mkoijn

input decoupling caps
« on: April 06, 2008, 09:54:26 AM »
Hi, I notice that on nearly all transformerless preamp schematics, the input caps are 47uF. As I've often read that elctrolytics are not desirable on the input path, what other possibilities exist for the input caps?


flintan

input decoupling caps
« Reply #1 on: April 06, 2008, 10:01:57 AM »
Maybe something like this?

Available up to 330µF, but big and expensive of course..  :cool:

jensenmann

input decoupling caps
« Reply #2 on: April 06, 2008, 11:02:11 AM »
If foil caps become huge then they get sensitive to EMR. Not so desirable in a preamp. I had to tinker a lot in my supergreen preamps to get rid of noise though the input caps were only 10uF
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

rodabod

input decoupling caps
« Reply #3 on: April 06, 2008, 11:13:20 AM »
DC servos?
Quote from: tv
punchy fat bastard chip

MartyMart

input decoupling caps
« Reply #4 on: April 06, 2008, 11:15:53 AM »
The good old SSL9k has just 2 x sets of 2u2 poly caps, can we follow this
with other designs and what makes that setup "work" in the 9k ??
Is it the following LM394 setup gain stage ?

MM.
"Success is the ability to go from one failure to another with no loss of enthusiasm"

JohnRoberts

Re: input decoupling caps
« Reply #5 on: April 06, 2008, 11:37:46 AM »
Quote from: "mkoijn"
Hi, I notice that on nearly all transformerless preamp schematics, the input caps are 47uF. As I've often read that elctrolytics are not desirable on the input path, what other possibilities exist for the input caps?


There has been some discussion here about DC coupling the input stage and shifting the cap coupling to post that gain stage where it can be managed with a better film cap.

Otherwise for good noise and LF CMR you are stuck with a 22-47uf typical input cap value that is only practical from electrolytic.

JR
It's nice to be nice....

mkoijn

input decoupling caps
« Reply #6 on: April 07, 2008, 04:05:47 AM »
Thanks, I may look into the dc input servo ides. Do electrolytics really degrade the signal path so much?

clintrubber

input decoupling caps
« Reply #7 on: April 07, 2008, 05:02:46 AM »
Quote from: mkoijn
Thanks, I may look into the dc input servo ides.

Quote
Do electrolytics really degrade the signal path so much?

You could spend ages discussing that, and it has been done, see the threads here and everywhere on the net.
Alternative is to read a bit, for instance this series:

C. Bateman, Capacitor Sounds, Electronics World, July 2002, pp. 12-18
C. Bateman, Capacitor Sounds 2, Electronics World, September 2002, pp. 16-22
C. Bateman, Capacitor Sounds 3, Electronics World, October 2002, pp. 12-18
C. Bateman, Capacitor Sounds 4, Electronics World, November 2002, pp. 40-50
C. Bateman, Capacitor Sounds 5, Electronics World, December 2002, pp. 44-51
C. Bateman, Capacitor Sounds 6, Electronics World, January 2003, pp. 44-51

To be found here, among other stuff:
http://www.groupdiy.com/index.php?topic=15803


Note that what comes out of the cap at the right is as as much the result of the very cap itself as well as the music you put into it at the left plate.
Better not overspend time & effort on the cap but keep making music as well.

MartyMart

input decoupling caps
« Reply #8 on: April 07, 2008, 05:10:28 AM »
Interesting input servo in this link :
http://www.groupdiy.com/index.php?topic=27119

Marty.
"Success is the ability to go from one failure to another with no loss of enthusiasm"

clintrubber

input decoupling caps
« Reply #9 on: April 07, 2008, 05:28:34 AM »
BTW, sorry for nitpicking, but let's call it input coupling caps.
Decoupling is to smooth supplies.

Cheers,

  Peter


rodabod

input decoupling caps
« Reply #10 on: April 07, 2008, 05:54:47 AM »
Quote from: "clintrubber"
BTW, sorry for nitpicking, but let's call it input coupling caps.
Decoupling is to smooth supplies.


Hey Peter. When do you decide which is which? I usually always call it decoupling as we are blocking DC components and hence they have been decoupled from the signal. I just call power supply filtering "filtering". I've never been that sure.
Quote from: tv
punchy fat bastard chip

clintrubber

input decoupling caps
« Reply #11 on: April 07, 2008, 06:25:23 AM »
Quote from: "rodabod"
Quote from: "clintrubber"
BTW, sorry for nitpicking, but let's call it input coupling caps.
Decoupling is to smooth supplies.


Hey Peter. When do you decide which is which? I usually always call it decoupling as we are blocking DC components and hence they have been decoupled from the signal. I just call power supply filtering "filtering". I've never been that sure.

That's how I always thought it without much further thinking, but I may not even be correct there, so let's check:

http://en.wikipedia.org/wiki/Decoupling_capacitor

http://en.wikipedia.org/wiki/Coupling_capacitor

Bye,

  Peter

input decoupling caps
« Reply #12 on: April 07, 2008, 08:48:23 AM »
Quote from: "jensenmann"
If foil caps become huge then they get sensitive to EMR. Not so desirable in a preamp. I had to tinker a lot in my supergreen preamps to get rid of noise though the input caps were only 10uF


This came up at Bruno Putzeys' forum recently: http://recforums.prosoundweb.com/index.php/t/21314/0/

Check the Sony photo.

Justin
Prepare yourself. You are about to become the voice of Interplanetary Parliament.

input decoupling caps
« Reply #13 on: April 07, 2008, 08:49:52 AM »
Quote from: "flintan"
Maybe something like this?

Available up to 330µF, but big and expensive of course..  :cool:


I have these in my power amps - nice!


Justin
Prepare yourself. You are about to become the voice of Interplanetary Parliament.

JohnRoberts

input decoupling caps
« Reply #14 on: April 07, 2008, 11:09:31 AM »
The "badness" of electrolytic caps is somewhat undeserved, for high impedance applications and can be made better or worse by how used.

The primary consideration for how much non-ideal behavior you will experience with electrolytic dielectric is a function of (AC) terminal voltage and series current.

The terminal voltage is reduced by using larger values. The series current will be impacted by where the nominal 1.5-2k mic termination is located. If the 2k termination is mostly on the mic side of the coupling caps, they only need to drive the much larger value resistors needed to establish a dc path for following high impedance electronics.  So the same exact capacitors can act differently in a similar preamp depending on where the termination is located.

Note: in a transformer input you can't change where the termination occurs, but you also don't need capacitor isolation of phantom DC.

Note2: In passive speaker crossovers you have both high current and high AC terminal voltage, worst of both worlds for electrolytic.

JR
It's nice to be nice....

rodabod

input decoupling caps
« Reply #15 on: April 07, 2008, 11:22:01 AM »
Quote from: "clintrubber"
let's check:

http://en.wikipedia.org/wiki/Decoupling_capacitor

http://en.wikipedia.org/wiki/Coupling_capacitor


You're totally right. Thanks for that. I shall try to use the correct terminology in the future!
Quote from: tv
punchy fat bastard chip

clintrubber

input decoupling caps
« Reply #16 on: April 07, 2008, 11:45:56 AM »
Quote from: "rodabod"
Quote from: "clintrubber"
let's check:

http://en.wikipedia.org/wiki/Decoupling_capacitor

http://en.wikipedia.org/wiki/Coupling_capacitor


You're totally right. Thanks for that. I shall try to use the correct terminology in the future!

No problem. As long as people don't mix up polarity and phase they can already be my friends. All the rest is optional  :wink:

It never hurts though to use the right wording.

MartyMart

input decoupling caps
« Reply #17 on: April 07, 2008, 12:13:31 PM »
I like to use the term "bypass cap" myself for de-coupling :-0

MM
"Success is the ability to go from one failure to another with no loss of enthusiasm"

bcarso

input decoupling caps
« Reply #18 on: April 07, 2008, 02:34:05 PM »
Agree with the wikis on decoupling/coupling---always the way I understood the terms, with in fact the greater emphasis for "decoupling" on reducing interaction among subcircuits via the power rails.

Some mention of a series R or L-R between the power rail and the shunt cap would be helpful---but then I guess the wiki would be more properly titled "decoupling network".

As far as the noise pickup issue, besides the electric field interception, I've had situations where the simple fact of the larger loop area of the coupling capacitor and nearby plane or trace of signal return, or the other capacitor in a balanced arrangement, did me in.  This was on the output (!) of a crossover system where the caps used were large copper foil/paper/oil types, and the charging current spikes were modulating the stray field of a nearby big toroid.

It seemed, in principle at least, that a compensating pickup coil could be arranged so as to cancel the fields.  Better was just to move the transformer away, but this meant a bigger box or two-chassis system.

The discussion in the other forum about magnetic shielding is correct but somewhat incomplete, as it doesn't emphasize the importance of having a complete magnetic circuit for the shield.  Just substituting ferromagnetic material in place of non-, where the latter was effective for electric fields, may well make the mag field pickup problem worse.

Sometimes you can contrive a given shape and size of such material to shape the field and get improvement though, as long as the interfering source is fixed in position.  This won't help for magnetic radiation from other units in proximity though.

Svart

input decoupling caps
« Reply #19 on: April 07, 2008, 05:01:50 PM »
Some of the folks I work with whom regard themselves as learned engineers mix the two terms.  This tends to lead to a lot of head scratching.  I tend to use the terms "DC blocking" or "AC coupling" for coupling caps to give it another level of clarity.  I leave DEcoupling as it is.  This seems to be working pretty well keeping the worker bees straight.
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.


 

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