Krell KSA 100mkII Clone

[pinkmouse]

It would be feasible, and something I considered at one point, to move the driver stage to the output boards, but I gave up the idea because of getting an output layout that could accomodate everyone's differing heatsink requirements.

[PWatts]

Layout is coming along nicely. Still need to go through a few rough iterations before I can post a preliminary layout picture. Quick poll about a few things though:

1) If one of the dimensions have to go up, should it be length or width?

2) Is it necessary for the capacitors to be so large? This is in particular referring to the bias transistor's 100nF CE cap, the input shunt and the 62pF's. Isn't standard 7.5 and 5mm lead spacings big enough? Only axial caps need that big spacing, and the ones I used were still much smaller.

3) Since the power dissipation between resistors is closer in this version I've decided to use the same footprint for all the resistors. Admittedly some of the low-power ones' size can be reduced, but a lot of people are presumably going to use those big Dales throughout. Tip: the PRP resistors from pcX are available in nice small packages and offer very good quality at a price not much higher than Dale.

4) Is it OK if I reduce the resistor length Al's used by 3mm from 19 to 16mm? That is still long enough for 0.5W resistors, including Dale. Same thing goes for the diodes and zeners, their footprints can be much smaller even to accommodate 1.3W ones.

5) Is the heatsink arrangement used for the four cascode transistors on the original Krell acceptable (i.e. four separate ones) or should I try making them all face in the same way for a larger shared sink?

6) Would people like the idea of having both the value as well as reference designator on the silkscreen for each component?

7) Still one zener or two in series for lower power dissipation (then 0.5W ones can be used without fear) and higher accuracy? 2x20V will work well; 1V higher than 39V will not make a difference.. besides a 39V zener is likely to drift anyway.

8) Has anybody yet made sense of the silver mica cap next to the feedback electro's on the picture of the Mk2 board vs the schematic? By looking at the routing of some of the components I suspect that there are small changes between the two. Another reason for suspicion is the power dissipation of the cascode transistors: for the schematic we have, one of each pair (those connected to the rails) dissipates almost a Watt, but the other one less than 50mW. The second one hardly needs a heatsink, but on the picture all four have equal sized, big, sinks. I'm probably going to service a Mk2 within the next two weeks or so, and then I'll take a good, long look at its circuitry.

[Mark A. Gulbrandsen]

1. More width is ok.

2. 7.5mm should work but I think the woder spacing was for WIMA caps.

3. I get the Dales for 10 Cents each now. Be sure the holes are large enough for the Dales!!LAst board the holes were tight.

4. Smaller resiistor foorprint should work ok.

5. They should face the same way so they can be on one sink. Sturdier this way. Trasistors supported by their leads and carrying a heatsink are not very sturdy.

6. Refrence designator is sufficient since some people drift away from the actual values anyway.

7. What ever on the zener... either is fine.

8. The mica cap could have been added to some amps to cure parasitic oscillation if an individual amp exhibited it.


Hope some of this helps.....

Mark

[jacco vermeulen]

the silver mica cap next to the feedback electro's

Frequency/gain compensation in the NFB line.
One lead of the silver mica is connected at the connection of the Roederstein electrolytic + pole and the feedback resistor lead.
The other silver mica lead can only go to the feedback resistor's other connection.
Which makes sense:
- that's where a silver mica is good for
- the technique is in line with the trend of the MKII years.

(the other bypass cap is a Roederstein MKC1860, btw. Metalised polycarbonate, not polystyrene. 10% accurate/100C max)

[Jozua]

Pierre


Personally I dont like PCB's that are too small and where you struggle to fit parts. Rather err on a size too big than too small. It also makes it so much easier to position and mount heatsinks on the PCB.

Maybe somebody (Mark G.?) can source a standard heatsink in bulk for which the board can be custom designed and that could possibly be sold with the board? It just looks so much better when a board is custom designed for a particular heatsink.

Who is designing the smaller PCB's that operate the relays in the Mk2 version?



Jozua

[PWatts]

A custom heatsink would be great, esp if it can be bolted down as with the original.

No need to go ultra-small on the board, but smaller reduces chance of oscillation and other nasty parasitic effects. Not too likely on this board though.

I also had the problem with tight resistor holes. I even had to shave some Holco H4P resistors with a sharp knife to make them fit - well at least they're damn sturdy now hehe.

I also figured the mica to be an additional feedback pole, was hoping for a confirmation. Judging from its position it's very likely. Will include it in the design, easy to leave out. COG ceramics are even more linear than mica btw.

The other feedback isn't possible to be polystyrene anyway; 100nF polystyrene would be one huge bulky affair. Polyprop/polyester/polycarb's the only choice there. I personally prefer Wima/Rifa MKP's for the 100nF's, mica for the 62pF's and polystyrene for the input shunt.

Mark, do you have any idea of how many people would be on the group buy list?

[jacco vermeulen]

Originally posted by PWatts
isn't possible to be polystyrene anyway

Sure,
just thought of mentioning it, there are some reading this who can't tell one from the other. Just the other day i dreamt of opening the front door to trip over a 1uF styroflex cap.

Probably a cosmetic revision for which Krell may have sent an update to service mechs. , but not an entire new service manual for just 1 cap. Krell had a number of revisions and optionals in those days, from exchanging PCBs to converting to KMAs.

[PWatts]

I've been wanting to experiment with those teflon REL-caps for the Miller caps but they're just too damn expensive and bulky.

What exactly is the difference between KSA and KMA anyway? I know of some refinements to the power supplies.

From the looks of the input stage on the picture it doesn't gel if the transistor pinouts have the collector in the centre (as 2S series do), but it's hard to judge without having the bottom side as well and the component values.

[jacco vermeulen]

Off topic but here goes:

Changes in the powersupply and all the main boards were swapped.

Example:
In 1990 Krell had a marketing offer for the KSA250.
For just the transport costs Krell converted a KSA250 to a monaural MDA500. Of course, unless you had 2 KSA250s you had to buy a matching second MDA.

(With the KSA250, Krell converted back to BJT only, the previous used FETs. Dan d'A personally did the KSA250 introduction overhere, unfortunately i couldn't get in, invitation only)

[Stuart Easson]

Based on what I've seen on other threads here, it seems (at least for some models) that a KMA-xxx is based on a bridged configuration using a stereo chassis of about 1/3rd the nominal power, ie a KSA-50 can be turned into a KMA-160...

Theoretically bridging should give a 4x increase into the same load (ignoring losses), but perhaps the specs are being convervatively stated...or the rail voltage is reduced to allow the idle current to be increased, giving more class A output...

Hopefully someone knows the details...

Stuart