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Old 5th February 2019, 05:54 PM   #5361
coolnose is offline coolnose  Europe
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Default Biasing after Feedback resistor change

Well,

so I finally changed my feedback resistors to 470R in parallel to increase the gain to +/-27.6dB.

Now I noticed (but it might not be related - I did not check before) that I get different readings depending if I measure the P or the N channel boards.

Also measuring on different resistor groups (the parallel source resistors) on the SAME board gives me slightly different readings (around 5mV difference)

I did measure the voltage drop on each single resistor with a LM317 current source set at 0.78A, the procedure was pretty constitant to me, and grouped those with the same measured voltage drop for ALL the source resistors on both channels - I did NOT match the resistors with a bridge or similar.

I am confortable accepting a measurement difference, but now am doubting about where to measure to consider they are biased +/- equally.

....

Thanks for you help in advance!

Cheers,

Max
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Old 5th February 2019, 06:02 PM   #5362
coolnose is offline coolnose  Europe
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Quote:
Originally Posted by uchu007 View Post
Hello, for the thermistors 4,7k what power must he choose and the precission please?
uchu007,

I am by far no expert but the types I found corresponding the BOM are 3% or 2% types, don't know if wattage is expressed on those.

I picked the first one on mouser, you can check if you find one you like better:

NTCLE100E3472HB0 Vishay / BC Components | Mouser France

Cheers,

Max
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Old 5th February 2019, 06:56 PM   #5363
uchu007 is offline uchu007  France
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Quote:
Originally Posted by coolnose View Post
uchu007,

I am by far no expert but the types I found corresponding the BOM are 3% or 2% types, don't know if wattage is expressed on those.

I picked the first one on mouser, you can check if you find one you like better:

NTCLE100E3472HB0 Vishay / BC Components | Mouser France

Cheers,

Max
Do you have the nomenclature of the pieces? Because I just that of the store but its still vague on these pieces
Cordialy
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Old 5th February 2019, 07:02 PM   #5364
coolnose is offline coolnose  Europe
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Thermistance à coefficient négatif (NTC) 4.7k Ohm 500mW
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Old 5th February 2019, 08:16 PM   #5365
Bfpca is offline Bfpca  Canada
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F5 Turbo Builders Thread
Quote:
Originally Posted by coolnose View Post
Well,

so I finally changed my feedback resistors to 470R in parallel to increase the gain to +/-27.6dB.

Now I noticed (but it might not be related - I did not check before) that I get different readings depending if I measure the P or the N channel boards.

Also measuring on different resistor groups (the parallel source resistors) on the SAME board gives me slightly different readings (around 5mV difference)

I did measure the voltage drop on each single resistor with a LM317 current source set at 0.78A, the procedure was pretty constitant to me, and grouped those with the same measured voltage drop for ALL the source resistors on both channels - I did NOT match the resistors with a bridge or similar.

I am confortable accepting a measurement difference, but now am doubting about where to measure to consider they are biased +/- equally.

....

Thanks for you help in advance!

Cheers,

Max
Hi Max

I’m not quite sure I understand what you are measuring that is different on the N and P boards. When it comes to the bias voltages across the source resistors there is one variable that will increase the spread in readings and that is temperature differences between mosfets. If the mosfets are not mounted with consistent pressure you can get differences in the thermal conductivity to the heatsink, which will show up as differences in voltage across Rsource. This also goes for your initial measurements with the current source. If you do not wait for the temp to stabilize on each fet you will have more variation when installed in the amplifier.
If those differences are in the order of a few milivolts it is nothing to be concerned about.
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Old 5th February 2019, 10:28 PM   #5366
Hikari1 is offline Hikari1  United States
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Quote:
Originally Posted by Bfpca View Post
Hi Max

I’m not quite sure I understand what you are measuring that is different on the N and P boards. When it comes to the bias voltages across the source resistors there is one variable that will increase the spread in readings and that is temperature differences between mosfets. If the mosfets are not mounted with consistent pressure you can get differences in the thermal conductivity to the heatsink, which will show up as differences in voltage across Rsource. This also goes for your initial measurements with the current source. If you do not wait for the temp to stabilize on each fet you will have more variation when installed in the amplifier.
If those differences are in the order of a few milivolts it is nothing to be concerned about.
Also, what about the differences between the N and P channel mosfets? There is no such thing as an 100% match.
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Old 6th February 2019, 10:40 AM   #5367
Bfpca is offline Bfpca  Canada
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F5 Turbo Builders Thread
Quote:
Originally Posted by Hikari1 View Post
Also, what about the differences between the N and P channel mosfets? There is no such thing as an 100% match.
You cannot expect a match in VGS between n and P channel mosfets. Have a look at the F5turbo manual written by Nelson Pass. I believe he goes into matching in that document.
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Old 12th February 2019, 05:13 AM   #5368
TheNuge is offline TheNuge  Germany
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Hi!
I've just finished building a pair of stereo F5s, using the v3.0 and PSU boards from the store and Modushop 4U enclosures - and all of the info available here obviously!
The amps biased OK with very little or no offset, and I've set P3 using an FFT to be the same on all 4 channels - so far so good!
The amps also sound fine, but do hum ever so slightly when the RCA cables from the preamp are connected. When the inputs aren't connected they're dead quiet! Any ideas what to do here?
Setup is: Source (CD, LP..) --> Sitronik Lucius Preamp / DSP --> Power Amps (2 F5s, 1 Thel AccuSound) --> Speakers (DIY Co-Axial horns + 15" bass; each channel has it's own amp, there's no passive crossover)


Cheers
Es
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Old 12th February 2019, 05:19 AM   #5369
6L6 is offline 6L6  United States
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F5 Turbo Builders Thread
Please pose some in-focus, well-lit photos, particularly of the wiring.
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Old 12th February 2019, 09:59 AM   #5370
Bfpca is offline Bfpca  Canada
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F5 Turbo Builders Thread
Sometimes a small amount of hum like you are experiencing is the result of where you connect the chassis ground to the audio signal ground. I had this issue on an Aleph J I built. By changing the location of the connection the hum was eliminated.
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