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Final Setup and Assembly Questions

beta24

Final Setup and Assembly Questions

Postby muskyhuntr » July 24th, 2017, 8:27 am

I'm finishing up my first mono bloc B24 (out of 3) and had a few questions before removing the 22R resistors.

On the MOSFET bias adjustments I am not able to secure a stable 40mV reading. The readings continuously rise and fall. On R47, they go between 38mV and 42.2mV. On R45, they go between 38.7 and 42mV. Is this to be expected or is there too much movement? Readings are taken after about 1 hour warm up. If this is too much voltage movement, what do I need to look at? If the fluctuation is okay, would it be better to increase the minimum voltage?

Using an infrared temp. gauge, I am reading around 140F on Q33 to Q40 as well as Q21 to Q24. (Take these as 'give or takes' as it is extremely hard to get an accurate reading with this gauge) Are these temps what I should expect? Amp is mounted to a 3U heatsink by its self. Fins are now 90 degrees from installed position to facilitate adjustments. I expect air flow to improve once properly mounted to rest of chassis, so temps may fall.

Should I expect these temperatures to rise after I remove the 22R's?

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Re: Final Setup and Assembly Questions

Postby amb » July 24th, 2017, 8:39 am

muskyhuntr wrote:On the MOSFET bias adjustments I am not able to secure a stable 40mV reading. The readings continuously rise and fall. On R47, they go between 38mV and 42.2mV. On R45, they go between 38.7 and 42mV. Is this to be expected or is there too much movement? Readings are taken after about 1 hour warm up. If this is too much voltage movement, what do I need to look at? If the fluctuation is okay, would it be better to increase the minimum voltage?

The small fluctuation is expected and acceptable. No need to tweak any further.

Using an infrared temp. gauge, I am reading around 140F on Q33 to Q40 as well as Q21 to Q24. (Take these as 'give or takes' as it is extremely hard to get an accurate reading with this gauge) Are these temps what I should expect? Amp is mounted to a 3U heatsink by its self. Fins are now 90 degrees from installed position to facilitate adjustments. I expect air flow to improve once properly mounted to rest of chassis, so temps may fall.

Sounds OK if it's right on the MOSFETs themselves. The heatsink temperature should be much lower.

Should I expect these temperatures to rise after I remove the 22R's?

The temps on Q37-Q40 should rise a bit because the heat dissipation on the current limiting resistors will now be borne by these MOSFETs.

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Re: Final Setup and Assembly Questions

Postby muskyhuntr » July 24th, 2017, 8:47 am

Thank you for your reply. Great news! Still have to finish wiring the inputs (may have a question or 2 on this) and final assembly.

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Re: Final Setup and Assembly Questions

Postby muskyhuntr » August 11th, 2017, 4:53 am

Finished the first of 3 mono blocks. I have not put the amp in my main system yet (waiting for the completion of the second one) so I do not have any comments on sound quality. I did hook up a 3" speaker and played a few tracks from my IPod. I also ran tones from my generator. Both sounded good. This is basicly a stock build with the exceptions of a custom Sumr transformer, Vampire connectors, and a bit of silver wiring. I have attached a few pictures.
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Re: Final Setup and Assembly Questions

Postby muskyhuntr » August 11th, 2017, 4:56 am

A few more pictures.
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Re: Final Setup and Assembly Questions

Postby muskyhuntr » August 11th, 2017, 4:58 am

And two more. Note that I spent a lot of time cleaning my work bench for these photos.
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Final Setup and Assembly Questions

Postby amb » August 11th, 2017, 5:05 am

Very nice! I like the lower profile version of the original front panel design.
I think this chassis can accommodate both channels of the lower powered (40W/ch class AB) β24 variant, if only one σ22 is used.
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Re: Final Setup and Assembly Questions

Postby muskyhuntr » November 27th, 2017, 12:59 pm

Working on setting up the second mono block. As usual, a few problems.

1) After getting all the settings close, I shut the amp down for the night. The next day -out was showing 0 mV across R47 while R45 was showing around 70 mV. The 22R resistors were scalding! I re-adjusted VR4 and the problem has not returned, yet. Any ideas why this happened?

2)Q45, Q47, Q46, and Q48 are getting very hot. I don't think this is normal. Where should I look?

3) Nether R45 and R47 readings are not very stable, varying by up to 6mV. Any ideas?

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Re: Final Setup and Assembly Questions

Postby amb » November 28th, 2017, 1:23 am

muskyhuntr wrote:1) After getting all the settings close, I shut the amp down for the night. The next day -out was showing 0 mV across R47 while R45 was showing around 70 mV. The 22R resistors were scalding! I re-adjusted VR4 and the problem has not returned, yet. Any ideas why this happened?

Hard to say, for some reason VR4 needed to be re-adjusted. If it doesn't happen again then don't worry about it.

2)Q45, Q47, Q46, and Q48 are getting very hot. I don't think this is normal. Where should I look?

The JFETs in the common mode feedback circuit? Are they soldered in the right way? They should be "twsited" just like Q1-Q4. Also inspect that area to make sure all the parts are the correct values and in their correct orientations (for polarized parts).

3) Nether R45 and R47 readings are not very stable, varying by up to 6mV. Any ideas?

That's not significant. The bias current through the output stage vary with temperature, so the voltage across these resistors will change as well.
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Re: Final Setup and Assembly Questions

Postby muskyhuntr » November 28th, 2017, 2:46 am

The JFETs in the common mode feedback circuit? Are they soldered in the right way? They should be "twsited" just like Q1-Q4. Also inspect that area to make sure all the parts are the correct values and in their correct orientations (for polarized parts).


Q45, Q47, Q46, and Q48 are the BJT's, not the JFET's. The JFET's are twisted. The heat problem is common to both sides. The JFET's appear to be warm, but not hot. Part orientations are correct. Is it possible to test the diodes in circuit?

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