Mofo

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Ray P
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#61 Re: Mofo

Post by Ray P »

Maybe consider the ACP+ preamp/HPA by Nelson Pass, voltage gain of 9dB and kit available from DIY Audio for less than £200.

https://www.firstwatt.com/pdf/art_acp.pdf
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#62 Re: Mofo

Post by pre65 »

Ray P wrote: Wed Nov 02, 2022 9:47 pm Maybe consider the ACP+ preamp/HPA by Nelson Pass, voltage gain of 9dB and kit available from DIY Audio for less than £200.

https://www.firstwatt.com/pdf/art_acp.pdf
Thanks Ray, but I was thinking more along the £20 figure. :lol:
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#63 Re: Mofo

Post by simon »

I'm not sure what you're getting at Ray? The gain, unity or a smidge less, is nearly good enough, I'd just like a little more.
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#64 Re: Mofo

Post by Ant »

Drive it with a 2a3
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#65 Re: Mofo

Post by simon »

That's perhaps more effort than a 26. IDHT would be easier, but would still need a HT trafo. The advantage of opamps et al is the easier (and cheaper) power supply. But would SS sound better? Is no driver stage better than any driver?
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#66 Re: Mofo

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simon wrote: Wed Nov 02, 2022 10:22 pm I'm not sure what you're getting at Ray? The gain, unity or a smidge less, is nearly good enough, I'd just like a little more.
Not getting at anything really Simon, just reminding or, perhaps, restating the obvious.
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#67 Re: Mofo

Post by ed »

simon wrote: Wed Nov 02, 2022 7:33 pm
Hi Ed, bit of an old one this that I've dug up but I'm interested in the 3x opamp you used and if you're still using it?
https://www.audio-talk.co.uk/phpBB3/vie ... php?t=7302

post #13 seems to tell the story Simon...
mofo1.jpg
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no longer using it, gave the bits to the late Mike Head(little eddy) and fully intended to build another but until now haven't got around to it.
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#68 Re: Mofo

Post by ed »

I've had these kicking around the workroom for a couple of years now.
It's part of my 'must use up all this clutter for something' endeavour that's driving the mofo revisit..
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#69 Re: Mofo

Post by simon »

Thanks ed
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#70 Re: Mofo

Post by Nick »

ed wrote: Thu Nov 03, 2022 9:03 am
simon wrote: Wed Nov 02, 2022 7:33 pm
Hi Ed, bit of an old one this that I've dug up but I'm interested in the 3x opamp you used and if you're still using it?
https://www.audio-talk.co.uk/phpBB3/vie ... php?t=7302

post #13 seems to tell the story Simon...

mofo1.jpg

no longer using it, gave the bits to the late Mike Head(little eddy) and fully intended to build another but until now haven't got around to it.
Why is that circuit using a input blocking cap AND split voltage rails? Seems an extra unneeded cap.
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#71 Re: Mofo

Post by pre65 »

At the moment, my MoFo is used with my Audionote (copy) preamp, feeding into a cheapo Chinese valve preamp and thence to the MoFo.

At the moment the volume controls on both are about half way to get a reasonable in room listening volume.

The resulting sound seems quite acceptable.

However, It's likely the cheapo Chinese item is less than optimal sound wise. :(

I've been looking at mono preamp modules to build into each MoFo unit, but I fear the signal level from the Audionote (copy) preamp might be too high for op-amp based units.

Oh, what is a thicko like me going to do ? :lol:
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#72 Re: Mofo

Post by ed »

Nick wrote: Thu Nov 03, 2022 10:37 am Why is that circuit using a input blocking cap AND split voltage rails? Seems an extra unneeded cap.
my bad, please ignore it......

it was a leftover from the previous design when I had a single ps and bias for the opamp and isolated it with a tl431 circuit. Isolation is best acheived(imo) witha seperate ps for the opamp.

For Simon......the circuit shows the ltc1150 but far better opamps can be found with respect to slew rate...the problem lies in finding an opamp with the sexy qualities that's still available in DIP format.....it requires much hunting and in some respects 'in this way lies madness'. If I don't fork out for the muses chip this time I may try something like an OPA2134.

to my mind the mofo is like any other piece of audio...put something/anything in front of it and somebody will argue that you're not listening to the sound of the mofo. There are lots of arguments in web land about the different flavours of opamp but in my naive mind I think if you're not going to run the mofo on it's own(0.9 gain) then the most likely transparency may be arrived at with an opamp. As usual the chancer that I am is only guessing.
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#73 Re: Mofo

Post by simon »

Phil - if I understand correctly you have two gain stages/preamps in front of the MoFo? In which you probably need to get rid of one.

Presumably you're using two preamps because neither has enough gain by itself? Having both set to half suggests you have too much gain overall.

So the question is how much gain are you using?
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#74 Re: Mofo

Post by pre65 »

Oh dear, I get annoyed with myself when people don't seem to understand my posts. :lol:

I use the Audionote (copy) by preference, but on it's own the gain is not enough to make MoFo sing, so the other pre is a stop gap to get realistic music levels.

I know the MoFo needs another (gain) stage built in, but I don't know (or can't sort out) how to go about it.

I was planning to make a stand alone gain stage to go with the 833a mosfet follower driver stage, using a 112a triode and interstage transformer, but my "powers" are not sufficient to attempt that at the moment. :(
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#75 Re: Mofo

Post by simon »

Thanks for your further thoughts ed. I'm using OPA2134 in the active crossovers but don't have any spare.

I'm sure I have some some LME49720 somewhere, if I can find them... So I'm tempted to knock something up to see what happens.

The comments about the cap and split rails interested me, being a SS newb. If I use the existing 19V LED SMPS I'm using for the MoFos can I just apply 19V to the V+ pin and 0V to the V- pin? (Plus the local decoupling caps.) Or do I need something further?
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