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Old 23rd January 2007, 01:59 PM   #1
h_a is offline h_a  Europe
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Default questions about passive preamp for Aleph-30

Hi folks!

I'm thinking of just putting a potentiometer in front of the Aleph 30. Its nice input impedance of 47kOhm should allow 25kOhm or below pots, right?

Any disadvantages to this? The source sees a varying load, so could this be a problem, if one uses a 10k or 25k pot?

Is the frequency response affected by this? Of course I want a ruler flat response, so better ask people that know things.

What would be the correct circuit then? If you look at this image

http://www.thel-audioworld.de/bauteile/regler/CP12.GIF

I guess the one marked with a "1" is more reasonable?

Well, I also read that the source impedance should be at least smaller than the half of the pot, so when connecting the pearl with I think 499Ohms source impedance should be perfectly ok then?

Any sideeffects? Something to take care of?

Any input is highly welcome!

Thanks as always for your great help!!

With kind regards, Hannes

PS: since I want to build the pot into the same enclosure as the A30 cabling is reduced to a minimum length.

PPS: I'll also put that info into the wiki!
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Old 23rd January 2007, 06:53 PM   #2
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A 10K to 25K pot will work just fine. There is very little
capacitance on the input, and the distortion and bandwidth
figures remain about the same.

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Old 26th January 2007, 10:54 AM   #3
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Thanks a lot for your reply!

Great that you could take the time to reply!

Have fun! Hannes
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Old 31st January 2007, 03:18 PM   #4
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Hannes,

I am very interested in passice preamp for a30. How many inputs due you plan to have? If more than one how will you implement?

Sorry for the basic questions.

Mike
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Old 1st February 2007, 08:03 AM   #5
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Well I thought of adding 6x inputs. Easily done by inserting a selector switch in front of the potentiometer.

Connect the selector switch with the 6 pairs of RCA inputs on the backpanel and you're done.

I'm by no means an expert, but I think it can't be easier. And in addition you've no additional preamp fets in the signal path, so no additional THD, no additional noise, nothing.

Just be sure to get a really decent pot or build a stepped attenuator, otherwise you suffer unequal channel balance (that is one channel being louder than the other by as much as 2-3 dB when using cheap pots!).

Especially with the Aleph30 this is a nice way of getting rid of a preamp, see in the Aleph 30 wiki for further details!

Have fun! Hannes
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Old 7th February 2007, 03:50 AM   #6
SteveCZ is offline SteveCZ  United States
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Default I want passive ...

...but I also want to use a Bottlehead Seduction phono preamp. Doc Bottlehead says it will run into a passive preamp *if* the attenuator is at least 50KOhms (and 100KOhms is better), and if, of course, there is enough gain in the amplifier. (In other words, the Seduction wants to run into an amplifier with at least a 50K input impedance.)

I'm new to this stuff. I'd like the simplicity and low cost of a passive preamp, but is that high input impedance going to give me problems with a source like CD?

Thanks!
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Old 7th February 2007, 08:29 AM   #7
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Well I read there are 2 rules:

1. pot resistance need to be roughly 5-10 times the source impedance to avoid loading the source too much. The lower the pot resistance the better in regards to signal to noise ratio.

2. pot resistance need to be at least equal to or better half of the amp input impedance to work at all - otherwise you can't attenuate volume at all.

So the last point is your problem since the Aleph30 has about 50kOhms inputimpedance - so you can only use the 50k potentiometer I guess.

And no, high pot values don't hurt other sources as CD.

Have fun! Hannes

PS: BTW how well does this tube phono amp perform?
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Old 28th November 2007, 07:08 PM   #8
mrRed is offline mrRed  United States
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So would it work fine to run something like the ALPS RK27, or would one be better off using something more along the lines of the RK097?

I'm thinking about wiring up a passive pre-amp (in a separate enclosure as the amp) with input switching and output switching between the aleph 30 and a headphone amp. What type of problems would I see using a set-up like this?
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