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Old 26th October 2005, 05:41 AM   #1
MrDodo is offline MrDodo  Mauritius
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Default Switchable law fake passive for Class-t

I intend to build this passive for my low power class-t amps, amp3, t-amp, Charlize (one day perhaps).
Idea behind being that I can switch to different pot value depending on the source chosen.
Easiest is a switchable law fake value.
Example I would choose 50K or 25K value for Monica2, 10K for output of CD player, whichever sounds best or gives the optimum attenuation without limiting HF response. I also think that Monica2 cannot a 10k pot correctly.

The bypass switch is specially included for Monica2 which gives low output. This way I can bypass all attenuation. I guess that I will also have to put the law fake switch to the unconnected position for this to work.
I intend to use Alpha 100k linear. I know that the 50k log is very good and very cheap. I'm not sure 100k is ideal.
Has anyone done this before. Will it work as intended?
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Old 26th October 2005, 08:21 AM   #2
Nuuk is offline Nuuk  United Kingdom
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I can only repeat my views that unless you are using a 10K volume control, you need to have an active buffer before the class-T amps!

And if you are using a 10K volume control, your Monica2 won't work optimally! In short, no amount of clever (passive) switching will allow you to use Monica2 (or DacKit) with a class-T amp.
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Old 26th October 2005, 08:59 AM   #3
MrDodo is offline MrDodo  Mauritius
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I'll build buffer for Monica2 when I am not lazy enough to search for the parts, specially the power supply.
So after that, will the Extinct Bird passive work?
I've got all the parts 100kpot, 1 toggle switch and selector switch.
All I need is to put it into box, but I don't want to go all the way only to find that it don't work.
I mean sometimes I read arguments as to whether 10k, 25k or 50k is better for this source or that as passive, so that I would be better off with swithable passive.
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Old 26th October 2005, 09:40 AM   #4
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In theory it will be a good idea to have a switchable volume control impedance.

But only if all the equipment that you are using will work on one of the switchable impedances. If two of the items in your chain won't work together without a buffer, then it doesn't matter how many options you have for the volume control, they just won't work optimally.

Once more. The Monica2 needs to work into 20K or higher. Without a buffer, the class-T amps need a 10K or lower volume control. There is NO impedance higher than 20K but lower than 10K!
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Old 26th October 2005, 10:12 AM   #5
MrDodo is offline MrDodo  Mauritius
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I know that Monica2 loves to be driven higher than 20K.
yeo once told me 50k is good for her.
As for the low power class-t, I have tried 10k with ipod (what? heresy!!) in my office and I prefered 50k. But then I have 82db speakers on my desk.
Ok this is not high efficiency system, just some musik to help sooth my soul when I'm working.
But again I'm just plain lazy to connect and disconnect all these pots to try all the different values.
That's why selectable pot impedance. Switchable values at my heart's content so that I can tell myself, see this value sounds better just by turning a switch. Except that I hope that it will not result in a big POP when I change the impedence.
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Old 31st October 2005, 08:32 AM   #6
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Hi, just to say that you have grounded both sides of your resistors in the diagram. Do not ground the output..
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Old 2nd November 2005, 06:05 AM   #7
MrDodo is offline MrDodo  Mauritius
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Thanks audio1st, I didn't realise that I grounded the output on the diagram. What was I thinking?

Nuuk, can your buffer work with +-12v instead of +-15v?
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Old 2nd November 2005, 08:52 AM   #8
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Quote:
Nuuk, can your buffer work with +-12v instead of +-15v?
I haven't tried it myself, but I can't see why not.
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