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LCR phono stage with cascode input - any obvious improvements?

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.
 

Attachments

  • LCR attempt 3 withCCS and cathode follower.asc
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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.
 
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.
 

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  • Final LCR circuit.jpg
    Final LCR circuit.jpg
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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.
 
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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 :)
 
Some good features. Always nice to see someone going LCR. I have all the parts ready for one myself, but too busy with speakers.
I have a few questions/suggestions -
1. What's the 'stuff' leading into the cascode doing ?
2. Why do you have a coupling cap into the final C.F. ? - each cap robs precious low-level signals.
3. Would prefer something nicer as an anode load there too - eg. DN2540 cascode CCS or IXYS equivalent.
4. How much current in the E88CC's ? - because the LCR chokes will need some current to drive transparently. I will be planning on a 5687 CF running about 25ma.
 
Hi!

Top of my head, here are the things you could try to improve on the design:

- go for a shunt cascode instead of a regular cascode (see Rod Coleman posts on this forum about it). Would greatly improve the PSRR. Albeit it would probably require recomputing the LCR values.
- go for Allen Wright's SLCF (Super Linear Cathode Follower), basically a CF with a CSS in the cathode and a constant Cathode-Anode voltage, instead of your regular CF. Also get rid of the coupling capacitor between the last gain stage and CF, as suggested earlier in the thread.
As an alternative to the tube CF, maybe just use a mosfet source follower with CSS in the source. Even simpler.

Btw, The SLCF is described in Allen Wright's "Tube Preamp Cookbook". A version with transistor is described on Ale Moglia's blog.

Also I remember reading somewhere that Tango LCR's were only working as designed (intended FR) when both sourced and terminated by the same impedance as the filter's (600 ohm in that case). I wonder if it is the case for all T-networks? In which case you could try to replace R18 by a value approaching the filter's nominal resitance. May be worth experimenting.

Regards,
M.
 
working as designed (intended FR) when both sourced and terminated by the same impedance as the filter's (600 ohm in that case). I wonder if it is the case for all T-networks?

This is a common misconception. Sourced or terminated is sufficient, you don't necessarily need both at the same time.

I would never understand the attraction of the CF. It is one of the worst sounding circuits according to my ears. Plenty of great triodes with Rp in the region of 1-2k. Is that not low enough? Use a step-down transformer then. Or even an SRPP.
 
The stuff pre the input is an inverse RIAA setup, setup so that the input is equivalent to a cartridge outputting 5mV/1kHz.

The cap between second gain stage and CF was there to keep the heater/cathode voltage low. The heater supplies I have from the PSU are at ground and at 40V (used for the cascode and the CF. Incidentally, when I tried it with 2.2uF there I got a large 0.5Hz oscillation.

The anode load is actually a 10M45 set at 4mA or so. I haven't got a model for that in LTSpice at the moment...

The cascodes work at 8.5mA, 2.5V bias of the bottom half.

I think I am set with 6.2k now, I don't have the available permission for funds to have new inductors wound at 300 quid a set from Sowter. The PSU has a total of 75H to the cascode stage and 400uF in it so there isn't an lot of ripple.

SLCF - I will have a look at that.

On the need for a CF, I haven't currently got tons of current to play with from the power transformer, so a high current stage wouldn't necessarily work. I haven't got anything against them personally.
 
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OK sounds alright. 8.5ma is decent. I just have bad experiences of driving any sizeable chunky components with low-current valves - especially transformers.
Inverse RIAA - should have spotted that !
Can you not somehow play around with the heater voltages to get rid of that cap ?
A cathode follower needs some current to back-up the low output impedance, so hopefully you have a few mA in there. They do work OK in phonos, I can vouch for that and from my mate Nick's 'Reference' phono ( for M.F. ) which uses two.