krell ksa5 klone

[luvdunhill]

http://www.cordellaudio.com/JFETs/LSK489appnote.pdf

[kevin gilmore]

input stage runs at 4ma, so yes it will work

 

the original part was higher capacitance and krell

used a 10k pot because of it.

 

10k part is still a better idea

Edited October 31, 2014 by kevin gilmore

[G600]

So LSK389A (hard to source in old Europe !).

[kevin gilmore]

is lsk489 any easier to source?

 

a pair of lsk170's or 2sk170's

also works

[G600]

Biggest problem is shipping fees.

The BOM points Trendsetter, but they are charging 4 times the parts price for shipping...

Everything works like that from here. Guess I'm not in the good person's little papers !

Nevermind.

[UFN]

So LSK389A (hard to source in old Europe !).

I bought from Micross in the UK (https://www.micross.com/ecommerce/shop-default.aspx) Their postage charge is still high, but more reasonable than trendsetter and there is no risk of additional customs charges since it is from the UK. Also, I got my parts delivered within the same week as I placed the order.

 

//UFN

[G600]

Thanks^^

[kevin gilmore]

notice that 2sk170 or lsk170 are not pin for pin, you have to bend the pins to get the correct pinout

[luvdunhill]

The drain and source are interchangeable. Seems odd, but most jfets have this property.

[kevin gilmore]

lsk170 and 2sk170 have the gate in the middle, not the either end like the dual.

[UFN]

Have now finished putting the amp together and without the opamp in place I get around 7mV offset on one channel, but around 1.5V on the other. The amp otherwise seems stable although I haven't listened to it yet. Any ideas as to what i need to check to find the problem?

 

//UFN

[kevin gilmore]

you have to use the servo on this amp. not designed to work without it.

you would have to very carefully match the input fets

[UFN]

So nothing out of the ordinary as long as putting the opamp in will zero it out? Was just surprised that the difference is that big based on previous posts here

 

//UFN

[spritzer]

1.5VDC?  That's a lot and I think the servo can't eliminate that much offset. 

[GrindingThud]

Were the inputs grounded (or the pot installed) when making the measurement?

So nothing out of the ordinary as long as putting the opamp in will zero it out? Was just surprised that the difference is that big based on previous posts here

 

//UFN

[CraigBuckingham]

UFN, there is obviously a fault.

 

For a 2N5564 with a data sheet specification for maximum offset of 5mV between the pair, this circuit should see a maximum of 25.6mV output offset for a worst case 2N5564 with the servo disconnected.

 

This thing doesn't need a servo if it's using a matched input pair like the 2N5564. The servo can be removed and a trimmer potentiometer used to trim R3 and R4 to shift the drain currents of the input pair to null the input offset and therefore the output offset.

 

R45 could be removed and R46 replaced with a 750Ω resistor to keep the gain the same. With the added benefit of removing the LF pole created by C8, C9 not the least the extra distortion caused by those electrolytics. So a true DC low frequency response.

Edited November 28, 2014 by CraigBuckingham

[CraigBuckingham]

I thought spritzer would be first to call me out on this.

 

Anyway, 1.5V offset is way too much, there is a fault.

[nopants]

Despite the departure from klone status, I am interested in your improvements. Have you implemented any of the modifications that you're outlining? I have a spare board which I might consider stuffing.

[CraigBuckingham]

Since it is off topic and is probably not what the majority of members would agree with I'll refrain further from any suggestions keeping with the spirit of the topic being a clone.

 

It would be more appropriate to create another topic or probably keep it to PMs.

[guzziguy]

I think creating a new thread (topic) for it would be better than PM, but it's up to you.

[Pars]

Agreed. Public record (and review) would be better than a bunch of PMs.

[muskyhuntr]

Finished my KSA5 and am very pleased after a relativly short listening session.  Board parts are standard from the BOM  with extra attention paid to closely matching the parts.  Goldpoint controls, Vampire RCA's, SUMR transformer.  A big thank you to Kevin Gilmore for making this project possible!

 

 

 

 

Me

[UFN]

1.5VDC?  That's a lot and I think the servo can't eliminate that much offset. 

 

 

Were the inputs grounded (or the pot installed) when making the measurement?

 

 

 

UFN, there is obviously a fault.

 

For a 2N5564 with a data sheet specification for maximum offset of 5mV between the pair, this circuit should see a maximum of 25.6mV output offset for a worst case 2N5564 with the servo disconnected.

 

This thing doesn't need a servo if it's using a matched input pair like the 2N5564. The servo can be removed and a trimmer potentiometer used to trim R3 and R4 to shift the drain currents of the input pair to null the input offset and therefore the output offset.

 

R45 could be removed and R46 replaced with a 750Ω resistor to keep the gain the same. With the added benefit of removing the LF pole created by C8, C9 not the least the extra distortion caused by those electrolytics. So a true DC low frequency response.

 

Thanks for the replies guys, I have been away and unable to retest until today. I was pretty sure I had the inputs properly grounded last time, but I removed the board from the case, reflowed a couple of suspicious joints and then retested it with better cabling. Now I get around 8mV on one channel and 22mV on the other without the opamps, and less than 5mV on both channels after fitting the opamps again.

 

Not sure what exactly the problem was, but it looks like I've fixed it 

 

//UFN

[UFN]

Finished my KSA5 and am very pleased after a relativly short listening session.  Board parts are standard from the BOM  with extra attention paid to closely matching the parts.  Goldpoint controls, Vampire RCA's, SUMR transformer.  A big thank you to Kevin Gilmore for making this project possible!

 

<snip>

 

Me

 

Nice work on that chassis!

 

//UFN

[jose]

Nice!!!