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RA12

Single Supply RIAA Amplifier.

Contents


Introduction.

RA12 is a compact, single supply RIAA amplifier with very good RIAA accuracy.
The reason for the design was the need to connect a gramophone to a multimedia AV processor.
To keep things simple, the RA12 can be powered from the TRIG output on most AV processors.
The amplifier is designed solely for use with Moving Magnet (MM) pick-ups or high-output Moving Coil (MC) pick-ups. It can not be used with "normal" Moving Coil (MC) pick-ups.


Amplifier.

RA12 Schematic.
Fig.1: RA12 Schematic.

To keep the supply current low, the entire RIAA equalizer is built around a single OP-AMP with a passive filter in the output to provide the last HF roll-off.
To obtain good RIAA accuracy, non standard component values and a wide bandwidth OP-AMP are required. With the component values shown, the basic RIAA accuracy is better than ±0.15 dB from 20 Hz to 20 kHz (±0.05 dB from 31.5 Hz to 20 kHz).
C13 can be increased to 2200 µF to obtain ±0.05 dB deviation from 20 Hz to 20 kHz, but start-up time becomes excessive as C13 has to charge through the OP-AMPs feedback network (start-up time with 470 µF is almost 5 minutes).
The OP-AMP is biased to half the supply voltage with divider R63, R64 and R12, R32. R11//R12 determines the input impedance ( 47 kΩ ). With a bi-polar OP-AMP, R63 and R64 are 100 kΩ. With a FET-input OP-AMP they can be increased to 1 MΩ reducing the sensitivity from hum on the power supply.
The capacitors C11 and C31 are normally not needed as this generally is the capacitance of the cable from the pick-up to the amplifier. If the amplifier is built into the the gramophone enclosure or a pick-up that requires a higher load capacitance than the cable provides is used, a capacitor can be mounted in this position.
C12, C32, C20, C40 are DC blocking capacitors for the inputs and outputs.
Capacitor voltage ratings are what I have; 10 V types are OK except for C61, C62 that must be minimum 16 V.
The output filter (C21, R18, R20) is shown for an unbalanced output. If the amplifier is used to drive a balanced input, R18 and R20 should both be 301 Ω.
R61 provides some filtering for the power supply and reduce the inrush current at start-up.
R62 is a simple "ground-lift" that can isolate the circuit ground from the power supply ground (to some extend). R61 + R62 should be kept around 50 Ω.

Table 1: RA12 simulated specification. All resistors are 1% types.
Lower -3 dB frequency2.5 Hz
Nominal voltage gain at 1 kHz41.8 dB
Voltage gain deviation at 1 kHz, C15, C16, C18, C19, C21 1% tolerance+0.18 / −0.16 dB
Voltage gain deviation at 1 kHz, C15, C16, C18, C19, C21 5% tolerance+0.32 / −0.29 dB
Nominal RIAA deviation+0.03 / −0.05 dB
Maximum RIAA deviation, C15, C16, C18, C19, C21 1% tolerance+0.13 / −0.18 dB
Maximum RIAA deviation, C15, C16, C18, C19, C21 5% tolerance+0.42 / −0.48 dB

Nominal RIAA deviation: The maximum deviation of the frequency response from 31.5 Hz to 20 kHz with 1 kHz as reference with nominal component values.
Maximum RIAA deviation: The maximum deviation of the frequency response from 31.5 Hz to 20 kHz with 1 kHz as reference with any combination of component values set to their maximum and minimum values. The maximum and the minimum value are not necessarily with the same set of component values.


Power Supply.

The amplifier will run from a regulated DC supply from around 10 V to 30 V as shown.
The amplifier is somewhat sensitive to mains hum on the supply line, so if you power it from rectified mains, I will suggest to use 2 LM317 regulators in series for best performance.
The voltage rating for C61, C62 must be at least as high as the supply voltage and the voltage rating for the other electrolytics at least half the supply voltage.


Powering from a Trig Output.

Most trig outputs from AV-receivers and audio amplifiers will deliver 12 V at 25 mA to 100 mA.
The output is most often on a 3.5 mm jack-socket, where the ring is GND and the tip is +12 V.
I do not think there is any standard for this and I have seen other variations:

Unless the manual for the receiver clearly states the voltage and connections for the trig output, it should be measured:
Enable the trig output.
Connect a resistor of around 1 kΩ between + and - on the trig output.
Measure the voltage from the trig output + to the receiver chassis. This should be around 12 V.
Measure the voltage from the trig output - to the receiver chassis. This should be around 0 V (some mV are acceptable).

Typical connection for Receiver with unbalanced inputs.
Fig.2: Typical connection for receiver with unbalanced inputs.

Typical connection for Receiver with balanced inputs.
Fig.3: Typical connection for receiver with balanced inputs.


Specification.

The specification for 2 versions of the RA12 are shown in table 2 below.
Column 1 is for RA12AAA (NE5532) and column 2 is for RA12ACA (OPA2134) with the amplifier powered from a low-noise lab power supply (<-92 dBu (18 µV) noise in the 22 Hz to 22 kHz bandwidth) and column 3 is measured through a Lyngdorf Audio DPA-1 with the RA12 powered from its TRIG output.
The unit dBu is dB referred to 0.775 V.
As you can see from the measurements, I have severe problems with low-frequency measurements in this room. The hum level changes around 4 dB if I turn on the lights in the room next to this. However, I have decided to publish the numbers as measured.
More detailed measurements are in the file RA12_Measured_Specification.ods (part of the download later).

Table 2: Specification for RA12AAA (NE5532A, unbalanced output), RA12ACA (OPA2134, unbalanced output) and RA12AAA powered by and measured through a Lyngdorf Audio DPA-1.
 RA12AAARA12ACARA12AAA + DPA-1
OP-AMPNE5532APOPA2134PANE5534AP
Input resistance47 kΩ
Input capacitanceDetermined by C11, C31
Output resistance600 Ω
Minimum recommended output load impedance10 kΩ
Start-up time5 minutes
Voltage gain, 1 kHz41.72 dB41.67 dB41.80 dB
Linearity ref. RIAA curve, 20 Hz - 20 kHz ref. 1 kHz+ 0.04 dB / −0.12 dB+ 0.05 dB / −0.13 dB+ 0.06 dB / −4.19 dB (Note 3)
Linearity ref. RIAA curve, 31.5 Hz - 20 kHz ref. 1 kHz+ 0.04 dB / −0.06 dB+ 0.05 dB / −0.13 dB+ 0.06 dB / −1.59 dB (Note 3)
Channel balance, 20 Hz - 20 kHz ref. 1 kHz± 0.09 dB± 0.09 dB± 0.1 dB
Maximum output level into 100 kΩ, 1 kHz, 0.1% THD+N12 dBu (3.1 V)13 dBu (3.5 V)11.7 dBu (3.0 V)
THD+N, 20 Hz, 10 dBu (2.45 V) output, BW : 10 Hz - 80 kHz0.018 % (Note 1)0.011 % (Note 1)0.062 % (Note 1, 4, 5)
THD+N, 1 kHz, 10 dBu (2.45 V) output, BW : 400 Hz - 80 kHz0.003 %0.003 %0.007 % (Note 5)
THD+N, 20 kHz, 10 dBu (2.45 V) output, BW : 400 Hz - 80 kHz0.006 %0.006 %0.007 % (Note 5)
Output noise, 0 Ω source, unweighted, 22 Hz - 22 kHz-77.5 dBu (103 µV) (Note 1)-76.1 dBu (121 µV) (Note 1)-77.3 dBu (106 µV) (Note 1)
Output noise, 0 Ω source, unweighted, 400 Hz - 22 kHz-85.4 dBu (42 µV)-83.0 dBu (55 µV)-83.6 dBu (51 µV)
Output noise, 1 kΩ source, unweighted, 22 Hz - 22 kHz-76.7 dBu (113 µV) (Note 1)-76.0 dBu (123 µV) (Note 1)-77.0 dBu (109 µV) (Note 1)
Output noise, 1 kΩ source, unweighted, 400 Hz - 22 kHz-84.1 dBu (48 µV)-82.1 dBu (61 µV)-82.6 dBu (57 µV)
Output noise, pick-up source, unweighted, 22 Hz - 22 kHz-74.6 dBu (144 µV) (Note 1, 2)-74.6 dBu (144 µV) (Note 1, 2)-71.8 dBu (199 µV) (Note 1, 2)
Output noise, pick-up source, unweighted, 400 Hz - 22 kHz-79.4 dBu (83 µV) (Note 2)-79.1 dBu (86 µV) (Note 2)-78.7 dBu (90 µV) (Note 2)
Output noise, 1 kΩ source, a-weighted-83.8 dBu (64 µV)-81.9 dBu (62 µV)-83.0 dBu (55 µV)
Signal/noise ratio, 1 kΩ source, a-weighted, ref. 5 mV81.7 dB79.7 dB81.0 dB
Channel separation, 1 KHz92 dB90 dB92 dB
Channel separation, 20 Hz - 20 KHz59 dB82 dB59 dB
Supply voltage+ 12 V+ 12 V+ 11.4 V
Supply current7.4 mA8.2 mA7.4 mA
Board size (length / width / height)67 mm / 54.8 mm / 18 mm

Note 1:Mainly 50 Hz.
Note 2:The pick-up source used for noise measurement is a Shure M75. Its impedance is 630 Ω in series with 720 mH.
Note 3:The "extreme nonlinearity" is due to the low and high-pass filters in the DPA-1.
Note 4:The level at 20 Hz had to be lowered to 9 dBu (2.18 V) due to the HP filter in the DPA-1.
Note 5:This is not really fair to the DPA-1. The signal goes through a noise shaping ADC and DAC. The proper way of measuring this is with a 20 kHz brick-wall filter (ref. AES-17), but I do not have one.

PCB.

Photo of PCB with components.
Fig 4: PCB with components.
This board has wires soldered directly into the PCB as there is not room for the solder-terminals.

Component selection:
Resistors must be metal film, preferably 1% types.
Filter capacitors must be film types, preferably 1% types, but 5% will work too.
Power supply decoupling capacitor (C63) must be a ceramic.
If you wish to use an OP-AMP other than NE5532A or OPA2134 you should use a type that is unity-gain stable and has a gain of 80 dB or more at 2 kHz.
An obvious candidate is LM4562, but I tested it and it is not stable in this design.
There are some suggestions in the part-list you can download, but they are mainly based on what I have in stock.

Download RA12A design files.

I have boards available for this project. See the PCBs page.


Construction.

The pick-up amplifier should be built in a metal enclosure for electrical and mechanical shielding (the film capacitors can be microphonic).
An enclosure made from 1..2 mm sheet metal is normally sufficient.
The circuit ground should be connected to the enclosure in one place only (near the input connectors normally works well).
For best performance, do not put mains transformers inside or near the amplifier enclosure (it is possible, but it can be very difficult to get to work without excessive hum).


Known Issues / updates.

RA12A:
The solder-terminals listed in the Bill of Materials are for 1.2mm holes, but the holes in the PCB are 1.3mm.

20160116:
Revisions updated:
RA12AAA to RA12AAB, RA12ABA to RA12ABB, RA12ACA to RA12ACB, RA12ADA to RA12ADB.
Fixed error in parts-list.
Minor cosmetic fixes in documentation.


References.

[1]Stanley P. Lipshitz "On RIAA Equalization Networks", Journal of the Audio Engineering Society, 1979 June, Volume 27, Number 6, Page 458.
Available from Audio Engineering Society for a fee. Search for "On RIAA Equalization Networks". Also available from some technical libraries.

Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
Poul Petersen home, Poul Petersen DIY index, E-mail: diy@poulpetersen.dk
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