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For RIAA preamp: Large value caps vs. Regulated/Stabilized PSU
For RIAA preamp: Large value caps vs. Regulated/Stabilized PSU
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Old Today, 03:41 PM   #271
rongon is offline rongon  United States
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For RIAA preamp: Large value caps vs. Regulated/Stabilized PSU
I wonder if the 'harsh' upper mids/trebles I heard from 12AX7s in this preamp are caused by the wheezy little 12AX7 triode trying to drive the gate capacitance of the MOSFET source follower?

I'm using an FQPF2N60 with Ids = 5.3mA for the source follower. Its Crss is listed as 4.3pF (5.6pF maximum). Let's say it's the maximum, 5.6pF.

In a DC-coupled source follower, that should equate to about 28pF Cin.

If we need to drive a 1V peak 50kHz signal into 28pF, that should require only 50uA (microamperes, 0.05mA). A 12AX7 with Ip = 850uA (over 15X the minimum requirement) should be able to sink that with no problems, so should not be anywhere near slewing driving a maximum 1V peak 20kHz signal.

Did I do that right? Is a 12AX7 OK driving the gate capacitance of an FQPF2N60 MOSFET source follower, or is it absolutely necessary to use a smaller MOSFET with ultra-low Crss like ZVN0545?

(The reason I ask is that I've noticed that the high frequencies sound far better if I put a 12AT7 in the same circuit. 12AT7 draws about 1.5mA, or almost double the plate current that a 12AX7 draws in that same spot.)
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Last edited by rongon; Today at 04:03 PM.
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Old Today, 06:26 PM   #272
rongon is offline rongon  United States
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For RIAA preamp: Large value caps vs. Regulated/Stabilized PSU
Quote:
Originally Posted by mashaffer View Post
Another possibility that intrigues me is to use a medium mu triode (thus lower Cmiller) and a small signal pentode instead of two high mu triodes. There just happen to be several such things in a single bottle.

This is a picture of the idea based on the above Marantz circuit. Note this is not an actual design but just a picture of the idea.
Absolutely!

The trick is to find one with a low-noise, low Cin triode in the bottle with the pentode. Perhaps use the pentode as the input stage, triode second stage? Perhaps if the pentode is a very high-gm one, triode-wire it by grounding the plate and using the screen grid as the 'plate'.

People like to use the 6F12P for phono preamps, but the triode (and the trioded pentode section) have very high input capacitance.

Actually, there are many of those triode-pentode tubes that have something like a 12AT7 for the triode section. That works well enough, in my (newfound) experience because the 12AT7 has just low enough Cin to get by with an Audio Technica or similar MM cartridge. (I don't know about Ortofon 2M, but I figure that's similar to the AT carts in the L and DCR departments.)
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Old Today, 06:43 PM   #273
MarcelvdG is offline MarcelvdG  Netherlands
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Quote:
Originally Posted by rongon View Post
I'm using an FQPF2N60 with Ids = 5.3mA for the source follower. Its Crss is listed as 4.3pF (5.6pF maximum). Let's say it's the maximum, 5.6pF.

In a DC-coupled source follower, that should equate to about 28pF Cin.
Why is that? Where does that factor of five come from?
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Old Today, 07:09 PM   #274
mashaffer is offline mashaffer  United States
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Unfortunately curiosity may dictate building one with AX7s and then another with AY7s and AU6s. I think we are getting dangerously close to a rabbit hole.
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Old Today, 08:03 PM   #275
rongon is offline rongon  United States
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For RIAA preamp: Large value caps vs. Regulated/Stabilized PSU
Quote:
Originally Posted by MarcelvdG View Post
Why is that? Where does that factor of five come from?
I think it's just 'headroom', or lots of slop factor so you don't end up running to close to the ragged edge.

I can't find the website where I read about slew rate limiting and how that relates to input capacitance of an amplifying stage, but the equation I got was

I = C * 2 * pi * F * Vpeak

Is that not correct?
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Old Today, 08:08 PM   #276
MarcelvdG is offline MarcelvdG  Netherlands
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That's perfectly correct, but I wonder how you come from 5.6 pF to 28 pF.
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Old Today, 08:35 PM   #277
rongon is offline rongon  United States
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For RIAA preamp: Large value caps vs. Regulated/Stabilized PSU
I did the equation with 100kHz, not 50kHz. A mistake. (oops)

The result I get from 28pF Cin, 1.414V peak, and 50kHz audio signal, is 10uA plate current.

Therefore, I have to assume that slew limiting is not an issue with a 12AX7 with Ip = 800uA driving the FQPF2N60 source follower gate.

Now, my calculation of how to find the Cin of a MOSFET source follower might not be correct. I'm using

MOSFET source follower Cin = Crss + [Cgs(1-gain)]

therefore

5.6pF + [265pF(1-0.92)] = 26.8pF

(I wrote 28pF from memory, but it's close enough)

Correct?
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Last edited by rongon; Today at 08:38 PM.
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Old Today, 08:41 PM   #278
MarcelvdG is offline MarcelvdG  Netherlands
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The equation is perfect, but something goes wrong with the calculation:

1-0.92 = 0.08
5.6 pF times 0.08 = 0.448 pF
5.6 pF + 0.448 pF = 6.048 pF

However, is Cgs = Crss? For a MOSFET in saturation, Cgs is usually larger than Crss.
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Old Today, 08:55 PM   #279
rongon is offline rongon  United States
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For RIAA preamp: Large value caps vs. Regulated/Stabilized PSU
In the online article I got this from...

Cgs was listed as equal to Ciss minus Crss.
Cgd was listed as equal to Crss.
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