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Old 20th February 2008, 07:57 AM   #11
decky is offline decky  Australia
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I tend to agree with you. But I am always for buffering the pot from the power amp input somehow. Might try with a MF X buffer stage or simmilar.

Considering the gain of this preamp - to continue thinking loudly - I might try it as a headphone amp (might not have enough gain for my HD600s) and use the passive preamp - relays + 100K pot.

I know that 100 K pot is a bit excessive but I might try - it wont hurt (a lot )
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Old 20th February 2008, 08:09 AM   #12
Nordic is offline Nordic  South Africa
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if it were a linear 100k pot, you could do a law fake... with a 10k resistor... 100k is way off for these amps...
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Old 20th February 2008, 08:54 AM   #13
AndrewT is offline AndrewT  Scotland
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Hi,
I too like the buffering idea to ensure minimum source impedance before the cables into the next stage.
But a buffer in this context can be a unity gain buffer.

If you cannot advance your volume pot off the stop without getting overload, then a little 10db of reduced gain is not going to solve your tiny volume control adjustment problem.

And yes, that preamp looks a though it might be worth using as a high impedance headphone amp.

I am perplexed as too why the front end uses lm394 as a line stage amplifier. This is more indicative of an amplifier for very low signal levels. Could it be the stage that follows a passive RIAA?
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Old 20th February 2008, 09:55 AM   #14
decky is offline decky  Australia
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You are probably right - DIYclub.biz is selling the same board as a phono-preamp kit but I do not know what are the differences (if any) between two boards.
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Old 20th February 2008, 10:07 AM   #15
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by decky
You are probably right -
That sounds like an admission of lack of research.
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Old 20th February 2008, 10:18 AM   #16
decky is offline decky  Australia
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You mean on my side - well my knowledge is not that great - but learning is always fun. Now I know better.

I guess I was too optimistic and wrongly assuming that everyone makes pre-amps with gains of up to 6dB or as unity buffers. I was obviously wrong. I recently heard a very nice valve pre-amp with a gain of over 30dB. Although paying almost 3K my friend had to sell it since it was over-loading the rest of the system (even with 10dB attenuation).

My lack of knowledge costed me only $50
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Old 20th February 2008, 10:46 AM   #17
AndrewT is offline AndrewT  Scotland
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Quote:
assuming that everyone makes pre-amps with gains of up to 6dB or as unity buffers
that is correct.
It's your high gain that is the problem, but not a write off if you can use it for headphones.
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Old 20th February 2008, 12:36 PM   #18
decky is offline decky  Australia
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What is your opinion - should I attempt to cut the gain down by replacing the R121 and/or R120 resistors. Or to better ask - what is a probability of pushing the amp in oscillations.

What would be better approach cutting 10K to 2k or increasing 1K to 5Kohm.
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Old 20th February 2008, 12:41 PM   #19
AndrewT is offline AndrewT  Scotland
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I wouldn't risk oscillation without the test equipment and skill to identify and correct the problem.

Fit a unity gain buffer instead.
Have you seen the simple discrete follower/buffer in Nuuk's site?
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Old 20th February 2008, 04:41 PM   #20
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Have you tried turning down the amp gain like BWRX and I both mentioned? Okay, its really weird that he and I agreed on something, especially the same gain setting. I'll recover from the surprise some day. Anyway, that adjustment will make the amp a little deaf. So unless your headphone amp is powerful enough to smoke the headphones. . .

Hey, it seems like it is too. You might want to ask the headphone amp designer what they recommend (from their experience) as an optimal approach to avoid headphone damage?

Clipping. . . I wonder if the chip amp is clipping while breaking the drywall and shaking the pictures off the walls. . . or if its clipping because its eating DC at the input and/or has too little voltage at the supply and/or has a ground loop?
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