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I decided to do
some serious tweaking of my RIAA amplifier. Armed with PSPICE this is a piece
of cake. First a schematic of my previously tweaked version of my old RIAA
amplifier:
I have removed the RIAA
network and created a base for simulation of different passive RIAA network
topologies. I looked around the net to see other designs and found a few
interesting.
The circuit is
intended to drive my bootstrapped cathode follower input in my line amplifier
without cable. The output stage can be changed to just about anything with
the same or more gain like SRPP, mu-follower or a
cathode follower compound. Some adjustment of the low frequency pole may be
needed.
Tweaks
has been run in PSPICE
with the enhanced triode model. I used 12AX7 with mu
lowered to 70 to emulate the 5751.
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This is the
original net I used in my amplifier. It’s designed by the late Jan Lodström many years ago.
It’s not far off
the RIAA curve, around +0.3dB at 20 Hz worst case. Simple
values, the 30nF is just 2 15nF in parallel.
It has two
advantages, there is no need for a grid leak on the second tube and C3 with
its large value slugs the miller capacitance of it.
The disadvantage
is the missing 50 kHz zero.
Gain 48dB @ 1
kHz.
I just love this
topology.
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This is my final
tweak of the above net. It’s now only +0.2dB off at 20 Hz worst case.
I also added the
50 kHz zero. Still no unusual values, all resistors are within E24 and the
30nF is still 2 15nF in parallel.
The miller capacitance
plays only a minor role, the lift at 20 kHz is there.
If you don’t want
the 50 kHz zero, remove R8 and ground C6.
Gain 50dB @ 1
kHz.
This is the one
I’m going to implement.
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This is Tweety by Maarten (link).
It’s his tweaked
version for his amplifier but it works very well in my amplifier above.
It’s no more than 0.2dB off anywhere.
The disadvantage
is the need for a grid leak, a large one which could cause troubles if the
tube is a little gassy. It also has an odd capacitor value.
I have added a
50kHz zero to it, R12.
Gain 53dB @ 1kHz
in my circuit.
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This is Loekie which I found on Maarten’s
site and tweaked by him. It’s also very good.
This one also
needs the grid leak resistor and has a few odd resistor values.
Gain 53dB @ 1 kHz
in my circuit.
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Finally the net
of Scratch by Triode Dick (link).
This one is also good but needs some tweaking at 20 Hz in my circuit. The
topology is the same as Loekie but without the 50
kHz zero. Adding the zero transforms the circuit into Loekie
above.
Gain 52dB @ 1 kHz
in my circuit.
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