Krell, KSP 7b, single ended, class A, dual mono pre-amp.

[sq225917]

I've been on a bit of a pre-amp trip of late. Ever since getting the EAR 509's, which have their own volume pots, I've been trying to find a pre for my Tag monoblocks that gives me the 'nothing in the way' feel of the EARs, but still delivers the bass that the Tag's are capable of providing. I love the Ear's they sound great from top to bottom, it's just that the bottom isn't quite as bottomless as the solidstate Tags can deliver- and I do like my bass.

I've been through a few of the usual suspect, Passlabs B1, basic and DC coupled versions, I've tried the Tom Evans Vibe +PSU which was good but lacked a little drive for my taste. I then stumbled onto a XTC Pre-1 (made by Orelle). This is a great sounding pre, flexible, solid top to bottom, it does great stereo and has a precise sound that appeals, but that last half hand of double bass and keyboard just doesn't carry the gravitas that it should. But it is a looker with that gloss front panel.

http://farm8.staticflickr.com/7048/7003906109_728a48ff7d_b.jpg


Enter the Krell KSP-b7 pre-amp. It's a 93 model in perfect condition, with a recently recapped off-board PSU. It's single ended, class A, fully dual mono and has a reputation for being handy in the bass dept. It also has a built in phonostage, so if it's good enough I could shed a few boxes, phonostage and ext PSU for starters.

http://farm8.staticflickr.com/7094/7008998627_463d11cf6f_b.jpg

I'm digging the pre-amp side of things, there is bass texture at the bottom end in spades. It's not overblown and loose, it's pretty damn tight and right. Tracks like Rickie Lee Jone's Easy Money are transformed as is the XX album. It's a great line stage.

Sadly I'm not digging the phonostage quite so much. For starters it has way more self noise than my Modded Prefix has. It is 20 years old though, and unlike the external PSU none of the electrolytic caps inside the pre-amp/phonostage stage section have been replaced and they are probably well overdue for some love. There's also 4 pairs of 2 watt carbon comp Holco resistors in there which might well be contributing to the higher than expected self noise.

The phono section does seem to have quite a different sound to the line stage, which makes me think it might be needing some love. I'm going to fiddle with cart loading values and then wire the Prefix through the line stage and see how that sounds so I can compare the two.

We'll see what happens, one thing is for sure though either the XTC or the Krell will be up for sale soon.

Does anyone else have any experience of the older Krell pre's, particularly the phonostages?

[Reid Malenfant]

A friend of mine has the KRC-HR which is a good bit younger than your KSP, it has a built in phono stage which was an option. Not the Reference stage, but the one just below it which could accomodate both MC & MM cartridges (the Reference was MC only).

Bit of a bugger as he also has a 1210, but the last time I went over to see him the record deck wasn't connected to the system which was a shame as I'd love to have heard it on a decent system.

From memory the pre certainly was capable of deep & clean bass as well as it being just the same accross the whole frequency bandwidth. I think if I wasn't running a multi channel setup that it may well be one of my pre amps of choice :D

Not the coolest running device though & along with his KSA250 a bit problematic in the summer :lol:

Sauna springs to mind...

[sq225917]

Yeh this one runs a touch warm in the psu. I'll crack it open tomorrow see if everything measures up as specced.

[sq225917]

krell internals-slideshow (http://www.flickr.com/photos/59684834@N08/sets/72157629656297809/show/)

[sq225917]

Well the replacement caps arrived yesterday, 36 of the buggers. I warmed up by fitting new 50v 1000uf Panasonic FM caps to the external PSU. They are higher temp co rated than the ones that the previous owner had the unit serviced with- it runs hot so it's a sensible precaution if nothing else. One thing became immediately apparent, the quality of Krell PCB's isn't all it might be. It's going to be a major task to replace all of these caps without lifting any pads or worse tearing a track on the pcb. The vias through the board are all 1.5mm wide which means they hold a lot of solder and take a lot of heating. I managed to remove and replace 8 caps in the PSU in about 2 hours, yeh it took that long to do it clean without doing any damage.

Here's everything in the main pre-amp disassembled and ready for removing all the electrolytic caps.
http://farm8.staticflickr.com/7263/7022226271_e16b92bf28_b.jpg

Rather than run the risk of lifting any pads I decided to take it over to Ynwans so he can press a soldering iron against one leg while I do the other- there's no way these caps will waggle out one leg at a time without breaking circuit tracks, no way no how. It took nearly 3 hours but we managed to do all the caps in the pre-amp section, L&R channels. I'll leave the phonostage for another day, there's a lot of caps in there and doing it after a few drinks at 11:30pm just wasn't an option.

Anyway here she is all built up agin and ready for a few measurements and an overnight soak.
http://farm7.staticflickr.com/6102/7022230715_235ffa0cc8_b.jpg

and she sounds fine.

[bogle111]

IIRC, and it was a long time ago (20 plus years), we tried one of these against others inc. an Audio Research SP9 all hooked to a KSA50. I liked the Krell because it was pretty analytical and had good bass, very dynamic. General consensus of those listening, was the AR was slightly better (which is pretty obvious really from a hybrid) which I then lived with for a while. I wish though that I had the Krell for longer as I prefer analytical instead of a pre putting in its' own stamp on things. It did though, like the SP9, get some mixed reviews.

Seem to remember that the PSU's on this model Krell did run hot, even US models and got more than a few comments. Phono section was pretty good though.

Please don't take this as gospel, though my memory is not too bad.

Regards
Pete

[pure sound]

Did it come with the Black Noble pot as stock? Good choice, shame they aren't making those any more. I'm not sure they track quite as accurately as the ALPS blue at low levels & I think they have more of a tendency to get noisy with time but they were very good sounding. (for those who believe such things can make a difference)

[YNWaN]

The pot has been replaced at some time in the past.

I've heard a pot change make an audible difference, and I would agree that the Noble is better sounding than the Alps (but as you say, less well channel matched at low levels).

[sq225917]

Guy,the original was a Penny and Giles but it had got noisy with age so the previous owner had it replaced. No argument from me about the low level tracking on the Noble pots, I had to rip two apart to get a decent match on my NAC72. The Krell however is spot on, so much so I've not even touched the balance pot.

If Mark's willing we'll swap out the phonostage caps tonight. With cocktail stick pressed against the upper PCB surface so as not to lift the tracks.

[sq225917]

My Mrs is a demon with the soldering iron... ;-)

So here it is all done, every electrolytic cap replaced with 105 degree temp rated Panasonic FM. These are the lowest ESR standard value caps I've come across, I can think of nothing better to use in this instance. I reckon there's 15+ hours of desoldering, cleaning, masking, and resoldering in this Krell pre-amp now. A proper labour of love.

http://farm8.staticflickr.com/7043/6887125308_0806137c60_b.jpg

It should be good for another twenty years now- and it's sounding bloody lovely.

http://farm8.staticflickr.com/7205/7033232431_62f9d4fd2e_b.jpg

It needs a quick brush and wipe clean to get rid of that dust before I close the case...

[bogle111]

Too bad it's not a perspex lid, seems a shame to cover up such mastery.

Regards
Pete

[sq225917]

I couldn't really give it a decent listen last night, but this morning it is sounding lovely. Solid and detailed in the bass, with greta stereo specificity and quite startling resolution. I might get round to plugging in the phonostage in a bit.

I've never heard Fess sound so good.

[nat8808]

Out of interest, what did you replace the ROE caps with and did you choose value based on the stated value?

I always think it's a leap of faith to assume the stated cap value on the can is the intended value as some manufacturers measure values from bulk purchases and then store according to more accurate measured values within their range so they can be matched. Might not apply to the lower end Krell though. Did it improve things?

Err what did you do with the removed caps? Might be handy for replacing definate failures within vintage gear as you know what it can be like sometimes re-selling gear that has non original parts, replaced by the owners.. even though it's probably fine, people don't always trust it. Certainly they are the same caps as in my 90's LFD phono stage.

[sq225917]

Yes I always assume manufacturers buy capacitors in values other than those required so that they can store them and use ones that have gone off- rather than simply buying the caps in the values that they actually require. Obviously the most sensible way to purchase parts for any manufacturer is to buy ones that they do not want and hope that some shelf magic makes them into the right parts.

To be honest I'm surprised that they even bother purchasing any specific parts at all. They could just throw any old shite in the box and hope that quantum physics makes it work for them.

Curiously my own experience with selling gear that I've serviced or modified doesn't seem to mirror your own, if anything i find that gear I've modded tends to carry a premium over unserviced examples of the same gear because of the reputation I have for lavishing the utmost care and attention on the gear that I own and only ever replacing like for like or with better specced parts where older alternatives are no longer available.

[sq225917]

Right then this one comes back to life.

As the phonostage was off in one channel and the transistors were utter unobtanium I had shelved the final steps of the restoration of this, not enough time and certainly not the inclination to hunt down remaindered transistors from the web, and match a suitable quantity. Time to farm the job out...

So I dropped it off with Jez@Arkless, the guy who designed some of the Alchemist amps. A week later and £135 lighter he's managed to source the transistors that i couldn't even find on the web they were so old and for the others (j74) he has stock of the part that Toshiba replaced them with- bonus.

So it's up and running again, completely re-capped in the PSU and the main pre-amp and the front end of the phonostage has all new transistors in it and it's all biased up to the correct spec and should be better than ever.

I'll pick it up some time this week and see how it sounds.

[YNWaN]

Cool, glad to hear that it's working again :).

I'm not really sure what Nat is on about regarding capacitors. Some manufacturers may match capacitors so that they are within tighter tolerance than the manufacturers spec, but the basic value is always the same.

[DSJR]

Those old Roe caps may have looked to be good what the Krell was first made, but they do seem to be intolerant of being run hard. Simon, am I right in saying that many of these were bulging on top? The Roe's in my 1980 vintage Quad 33 were fine, but I've seen similar in old B&O gear where they'd all but exploded with thermal? stress.

[sq225917]

Dave, You'd struggle to make the ROE's bulge, they are capped in phenolic, they just pop.

[Reid Malenfant]

Well done getting it sorted Simon :)

The chap I mentioned that had the KRC-HR apparently took a look in the PSU & decided he'd better get something done about it before it expired :eyebrows: From the description he gave me I thought it had actually packed up, but no, even though he said it looked like some kind of forest fire had visited it was still working :rolleyes:

He sent it away to a place that'll remain nameless (though is very well known) & still hadn't got it back the last time I happened to be over his way, I hope he's actually got the thing back now as I'm off over with a friend some time this coming weekend.

If only he'd have mentioned it I'd have offered to do the re-cap myself...



I hope yours gives years of trouble free service now it's all good again :)

[nat8808]

Cool, glad to hear that it's working again :).

I'm not really sure what Nat is on about regarding capacitors. Some manufacturers may match capacitors so that they are within tighter tolerance than the manufacturers spec, but the basic value is always the same.

I just meant that in order to get tight tolerance components, some manufacturers have been known to buy looser torerance components (say 5%) and measure each individual component value so they can then sort them into 1% catagories. Saves buying the more expensively manufactured components whilst still providing close tollerance.

Certainly Kef used to do this for crossovers in their reference series and Stan Curtis used to do this so that their standard TDA1541s were effectively closer to 'spec' than the official (expensive) S1 and S2 close tolerances. For example anyway.

So the values stated on the components could perhaps be 10% from their measured and deliberately used values.

I guess it would be a bit anal to take that into account though if the bits really do need changing.. and they might drift out over time too.

[sq225917]

That's how most brands do it, i did the same thing when I was replacing the caps, bought twice as many as I needed and sold the poor matches on eBay. I'm going to pick it up today...

[MartinT]

Nice looking circuit board design there, Simon. I'll bet it was a pig to recap but well worth the effort.

[sq225917]

Through hole, double sided with tiny pads and vias that were way tight. I managed it without savaging the board, but it took me about 6 hours to replace all the caps.

[sq225917]

Yeh, sadly I'm not keeping it, the Muse has rendered it surplus to requirements.

[sq225917]

Now in the classifieds for £800