[33na3rd]

I came across a short article written by Steve McCormack, who brought to market one of the first Passive Preamps that I was aware of, the Line Drive.

http://www.hometheaterhifi.com/volume_2_1/passive.html

What caught my eye was his remark that while the length/capacitance of the Interconnect from the Passive Pre to Power Amp were critical, the length of the interconnect between the Source and Passive Pre was not.

Would this apply to our LDRx's as well?

If one had a long Interconnect and a short Interconnect, would the longer one be better suited between the DAC and LDRx, and the shorter IC between the LDRx and Amp?

[tortugaranger]

I came across a short article written by Steve McCormack, who brought to market one of the first Passive Preamps that I was aware of, the Line Drive.

http://www.hometheaterhifi.com/volume_2_1/passive.html

What caught my eye was his remark that while the length/capacitance of the Interconnect from the Passive Pre to Power Amp were critical, the length of the interconnect between the Source and Passive Pre was not.

Would this apply to our LDRx's as well?

If one had a long Interconnect and a short Interconnect, would the longer one be better suited between the DAC and LDRx, and the shorter IC between the LDRx and Amp?

That is all correct. The preamp and the downstream cabling form an RC filter low pass filter so it's possible to roll off the highest frequencies if C gets big enough. The higher the R (impedance) of the preamp the more important to have a lower C cable which means either shorter, lower specific capacitance  (pF/foot)  or both. Per my own calcs anything 6-8 feet or less should be fine.

Then again I once had a reviewer who plugged one of our LDR units into his tube amp with 20 feet of IC with no issues. 

I touch on this subject in an article I published a while back which can be found here:  http://www.tortugaaudio.com/what-is-a-passive-preamplifier/

Cheers,
Morten

[bonsai]

I am a happy owner of a tortuga passive.  I run mine into a Purity Harmony - http://www.purityaudiodesign.com/purity%20buffer.htm

Absolutely killer combo..  The Harmony buffer has 1dB of gain and an excellent sound...  corrects the impedance on the way to the monoblock amps I have.  with a short connection to the buffer you can go long connect from the buffer to the amps. 

Here is my harmony

[33na3rd]

I guess the part of the article that I found interesting was Mr McCormack inferring that cable length before the Passive Pre was of no concern, only after.

So Morten, when you mention 6 to 8 feet, you are talking about the IC after the LDRx, or the total of the IC's from source to LDRx to Amp?

I apologize if I am being overly dense!

[Davey]

I guess the part of the article that I found interesting was Mr McCormack inferring that cable length before the Passive Pre was of no concern, only after.

He's sort of correct.

The cable length from the source could be important as well.  Much depends upon the output resistance of the source.  If it's low then it shouldn't be a problem to drive long interconnects.  If higher you can start running into problems with long IC lengths

The preferable location for a passive attenuator would generally be closer to the load than the source.

Dave.

[33na3rd]

He's sort of correct.

The cable length from the source could be important as well.  Much depends upon the output resistance of the source.  If it's low then it shouldn't be a problem to drive long interconnects.  If higher you can start running into problems with long IC lengths

The preferable location for a passive attenuator would generally be closer to the load than the source.

Dave.

Thank you!
I'm starting to get it!

[bonsai]

I'm not sure I agree with the author of that write up...  Some "depends" on the passive preamp....

Say you had a harder to drive 20k stepped attenuator in the passive preamp then I think you want the passive close to the source....  Say you have a harder to drive 100k attenuator then you want the passive close to the amp.

Question for Morten...  What is the k value for the LED attenuator you now sell?

[tortugaranger]

I'm not sure I agree with the author of that write up...  Some "depends" on the passive preamp....

Say you had a harder to drive 20k stepped attenuator in the passive preamp then I think you want the passive close to the source....  Say you have a harder to drive 100k attenuator then you want the passive close to the amp.

Question for Morten...  What is the k value for the LED attenuator you now sell?

It does indeed depend on the passive preamp as well as the downstream cable.

The 3 dB roll-off (high pass filter) frequency is governed by f = 1/(2*pi*R*C).  Units aside, both the preamp (R) and the cable (C) contribute to the potential filtering within the audio frequency range. The LDR3x.V2/LDRx preamps have a nominal 20k impedance (40k for balanced). Let's assume a truly crappy 6 foot cable with 100 pf/foot of capacitance. f = 1/(2*3.1416*20000*6*100x10^-12F) = 13.3kHZ.   Yikes!  On the other hand a low capacitance 6 foot cable of say 15pf/foot results in f = 88.4kHZ. No problem. But 40 feet of that cable and we're back to 13.3 kHZ. Not good.

Conversely, let's take that 6 feet of nice 15pf/foot cable and connect it to a passive preamp with 100k impedance. Now f = 17.7 kHZ.  Most of us over 30 years or older can't hear that far up but phase shift is another consideration which will start to smear the higher frequencies. Cut that cable in half and you're back up to 35.4 kHZ. Safe. 

Hence the moral of story is use short, high quality low capacitance cables between the passive and amp and you're good to go.

As a practical matter most decent cables of 6-8 feet or less should be fine but 3 feet is better/safer. 

Cheers,
Morten

[robertopisa]

I have a dual-enclosure mono DIY Tortuga pre and I can play with cable length. Thus I can use 30cm cables from the pre to the load and 150cm from the source to the pre.  But I preferred the sound when using same length 90 cm cables on both connections, tighter and deeper bass control.

I agree with what said above, so I do not have an explanation why  it sounded better with equal length. Since I employed custom DIY cables in both cases, it was not due to changing cables. Capacitance was low.