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Old 25th December 2010, 06:53 PM   #1
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Default for vintage SS amps - block DC offset leaks

photo-1.JPGThis is no joke. This works. Money back minus shipping if you don't like it.
Here is a device I used successfully to get a handle on trouble amps. Amps that have DC offfset issues. Amps that run hot, fad in and fade out, take a while to warm up and stabilize, amps that won't turn on due to having their short circut protection activated, or that blow fuses either in the amp or in the speakers. It's basically a 250uf 250V film capacitor. 6" long and about 1.5" in dia. This is not an electrolytic cap, it's a film cap. It block the DC from going thru your speakers and shorting out your amp. amps run cooler since less current is flowing thru them. Granted, you ideally want to get your amp re-capped or repaired, but if you don't want to spend a few hundred dollars on repairs, this will do the trick. its good for about 100watts. If you need more watt handling, you will need 4 of them since you will have to series parallel them. Of course, the 250uf cap, will roll off the low end in the 70hz area. You can parallel 2 of them if you want to recover that. if you have 12inch speakers in a 2 cubic cabinet, this is a non issue since your speakers can't reproduce below 70hz anyway. if you get the series parallel version you can battery bias them. I have a few of these and I am selling them dirt cheap to my forum buddies. In the pic, the cap is installed in line, on some 6" woofer bookshelf speakers powered by a 200W class A amp that has a DC offset issue. it's simple and it works. you can build a box for it, install it in your speakers, or just connect it in line like the photo below. keep that vintage amp going , buy your self some time.

1 pair, 20 bucks plus 7 bucks to ship - great for 100watt amps and 2 cubic foot speakers that house a 12" woofer.

2 pairs, 40 bucks plus 7 bucks to ship - if you have a large speaker with 15" woofers, you can reproduce probally down to 40hz, so you will have to parallel 2 of them on each speaker.

4 pairs, 80 bucks plus 7 bucks to ship - if you have 12" or less woof's in a 2 cubic ft or less can and need to handle more than 100 watts, you need 4 per side to series parallel them. This will get you to a little ove 200 watts per channel.

8 pairs, 160 bucks plus 7 bucks to ship - you need 8 per side if you have 15" woofers in a large cabinet and want to recover frequencies down to 40hz and you need more than 200watts power handling.

email mail me if you want one.

Since there are no selling fees on this great forum, I'm donating 5% of what ever gets sold to the forum to help keep it going.

Last edited by speakerfritz; 25th December 2010 at 06:58 PM.
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Old 25th December 2010, 08:31 PM   #2
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Fc @ 8R = 80Hz
Fc @ 4R = 160Hz

Important info, no? I fail to see what the size of the speaker or it's enclosure has anything to do with it.
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Old 25th December 2010, 08:53 PM   #3
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Quote:
Originally Posted by theAnonymous1 View Post
Fc @ 8R = 80Hz
Fc @ 4R = 160Hz

Important info, no? I fail to see what the size of the speaker or it's enclosure has anything to do with it.
the frequency response is addresses by the examples.

cab size is very relevant. you don't need a 40hz solution, if for example, you have a 12" woofer in a 2 cubic foot cab, the speaker will only have a response of 70hz anyway. a 15" woofer in a 9 cubic foot cab will have a response near 40hz, and you would need a 40hz solution. when we talk about frequency response....not talking about -5db, -10db specs most speakers use. Using this capacitor has a -6db impact.

and sure, if you have 2 ohm or 4 ohm speakers, the examples change . I can deal with that on a case by case basis.

you can add as many as you like to get down to what every frequency response is needed.

main thing, your amp will work....and theres no debate about that.

maybe I'll do a chart with every possible senerio and indicate the optimal number of caps needed.

Last edited by speakerfritz; 25th December 2010 at 08:55 PM.
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Old 25th December 2010, 08:56 PM   #4
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Wouldn't it make more sense to repair the broken amplifier rather than add a bunch of Band-Aids?

Merry Christmas
G
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Old 25th December 2010, 09:01 PM   #5
adason is offline adason  United States
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"cab size is very relevant. you don't need a 40hz solution, if for example, you have a 12" woofer in a 2 cubic foot cab, the speaker will only have a response of 70hz anyway. a 15" woofer in a 9 cubic foot cab will have a response near 40hz, and you would need a 40hz solution. when we talk about frequency response....not talking about -5db, -10db specs most speakers use. Using this capacitor has a -6db impact."

this statement is false...

you can have small woofer (like 8") with great extension to 30Hz, even in small box, if it was designed to do so, in exchange for efficiency, look for low fs, expect low dB/w/m

you can have big woofers (15", 18" or even higher) with fast drop below 50Hz if that's what they were designed to do, look for PA woofers with high Fs, high efficiency, high power handling and so on...

Last edited by adason; 25th December 2010 at 09:07 PM.
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Old 25th December 2010, 09:09 PM   #6
adason is offline adason  United States
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btw speakerfritz, that output cap does not need to be 250V
50-100V would do and you can make it bigger value for the same money
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Old 25th December 2010, 09:10 PM   #7
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Originally Posted by stratus46 View Post
Wouldn't it make more sense to repair the broken amplifier rather than add a bunch of Band-Aids?

Merry Christmas
G
sure....if you can do it yourself...and/or can afford the rates. recent example...an amp bought used for 300 bucks.....costs to repair in the nyc area 385.....or use a dc blocking capacitor solution....medium size speakers at 8 ohms...1 pair of caps ....costs of 20 bucks plus 7 to ship...amp has been working for months with no issues.

it's not the ideal perfect world solution...but it works.
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Old 25th December 2010, 09:18 PM   #8
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Quote:
Originally Posted by adason View Post
btw speakerfritz, that output cap does not need to be 250V
50-100V would do and you can make it bigger value for the same money
sure it would work with a smaller cap, but the lower the voltage, the lower the power rating.

as far as costs, take a quick look to find a film cap in the 250uf value range, it will surprise you. I bought 4 30uf film caps for 40 bucks recently, 10 6uf film caps for 25 bucks. I think, you will find, 10 bucks for a 250uf 250volt film cap is a bargain. you don't want to use electrolytic caps. the sound quality is very poor and they break down very quickly. If you want to use electrolytic's, you can get a 250uf 100volt bi polar cap from partsexpress for 4 bucks.
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Old 25th December 2010, 09:23 PM   #9
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Quote:
Originally Posted by adason View Post

this statement is false...

you can have small woofer (like 8") with great extension to 30Hz, even in small box, if it was designed to do so, in exchange for efficiency, look for low fs, expect low dB/w/m

you can have big woofers (15", 18" or even higher) with fast drop below 50Hz if that's what they were designed to do, look for PA woofers with high Fs, high efficiency, high power handling and so on...
okay, I'll byte, show me a consumer speaker system with an 8" woofer with a flat response down to 30hz. Speaker system with an 8" woofer would be -10db down at 30hz at best.

but anyway, if you have a speaker that can do 30hz, we can figure out how many caps you would need. probally 2 on each channel, if you can keep it under 100watts.
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Old 25th December 2010, 09:31 PM   #10
adason is offline adason  United States
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I do not need to use caps, I use either transformer coupled amps or DC sensing circuit.
I do not think its wise to run DC coupled amp with cap on output and significant DC before the cap. It simply means its not working well. The cap is only temporary solution.
My suggestion to drop the voltage and increase the size of the cap was simply to lower the cut off, so you can avoid the mumbo jumbo in the intro.
Otherwise I agree, what you ask is a good deal for 250uF/250V.
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