Need Help From The Resident Experts


My problem:

My preamp puts out 30db of gain and the output impedance is 1.2K ohms.  My power amp input impedance is 22K ohms.  I can only use 1/4th of the volume controls (9 o'clock) and it is too loud after that point.  What would be the ideal output impedance for the preamp to mate well with the power amp?  Components in question are an Audible Illusions Modulus 3B and an Odyssey Stratos Extreme+++.  I would like to use more of the volume range and am open to a different pre.

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The Modulus is well-known to have excessive gain, it's design dating back to the pre-CD era. Most modern pre-amps provide around 12dB. There are attenuators available for inserting between pre and power, or resistors can be installed on the inputs of your power amp to lower it's input sensitivity, I believe. I could be wrong!

The output impedance of your pre and input of your power has nothing to do with the gain, however. The old rule regarding pre-amp output impedance to power amp input resistance is a ten-fold one as a minimum: 1k output to 10k input. More is better.

The lack of range on the volume control is due to the extremely and unusually high 30 db of line-stage gain. Impedance compatibility is a separate and unrelated issue.

The majority of active line-stages provide gains in the area between about 6 db and 15 db. Anything in that range will probably cause the volume you presently have at 9 o’clock to come out above the 12 o’clock position, and gain in the lower part of that range would probably bring it to the vicinity of 2 or 3 o’clock.

An impedance incompatibility, if present, would mainly affect frequency response flatness, especially in the deep bass region. The 1.2K spec is probably based on a mid-range frequency such as 1 kHz, and may rise to considerably higher values at deep bass frequencies due to the coupling capacitor that is used at the output of most tube-based preamps (the impedance of a capacitor rises as frequency decreases). Whether or not a problem occurs depends on the **variation** of output impedance over the frequency range.

To assure that an impedance incompatibility does not occur the commonly accepted rule of thumb, properly stated, is that the amp’s input impedance should be at least 10x the output impedance of the preamp, at the audible frequency for which the preamp’s output impedance is highest. Although non-compliance with that guideline does not mean that a problem will necessarily arise; compliance with that guideline assures that a problem won’t arise. For most tube-based preamps the highest output impedance at any audibly significant frequency is likely to be at 20 Hz.

If the 20 Hz output impedance can’t be obtained from the manufacturer or determined in some other manner, I suggest using a ratio of 75x to be safe. To assure compatibility with the Odyssey that would mean a nominal (mid-range) output impedance of less than or equal to 22K/75 = 293 ohms.

If the maximum output impedance at any audible frequency can be determined (and if Stereophile has reviewed the product it will be indicated in the measurements section of the review), then 22K/10 = 2.2K would be a safe upper limit.

Good luck. Regards,
-- Al

P.S: Per bdp24’s good suggestion, you might consider purchasing a pair of Rothwell Attenuators in the 20 db configuration that is offered. Some people have reported compromised dynamics or other sonic issues with these and similar devices, while others (including me) have found them to provide excellent results. I’m fairly certain that the Rothwells will provide a load impedance that is high enough to be suitable for use with the AI preamp; I know that some competitive units from other manufacturers will not.
@almarg your wealth of information about all things audio related never ceases to be impressive, informative and rather well articulated. 
+1 almarg (sort of redundant).  No question that the problem is too much gain.  FWIW, I've used a pair of 10 dB Rothwell attenuators successfully in the past. 
I appreciate all the responses, but the bottom line is I want a different preamp.  I am much older now and I would like remote control, phono input and either a tape monitor or processor loop.  I have been looking at the Rogue RP-5 and wonder if it will work well in my system.  What do you guys think?  Keep in mind I have had 3 Modulus preamps in my system since 1992 and I want to try something different.
The RP-5's nominal output impedance of 500 ohms doesn't quite meet the guideline I suggested, and I couldn't find any measurements indicating how high that number may become over the audible frequency range.  So you might want to contact Rogue and ask them what its output impedance is at 20 Hz, and if they would recommend it for use with a power amp having an input impedance of 22K.

Otherwise I don't see any technical issues that would arise with the RP-5.  I have no particular knowledge of its sonics, though.

Hal, thanks very much for the nice words.

Regards,
-- Al
 

Thank you Almarg........

I plan on calling them tomorrow but I wanted to know what you guys think.

What's your budget?
My problem:
My preamp puts out 30db of gain and the output impedance is 1.2K ohms. My power amp input impedance is 22K ohms. I can only use 1/4th of the volume controls (9 o'clock) and it is too loud after that point. What would be the ideal output impedance for the preamp to mate well with the power amp? Components in question are an Audible Illusions Modulus 3B and an Odyssey Stratos Extreme+++. I would like to use more of the volume range and am open to a different pre.
stereo5
All to common, no need for a preamp with gain, try one of these for $49 and don't look back.
And you can sent it back, if you wish, I doubt you will.
http://schiit.com/products/sys

A Quote from the master Nelson Pass

Nelson Pass,
We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more.
Think of it this way: If you are running your volume control down around 9 o’clock, you are actually throwing away signal level so that a subsequent gain stage can make it back up.
Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control.
What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection.
And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp.
Cheers George

George.....

Thanks but not interested. I need a full function pre with remote at least 3 line inputs a tape loop and input for my turntable. The rogue RP-5 seems to fill the bill, plus it’s tubes. I have always had a tube preamp in my system and like the sound combined with the solid state amp. I listen to mostly rock and  it works out well for me.

Prima Luna preamps are designed specifically with low gain for just that reason. :)

And quite easy to mod the caps with....hehehhe

Not sure about remotes.

Best,


Erik
Viva Almarg!!!

As a horribly scarred past M3A owner I hope you will post the outcome of your experience.



 


  
Erik................

i looked at at them but they don’t have a full tape loop or full processor loop. I would love a prima Luna dialogue preamp with the tube rectification but they don't offer the features I would need.   I also would Ned a input for my turntable.  
Parasound P7 then, or the JC 2 BP

I can attest that the P7 is pretty neutral and quiet, with an excellent remote and useful bass management. Kal Rubinson I think at Stereophile really likes it and uses it when his ARC multichannel pre is out.

The volume is digitally controlled, but analog. Steps are about 1.1 db or so per step. I measured this a long time ago and am hazy on the exact details. :)
Rogue preamps that use the 12AU7 tube have a rather neutral sonic signature with good bass extension. They respond well to tube rolling.
Very well built products.

...another reason to get the amp and preamp from the same manufacturer
Cary AES has 6db gain or unity.
Not sure of output impedance, but it's very transparent and clean.

I spoke with John Rutan at Audio Connection and I will be traveling there Saturday to audition the Belles Aria preamp.  I  may also audition the Rogue RP-5 if the Aria doesn't float my boat.  Thanks to all for their input, the information I received was very valuable to me.  Truly appreciated.
you'll be in good hands and happy. JohnnyR is best.
Agreed, he is.  Unfortunately, my road trip will be postponed for 3 weeks due to my wife working the next 2 weekends and Columbus day.  I won't get down there until the weekend after Columbus Day.  I am so impatient though.  Kind of sorry I asked my wife to ride shotgun or I could have gone over this weekend.   I promised my wife we would stop and see family in Bergen County (Demarest, NJ) so it'll work out.  Can't wait to see Johnnys store, I haven't been in a high end shop in at least 5 years.