Parallel-Feed (Parafeed) Output Transformers: A Practical Guide

Jul 27, 2026 | 0 comments posted by Vincent Zhang

Published by iwistao · Tube Audio Engineering

How splitting DC and audio current across two magnetic parts changes the trade-offs of single-ended tube amplifier design.

In a single-ended (SE) vacuum-tube amplifier, the output transformer does double duty: it carries the tube's DC plate current and the AC music signal at the same time. That DC bias pushes the transformer core toward saturation, so the component must be large, air-gapped, and expensive to keep distortion low. The parallel-feed — almost always called "parafeed" — topology removes the DC from the output transformer entirely by routing it through a separate choke, leaving the transformer to handle only the audio signal [2][3].

This guide explains how parafeed works, where it helps, where it doesn't, and how it compares with conventional series-fed single-ended output stages. It is written for builders, hobbyists, and buyers who want the engineering substance rather than the marketing.

What Is a Parallel-Feed (Parafeed) Output Transformer?

The term "parafeed" is a contraction of "parallel feed," but the name can mislead as a circuit description. In this arrangement the plate choke provides the DC feed path from the supply to the tube plate, while the output transformer is AC-coupled from the plate node through a parafeed capacitor. The two are not simply two components wired directly in parallel; the choke carries the static DC current, and the capacitor feeds only the AC signal into the transformer primary [1]. The output transformer is therefore freed from the DC magnetization that defines conventional SE design.

In a standard transformer-coupled SE stage, the average DC plate current must be supported by the transformer, which forces a relatively large core so it does not saturate under DC alone [1]. Parafeed changes that constraint at the cost of adding a second magnetic component.

How the Parallel-Feed Topology Works

A parafeed output stage has three core parts:

A plate choke (a large air-gapped inductor) connected between the power supply and the tube plate. It carries the full DC current and presents a high impedance to audio frequencies.
A coupling capacitor in series with the output transformer primary.
The output transformer itself, now free of DC current.

Because the DC plate current flows through the choke and returns to the supply, the output transformer's primary sees only the AC signal. The capacitor blocks DC, while the choke's high AC impedance keeps most signal current in the transformer rather than the choke [1]. The transformer's secondary then drives the loudspeaker as usual.

This single change reshapes the engineering trade-offs. In a conventional SE stage, one component must satisfy opposing demands: it needs enough iron to avoid saturating under DC, yet a small core and tight windings for wide bandwidth and low capacitance. Parafeed lets the choke and transformer each be optimized for their actual job [5].

Figure 1: Simplified parallel-feed (parafeed) output stage. The plate choke carries DC from the supply; the coupling capacitor feeds only the AC signal to the output transformer. (Diagram by author)

Key Advantages

No DC in the output transformer. Removing the DC magnetization means the core needs no SE air gap, which can raise the primary inductance and avoid the core-bias problems an air-gapped SE transformer faces. As a rule of thumb, an OPT built for parafeed may be similar in size to a push-pull unit of the same power, whereas a conventional SE transformer is typically larger because it must also accommodate the static DC flux [4]. This is an empirical guideline rather than a fixed law; actual size and performance depend on the specific OPT, plate choke, and coupling-capacitor design.
Potential for lower transformer distortion. With the DC bias removed, the transformer no longer walks asymmetrically into saturation on signal peaks — a mechanism that contributes odd-harmonic distortion in conventional SE stages [2]. This does not by itself guarantee a lower-distortion amplifier; the final result depends on how the OPT, plate choke, and coupling capacitor are designed and how their resonant behavior is managed.
More design freedom for bandwidth. Without DC forcing an air-gapped core, the transformer designer has greater freedom to optimize leakage inductance, primary inductance, and winding capacitance [1]. Combined with the coupling capacitor and choke inductance, this adds design "degrees of freedom," and their resonance can be used to extend low-frequency response. Whether the actual bandwidth is wider still depends on the complete OPT, choke, and capacitor combination.
Better power-supply hum isolation. In a standard transformer-coupled stage, supply ripple divides across the plate resistance and load, coupling hum to the output. In parafeed, the ripple drops across the choke and is largely kept away from the transformer, reducing hum — especially valuable with low plate-resistance tubes [1].
Option to use an autoformer. Because no high-voltage DC sits across the output winding, designers can substitute a tapped autoformer, a further optimization [1].

Trade-offs and Limitations

Parafeed is not free of compromise:

Two magnetic elements instead of one. You trade one large, air-gapped SE transformer for a choke plus a smaller OPT. The combined iron, weight, and cost are often similar or slightly higher — Jacmusic's 300B example shows a parafeed pair at about 9% more cost than the equivalent single SE transformer, for lower distortion and roughly double the power-handling headroom (55 W vs. 25 W transformer rating) [2].
A capacitor sits in the signal path. The coupling capacitor carries significant AC current, and its non-linearity can add coloration if poorly specified. It must be a high-quality, generously rated part [1].
Careful tuning required. The parallel inductance of choke and transformer, plus the coupling capacitor, creates resonant behavior that can cause frequency-response dips or peaks if not designed deliberately. Choke and OPT are usually specified and sold together for this reason [3].

"Parafeed splits the opposing demands on the typical SE transformer to handle both AC flux and DC flux… Each component is designed for its current load, without the offsetting compromises from an OPT that must handle both." — AudioCircle builder discussion [5]

Design Considerations

Plate choke selection. The choke must handle the full DC plate current without saturating and provide high inductance (typically several henries) at audio frequencies. It tends to be physically similar in size to a conventional air-gapped SE transformer because it carries the DC [4].
Coupling capacitor value. The parafeed capacitor does not follow a "larger is always better" rule. Together with the plate choke inductance and the load, it forms a low-frequency network with an optimal value or design range rather than a single maximum. Paul Joppa of Bottlehead gives the rule C = 2·L / R² (L = plate choke inductance in henries, R = OPT nominal primary impedance in kΩ, result in µF), describing it as a compromise between small-signal bandwidth and power bandwidth [6]. The capacitor's AC current rating also matters more here than in a typical line-stage position, so film types or high-quality bipolar electrolytics are common choices.
Output transformer rating. Size the parafeed OPT like a push-pull transformer of the target power, not like a conventional SE unit [4].

Parafeed vs. Conventional Single-Ended: A Comparison

Aspect	Conventional SE (series-fed)	Parallel-Feed (Parafeed)
DC in output transformer	Yes — requires air gap	No
Relative OPT size (same power)	Typically larger (must also hold DC flux)	Similar to a P-P unit (rule of thumb)
Typical distortion	Higher (odd harmonics from core bias)	Potentially lower (depends on design)
Power-supply hum coupling	Directly divides to output	Largely isolated
Magnetic parts count	One large transformer	Choke + smaller OPT
Cost / weight	One heavy part	Two parts; ~similar or slightly more

Table 1: Practical differences between conventional series-fed SE and parallel-feed output stages, based on documented measurements and builder experience [2][4].

Is Parallel-Feed Right for You?

Parafeed suits builders and manufacturers who prioritize low distortion and clean low-frequency behavior and are comfortable with the extra parts and tuning discipline it demands. It gained modern popularity through Bottlehead's Paramour 2A3 kits — engineered with Paul Joppa's input and MagneQuest's choke work — and remains a favorite among DIY single-ended triode enthusiasts [5]. For mass-market products, conventional SE is often chosen simply because it is easier to explain and market [5].

Frequently Asked Questions

Does parafeed eliminate the need for a large transformer entirely?

No. It replaces one large air-gapped SE transformer with a plate choke (similar in size) plus a smaller OPT. Total iron is often comparable [2][4].

Isn't the coupling capacitor a problem?

It does sit in the signal path and must carry substantial AC current, so part quality matters. Properly specified, its effect is generally considered linear; the main risk is low-frequency rolloff from too small a value [1].

Can I use a push-pull output transformer in a parafeed design?

Potentially, yes. The core reason is that the OPT carries no static DC, so it does not need the air gap that a conventional SE transformer requires to hold one-directional DC flux [4]. A push-pull transformer can therefore serve as the parafeed OPT — but only if its specifications fit the circuit. Primary impedance, power handling, primary inductance, frequency response, and turns ratio must all be appropriate for the application, so suitability should be checked case by case rather than assumed.

Who popularized parafeed in modern hi-fi?

Modern commercial parafeed traces to Bottlehead's Paramour 2A3 amplifiers, with key contributions from Paul Joppa and MagneQuest's Mike LaFevre [5].

Is parafeed only for single-ended amplifiers?

The principle applies to both SE and push-pull output stages, though it is most discussed in the SE context where DC magnetization is the central pain point [1].