AR3a crossover caps

[stan461]

This is really a general question but let me get specific. There are 3 capacitors in the AR3a crossover network.

150uf 50vdc

6uf 50vdc

50uf 50vdc

How close do I have to match the caps in both uf and volts. A lot of vendors have crossover caps but almost none match exactly. For example the caps in the AR3a are 50vdc. Can you go with 100vdc? Is is better to go under or over? If the cap is a 6uf as one of them is, is it better to go with a 5.6uf or a 6.8uf?

Polarized or non-polarized? (I am almost sure non-polarized)

For the 50uf & 6uf I planned on getting metallized polypropelyne.

For the 150uf, electrolytic.

I love reading this forum. You guys are great.

A happy and healthy Thanksgiving to all.

[r_laski]

Stan,

Many are wondering why you are replacing the caps in your AR-3A's.

>>How close do I have to match the caps in both uf and volts. A lot of vendors have crossover caps but almost none match exactly. If the cap is a 6uf as one of them is, is it better to go with a 5.6uf or a 6.8uf?<<

If you look at the caps the rating should be XX uF +/- X% (150 uF +/- 10%) and a voltage rating. Stay within that percentage. In your 6 uF example -- 5.4 uF to 6.6 uF is the "acceptable" range. 5.6 uF is within 10%, 6.8 uF is not.

If you can’t find something close enough, you can parallel two (or more) caps to get the desired capacitance. (parellel two (2) 3 uF caps to get 6 uF).

>>For example the caps in the AR3a are 50vdc. Can you go with 100vdc? Is is better to go under or over?<<

You should not go below the voltage rating. I may not explain this exactly technically correct, but look at the voltage rating as the DC voltage at which the cap will be “overloaded”. Think “higher” is “better”, but this is a generalization. 50 VDC caps are hard to find. 100 VDC will work fine unless you are using VERY high powered amps (several hundred wpc) to drive your AR-3a’s. All the caps do not necessarily have to have the same voltage rating.

>> Polarized or non-polarized? (I am almost sure non-polarized)<<

Yes, non-polarized is the way to go.

>>For the 50uf & 6uf I planned on getting metallized polypropylene. For the 150uf, electrolytic.<<

This is OK, but if you can afford it, go all polypropylene. Besides being very large physically, large cap polypropylenes can be relatively expensive.

For some more information and a good source for some VERY high quality MADE IN THE USA caps at (IMO) reasonable prices go here: http://www.northcreekmusic.com/ George has 6 UF, 50 uF, and 100 uF (for paralleling with a 50 UF to get 150 uF).

>>A happy and healthy Thanksgiving to all.<<

Dittos!

[soundminded]

The voltage rating is the maximum voltage the capacitor is rated to withstand before the diaelectric (insulator between the conducting plates) is at risk of breaking down. So if you can't get 50 volts, get 100 volts. DO NOT get lower.

Crossover network capacitors are always non polarized. That is because either side could be positive with respect to the other. Disregarding a very slight dc offset voltage, the output of an audio amplifier is zero volts with no input signal present and will swing both ways. Polarized electrolytic capacitors are used only in circuits where there is a dc bias where one side is ALWAYS positive with respect to the other.

Try to match the value of the capacitors as exactly as possible. Changing the values will change the crossover frequency and possibly the sound of the speaker with it. The equivalent capacitance of capacitors in parallel are C(eq) = C1 + C2 + C3 +....+ Cn. The eqivalent capacitance of capacitors in series is 1/C(eq) = 1/C1 + 1/C2 + 1/C3 +...1/Cn. Personally I would try to avoid wiring capacitors in series but in parallel, there should be no problem.

Good luck.

[stan461]

>Many are wondering why you are replacing the caps in your

>AR-3A's.

Excellent point.

On about the 5th time I fired them up the woofer cone went inward for a bit and then finally released. It made some sort of noise. Thats how I noticed the cone. Anyway when it started playing again the woofer was distorted. I swapped the speaker left and right and I'm sure the woofer is damaged.

I have already purchased a good used woofer.

The owner never touched these. The grills were a delicate operation

to remove. I bought this pair from an original owner. Boxes were included with the seller's name on it.

I know swapping caps is an inefficient way of diagnosing the problem, if there is one. Could have been a bad woofer. Maybe the caps are good. I have no way of testing them currently.

I am open to suggestions.

Thank You.

[soundminded]

My first reaction is that somehow DC from your amplifier was applied to the speaker. This could certainly damage the woofer since there is no low frequency cutoff between it and the amp as far as I know. The midrange and tweeter however would be protected by the series capacitors in the crossover networks. Unless you find the source of this problem and correct it, you may continue to damage woofers. Try a DC voltmeter across your amplifier's speaker outputs. It should read almost zero. If it's a tube amplifier, it indicates shorted output transformer windings between the primary and secondary. If it's solid state, it indicates a shorted electrolytic output blocking capacitor or if it's DC coupled, a defective output transistor. Careful, the problem could be intermittent which makes it very difficult to find.

[r_laski]

Stan,

I had the same initial thought as Soundminded, when I read what happened. I want to ask more questions:

>> The owner never touched these. I bought this pair from an original owner. Boxes were included with the seller's name on it.<<

1) Were these shipped to you? Possible damage to woofer during shipping – surround comes apart due to age/dried out, and or spider detached from basket/cone. (Been there)

>> On about the 5th time I fired them up the woofer cone went inward for a bit and then finally released.<<

2) Did this happen right when you turned on your amplifier? If so, did you have the preamp on and the volume turned up? (These questions apply to integrated amps and receivers as well)

3) Does you amp “thump” your speakers when you turn it on? In other words, do you know if your amp has a turn on delay function to prevent unwanted DC voltage from going to the speakers when you first power up the amp? This is what Soundminded was talking about. And the way you describe it, sounds like what happened.

4) Do you know if these are Alnico (would be very early, pre-1969, AR-3a’s) or ferrite magnet woofers? Trying to get an idea of age and which woofers you’re working with.

5) What is/was the condition of the woofer surrounds? From what you’ve described so far, it sounds like the woofer voice coil bottomed out and is damaged. But, need to figure out what caused it to bottom out if that’s the case.

*DO NOT trash the original woofer. It can most likely be repaired (re-coned).* I just had one re-coned that had the same type thing happen to it, except the voice coil was frozen at full extension.

>>I have already purchased a good used woofer.<<

6) Have you installed it in you AR-3a and tested the speaker? If you haven’t done it yet, that’s Ok. I’m just trying to get an idea of what you’ve done so far. I’m not recommending this yet. The problem may not be with your loudspeakers. Can you borrow a different amp from a friend? One that you know is in perfect working order.

7) Have you looked at the woofer in your other AR-3a? I know it’s a pain to remove the grill cloth. Wondering about the condition of its suspension – spider and surround, and whether it needs to be re-foamed before you use it again.

>> I know swapping caps is an inefficient way of diagnosing the problem, if there is one. Could have been a bad woofer. Maybe the caps are good. I have no way of testing them currently.<<

If it’s just a bad woofer, you really don’t want to go through the trouble of replacing the caps. From what you described so far, I would look at the woofers themselves and at you amp before going into the crossover. Visually inspect your woofers and borrow a “know good” amp to test your loudspeakers and test your amp or have it tested as Soundminded suggests.

Rich Laski

[stan461]

Rich,

>1) Were these shipped to you? Possible damage to woofer during

>shipping – surround comes apart due to age/dried out, and or

>spider detached from basket/cone. (Been there)

I picked these up myself and carefully loaded them into my car and have never jostled or dropped them.

>2) Did this happen right when you turned on your amplifier?

yes.

>If so, did you have the preamp on and the volume turned up?

No.

>3) Does you amp “thump” your speakers when you turn it on? In

>other words, do you know if your amp has a turn on delay

>function to prevent unwanted DC voltage from going to the

>speakers when you first power up the amp? This is what

>Soundminded was talking about. And the way you describe it,

>sounds like what happened.

Yes. At that time I believe I was using a NAD 3020 as a preamp. If I wasn't using that then I was using a Forelay tube preamp (its a kit from Bottlehead). I just don't remember which one. The NAD always "thumps" when powered on. I think thats a trait of them. By the way my power amp at that time was an ADCOM 545. I always thought that was rock solid.

Between the thumping NAD and the inconsistency of the "foreplay", I wound up buying the ADCOM GFP555. It has never given me a problem "thumping" like the NAD and the stereo image never wanders nor does it microphone as did the tube amp. I'm not knocking them, just offering my observations about the particular pieces I own.

>4) Do you know if these are Alnico (would be very early,

>pre-1969, AR-3a’s) or ferrite magnet woofers? Trying to get an

>idea of age and which woofers you’re working with.

Definitely alnico. I have seen pictures of the alincos & the ferrite.

>5) What is/was the condition of the woofer surrounds? From

>what you’ve described so far, it sounds like the woofer voice

>coil bottomed out and is damaged. But, need to figure out what

>caused it to bottom out if that’s the case.

They look OK. I would agree that the woofer voice coil bottomed out. Thats what it looked like.

>*DO NOT trash the original woofer. It can most likely be

>repaired (re-coned).* I just had one re-coned that had the

>same type thing happen to it, except the voice coil was frozen

>at full extension.

I will keep it and have it re-coned.

>>>I have already purchased a good used woofer.<<

>6) Have you installed it in you AR-3a and tested the speaker?

Not yet.

>If you haven’t done it yet, that’s Ok. I’m just trying to get

>an idea of what you’ve done so far. I’m not recommending this

>yet. The problem may not be with your loudspeakers. Can you

>borrow a different amp from a friend? One that you know is in

>perfect working order.

For the past year and a half I have been using an Adcom GFP555 with a Hafler DH500. I have hooked up Klipsch KLF20's, Klipsch Heresy II's, EPI 100 & Advents and never had any problems. I used the KLF20's for almost a year with this combo without a hitch. I am very happy with this preamp/amp combo. It gives me a lot of very clean and quiet power.

>7) Have you looked at the woofer in your other AR-3a? I know

>it’s a pain to remove the grill cloth. Wondering about the

>condition of its suspension – spider and surround, and

>whether it needs to be re-foamed before you use it again.

I don't remember but I will take a look at it.

> If it’s just a bad woofer, you really don’t want to go

>through the trouble of replacing the caps. From what you

>described so far, I would look at the woofers themselves and

>at you amp before going into the crossover. Visually inspect

>your woofers and borrow a “know good” amp to test your

>loudspeakers and test your amp or have it tested as

>Soundminded suggests.

I think the pre-amp/amp setup I have is good. If you think after these questions that its the woofer then I'll put the replacement woofer in and give it a try.

Thank You so much for your help.

Stan

[soundminded]

I would install a 3 or even a 2 amp slow blow fuse in series with the speaker temporarily and not play it too loud for an extended period of use including many on/off cycles. If the fuse blows, I'd take a very hard look at the amplifier. Unless the amplifier is dc coupled end to end, the preamp is not a good suspect. This still sounds like the dc bias for the output stage being directly applied to the speaker. We had someone with a Krell amplifier on another board which kept blowing up periodically. Krell kept repairing it without finding the root cause until eventually they found a cracked capacitor in the power supply. Stuff happens.

[r_laski]

Stan,

Just a few tips to add.

If you need info on installing a fuse on your speakers, as Soundminded recommends, check out the AR-11 through AR-18 User Manual, page 14-15. Its in PDF format in the AR – ADD/Truth in Listening section over on the Classic Speaker Pages.

An aside, if I may --- I think this may be the fuse (optional and user installed) that shows up on the AR-11 crossover schematic.

I’ve never used an Adcom amp, but they are usually pretty reliable. As Soundminded says, “Stuff Happens,” even to Krells. I got a used Adcom GFP 565 preamp about a year ago. It has a short in the bypass output RCA jacks that causes a devastating hum to the loudspeakers. I have to use the normal or lab output jacks.

I’ve borrowed my Dad’s Hafler DH500 and his DH110 on occasion to do troubleshooting. I’ve used it (DH500) recently and if I remember right, it also has a turn on delay circuit.

If I followed everything you wrote, you did not use your Adcom pre / Hafler amp combo with your AR-3a’s. I’d hook them up to your AR-3a’s after you replace the woofer and install the fuses. The GFP 565 / DH500 works VERY well with AR-LSTs, AR-9s, and AR-11s your GFP 555 / DH500 combo should work just as well with your AR-3a’s. As suggested, keep the volume low for a while.

If all goes well, the woofers and the fuses stay intact, have your GFP545 amp looked at. I realize I recommended changing both the amp and preamp. Normally, in troubleshooting you only change one thing at time, but AR woofers aren’t cheap to buy or have re-coned. IF you keep the NAD preamp and swap amps only, turn the volume control all the way down and power it up BEFORE you power on the amp. You should not get a “thump” from the preamp with the amp off. Of course, volume off at preamp, amp off first, then preamp off for the same reason.

Let us know what your results are.

Rich Laski