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 April 22, 201412 yr by DefQon

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?

You can do a diode test but they seem to be damaged but still functional.  Personally I'd just replace the entire output stage but then again I like scorched earth as a policy...

Don't know if anyone has an extra set of boards -- Amp and PSU -- but if you do please PM me. This looks like a fun project.

Looks like spritzer has some of the original style psu boards still available: 

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?

 

Edited April 22, 201412 yr by jboehle

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 April 23, 201412 yr by GrindingThud

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!

when tom's real ksa5 suffered damage due to a stupidass company making a stupidass

and incorrectly wired power supply cable and the + and - voltages were flipped, the

total of the damage was 2 bad resistors and one transistor.  Circuit is highly resistant

to abuse.

Circuit is highly resistantto abuse.

 

Or the PSU is too dinky to fry it all up! Still, there is a lot of frying to do on the KSA5.

The PSU isn't current limited so it will easily fry the amp given the chance. 

Got the replacement 1N914 diodes installed, DC offset is now less than 1mV DC in both channels with opamps in.  Letting it warm up now.  Gotta get back to work, will listen to it later.  Really hope that's all it was!

That is good news! Now stop probing and get to listening.

Got the replacement 1N914 diodes installed, DC offset is now less than 1mV DC in both channels with opamps in.  Letting it warm up now.  Gotta get back to work, will listen to it later.  Really hope that's all it was!

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!

its krell. it is supposed to run hot.

 

touch a fpb600 after its been on for a while.

Edited April 24, 201412 yr by kevin gilmore

I bet!  I was just trying to make sure nothing was getting too hot (beyond operating specs) while the case was on.  

Stop fondling it you perv. Just listen.

Sounds great!  Tried my cheapo Monoprice 8323 first, I get a little hiss at 50%+ volume with those, but I'm pretty sure that's coming from my computer or the pupDAC.  Listening now with Senn HD650, I'm jamming!

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. 

 

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

 

Me

 

Thanks Kevin for a great project.

DT880 sounds more balanced and musical than HD800?

It is worth getting your hearing checked.

DT880 sounds more balanced and musical than HD800?

It is worth getting your hearing checked.

Yup, make that a priority.

Having owned two HD800s and three B22s over time, I can believe better pairings. Even the 880 being more balanced (though it's not).

My experience with the term "musical" is the popular usage almost always means I wont like it. So, seems an apt usage.

Feel similar, especially if proceeded by 'CATS' or 'Mamma Mia.'