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pxr5 19th November 2004 05:42 PM

simple preamp
i've built a simple preamp to replace a passive stepped attenuator,
sound has much improved.
now i want more,so build a mark2
the pre amp is merely an opamp input buffer then balance pot then volume pot then line driver.
the inputs to + and - on each op amp are impedance matched for sorce resistance and capacitance, and have unity gain.
now, i'm thinking of investing in four buf634 to use as unity buffers, and not using opamps at all.
the specs for buf634 configured as unity buffer, wide bw seem to be vastly better than even opamps i've used such as opa627 and ad843.
has anyone tried this, ie buf634 on its own not opa627+buf etc
I'c just like to know if its worthwhile as each buf634 ius abour 8 from rs and I'll probably do some improved pcb and power supply work at the same time, so it could prove expensive (but interesting!)
just an idea

pxr5 19th November 2004 06:25 PM

no one tried this?
guess i'll build it, might just be the best preamp ever!

joensd 19th November 2004 06:51 PM


has anyone tried this, ie buf634 on its own not opa627+buf etc
If you dont need any gain, this is of course an option.
The BUF634 is a very nice chip. Built some headphone amps with it.
But why do you need 4?
And yes, a nice power supply to go with it is worthwile. (LM317 for example is not so expensive)

pxr5 19th November 2004 07:20 PM

thanks, its good to know that buf634 is a useable chip,
i'll need 4 as i work in stereo so (one for input buffer and one for output line drive( and maybe headphones two (bonus))*2(for stereo)
at present have lm317/lm337 split supplt with plenty of bypass caps dotted around. have been reading some of walt jung's papers and he has a schematic for super low noise, but it seems a little ott.

i was thinking more along lines of localised regulation. ie +-20v central psu regulated then next to chip using to92 lm's and silvermica caps just to make things super smooth.

have found that opamp class a bias is worthwhile and certainly feedback impedance matching.

i've noticed that buf634 in wide bw mode is already class a

the circuit should become a lot less component intensive

the daunting prospect for the future is applying this concept to my active xo if the new preamp is a sucess.

thanks again

joensd 19th November 2004 11:07 PM


the daunting prospect for the future is applying this concept to my active xo if the new preamp is a sucess
Any hints to your active XO.
Is it a DIY-project or commercial?
Im still using breadboard 12dB-LR and want to try something else with a nice PCB-layout.

pxr5 20th November 2004 12:52 PM

active xo
i've done a lot of work on my active xo
its mostly home built
20 year ago i bought some crimson electric 2 way active xo boards after getting fed up with inductors and capacitors which seemed very vague.
i was working for wilmslow audio at the time so all pieces could be played with freely.
the only thing we didn't have was proper accoustic measurement equipment. anyway speakers sounded ok with stock 24db lr at 2.1khz and i left them at that only swapping some tl081 for ne5534.

now, with modern computer, soundcard, mike and software its possible to get all sorts of information.

thus the crossover has evolved. firstly good input buffer. opa unity gain.

then high pass 24db lr set at -6db 1800hz with no further eq for my present tweeters, north d28-06s

also low pass 12db butterworth set at -3db 1600 hz, this with bassmid accoustic rolloff gives close to lr 1800hz lp.

also the phase seems to have come out just right to take into account voice coil offset. (had to use allpass filters before when it was standard 24db lr high and low pass at same frequency.

then bass channel goes to a linkwitz transform circuit to give total Qtc of 0.495 at 16 hz.

then opamp based baffle step eq set to +5 db at approx 330 hz, bit hard to get this right . used trimpots instead of fixed resistors so i could measure, fiddle , listen to an infinite variety of steps.

then a simple pot for level adjustment. the bass driver is very efficient so it needs bringing down by about 3db. I'm not happy with this bit and will eventually adjusted bassa mp gain to suit.

i cant play these speakers at full volume as spl is near 112db with 100w amps and my room is small.

of course all opamps have good psu but general layout could be improved. now working on last 10% using salen key impedance matching as ad743 datasheet etc.

best to start with general flow diagram with required stages. its supprising though how much the sound can be improved by paying attention to fine circuit details.

mostly built on tripad board, no noise or hum problems, and no oscillation even with opa627,you must us 100 ohm ouput resistors on opamps even for short cable runs.

i'll post circuit diagram latter, just got in from work and having a beer.

Upupa Epops 20th November 2004 01:43 PM

Simplest preamp ? Use TPA 6120 A with input amp, like make it peranders ;) ( or buy his PCB ) , or use directly this circuit, but with DC servo. I mean, that not exist better circuit at this time :cool: .

pxr5 20th November 2004 02:19 PM

xo schematic
2 Attachment(s)
rough xo using circuit maker student

pxr5 20th November 2004 02:24 PM

thanks tpa 6120 looks interesting, but is it as good sound wise as buf634, sound is more important than simplicity for me

mandat 20th November 2004 07:29 PM

Look at OPA1632 specification, Walt Jung's buffer or link below
and make the final decision.

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