Custom Phono Playback Equalization

The Bugle phono stage from Hagerman Technology LLC is designed to offer a flexible and customizable equalization. Figure 1 shows the simplified circuit. Each RC filter section implements a pole-zero combination that has a frequency response of the form shown in Figure 2.

Figure 1. Simple, passive, split phono equalization.

Figure 2. Frequency response of each filter section.

The transfer function is calculated mathematically by

where the characteristic time constants are given as

Knowing this, it is a relatively straightforward procedure to calculate the required component values for a desired response. The following table is an online interactive calculator that will do the math for you. Just enter standard capacitor values and the desired frequency breakpoints (turnovers). The default values shown are for modern RIAA. Table 1 lists the parameters for many of the "older" equalization curves. If the treble turnover is specified in terms of dB cut at 10kHz, use Figure 3 to convert to turnover frequency.

Calculate Filter Values

Reference Information

78 rpm Coarse Groove
Type	Bass Corner	Bass Turnover	Treble Turnover
Decca		150	3400
ffrr	40	250	6360
Westrex		200
Blumlein	50	250
BSI	50	353	3180
33 & 45 rpm MicroGroove
Type	Bass Corner	Bass Turnover	Treble Turnover
ffrr	100	500	3000
EMI	70	500	2500
NAB		500	1600
Columbia	100	500	1590
RIAA	50	500	2122

Table 1. Equalization parameters for popular formats

Figure 3. 10kHz dB-cut to frequency conversion.

Standard 1% Resistor Values
10.0	14.7	21.5	31.6	46.4	68.1
10.2	15.0	22.1	32.4	47.5	69.8
10.5	15.4	22.6	33.2	48.7	71.5
10.7	15.8	23.2	34.0	49.9	73.2
11.0	16.2	23.7	34.8	51.1	75.0
11.3	16.5	24.3	35.7	52.3	76.8
11.5	16.9	24.9	36.5	53.6	78.7
11.8	17.4	25.5	37.4	54.9	80.6
12.1	17.8	26.1	38.3	56.2	82.5
12.4	18.2	26.7	39.2	57.6	84.5
12.7	18.7	27.4	40.2	59.0	86.6
13.0	19.1	28.0	41.2	60.4	88.7
13.3	19.6	28.7	42.2	61.9	90.9
13.7	20.0	29.4	43.2	63.4	93.1
14.0	20.5	30.1	44.2	64.9	95.3
14.3	21.0	30.9	45.3	66.5	97.6

Table 2. Standard resistor values.

The accuracy of equalization depends on component tolerances. Here is a plot of what happens when nominal capacitances vary by +/-5%. Resulting error is roughly +/-0.4dB.