56 | April 2014 | audioxpress.com
This can be achieved using IXYS's constant-current
source (CCS) circuitry. I did not recommend this circuitry
information in my original article because it was not
widely available (even though the original prototype
was tested that way). Many audio enthusiasts have
discovered this CCS, IXYS's IXCP-10M45S, which can
be used as a valid alternative to cathode resistor R7.
The CCS can also be used with the existing circuitry,
PCBs, and power supplies.
I built my Mugen ampliﬁer in 2007 and originally
published the design in Elektor (October 2007). The
Mugen Hybrid Ampliﬁer design replaces the output
valves and transformer with a solid-state circuit
using modern transistors, which can directly drive
a loudspeaker (see Photo 1). However, the original
prototype used well-priced uncommon and unknown
tubes that had great speciﬁcations. The prototype
used a 5965 tube for the long-tail driver and a 6BQ7A
The 5965 is well suitable for audio purposes. I used
as long-tail driver (tube V1 shown in Figure 1). It pro
vides enough gain (mu of 47), relative low output imped-
ance, and low distortion. The 5965 is also pin-compat-
ible with the ECC83 family. For optimal use the anode
(R3 and R4) is changed to 47K and the anode
current is set to 3.5 mA. I used P1 to set the test point
TP3 to 3 V. As a long-tail driver, the 5965 does not
provide enough gain to be used with overall feedback.
This is ﬁne because I don't think overall feedback is
necessary using these conﬁgurations (see Photo 2).
The 6BQ7A has good specifications and is an
excellent choice as cathode follower (see Photo 3).
Photo 1: The Mugen
got its name from
the Japanese word
inﬁnite or endless.
Photo 2: The ampliﬁer design uses a combination of transistors and valves.
Wim de Haan
The Mugen Hybrid
Ampliﬁer's Driver Update
Figure 1: The design details for the updated Mugen Hybrid Ampliﬁer are shown.
My Mugen Hybrid Ampliﬁer project published in audioXpress October 2013
relies on common, widely available tubes—the ECC83 and the ECC88—for the
driver section. In this article, I am providing an alternative that will enable the
distortion to become less than 0.1%, up to 80 W, without overall feedback.