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Old 26th May 2006, 09:32 PM   #1
st2_998 is offline st2_998  Italy
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Default Passive pre

Hi everybody
I'm planning to build a pseudo-passive preamp. I would like to have an outbut buffer to avoid getting too fussy about cables and impedances (this IMHO, any different is welcome). I am discovering Mr. Pass' work and I'm wondering which design could suit my needs.Very boring topic for most of you, I fear, since I'm a newbie and I know little about Mr. Pass' designs. For example I found "Balanced line stage" (is that BOSOZ? and what is X-BOSOZ then?) very interesting, but:

I don't think I'll need 10 dB gain, do you? I thought unity gain was enough, but perhaps I'm out of way

I didn't fully understand how tu run it with both inputs and outputs unbalanced (negative input to ground? and negative output?)

Sources will be ordinary Cd players or phono preamps, amps will be a D class (DIY, probably UcD) and possibly a Dynaco ST70.
Anyone can give me advice?
Thanks
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Old 26th May 2006, 11:00 PM   #2
Mad_K is offline Mad_K  Norway
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Take a look at the BUF634. http://www-s.ti.com/sc/ds/buf634.pdf
Run it of a couple of 12V SLA batteries and add a DACT CT2 in front and you've got a hell of a "pseudo-passive" preamp

http://www.dact.com/html/attenuators.html

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Old 27th May 2006, 02:06 PM   #3
st2_998 is offline st2_998  Italy
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Thanks
I read the BUF634 spec sheet, seems interesting.
Have you got a schematic idea for this?
I saw on the forum that something with OPAxxx + BUF634 was made, I suppose has much more gain

And what about something without op-amps? Discrete perhaps? No Pass designs?
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Old 27th May 2006, 02:21 PM   #4
Mad_K is offline Mad_K  Norway
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Take a IRF610 and use it as a follower. Like my SEWA design, only with lower idle current. Look here:http://www.diyaudio.com/forums/showt...796#post912796

here are some more ideas: http://www.muzique.com/lab/buffers.htm
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Old 27th May 2006, 04:17 PM   #5
Mad_K is offline Mad_K  Norway
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Quote:
Originally posted by st2_998

I read the BUF634 spec sheet, seems interesting.
Have you got a schematic idea for this?

Here's what you are thinking about.

http://home.broadpark.no/~viiaudio/p...enixRA.SCH.pdf

For unity gain, simply delete U1 and run your signal into R3/R9. There will be some offset.

Or, if you want feedback and low offset, delete R1/R7 and use 220pF for C1/C2.
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Old 27th May 2006, 04:27 PM   #6
Mad_K is offline Mad_K  Norway
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Something like this is interesting:
(JFETs are really popular these days)

http://www.muzique.com/images/buff4.gif

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Old 27th May 2006, 04:33 PM   #7
adason is offline adason  United States
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try three tranistor buffer
http://myweb.tiscali.co.uk/nuukspot/...ainclone2.html
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Old 27th May 2006, 04:44 PM   #8
jleaman is offline jleaman  Belgium
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I've attached a photo of what i use for a passive Pre-Amp. I run a few Mini Aleph's Right at the moment i'm using the bozos Pre-amp Built from Twisted Pearl Audio. In some cases i can say that just running my cd player into a volume ( passive Pre-Amp ) and into a set of aleph's It sounds way better more controlled. I have tried DACT switches and didn't like them Over time they fail and sound horrible. BrianGt had a 150$ on in his gain-clone and when i was using it it sounded like HARSH CRAP! a wasted 150$ on a dact EW. Right now in this cheap YES cheap passive is a peter daniel volume pot it's a Alps. Im happy with it till i get more money to buy a way better one.

Do we really need a pre-amp to run these amps ? I was told we do but i just proved to my self that we don't. NOW if you need lots of output and hi volumes you might need a pre-amp. I have found running the alephs in Se mode they are pretty loud running a passive Pre-Amp.

Maybe i can try that simple circuit that Mad_K has attached.! Have you run it yet Mad_K ?

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Old 27th May 2006, 05:22 PM   #9
st2_998 is offline st2_998  Italy
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Lot of ideas Mad_K

As for "Phoenix" when using only BUF634 perhaps an input cap would be needed?

I'd like a "pure passive" pre, but I'd like also to play with different cables and amps, so I think I need a buffer stage

Well I'll build a pre with opamp, mosfet, jfet and tube output buffer, strike repeat on my CD and A-B-C-D'ing all the time 'till my girlfriend kicks me out of home
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Old 27th May 2006, 05:33 PM   #10
st2_998 is offline st2_998  Italy
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Back and serious...

I read on passdyi about discontinued preamps like Aleph P, L etc.
Browsing the forum I understood there were some problems about technical infomation on these designs.
I know little or nothing about this point but I'm really impressed by Mr. Pass' approach in sharing his work with DIYers, and it is sad to see that somebody took advantage on his generosity.
I would be interested in these designs, I saw there are also PCBs out there...
The matter is that I would like to understand them better prior to build one, even if it is a kit. Are there descriptions, white papers, product sheets about?

Thanks
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