Friday, 27 February 2009

Darius RIAA 2007

coupling triode fundamentals

This blog explains the RIAA 2007. In this RIAA pre amplifier the RIAA equalization is made by RIAA Triodes a special kind of coupling triodes.

Searching for a DC coupled valve amp I came up to the following idea:

The red line shows the signal path. Note that there are only triodes in the signal path.

A RIAA preamp is one of the most complicated things in audio technology. It must be low noise, make high gain and reproduce the RIAA curve. Thus it is a good example to explain the coupling triode. You can see the "making of" forum thread here .

After careful consideration I decided to build a passive RIAA in two stages.

To get a gain of 50dB at 1KHz the total gain without equalization must be 70dB. Thus each amplifier stage must make a gain of 35dB. This is possible with one triode section of the ECC83. The first RIAA filter provides attenuation of the frequencies above 50Hz, maximally 20dB. At the output of the first RIAA filter the signals that are attenuated about 20dB still have a gain of 15dB because the gain of the amplifier is 35dB. Thus only the first amplifier stage is relevant for the self-noise of the whole RIAA pick-up amplifier.

Each amplifier stage consists of a asymmetrical cathode coupled amplifier.

I designed it that the triodes are self linearizing. Since this circuit is unknown, I want to write a few words in addition. The asymmetrical cathode coupled amp is a kind of differential amplifier. Here a cathode follower (V1, V5) drives a grounded grid amplifier (V2, V6).

The cathode follower is made with one half of an ECC83 µ=100. At 2mA DC current its small signal output resistance is approximately 400 ohms. The negative supply voltage makes it possible to take a relatively high cathode resistor (R4, R16) of 22KOhm. The small signal input impedance of the following grounded grid stage amounts to at least 8KOhm. Thus the cathode follower operates in almost no-load and its anode current changes remain small because the signal voltage is substantially smaller than the DC voltage drop at the cathode resistor (R4, R16). Thus the input signal is passed through linear and with low loss (µ=100) to the cathode of the grounded grid stage.

The grounded grid amp is made with one section of an ECC83 µ=100. Its working resistor is approximately 800K Ohms. The anode voltage is approx. 110V DC. It is below the half operating voltage. The anode current is approx. 180µA. These operating conditions provide a high voltage gain and enables the triode to self linearize. The working resistor for the grounded grid stage is made by another triode operating as a resistor multiplier in a grounded anode circuit (“SRPP” - arrangement).

The resistor multiplier triode multiplies its resistor between grid and cathode µ times to its anode. Thus the triode's AC resistance has about µ times of the cathode resistor value (r). In this configuration µ of the resistor multiplier triode is not very important. For example if an ECC83 µ=100 is used for V3/V7 then its cathode resistor R8/R15 must be much lower resistance than for an ECC82 µ=17 to get the same current and voltage drop at the whole arrangement. The product of the cathode resistor times µ sets the small signal resistance. This will be nearly the same regardless of whether ECC82 or ECC83 is used.

This is helpful, because the cathode of the resistor multipler triode is not loaded by the current of the bottom triode only. This can be seen in the simplified diagram on the left. It has to handle the coupling triode's anode current too. The anode current of V3 and V7 is three times higher than the current in V2 and V6. As a low µ valve the ECC82 provides a higher current with lower anode voltages than the ECC83. This is why the ECC82 is used for V3 and V7. The current changes in the top triode are low because the current in the coupling triodes (V4 , V8) remains constant and thus the upper triode can work with high linearity. The following coupling triode makes the DC level shift from 110V DC to 0V DC and the RIAA equalization. This will be explained later.

The input stage can be seen as an operational amplifier. The grid of V1 is the non inverting input and the grid of V2 is the inverting input. The output of this triode operational amplifier is the cathode of V4. The grid of V4 is the offset adjusted input. All well known circuit topologies for operational amplifiers can be used to get the desired transfer function.
The triode offers us additional possibilities too. This triode opamp can be operated without negative feedback because its open loop gain is set by µ (V2). The desired transfer function can be adjusted with the coupling triode V4 also indirectly. In the RIAA 2007 the coupling triode in the filter mode is used to get the RIAA transfer function. This coupling triode is called RIAA triode. What makes me fascinating is that the signal flow only passes triodes in this topology. The signal flow is shown by the red line in the schematic at the beginning. Since there are no coupling capacitors or transformers there is no limit at the low frequencies. Thus one has to define the RIAA curve extended to the infrasonic region. In this improved RIAA we have two additional frequency breakpoints to adjust, later more about this.
The picture shows the RIAA graph measured at the RIAA 2007.

The RIAA equalization is exactly defined between 20Hz and 20KHz. The RIAA transfer characteristic can be reached exactly with two R_(C+R) low pass filters one after the other. The small signal schematic below shows the analogy between an R (C+R) low pass and a similar low pass made with a coupling triode in the filter mode.

This low pass works in the following way. Put a very low frequency at the input. C1 is high Z and the low frequencies can pass R1 without attenuation. In the RIAA triode it is the same. C6 is high Z and the grid is decoupled by R12 to cathode. The RIAA Triode works in the diodemode and the low frequency signal is passed from anode to cathode. For high frequencies C1 is short circuit. The high frequency signal passes the voltage divider made by R1 and R2. It is attenuated by this divider. In the RIAA Triode topology C6 decouples the grid to ground. The RIAA Triode works in the transformer mode for high frequencies. The high frequency signal at the anode is divided µ times at the cathode of the RIAA Triode . Breakpoint 3 in the RIAA curve is set by the edge frequency of the first low pass and breakpoint 4 by the dividing ratio of the low pass for high frequencies. What I like very much is in the RIAA triode topology the triodes µ sets the position of breakpoint 4 in the RIAA curve.
It is the same with the breakpoints 5 and 6 that are made by the second RIAA triode . Breakpoint 5 is set by the low pass edge frequency of the RIAA triode and breakpoint 6 by µ of the second RIAA triode . Theoretically the enhanced RIAA curve goes straight horizontally right from breakpoint 6. In practice the upper frequency limits of both amplifiers are setting the gain in the ultrasonic region.

The small signal schematic on the left explains how a coupling triode with low pass characteristic and a coupling triode with high pass characteristic are combined into a coupling triode with band pass characteristic in the RIAA 2007.
The coupling triode's grid in the filter mode gets its DC biasing via R11/R22. These are the voltages U_bias_1 and U_bias_2 in the schematic.

continued ...


Parts description for the dc coupled triode RIAA preamp 2007

R1_ pick up load resistor
R2_ RFI Filter resistor (optional)
R3 / C2_ positive supply voltage smoothing
R4_ kathode resistor cathode coupled amp V1 V2
R5 / C3_ negative supply voltage smoothing
R6 / R10_ kathode resistor V4 C8 bootstrap capacitor
R7 / C4_ positive supply voltage smoothing
R8_ bias resistor V2 V3
R9 / C5_ offset voltage smoothing first stage
R10 /R6_ kathode resistor C8 bootstrap
R11_ coupling valve V4 bias voltage feed resistor
R12 / C6_ RIAA timing components for RIAA Valve V4
R13 / C9_ positive supply voltage smoothing
R14 / C12_ positive supply voltage smoothing
R15_ bias resistor V6 V7
R16_ kathode resistor cathode coupled amp V5 V6
R17 / C10_ negative supply voltage smoothing
R18 / R20_ kathode resistor V8 C15 bootstrap capacitor
R19 / C11_ offset voltage smoothing second stage
R20 / R18_ kathode resistor V8 C15 bootstrap capacitor
R21 / C14_ RIAA timing components for RIAA Valve V8
R22_ coupling valve V8 bias voltage feed resistor
R23 / C16_ positive supply voltage smoothing
R24_ cathode resistor for current source V9
R25 / C17_ negative supply voltage smoothing
R26 / R27_ grid bias divider current source V9
R27 / R26_ grid bias divider current source V9
R28_ output series resistor (optional)

C1_ pic up load cap (optional)
C2 / R3_ positive supply voltage smoothing
C3 / R5_ negative supply voltage smoothing
C4 / R7_ positive supply voltage smoothing
C5 / R9_ offset voltage smoothing first stage
C6 / R12_ RIAA timing components for RIAA Valve V4
C7_ bias voltage decoupling to kathode of V4
C8_ bootstrap capacitor for coupling valve V4
C9 / R13_ positive supply voltage smoothing
C10 / R17_ negative supply voltage smoothing
C11 / R19_ offset voltage smoothing second stage
C12 / R14_ positive supply voltage smoothing
C13_ bias voltage decoupling to kathode of V8
C14 / R21_ RIAA timing components for RIAA Valve V8
C15_ bootstrap capacitor for coupling valve V8
C16 / R23_ positive supply voltage smoothing
C17 / R25_ negative supply voltage smoothing
C18_ grid bias voltage smoothing V9

V1_ cathode follower, V1 V2 cathode coupled amp
V2_ cathode input amp, V1 V2 offset voltage difference amp
V3_ high impedance load for V2, kathode follower, plate voltage source for V4
V4_ RIAA coupling triode
V5_ cathode follower, V5 V6 cathode coupled amp
V6_ cathode input amp, V5 V6 offset voltage difference amp
V7_ high impedance load for V6, kathode follower, plate voltage source for V4
V8_ RIAA coupling triode
V9_ high impedance current source for V10
V10_ low output impedance cathode follower, output stage

Schematic of the RIAA_2007.

R1_ 47K
R2_ 470R (optional)
R3_ 22K
R4_ 22K
R5_ 1K
R6_ 47K
R7_ 10K
R8_ 47K
R9_ 1Meg
R10_ 47K
R11_ 1Meg
R12_ 22K
R13_ 22K
R14_ 10K
R15_ 47K
R16_ 22K
R17_ 1K
R18_ 47K
R19_ 1Meg
R20_ 47K
R21_ 27K
R22_ 2M2
R23_ 10K
R24_ 2K2
R25_ 470R
R26_ 2K7
R27_ 11K (two 22K resistors in parallel)
R28_ 2K2 (optional)
RV1_ 22K
RV2_ 22K

C1_ 220p (optional)
C2_ 4µ7
C3_ 100µ 63VDC
C4_ 4µ7
C5_ 1µF MK 50VDC
C6_ 22nF
C7_ 1µF MK 50VDC
C8_ 1µF MK 50VDC
C9_ 4µ7
C10_ 47µF 63VDC
C11_ 1µF MK 50VDC
C12_ 4µ7
C13_ 470nF MK 50VDC
C14_ 220pF
C15_ 1µF MK 50VDC
C16_ 4µ7
C17_ 47µF 63VDC
C18_ 100µF 16VDC

V1, V2, V5, V6, V9, V10_ ½_ECC83_12AX7
V3, V4, V7, V8 _½_ECC82_12AU7


The gain of a cathode follower is simplified calculated in the following way:

vu = (1–1/µ) x (r(r+1/s))

vu_ [this is the voltage gain of the cathode follower]
(1–1/µ)_ [open output gain]
r(r+1/s)_ [gain reduce caused by load]
s_ [transconductance]
r_ [small signal load resistance at cathode]


Voltage gain of the asymmetrical cathode coupled amplifier calculated in three steps:

Step one,
gain of the cathode follower:
Input resistance of the grounded grid stage is 8KΩ. This resistance is in parallel[ll] to R4 =22KΩ.
R load 8KΩ ll 22KΩ = 5900Ω
The output resistance of the follower is 1/mutal cond. = 400Ω
This value comes from the data book ECC83 @ 2mA.
The output resistance of the follower and the load resistance are dividing the no load output voltage of the cathode follower:
5900Ω / (5900Ω+400Ω ) = 0,94 [gain reduce caused by load]
The no load output voltage of the cathode follower is reduced by the amount of signal voltage between grid and anode divided by µ. See triode transformer
mode. This is calculated by 1-1/µ [ECC83 µ=100]
1-1/100 = 0,99 [open output gain]
The no load gain multiplied by the gain reduce caused by the load gives the gain of the cathode follower stage:
0,99 x 0,94 = 0,93 [this is the voltage gain of the cathode follower]

Step two,
gain of the grounded grid stage:
The mutal conductance [s] is 0,5mA/V. This value comes from the data book.
The anode load resistance [ra] is 800KΩ it comes from the resistor multiplier
triode V3 of the SRPP arrangement. The no load resistance [ri] at the anode is:
ri = µ/s = µ x 2KΩ = 200KΩ . Both in parallel:
ri ll ra = 200KΩ ll 800KΩ = 160KΩ
The voltage gain is (ri ll ra) x s = 160KΩ x 0,5mA/V = 80

Step three,
to get the total gain of the asymmetrical cathode follower the gain of the cathode follower and the gain of the grounded grid must be multiplied:
vu = 0,93 x 80 = 74
[this is the voltage gain of the asymmetrical cathode follower amplifier]
74 = 37dB

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