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krell ksa5 klone


kevin gilmore

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Let the amp warm up a bit around 5 minutes or so then adjust the bias/offset, values tend to drift a bit if you adjusted on a cold start up compared to warm up period, at least the procedure I take with all ss amps when making adjustments with bias and dc offset.

Edited by DefQon
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Okay, I left the amp on for 15 min or so, adjusted the bias on channel 1 to 99mV.  I believe I read earlier in the thread that the bias should be 125mv cold, or 90-100mV after warmed up.  To get channel 1 to 99mV, I had to turn the bias pot all the way down (counterclockwise), so that was as low as I could get it.  On channel 2, I was able to adjust the bias down to 95mV without reaching the limit of the bias pot.  This gives me pretty much the same DC offset in each channel, without opamps installed, as I posted last night.  Channel 1 = 6mV, channel 2 = 56mV DC offset.  I also get the same results as last night if I turn the amp off, install the opamps, turn it back on, then my DC offsets are channel 1 = 60mV, channel 2 = 0.4mV.

 

Are there any measurements I can make to verify each output transistor is working correctly?

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Personally I'd just replace the entire output stage but then again I like scorched earth as a policy...

 

It won't be fun :) but I guess that's the only way to go.  What all should I replace?  Just the stuff in the red box?  Or should I replace any of the stuff in the yellow or blue boxes as well?

 

VtZFYIS.jpg

Edited by jboehle
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Before you pull it apart, check the diodes at the input to the op amp (1N914 between the heat sinks). There are a number of resistors between your accidental short and the input/output. I doubt you fried a transistor. The diodes however.....zap, straight from V- to ground. I bet you fried that. Worth a look. Also, listen to it......scab some earbuds on it and hook up a sleazy input source.....you'll know if it's broke. :)

Edited by GrindingThud
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GrindingThud, thanks for the suggestion!  I pulled the 1N914 diodes out of both channels so I could compare the good to the bad.  Both of the channel 1 diodes were bad as you suggested.  Both of my channel 2 ones were still good.  Luckily my local Rat Shack stocks 1N914 diodes (at least according to the website), so I will be picking some up first thing tomorrow.  Thank you so much!

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After biasing to 125mV at cold startup, then warming up with the case lid on, ch 1 settled to 104mV and ch2 settled to 100mV.  I turned down ch 1 to 100mV.  Amp heatsinks range from 45-50 deg C.  I am using the 2" heatsinks.  The PSU TO-3 cases are at 55 deg C, PSU Opamps at around 50 deg C, amp Opamps at around 35C.  Looks to be running good!  Now to hook up a source & some 'phones!

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  • 3 weeks later...

Hi All,

 

Been working on a couple of Gilmore projects at the same time.  Shown here is my compleated KSA5 power supply (new version).  Waiting on SUMR transformer before I test it.  I also have a good start on the amp board.  Still need to populate the 475 ohm resistors before I go on to the transistors. 

post-4368-0-47223400-1400104859_thumb.jp

post-4368-0-07964200-1400105400_thumb.jp

 

Love this project.  For those interested, I have one amp board and 2 power supply boards I would like to get rid of.

 

Me

 

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  • 4 weeks later...

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