..... single layer PCB , first try .......
Regards Alex
Great layout. Your getting better and better. I always enjoyed your layouts since first building a Blame ST.
Simon
Also LazyCat had noticed some sibilance in VSSA before, when the regular PS was replaced to SMPS the "hiss" was totally gone.
By the way, has anyone tried SKA with SMPS?
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I won't say I prefer the SKA to the VSSA. The VSSA is more detailed than the SKA. Neither are really broken in yet. I am just finishing 5 amps. SKA, TO-3 VSSA, PMI Through Hole VSSA, DX Super A and DX Blame MKIII. So far I've only been able to devote a couple days listening to each of them. Built a case for the SKA today but didn't quite finish all of the install and leave for Hawaii tonight for a week so 'it will be a few days before I can give it a real workout.
Blessings, Terry
Blessings, Terry
Hi Paulo,
Doesn't Jim's use some altered resistor ratings to accommodate the 2SC/SA's? I am just thinking of the earlier post in this thread where someone mentioned him alerting buyers to the changed transistors and that the boards were made for these.
Stuey
Hi Stuey,
No, the resistors are the same. The difference is in the pinout of the board. If you want to use the BC's you'll have to twist their legs so they match the holes on the pcb.
I have had a dual mono SKA for several years now with almost daily use. When I first finished building it I remember it took many hours to burn in. At first the amp sounded dull, dark. Lack of detail. Once burned in it sounded totally different, plenty of detail, very liquid, nice amp. I was never a big proponent of burn in before this experience, but the difference was so large it was impossible to argue.
PJN
PJN
Member
Joined 2009
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Another thing to bear in mind, the sensation of 'detail' can be exaggerated by a bit of distortion at the top end. In particular it can depend on your speaker, but when comparing amps we usually have the same speaker. Anyhow, for me I find it helps to have a CD track with some cymbals or bells or something similar - this allows me to listen carefully to the top end of the treble; the cymbals should be clearly heard but also very clean, not 'splashy' nor 'muted'. My first listen to the SKA was similar - it was not 'sparkly' at the top end, just surprisingly smooth treble compared to all my other amps, a little dark as you say - and no time given for break-in. I'm waiting for new pcb's before I can do more listening.
I once experienced just the opposite. Long time ago I have built a kit from Heed Audio called Canopus. (If I remember correctly gaborbela knows this amp) It has CFP output and 10db feedback only. At the beginning of the burn in period it had uncommon extended top end, very crisp and open sounds, but two weeks later I felt it has lost many of its sheen. This phenomenon was perceived with Hitachi transistors only. The tonal balance remained same, but each musician took a few steps back. Not so direct not in my face. and very easy to listen It think this called "laid back" in english. It was hard to choose which one is better and also depends on the music genre. I think SKA is closer to laid back sound.
Regards, egra
Thanks, that makes more sense. I thought it must have involved more, because it would be easy to tell buyers to beware of pinout. I guess it's a bit risky though and they didn't want any disasters...
Hi Guys,
I need a little help. The amp was playing fine for a couple days so I finally got around to building a case for it and once I had everything bolted in I brought it up on the variac through a light bulb and see the bulb getting brighter so I know I have a problem. Once the rails reached about 25V R8 smoked on one channel so I shut it down and pulled that channel and started testing things and found a shorted IRFP240. Seems somewhere along the way a metal sliver shorted it to the heatsink. So I pulled the two outputs on that board and installed new matched parts and replaced R8. I dropped the bias pot to zero and brought it back up. R8 still starts smoking so I lower the voltage and start measuring things. I see that though R8 is dropping a lot of voltage R6 is not so I checked Q7 and see that it looks shorted so I replaced it. Now R8 & R6 both smoke at about +/-25V. I don't want to just willy-nilly keep replacing parts. Any suggestions for how to test from here would be greatly appreciated.
Thanks, Terry
I need a little help. The amp was playing fine for a couple days so I finally got around to building a case for it and once I had everything bolted in I brought it up on the variac through a light bulb and see the bulb getting brighter so I know I have a problem. Once the rails reached about 25V R8 smoked on one channel so I shut it down and pulled that channel and started testing things and found a shorted IRFP240. Seems somewhere along the way a metal sliver shorted it to the heatsink. So I pulled the two outputs on that board and installed new matched parts and replaced R8. I dropped the bias pot to zero and brought it back up. R8 still starts smoking so I lower the voltage and start measuring things. I see that though R8 is dropping a lot of voltage R6 is not so I checked Q7 and see that it looks shorted so I replaced it. Now R8 & R6 both smoke at about +/-25V. I don't want to just willy-nilly keep replacing parts. Any suggestions for how to test from here would be greatly appreciated.
Thanks, Terry
Attachments
Hi,
testing MOSFETs can be tricky, did you check it out of the board? There is gate-drain capacitance, if gate is positively polarized N chanel MOSFET will be low resistance between drain and source. And when polarized opposite it will be open circuit.
For bias adjusting, vr1 regulates bias current. For this topology, when vr1 is zero, current through MOSFETs is max. You should start with vr1 max resistance position.
You should measure voltage R5, R6, R7, R8 and output for further conclusion.
Vr2 is for output offset, start from central position.
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Thanks,
Only one MOSFET was shorted but it requires a matched set and I didn't want to go through a big matching session to try and find a match for the good one so I just replaced both with a known matched pair.
I found out the hard way, at the first start up, that VR1 has to be all the way counter clockwise for low bias. You can see that earlier in the thread. I have a feeling that Q1 and/or Q3 have gone bad just not sure how to test for that in circuit. These boards have fairly tight through hole cladding so part removal is tough, even with a solder pump.
Thanks, Terry
Only one MOSFET was shorted but it requires a matched set and I didn't want to go through a big matching session to try and find a match for the good one so I just replaced both with a known matched pair.
I found out the hard way, at the first start up, that VR1 has to be all the way counter clockwise for low bias. You can see that earlier in the thread. I have a feeling that Q1 and/or Q3 have gone bad just not sure how to test for that in circuit. These boards have fairly tight through hole cladding so part removal is tough, even with a solder pump.
Thanks, Terry
Well, it points to q9 and q10, I would change it also. There was strange breakdown.
