[srb]

The Hypex UCD-400 modules are delivered with a default gain of 26dB, but can be adjusted.  The Rogue Medusa adds their own tube input stage, so I don't know if they get increased gain from that, from adjusting the UCD-400 modules or a combination of both.  Also, the input sensitivity of the complete amp is 1.0V.

Like you say, with all the variations of sources and speakers ultimately you'll just have to try a passive volume control and see how it works with this particular system, but it sounds like the Medusa is on the right side of the tracks.

Steve

[undertow]

1.0 volt, 31 db plus gain... Should be great. But to be honest even with a 89 db speaker depending on the source material... You might be looking for another 6 db of power unless turning well up past 1/2 volume.

[wilsynet]

I have NC400 monoblocks and have been driving it direct from my NAD M51 DAC for some time against only moderately efficient Selah Tempesta speakers.  They aren't difficult to drive, but they aren't exactly super efficient either.

Either I have tin ears or maybe my room is just smaller.

[sruffle]

Thanks to the kind offer from wilysnet I will try the Music First volume pot in my system.  If the volume situation works out, I plan to try a passive preamp.  I will post an update after getting the initial impressions.  Happy Memorial Day to all.

[wilsynet]

Correction: it's a "Music Reference" not a "Music First".  It's important because Music First actually makes a high end TVC based passive preamp.

The Music Reference is a pot-in-a-box resistor based volume control that Roger sells for around $150.

[ssglx]

I tried the LDR1 in my system for 2 weeks. The result was excellent. Read my review under the Tortuga demo tour.

Chris

[Big Red Machine]

If I wasn't so attached to my Dude, I would probably have an LDR for an extended time.

[jtwrace]

If I wasn't so attached to my Dude, I would probably have an LDR for an extended time.

You could always put an LDR in the Dude...

[tortugaranger]

You could always put an LDR in the Dude...

Indeed! In all likelihood the attenuator in your Dude is high quality pot which is just a voltage divider with a fixed input impedance. The LDR3x is just a voltage divider with a mostly fixed input impedance. If you like the Dude with a pot, there's a very good chance you'll like it even more with an LDRx.  You can argue there shouldn't be any difference because a voltage divider is a voltage divider but the resistive material in LDRs turns out be superior to a pot plus there's no mechanical wiper interface point with LDRs.

[tortugaranger]

I had never thought about passive preamps but have recently become really intrigued.  The LDR3 looks very interesting.  I think a passive would fit in my system but am not sure. It would go between a Rega DAC and a Rogue Audio Medusa power amp.

The Rega has a 600 ohm output impedance and goes 2.175 V.  The Medusa has an input impedance of 200,000 ohms.  It seems that a passive would be fine but I am not sure if here are any other specs, I need to check.  My interconnects are short so that is not an issue.  Are there any other factors  I should check? 

Thanks in advance for any responses.

Your system sounds like an ideal candidate for a passive attenuator.

[rajacat]

It would be helpful if there could be a suggested range of impedances for the source and the amp. for which the LDR3 would be compatible.

[tortugaranger]

It would be helpful if there could be a suggested range of impedances for the source and the amp. for which the LDR3 would be compatible.

I posted the following response on another thread (http://www.audiocircle.com/index.php?topic=119407.msg1332161#msg1332161 ) but am repeating it here since the question is similar.

The established guidance when connecting audio devices is to either match impedance (output impedance of source = input impedance of amp) or to have sufficiently high bridging impedance ratio (10:1 where amp input  impedance at least 10x greater than source output impedance). Matching impedance may be ideal but it's impractical so most sources have less than 1k output impedance and most amps have input impedances of 10k or greater.

When you place a preamp between a source and amp you now have 2 instances of impedance bridging: between the source (DAC, phono stage, CD etc. ) and preamp, and between the preamp and amp.

When we were able to raise the input impedance of our LDRx preamps to 10k last year (the HiZ upgrade), this improved matters between the source and preamp.

However, as the graph below shows, any passive attenuator employing voltage division (like pots or LDRs) has a varying output impedance that reaches a max of 25% of the input impedance at -6dB (loud!!!). In the case of a 10k unit, that's a max output impedance of 2.5k. For a 20k unit, it's 5k. This would then argue for amps having at least 25k input impedance for a 10k passive attenuator or 50k for a 20k passive attenuator.

Then again, as a practical matter few of us listen to music at -6dB attenuation. In the more likely range of -40 to -20 dB attenuation, the passive preamps output impedance is more likely to be in the 1-2k range which means that an amp with 20k input impedance of greater should suffice but arguably 40-50k or higher may be better (that's "may", not "will").

Beyond this level of general guidance you get into more subtle and equipment specific issues.

[sruffle]

The Volume pot is expected to arrive early this week so I will be eager to test it out.  This is another newbie question:

There have been several mentions that some people don't like listening while volume knob is at 50%.  Is this  a psychological hurdle or does this indicate that your system doesn't have enough power to give proper dynamics?

[tortugaranger]

The Volume pot is expected to arrive early this week so I will be eager to test it out.  This is another newbie question:

There have been several mentions that some people don't like listening while volume knob is at 50%.  Is this  a psychological hurdle or does this indicate that your system doesn't have enough power to give proper dynamics?

Been scratching my head on this post     Are you sure you posted your question in the right forum? 

Our attenuator isn't a pot and while you can use a rotary encoder/knob to control it, there's no 50% position - again, because it isn't really a pot. Also, to my knowledge nobody has mentioned anything about not liking to listen at mid-range. Lastly, being essentially a passive volume control device, our attenuators don't impart any power into the audio signal. Although the issue of dynamics is relevant this has more to do with impedance matching/bridging than power per se. Fortunately, our semi-fixed impedance design yields very nice dynamics. 

[sruffle]

Been scratching my head on this post     Are you sure you posted your question in the right forum? 

Our attenuator isn't a pot and while you can use a rotary encoder/knob to control it, there's no 50% position - again, because it isn't really a pot. Also, to my knowledge nobody has mentioned anything about not liking to listen at mid-range. Lastly, being essentially a passive volume control device, our attenuators don't impart any power into the audio signal. Although the issue of dynamics is relevant this has more to do with impedance matching/bridging than power per se. Fortunately, our semi-fixed impedance design yields very nice dynamics. 

Sorry for the confusing post.  The volume pot refers to the Music Reference volume pot that I am borrowing to test whether or not my system has enough gain to use a passive.  The package arrived today so I will be trying it out tonight.  I am looking forward to testing it and will report back.

[sruffle]

Thanks to the kindness of wilysnet, I tested out the Music Reference volume pot in my system.  The verdict is what I had hoped for - I was able to generate sufficient volume with no issues.  I am now going to think about this for a while.  My instinct is that I will try a Tortuga - either new or from a DIY build.  The concept of a passive peamp makes so much sense that I feel like it is something everyone into hifi should try at some point.

[ssglx]

Good choice. I'm pretty sure you'll like it.

[tortugaranger]

The concept of a passive peamp makes so much sense that I feel like it is something everyone into hifi should try at some point.

Couldn't agree more. 

[werd]

Thanks to the kindness of wilysnet, I tested out the Music Reference volume pot in my system.  The verdict is what I had hoped for - I was able to generate sufficient volume with no issues.  I am now going to think about this for a while.  My instinct is that I will try a Tortuga - either new or from a DIY build.  The concept of a passive peamp makes so much sense that I feel like it is something everyone into hifi should try at some point.

Passives make great primary pre amps. They are also great secondary pre amps too. I agree, every one should have a passive in their inventory. Aside from using them for listening, they can tell you a lot a out of what your front-end sounds like. So they are helpful in system building. That is if you can afford having one for this use.

[sruffle]

Well, I made the leap.  I replace my modded Acurus RL-11 with DIY passive that uses the Tortuga LDR3V2 board.   I received it this weekend and gave it a trial run Saturday and Sunday.  Because I was juggling taking care of the kids and getting used to the remote I didn’t really get a good feel for the sound but I will say that I wasn’t bowled over right out of the gate.  The second night I was able to sit down and really dig in.  I am not a reviewer but my impressions were:
 
·        Lower noise floor – I listen to a lot of live Grateful Dead and this was particularly noticeable on several live recordings.
·        Increased detail
·        Better tone and timber - on jazz recordings, horns and saxes seemed more pure and free flowing.  Additionally some glare was removed from my system.
 
This was exactly what I was hoping for.  One thing that I did realize is that the Acurus has a great remote.  I am going to order an Apple remote to see how I like it.  I was looking for some of the typical characteristics that critics of passives highlight.  Other than not liking the remote as much, I didn’t sense any deficiencies  -   maybe I will in the future but for now I am very happy with the decision to go this route.  This is a great component to build a system around.  The money saved from upgrading the preamp can now be put into a DAC upgrade somewhere down the line.
 
Again, thanks to all the responses on this thread.   This has been a significant upgrade for relatively minor dollars that will give me much listening pleasure going forward.  I may post some more impressions after further listening