some years ago an English Hi-Fi magazine reviewed a locally (UK) made low power
amplifier (I think it was 15 Watts) powered by batteries. This was not a cheap
design, it sold for £1500. Internally
there were only two devices, one monolithic power amp chip per channel
(from here on will be called the Gainclone IC).
Now there has been amplifiers, especially integrated types from oriental
sources, that have used IC power amp modules. But what made this different
was the rave review, it was to all who heard it, evidently a
real surprise packet.
Again, in more recent times, an unusual small scale
specialist Japanese manufacturer also brought out a product using the
exact same IC power chip. But this time it was presented as a one only
input integrated amplifier. This was done by adding a 12 position
attenuator ( 24 position would have been better). The asking
price for this combo including external power supply (not battery operated) was
then US$2700, but now US$3300.
It is apparent these products were aimed at the hard core audiophile
fraternity, especially for those who may also be interested (even
intrigued) by possible alternatives to SE Triodes. These products were
The pricing of these products were not
based on the basic costs of parts, but rather that the sound quality
made it possible to exploit such a situation. Some
have called it a rip-off. Those are harsh words.
Could someone bring out a reasonably priced version, not just a copy
but an attempt to further the art?
The story does not end here. This essay is also about tubes and how
does this fit into this scheme? Bear with me.
Here is a basic recommended circuit of the
I would now like to analyse the above. The circuit uses
conventional shunt feedback. This has some tricky drawbacks.
For one, notice C2, this has to be an electrolytic
cap, and ideally a larger value than shown. Why is C2 used? This is part
of keeping the DC Off-Set into the speaker load down to a few mV. C1
also decouples non-inverted input (+ pin 1) to DC. There are good reason
why R2 and R4 has similar values, the input current on both inputs are
almost the same (I measured it) and consequently the difference is not
amplified by the gain set by R4 & R3. So in this way DC Off-Set is indeed
How can we get rid of C2? We can by using series
Let us look at the alternative, the inverted variation:
Looks a wee bit different (thanks to Thorsten Loesch
for this circuit). Now our signal is
going to the inverted input (- pin 2) but it itself has also become part
of the feedback path. We have also eliminated the Electrolytic cap.
The 2.2uF input film cap is the minimum value and can even be made larger. Again notice
both 220K caps, the fact that they are identical, along with our input cap, ensures
very low DC-Offset. The 0.1uF is there to ensure our non-inverted input
(+) is AC signal grounded.
So what are the pros and cons?
It has already been established that series feedback (inverted)
has the potential to sound better than
equivalent shunt version (non-inverted) - the reasons for this was
established by Lindsay Hood and others. Yet, for its superior qualities, it is rarely used,
because the low value input resistor defines the
input impedance. So
by nature the input impedance is low. Hence feedback values needs
to be scaled up much higher, typically ten times as much. This makes for pour
noise figures in low level circuits and that is why series feedback is
rarely used in circuits for mics or phono cartridges. But in power amps,
this is not problematic where signal to noise is good to start with, due
much higher signal levels.
There is also the question of the source (preamp or CD in some
cases). This now becomes, arguably I know, part of the series feedback loop, so it has to be
Lo-Z. This means that using 1 Meter interconnects the feedback loop is now
more than 1 Meter long... more about this later.
The other con is that, in practice, the series feedback amplifier is
far more tricky, the layout, the earth arrangement and wiring become even
So what is it about the Gainclone
monolithic chip that makes it special. Let us take a look at its internal
circuitry. This is the simplified circuit:
Do we note anything? Starting at the inputs, emitter
followers with individual current sources, then a differential stage also
current sourced, indeed everything up to the final output stage is current
sourced! In fact you can count no less than six of them. For those not in
the know, this generally implies good things and also that all stages up
to the output stage are in Class
This helps to explain why
the Power Supply Rejection (PSR) is typically 120dB. That is a ratio of a
million to one! Finally the output stage itself is not fully
Complimentary, but Quasi! This avoids the use of PNP bi-polar devices.
There are still those that say NPN/PNP complimentary
'power pairs' are never truly
such. They say that the NPN will always be superior. No such problem here. So our
internal view of the Gainclone
IC shows some significant reasons why it has
the potential to sound as good as it can. Many discrete circuits are less
interesting than what is on show here.
So What About Tubes & Gainclones Together?
Now we come to the crux of the matter.
This is the current JLTi
Mark 2, simplified circuit.