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Class A Single-Ended Amp Choke Calculator

Design calculator for single-ended Class A amplifier chokes with DC bias

Core Dimensions

Standard imperial EI is named for the tongue: EI-87 = 0.875", EI-100 = 1", EI-150 = 1.5"
Typical: 0.0625" (1/16") to 0.125" (1/8")
Typical 0.90-0.95 for most laminations
View EI Lamination Reference

Design Parameters

Typical: 0.008-0.040"
1.0 = perfect, 1.15 = hand wound
Check if using spacer between entire E and I pieces. Uncheck if spacer covers center leg only.
The choke plays a different part in each, so output impedance is worked out differently.
0 dB = no feedback, typical 10-30 dB. Open-loop output impedance is the choke’s reactance, which feedback divides by 10dB/20.
Large-signal low end is a current limit, so how low it reaches depends on how hard you drive it. This applies to either topology: the device feeds the load and the choke from the same bias. Class A tops out at Idc²RL/2.

Design Guidelines

Formulas Used

Standard scrapless EI proportions (derived from Tongue = T):
Window width = T/2, window height = 3T/2 — the height is three times the width. Drawn upright, the width runs across the legs and limits how far the coil can build; the height runs along them and is the winding traverse, so it sets turns per layer.

Core Cross-Sectional Area:
Ac = T × Stack Height × Stacking Factor

Magnetic Path Length:
lc = 6 × T (where T = Tongue)

Effective Air Gap:
lg,eff = lg (center leg gap only) or 2 × lg (E-I spacer on all legs)
With E-I spacers, flux crosses two gaps in series

Apparent Permeability:
μapp = μr / (1 + μr × lg,eff / lc)
Effective permeability reduced by air gap

Inductance Factor:
AL = (μ₀ × Ac) / (lc/μr + lg,eff)
where μ₀ = 4π × 10⁻⁷ H/m

Required Turns:
N = √(Ltarget / AL)

Mean Length of Turn (MLT):
MLT = 2 × (T + D + 2 × tbobbin) + π × (hbuild / 2)
where T = Tongue, D = Stack Height, tbobbin = bobbin wall thickness
hbuild = number of layers × insulated wire diameter

DC Flux Density:
Bdc = (μ₀ × N × Idc) / (lc/μr + lg,eff)

-3dB Frequency:
f-3dB = Rload / (2π × L)