Passive pre and Paramounts??

[shelby1420]

Hey all, I recently had a chance to hear a passive hooked up to my Paramounts and really liked what I heard!!  I have a design that calls for jacks 1 watt resistors and 100k pott.  I am thinking of trying those naked Vishays in there for my resistors and the Audio Note 100k pott ( along with the wiring provided by a buddy of mine from one of his old tube Guitar amps gone to amp heaven......)  - here is my question......do I need to worry bot impedance matches or mismatches or anything else to go along with my Paramounts/Seduction/Denon AU-300LC step up/Benz Glider s class?  I have decided that the Stereomour will go in the living room with CD player and Tuner soooooooo the only source my new passive will see is the TT.

[Jim R.]

Hi Rick,

If I remember correctly, the Seduction has a fairly high output impedance and also 100k seems quite high to use in a passive setup, even if you mount it directly to theinputs on the paramounts a la Scott Endler attenuators.

You're probably going to have to play with the pot and resistor values a bit because you want to shoot for a lower input impedance to feed the amp, yet on the other hand it will have to be high enough to be a good match for the seduction.  So in essence you have two conflicting requirements here and the typical 10-15 k that I'd use for a pot in a passive design would probably not work so well in this situation.  Also, you'll want to keep the leads out of the pot that go to the amp as short as possible, and you'll need to keep them short on the other side too, but that's somewhat less critical than the output side.  I assume the resistors are for level matching?  If you don't need that I'd almost be inclined to leave tem out altogether -- just jacks, wire, and pot, and a very light gauge, cotton insulated silver wire works really well  for wiring these kinds of passive pres.

Hope this helps,

Jim

[shelby1420]

Helps alot!!  This is the kinds stuff I am looking for, I know very little bout this kinds stuff so trying to gather any info I can before buying items for the pre......... not sure why there is a resistor and was thinking of maybe a 50k instead, make more sense??

[xcortes]

you can substitute the 249k resistor in the paramounts with 100k pots. 100k impedance is more than fine with the seduction.

[glynnw]

I have been playing around with passives for a while, but remain ignorant of the math involved.  With only PC as a source, going to a Wavelength Brick 24/96 DAC, there is no need for input switching.  Several of my experiments involved using nude Vishays in path (used  both Ultimate Attenuators and Welborne Remote Volume Control), resulting in a very slight improvement over the Foreplay 3, but lack of volume always brought me back to the Foreplay.  A couple of months ago I purchased a used Bent Audio Tap-X (TVC) and am now very happy using it in the place of the Foreplay in my system.  This may not work in other systems, but with the Tap-X placed between my tube DAC and tube amps, it sounds effortless and will play louder than I can use.

[shelby1420]

you can substitute the 249k resistor in the paramounts with 100k pots. 100k impedance is more than fine with the seduction.

Hmmmmmmmmm, sounds interesting, can you elaborate, not the most knowledgeable about this stuff, how would I do this??

[Doc B.]

A lot of the issue with passive "preamps" is that they don't work too well with low impedance loads, particularly when driving the load via long cables. If you put the passive attenuator in the Paramount you eliminate that long cable issue. Aside from having to adjust the level on each amp individually this would probably be best the best solution sonically. I agree that a 100K attenuator is probably fine. If you decide instead to make a passive box and use cables to connect it to the Paramounts I would suggest maybe 20K-25K ohm pots or attenuators, still a relatively high load for the 4K output impedance of the Seduction, and not so high as to create problems with the cables running to the amps.

[Paul Joppa]

For what it's worth, back when we developed the "Sweetest Whispers" we made up a passive preamp using that 15K control as a conventional potentiometer. A Seduction could drive it as is (I would probably increase the output capacitor from 0.47uF to 1.0uF myself, but it works OK as is). There is not enough gain to use a shunt-mode control; stick with the conventional potentiometer. 10K is getting pretty marginal, but 20K-25K as Doc B suggests is about the best compromise.

[xcortes]

In the past weeks I've also been experimenting with passives. Way better than just a pot is, IMHO, a transformer coupled passive,  Magnequest makes a 10k:10k unit that is very nice with a 10k pot but, as PJ says, 10k is marginal with the Seduction. My friend keto is looking for a 50k:10k transformer to load the Seduction correctly and use his 10k Penny and Giles pot. I have his 15k: 600 ohm MQ B7s and ordered a 1k P&G pot. Negative gain is perfect in my system with 2A3 Paramounts but my speakers are 106 db efficient. Nothing like experimenting in your own system.

[shelby1420]

you can substitute the 249k resistor in the paramounts with 100k pots. 100k impedance is more than fine with the seduction.

Ok, last question (maybe) where is the "249k" resistor found??

[xcortes]

The 249 k resistor is at the input of the signal. I'm not sure if it's at the RCA jack or at the terminal strip where the wires from such jack are connected to (at the front left side of the amp). It connects the positive of the signal to ground. That's the resistor you replace with a 100k pot. I use pec pots from digikey. In my particular application I don't require access to it from the top but if that's going to be your volume control you will need to drill a hole.

When installing these pots you need to also disconnect the input signal wire so that the signal goes through the pot. That is, instead of the signal going directly in and having a shunt resistor you will wire the pot so that the signal sees some resistance before the shunt creating a voltage divider. When you turn the volume up the resistance before the shunt goes down and the shunt one goes up and viceversa.

I hope I made myself somewhat clear.

[Grainger49]

Xavier is right.  I searched the manual for the value and I think you will find it on the bottom of page 52, next to the last step.  The picture shows it well.  The "finished picture" of this resistor is obscured by the C4S board.  The STP that feeds the input signal should be split.  From the input RCA jack it goes to the pot then from the pot to the same terminals it goes to now.  But the resistor on page 52 is replaced by the pot.

[keto]

Hey Shelby, why not try going from the Seduction to the Stereomour? Perfect impedance matching, at least. As xcortes mentioned, I'm working on a similar "passive-pre" and Paramount combo (at the moment, seduction -- 10K attenuator -- 300B Paramounts). To get the right impedance and gain combos, I've come up with the idea of Eros -- 50K:10K transformer -- 10K stereo potentiometer -- Paramounts. That way, a nominally 3mV MM cartridge will yield almost 1V at the output of the Eros, stepped down to about .4V, which is what the 2A3 Paramount requires for full output. Similarly, the seduction--stereomour combo would take a 3mV cartridge output to .3V, which is not quite enough to drive the latter to full output. I guess the answer to my first question is "maybe not enough gain", though that would depend on the output of your MC cartridges + step-ups. If you do try the 100k pots added to the input of the Paramounts, make sure they are stepped! The combo that makes the most sense to me, though would be the Eros connected to either the S.E.X. amp or Stereomour, since that would be a good impedance and gain match, almost as if it were MEANT to be :-)

[Paul Joppa]

Quick note - cartridges are rated for nominal output; instantaneous peaks will usually be some 14dB (5 times the voltage) greater. Depends on how the record was mastered, of course.

[shelby1420]

Whew, tons of great info guys, thanks a ton!!!! I have decided to build the passive separate from the amps using " http://www.partsconnexion.com/KHOZMO-74446.html " as the attenuator and for my resistors I am going to use " 1K Vishay VAR-Series "naked" Z201 Z-FOIL RESISTOR 1% " and then I have a set of extra cables that I will solder directly to he attenuators........  any issues with this plan??  Again, thanks a ton for the insight all!!!