I recently purchased a tube headphone amp, and the small PSU filter cap went bad within 10 minutes up using it. After it bulged, the amp would not start up. I would have to remove the tube, turn it on, and then install the tube. Otherwise it would just flash on and off.
It is a Rubycon BXA 6800uf 25v cap. I measured across the terminals and it is 24.2 volts, and the same coming right out of the external psu.
Could the cap be defective? or is running it that close to the rating, bad for it?
Thanks
It is a Rubycon BXA 6800uf 25v cap. I measured across the terminals and it is 24.2 volts, and the same coming right out of the external psu.
Could the cap be defective? or is running it that close to the rating, bad for it?
Thanks
If it fits, use it.
In principle, you would expect a capacitor with higher voltage rating to have thicker dielectric and therefore also higher ESR. This would then be bad in a PSU. However, in my actual real-life experience, higher voltage rating is always better; the only downside is the larger size.
Thicker dielectric would also be more linear, but I don't think that is the main point of interest here.
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Greetz,
MatchASM
Please check:
- if the cap was put in backwards (it seems like that)
- check the voltage at power on, maybe it is way too high
- if the cap is really a BXA or a fake ( cut it open ).
Not being a "melting glue mechanic" I would use a cap with the same physical properties ( it should fit mechanically) and preferably a 35V type. However, first action should be to make a good diagnosis of what is the problem exactly. It can't be the 24,2 V. A 100V type will be very large and not necessary. Better buy a good industrial cap with good properties like Pana FC/FM. Only use the cap you have if it is fresh ( recent production) and fits perfectly.
- if the cap was put in backwards (it seems like that)
- check the voltage at power on, maybe it is way too high
- if the cap is really a BXA or a fake ( cut it open ).
Not being a "melting glue mechanic" I would use a cap with the same physical properties ( it should fit mechanically) and preferably a 35V type. However, first action should be to make a good diagnosis of what is the problem exactly. It can't be the 24,2 V. A 100V type will be very large and not necessary. Better buy a good industrial cap with good properties like Pana FC/FM. Only use the cap you have if it is fresh ( recent production) and fits perfectly.
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I checked that also, it is the correct polarity. My only guess that maybe it's a fake cap and is not up to original spec.
Thank you, I was going to ask what brand of cap would be good, I was looking into the panasonic and alsi the nichicon gold.
Yep, I checked, polarity is fine. Thanks all.
I installed the spare 10,000uf 100v. It's been on for 30mins, and it is very slightly warm, but I think that is from the tube being so close.
Sounds better than before, bass is tighter and definitely less rolled off. Before I had to use the eq and boost up the treble, now I have it flat.
This was a christmas present from my Wife. I love her!
Sounds better than before, bass is tighter and definitely less rolled off. Before I had to use the eq and boost up the treble, now I have it flat.
This was a christmas present from my Wife. I love her!
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Hey Matt,
Wow that looks sweet! That's a nice present for sure.
Is the new cap warm all over or more on the side nearest the tube? I shouldn't think it would heat up much at all, unless it was dealing with some pretty intense voltage ripple. How warm is it getting?
Looks nice though, good work on the repair! 10,000 uF should be ok if the original was 6,800 uF. I wouldn't go too much higher as I am not sure how much in-rush current your power supply was designed to expect. As you go bigger with the filter cap, the initial current needed to fill it up goes up (in duration at least). That should be fine though, it's not too much higher. I am surprised they were running the old one at close to 100% of its voltage rating. Usually the value chosen should be de-rated a little for saftey margin, and even more so when heated. A 25 v cap when heated by a pretty tube might really be a 23 v cap!
Jim
edit: I just read your last post - hmmm, if the sound quality is changing, maybe something else is going on as well? I wouldn't think a filter cap would change the tone as you've described, but like you said it might be worth investigating by going back to a 6,800 uF just to see.
Wow that looks sweet! That's a nice present for sure.
Is the new cap warm all over or more on the side nearest the tube? I shouldn't think it would heat up much at all, unless it was dealing with some pretty intense voltage ripple. How warm is it getting?
Looks nice though, good work on the repair! 10,000 uF should be ok if the original was 6,800 uF. I wouldn't go too much higher as I am not sure how much in-rush current your power supply was designed to expect. As you go bigger with the filter cap, the initial current needed to fill it up goes up (in duration at least). That should be fine though, it's not too much higher. I am surprised they were running the old one at close to 100% of its voltage rating. Usually the value chosen should be de-rated a little for saftey margin, and even more so when heated. A 25 v cap when heated by a pretty tube might really be a 23 v cap!
Jim
edit: I just read your last post - hmmm, if the sound quality is changing, maybe something else is going on as well? I wouldn't think a filter cap would change the tone as you've described, but like you said it might be worth investigating by going back to a 6,800 uF just to see.
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Hey Matt,
Wow that looks sweet! That's a nice present for sure.
Is the new cap warm all over or more on the side nearest the tube? I shouldn't think it would heat up much at all, unless it was dealing with some pretty intense voltage ripple. How warm is it getting?
Looks nice though, good work on the repair! 10,000 uF should be ok if the original was 6,800 uF. I wouldn't go too much higher as I am not sure how much in-rush current your power supply was designed to expect. As you go bigger with the filter cap, the initial current needed to fill it up goes up (in duration at least). That should be fine though, it's not too much higher. I am surprised they were running the old one at close to 100% of its voltage rating. Usually the value chosen should be de-rated a little for saftey margin, and even more so when heated. A 25 v cap when heated by a pretty tube might really be a 23 v cap!
Jim
edit: I just read your last post - hmmm, if the sound quality is changing, maybe something else is going on as well? I wouldn't think a filter cap would change the tone as you've described, but like you said it might be worth investigating by going back to a 6,800 uF just to see.
Hi Jim,
Thank your for your reply.
Yes, the cap is warm all over, but more so on the bottom than the top.
Also, I still need to remove the tube before powering the amp on, or else the thing just cycles on and off.
How can I measure the voltage ripple, do I need scope for that?
Maybe the external psu is the problem? I know this company sells a bunch of these, they can't all be going bad after 10 minutes.
Hi Matt,
This might be getting outside of my comfort zone a little, so please take all of this with a grain of salt!
About the heating-up of the capacitor: how long does it take to warm up, and is this with the tube in place? I am wondering if maybe the heat of the tube is making it to the capacitor via a ground pour on the circuit board. If the capacitor is warmer at the bottom, perhaps it is being heated externally from the bottom, via a warm ground pour.
One test you might try is letting things heat up, shut everything down, disconnect the power for safety and touch around on the circuit board. If you feel as though the board has been warming up the cap, the cap might be ok. Be careful with this step as the cap will still be charged, and the tube will still be hot!
About the voltage ripple: you would need a scope unfortunately to check this accurately. However, serious ripple would likely be in the audio range, and would manifest as hum of some sort in your head-phones. If it is not humming it is probably ok.
About the power cycling: this is speculation, but I suspect the power supply is going into a current limiting protection mode. Did it do this with the old 6800 uF filter cap? Did this power supply come with the headphone amp?
Jim
This might be getting outside of my comfort zone a little, so please take all of this with a grain of salt!
About the heating-up of the capacitor: how long does it take to warm up, and is this with the tube in place? I am wondering if maybe the heat of the tube is making it to the capacitor via a ground pour on the circuit board. If the capacitor is warmer at the bottom, perhaps it is being heated externally from the bottom, via a warm ground pour.
One test you might try is letting things heat up, shut everything down, disconnect the power for safety and touch around on the circuit board. If you feel as though the board has been warming up the cap, the cap might be ok. Be careful with this step as the cap will still be charged, and the tube will still be hot!
About the voltage ripple: you would need a scope unfortunately to check this accurately. However, serious ripple would likely be in the audio range, and would manifest as hum of some sort in your head-phones. If it is not humming it is probably ok.
About the power cycling: this is speculation, but I suspect the power supply is going into a current limiting protection mode. Did it do this with the old 6800 uF filter cap? Did this power supply come with the headphone amp?
Jim
Hi Jim,
thank you for your reply.
I've determined the cap is getting warm from the tube, and also the two heatsinks in the rear. I can only hold my finger on it for 5 seconds, so it is pretty hot. I've had the amp running now for 4 hours, and the cap is barely warm.
Yes, the psu came with the amp... it looks like a laptop brick psu.
I agree with you, I think the stock psu is crap, and it is going into current limiting.
I just ordered a heavy duty regulated switching psu on ebay. 24v at 3amps. the stock one is 2amps, but I'm guessing it is very over rated.
Here is a rear view of the amp, you can see the two black heat sinks.
Anyhow, I think my problem is a combination of things. Too much heat around the stock cap, running too close to the rated 25v, and a weak stock psu.
I think I'm going to get a 6800uf 35v cap, and install it on top of the case, like I did with the 10,000uf cap.
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