @ about 10 Hz. All are below the level of audibility and probably below the level of frequencies that the output transformers can reproduce especially at higher volume levels. You can also run into issues with too high a value. Excessive capacitance in a coupling cap, especially if the amp has negative feedback, can cause bass peaks and/or "motorboating" or amp instability driving difficult loads. Check the link below from the DIYAudio forum on this subject.

    .10 uF uF, .22 uF or .47 uF coupling caps?

    Bob

    sailor

    Posts: 249
    Join date: 2011-04-04

    Re: Coupling capacitor values for MK-IV build.

    Post by sailor on Wed Aug 08, 2012 7:43 pm

    If you know the load ahead {usually a resistor going to ground} of the capacitor then you can figure the lowest value in MFD that you can use. I found the below calculator. Remember you will be down -3db at the hz you put in. To be flat you need to put in one octave lower. If you want flat to 20 hz put in 10 hz. My SCA has 470k resistors after the cap. so a .03 mfd is all that is needed. The .1 mfd that Dynaco installed were more than enough to get full response. If I remember correctly my Stereo 70 has a 270,000 so I need at least a .06 mfd again Dynaco installed a .1 mfd. I use to use way oversized Caps. now I think a little over is better. The only exception that I can think of is preamps because the designer does not know if the amp. you hook it to has an input impedance of 20k or 400k. So he has to put in a cap for the 20K.

    http://www.v-cap.com/coupling-capacitor-calculator.php

    If I didn't make this clear please let me know.

    sailor

    Posts: 249
    Join date: 2011-04-04

    Re: Coupling capacitor values for MK-IV build.

    Post by sailor on Wed Aug 08, 2012 7:55 pm

    I just calculated for the Dyanco Mark 4. The .1 caps with the 270,000 load resistors will take you flat down to 12 HZ. So .1 should be fine .2 will lower it to 6 HZ.

    Stevedel

    Posts: 5
    Join date: 2012-02-15

    Re: Coupling capacitor values for MK-IV build.

    Post by Stevedel on Thu Aug 09, 2012 11:37 pm

    Thanks guys.
    I guess that there is no real reason to second guess the MK-IV designers in this instance. I will stick with the original .10 MFD value.
    I'll figure out something else to change. Maybe to super-duper low noise resistors, or some such thing.
    Any suggestions are more than welcomed.
    Regards, Stevedel

    Jim McShane

    Posts: 122
    Join date: 2011-10-19
    Location: South Suburban Chicago

    Re: Coupling capacitor values for MK-IV build.

    Post by Jim McShane on Fri Aug 10, 2012 3:15 pm

    sailor wrote:I just calculated for the Dyanco Mark 4. The .1 caps with the 270,000 load resistors will take you flat down to 12 HZ. So .1 should be fine .2 will lower it to 6 HZ.

    You might want to double check your math on this one.

    The formula for the corner frequency (-3 db point) is:

    159155/(resistance in Ohms * cap value in uf)

    The "magic number" 159155 is derived from 1000000 / (2 * pi). The 1000000 eliminates the need to state capacitance in farads, we can use the much more common microfarad (uf).

    The resistor is the usually the grid resistor of the next stage (if we are talking about output tube coupling caps).

    So if you plug in the numbers you get 159155/(270,000 * .1), which is 5.89 Hz. Keep in mind that's the point at which the frequency response is down 3db - so there is some drop at frequencies immediately above our calculated number. So if indeed the corner frequency had been 12Hz there would likely be some loss of deep bass.

    I went to the referenced V-Cap site to see their calculator for cap value. My advice is take the idea of a corner frequency that is 1/10 of the lowest frequency of interest (as they call for) with a large grain of salt. A corner frequency too low has some problems just as a frequency that's too high has some. But if you want the lowest frequency of interest to be 20 Hz, then the V-Cap recommended low frequency corner would be 2 Hz! That's quite problematic IMHO for a number of reasons.

    My rule of thumb is the low frequency corner should be 1.5 to 2 octaves below the lowest frequency of interest. In the Mk IV it is 6 Hz - just a bit over 1.5 octaves below 20 Hz. In a Harman-Kardon Citation II it's 7.2 Hz - again about 1.5 octaves below the lowest frequency of interest (20 Hz).

    One reason Bob Latino might not have heard a difference when he changed coupling cap values is due to the global negative feedback used in the Dynas/VTA amps. The feedback tends to minimize differences such as those. If the amp was run with no feedback it's likely Bob could have heard a very minor difference - but the amp uses NFB, so he didn't. As well, the stock 6 Hz corner frequency is plenty low enough.

    Follow Bob's excellent advice - stick with the coupling cap values he recommended. Too LARGE a coupling cap can cause at least as many problems as it cures - for instance, look up "blocking distortion" when you get a chance...


    Last edited by Jim McShane on Fri Aug 10, 2012 3:19 pm; edited 1 time in total (Reason for editing : Clarification of two points)

    Bob Latino
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    Re: Coupling capacitor values for MK-IV build.

    Post by Bob Latino on Fri Aug 10, 2012 3:48 pm

    Jim,

    Many thanks for your clarification on this subject ...

    Bob

    sailor

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    Join date: 2011-04-04

    Re: Coupling capacitor values for MK-IV build.

    Post by sailor on Fri Aug 10, 2012 9:05 pm

    Jim, I never said 12 HZ. was the -3db frequency. I took the number you came up with 5.89hz and moved it up one octave and rounded it up to 12 Hz. The exact answer is 11.78 HZ a .2 cap would be 5.89 for flat response.
    Why make this more difficult than it needs to be. A cap blocks AC the same way if it is in the circuit of an amp or in a speaker. No one would ever say I need my tweeter to roll off at a certain frequency which say requires a 4 mfd cap but I think it would sound better if I changed it to a 8 MFD cap.
    I think most people, including myself up until a year ago, have installed larger capacitors than necessary because most people don't know what happens when you increase the MFD value of a cap. After all bigger must be better.
    The site above makes it simple to understand and easy to figure for everyone.
    I also use calculators on the internet to build crossovers, to figure Ohms law and a dozen other kinds of electronic problems.

    sailor

    Posts: 249
    Join date: 2011-04-04

    Re: Coupling capacitor values for MK-IV build.

    Post by sailor on Fri Aug 10, 2012 9:37 pm

    Ok, I used mfd instead of uf. It's the same thing. MFD is the industrial symbol for microfarad.

    Jim McShane

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    Join date: 2011-10-19
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    Re: Coupling capacitor values for MK-IV build.

    Post by Jim McShane on Fri Aug 10, 2012 10:09 pm

    sailor wrote:Jim, I never said 12 HZ. was the -3db frequency. I took the number you came up with 5.89hz and moved it up one octave and rounded it up to 12 Hz. The exact answer is 11.78 HZ a .2 cap would be 5.89 for flat response.

    Thank you for clarifying - it wasn't clear from your prior post (or I simply misunderstood your statement), so I wanted to clarify it in mine.

    Also - a single C-R section rolls off the signal at a rate of -6db per octave, so if the "flat" frequency - the beginning of the rolloff - was 12 Hz, then the signal would be down 6db (not 3db) at 6 Hz, 6 Hz being one octave below 12Hz. If the rolloff started at 12Hz the -3db point would be about 8.5 Hz.

    Why make this more difficult than it needs to be.

    If I may... why be so defensive? I was only trying to help by sharing some useful info. I'm sorry if you felt I was calling you out for some reason - that was certainly not my intent. I just wanted to share some things I hoped would be interesting and worthwhile with all the forum readers.

    But it IS more complex than it first appears. For instance, what if the original designer used a smaller cap value to limit the low frequency energy to an output trafo that can't handle a lot of deep bass? Or in an amp with more than two stages, maybe the amp won't be stable unless the two poles (another term for corner frequency) are staggered a good ways apart in their -3db frequencies? One rule of amp design is that the farther the multiple poles are apart the more stable (all else being equal) the amp. Often that requires some compromise to get adequate spacing to ensure stability.

    A cap blocks AC the same way if it is in the circuit of an amp or in a speaker.


    I think you meant DC, not AC, since a cap will pass an AC signal but block DC.

    In any case, as long as the cap is part of an identical network you've pretty much got it. There are some differences due to the inductive nature of the load seen by the C-R circuit in a speaker crossover network, but for our purposes here we can ignore that for now.

    No one would ever say I need my tweeter to roll off at a certain frequency which say requires a 4 mfd cap but I think it would sound better if I changed it to a 8 MFD cap.

    Actually that sort of thing happens all the time - not only the frequency but the rate of the rolloff has to be carefully considered when you work with a tweeter. Remember, a 1st order (single C-R section) high pass filter only rolls off at 6db per octave. So the tweeter will see significant energy a full octave lower than the frequency at which the rolloff begins. The tweeter may be fine at 3K - but an octave lower (at 1.5K) it could have performance and/or durability problems. There are also phase issues involved.

    I think most people, including myself up until a year ago, have installed larger capacitors than necessary because most people don't know what happens when you increase the MFD value of a cap. After all bigger must be better.

    There was a time I did too!

    The site above makes it simple to understand and easy to figure for everyone.

    Yes, the site calculator does the math for you very well. But the recommendation for determining the low frequency corner (1/10 the lowest frequency of interest) might well cause trouble in a variety of ways. So use the calculator, but I would recommend you not use the "1/10th" recommendation.

    I also use calculators on the internet to build crossovers, to figure Ohms law and a dozen other kinds of electronic problems.

    That's great, the web is FULL of useful tools like that. But use the results carefully to get the best results.

    Incidentally, if you want to read a NICE piece explaining blocking distortion, amp designer Randall Aiken has a great page on it here:

    Blocking Distortion Explained

    Jim McShane

    Posts: 122
    Join date: 2011-10-19
    Location: South Suburban Chicago

    Re: Coupling capacitor values for MK-IV build.

    Post by Jim McShane on Fri Aug 10, 2012 10:26 pm

    sailor wrote:Ok, I used mfd instead of uf. It's the same thing. MFD is the industrial symbol for microfarad.

    It is one of a number of abbreviations for microfarad, but again just FYI - it is often considered obsolete. Kind of like the term micro-microfarad (abbreviated mmf) is now picofarad (pf), the abbreviation now is the Greek letter mu followed by a capital F. But on the web it can be difficult to insert the symbol for mu, so the "uf" or "uF" abbreviations have kind of become de facto standards.

    IIRC, the reason for the change was that MFD was sometimes misinterpreted as millifarads (1 millifarad = 1,000 microfarads). But your post was clear, no problem knowing what value you meant.

    anbitet66

    Posts: 119
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    Location: Valley Stream, NY

    Re: Coupling capacitor values for MK-IV build.

    Post by anbitet66 on Sat Aug 11, 2012 10:28 am

    When I was in high school taking electronics in a vocational program my teacher wouldn't accept mf, MF, mF, etc... It had to be µF. How to go about it on a computer? Press and hold the alt key then type in 230, followed by capital F. tongue Tony

    Jim McShane

    Posts: 122
    Join date: 2011-10-19
    Location: South Suburban Chicago

    Re: Coupling capacitor values for MK-IV build.

    Post by Jim McShane on Sat Aug 11, 2012 2:01 pm

    Tony, thanks a lot for the tip!!


    Stevedel

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    Join date: 2012-02-15

    Re: Coupling capacitor values for MK-IV build.

    Post by Stevedel on Sun Aug 12, 2012 12:46 pm

    What is the input impedance of a MK-IV? I thought it was 470,000 Ohms.
    Forget what I said about accepting the designers specs.
    I checked the V-cap website and I want to play.
    Thanks all.

    Bob Latino
    Admin

    Posts: 1970
    Join date: 2008-11-26
    Location: Massachusetts

    Re: Coupling capacitor values for MK-IV build.

    Post by Bob Latino on Sun Aug 12, 2012 12:58 pm

    Hi,

    On a stock Mark IV the input impedance is 470,000 ohms. On a Mark IV with the VTA driver board the input impedance is 270,000 ohms. You can see the 470K resistor on the schematic between eyelets 1 and 2 on the Mark IV driver board. It is interesting that on a stock ST-70 these two 470K resisitors (one for each channel) are on the input jacks/stereo mono switch and not on the ST-70's driver board.

    Bob



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