Using bipolar caps in place of polarized ones?

[MisterFishey]

I am recapping a Technics SL-P2 CDP, and what the service manual said was different from what was in the unit. :dammit: Lesson learned! But, anyway, I ordered bi-polar caps, but there are currently polarized ones on the board. Both are electrolytic. Is it ok to use the bipolar ones?

 

[EchoWars]

Bipolar electrolytics are, in general, inferior to standard electrolytics. If the cap actually has a decent polarizing voltage in it when in-circuit, a standard electrolytic is much preferred.

 

[ecluser]

"decent polarizing voltage"...

In some applications, a bipolar cap is far superior to a polar cap. For instance, on the input side of many power amplifiers, there is a capacitor just before the base of the input transistor. The DC voltage at this point is quite low, usually only a few mV.

The AC input signal at this point can be as high as ± 500mV. Thus, a polar cap is reversed bias 50% of the time, by the audio signal itself (at very low frequencies). This is one cause of distortion at low frequencies in audio amplifiers.

 

[Schnitzer]

I saw a posting in another forum a while back that recommended using bipolar caps in the signal path. His argument was that, although bipolars exhibit higher distortion levels than polarized caps, they produce even harmonics which are audibly less objectionable than odd harmonics produced by polarized caps. Judging by that writer's other postings, he seemed to be quite knowledgeable.

I have done this to a vintage HK630 receiver and am pleased with the sound it delivers.

Comments?

My intention is not to get emotional and off-the-wall responses, just some realistic feedback.

 

[Tripqzon]

The OP doesn't mention whether the caps he speaking of are in the signal path or not.

 

[avionic]

I am recapping a Technics SL-P2 CDP, and what the service manual said was different from what was in the unit. :dammit: Lesson learned! But, anyway, I ordered bi-polar caps, but there are currently polarized ones on the board. Both are electrolytic. Is it ok to use the bipolar ones?

Personally ,I would stick to what ever was originally in the circuit and working.:scratch2:

 

[MisterFishey]

The OP doesn't mention whether the caps he speaking of are in the signal path or not.

The schematic is somewhat ambiguous, but I believe they are in the signal path. Pretty sure.

 

[EchoWars]

"decent polarizing voltage"...

In some applications, a bipolar cap is far superior to a polar cap. For instance, on the input side of many power amplifiers, there is a capacitor just before the base of the input transistor. The DC voltage at this point is quite low, usually only a few mV.

The AC input signal at this point can be as high as ± 500mV. Thus, a polar cap is reversed bias 50% of the time, by the audio signal itself (at very low frequencies). This is one cause of distortion at low frequencies in audio amplifiers.

I didn't know that we were talking specifics. He asked about bipolar electrolytic caps, which are, by design, essentially two back-to-back polarized electrolytic caps in one body. There aren't many applications where two electrolytic caps in series might be considered superior to one electrolytic cap.

In the application you mention, a film cap is best.

 

[d3imlay]

In many amplifiers the feedback loop consists of a voltage divider made up of primarily two resistors and an electrolytic cap. This cap is almost always a polarized cap. Shouldn't it be non polar? It may only be millivolts, but the cap has to go below zero. See attached pdf.

 

[soundmotor]

I didn't know that we were talking specifics. He asked about bipolar electrolytic caps, which are, by design, essentially two back-to-back polarized electrolytic caps in one body. There aren't many applications where two electrolytic caps in series might be considered superior to one electrolytic cap.

In the application you mention, a film cap is best.

That is a major statement considering nearly every loudspeaker use bipolar capacitors somewhere in the signal path.

:scratch2:

 

[ecluser]

In many amplifiers the feedback loop consists of a voltage divider made up of primarily two resistors and an electrolytic cap. This cap is almost always a polarized cap. Shouldn't it be non polar? It may only be millivolts, but the cap has to go below zero. See attached pdf.

You are right, but in this position the cap must have a very large value for extended bass response. Unfortunately, non polar cap are physically larger than polar cap of the same value. It is not always practical to use a non polar cap in this application.

We can have very good results if we bypass the actual polar cap by a small (capacitance 1/1000 to 1/100 of the actual cap) film cap.

See this paper about capacitors:

http://www.reliablecapacitors.com/pickcap.htm

Good readings!

 

[ecluser]

I didn't know that we were talking specifics. He asked about bipolar electrolytic caps, which are, by design, essentially two back-to-back polarized electrolytic caps in one body. There aren't many applications where two electrolytic caps in series might be considered superior to one electrolytic cap.

In the application you mention, a film cap is best.

I saw a posting in another forum a while back that recommended using bipolar caps in the signal path. His argument was that, although bipolars exhibit higher distortion levels than polarized caps, they produce even harmonics which are audibly less objectionable than odd harmonics produced by polarized caps. Judging by that writer's other postings, he seemed to be quite knowledgeable.

I have done this to a vintage HK630 receiver and am pleased with the sound it delivers.

Comments?

My intention is not to get emotional and off-the-wall responses, just some realistic feedback.

Bipolar electrolytic capacitors, or back-to-back polar caps, or side-by-side polar caps in opposite directions (less preferable) exhibit lower THD than polar caps. They are more symetrical devices and they have even order distortion.

Polar caps can be seen as good caps shunted by an imperfect diode. Anyone who ever measured the leakage current in a polar cap knows this. Thus, they have a non symetrical behavior and they have much more odd order distortion than non polar caps.

If the voltage across a cap is always the same polarity, whatever the amplitude of the audio signal, like the output of a cap coupled amplifier (Marantz 1060 as an example), a polar cap is better. It is a good idea in this application to bypass the polar cap by a film cap.

 

[EchoWars]

That is a major statement considering nearly every loudspeaker use bipolar capacitors somewhere in the signal path.

:scratch2:

You totally missed the operative word here: electrolytic. Your reply to my previous post borders on a non-sequitur.

 

[SolderIron]

>Shouldn't it be non polar? It may only be millivolts, but the cap has to go below zero.

I have not verified this, but I read somewhere that for small negative volts the cap still works normal. Maybe it is true as a regular cap is used in many feedback. I would have put in a bipolar.

 

[soundmotor]

You totally missed the operative word here: electrolytic. Your reply to my previous post borders on a non-sequitur.

No need for the 'tude oh surly one.

What do you think a bipolar capacitor is in a loudspeaker?

It can be film, it is more often an electrolytic.

 

[ccheath]

It can be film, it is more often an electrolytic.

I agree. Electrolytics are cheaper. Film are preferred but pricey and sometimes very pricey.

 

[dnewma04]

No need for the 'tude oh surly one.

What do you think a bipolar capacitor is in a loudspeaker?

It can be film, it is more often an electrolytic.

Well, he did say there aren't many applications where required. A bipolar is obviously superior to a cap that would not work in an application. However, his advice is dead on that in the case where bi-polar electrolytics are used, it would be nearly universally accepted that a film cap would be superior to electrolytic, would it not? Aside from an accountants or engineers POV.

Am I missing something here?

 

[ecluser]

...in the case where bi-polar electrolytics are used, it would be nearly universally accepted that a film cap would be superior to electrolytic, would it not?

Not necessarily. For the same capacitance, film caps are usually much larger in size. There may be more RFI picked up by the cap's external layer. If this side of the cap is connected to ground, it's not so bad. But think about the (DC blocking) input cap in an audio amp, this cap is not connected to ground. There may be more RFI problems in an amplifier if you replace the electrolytic input cap by a film cap of the same value. You may have better performances if you keep the electrolytic cap and bypass it by a small film cap.

 

[dnewma04]

I was going to mention space/money as the two constraints but decided to just say money because they go hand in hand. The RFI thing I hadnt considered, but I don't know enough about the effects to agree or disagree with you. Thanks for pointing it out.

 

[EchoWars]

No need for the 'tude oh surly one.

No 'tude here, was just trying to figure out what the hell you were talking about.

RE: Swapping film for electro's. RFI is rarely a problem. Good circuit design should preclude it from being so.

In a speaker, electro's should be avoided at all costs, if possible. And RFI doesn't even enter into the equation.