[www.keith-snook.info]

~ RIAA & BS1928 Replay Amplifiers ~

Still Under Construction
More than 35 years after the introduction of the Compact Disc and now during a time when music is downloaded or streamed over the internet there still appears a need for a pre-amplifier that can equalise the low voltage output from a phono cartridge for the playback of vinyl phonograph discs recorded with RIAA equalisation or records as I knew them

Prior to 1954 numerous different playback equalisation or EQ curves were required to correct different manufacturers recording EQs to provide a level playback response for Mono recordings ~ With the introduction of a new fine groove EQ in 1955 and stereo in 1957 things became more standardised for fine groove vinyl disc replay

The need for record and playback EQ is partly explained by me here in the context of the 1959 QUAD 22 pre–amplifier and at length elsewhere on the internet and in some of the references at the bottom of this page which you should be read before proceeding further especially Analogue Sound Restoration by Peter Copeland

The subject of this webpage is RIAA EQ as applied to low noise amplifiers for vinyl replay ~ RIAA here refers to the equalisation characteristic for vinyl record and playback ~ BS1928 refers to the first edition of the British Standards Institution document BS1928 : 1955 (published in 1955) which covers other aspects of vinyl disc recording

A pole at 3.18µs (50.05kHz) "fits in" with the other RIAA time constants but why not choose 45kHz or 55kHz ? ~ For "normal" audio recording there is very little energy above 10kHz and the recording chain of microphones mixing consoles and tape machines ensures that if there were any energy above 20kHz it would be 10s of dB lower before it reached the cutting head and would most likely be distortion products

Even if some record cutting lathes have a pole above 20kHz ~ which some do ~ attempting to correct for such at replay would be a waste of time — The signal to the cutting head falls naturally above 20kHz due to drive circuitry and there is often a "formal pole" beyond 40kHz which is 2nd order and corrects for phase up to 20kHz

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Passive RIAA EQ or Active feedback EQ ?
•     •      •      •      •
Separate time constants or an all in one EQ Block ?

The circuit above was copied directly from the National Semiconductor (now Texas Instruments) data sheet for their "Hi-Fi Audio operational amplifier" the low noise and very low distortion LME49710 and LME49860 etc. and although it is only offered as a typical application it demonstrates how little thought goes into these designs

It looks like the capacitor and resistor values have been carefully crafted to provide the most accurate RIAA EQ ~ The resistors are chosen from the E96 series and as stated are 1% tolerance but I have seen an implementation using 0.1% ! ~ The capacitor type should be questioned and the tolerance 5% 'puts paid' to any attempt at precision

26.1kΩ + 909Ω is 27.009kΩ and 3.83kΩ + 100Ω is 3.93kΩ so why not use single E24 series 1% values 27kΩ and 3.9kΩ ? ~ Calculated from 27.009kΩ the capacitor 47nF||33nF should be 80.973nF not 80nF ~ The other RIAA capacitor 27nF||4.7nF||500pF should be 27.769nF so rather than 5% polypropylene why not use 27nF||750pF 1% polystyrene which are readily available and I would say are better capacitors for this application

The solution shown above using standard E12 values (E24 1% in practice) gives a more accurate RIAA equalisation than the Texas Instruments "design" with its modelled peak–peak deviation of 0.16dB between 20Hz and 20kHz using perfect components ~ The "age old" standard values of 47k 6k8 16n and 47n have a peak–peak deviation from the perfect RIAA curve of only 0.05dB

The circuit has a higher impedance passive RIAA EQ block R1—C2 than the TI circuit but this does not affect the noise performance which is dominated by the source S/N and op-amp input noise voltage ~ Both circuits as shown have the same overall gain which is about 530x or a 3mV cartridge input gives about 160mV equalised flat output

The open loop or Ao gain and GBW of the LME49710 will allow the gain of each stage to be increased without affecting the S/N greatly ~ If R4 in my circuit were changed to say 100Ω a 3mV cartridge input would give 320mV output with slightly better S/N ~ Then changing R6 to 1k would give about 0.7V out for 3mV input

Both circuits are d.c. coupled which is not desireable for a phono stage ~ Rather than use coupling capacitors which would also require additional resistors for d.c. biasing of the op-amps it is possible to place capacitors in series with R4 or R6 or both ~ the values should be calculated to provide a high pass response say -3dB at 20Hz or lower

With capacitors in series with both R6 and R4 the final slope of the the high pass step response will be 12dB/oct and will provide a reasonable rumble filter which may not be appreciated by some critics even though it keeps the through signal path d.c. coupled ~ The input offset voltages of the op-amps are now only amplified by 1x so d.c. at the output is near zero

Comparison between the 2 circuits was made with PSpice computer modelling using an Analogue Behavioural Model (ABM) block to provide a perfect inverse RIAA function source with a 0Ω output as shown on the right

The circuits were modelled with various op–amps and the other components shown in the schematic so the impedances around the RIAA EQ section were accounted for

Whether the RIAA EQ is achieved in a single block or in separate sections or is passive or in a feedback loop the source and load impedances will have an affect on the accuracy of the calculated values

When the source or load impedances around passive RIAA sections or an EQ block ~ as shown above and left ~ are incorporated into the calculation not only is the overall response correct but gain wasted due to coupling is minimised

The valve circuit on the left uses the same lumped passive EQ as my LME49710 circuit above (RIAA-1 in ref.3) but configured for current drive ~ The output of the triode can be considered as a current source with an internal shunt resistance ra of about 31kΩ for Ia = 1mA

The unbypassed cathode resistor R11 raises ra to ra' ≈ 126kΩ so the resistance affecting the EQ is 126kΩ||75k which is the required 47kΩ ~ Any load of the next stage must also be incorporated in the EQ calculation but as the total value of R1 is the only parameter that needs adjustment the calculation is easy and the response of the EQ block predictable

In practice a valve stage configured as shown will require a pre stage for best S/N and for sufficient output level and will need a high input impedance following stage to prevent loading of the EQ block ~ C2+R2 and C1 would best be connected to ground and depending on the HT supply used the impedance of the EQ block could be made higher for more gain provided the S/N is not compromised by the Johnson noise of R1

Using a separate current source as the valve anode load with the EQ block returned to ground offers little or no advantage and would introduce semiconductors and or additional noise ~ becasue some amplification is required before the valve EQ stage for best S/N this may lead to the conclusion that separate time constant gain stages would be easier to use with valves

3 or more stages are often required to amplify the range of signal levels from magnetic cartridges and the RIAA TCs could be split across 2 or 3 of them but each stage has to provide correct loading for the EQ elements ~ When a single EQ block is used on the 2nd stage the 1st stage can be made a very high gm valve with high Ia for best S/N

High gm valves tend to have a low ra which is never well defined and varies with the slightest change of heater or HT voltage and with age ~ An unbypassed cathode resistor gives a higher ra' but at the expense of gain ~ The 1.8kΩ cathode resistor of the ECC81 stage above makes ra' very predictable and stable but the 1kHz gain is about 6dB

By placing all 3 (not 4 or 5) RIAA TCs in a single EQ block after a flat response high gain 1st stage the construction of a good RIAA pre amplifier is actually easier than using separate EQ sections ~ Using op-amps or other devices with large amounts of feedback

Panasonic Matsuhita Technics EPC-450CII

References:

ref.1 ~ Peter M. Copeland ~ BBC ~ Analogue Sound Restoration

ref.2 ~ J. D. Smith ~ W.H. Livy (EMI Studios Abbey Road London) ~ Wireless World Nov 1956 & Jan 1957

ref.3 ~ Keith Snook 1982 ~ RIAA Lumped CR equalisation calculations

ref.4 ~ E. A. Faulkner ~ The design of Low-noise audio frequency amplifiers

ref.5 ~ Editor S.W. Amos ~ BBC ~ Radio TV and Audio Reference Book published by Newnes-Butterworth Ltd

ref.6 ~ Allen Wright ~  Secrets of the phono stage


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