Humming LW Erath Model 288 amp - filter caps (help!) - diyAudio
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Old 15th January 2007, 03:30 AM   #1
sam562 is offline sam562  United States
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Default Humming LW Erath Model 288 amp - filter caps (help!)

Hi,

I just recently got an old LW Erath 288 amp for my LWE-I speakers and when I first power it up it makes a loud hum that quickly fades away to where its barely audible unless you put your head up against the speaker.

The amp sounds great and I don't notice the hum while playing music but I've learned from these boards that it's 40-year-old filter caps should probably be replaced before firing it back up. I'm guessing that it might be especially important to fix this, since this amp works with the speaker's negative feedback circuitry.

I was going to see if a local electronics repair shop could do it but they didn't want anything to do with a hand-made, 40 year old amplifier with no schematic -- they couldn't get me out of there fast enough!

I'm certain I could do it myself as the filter caps are right up front (err, back) on this amp.

Unfortunately, the big Mallory filter caps it uses aren't available anymore (Mallory CG452U50D1, 4500 MFD 50 VDC, MAX SURGE 75 VDC) and while I have found some close matches (4700uf/100V) with brands like Nichicon, Black Gate (good god! $$$), Vishay/Sprague, Panasonic, etc... they have subcategories that I don't feel qualified to discern: computer grade, wide temp range, rapid charge/discharge, etc...,

I hope that some of you old hands at recapping might have some suggestions/tips for a novice like myself about particular brands that you've had good luck with. Also, I'm guessing I should probably replace some of the other caps,...at least all of the electrolytic ones? Any ideas about how to proceed would be great. I know I need to discharge the caps (important!).

Here are some snapshots of the insides of my amp, as you can see it is a small, simple amp that fits in the back of a speaker:
(pictures are big)

Amp Innards 01
Amp Innards 02
Amp Innards 03
Amp Innards 04
Amp Innards 05


Amp Face
Amp Cage
Mounted in speaker compartment


Thanks in advance, ;-)
-Sam
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Old 15th January 2007, 06:30 AM   #2
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Smile I hope this will help!

Please don't flame me for this as I am a noob to this site but an old man in the area of electronics and more specifically as they have to do with amplifiers etc. As long as the working voltage is the same or HIGHER and the uF (micro farads) is the same or higher it will be fine. I really doubt you'll be able to discern any audible difference if you follow these guidelines. Filter capacitors serve to "store" DC voltage for use when the power line is wavering. That would be all the time if they are on the receiving end of a full wave (or especially half wave) rectified AC to DC power supply circuit (which yours are if they are in fact the filter caps). There are a couple of caveats to this though... One being physical size and footprint. Should not be a problem though as your old parts were likely much bigger than those available today. The second caveat (and likely the reason why the repair shop declined to repair your amp) is because in some amplifier designs the outputs are actually couple by way of the filter capacitors. QSC likes this odd means of design. I personally hate it as it guarantees bizzarre coloration IMHO. I really doubt though you have this design constraint to contend with. If you do however, then you will want to understand they means by which the capacitors were manufactured (electrolytic, etc). The specs you offered up are not uncommon specs and I believe you can afford to try any number of them to determine the best sounding ones. Just be sure you keep in mind the polarity and remember to discharge them COMPLETELY using a 100 ohm resistor so as to not pose an unneccessary defibrillation on you! I hope any of this is helpful!
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Old 15th January 2007, 06:52 AM   #3
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get the Nichicons as long as you can mount them..... replace those 3 axial electrolytics as well, especially the ones with the orange cardboard tubes around them, and the ones on the amp boards behind the big filter caps...... from the pix it looks like the large aluminum ones are your + and - supplies for the output transistors (the grey wires go to ground, and the red on the left goes to the positive side of the cap, while on the right side the purple wire goes to the neg side of the cap.... tnx, nice detail pix). the ones you mentioned called "computer grade" should be fine, and is more of a description of their form factor (sizes and proportion) than anything else. it also implies that they have low ESR, which is another thing you want for an amp power supply. i don't know why a shop would refuse to work on an amp without a schematic, especially one with the problem you describe. power supplies are very simple things. the only time i really need a schematic is when there are burnt resistors and semiconductors (not that a schematic wouldn't be helpful, but i don't always need one).

you don't need to replace the orange mylars as they should last a lifetime.

amp looks like about mid 60's design????? the gumdrop transistors and the heatsinked TO-5 transistors are a pretty good indication.
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Old 15th January 2007, 02:53 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
you can use 50V or 63V elecrolytics. Anything with higher voltage rating is wasting money and space.

4500uF is now available as 4700uF. Modern version are likely to be MUCH smaller than the originals. So this should be an easy replacement.

Electrolytics can only be fed with DC supply one way round. Fit then back to front and there is a possiblity that they could explode.

I would like to suggest you double the smoothing capacitance but without details of the amplifier that wound be rash of me. Get it working first.
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Old 16th January 2007, 03:51 AM   #5
sam562 is offline sam562  United States
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Thanks guys for all of the responses.

I did a bit more searching and it turns out that Cornel Dubilier (cde.com) still makes a version of the big caps that is very close to the original Mallorys:

close match for silver caps

so I'll see if I can grab a couple of those. I spent alot of time looking and learned that more ripple current is good and less ESR is best. (right?) there were a few other options out there that looked promising, but if I can get basically the same caps, I'm going for that.

I'm pretty sure you all were right when you mentioned this amp having some sort of odd design: When I first got the amp, I talked to Mr. Erath and he didn't remember building it,...until I sent him the pics from this post. He then remembered having a guy build the transformers and that this amp was a 2nd generation of his original LWE-I amp; this one having more power 80-100w and he did mention that he "directly coupled" something to do with the capacitor(s)..or was it "decoupled"? Anyway I can't understand what I wrote in my notes, so I'm guessing it is "directly coupled".

I read the wikipedia entry and *sort of* understand what it means.

Yes the amp is approx mid sixties - it has Nov 7 1967 stamped on the bottom - you are good! ;-)

If possible, I would like to try and repclicate the properties of the other smaller electrolytic capacitors with modern replacements. I know I'm getting a bit anal about this, but I really love this amp's sound and want to keep it:

(2) Orange Mallory 100uF/50v axial
(3) Black Cornel Dubilier "Beaver" 250uF/50v

Thanks again for all of your tips and suggestions!
-Sam
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Old 11th February 2007, 08:47 PM   #6
sam562 is offline sam562  United States
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Hi again.

Thought I would put a cap on this recapping thread to help all of the many, many LWE-1 loudspeaker owners that are looking to recap their beloved LWE amplifiers. ;-)

Because I'm gullible, naive and insecure I went with all Black Gate capacitors - even thought I opted for the "standard" ones (!?)...lol Anyway, I ended up replacing the 250uF with 220uF because it they were easier to find in the "magic" Black Gates - but I had to switch to radial leads instead of axial. I used shrink tubing to cover the longer lead runs and luckily had plenty of room for clearance. I read it that it was okay to use different values as long as you stay within 20% --with greater uF being preferable; but as I mentioned before about my insecurity: it was telling me to not go higher with this old system...lol I love the sound of this little amp and didn't want any extra bass response or to have the old transisters get more DC or whatever.

The two large orange caps turned out to be non-polar, so I got some exact -match nichicons and stuck them to the chassis with double sided padded tape. I didn't bother shrink-tubing those leads as it is sort of a birds nest down there anyway and those caps arent moving.

The two largest caps are exact matches aside from being a revolting baby blue color instead of the almost-tube-looking shiny aluminum ones that they were replacing. They are only 10 years old (1996) - "new old stock"

It turned out well! - the amp still has a slight, momentary "brrmp!" sound when starting up, but immediatly goes whisper quiet. This amp doesn't have a power switch or any relay to mute the power up. It does have a mystery 1/8" jack one the faceplate with one wire going to the amp innards. Louis Erath doesn't remember what it is for, he said it might be for a test probe for setting the idle current or something.

Anyway, I appreciate all of your help and love the results! -Sam

Pics of the new caps:

http://www.dontgethungup.com/wbcam/LWE_288recap00.jpg
http://www.dontgethungup.com/wbcam/LWE_288recap01.jpg
http://www.dontgethungup.com/wbcam/LWE_288recap02.jpg
http://www.dontgethungup.com/wbcam/LWE_288recap03.jpg
http://www.dontgethungup.com/wbcam/LWE_288recap04.jpg
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Old 12th February 2007, 02:54 AM   #7
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Looks good = dun right! ...

Did Mr. Erath mention anything about replacing the carbon resistors in the signal path(s)?

(" the amp still has a slight, momentary "brrmp!" sound when starting up ..." = expected since the circuit probably doesn't have any time delay pre-loading the caps.)
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Old 12th February 2007, 08:25 PM   #8
sam562 is offline sam562  United States
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Quote:
Originally posted by FastEddy
Looks good = dun right! ...
Thanks!

Quote:
Originally posted by FastEddy
Did Mr. Erath mention anything about replacing the carbon resistors in the signal path(s)?
No he didn't. I didn't really consult with him much on this project as not to bother him. When I first got the amp he recommended connecting it to a variac and adjusting the idle current to 15-20 (millivolts?) so that the transistors would warm up sufficiently.

I don't have a variac and the electronics shops around here (Dallas) only do warranty work - its really strange.

I'll google the carbon resistor thing. I do have a multimeter and I would guess I could just check them all.

Quote:
Originally posted by FastEddy
(" the amp still has a slight, momentary "brrmp!" sound when starting up ..." = expected since the circuit probably doesn't have any time delay pre-loading the caps.)
Cool. Thanks again for your encouragment. - Sam
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