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Old 7th December 2008, 02:40 PM   #1
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Default Cascode RIAA

This is a funny way of using a cascode as phono-amp. I used it in the 80s, came to thinka about when discussing cascodes with SY.

Originally it was with a cascoded 12AX7, with in my eyes to high ressitance values and also a ECC82 White follower. So I added the extra current feed to the lower cascode triode and simplified the CF.

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Old 8th December 2008, 01:28 AM   #2
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Lars,
What is funny about it?

First stage has Zout tied down reasonably tight, suitable for driving the RIAA equalisation, although I would run lower RL than the 120K (R5) you show and up the current through the cascode. That may well make R4 redundant. Then adjust the RIAA components as required.
That is as long as you can get enough gain out of the stage.
(Hint for others - gain of a cascode is approximately the RL on the top triode X the gm of the lower triode, Thats why R5 is higher than you might expect and why R4 has been added to increase the current in the lower triode and hence get the gm up a bit).

I'm not that fussed about the RIAA components going back to rail like that rather than down to 0V. A good demonstrator to newbies about what we mean when we say that the high voltage rail IS AC ground.

The output circuit would be a really good test of one theory/opinion that is floating around. That theory is that the cathode follower is analagous to a common collector transistor circuit and that you need (for in depth analysis) to consider its input as being between grid and anode rather than between grid and cathode. The "required" current source load for the cathode follower is there.

I should think that it would work very well.

A bit of lateral thinking in there - I'm in favour of that.

So if you were expecting "flames" you may still get them but not from me.

Cheers,
Ian
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Old 8th December 2008, 01:36 AM   #3
SY is offline SY  United States
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Nor me. Though... you could profitably sub an IR LED for the cathode bias resistor in the first hole to kick the gain up.
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Old 8th December 2008, 02:25 AM   #4
Salas is offline Salas  Greece
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Another thing is that in practice, beyond the simulator, the passive RIAA components may need readjustment. It seems that most tube models aren't that real with Ri especially when cascoding them. Had to realign the Steve Bench 12AY7 paralleled with ECC88 cascode on top. A makeshift Hagerman inverse Riaa will prove most practical to feed an FFT impulse to the phono input, so to check for a flat output.
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Old 8th December 2008, 03:44 AM   #5
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Well, since it's a cascode, effectively the source impedance is just the plate resistor- the source impedance off the upper tube plate is extremely high. Still not a bad idea to trim, given real-world component tolerances, but the cascode connection does simplify things in that regard..
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Old 8th December 2008, 05:39 AM   #6
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Cascode makes awesome input stages

http://geek.scorpiorising.ca/GeeK_Zo...33448#msg33448

Cheers!
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Old 8th December 2008, 07:06 AM   #7
Salas is offline Salas  Greece
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Some tend to sound weird in my experience. Especially if they repeat cascoding stage after stage. They tend to highten detail and lose flow. That is subjective experience of course, you may pass it. For input is good to cascode (bandwidth) or even essential, to keep Miller low (MM carts). Best I ever experienced is Steve Bench circuit. For other schematics that look good or better on paper, can't never be sure of the final result. Iron must burn, solder flow, vinyl spin.

P.S. Many fine RIAA circuits about, those two here look expertly nice too. What we miss here in the tube forum is a clever and rather simple current sourced shunt regulator. Adjustable from 200-400V. Especially for RIAA, the PSU is more than 50% of the sound.
The solid state guys have put a big group buy for hundreds of Toolreg boards. They have heard the niceness. They have made the simple good thing, they roll. We must develop one too. Maybe Mosfet controlled by tube. Any initial circuit ideas?
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Old 8th December 2008, 07:30 AM   #8
Salas is offline Salas  Greece
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Quote:
Originally posted by SY
Well, since it's a cascode, effectively the source impedance is just the plate resistor- the source impedance off the upper tube plate is extremely high. Still not a bad idea to trim, given real-world component tolerances, but the cascode connection does simplify things in that regard..
Its as you say for circuit analysis. But in practice, when we changed 6DJ8's on the Bench, the Riaa was changing on the FFT too. Also it was off anyway, but Bench had measured it well. Never happened on non cascoded cascade circuits with passive RIAA. Weird.
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Old 8th December 2008, 08:10 AM   #9
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Ian,
With "funny" I ment: "Unusual" with a, not so common, little twist. Also remember I have used and played this stage both with ECC83/ECC82, 6CW4 and E88CC. This was to show an interesting circuit from the 80īs left in the closet that sounded OK in those days! It will still remain there as it isnīt so "funny" anymore.
And Ian, why should I expect flames from you, you havenīt got any from me either!
The reason for publishing it where to discuss it as the subject of cascodes where brought up in another thread where SY participated.
About the RIAA going to rail it is the natural choice here as gain is calculated Ra*Gm and Ra is frequencydependant altered.

SY,
Didnīt expect any ovations and didnīt want any though you seem to like cascodes. You are also aware I am not in favor of them! I used unbypassed cathode for PU with higher output. I used it with bypass caps for lower outputs as diodes wherenīt in fasion at that time. Gain is 6dB higher, 35dB, with diode or bypassed. You have to be careful with choosen gain as overload reserve is small.

Salas,
The inverse RIAA at the the input is based on Hagermans model right out of the box. By the way Hagermans ideas arenītotally correct about the fourth timeconstant as Neumann uses a second order filter at 50kHz. As I have the RIAA built up in three versions now standing in the closet I know this one is close to correct. Havenīt heard of RIAAs that didnīt need trimming. So feeding the input as you say is not more than theoretical correct. A RIAA must be trimmed in combination with PU and player from recorded frequency tracks. I have an old DECCA, maybe there are better ones out there nowadays.

Good idea about PSU, though. This one is fed by a sand, regulated PSU from Erik A.(Audion).

Guys,
From a theoretical view you could call the cacode "cascade" as the cascode could be looked upon as a DC-coupled cascaded GC into a GG!
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Old 8th December 2008, 08:22 AM   #10
Salas is offline Salas  Greece
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I avoid the ''lost'' constant too. It peaks ultrasonically, disrupts circuits, does nothing good. I have designed a jfet Riaa, had checked it with Telarc Omnidisc and FFT, and tuned the Bench in comparison. The Hagerman, I meant to really build it and feed FFT. Not only to help modeling. Its alternative to PU. And yes, the shunt PSU we must make. What HV Mosfet you would pick for CCS and main shunt element?
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