F5X -- the EUVL Approach - The Build Thread - Page 28 - diyAudio
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Old 31st December 2012, 07:14 AM   #271
EUVL is offline EUVL  Europe
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1) None of the above. The diode is soldered electrically in parallel with the relay coil with the cathode to the positive side of the relay coil.
Please kindly read the datasheet of the relay to find out which pins are for the coil, and which is positive.
2) The connection to the Mute board is correct. 4 connections altogether. I do not have these boards, as they are Dave's layout. So he should confirm.
There is also one green line missing going from the input board to the RHS of your picture.
So 6 connections altogether between main & input board.

Dave please cross check.


Patrick
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Old 31st December 2012, 04:02 PM   #272
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Default Mute PCB

My (limited) understanding:

Connections to input PCB in red.
Relay data sheet indications in green.
Connections to control board in blue.

When the relay is unpowered (green lines) the amp inputs (1, 2) are grounded via a 10R resistor (R101) on the input PCB.
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File Type: jpg MutePCBconnections.001.jpg (40.7 KB, 396 views)
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Old 31st December 2012, 05:55 PM   #273
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Almost perfect, except that you connected the power to the relay inexactly the opposite way !!!!!!!!!!!


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Old 1st January 2013, 10:59 AM   #274
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How do I best "disconnect" the optional protection circuit I have installed on my boards? I have current (16V/10k = 1.6 mA) flowing to the output over R13 and I don't know exactly how that confuses my measurements.

2013: the year I finished my F5X, F5X-Pre and super-SEN
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Old 1st January 2013, 11:13 AM   #275
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Remove the bipolar transistor.
It probably also cost the least.

People say that removing the current limiter will improve sound drastically.
So maybe you don't want it anyhow ... ??




P.
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Old 1st January 2013, 10:51 PM   #276
NicMac is offline NicMac  Italy
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Ok. Removed the bipolar transistors, but also R13 and family.

The current draw from the supply lines now quantitatively corresponds to the predicted current draw from the degenerated transistors (measured on very similar Idss/Yfs transistors not in the amp).
I however still cannot make current over the 11R resistor fit exactly.

While measuring I realized that I can calculate Yfs, without the tracer measurements, as long as I have Id measurements with two different values of degeneration (for example 0R and 11R).

The relationship is the following:
Yfs = (Id(Rdeg1)/Id(Rdeg2) - 1)/(Rdeg2-Rdeg1)

Could be useful if one would like to transconductance match transistors with a simple DMM.

Cheers,

Nic
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Old 2nd January 2013, 05:11 AM   #277
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This is an approximated value, but nevertheless useful.
Remember Id vs Vgs is a curve, not a straight line.

Nic,
Why not try to short the 11R resistors temporary and see if you get Idss through the FETs ?


Happy New Year to all,
Patrick
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Old 4th January 2013, 10:09 PM   #278
NicMac is offline NicMac  Italy
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Shorting out the Q9 11R resistor(s) increases current by 1.74 mA, which is similar to what I predict from FET measurement (6.79 - 5.15 (11Rdeg) = 1.64 mA). I think this is close enough and conclude that most likely everything is 0k.

Next question:
Dave suggests to start with a start bias of 1.5A and I know both Vgs and Yfs @2A for my MOSFET's. Therefore I guess I can estimate my start Vgs by the following relationship:
Vgs@1.5A = Vgs@2A - (2A-1.5A)/Yfs@2A
I know I will be underestimating as the curve is not a straight line, but anyway
In my case the result is just 63 mV lower than Vgs@2A so maybe a thump rule of 0.1-0.2V below Vgs@2A is safer.

Last question:
any recommendations for bolting down the MOSFET's? I must admit that this is my first class A amp build, where thermal conduction obviously is critical.
How much "muscle" should I put into the tightening?
Should I use washers/bars to distribute the pressure on the FET?
Re-tightening when warm?

Cheers,

Nic
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Old 5th January 2013, 03:12 AM   #279
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> Vgs@1.5A = Vgs@2A - (2A-1.5A)/Yfs@2A

Close enough as first approximation.
But have you not forgotten about the voltage across Rsource ?

> In my case the result is just 63 mV lower than Vgs@2A

Really ?
Your FETs have 8 Siemens transconductance at 2A ?
You have them all curve traced, So why not look up the actual Vgs at 1.5A

> any recommendations for bolting down the MOSFET's?

Single M3 is provided on the heat sink.
Use stainless steel (non-magnetic) cap head screws with Torx or Allen key (hex) drives.
Use spring washer plus plain washer between screw head and MOSFET.
Tightening torque ~0.5Nm (the tapped hole is AL and not steel).

> Re-tightening when warm?

In theory the spring washer will compensate for any thermal expansion.
You are not using any grease or phase-change thermal compound.
So I don't bother myself.
But check it to convince yourself, by all means.


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Old 5th January 2013, 09:00 AM   #280
NicMac is offline NicMac  Italy
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>Really ?
Your FETs have 8 Siemens transconductance at 2A ?
You have them all curve traced, So why not look up the actual Vgs at 1.5A

Calculation error. The number is 0.125V (=(2.0-1.5)/4)). The GB mosfets have Yfs@2A in the 3.5 - 4.5 Siemens range.

Of course I can look up the measured value for Vgs@1.5A. And of course I can do this for all the GB members........
Interested members can PM me their FET numbers (one will be sufficient as they are matched devices).

Thank you for the useful info on bolting down the FETs. Maybe the bolts are included in the chassis kit.

Cheers,

Nic
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