AR Speaker Protection from DC Current

[xpat]

Background:

I've been reading about amplifiers (without protection circuits) that fail and send DC output to the speakers. The woofer gets fried but the mid and tweeter are OK because capacitors (CAPs) in the crossover block DC current. Also, some amps with relay protection circuits also fail and send DC current to the speakers. Additionally, fuses will typically not help with the DC problem. As an owner of AR9s, AR5s and AR6s, I'm concerned.

Questions for engineers in this group:

1) Is it possible to put a large capacitor between the amp and speaker to protect the woofer from DC current?

2) Would this be detrimental to a solid state amplifer in the short or long term?

3) If CAP protection is feasible, what size would need to be used so the quality of the sound would not be affected? I read that at least 1000uf would be needed.

Your help would be greatly appreciated.

[Carlspeak]

Take a look at the AR LST2 schematic in the library. It has a very high cap value ahead of everything (on the neg. side). There's proof it's already been done. However, the LST's seems an isolated case and I don't know why. http://www.classicspeakerpages.net/library/acoustic_research/original_models_1954-1974/original_models_schematicss/

This raises an additional question. With the 1000 uF cap on the neg. side, what protection is there if the DC current surge happens on the plus side? I suspect, hopefully nothing because the cap on the neg. side prevents the current from completing its journey thru the speaker.

[xpat]

Hi Carlspeak,

Thanks for the information - I never would have found it on my own!

FYI, I'm not an engineer and barely can read a circuit diagram. However, I did find the "C1" cap in the diagram. I'm hoping you can help again. From the diagram, how is it determined that C1 is 1000uf? (I see it in the parts list, but the list doesn't show equivalencies (e.g., c1=1000uf).

Secondly, I noted a 10 ohm resistor on the negative side. Do you have any idea why that resistor is way ahead of everything else?

Regards,

xpat

[Carlspeak]

You'll have to trust me because I have a copy of the LST schematic and it's wired basically the same way.

Note the fuse also in series with the caps.

There may be a LST schematic somewhere in a thread, but I don't have time to find it. Kent?

[RoyC]

LST schematic:

[xmas111]

Here's the schematic.

John

looks like Roy beat me to it....

[xpat]

Carlspeak, RoyC and xmas111,

Thanks for all of the information you provided - it is appreciated!! I guess my next step is to buy some 1000 mfd caps from Parts Express and see what happens. I have a big learning curve here and need to do some tests - it looks like fun (if I don't blow my amp). I can let you know the results if interested.

Regards,

xpat

[ben76]

Two points are worth mentioning:

1. Yes, the capacitor on the negative side is enough to prevent the DC from completing the circuit. It can also be placed only on the positive side, with the same result.

By the way, calling the connections positive and negative is somewhat misleading since the speakers are being fed AC which changes polarity all the time. In other words, on average, half the time the 'negative' connection is actually electrically more positive than the 'positive' one.

The main reason the two connections are given different names and are usually colored black and red is so that one can keep the different speakers in a system (for example the two speakers in a stereo configuration) in phase. I.e., so that when the diaphragms of the drivers in one speaker move forward those in the other speaker also move forward and not backward. Ideally, even with one speaker, one would want the diaphragms to move forward at points in the music when the recording microphone was pushed in by the sound, and move backward at points when the microphone was pulled out.

2. Please note that, as usual in crossover circuits, you need a non-polar capacitor (i.e., a capacitor that does not have + or - marked on it) because during normal operation the speaker is fed AC which changes polarity all the time.

Film capacitors are non-polar, but a 1000 microfarad one would be physically very big (and expensive).

Electrolytic capacitors are usually used when such large values are needed. However, they are polar and *will explode violently* if used in an AC circuit like this. The solution though is very simple: by connecting two identical electrolytic capacitors in *series* with their positive sides connected to each other and the negative sides connected to the outside circuit (or negatives together, and positives to the rest of the circuit, it does not matter) one gets a non-polar capacitor of half the capacitance (and the same voltage rating as one capacitor). For example, connecting two polar capacitors of 2000uF will give you a non-polar one of 1000uF. Please note that you must make sure that you do not connect the capacitors in a way that the outside circuit sees one positive terminal and one negative terminal, as this results in a polar capacitor. One can also buy ready-made non-polar electrolytic capacitors that are simply 2 polar capacitors in series in the same housing.

You can see in the LST schematic that this is exactly what AR did: the parallel combination of C4 and C5 behaves like one polar capacitor (with a capacitance which is the sum of C4 and C5) and it is connected in series (positive terminals tied together, negative terminals to the outside circuit) with the parallel combination of C6 and C7, resulting (assuming all 4 capacitors are of the same value) in one non-polar capacitor of the same value and voltage rating as each of the individual 4 capacitors.

[xpat]

Ben76

Thanks so much for the information - and for making it understandable!! I'll go with the Electrolytic capacitors and connect them as noted in your comments.

Just curious, do you have any idea if using a capacitor in this manner can harm the power amp? From everything I've seen so far, it doesn't seem to be detrimental. But I would appreciate your input.

Regards,

xpat

[Carlspeak]

IMO, using polar lytic caps is confusion and could lead to problems if everthing isn't oriented correctly. OTOH, using non-polar caps negates the need to worry about +'s and -'s and which direction they are in. I suggest you get your crossover caps from suppliers like Parts Express or Madisound or Meniscus. ALL of what they offer will be non-polar for crossover applications because the current passing thru loudspeakers is AC.

[ben76]

xpat, I have yet to see a power amplifier that will be damaged by this. Many power amplifiers actually have exactly such a big capacitor in their output, and with these amps one does not have to worry about DC hitting the speakers. It is theoretically conceivable that a power amp may be designed to require a DC path (i.e., a path without a capacitor) through the speakers, but I have never seen one (even then I fail to see how putting the capacitor would damage anything). When I think of how power amplifier output stages are constructed it is hard for me to imagine such a situation. In short, if it were me I would not hesitate to put that capacitor in. Obviously, the usual disclaimer that it is your decision and your responsibility applies...

I agree with Carl that the safest way is to buy a ready-made non-polar crossover capacitor from a reputable supplier. This way you can not make any mistake. However, as long as you trust yourself not to make a mistake in wiring the capacitors (like AR trusted themselves when constructing the LST), and if you triple check your work, you will be fine.

Good Luck.

[ben76]

One more thing. I see that your original question regarding the size of the capacitor was not directly addressed (except indirectly by providing the LST diagram which uses 5000uF). In my replies I have used 1000uF as an example in the calculations, but this value is way too small .

The capacitor and the speaker act as a voltage divider where the AC voltage from the amplifier that reaches the speaker is proportional to the impedance ratio between the speaker and the capacitor. The bigger the capacitor the lower its impedance and the more 'invisible' it is, i.e., the more energy goes to the speaker as intended.

The impedance of both the capacitor and the speaker is different at different frequencies. For the speaker the connection between frequency and impedance is complicated. Saying that a speaker has an impedance of 4ohm or 8ohm is a crude simplification. For some speakers AR actually provided a graph showing impedance vs. frequency. For practical purposes at audio frequencies, if we ignore the parasitic ESR of a capacitor (don't worry about ESR - it should be pretty low for a new modern capacitor - I mention it so that the experts will not complain), the impedance of a capacitor goes down linearly with increasing frequency. I.e., double the frequency, half the impedance.

So, what does this all mean? Very simple: we have to design for the worst case (the frequency for which the capacitor impedance is highest compared to the speaker's impedance) which would for all practical purposes be the lowest frequency the speaker can produce.

Let's consider your toughest client: the AR9 (because it has low impedance and can sound very low frequencies), which has appreciable ability to go as low as 20Hz. According to the AR9 manual its impedance at that frequency is 8ohm (it is worse - i.e. lower - at higher frequencies, but at these frequencies the capacitor will have a very low impedance, so we are fine). If you use a capacitor of 1000uF it will have an impedance of almost 8ohm at 20Hz. Thus, the ratio of impedances is 1:1 and the speaker will get only half the voltage the amplifier is outputting at the 20Hz portions of music (I remind you that most music has negligible components at 20Hz, but it makes no sense to cripple an AR9 with such a capacitor). At 40Hz the capacitor's impedance will be 4ohm and 2/3 of the amplifier's output voltage will go into the speaker. Not too terrible, but not good. Note also that we have created a non-linear response curve for our combined speaker-capacitor - the lower the frequency the lower the volume we hear. Not very good.

If we go with a 5,000uF capacitor (like the LST) its impedance at 20HZ is only 1.6ohm, and the speaker will get 5/6 of the amplifier's output voltage. At 40Hz it will get 10/11 of the voltage. This is very good. If you go with 10,000uF the speaker will get 10/11 of the voltage already at 20Hz. There is hardly any point trying to improve on this.

In short, 5,000uF is very good. 10,000uF if you want to go all the way. Also, I suggest that you go with capacitors rated for at least 100 Volts, which by the way is what the AR9 uses in its crossover.

[fran604g]

Great topic, folks! I've wondered aloud many times in the past about the caps in a crossover preventing damage to the speakers if an amp shorts and sends DC to the speaker. I've long had a theory that the cap(s), if present, should prevent damage, however, I don't have but the very slightest knowledge to prove my assumption.

Thanks for the lesson!

[Gerry S]

How about in-line fuses for protection? Didn't AR recommend a slow-blow Fusetron for the AR3a and 10Pi?

[xpat]

Ben76,

Once again, you provided some great information on capacitors, their effect on impedance, and how they can impact the sound emanating from a speaker. I’m looking forward to getting this project moving after the holiday(s). It looks like 5000 mfd is the way to go – not much impact on the sound and probably less expensive. I’ll keep you and everyone else posted on progress.

To everyone who has contributed, a hearty Thanks. I’ve learned so many things over the past few days. I really appreciate everyone sharing their knowledge and providing guidance. Hope everyone has a safe and happy Thanksgiving!

Regards,

xpat

[xpat]

Hi Gerry,

I agree that inline fuses are good protection and were recommended by AR. However, while surfing various Internet sites, I heard individuals blog how their amp output DC and fried their speaker even with fuses. They said the woofer got fried but the mid and tweeter were ok because they were protected by the crossover caps. Those types of comments are what got me started on this "quest" for better protection. I frankly don't know why a fuse wouldn't protect against DC. My best (uninformed) guess is that the fuse actually is protecting against too much current. If DC was output at a lesser current than the fuse rating, the fuse would'nt blow and result in anywhere from incremental to catastrophic damage to the woofer. As I said, this is only a guess. I'm a solid Classic AR fan, and the thought of frying my woofers is very unsettling.

Regards,

Phil

[Gerry S]

Phil, I believe you are correct; fuses can only do so much. And fuses can alter the frequency response as the fuse heats up. The D.C.R of the fuse increases from it's theoretical zero value and acts as a voltage divider that changes with frequency. Wheather this is audible to the listener is something I don't know about.

Also, the amps of the 70's and 80's can suffer from excessive "dc offset" (my Dynaco 400 did), which at low levels could cause the woofer's cone to migrate from it's theoretical "neutral" or "resting position". I suppose if this offset was high enough, it could heat up the woofer's voice coils and the owner wouldn't even be aware of it happening (since DC isn't audible). Again, I don't have personal experience of woofers failing from this. Perhaps others here can chime in.

I wonder if today's amplifiers suffer from this. It's easy enough to check with a dc voltmeter or by looking at the woofers as the amplifier is switched on; woofers should show no displacement when there is no music being played.

[xpat]

Gerry S,

I have an Adcom GFA555II which was released about 1990. I've read a few Internet posts indicating that the designer (Nelson Pass), said that once the amp is 20 years old, the caps may begin to fail and cause DC related problems. According to the posts, the cap problem also depends on how much the amp has been used over that time period. The 555 is running perfectly, but I am starting to get nervous because of its age. As a result, I've recently checked the output with a DC voltmeter and everything measured fine. I also pulled the cover off the amp and inspected the boards and caps - again everything looks fine. (In fact, the insides look brand new - I was somewhat surprised how clean and new everything looked. Also, the amp is built like a tank - I hope it lasts many more years but I think a DC firewall is needed "just in case". ) I like your comment about looking at the woofers as the amplifier is switched on to ensure the woofers show no displacement when no music being played. I'm going to try that test tomorrow.

Regards,

xpat

[SHOKDU]

Madisound sells 1000uf bi-polar Bennic capacitors, which are electrolytics.

Note that even though the inside of your amp may look new, most times capacitors don't show external signs of age, but internally have started to dry up, and/or drift from original spec. Visible external damage could happen in the form of a bulging or leaky capacitor, but I've only seen this happen rarely. It is very good practice to change capacitors that are beyond an expected 15-20 year service life. A re-cap is cheap, for electrolytics found in amplifiers, and a good investment. The only re-caps I've done that get expensive are crossover networks within speakers, film types tend to get expensive.

Most times when I refurbish an amplifier I will change out most, if not all, semiconductors as well. If a protection circuit is within the unit I will pull relays and burnish all contacts as well.

Just food for thought.

[xpat]

Hi all,

I'm providing a link to the capacitors I'm thinking of purchasing. I like the idea of only one capacitor in the circuit for each speaker that will not affect the bass because of the high capacitance (as eloquently stated by Ben76). Also, the price looks reasonable. If the picture of the cap is blown up, the positive terminal is clearly marked which hopefully will negate any mistakes on my part. Any thoughts would be appreciated.

http://www.digikey.c...3299-ND/2095913

Regards,

xpat

[Carlspeak]

That's a polar capacitor. Just one polar cap won't work. If it's 10K uF you want, then get two 5k's and wire them so the two become one non-polar cap.

Remember you're dealing with an AC signal, not DC. So you must have some form of non-polar capacitive elements in the circuit.

[ben76]

xPat, as Carlspeak said the capacitor you linked to in DigiKey is a polar one and you will need two to make a non-polar one!

Let me repeat:

********

One polar capacitor per speaker would explode violently in this application!

********

Unfortunately, non-polar capacitors of this size (10,000 uF) are hard to come by. For example, the largest non-polar capacitor that PartsExpress currently has to offer is only 500uF. Thus, you will probably have to connect two capacitors as I explained.

[ben76]

I have just looked in DigiKey at capacitors of 4700uF / 100Volt (5000uF is a non-standard value).

Remember that you will need two of these per speaker.

If you do not mind soldering these should work fine:

http://www.digikey.c...6110-ND/132316.

If you do not want to solder you can use these (they cost double though):

http://www.digikey.c...3302-ND/2095916

One more thing, I have just noticed that in post #8 I wrote by mistake that when connecting two identical electrolytic capacitors in series to create a non-polar capacitor one gets a capacitor of double the voltage rating of each individual capacitor. This is obviously not correct since the whole point of this configuration is that when faced with AC voltage only one capacitor carries the full voltage burden (the capacitor whose polarity matches the instantaneous AC signal polarity).

In short, when connecting two identical electrolytic capacitors in series as described in post #8 one gets a capacitor of half the capacitance and the same (not double) voltage rating.

Please note that I have fixed post #8 so it is now correct.

[fast_eddie_72]

I used the Bennic 1000 uF caps from Madisound when I recapped my LST2s. They perform well.

I have to say, however, that those caps do not simply sit there ahead of everything else protecting the speaker from DC. They extend the low bass of the speakers. I won't pretend to understand how, I'll simply refer you to the source of my information on the subject.

http://www.audiokarm...ad.php?t=347831

On page two of that thread, Gordon W, who has forgotten more about speaker design than I'll ever know, explains this.

"What the huge caps did, was to cause a change in the damping applied to the woofer, so that there was less damping as frequency went down... which acted to ADD low bass output to the woofer, at or below its resonant frequency. In effect, it "boosted" the bass output, at and just below resonance in-box.

"Ultimately, starting about a half-octave below the woofer resonance in-box, the cap added a high-pass function... so, you'd start out with an over-damped second order (large sealed box) response, and add another first-order (the cap) to create what's called a 'third order assisted bass' alignment. It rolls off at 18dB/octave, but quite a bit LOWER than if the cap wasn't there..."

I can tell you, in my experience, replacing that cap in my LST2s made a noticeable difference in deep bass extension.

[xpat]

Hi All,

Hope everyone had a great Thanksgiving Holiday.

Thanks for the information - seems like I would have caused myself a lot of headaches if I wouldn't have consulted with you first! I guess I'm in over my head on this. ...... Anyway, I'll keep trying. I'm attaching a layman's version (diagram) of how I believe the caps should be connected between the amp and the speaker. Somehow it looks odd that only one side of the cap is connected but I'll let you folks review and comment.

Regards,

xpat

Note: Incorrect Drawing deleted on 11/29/2012. See next post