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Old 17th June 2019, 05:47 PM   #2331
csample is offline csample  United States
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Default Source Resistor Question

I am in the middle of my BA-3 build, and I am basically just duplicating the build in 6L6's build guide (thank you for writing this). There are a couple small differences, I have a 4U 400mm chasis, and 22,000 uF 50V power supply filter caps, and I am planning on using 1 ohm source resistors, but when I read the following from Papa's BA-2 write up, I wonder if I should be using a lower value:


"For fewer output pairs, consider using proportionally lower values of Source resistors on the power Mosfets. For example, with 2 pairs, 0.33 ohm might be a better choice than 1 ohm."


If I am following this correctly, I have 3 output pairs per channel, so I would use .5 ohm source resistors.


Any thoughts or suggestions? I already purchased 1 ohm, 1%, low ppm resistors, but I would certainly purchase new resistors if that is the right thing to do.
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Old 17th June 2019, 06:15 PM   #2332
Itsmee is offline Itsmee  England
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0R47 is used in the 3 deep F4.
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Old 17th June 2019, 07:54 PM   #2333
Hikari1 is online now Hikari1  United States
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It really depends on how well matched your mosfets are.

You could try .5R and se if there is any "bias hogging" (imbalance of current among mosfets). This can be done by measuring across all the mosfet source resistors and comparing current draw.

1R is a safer choice and provides more of a buffer if the balance is not perfect between the mosfets.

And yes, a lower value will sound better. But the lower the value the more likely bias hogging and thermal runaway will occur.
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Old 17th June 2019, 08:10 PM   #2334
6L6 is offline 6L6  United States
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It’s my understanding that the 1ohm were specified because the original test chassis(s) way back when had not particularly matched transistors, and the big source resistor helps makeup for that.

If you have really tight matches you could go as low as 0.22ohm. 0.47ohm is very safe if you know that the mosfet are nicely matched.
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Old 17th June 2019, 10:06 PM   #2335
csample is offline csample  United States
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Thanks for the informative replies. I really like the term "bias hogging" it helped me understand the purpose of the source resistors. I purchased my mosfets as a matched set from the diyAudio store, so I am guessing they are pretty well matched. However, I would like to check the matching so I know that I am good to go with a lower source resistor. Would checking with a low current level as shown in the thread: "Nelson Pass' Easy Peasy MOSFET Vgs Measurement" be sufficient to verify mosfets that are already believed to be matched, or is matching at the suggested 500mA still required?


Nelson Pass' Easy Peasy MOSFET Vgs Measurement
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Old 17th June 2019, 11:25 PM   #2336
6L6 is offline 6L6  United States
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Store mosfets are matched to at least .01 Vgs. I'd use 0.47ohm resistors without worrying about it at all, and use 0.22 if you wanted to, but at that point you'd probably want to match your source resistors since that will give more variation than the mosfets. And it's likely you don't have a meter that can reliably measure 1/5th of an ohm.

Last edited by 6L6; 17th June 2019 at 11:28 PM.
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Old 18th June 2019, 11:42 PM   #2337
csample is offline csample  United States
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Quote:
Originally Posted by 6L6 View Post
use 0.22 if you wanted to, but at that point you'd probably want to match your source resistors
Thanks - do you think .22ohm 2% rated resistors would be matched close enough? The resistance range would be .2156 thru .2244 ohms, and you are right this is definitely not something my multimeter could measure.

If it makes a difference, the resistors I am looking at are Vishay/Dale CPF3R22000GNB14.

CPF3R22000GNB14 Vishay / Dale | Mouser
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Old 18th June 2019, 11:48 PM   #2338
Zen Mod is offline Zen Mod  Serbia
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no need for overthinking that

use 0R47 , and be happy , knowing that TL431 bias thingie is well suited with that value to keep Iq at bay
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Old 19th June 2019, 12:29 AM   #2339
csample is offline csample  United States
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Thanks ZM for pulling me back to reality - 0R47 and be happy it is.
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Old 26th June 2019, 06:13 PM   #2340
Russellc is offline Russellc  United States
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Are there plans to restock the complementary output boards+bias cards in the store? Somehow I managed to get the output boards some time ago, but more recently discovered I don't have the bias cards to go with them.

I suppose there is even a smaller chance that some extra of those bias cards alone would be left over at store?

Russellc
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