Ageing Amplifier: Concerned about Damaging Speakers?

[admin_exported]

*Cheap Speaker Protection*

Throw an electrolytic capacitor in series with each speaker. Taking advantage of the fact that a capacitor will pass AC but block DC means that there can never be a power source to burn out the speaker coil. Choose a large value, say 2,200uF. Current flows from positive to negative, so connect the positive lead of the cap to the output of the amp, and the negative side to the speaker.

*Side Effects*

Attentuation (mild) of some frequencies determined by capacitive reactance of the cap and XL (inductive reactance) of the speaker.

 

[visionary]

So, we have folk on here getting all excitable about the capacitance of speaker cable (measured in pF) and you're recommending putting a great big electrolytic (measures in uF) in series with the speaker? REALLY?!

 

[chebby]

Ageing amplifier. Get it serviced.

 

[Anonymous]

So, we have folk on here getting all excitable about the capacitance of speaker cable (measured in pF) and you're recommending putting a great big electrolytic (measures in uF) in series with the speaker? REALLY?!

It comes down to XC with respects to what and how much of the signal it will alter. Chances are that your amp is powered by a single voltage rail, and in such case the ouput will be already directly coupled to a rather large capacitor. XC can be obtained from 1 / 2 * pi fc XL (your speaker coil) 2 * pi (fl) Do the maths to understand the attenuation.

 

[The_Lhc]

So, we have folk on here getting all excitable about the capacitance of speaker cable (measured in pF) and you're recommending putting a great big electrolytic (measures in uF) in series with the speaker? REALLY?!

It comes down to XC with respects to what and how much of the signal it will alter. Chances are that your amp is powered by a single voltage rail, and in such case the ouput will be already directly coupled to a rather large capacitor. XC can be obtained from 1 / 2 * pi fc XL (your speaker coil) 2 * pi (fl) Do the maths to understand the attenuation.

I think you're seriously overestimating your audience. With a small number of exceptions this isn't really a technical forum.

 

[ID.]

So, we have folk on here getting all excitable about the capacitance of speaker cable (measured in pF) and you're recommending putting a great big electrolytic (measures in uF) in series with the speaker? REALLY?!

It comes down to XC with respects to what and how much of the signal it will alter. Chances are that your amp is powered by a single voltage rail, and in such case the ouput will be already directly coupled to a rather large capacitor. XC can be obtained from 1 / 2 * pi fc XL (your speaker coil) 2 * pi (fl) Do the maths to understand the attenuation.

I think you're seriously overestimating your audience. With a small number of exceptions this isn't really a technical forum.

I think I tend towards the paranoid. I assumed he knew that and was somehow rubbing it in.

 

[Anonymous]

So, we have folk on here getting all excitable about the capacitance of speaker cable (measured in pF) and you're recommending putting a great big electrolytic (measures in uF) in series with the speaker? REALLY?!

It comes down to XC with respects to what and how much of the signal it will alter. Chances are that your amp is powered by a single voltage rail, and in such case the ouput will be already directly coupled to a rather large capacitor. XC can be obtained from 1 / 2 * pi fc XL (your speaker coil) 2 * pi (fl) Do the maths to understand the attenuation.

I think you're seriously overestimating your audience. With a small number of exceptions this isn't really a technical forum.

I think I tend towards the paranoid. I assumed he knew that and was somehow rubbing it in.

So you want fiction not fact?

 

[The_Lhc]

So, we have folk on here getting all excitable about the capacitance of speaker cable (measured in pF) and you're recommending putting a great big electrolytic (measures in uF) in series with the speaker? REALLY?!

It comes down to XC with respects to what and how much of the signal it will alter. Chances are that your amp is powered by a single voltage rail, and in such case the ouput will be already directly coupled to a rather large capacitor. XC can be obtained from 1 / 2 * pi fc XL (your speaker coil) 2 * pi (fl) Do the maths to understand the attenuation.

I think you're seriously overestimating your audience. With a small number of exceptions this isn't really a technical forum.

I think I tend towards the paranoid. I assumed he knew that and was somehow rubbing it in.

So you want fiction not fact?

All I'm saying is that your facts aren't going to be of a great deal of use to most of the members here. Most of us aren't the sort to get a soldering iron out and start fiddling about with our equipment.

 

[Anonymous]

Perhaps I might have been planting a SEED. If you want to put another bullet in capitalism, then make up a box with a capacitor inside and sell
it as a miracle on ebay for 20 bucks + FREE SHIPPING.

 

[TrevC]

So, we have folk on here getting all excitable about the capacitance of speaker cable (measured in pF) and you're recommending putting a great big electrolytic (measures in uF) in series with the speaker? REALLY?!

It'll work just fine. I did the same thing with my test speakers when checking out amplifiers. Capacitance isn't an issue with speaker cables in any case.

 

[Anonymous]

So, we have folk on here getting all excitable about the capacitance of speaker cable (measured in pF) and you're recommending putting a great big electrolytic (measures in uF) in series with the speaker? REALLY?!

It'll work just fine. I did the same thing with my test speakers when checking out amplifiers. Capacitance isn't an issue with speaker cables in any case.

The input coupling cap is usually like about 2.2uF right? Seems like another con.

 

[proffski]

2,200µF electrolytic eh?

Bit of a tall order, the terminals may well be marked red and black but what flows is AC, otherwise the speaker voice coil would have evaporated at first switch on from new! Will sound interesting... do not confuse DC blocking capacitors in single rail power supply amplifiers, they are kept biased correctly.

 

[TrevC]

2,200µF electrolytic eh?

Bit of a tall order, the terminals may well be marked red and black but what flows is AC, otherwise the speaker voice coil would have evaporated at first switch on from new! Will sound interesting... do not confuse DC blocking capacitors in single rail power supply amplifiers, they are kept biased correctly.

You could use two 4700uF wired back to back.

 

[eggontoast]

Current flows from positive to negative,

With an AC signal ?

so connect the positive lead of the cap to the output of the amp, and the negative side to the speaker.

Since the audio signal is AC it doesn't matter. If your protecting against amplifier failure and a massive DC offset it would be wiser to use a bipolar cap since you don't know if it will be a + or - DC offset. After all you don't want exploded cap everywhere too........

But overall it is totally unnecessary, anyone who is running vintage gear worth any value would have it serviced unless they are stupid, in that case they deserve to have their speaker coils fry ;-)

 

[lindsayt]

In that case I'm stupid. I'm running vintage amps that I haven't had serviced. I have no intention of having them serviced. Time will tell if I ever get fried speakers as a result.

 

[Anonymous]

Current flows from positive to negative,

With an AC signal ?

so connect the positive lead of the cap to the output of the amp, and the negative side to the speaker.

Since the audio signal is AC it doesn't matter. If your protecting against amplifier failure and a massive DC offset it would be wiser to use a bipolar cap since you don't know if it will be a + or - DC offset. After all you don't want exploded cap everywhere too........

But overall it is totally unnecessary, anyone who is running vintage gear worth any value would have it serviced unless they are stupid, in that case they deserve to have their speaker coils fry ;-)

Don't question standard procedure. AC is the hardest part of electronics. Very complex. Hard enough to digest it let alone question it.

 

[eggontoast]

Well that's a cop out if ever I've seen one.......

 

[Anonymous]

Well that's a cop out if ever I've seen one.......

I formally studied this with pass marks back in the 90s. How is it a 'cop out' to adhere to standards?

 

[Bodfish]

Well that's a cop out if ever I've seen one.......

I formally studied this with pass marks back in the 90s. How is it a 'cop out' to adhere to standards?

oh good, more edjookashun.

 

[eggontoast]

Well that's a cop out if ever I've seen one.......

I formally studied this with pass marks back in the 90s. How is it a 'cop out' to adhere to standards?

That is not a standard !

 

[Anonymous]

I will totally disregard all the schematics I've ever read. Forget about the two large books. To hell with what the guy with a diploma said to me. How does that sound?

 

[MajorFubar]

As The_LHC said, you're probably wasting your time preaching those kind of D-I-Y mods round here. Not long after I joined, someone came on here wanting a certain type of interconnect with non-standard plugs at one end. I can't remember exactly what he needed, but I remember thinking he could easily buy the plugs from Maplins, eBay and a million places elsewhere, so I suggested he soldered-up his own cable like I have on countless occasions. I didn't realise it was classed as a speciality job, I though it was just what people did as a matter of routine. No one was rude, but the tone of the responses I got was like I'd suggested he performed brain-surgery on himself or assembled his own space-rocket.

 

[proffski]

2,200µF electrolytic eh?

Bit of a tall order, the terminals may well be marked red and black but what flows is AC, otherwise the speaker voice coil would have evaporated at first switch on from new! Will sound interesting... do not confuse DC blocking capacitors in single rail power supply amplifiers, they are kept biased correctly.

You could use two 4700uF wired back to back.

So, do we have electrolytics without leakage, and dielectric absorbtion accross the entire audio bandwidth and at all drive levels?

 

[TrevC]

2,200µF electrolytic eh?

Bit of a tall order, the terminals may well be marked red and black but what flows is AC, otherwise the speaker voice coil would have evaporated at first switch on from new! Will sound interesting... do not confuse DC blocking capacitors in single rail power supply amplifiers, they are kept biased correctly.

You could use two 4700uF wired back to back.

So, do we have electrolytics without leakage, and dielectric absorbtion accross the entire audio bandwidth and at all drive levels?

We don't, but in this application we don't worry about it.

 

[andyjm]

Current flows from positive to negative,

With an AC signal ?

so connect the positive lead of the cap to the output of the amp, and the negative side to the speaker.

Since the audio signal is AC it doesn't matter. If your protecting against amplifier failure and a massive DC offset it would be wiser to use a bipolar cap since you don't know if it will be a + or - DC offset. After all you don't want exploded cap everywhere too........

But overall it is totally unnecessary, anyone who is running vintage gear worth any value would have it serviced unless they are stupid, in that case they deserve to have their speaker coils fry ;-)

Don't question standard procedure. AC is the hardest part of electronics. Very complex. Hard enough to digest it let alone question it.

Back to the textbook, Madhouse. Eggontoast is quite right, assuming you want to put a DC block in place then an electrolytic cap is very bad idea. The plus and minus (or red and black) markings on an amp's speaker outputs are primarily to help you get speaker phase correct. The output signal is AC and the plus and minus outputs will swing both positive and negative realtive to each other every AC cycle.