F5X -- the EUVL Approach - The Build Thread

[bkdog]

Originally Posted by NicMac

Just joking......
These are fakes that I picked up on the way

Perfect for practicing

Back to business: from what I can see/measure the inner set of holes on the PCB will make the best fit (or bending will have to exceed 90˚)

Cheers,

Nic

That gave me the best laugh of the day - thanks Nic! I particularly like the one on the right with the legs bent in different directions...

[NicMac]

Ok - I'm ready to plug in mosfets

Of (possibly) useful note I found that:

1) It is highly recommended to dial in the Vgs voltage before soldering in the MOSFETs. With the factory "settings" of the trim-pots I would have had 2.8V Vgs on my MOSFETs which would have caused a current draw way exceeding several ratings of this amp (possibly frying the MOSFETs to start with). I dialed the Vgs down to 2.15V (0.15V below the measured Vgs@2A for my FETs) thinking that this will yield about 1.5A bias current.........

2) There are two rows of holes for the MOSFETs. I soldered in a small solder tag in the free source hole as this is very practical for clipping on a voltmeter probe when biasing.

In the photos please ignore:
Cap orientation in the CAP banks - it is wrong
Wire colors

Sorry about the photo quality. I'm still hoping Patrick can help me getting my hands on a Leica for a fair price

Cheers,

Nic

[ashaw]

re the 0.1uF capacitors. I am certain that I shipped you 1uF caps. if I did ship you 0.1uF caps though it would not be a problem to use them in place anyway.
re post #229: the mute boards use a NO relay and as such they should be open without the relay. the HJ4002 are in fact the markings on the 74HC4002, as far as my knowledge, they are the part on my prototype and work well.

Re post #304: I will post a more in depth explanation once I have finished an EE assessment I have due monday.

[EUVL]

You should use a lab power supply with a current limit.
Then all will be safe.


Patrick

[NicMac]

In the amp BOM the two P MOSFETs are designated Q1 and Q8. I think this should be Q1 and Q6. Q8 is a JFET and the PCB is correctly labeled.
I may have an old BOM and I did not check the schematics.

Q: In amplifier/circuit jargon what does "top" and "bottom" refer to?
Are "opposite corners" referring to top-left versus bottom-right?

I'm just trying to understand the terminology of the excellent bias description by Dave.

[NicMac]

[CeeVee]

Originally Posted by NicMac

magic moment...Nic no ? ufff !!! hehehe.

Let us know how it sounds later on.

[NicMac]

After about 1 hour of warming up and without adjusting anything I get:
378, 378, 379 and 379 mV drop over 0.22R (= 1.72A bias)
3.5 and 0.6 mV offset on the outputs.
Heatsink: 53˚C

I guess I can be happy with this even if I had been calculating to hit 1.5A (and not 1.7A).

I guess having MOSFETs measured at operating temperature also has some practical advantages

I will later dial it up to 2A and then maybe learn to use my PicoScope to find out if this is indeed an amplifier and not just an expensive heating block

One question: if the current is essentially equal for all 4 quadrants, how will I come about adjusting down offset on the outputs

[EUVL]

I'll have to let Dave answer those questions as it is his documentation and numbering system ....


Sorry,
Patrick

[EUVL]

How equal are your currents, to mA ? Even 1mA x 8R load is 8mV.
If you have no load, any imbalance in current even in mA has no where to go other than the feedback network.
So you still need to adjust for zero voltage offset.
That is why the trimmers are there.
You can pick any one you want, as long as in the end the offsets are zero.

Otherwise good progress.


Patrick