Trafomatic RIAA network. Any good?

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JordiMB

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Joined 2014
#1
While studying different RIAA deemphasis sistems (because learning is fun) I've found this phono stage that uses a different aproach:

6moons audio reviews: Trafomatic Audio Reference Phono One

The RIAA network is neither in the feedback path nor in the classic passive filter between stages. It's in the plate load. Maybe some of you were aware of it, but for me it's the first time I see something like this. Well, it makes sense, and it seems to work allright.

I've made a very quick simulation on LTSpice, using jfets instead of tubes, and with some tweaking of resistors and capacitors values I've obtained a decent result (not accurated, just promising). What it can be appealing is that you lose a lot less signal than with a passive filter.

Can it be a good alternative system, or has it been tried before and has it been rejected for not being good enough?
 
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#2
Check out the last circuit in this thread

:: View topic - Pentode Phono

It is about the only way to build an entire phono preamp in a single stage. Having listened to a proto based on the idea i can attest to its viability. In fact it was remarkably good.

The idea is of course not new. Historic phono preamps have been built using frequency dependent loaded pentodes. If not mistaken one of Creek Audio's phono stages employed the same idea using transistors.
 
#3
> nor in the classic passive filter between stages. It's in the plate load.

Electrically it is the same thing.

With triodes it is dubious because internal plate resistance is not known exactly. With pentodes the internal resistance is generally so much greater than the external DC resistor that we can work with resistor-based accuracy.
 
#5
PRR said:
> nor in the classic passive filter between stages. It's in the plate load.

Electrically it is the same thing.

With triodes it is dubious because internal plate resistance is not known exactly. With pentodes the internal resistance is generally so much greater than the external DC resistor that we can work with resistor-based accuracy.
Click to expand...

Interesting. That's the reason they used pentodes. Worth exploring, even you can use jfets or mosfets...
 
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#7
From what I've seen in the simulation, you don't lose as much gain as with the classic passive filter. So maybe it's possible to make a phono preamp with only one gain stage. Interesting what analog_sa says: this kind of riaa network can prevent from overload at high frequencies, no?
 
#8
If you use a voltage amplifier with low output impedance (e.g. a triode) then you need to add a series resistor to define the source impedance, then add the RIAA network.

If you use a transconductance amplifier with high output impedance (e.g. pentode or cascode) then you need to add a shunt resistor to define the source impedance, then add the RIAA network.

I can't see much difference between these two approaches. With the latter you may have to be more careful about PSRR because you probably want to take your final output voltage referenced to ground but the pentode version uses the supply rail as reference.
 
#11
Thanks for your ideas. I think I'm going to try a simpler jfet version (easier to do, lower voltages) just to see how it works. Being high output impedance devices, the shunt resistor version would be the way to go. Do you think I can use another jfet as the shunt load, instead of a resistor?
 
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