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Muddy Bass Vintage JVC JA-S31 Before and After complete recap

Hey this is for any techs or hobbyists out there. I have a flly functioning JA-S31 that finished a total recap on. I used all Nichicon, audio series for as much as I could and general purpose for the ones that they didn’t have in Audio series. The exception was the power caps which I went with Chemicon 12000uF 80V (up from 10000uF 50V). Overall the sound is improved. Highs are superb. However, the biggest issue that I was having before the recap, as well as after is thick muddy bass. I can trim it down almost all the way on the bass pot or maybe one click from the bottom, which will take the muddiness out, but then I lose a lot of midrange and vocals become tinny. I had heard really good things about the amp and I don’t think that was an issue from the factory.

What avenues should I explore to help narrow down the cause of the muddy bass? Are there any known weaknesses with the electrical design and/ or are there particular components that could fail leading to this issue? Also, are there any mods one can do in general to help attenuate the bass without sacrificing mids? I’m fairly new to this, I’ve recapped 3 amps and thats the extent of my knowledge. BTW the other 2 came out wonderfully.

Any help or advice is sincerely appreciated.
Manual Download available here
https://www.hifiengine.com/hfe_downloads/index.php?jvc/jvc_ja-s31_service.pdf

charnich
millercarbon's avatar
millercarbon
324 posts
 
With your experience I’m sure you already know the answer, you’ve pretty much answered it yourself, and just need a little confirmation. Because you don’t like the answer. I don’t blame you.

The other two that you recapped came out wonderfully because recapping doesn’t really change the fundamental character of a design, it simply allows that character to be realized more fully and heard more clearly. Recap an amp with dynamic extended highs and the highs will be just as dynamic and extended, maybe even a bit more so, only with less grain and glare. Recap an amp with tight powerful tuneful bass and the bass will be just as tight and powerful and tuneful, maybe even a bit more so.

The problem you’re having is this time you didn’t recap either one of those amps. You recapped the one with too much wooly bass. So now you have even a little bit more wooly bass.

Certainly there are all kinds of workarounds. My experience however is that in situations like this, rather than try and go off balance elsewhere attempting to correct what is off balance here that you are far better off to chalk it up to experience and move on.
erik_squires
5,268 posts
 
This could be from having too low a damping factor/too little feedback.

Low damping factor can be caused by poor speaker cable connections. Try using a jumper from inside to the speaker cable directly.

Of course, this assumes the problem is really between the amp and the speakers.

Otherwise, it's the room.
charnich OP
6 posts
 
"otherwise, its the room" ...I got a kick out of that. I'll try a direct speaker connection. It does have those abysmal screw terminals after all. Thank you for responding!
charnich OP
6 posts
 
Millercarbon: I have considered that perhaps its just the character of the amp. I hope not but I have never heard one of these before so I have no basis for comparison. I have read others' opinions regarding the model and it has been characterized as a sweet sounding amp.... currently I don't agree, at least with mine. I would say that the bass is excessive for an amp of this vintage. It just sounds wrong.  I wouldn't say that the recap made the bass problem any worse than before, I was just hopoing it would clean it up a bit. I'd like to try to do some diagnostics, at least as much as I am capable of, before I chalk it up to design. The good news is that this is all for fun and I have no sentimental attachment. For me the enjoyment has been the learning and tinkering so I have the time and patience to pursue it more. Worst case scenario, the amp is very loud and is good for a big open space so I'll at least make room for it in my garage. At present it seems to sound like more of a PA than an integrated amplifier. I appreciate your response. Thank you.
birchoak
1 posts
 
Charnich, did you do the amplifier section adjustments outlined (hopefully) in the service manual?  That can affect the sound.  As others have said, that may be the actual sound of the amplifier, as intended.  Despite looking impressive, many mid- to late-70s receivers and amps were overdesigned (trying to get specs that looked good on paper but didn't necessarily actually sound good--your proverbial lab queens)--take a gander at the hi-fi section at select45rpm.com--that dude knows his stuff and has a very different take on when the Golden Age of Audio actually was.  Every receiver I've gotten my hands on, based on his comments, has sounded fantastic, head and shoulders above what was previously my gold standard.  For example, the JVC 5010 with it's ugly graphic equalizer and measly 14 watts seems like a joke and looks terrible on paper.  Until you turn it on.  It betters my recapped and biased Marantz 2230, and has more low-end grunt that just about anything I've ever heard, with the possible exception of a Sansui 800.
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