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

[HappyChuck74]

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.
Circuit diagram attached as pdf.
Any help or advice is sincerely appreciated.

 

[c_dk]

Sounds like you either have the loudness compensation switched on or speakers with a real issue.

I don't suppose you can do a proper bench test to measure the amps output for linearity.

 

[HappyChuck74]

Hi. Thank you for your response. The loudness switch functions fine but it really makes the bass even more thick and muddy if I turn it on. Bass pot seems to be functioning fine aas well.. Speakers are fine too. I wouldn't know how to do a linearity test. All I have is a multimeter. Any other ideas?

 

[gadget73]

might be worth a frequency response test. Basically input a signal at 1khz and set the output to 1w. Sweep up and down from there and see what it does. I expect you'll find a bump in the upper bass region. Some things were designed like this on purpose. I rebuilt a Pioneer with Allied badging and found that "flat" was about 1/4 down on the bass control. I set it flat and just put the knob on at the proper spot. I also made some marks on the shaft so I knew where it was actually right. I suppose I could have meddled around inside to correct it, but just re-installing the knob was a lot easier.

 

[Hobie1dog]

What speakers?

 

[HappyChuck74]

I'm using the same bench speakers that I've used for the past several projects. A pair of Onkyo front channel speakers from a home theater in a box set. I've used those to also test a NAD 3020 and a Pioneer SA-500A, both of which produced great sound though those particular speakers.

 

[HappyChuck74]

might be worth a frequency response test. Basically input a signal at 1khz and set the output to 1w. Sweep up and down from there and see what it does. I expect you'll find a bump in the upper bass region. Some things were designed like this on purpose. I rebuilt a Pioneer with Allied badging and found that "flat" was about 1/4 down on the bass control. I set it flat and just put the knob on at the proper spot. I also made some marks on the shaft so I knew where it was actually right. I suppose I could have meddled around inside to correct it, but just re-installing the knob was a lot easier.

It may be a worthwile test but unfortunatley I don't have a signal generator or a scope. I do know about where to set it to get the least obtrusive bass. I'd ust like to figure out why. Thanks for your input!

 

[gadget73]

you can get signal generator programs for the computer, and a decent voltmeter will work for the output. It just needs to be something with a wide frequency response otherwise it won't read properly. I generally prefer to use a dummy load just to rule out any nonsense from the speakers (and so I don't have to listen to it) but speakers will work if you keep the level low enough to not drive you insane.

 

[elnaldo]

It may be a worthwile test but unfortunatley I don't have a signal generator or a scope. I do know about where to set it to get the least obtrusive bass. I'd ust like to figure out why. Thanks for your input!

you can even use your computer and ARTA software to do a freq response measurement, using the headphone output (not speakers output, to protect your sound card). the graph will look like this: (this is a measurement to check the effect of a single tone control potentiometer. In green, the response at "flat" position, yellow is all the way to bass, white is all the way to treble)

 

[gadget73]

thats even better. I see the classic V shaped response on that amp too. Its just something a lot of them do, especially with the loud switch turned on. Some loud circuits are just too much though. My Sherwoods are way over the top, and its something I've meant to fiddle with for a good while now.

 

[BinaryMike]

It may be a worthwile test but unfortunatley I don't have a signal generator or a scope. I do know about where to set it to get the least obtrusive bass. I'd ust like to figure out why. Thanks for your input!

I think you have all of the ambition and curiosity that's needed to derive huge benefit from owning a scope and signal generator. Suggest you acquire them without delay, because you can learn more from fooling around with them for a few hours than you will from months of online chat.