Krell KSL Pre Amp / Phono Board Recap

SCompRacer

My recent DIY DAC was designed for balanced out and already having balanced active crossover/amps I wanted a balanced pre to connect it all up. Ed (Savinon) has been trying to get me Krell-ed for quite some time so I got this old KSL from him complete with KSL phono board.

We are reaching back 23 years with this old Class A design. The pre sounded good, was quiet, but it had some phono stage noise issues on the left channel only with the phono board set to high gain MC. The phono board is a Fairchild J309 jfet N channel RF amplifier / MPS8599 PNP /MPS8099 NPN transistor design. I had feared the worst (bad jfet or transistor) but it just turned out to be a bias issue. Once the phono board was re-biased all was well.

There are different recap camps. Some say only the caps that are bad, many have been telling me get rid of them 23 year old caps right now. OEM caps were Sprague and Rhode. No idea what happened to Rhode, but Vishay bought Sprague and they are just a Tantalum division now so matching brands was out. In circuit testing is not 100% reliable so I pulled some caps to test and found them all over the place. Some were worse than their 20% spec, some were closer. I just decided to recap the entire power, main and phono board to be safe.

Fred was a big help (as usual, thanks Fred!) and had me go with United Chemi-Con caps. That name isn't bandied about but after some research I learnt they are a well respected cap. I used all 105C caps, UCC 1000uf KME's for the main board, UCC KMH for the 4700uf power supply and four extra long life 470uf UCC LXY's for the hot spot between the sinks.

On the phono board I used 105C Vishay AML 138 series. Fred also surprised me with some long OOP 1% NOS Siemens Styroflex polystyrenes to replace the four .033uf poly caps. These are either part of the RIAA filter or some signal path thing. (No schematic available, only $125 an hour for Krell service). I have two Philips .0016uf polystrenes on the way. Word is Siemens and Phillips were the best polystrenes made.

After washing the boards with a 99% isopropyl alky wash and reassembly, I found it survived surgery and is good for another 20 years.

TNHNDYMAN

Nice work! I'd love to listen to it w/ your vinyl rig playing. Your DIY is always inspiring, keep it up.

michael1947

That is such a beautiful job! You are so fortunate that you have the talent to upgrade and repair your gear.

FTGV

Beauty job as usual ,it's a perfect spot for those styrenes, glad they found a good home.

agfrost

Well done Rich. I'll let my ears be the final judge.

SCompRacer

Thanks very much for the kind words. This stuff ain't that hard once you learn how to solder/desolder. If it don't work upon reassembly I just send it to Fred. He can fix anything!:cheesygrin:

agfrost wrote: »

Well done Rich. I'll let my ears be the final judge.

Kewl! Looking forward to your opinion.

My 1TB music drive puked 5 days out of warranty so I stuck a spare in. Thank goodness I had it backed up or we'd be having to do digital the old way.:eek:

FTGV

SCompRacer wrote: »

He can fix anything!:cheesygrin:

I wish,there are a couple of pieces in my basement that disprove that.

My 1TB music drive puked 5 days out of warranty so I stuck a spare in. Thank goodness I had it backed up or we'd be having to do digital the old way.:eek:

Ouch only 5 days that sux..

ESavinon

I want it back!

ESavinon

Great work as usual. I hope it serves your purpose well.

george daniel

Nice work Rich,, atta'boy Fred!

SCompRacer

ESavinon wrote: »

I want it back!

Sure! I'll trade you for that Krell KRC-HR.:eek::cheesygrin:

ESavinon wrote: »

Great work as usual. I hope it serves your purpose well.

Thanks Ed. After the tune up it sounds really good. Adjusting the bias did the trick with the phono stage. It should have occured to me sooner (before removing and testing the 22 jfets/transistors on the left channel:redface:) as the right channel bias voltage was different every time I checked it, usually after I adjusted the left channel to match. A schematic would have helped as Fred would have read it and said go here and check this and that.

The folks smarter than me tell me that a different bias on each channel (-.569 versus -.612) is not a big deal as they doubt Krell matched jfets/transistors exactly between the R and L channels; only components on each channel would be closely matched. After turning the bias pots a couple of turns either way while off, bias adjusts smoothly and have been holding steady. Before there was too much - bias voltage on the left channel which caused the MC hum and gain issues. The jfet/transistor that are biased by the pot set voltages on other transistors. They was juiced up too high, causing the hum and excess DC on left channel output.

I have even more deeper and articulate bass. Extended highs as some folks felt the ModWright rolled them off (could have been the tube CD player used at the time). Digital with my new DAC is closest to analog sounding as I have ever heard. The pre just gets out of the way and does not color the DAC. The vinyl, incredibly dynamic. I never had vinyl bass like this before. Most likely all due to the combination jfet/transistor design (I've had balanced jfet gear here before). Could be my speakers, but the pre does not sound strident, or analytical, at all. I had asked Jay over as I have been heavily invested in time and effort with this pre and the DAC; I wanted some verification that I was hearing correctly. I am now tubeless in my two channel.

george daniel wrote: »

Nice work Rich,, atta'boy Fred!

Thanks George! Good to hear from you! Fred is # 1!

EndersShadow

Man you IL boys are just close enough to tempt me to drive out there to see/listen to your gear sometime lol.....

That or be able to buy it when you move on and save on shipping hehehe.....

[Deleted User]

Very nice work!

hochpt21

Wow, great work. My head spins just looking at those pics!

hochpt21

Edit. Whoops...double post. Well deserved double post though!

agfrost

I can attest to how good Rich's system is currently sounding; this revived Krell has certainly brought his analog rig up another notch or two (and it was pretty damn nice where it had been before). I'm happy that he was able to diagnose and fix the electronic mischief the MC phono section had been making, it's really singing now!

Then the Krell's balanced design allows the DAC he built to perform at its best. He set out to make a no-compromise run at bringing his digital as close to his analog as possible and has come admirably close--in spite of the fact that his analog has improved with the Krell as well. Nothing about his digital screams digital at this point.

Thanks as always for your hospitality and the good tunes, Rich. It's a treat to sit in the sweet spot at your place and to shoot the breeze as well.

Jay

F1nut

Hey Rich, nice job!

SCompRacer

Thanks for the kind words! Jay, you are welcome anytime! Thanks for lending your ears....:cheesygrin: Ender, come on out!

I needed two .0016uf Styroflex caps to totally recap the phono board and compliment the four Fred had supplied. A fellow in the Netherlands had a bunch of Phillips 1% Styroflex polystrenes on ebay and had the values I needed. He was kind enough to check they were as stamped (these Styroflex are spot on!) and I found them to be exact. Now its done and I can put the screws back in the cover.

The two little silver jobs near the top, middle board between first row of resistors.

ZLTFUL

You should update those resistors next. :eek:

LOL

Beautiful work man. I have done recaps on some much less elaborate boards but even so, I can appreciate the amount of work you put in there.

SCompRacer

ZLTFUL wrote: »

You should update those resistors next. :eek:

LOL

You mean them things that look like the back side of bees? I already tried making the round things with flat sides face the same way and that didn't go well.:eek::cheesygrin:

FTGV

ZLTFUL wrote: »

You should update those resistors next. :eek:

LOL

Yup,time to man up Rich and replace the Dale's with some nice Caddock or Vishay bulk film resistors.

Face

Vishay Z Foil resistors.

headrott

Face wrote: »

Vishay Z Foil resistors.

Yep. Without a doubt.

SCompRacer

Another challenge! The six Dale's are probably the only ones doing the work, the rest for show.:rolleyes::cheesygrin: I dunno, the phono board already has a high FW, er, mess with factor. It is at a point where the audio gods start spinning the wheel of chance, heavily populated with doom.:cheesygrin:

Hmmmmm .... z foils.... .01-.005% tolerance? That's tight.:exclaim:


This is how/when I figured out what was wrong with it. Left channel hum/buzz when set to MC. High DC in left channel output. This poor board has so many stab marks in the solder from the sharp pointy probes.....

headrott

SCompRacer wrote: »

Hmmmmm .... z foils.... .01-.005% tolerance? That's tight.:exclaim:

They sound "tight" too! I have them in my DAC. Just the .01% as I needed 6 of them and the price of the .005% would add up way too much.

SCompRacer

headrott wrote: »

They sound "tight" too! I have them in my DAC. Just the .01% as I needed 6 of them and the price of the .005% would add up way too much.

I've got two Legato's...that would get pricey....:eek:

FTGV

Well in that case while only 1% the the MK132 Caddock's are a better sonic bargain.YMMV

SCompRacer

If we're only talking six.....I'd splurge.:cheesygrin: Wish I could get a schematic so you could read it.:twisted::cheesygrin: Someone at diy thought it odd to see 8 jfets at the input side....I dunno but it works pretty good. The other four jftes are two per side at the edge by the bias pots.The two inner pots are to null any DC.

I called it a multilayer board but somene at diy said it was just double sided. But I can see traces with a strong backlight through the fiberglass that are not on either side of the board. There is no mask covering traces, and there is a seam on side of board. I guess it could be a sandwich of double sided boards, thus multilayer. Anyway that makes it hard to follow the circuit path.

headrott

SCompRacer wrote: »

I've got two Legato's...that would get pricey....:eek:

Complainer.:razz:

SCompRacer

headrott wrote: »

Complainer.:razz:

I'm a wino.....I mean whiner.:cheesygrin:

danielsu

Hi, I am new to the forum. I have a Krell KSL pre.
Where can I buy the caps you installed in your Krell?

Thanks