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Old 6th May 2012, 03:18 AM   #1401
Rush is offline Rush  United States
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Had an issue with the cascode transistors.
When I first powered the amp with my variac, set on the 2.5 amp fuse, no bias could be set. I was pretty sure where to start looking. Checked the data sheet on the 2SC4793 and sure enough the emitter was not pointed at the JFET source, DOH! Either I don't know how to install them or they are marked wrong on the board. The back side should have the double line right? I believe they are marked wrong on the board, Andy. After trying to fry them (it didn't work) I had to unsolder them (awful job, pulled up 2 traces) and put them back in hoping they were OK, they are OK. The amp pass' a signal too!
It biased up just fine the second time. Set at .3 VDC.

Being a prototyper I should have checked everything first, but I only checked most things, not everything! DOH again.
I am running 33.5 VDC on the rails and the heatsink isn't getting that warm, but it has only been on for about 20 minutes and biased at 2.4 amps total, which is all a standard F5 runs at.

Let know if you have any questions.

I fretted over drilling and tapping the heatsinks, but that was an easy job. I drilled the pilot holes all the way through (broke only 1 drill bit when it exited the side of a blade and no tap was broken because I only tapped 1/2 way down the hole). I used fairly short #4 - 40 screws.

I have been thinking about this diode and low bias issue and thinking maybe do away with the diodes altogether. Of course the DIYAUDIO Store has boards that do just that and I'll bet they match the standard spacing (which is too close together, isn't it!)
The proof is in the pudding and I will compare this amy to my standard F5 and see which one sounds the best. This of course will take twice as long as I think it will and somehow cost twice as much.
Thank you Andy for your hard work, I see you have incorporated some of my suggestions already. The terminal blocks need to be able to take 12 gauge wire, currently your BOM has a block that only takes 16 gauge wire, make the speaker output hole larger, so it too could take a 12 gauge wire (there is plenty of room) and mark cascode transistors correctly and you will be good to go.

Rush
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Last edited by Rush; 6th May 2012 at 03:34 AM.
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Old 6th May 2012, 10:27 AM   #1402
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Put the diodes in and fire it up and just see hoe hot they get with just a small hearsink attached. Mine didnt eben get real warm in free air with no heatsink. If you consider their very poor ability to dissipate heat into free air with no heatsink, then you realize they are not putting out hardly any heat.

It could be argued, that if you want to hear the effects of having the diodes, you have a better chance of doing so with less pairs, where it is more likely they will push beyond their bias point. This assuming you are not driving big or inefficient floorstanders. In that case you will probably be using them regularly.
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Last edited by buzzforb; 6th May 2012 at 10:41 AM.
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Old 6th May 2012, 01:54 PM   #1403
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At idle, with diodes on cheapo 25C/W+ heatsinks, I can get the bias up to almost 1A (.485 across Rs and diodes)before they start to want to jog a little. This is with the little heatsink sitting at 31-32C. All the little heatsink is doing is keeping it from having to dissipate in free air, not much more. The large heatsink are at 42C roughly. I will mount the next version with diode on main sink, which will raise the temp on it and we will see how that affects bias point. Next I will run a signal through it into 2OHM load to push it and see what happens.
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Old 6th May 2012, 01:58 PM   #1404
AndrewT is offline AndrewT  Scotland
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Biasing the output stage to the the point and beyond where the diodes are passing just seems wrong to me.

My feeling is that the diodes should only pass when the speaker demands extreme currents and that should only happen when extreme transients are being processing. NOT when the amplifier is quiescent and just bias current flowing, i.e. none to the speaker load.
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Old 6th May 2012, 01:59 PM   #1405
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Tiny faux pas you make is that the diodes + heatsink will not operate out in the open, but undercover.

(you might try a cardboard box for cover. now that it's spring, you can have mine)
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Old 6th May 2012, 02:03 PM   #1406
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I am just seeing about general settings here. when i run it with a load, I will have it set much lower. I will try it at .450 in open air. I am trying to push then to see when and at what temperature they will react at. After open air test, they will go in a case. BTW, I had them over 1A bias and stable, in open air, but that is just playing with fire.

Andrew, they are not biased beyond their passing or they would take off, correct. I am still below threshold, although on very shaky legs.
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Last edited by buzzforb; 6th May 2012 at 02:06 PM.
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Old 6th May 2012, 06:14 PM   #1407
Horio is offline Horio  United States
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Old 6th May 2012, 06:35 PM   #1408
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Little note for perspective builders. The idss value of the jfets is important with things set as they are in the article. I fired up the second channel and couldnt get bias beyond a certain point. Problem was low idss k170. I replaced it with 9.2mA and all was fine. If you have only low idss jfets, you might have to consider lower Rs than 10 in ordr to have enough current to bias output fets. You could increase 5K pot, but this is less effective, from what i remember. My vdrp across jfet sorce resistors is 50mV for sj74 and 61mv for N channel.
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Old 6th May 2012, 08:56 PM   #1409
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Quote:
Originally Posted by buzzforb View Post
Little note for perspective builders. The idss value of the jfets is important with things set as they are in the article. I fired up the second channel and couldnt get bias beyond a certain point. Problem was low idss k170. I replaced it with 9.2mA and all was fine. If you have only low idss jfets, you might have to consider lower Rs than 10 in ordr to have enough current to bias output fets. You could increase 5K pot, but this is less effective, from what i remember. My vdrp across jfet sorce resistors is 50mV for sj74 and 61mv for N channel.
Yep, I did 10K pots since I have a shortage of JFETs. Or change 1K to 2.2K R5-R6. Gives a little more breathing room. I believe this will likely affect cascade building people more.
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Old 7th May 2012, 01:12 AM   #1410
eyoung is offline eyoung  United States
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hola guys,

correct me if I'm wrong, but the MUR3020 has a V forward of 1.05V@25c and .85V@150c. So at idle the voltage drop across the source resistors is somewhere between 450mv to 600mv and at that voltage the MUR3020 should not conduct till the amp is rocking at near full tilt. The purpose being to allow the outputs to swing closer to the rails. At least that was my understanding.

Regards, El
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