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LCR phono stage with cascode input - any obvious improvements?
LCR phono stage with cascode input - any obvious improvements?
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Old 17th January 2019, 09:43 PM   #21
euro21 is offline euro21  Hungary
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LCR phono stage with cascode input - any obvious improvements?
Optimized LCR parts.
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Old 18th January 2019, 12:22 PM   #22
Rod Coleman is offline Rod Coleman  United Kingdom
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LCR phono stage with cascode input - any obvious improvements?
Quote:
Originally Posted by vmotion View Post
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.
Yes, a shunt cascode first stage would improve the power supply isolation, both from conducted and radiated noise. The input and output are both referred to ground, which helps, and no part of the signal current runs through the power supply capacitor, so any old electrolytic can be used without penalty.

More importantly, the Shunt Cascode's output is a current, so you can terminate this with a (example: 6.2K) resistor to ground, and the driving impedance to the EQ becomes 6.2KΩ regardless of the ri of the E88CC, with its static and dynamic variations.
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Old 18th January 2019, 06:13 PM   #23
vmotion is offline vmotion
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Hi Rod,

I really appreciate the tremendous information you posted on the folded cascode, it really inspired me (I'm working on a RIAA amp design based on it, which I'll hopefully post here in a couple of weeks).

I'd like however to point out that though the output impedance is going to stay constant (determined by RL), the gain of the cascode being gm x RL, it is going to vary substancially over the lifespan of the tube. To me, this is the biggest drawback of the cascode.

Cheers,
M.
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Old 19th January 2019, 02:20 PM   #24
Rod Coleman is offline Rod Coleman  United Kingdom
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LCR phono stage with cascode input - any obvious improvements?
Hi M,

Glad the Shunt Cascode has got the design ideas working!

Yes, ordinary (series) cascode stages can present some variations of drive impedance (that's what I meant about static and dynamic variation) but the Shunt cascode drive impedance avoids the problem of ageing, and variations due to operating conditions.
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Old 19th January 2019, 02:37 PM   #25
Rod Coleman is offline Rod Coleman  United Kingdom
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LCR phono stage with cascode input - any obvious improvements?
To see some of the improvements that Shunt Cascode offers, here is a worked-out LTSPICE version.

The Upstairs ECC88 of the original (series) cascode is removed, and used for the output stage.
The Shunt Cascode has the 6.2KΩ output resistor that provides a very stable driving-impedance for the LCR network. The second stage loses the poor-performing cathode-follower, and uses the standard DN2540 cascode CCS. Taking the source output from this drops the small-signal output impedance from 100Ω (cathode follower) to about 10Ω.

Now, we have a RIAA amplifier with 46dB gain (1kHz, inc. EQ), very low distortion, and low output impedance.

Chokes are no longer required for the power supply, and the supply voltage can be reduced to 125V, with regulation of the supply absolutely not needed.

The two bottles are reduced to one ECC88 per channel, and heater biasing problems vanish. Also, both halves of the ECC88 are run at 90V 11.5-12.5mA - almost exactly per data sheet nominal, often a good choice in itself.

The first stage power supply cap, the two cathode bypass caps, and the extra 2nd stage coupling cap are now removed, or out of the signal path. Yes, four caps down, per channel!

If my Shunt Cascode Explorer is used for the first stage, the whole circuit can be made very neatly, and at low cost.

It is worth mentioning that the stable drive-impedance of the Shunt Cascode can be used for standard RC EQ, and this makes a superb improvement compared to most other solutions. It can also have higher gain that with 6.2K LCR, if desired. In this case, spending a little of the money on some top-flight caps (Charcroft Micas do it for me) can make a very enjoyable RIAA, that measures well.
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File Type: png riaashcasc-LCRs.png (32.8 KB, 180 views)
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File Type: zip RIAA.zip (13.4 KB, 27 views)
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Old 19th January 2019, 05:07 PM   #26
MrKettle is offline MrKettle
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Interesting. To put it in my context, I already have a 250V supply (a WD PSU3 on steroids) with the rated maximum of the two Hammond 157Gs which feed this and two 82s in the pre being 40mA. It also already has the lifted heater supply, but that can be modified fairly easily.

What might be interesting is trying different valves in the first stage with lower current requirements (if any are suitable from a noise perspective), and seeing what the overall gain is.

P.s. I appear to be about 15 mins away from your business address.
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Old 19th January 2019, 05:56 PM   #27
euro21 is offline euro21  Hungary
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LCR phono stage with cascode input - any obvious improvements?
Rod,
IMHO 10R output impedance is very optimistic estimation for DN2540 CCS. :-p
Rather 300-400R (which is very correct value) is acceptable.
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Old 20th January 2019, 12:07 PM   #28
Rod Coleman is offline Rod Coleman  United Kingdom
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LCR phono stage with cascode input - any obvious improvements?
Quote:
Originally Posted by euro21 View Post
Rod,
IMHO 10R output impedance is very optimistic estimation for DN2540 CCS. :-p
Rather 300-400R (which is very correct value) is acceptable.
Bela, OK - I know you will have measured it properly. Thinking about it again, I did not account for the low voltage VDS in the case of cascode pair.

But 300Ω is too much for my taste, if the load is 10K, and I might prefer a non-cascode for this reason. I will look at it again and find a solution!
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Old 20th January 2019, 12:17 PM   #29
Rod Coleman is offline Rod Coleman  United Kingdom
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LCR phono stage with cascode input - any obvious improvements?
Quote:
Originally Posted by MrKettle View Post
Interesting. To put it in my context, I already have a 250V supply (a WD PSU3 on steroids) with the rated maximum of the two Hammond 157Gs which feed this and two 82s in the pre being 40mA. It also already has the lifted heater supply, but that can be modified fairly easily.

What might be interesting is trying different valves in the first stage with lower current requirements (if any are suitable from a noise perspective), and seeing what the overall gain is.

P.s. I appear to be about 15 mins away from your business address.
The STC 6BR7 (CV2135 and other CVxxxx variants) is low current and low noise, low microphony purpose built for audio. Ia=2mA, S=1.2mA/V. It is similar, but superior to the EF86. I have used it it shunt cascode for RIAA, and it works quite well, but the E88CC, E180F, PC86, and various Russian valves with anode current 12-30mA, and gm 10-50mA/V give better performance than any low-current triodes, because the gm is greater, and the current in the PNP should be reasonably high for low distortion.

In a series-cascode, the low-current valves will usually need a high anode load, making drive of the EQ section more difficult.
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Old 20th January 2019, 07:13 PM   #30
uptick is offline uptick
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Quote:
Originally Posted by MrKettle View Post
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.
I'm late posting to this, but I'm not a fan of cascodes, you are just making a pentode of the two triodes, and the input triode has a very low plate load. Why not just use a pentode?

The output is still a plate loaded resistance, which feeds a RIAA filter circuit without a low Z buffer. Filter calc needs defined source and load impedance.

It will work but is it the best option? Old school tube circuits were cheap, as components were expensive. Using an big choke/inductor in the power supply is a waste, these high voltages can easily be filtered/regulated with a HV transistor + LM317 and filtered with massive capacitance, effectively getting rid of all noise out of the power supply.

Some diode rectifiers can generate switching noise, so it may be a good idea of resistive, or capacitive, bypassing of these to reduce the switching noise. Check for jaggies on the scope and/or use your ears. Sharp on/off transitions can excite resonances from the leakage inductance in the AC mains transformer. If you can do an FFT of the noise you may want to try making an LC filter tuned at those frequencies. Not generating the noise is better than filtering it. A typical common mode choke used on 120V AC lines may be useful here. This would also be useful for filtering incoming AC line noise. Switching power supplies are everywhere, so some filtering cannot hurt on the AC input. Some HF noise may couple thru the capacitive primary/secondary leakage. A resistive loaded plate circuit will pass half of the power supply noise to the output, so the power supply is as important as you amplifier circuit itself. I though I was crazy to use W.W. resistors in plate loads until I saw the same in a Jensen microphone tube transformer preamp appnote. Don't be afraid of wire-wound resistors of high value. Inductance problems can be an issue for lower values, so keep an eye at those. W.W. noise is better than metal film.

I would prefer a constant current load for a triode gain stage, easy to do with an LM431 and a HV MOSFET. The gain and temp stability of the LM431 is great for a very constant current. You can extract max gain from a tube at the most linear part of the plate/grid curve, a horizontal line on the chart. Just pick a current on the chart where the tube have the most linear plate current spacing. (Don't bother with pentodes for CCS. ) You will get maximum, and signal independent gain from a triode with a CCS, that is not possible with a resistive load, but if the voltage output is small, you will not really use all the available voltage swing anyway, so a resistive load should be OK for a low level input.

CCS noise may be an issue. I have to make some measurements. CCS will also isolate the gain stage from the power supply.

Using a microphone transformer for a moving coil input is not a bad idea, the voltage gain is noise-less. The signal is only maybe 0.5mV so you do not need a big core. A mike transformer should be OK. HF roll-off is included for free. Bass response may be an issue with some transformers, so keep an eye on that.
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