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Supercaps and DC offset - Page 2

post #16 of 26

It sounds like the leakage current is interacting with the resistance of whatever is connected after.  It sounds like you measured DC offset with the amp (or whatever it is, it's not clear) plugged in.  It sounds like your amp has no DC blocking caps.  If the amp has a potentiometer, it's probably interacting with it.  Depending on the potentiometer configuration, the resistance may change depending on the volume control position.

 

You'll never solve the problem with supercaps.  The leakage currents are too high, in the 10s of uA range.  "normal" caps used for DC blocking leaks in the nA range.  If you put a low value bleeder/pull-down resistor, you'll increase the current load on the ipod's output stage.  If you put it too high, nothing changes.

post #17 of 26
Thread Starter 
Quote:
Originally Posted by holland View Post
 

It sounds like the leakage current is interacting with the resistance of whatever is connected after.  It sounds like you measured DC offset with the amp (or whatever it is, it's not clear) plugged in.  It sounds like your amp has no DC blocking caps.  If the amp has a potentiometer, it's probably interacting with it.  Depending on the potentiometer configuration, the resistance may change depending on the volume control position.

 

You'll never solve the problem with supercaps.  The leakage currents are too high, in the 10s of uA range.  "normal" caps used for DC blocking leaks in the nA range.  If you put a low value bleeder/pull-down resistor, you'll increase the current load on the ipod's output stage.  If you put it too high, nothing changes.

 

What I measured is the immediate input to amp (no amp is connected). I simply turned on the ipod and plugged in a LOD cable, then measured the two channels from the 3.5mm plug end.

 

I am OK with the presence of the dc offsets as I don't hear anything unusual. I also checked with my amp manufacturer concerning the dc offsets and I am assured there is no harm done to the amp. Below is the response I got from the amp maker:

 

"A couple hundred millivolts won't make too much of a difference. The output of the amp does not have DC blocking caps in the signal path. Instead, there is an active circuit which compensates for any offset seen at the output, driving it to near zero."

post #18 of 26
Thread Starter 
I also tried 1F caps and measured less dc offsets. Yes it appears the dc offsets correlate with the capacitance.

Edit: Update on fluctuating DC offsets

OK, I have been playing this ipod (with the 3F caps) all day today at office. So I decided to measure the DC offsets after I came back home. Guess what? The DC offsets on both channels dropped to 50 mV's. Immediately, I grabbed the other ipod with the same caps (but stayed idle today) and the measured DC offsets stayed at 250 mV's.

Just a couple days ago, the measured DC offsets from both ipods were 250 mV's. Wow that's something nice to know.
Edited by DMinor - 9/16/13 at 10:29pm
post #19 of 26
Hi DM- as I mentioned before on the DIY iMod thread, and by PM, there is a formula used to select the proper value capacitor for this mod. Part of that formula is the input impedance of your amplifier.

Using this formula, there is no scenario where you would need more than 8uf capacitors in the signal path! In many instances much lower values are sufficient. The other spec you need to look at is the voltage spec on the cap. Almost any cap you choose, especially the film caps used with the mod have more than enough voltage rating. There is no harm in the voltage rating being higher than needed.

The problem with these "super" caps is a ridiculously high capacitance, and very low voltage rating. Neither is beneficial to the application. In fact damage to downstream gear is possible.

Unless you are mesmerized by the "super" moniker, ditch these and go with the proper rating. Science is your friend! The solution to the problem you are experiencing is easily remedied by going back to the film caps you used before.

I posted this also for the benefit of anyone researching the mod.....maybe they find this info useful. Lower value film caps of proper spec work beautifully.
post #20 of 26
Thread Starter 

Thanks my friend for stopping by. Yeah I know I am crazy about these 3F caps. :wink_face:

 

You are correct those 4.7uf film caps are doing just fine without any dc leaking. But once you start doing DIY stuff you can get crazy like me, as you said I am mesmerized by the "super" moniker. :p

post #21 of 26
Quote:
Originally Posted by thegrobe View Post

The problem with these "super" caps is a ridiculously high capacitance, and very low voltage rating. Neither is beneficial to the application. In fact damage to downstream gear is possible.
 

 

I disagree that high capacitance or low voltage rating (provided the parts are rated for more voltage than the application) could hurt downstream gear in this app. The poor leakage spec might, but it can be dealt with. 

 

The issue with supercaps is that they are designed to replace small batteries as I posted previously and basically no thought is given to how well they pass AC. Since this application is purely based on how well the cap passes AC they are a poor choice. 

 

To put it bluntly nothing about these caps except their capacity is super. Since you don't need the capacity, the only thing super about them is their name. 

 

A super cap for this application would have the following characteristics:

Nominal DC leakage

Low AC distortion

Reasonable physical size

Low ESR (ESR is not very important in this application, but maybe someone will find this and be saved from the poor choice of using a supercap for the power rails of their amp after considering ESR)

post #22 of 26
Quote:
Originally Posted by nikongod View Post
 

 

I disagree that high capacitance or low voltage rating (provided the parts are rated for more voltage than the application) could hurt downstream gear in this app. The poor leakage spec might, but it can be dealt with. 

 

The issue with supercaps is that they are designed to replace small batteries as I posted previously and basically no thought is given to how well they pass AC. Since this application is purely based on how well the cap passes AC they are a poor choice. 

 

To put it bluntly nothing about these caps except their capacity is super. Since you don't need the capacity, the only thing super about them is their name. 

 

A super cap for this application would have the following characteristics:

Nominal DC leakage

Low AC distortion

Reasonable physical size

Low ESR (ESR is not very important in this application, but maybe someone will find this and be saved from the poor choice of using a supercap for the power rails of their amp after considering ESR)

 

Hi Nikongod - Completely in agreement with your thoughts about super caps and what would be truly super in this application.
 
The reason I mentioned damage to downstream gear, is that on the DIY iMod thread, the voltage rating of these caps were posted. I can't remember the spec off the top of my head, but it was very very low, like a volt or two. Please anybody correct me if I am remembering this wrong, I'm not going to look it up on the thread. While the iPod isn't running any high voltages on the output, I would be more comfortable having it a little higher than a volt or two. I admit that I do not know the actual voltages in play on that circuit, but it  would stand to reason that if the proper spec cap (4.7uf film cap for example) has a voltage rating of 50v why not use that? Then you know you are covered. Mostly any cap, film or electrolytic that is sensible to use in the application will have much a much much higher voltage rating than the "super" caps. And you mentioned there are ways to deal with the poor leakage spec...Why bother? My point of this is why not use the proper cap? It has been proven by many others performing iPod mods that using the proper part works. 
 
The point of using a cap in the signal path is to block voltages from going downstream. (yes, which can damage gear) If you are using a cap that does not provide this basic function, just don't use a cap at all. If you don't care, and want to experiment with your own stuff, fine, but please don't show up to a meet with that gear. And please don't tell someone wanting to try the mod themselves that this is a a part they should use. It's obvious that the super cap is not a good choice in this application. Why search for a workaround just to have the word "super" attached to a part? 
 
I am not meaning to disagree, I just don't want someone who is researching doing the mod themselves to get confused thinking this super cap thing is a viable option. There is a formula of what value to use, there is no reason to go willy-nilly selecting anything outside those parameters. Say your car needs new tires. Can you put square tires on? Sure. But why use square tires? Round is what you need. 
post #23 of 26
Thread Starter 

I agree the 3F capacitance is theoretically overkill and unnecessary, in fact the high capacitance is the reason behind the dc offsets - no doubt about that. Thegrobe is correct. Don't try these caps unless you know what you are doing or willing to risk your gears. The main purpose of this thread, for me personally, is to find out whether the amount of dc offsets would do harm to my gears. I know it may sound stupid - if it is not necessary and only causing dc offsets, then why use it?

 

Thanks guys for the informative discussions. I always keep an open mind and listen when it comes to learning. :wink_face:

post #24 of 26
Quote:
Originally Posted by thegrobe View Post
 
The reason I mentioned damage to downstream gear, is that on the DIY iMod thread, the voltage rating of these caps were posted. I can't remember the spec off the top of my head, but it was very very low, like a volt or two. =

 

I did not realize that they may be under speced for voltage. 

My normal standard is the maximum voltage found anywhere within the device, or 5V, whichever is higher. 

 

Quote:
Originally Posted by DMinor View Post
 

I agree the 3F capacitance is theoretically overkill and unnecessary, in fact the high capacitance is the reason behind the dc offsets - no doubt about that. Thegrobe is correct. Don't try these caps unless you know what you are doing or willing to risk your gears. The main purpose of this thread, for me personally, is to find out whether the amount of dc offsets would do harm to my gears. I know it may sound stupid - if it is not necessary and only causing dc offsets, then why use it?

 

Thanks guys for the informative discussions. I always keep an open mind and listen when it comes to learning. :wink_face:

 

I would say that high leakage currents are the cause of the DC offset. 

 

"Don't try these caps unless you know what you are doing" is kind of a trick phrase. Could you restate it? 

 

I have stated several times what these caps are intended for. plox go read. 

post #25 of 26
Thread Starter 
Quote:
Originally Posted by nikongod View Post

 

"Don't try these caps unless you know what you are doing" is kind of a trick phrase. Could you restate it? 

 

 

 

What I was trying to say is I didn't know what I was doing other than experimenting it. The potential harm to gears from the dc leaking can be gear specific. So just because it hasn't happened to mine doesn't mean it won't do harm to others'.

 

Thanks

post #26 of 26
Thread Starter 

Just to update the thread. I now have zero dc offsets using a pair of different supercaps (without using resistors). Apparently the presence of dc offsets is not necessarily due to the high capacitance. This is the best experiment I have had.

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