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Old 29th July 2013, 10:17 PM   #1
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Default RIAA Second Stage Amp

Attached is a circuit using my current depletion mode darling, the Infineon BSS159 (it's surface mount, but you can't have everything). Luckily, there is a model for it already in my PSpice libraries, so simulating its behavior wasn't a big problem. The concept I'm using here is something I was trying beck in the 2007-2008 time frame, though with different devices. I wasn't aware of the BSS159 back then - It makes biasing much easier. Cascoding the BSS159 gain device reduces the effect of the Miller capacitance. It also allows the device to run at high bias current without a lot of power dissipation, making it possible for me to pump in a bunch of extra bias currrent below the cascode device to improve linearity. Think of it as a folded cascode unfolded...

As can be seen, the harmonics are almost entirely 2nd order. In practice, I would use another depletion mode fet as the current source, with perhaps a drain resistor to soak up some of the power dissipation. A Supertex DN2530N3 would work just fine. A simple cascoded common source amp or a folded cascode would work well for the first stage.
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Old 30th July 2013, 08:26 PM   #2
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This guy might be a candidate for front end section. The topology (a basic folded cascode) was brought to my attention in a thread I did called "Open Loop Follies" back in the 2007 time frame when I was first experimenting on modifications to the basic Pacific RIAA circuit, mostly by adding cascoding and buffers, but also using modifications to the second stage to reduce distortion. Folded cascode circuits have also been discussed in Sala's thread on his modified Pacific RIAA preamp, but I haven't looked at that particular folded cascode implementation.

This circuit would need to run with a lower supply voltage if one of the low-noise Japanese fets is used, as their gates will be very leaky at the drain voltages used in this circuit. This may be a little higher than optimum for even the PN4393 - I'll have to check. At any rate, this looks like a start.An appropriate resistor could be used in place of the J175, but I chose the jfet current source to add some power supply noise imunity.

The RIAA equalization would be added across the collector load resistor for the 2N4403.
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Old 31st July 2013, 03:51 AM   #3
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In this application, probably the best usage that could be made of the usual low noise JJ-fets (2SK170, 2SK117, and 2SK369) would be as cascoded common source amps. That way, they are run at low drain voltages where exponential gate leakage is not a problem, and the distortion profiles will be very benign and low order, with a THD similar to the first circuit presented in this thread.

Edit - if you have them, running some 2SK170Vs with an extra current source like used in the first circuit here might be interesting.
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Old 3rd August 2013, 10:44 PM   #4
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Attached are a couple of circuit variants, one using a folded cascode front end, and one using cascoded common source. Some values would need tweaking depending on the characteristic of the parts on hand.
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Old 4th August 2013, 05:02 AM   #5
mlloyd1 is offline mlloyd1  United States
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wrenchone,

good stuff!
keep it coming ...

mlloyd1
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Old 19th September 2013, 07:54 PM   #6
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Just for grins, I tried the BSS159 in a folded cascode RIAA first stage. Gain is about 40X (where it needs to be), and the simulated distortion is impressive. The only question is the noise, which I suspect might be ok for MM but not for MC.
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