~ Bicycle Dynamos and RIAA Replay Amplifiers ~

I often rode when it was dark so I fitted a 6 volt dynamo similar to the one pictured to the rear wheel of the bike that powered a red rear light and dim front one for free

No matter how fast I rode or turned the wheel with the bike upside-down the lights did not appear to get much brighter but upside-down and spinning the wheel really fast you could get a shock from the dynamo when the lights were not connected

I tried various 6V lamps and eventually found a pair that gave a good front and rear light balance but they were different wattage

The a.c. output from the bicycle dynamo can be far higher than its 'rated' voltage when not loaded but with the correct bulbs attached the light is fairly constant above an initial slow speed because the source impedance is mainly inductive and this impedance increases as the speed or frequency of current increases

In the case of our dynamo the the intention is to only have a coil of wire that generates an a.c. voltage but the wire also has series resistance R_D and winding it also produces capacitance across the coil ~ its impedance Z will be useful when analysing the dynamo looking into point b relative to chassis and more so at high frequency if X_C is included

Standard wire wound resistors also have some series inductive reactance X_L and to a lesser extent some parallel capacitive reactance X_C but are mainly resistive ~ The reactive parts of the resistor do not dissipate any power and neither does the reactance of L_D in the dynamo but they do influence the flow of a.c. current as frequency increases

The reactances X_L and X_C are measured in ohms Ω and like resistors oppose the flow of current but only alternating current ~ The value of X_L increases as frequency rises and X_C decreases ~ The actual values are given by X_L = 2πfL Ω and X_C = 1/2πfC Ω ~ but current flowing through either reactance will not produce power in it ?

The white trace below represents the current flowing in a 1Ω resistor in series with an inductor that has 1Ω reactance ~ The white trace can also represent the voltage across the 1Ω resistor because the voltage and current are in phase ~ 1V peak-peak produces 1A peak-peak and the real power in the resistor is 1W peak or 0.5W average ~ click the link to see I_R * V_R shown red

Where the resistor current and voltage are zero 3 times per cycle there is no power generated and in the inductor this occurs 5 times per cycle but what happens in between these points ?

Inductors oppose the flow of a.c by generating a 'back emf' which is maximum at the maximum rate of current change ~ in effect it generates its own rotating magnetic field and because of this a dynamo inductor cannot produce more power simply by turning faster even though the higher speed will have more energy

The formula for back emf with rate of change of current is V = –L(∂I/∂t) as the current is falling the voltage rises and multiplying current and voltage we get the power in the inductor ~ There is power generated between the 5 zero crossing points but unlike the resistors real power half of it is negative and so the average is zero

If the resistive power is said to be 'real' the inductive or reactive power can be referred to as 'imaginary' ~ The current through a capacitor is also 90˚ out of phase with the voltage across it and there are also 5 zero crossings and it too can be said to have imaginary reactive power when I_C * V_C is integrated over a whole number of cycles

An ideal inductor has only inductance and zero resistance ~ An ideal capacitor has only capacitance and infinite resistance ~ At any frequency including zero Hz the inductor cannot dissipate any power ~ the current through it would have to be infinite ~ In the case of an ideal capacitor the voltage across it would have to be infinite ~ This condition for zero power despite a current flowing only occurs when the voltage and current are 90˚ apart

complex easy

leave d.c. step to a.c.

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

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 circuit has a higher impedance passive RIAA EQ block 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 output

The open loop or A_o 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 ~ And also changing R6 to 1k would give about 0.7V out for 3mV input

Both circuits are d.c. coupled which is not desirable 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

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

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 can be 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 r_a of about 31kΩ for I_a = 1mA

The unbypassed cathode resistor R11 raises r_a to r_a' ≈ 126kΩ so the resistance affecting the EQ is 126kΩ||75k which is the required 47kΩ for this lumped RIAA block ~ 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 simple and the response of this EQ block predictable

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 ~ because 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 g_m valve with high I_a for best S/N

High g_m valves tend to have a low r_a 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 r_a' but at the expense of gain ~ The 1.8kΩ cathode resistor of the ECC81 stage above makes r_a' very predictable and stable but the 1kHz gain is only about 6dB

By placing all 3 (there are 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 whether using op-amps or other devices with large amounts of feedback

References and further reading:

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

ref.7 ~ Stanley Kelly ~ Stereo Gramophone Pickups (The State of the Art at the end of 1969)