Stuart and Neil Build a KSA 50 (tweaked for 100 watts)

[john65b]

Getting the 100 wpc from the KSA-50 - another pair of output devices? What did you use for output devices? What was bias current set to?

Sorry for the probing questions...

[trilok6348]

Hello,
Congratulations for these ones !

Have you compared it to a KSA 100 mkII (clone or original one) ?
I'm wondering if I'm going to build a KSA 50 MKII tweaked for 100 watts or a KSA 100 mkII.

Would you share on this ?

Thanks for your replies.

[Stuart Easson]

Hi,

Neil suggested I chime in with some basic operational parameters:

Power supply: +/- 40v, +/- 20000uF, 2.5A bias current from a 650VA toroid. The toroids were cores I had that we decided to use. I rewound them with dual tri-filar secondaries to get the voltage and current we wanted.

6 output pairs mjl4281/4302 with 2ohm Re. Single drivers are mjl15031/2. The Re sounds high, but actually keeps the drivers at the same bias point they were in the KSA50. I *think* this is important to the sound quality these amps have. I've built quite a few channels of this amp, more driver current has always had them sound nicer to my ears, more effortless and open. I'm sure there are limits. We figured the KSA50 was very well regarded and so we'd start there.

Output stage is capable of taking everything the transformer can provide, until either the 75c breaker opens, or the fuse blows. The speaker is protected by a DC offset detector with a delay etc.

In the course of making these we had occasion to test all the safety features:

At one point we thought the ZD5 was taking enough power to blow the 3.15A fuse...we kept replacing it for a while, but eventually realized the amp was slightly mis-wired and we were driving the output to ground (in addition to the speaker) via a CL60 thermistor 10ohms cold, 1ohm hot)...I estimate it was dumping >30A peaks by the time the fuse blew, all the time driving the ZD5 with no evidence of strain.

We also found out the heatsinks will reach about 170f after 10 mins of playing with the fans not spinning, at which point the main thermal breaker cut out...again no sign of problem at the output...

So far no serious comparisons with anything, neither Neil or I have anything of comparable capabilities. He may have compared them to an F3, but I've not heard the results if that has happened.

HTH

Stuart