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New Millett Hybrid Maxed Amp - Page 424

post #6346 of 6600

 

Quote:
Originally Posted by funch View Post

They can be jumpered, used to reduce gain, or tailor the sound.


I recall that they are absolutely required for the MOSFET Max in order to prevent oscillation.

post #6347 of 6600

Thanks guys. The BOM said optional, but I recalled seeing elsewhere that they were mandatory.

post #6348 of 6600
Quote:
Originally Posted by scootsit View Post

Thanks guys. The BOM said optional, but I recalled seeing elsewhere that they were mandatory.


Sorry, but Beefy is absolutely correct.  You need a minimum of 22R to keep the MOSFETs from oscillating.
 

 

post #6349 of 6600

Can I see a picture of how you guys routed the wires for your biasing contacts?

Thanks!

-Scott

post #6350 of 6600

Post #6110.  http://www.head-fi.org/t/233941/new-millett-hybrid-maxed-amp/6105

 

Just run the wires straight back from the solder holes and bring them up through the notch in the back of the PCB.

post #6351 of 6600
Quote:
Originally Posted by scootsit View Post

Can I see a picture of how you guys routed the wires for your biasing contacts?

Thanks!

-Scott


This is from the MAX website for the BJT MAX, but the wiring for metering the buffer bias would be no different:


 

MAXcasework31.jpg

 

EDIT: Oops - looks like Funch and I were responding at the same time!


Edited by tomb - 2/18/12 at 8:06pm
post #6352 of 6600

Awesome, thanks guys. I searched, but I guess I didn't go far enough through the posts to find that. Last question, what do you recommend biasing the Mosfets to? I saw the range on the website of 176-264mV, and 220-275mV elsewhere. I've got it biased at 180mV to start, but what would be optimal, or is there a page explaining it similar to the one for tube bias?

Thanks!

post #6353 of 6600
Quote:
Originally Posted by scootsit View Post

Awesome, thanks guys. I searched, but I guess I didn't go far enough through the posts to find that. Last question, what do you recommend biasing the Mosfets to? I saw the range on the website of 176-264mV, and 220-275mV elsewhere. I've got it biased at 180mV to start, but what would be optimal, or is there a page explaining it similar to the one for tube bias?

Thanks!


http://www.diyforums.org/MOSFET-MAX/MOSFET-MAXsetup.php

 

It says right in the table, 220-275mV (125ma is max, IMHO).
 

 

post #6354 of 6600

I see that, and understand that. But, that's a 50mV range. What value in that range is recommended? What determines where in the range is best, etc?

post #6355 of 6600

Sorry for all of the bone-headed questions. I've got it now. It's been a long week. Tomb, your patience is incredible and appreciated. Sorry again guys.

Thanks!

post #6356 of 6600

I the setup and Biasing the same for the Mini MAX and the Millet MAX?

post #6357 of 6600

The bias points are the same. The values for the output buffer are not. Both are listed on the website:

http://www.diyforums.org/MAX/MAXsetup.php

 

If you are building the Mosfet, they have updated the Mosfet values a little further on the page TomB linked to above.

post #6358 of 6600

I just biased my Mosfets. It's weird. In order to obtain 100mA, I need to get them to 282mV. It seems weird that they wouldn't be exactly the same as the values on the website. Any thoughts on what I did wrong?

post #6359 of 6600



 

Quote:
Originally Posted by scootsit View Post

I just biased my Mosfets. It's weird. In order to obtain 100mA, I need to get them to 282mV. It seems weird that they wouldn't be exactly the same as the values on the website. Any thoughts on what I did wrong?

You can't read milliamps with your meter - not unless you broke the circuit to the MOSFETs somewhere and wired the meter in series with the MOSFETs.  It can't be done without screwing up the PCB.

 

The whole point about biasing the buffer is that we're trying to achieve a certain value in current: about 125 milliamps.  The way we do that is to measure mV across one of the power resistors that are connecting the MOSFET pairs, which your meter can do quite easily without having to be placed into the circuit.  RB10 or RB11 is 2.2 ohms.  So, if you divide the measurement of millivolts by 2.2, you get the value of current in the circuit in milliamps.  Thus, to have 100ma bias in the buffer, you should adjust RB12 (the buffer trimmer in each channel) until you read 220mV (2.2 x 100ma, I = V/R).  So, right now you have 282mV/2.2 = 128ma. (That's pretty close to 125ma - I'd leave it there.wink.gif)

 

By the way:

  • for TB1R or TB2R with TA2R: TB1R with TA2R measures across RB11R, TB2R with TA2R measures across RB10R - take your pick for the Right channel.
  • for TB1L or TB1R with TA2L: TB1L with TA2L measures across RB11L, TB1L with TA2L measures across RB10L - take your pick for the Left channel.

If you try placing your probes across the leads of one of the resistors above, you'll be reading the same thing as those test points.

 

Looks like I had the English screwed up on the webpage's table for this stuff, so I've corrected that.

 

P.S. Anytime someone mentions measuring the current on a built PCB, they are more often than not talking about measuring the voltage across a resistor of known value.  The measurement in voltage divided by the resistor's ohm value is the current through that resistor.  Wiring a meter in series within the circuit is always the last choice for measuring current, because you can very easily blow up a meter if the current range setting is exceeded.


 

 


Edited by tomb - 2/20/12 at 6:58pm
post #6360 of 6600

TomB, let me reiterate my last posting - your patience and help is spectacular. Thanks so much. A Millett Mosfet lives and sounds INCREDIBLE! I couldn't have done it without your help! Thanks!

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