They are simply a different method of getting the EQ required in a phono preamp to change the signal from a record into flat frequency response.
Instead of using C&R , as is usually used an inductor L is added.
Tango, the famous Japanese transformer company, made a unit that had multiple inductors plus the necessary caps and resistors in it, quite a few years ago .
It was the inspiration for many diy LCR phono stages but rarely available commercially as they are expensive and tricky to implement .
Tango Circuit
I built quite a few LCR's 15 years ago but always found them a little inaccurate and prone to hum, with the inductors picking up any stray AC fields.
A good tube CR phono can also sound very good so why make it harder.
In conjunction with Kevin Covi I also built a lot of LR phono prototypes, which I think is a better approach, as this eliminates the C , which is mostly a good thing.
These are tricky though, just a slightly wrong value of L or R and frequency deviations are more than what is tolerable.
And building an active tube phono that can give enough gain for a 0.2mV moving coil cartridge is tricky enough without complicating it with noisy inductors.
Which is why commercial LCR phonos released over the last few years are for MM cartridges only or require a step up for MC cartridges.
Michael Fremer , the influential analogue reviewer has a LCR phono in his system, and his acclaim
of its sound has introduced LCR phonos into the audiophile mainstream.
The unit he uses is very expensive, and is confined to MM use or MC with an overly expensive step up.
Its a very nice design and very well made, but I have no love for step ups , and I decided to re-investigate the LCR approach.
Fortunately , all the ground work was done years ago , and our hybrid LCR phono , using CR and LCR stages , with a few innovations and Lundahl inductors works and sounds perfectly.
And it is capable of gain for the lowest output MC cartridges, with no step up, so dynamics - micro and macro are stunning.
As Fremer says, a good tube LCR phono doesn't sound "tubey" , their forte is revealing micro-dynamics . I dont think CR tube phonos are shamed by them, but there is something inherently nice about them.
Now I need to go back and do some work on a LR phono , which could be even better again!
