MoFo

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little eddy
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MoFo

Post by little eddy » Wed Jul 18, 2018 10:28 pm

With the LM3886/F4 on hold, primarily because of no availability of the now almost obsolete complementary JFETs needed for most of the Pass amp builds, it is time to embark on a different SS amp build prompted by Ed's Lateral FET build and his reference to the MoFo thread. I also think Ray may have posted his interest in this design but the MoFo seems a good alternative to the F4, driven by my Niobium 41MP pre-amp.

So starting at almost the 'entry' level. I have a pair of Lenovo 20V 5A power supplies, some 0.5 C/W heatsinks and a pair of Hammond 159ZC chokes from Ed. It should yield around 13.7W of Class A when biased at 1.9A.

All of the bits are on order and I am already thinking about 'phase 2' should the initial build sound promising. Below are some of the links you may have seen before:
MoFo article: http://blog.audiomaker.tech/download/build-this-mofo/
MoFo DIYAudio thread: http://www.diyaudio.com/forums/pass-lab ... -mofo.html
MoFo pcbs https://diyaudiostore.com/collections/p ... ducts/mofo
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

little eddy
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Re: MoFo

Post by little eddy » Thu Jul 19, 2018 8:55 pm

This could be a really compact design for the initial build, even smaller if the pc psus were off-board.
Initial layout small.jpg
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

little eddy
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Re: MoFo

Post by little eddy » Fri Jul 20, 2018 12:04 am

A question regarding the dc blocking capacitor C1.

My Niobium pre has an 'isolated' transformer output in that it is connected directly to the RCAs and not connected to 0V in the pre

Can I bypass the input cap C1 within the MoFo, and connect the input either directly across R3, or from the top of R3 and the 0V reference?

Circuit can be found in the first link of my original post.

Thanks in advance,
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

little eddy
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Re: MoFo

Post by little eddy » Fri Jul 20, 2018 9:12 pm

So here's my attempt at a simplified input, designed to match the Niobium transformer 'non-grounded' output.
18-07-20 MoFo Simple.jpg
Any comments would be appreciated.
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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ed
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Re: MoFo

Post by ed » Fri Jul 20, 2018 9:23 pm

mmmm...I'm scratching my head on this one..

how are you setting the bias on this?
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Nick
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Re: MoFo

Post by Nick » Fri Jul 20, 2018 9:52 pm

Yep, that should work.
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little eddy
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Re: MoFo

Post by little eddy » Fri Jul 20, 2018 10:09 pm

ed wrote:
Fri Jul 20, 2018 9:23 pm
mmmm...I'm scratching my head on this one..

how are you setting the bias on this?
Bias is set by P1 and R4. There will be continuity through the Niobium secondary winding connected to the RCA socket.
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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Re: MoFo

Post by ed » Fri Jul 20, 2018 11:37 pm

ah ha....cunning plan

bit trixy trying other pres though, no?

what is the benefit of running the bias dc through the secondary of the pre amp?
I am being a bit slow here, but wouldn't that close couple the output swing on the pre with the ps on the output stage?
on reflection, no more than an ordinary bias I suppose.............

mmmm time for bed I guess
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Nick
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Re: MoFo

Post by Nick » Fri Jul 20, 2018 11:53 pm

ed wrote:
Fri Jul 20, 2018 11:37 pm
ah ha....cunning plan

bit trixy trying other pres though, no?

what is the benefit of running the bias dc through the secondary of the pre amp?
I am being a bit slow here, but wouldn't that close couple the output swing on the pre with the ps on the output stage?
on reflection, no more than an ordinary bias I suppose.............

mmmm time for bed I guess
Avoids a cap. I would use the cap and be happy knowing it will work with any pre. But YMMV,
Resistance isn't futile it's V / I.

little eddy
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Re: MoFo

Post by little eddy » Sat Jul 21, 2018 10:32 am

One of my questions is about the 50k 'load resistor'. Will the Niobium output stage see this as a load? The attached probably closer resembles a valve inter stage fixed bias arrangement?
18-07-21 MoFo Simple.jpg
Don't know why but I'm thinking about a dc voltage developed across R3 (if there is any current flow due to the biasing) although that might be small when compared to the potential divider R4 and P1.

This second scheme would allow me to have 2 rcas, the second one via the ac coupling capacitor quite easily.

Which of the two schemes would be recommended? Should I consider increasing R3 to say 100k if this is the preferred topology?
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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Nick
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Re: MoFo

Post by Nick » Sat Jul 21, 2018 6:17 pm

In the first case the output transformer will run without a load, in the second a 50k load. Not sure what current you expect to develop a voltage across R3 in either cases.
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little eddy
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Re: MoFo

Post by little eddy » Thu Jul 26, 2018 5:46 pm

Could be a silly question (as often they are from me), but in the MoFo schematic and pcb, why is there a 200k grounding resistor proposed before the signal dc-blocking capacitor, that for ac is effectively in parallel to the 50k load resistor? I'm considering leaving this out unless I'm missing something.
MoFo Schematic.jpg
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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Nick
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Re: MoFo

Post by Nick » Thu Jul 26, 2018 5:49 pm

It stops the other side of the cap from floating.
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little eddy
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Re: MoFo

Post by little eddy » Fri Aug 17, 2018 9:12 pm

Steady at 1.7A.
18-08-17 Mofo small IMG_20180817_204044837.jpg
Next the other channel and then it's time to have a listen.

If it's any good, this is a remarkably easy Class A amp to build.
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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