Build This MoFo!

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#3,442
Mofo is definitely worth the money, perhaps with a step-up-inductor. The pcb is for those who can’t handle p2p. Mofo is definitely a worthy p2p project. The down side with open, not metal box, is shielding but have not noticed disturbence.
 
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#3,444
I am using ra7's variation of this amplifier using THF51s.

No extra resistors are used just the resistance of Lundahl LL2733 with coils in parallel.

I know I am close to 3 amps at 38 volts so I am not surprised that the chokes get hot. Considering Mr. Pass's five seconds rule there are about that hot.

Is this a problem? It sounds good. I am concerned about longevity. Hoping someone here has an opinion on this.

If I used two of these wired in series and then paralleled those I would end up with the same resistance and twice the inductance, which is not important for me - would this have any effect on lowering the temperature?

If this could be worthwhile for that reason would there be sonic consequences to paralleled chokes here?

I would go ahead and try if it did not require lots of revisions to the layout. Hoping for encouragement or discouragement, either would be appreciated.
 
#3,449
In the past I make a headphone amp with IRF610 albeit CCS loaded. Sounded excellent. This was my option before going to tubes. Certainly works with a choke. Question is to find one. Perhaps trying a choke for PSU, with more that 1H, with no excessive dcr (is worth to check manufacturer's list of disponible inductors; maybe some for tube amplifier's PSU). Or if someone know about some another commercial options.
 
#3,450
J

Thread 'Upcycling Microwave Transformers into Output Transformers'

I recently decided to try to upcycle a microwave oven transformer into an output transformer with roughly a 2500:24 ohm impedance ratio. I chose a turns ratio of 1500:150 merely because those are round numbers. I didn't want to make this any more complicated than strictly necessary since I've never tried it before. It took a fair amount of time and patience, but I have both. Microwave iron is so readily available that I figure that someone else may have a need to know how this turned out for me. I challenge anyone to try the same instead of sending yet another dead microwave oven to...
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#3,451
Sorry, a bit of a noob here. How does one choose the protection zener, and Is there a schematic showing how to include a cap with the zener? Is it just in series with the zener (presumably with the + toward the source - that would seem to be the case if, like with tubes, the gate is more negative than the source, but I'm really noob with transistors)? And lastly, it seems like the board in the store has been out of stock for a long time. I may P2P my build, but a board would be easier and a bit neater (my P2P layout and wiring skills aren't great). So wondering if there is any news on these boards in the store. Thx!
 
#3,452
A while back when the store was out of PCB, Prasi was kind and made one for me. I can send you the Gerbers, drop me a PM.

I had R6 added (0.1R) to measure your bias and also 2 terminals across the output cap to add a nice polypropylene cap.
IMG_4497.png
 
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#3,454
Be sure to use a strong zener for mosfet protection: 1w is not enough if the DC power supply suddenly disappears - use a bigger/fatter 5w 1N5357.

An alternative solution is to have a local PSU cap at the power input jack so power bleeds down rather than abruptly stopping - this slows the collapse of the coil. 1,000uF cap is sufficient to protect the mosfet from coil collapse.
 
#3,456
Good evening, gentlemen
Can I replace the IRFP 250 with an IXFN 140N20P? , if yes ,the value of the 221 ohm grid resistance on IRFP 250 should be 22 ohms on IXFN 140N20P?
Should the 20 volt/5W Zener diode with IRFP 250 be 9.1 or 8.2 volt/5 W for IXFN 140N20P?
Thank you in advance for your answers.
 
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#3,457
Although it have higher input capacitances, I think to maintain the original value, since it is a follower operating naturally in a linear region, and for audio bandwidth, I think the 221R serves well for it. Lower and we risk some oscillation "perhaps" (layout dependent).
 
#3,458
e_fortier said:
A while back when the store was out of PCB, Prasi was kind and made one for me. I can send you the Gerbers, drop me a PM.

I had R6 added (0.1R) to measure your bias and also 2 terminals across the output cap to add a nice polypropylene cap.
View attachment 1383047
Click to expand...
With many thanks to e_fortier and Prasi, I ordered the boards and they arrived the other day. Busy with upcoming travel and holiday cheer, so I won't start building until late December or January, but happy to be making progress. I went the board route partly to try my hand at getting boards made, and assuming they work, I will likely have a few boards to offer up here. Thanks again!
 
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