[suicidallabrat]

As someone truly excited to experiment with removing any preamp/processing from my audio pipeline, I was discouraged to see the data plotted on the Roll-Off graphs posted at http://www.tortugaaudio.com/what-is-a-passive-preamplifier/; considering the length of the cable runs to my mains (35’). 
I don’t currently have a passive pre, but my source has digital volume control in its dac’s, so I was experimenting with a source direct set up (Oppo 105 --> Pas Labs XA200.5 mono’s).
Being that I am not an engineer, can anyone enlighten me with regards to whether I am likely to encounter roll-off in the audible range, considering the spec of my current system?

What I’m running:
Source peak output level: 2.1Vrms
Source output impedance: 100Ω
Balanced XLR Length: 35 ft.
Balanced XLR capacitance conductor to conductor: 39.372(pFm)  Note: Meter, not ft.
Balanced XLR capacitance Cond. to other conductor and shield : 85.306(pFm)
Power Amplifiers input impedance: 30,000 Ω

Any other pointers that might help guide me in weigh the benefit/deficit of losing the pre/pro?

On an aside, can anyone share the formula used to plot the graphs I referenced?

[WireNut]

Balanced XLR Length: 35 ft.

 
.
For someone like myself who tries to keep cable length at 1 meter or less, 35 feet seems like an awful long run. Roll-off due to cable length would be my first concern and even more so if I were trying to drive that length with a passive. But that's just me.








 
 

[tortugaranger]

As someone truly excited to experiment with removing any preamp/processing from my audio pipeline, I was discouraged to see the data plotted on the Roll-Off graphs posted at http://www.tortugaaudio.com/what-is-a-passive-preamplifier/; considering the length of the cable runs to my mains (35’). 
I don’t currently have a passive pre, but my source has digital volume control in its dac’s, so I was experimenting with a source direct set up (Oppo 105 --> Pas Labs XA200.5 mono’s).
Being that I am not an engineer, can anyone enlighten me with regards to whether I am likely to encounter roll-off in the audible range, considering the spec of my current system?

What I’m running:
Source peak output level: 2.1Vrms
Source output impedance: 100Ω
Balanced XLR Length: 35 ft.
Balanced XLR capacitance conductor to conductor: 39.372(pFm)  Note: Meter, not ft.
Balanced XLR capacitance Cond. to other conductor and shield : 85.306(pFm)
Power Amplifiers input impedance: 30,000 Ω

Any other pointers that might help guide me in weigh the benefit/deficit of losing the pre/pro?

On an aside, can anyone share the formula used to plot the graphs I referenced?

When you distill is all down, most passives represent a variable resistor between your source and your amp. Add in the capacitance of the downstream cable etc. and you have a classic RC low pass filter. "low pass" means the lower frequency stuff gets "passed" through and the higher frequency stuff gets filtered out. Depending on the quotient of R*C, it's possible to roll-off some of the signal below 20khz not to mention the phase shift that comes along with this.

The equation to calculate the roll-off frequency where the signal is down 3dB due to this effect is  f = 2*pi*R*C; where R is in ohms, C is in farads, and pi = 3.1416.

Let's take your 35 foot cable. At 85 pf/meter that's roughly 28.4 pf/ft (i divided by 3, yes I know that's not exact, roll with me here, I didn't want to look it up). Not the best but not the worst by way of cable capacitance. 35*28.4 = 9.95E-10 (a very small number indeed).

Now let's look at the passive. Say it's running around 2k ohm at normal listening levels. Run the calc and you get a roll-off freq of ~80kHz. Way above 20kHZ.  No worries.

Now let's turn down the volume a fair amount and the series resistance of the passive goes up a lot. Let's say it goes to 50k. Run the calc again and you're at 3.2kHz. Woah! Where did all the highs go? Flat sound indeed.

Next let's cut your loooonnnnng cable down to 6 feet. Rerun the calc and now f = 19.6kHZ.  Much better and you probably won't even notice it. Cut it in half again - 3 feet - and presto!  f = 37.3 kHZ. No worries again. Or alternatively, rather than shortening your cable, the same effect can be achieved by throwing out your cable and buying a better one with half the capacitance. If it were me, I'd just cut the cable but that's me.

In my view, realistically, you're not going to spend much time listening at attenuation levels where the R of the passive is high enough to cause a problem.

I spent quite a bit of time recently with a reviewer who was working with our LDR1. His cables were 20+ feet in length and I cautioned him on this issue. After extensive critical listening, he was convinced that there was no discernible roll-off of the highs notwithstanding his long cables. I think that's because at the attenuation levels his was at, the R of the passive was too low to matter.

[*Scotty*]

If I am not mistaken the LDR is an active device. It should have a
low enough output impedance that HF roll off is not a problem for most cables and any thing like reasonable
cable length. If it behaved electrically like a simple pot there would be no advantage to using it.
Scotty

[poseidonsvoice]

If I am not mistaken the LDR is an active device. It should have a
low enough output impedance that HF roll off is not a problem for most cables and any thing like reasonable
cable length. If it behaved electrically like a simple pot there would be no advantage to using it.
Scotty

Scotty, obviously you are mistaken  . Whether it is active or passive is immaterial, to this discussion (i.e. there are some 'active' preamps with very high output impedances at all frequencies from 20 Hz to 20khz, i.e. poor designs in my book!!!). What is material is how the output impedance of this device changes with frequency and with volume attenuation. Around the middle of the range, it's output impedance is 2,000 ohms. And at the top of the range, the output impedance is 50,000 ohms. Morten has clearly shown the math. Of course, whether the user will be using the LDR in the middle of the range, thereby having an output impedance of 2K ohms is anyone's guess as that depends on the size of his room, the output voltage of his source, the sensitivity of his speakers, the gain of his amplifier and finally how loud he listens which is dependent on his listening habits and distance he is sitting from his speakers. Lots of variables. Cutting the cable to less than 8 feet may not be an option for him depending on how his system is configured.

A dilemma indeed. Maybe he should give it a try in the tour, that would be ideal for him... 

Best,
Anand.

[tortugaranger]

If I am not mistaken the LDR is an active device. It should have a
low enough output impedance that HF roll off is not a problem for most cables and any thing like reasonable
cable length. If it behaved electrically like a simple pot there would be no advantage to using it.
Scotty

Ah yes but you are indeed mistaken.   If we define "active" to mean that the power supply of a powered device (preamp in this instance) interacts directly(electrically) with the audio signal then the Tortuga Audio LDRx preamp (and any other LDR based passive preamps) is passive and not active. It's true that the LDRx preamps actively regulate the resistance levels of their LDRs but this regulation is optically isolated from the audio signal. The only thing the audio signal "sees" is a variable resistor, not unlike a pot.

Not unlike a pot, but not a pot. There's no mechanical wiper interface point and the variable resistive material in an LDR is classified as a solid state material and as such is very much different from materials used in any conventional resistor. It's the combination of materials and absence of a wiper interface point that give LDRs a distinctively different audio characteristics compared to pots, stepped attenuators and even transformer based volume controllers. LDRs configured in a series/shunt arrangement more or less emulate a pot but they definitely don't sound like one.

Cheers,
Morten

[*Scotty*]

I thought the LDR had a transfer function and the associated THD and IM that go with an active device. I guess I also thought it had lower output impedance than that. That being said a 2kohm output impedance into 25kohm load still maintains a 10 to 1 ratio. I would rather see 200 ohm or a 20ohm output impedance but
you get what you get. I would be a poor candidate for this beast as I am driving 2amps and 2sub amps none with less than 22 ft. long ICs
Scotty

[Davey]

It's not the 2k source with a 25k load that's the issue......it's the 2k source with the shunted capacitance of long interconnect cables.  That can be a fairly effective low-pass filter. 

Conceptually, these LDR attenuators are just like passive attenuators.....unless a buffer follows.  Only then would you have an "active" device.

Dave.

[poseidonsvoice]

It's not the 2k source with a 25k load that's the issue......it's the 2k source with the shunted capacitance of long interconnect cables.  That can be a fairly effective low-pass filter. 

Conceptually, these LDR attenuators are just like passive attenuators.....unless a buffer follows.  Only then would you have an "active" device.

Dave.

Exactly. You cannot underestimate the capacitance of the long interconnects as Dave states and as Morten has shown on his website, when evaluating whether the LDR is right for you. I've always thought for those who really want the LDR but need to have long interconnects/or high capacitance interconnects, to build the LDR with a DCB1 buffer or similar. But then, others may argue that you defeat the simplicity and transparency of a Tortuga LDR that way.

Best,
Anand.

[WireNut]

I just don’t understand the need for a long interconnect run unless I wanted to control all my source components from the bathroom. But then the Tortuga has a remote .  I guess if I had to do that I'd be rethinking my whole setup.

 

[Yucca06]

Hello, I maybe should have opened a new thread, but my question is quite the same as the first one, the thing is I have "only" 16ft XLR length...and soon a tube buffer between my (future) preamp and my active speakers. So DO YOU FEEL would the Tortuga LDR1B v.25 fit my system ?

That's what I plan :
PSaudio Directstream Jr -> 2ft 1/2 XLR -> Passive Preamp (Tortuga ?) -> Active tube crossover/buffer - > 16ft XLR to active speakers / 18ft + 30ft RCA RCA to 2 stereo subwoofers...(I believe the low bass would not suffer from long cables run, even single ended...)

[tortugaranger]

Hello, I maybe should have opened a new thread, but my question is quite the same as the first one, the thing is I have "only" 16ft XLR length...and soon a tube buffer between my (future) preamp and my active speakers. So DO YOU FEEL would the Tortuga LDR1B v.25 fit my system ?

That's what I plan :
PSaudio Directstream Jr -> 2ft 1/2 XLR -> Passive Preamp (Tortuga ?) -> Active tube crossover/buffer - > 16ft XLR to active speakers / 18ft + 30ft RCA RCA to 2 stereo subwoofers...(I believe the low bass would not suffer from long cables run, even single ended...)

With passive alone I recommend limiting IC length from preamp to amp to no more than 8-10 feet. However, with a buffer downstream of the passive preamp the length of IC cable becomes moot. I had one review done where the reviewer used a 25 foot cable between the passive preamp and his amps and no roll-off of high frequencies was discernable. That probably said more about the reviewer's ears than anything else.

[Yucca06]

I don't have young ears, but also not so old (50's). There's absolutely no discernable high requencies roll off between my Promitheus passive (TVC) and an active Parasound P7. Without buffer, the Parasound has only more "body", and the Promitheus less dynamics, but a very high transparency with slightly more 3D feeling. But I believe the advantage of TVC models are the adaptative impedance, which is not the case with LDR ?

[tortugaranger]

I don't have young ears, but also not so old (50's). There's absolutely no discernable high requencies roll off between my Promitheus passive (TVC) and an active Parasound P7. Without buffer, the Parasound has only more "body", and the Promitheus less dynamics, but a very high transparency with slightly more 3D feeling. But I believe the advantage of TVC models are the adaptative impedance, which is not the case with LDR ?

The issue of potential hi freq roll-off with a resistive passive is the combined R(esistance) of the passive attenuator plus the cable together with the C(apacitance) of the cable. If the combined RC value gets sufficiently large it can impinge on the upper audible range. This is easily mitigated by both limiting the length of the interconnect cable between the preamp and amp together with using high quality low capacitance cables, i.e. limit cable length to 8-10 feet max. Got longer cables and don't need the length? Shorten them! In practice this is rarely a problem unless you have unusually long IC's that are also made of crappy cable.

Meanwhile with any TVC you have electromagnetic isolation between source and amp so cable length and potential IC rolloff is not a concern. So no surprise that you've had not issues with your Promethius.  If you must use long cables for whatever reason you could argue advantage goes to the TVC on cable length.

Many of our customers have owned various TVCs and after trying a Tortuga LDR passive their feedback has been the LDR passive beats their TVC passive. I'm agnostic on the subject since I've never owned nor listened to a TVC but customer feedback has been consistent on this.

Adding our TPB.V1 Tube Preamp Buffer downstream of the LDR preamp cures all cable ills, adds some body and dynamics, all without losing the clarity and transparency inherent with the LDR attenuator.

[Yucca06]

Ok...I’ll get soon a tube preamp buffer (in fact a crossover, with a tube buffer output) from a local French manufacturer, so I suppose it will act the same as your buffer...i’d just want not having to return your preamp once ordered, but I suppose no one can be sure before connecting it to my rig and hearing what’s happen...

Unfortunately, I can’t have really shorter IC lengths, as my system is active, and there’s a wood stove right in the middle behind speakers, so I have to put electronic devices at one side -> long IC. Or change my speakers for passive, that I don’t want at all...
I told 16, but it’s in fact 15ft for the left side, and 10ft for the right one (I know, not ideal at all, but no concerns yet)

[milpai]

@Yucca06,
I was looking for the difference between a TVC and an active preamp and your description of "body" vs "transparency" makes sense. So, Thanks for that!
I should say, I have a dual mono TVC from Promitheus and have been happy with it for the last 8-9 years. But with my upgraded loudspeakers I am looking to upgrade my preamp.

@tortugaranger,
I have sent you a PM via your main site.
Seriously looking to upgrade my preamp.
Forgot to mention in that email that I have been suggested the following active preamps, and in the order preference as listed:
1. Jeff Rowland Capri S2
2. Bryston BP-17 Cubed
3. Parasound JC2

Thanks!