Are LDR pres the ultimate passive?

[Figlet108]

Yesterday I was over at Macca's gaff where we had ourselves a mini passive pre bake-off.

Both of us seem to be attracted to the concept of how LDRs work and certainly on paper from an objectivist perspective they should sound very transparent and uncoloured. We had both bought a cheap Stereo Coffee single input LDR pre kit (~£50) about 8 months ago! and it was high time to try them out and see if Objectivist theory and Subjectivist reality would converge...

I made mine into a 2cm thick limestone box:



Meanwhile I'd also bought a Tortuga LDR3x.V2 kit (~£250) with dual LCD displays and manual and remote control.

Not made the case for this yet, so here it is languishing naked next to the 12v power supply I made for it:



Macca has an NVA P90SA pre (£640) which uses these Shinkoh/Elms 24 stepped attenuators and I can imagine it's as good a stepped attenuator passive as any out there and probably better than many. (I also used to have one of these for a couple of years before I moved it on and liked it).

We used the excellent Red Hot Chillies album 'Blood, Sugar, Sex, Magic' as music fodder and tried to level match as best as we could with the sound meter on my phone (not perfect but close enough).

It was clear pretty quickly that both LDRs were very close to each other and a step up on the stepped attenuator. The stepped attenuator was more fuzzy, less detailed, less transparent than both LDRs. The LDRs had better slam on the bass and more of a sparkle at the top end with cymbals and high-hats etc. We both remarked on the excellent reverb and decay.

In the end the LDRs were addictive to listen to and we didn't want to turn them off, in a way that the Stepped Attenuator wasn't.

While the 2 LDRs were very similar we both thought that maybe the Tortuga just had the edge by a whisker. The other thing about the Tortuga is the awesome engineering and attention to detail that's gone into it as a product. It's clearly a labour of love by a genuine hifi geek and it's the kind of thing I'd love to be doing if I had the skills.
The stand out features are that it has built in and automatic channel matching controlled by the embedded software. The Apple Remote control option and the dual LCD displays are icing on the cake. I also understand that in the near future it will be software update-able via it's built in USB interface and there will also be an android/iphone app to control it.

On the other hand, the Stereo Coffee LDR is wonderfully affordable at only £100 for 3 inputs and considering it sounds 95%-99% as good as the Tortuga it's also a very compelling proposition.

I needed 2 pres so I didn't have to choose between them, but if I had to rank them on their overall package of sound and features it would be thus:
1. Tortuga LDR3x.V2 (~£250 for 3 inputs and with all the options: manual and remote control, dual LCD displays)
2. Stereo Coffee LDR (£85 for 3 input version)
3. NVA PS90SA (£640 with 6 inputs)

To make sure these comments are put in the right perspective, all 3 pres were excellent and it wasn't night and day between any of them. The comments are relative to 3 very good passive pre amps.

I'm looking forward to some long term listening with the 2 LDRs and will post more comments, but I'm now wondering if LDRs have matured to the point that make them the ultimate passive pre?

[Sovereign]

Great read, thanks

[kenworthy100]

Fascinating stuff, very tempted to try one of these in my office system.

[Figlet108]

It'll be well worth it John, and both are super easy to connect up - no compulsory soldering involved.

It's also worth mentioning that Tortuga also have fully made up products as well as DIY kits, and that Stereo Coffee are also planning a fully made up product in the future. Obviously these are more expensive, but sometimes not having to do DIY is worth the extra money...

[Puffin]

I made several LDR pre-amps both for myself and others following the disclosure by Mr Stanscheff of his circuit. There was a long thread on Diyaudio about 3-4 years ago. The one that I use in my system now is the first one I built on a piece of perfboard and cost about £15 including a simple 7805 circuit to power it. I was amazed by the sound then and I still am. I later added a motorised Alps pot as I had a remote board and controller from a previous project. IMO it has a lot of the quality's of a good active pre. I am also amazed that it is still working perfectly (tempting fate here!)

[Macca]

It was very hard to tell the difference between the Stereo Coffee and the Tortuga. If we had more time to swap them in or out we probably could have worked it out but it was very close. I do wonder if the differences between the two LDRs and the NVA stepped attenuator might not have been down to the LDRs being a better match for the power amp, especially as for me the main difference was in the bass, with the LDRs offering just a little more of everything, slam, drive and tone. When we hooked up the Coffee pre to a different power amp from my usual XTZ I though the bass performance deteriorated noticeably.

Of course there is no such thing a free lunch and we have yet to see how these pre amps fair up to daily use over a period of time and if they will exhibit any of issues that have previously hampered this technology from delivering consistently.

[Marco]

Pity there wasn't a good valve preamp in there to add another interesting and valid facet to the test... If you guys had said you were doing this, I'd have popped down and brought the Croft!

Marco.

[Macca]

Pity there wasn't a good valve preamp in there to add another interesting and valid facet to the test... If you guys had said you were doing this, I'd have popped down and brought the Croft!

Marco.

We can certainly arrange another bake off at some point in the near future if you want to do that.

[Marco]

We defo should... I could probably get to your gaff in less than an hour!

Or you could bring your passive up here... I don't believe you drive, Martin? However, I bet that you could get a train straight to Chester station, where I could pick you up.

Marco.

[Macca]

Yep when I've got the LDR passive up and running and I'll bring both.