JoLida SJ-502a mods

bikerboy

Hi,

I bought this JoLida 502 a couple of years from Doro. I like it a lot. Response Audio has been doing mods to this amp for years and most of them are to make the integrated amp into a straight power amp. The volume is just a cheap passive volume pot. Within weeks of owning the amp I removed the volume, balance, and selector controls because I already owned a TAD-150 Signature preamp. Doing that made the amp quieter and cleaner sounding. I also switched out the diodes for Fairchild Stealth diodes and installed Auricaps in the signal side. I also used VH Audio wire and new rca connectors on the rear in the stock location for the input.

Well 4 months ago a resister went bad in the power supply so I decided it was time to rebuild the PS of the amp. It has developed a hum that is most noticeable when it is on but no signal through it. So last week I finally got the parts to replace all the rest of the parts in the PS. I increased the main filter caps from 470 uf to 820 uf. The B supply caps were increased a little and all the resistors are Mills MRA. I havent added any bypass caps yet, just replaced caps in the stock locations. Schematic is at the bottom.

The amp now sounds like a new amp. It has only 6 hours on the new parts so I cant be very critical but it sounds great. Much more dynamic and way more kick in the mid bass. The smoothness of the upper range is so nice and easy. I will wait till it has at least 100 hours before I do any tube rolling. It still has the hum so I may test a different input connection by using a 2" lead on the front of the amp and the rcas on the front panel. That will remove the input that is now very close to the transformer at the back. But then it is a pain to use in a rack. I'm not sure how that will work but a lot of tube amps have the input in the front or top to shorten the input connector. Any ideas on this hum issue would be a big help.

I bought new russian teflon caps from tubesforever on the Hagerman forum and will install those as soon as I can get some hookup wire from VH Audio. Those will take hundreds of hours to settle in. Then I can say that this amp will be very close to done as far as mods go. Maybe some VH Audio caps someday! It sounds so much better and it didnt cost too much. I have learned a lot about tube amps in the process and am happy with the outcome so far. Any ideas about the hum issue or any other improvements will be appreciated. Thanks for all the motivation to audio nirvana. It has been fun. More to come.

http://www.drtube.com/schematics/jolida/sj-502a.gif

dorokusai

Awesome!

NJPOLKER

WOW beautiful Did I read this correctly? You did all the work! Congratulations
By the way the cables are on the way.
Drew

zingo

That is the best looking set-up I've seen in a long time! Nice work on the amp and I'm glad it's back to sound great.

bikerboy

Thanks,
I did all the work myself. It was very time consuming to figure out what caps(correct pin spacing, hight, width, rating from digikey) that would work. I somehow ordered one set too large in diameter and had to use a 1/2" long jumper lead. I guess thats not bad for a newbee. Dont expect quick turnaround if you order from Percy Audio. I had to wait 10 days for Percy Audio to respond to my order request for all the resistors. Then another week for it to show up. Does anyone know about using caps in a super-e configuration(+to-,-to+) that is used to eliminate esr distortion in respect to power supplies? Just a thought. Ordered the hookup wire from VH Audio so the new rusky caps will be installed soon. Thanks NJPOLKER I am looking for the mits. More good things. The Esoteric P-10 is sounding great in the system and I just ordered a used 275mhz o-scope from ebay to mod the coax out to solve the warm up issue. The Lamizator site is dangerous. Back to work.

Early B.

Great job.

Give Bill at Response Audio a call regarding the hum. He may be able to help you track it down.