LCR phono stage with cascode input - any obvious improvements?

[MrKettle]

Recently finished putting together an LCR phono stage as per the attached schematic, apart from the CCS for the second stage being a 10M45 run from the full B+ rather than reduced amount(haven't got round to sorting out a setting for that on my LTSpice).

The input stage is an ECC88 cascode with the output impedance used to match the LCR network. Given that there is more gain than I need and given that the load resistor needs to stay as it is to keep the frequency response right, are there any obvious changes? The things I am pondering are LED bias of the first stage (it is set up for 2.5V with the resistor as it is) or possibly having only a partial bypass of the cathode resistor (say 220ohm bypassed with 110ohm unbypassed to add a little local feedback) as the schematic appears to give 1.5V out for 5mv/1kHz which is pretty huge. Would these be worth looking at, and is there anything else people would suggest?

As a background, the project was to have a look at what an LCR stage would be if it was made to fit in with the World Designs valve kits.

[rayma]

Originally Posted by MrKettle

The input stage is an ECC88 cascode with the output impedance used to match the LCR network.

If the RIAA is passive, any change to the first tube stage will alter its output impedance,
at least to some degree, and hence the eq.

[analog_sa]

Originally Posted by MrKettle

The input stage is an ECC88 cascode with the output impedance used to match the LCR network.

Some symbols missing and cannot run your sim without a bit of work, so if you could please explain it a bit. There are two ways of running an LCR riaa: either by driving it from the riaa impedance, or loading it with the same. If this is a standard 10k riaa, then i don't see a 10k source or load. So, perhaps it is not 10k. 6k2?

Anyway, it is high impedance. Why don't you load the output of the riaa with the appropriate load as this will allow you to modify the front end without worrying about its output impedance?

Ok, so if the RIAA is indeed 6k2 then loading so low won't be appreciated by the input stage.

You can certainly try a partial or absent cathode bypass on the first and second stage and see if the reduction in gain is sufficient. Some commonly used stages like the EAR 834 have an even higher gain and few people seem to mind.

[MrKettle]

Apologies, wasn't sure how much came with the .asc file. The circuit is as per the attached image, with the only other relevant details being the DCR of the jnductors which are 118ohms and 30ohms respectively.

[analog_sa]

Could you confirm the riaa impedance?

[MrKettle]

It is based on 6.2k, which seemed to work well with standard component values. There only needed to be one combination of caps and one combination of resistors to make the response as it should be.

The idea of loading the RIAA with the output impedance means that the gain from that stage can be higher. Subjectively the sound is very good, much better than a World Designs Phono 3, but it was just a question of whether anything else would be beneficial.

[analog_sa]

I cannot suggest anything if you need to keep the valve complement. Different valves would allow you to tailor the gain to your liking without introducing unbypassed cathode resistors. Some valves will also allow you to get rid of the cathode follower and that unneeded cap

[euro21]

There are the basic error in LCR RIAA.
C14 is unnecessary, instead of this use appropriate R.

The "original" -untested- schematic here:
In which an idiot prats about with LTSPice... - DIY - HiFi WigWam

[MrKettle]

C14 and C7 form effectively a 0.012uF, a value not available as standard. Just to note, the stage does have accurate EQ both in sim and practice (using reverse EQ'd test tones from Audacity).

[euro21]

Originally Posted by MrKettle

C14 and C7 form effectively a 0.012uF, a value not available as standard.

10n +1n +1n

Using parallel capacitors in RIAA is better practice, than series ones.