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Old 9th March 2011, 05:16 AM   #1
spgjmf is offline spgjmf  United States
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Default PAS-3 phono board as stand alone preamp

hi folks,

well after a year of listening to my lm4562 phono pre, I am interested in somethng different -a tube based solution. Hoping for more lively listening. The current preamp is clear, strong and sort of lacks "fun". Am I looking for some tube distortion...no sure - just some magic and more life to the playback.

I was thinking of taking one of the various dynaco pc6 phono upgrade boards, add a PS and use that. Seems to be an under $100 solution. Triode electronics has the pas replacement tranny for $35 and the boards are from 30 to $70 depending on who and what they are.

my question is, will the hi output impedance of the preamp be able to drive an amp? Could or should a buffer be added to the output - cathode follower?
will it matter for a 3 foot cable run?

thought, suggestions ??- looking for low cost tube solution - the pacific FET phono amp is also under consideration, but tubes are so much fun .oh, my amp is JE labs 2a3.

thanks for you thoughts and time,
Jeff
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Old 10th March 2011, 07:06 PM   #2
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When dynaco produced their first transistor amp, they bought a special 150 gain input transistor to cope with the low output of the PAS3. My ST120 has way more gain than my CS800s even with the DDT knobs all the way up, as does the ST70 tube amp. What is different is the headroom of the CS800s, but on CD source it really isn't loud enough to shake the windows with JSBach pedals, for example. If your power amp has op amp input like the Peavey CS800s, getting more gain out of it may just be a matter of changing the feedback resistors. But if it is another design, you'll have to ask someone more knowledgable than me how to do it. Incidently, the CS800s has 29db of gain into 4 ohm full gain. I have 6' cable run on the PAS2 with the 2nd tube output stage, still 250k output impedance according to the tube datasheets.
I don't think my PAS2 has noticeable distortion of any kind. Not with a .003% HD power amp and with SP2 speakers whose distortion chart you can look at on the Peavey website- pretty **** low at 1 W which is where I listen in my living room.
Note the double ferous steel box of the PAS is an important hum attenuation tool. 120 VAC one side of the steel bulkhead, low voltage the other side. I had to pull the power transformer right out of the op amp disco mixer 6' away to get anywhere close to PAS2 on hum.
I've got the RA88a disco mixer (op amps) sounding very like the PAS2 after several upgrades of both, except the PAS2 has slightly less hum. Since I can't find 250k log stereo volume pot with the loudness feature, and my dynaco bankruptcy replacement pot has a stuck shaft, the PAS2 is in the attic.
Have fun.
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Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 10th March 2011 at 07:14 PM.
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