Eros with LCR modules?

[johnsonad]

Team Bottlehead,

I've been reading more and more about LCR RIAA and had to pose the question.  Please forgive my non-tech background but this is the place to ask it; is it possible to replace the passive RIAA network with a LCR RIAA in the Eros and how much work would it entail if the circuit would allow it?  From my prospective it has all of the makings of it: direct coupled pentode to triode, very nice power supply and more. 

Regards,

Aaron

 

[Paul Joppa]

In a word, no.

The original standard for LCR equalization is a passive module that goes between a high-gain linear amplifier and the rest of the system. Because of the way inductors are made and perform, it is not practical to operate them at high impedances, and the standard modules are all designed to operate at 600 ohms. Hence a studio mic preamp could easily be used since it is designed to drive a 600 ohm impedance, and it has very high gain, 60dB or more. The LCR module attenuates the midrange by 20dB, so high gain is needed in front of the module.

Eros, Seduction, and most phono preamps operate internally at much higher impedances, 10 to 1000 times higher. To get these low impedances with tubes usually requires a step-down transformer to drive the LCR module. Sometimes a modern LCR phono preamp will have lower gain before the module, and another gain stage after.

I think all this came about as electrical phonographs were first becoming available; there were no dedicated phono preamps in radio studios, but plenty of mic preamps and the usual gain controls, source selectors, etc.

It's an attractive purist approach to phono reproduction, and I'd love to explore it further. But because of the step-down transformer, it would require a third gain stage as well as the output transformers, and would necessarily be expensive. The configuration is basically similar to our high-end Tape Repro amps.

 

[johnsonad]

Thank you Paul for the reply.  I've seen modules with higher resistance.  Dave Sladge, Stevens & Billington and Silk also make 10k modules.  From your post I'm assuming these would be easier to work with?  With the configuration being similar to the Tape Repro, could it be adapted to use LCR? 

Regards,

Aaron

 

[Paul Joppa]

Yes, there are some available at 10K impedance. I've not yet heard many reports on experience with them.

It's possible a Seduction circuit could work with a 10K LCR, in between the two stages. The Repro relies on the attenuation of the EQ between the stages to maintain the proper gain distribution and would not be a good candidate for simple revision to use 600-ohm LCR. A whole new circuit optimized for that design would probably be easier.

By analogy, this change is more like replacing a gasoline engine with an electric one, as opposed to replacing the carburetor with an after-market one.

 

[johnsonad]

Thanks Paul.  I'm in no hurry to crack this egg but exploring the possibility using the Bottlehead mindset and quality of products would be nice.  The units available on the market now are expensive to astronomical depending on who builds it (DIY Hi-Fi Supply, Alnic, Experience Music).

 

[Paul Joppa]

I must admit, the idea is attractive. We are exploring quite a variety of ideas currently, and I'll keep this one in mind.