Compound Power Amplifiers

[Antoinel]

The F6 Amplifier thread is an intellectual stimulation and true inspiration. I understood the operation of its output stage, and thus generated a model Compound Power Amplifier. This is not a simulation; but a working power amplifier which can be assembled in a short time. Take a look at post #38 in the F6 Amplifier thread. In it I reported the schematic OTLAmp1.pdf of an amplifier [attached below]. Basically, I replaced the NPN output BJT [Q3] with one channel of a THRESHOLD S/150 stereo power amplifier. I characterize [S/150] as a phase non-inverting voltage source amplifier or [VSA]. Conceptually, [S/150] is a building block which models the upper JFET in the simplified F6 schematic. I also replaced the NPN output BJT [Q4, in OTLAmp1 schematic] with a phase inverting transconductance power amplifier [TCA]. This TCA happens to be a diy. Other famous TCA building blocks in the hands of DIYers will work equally well and better. Conceptually, this phase inverting TCA models the bottom JFET in the simplified F6 schematic.

The schematic of this Compound Power Amplifier is shown below as OTLAmp6A.pdf . I'll pause my discussion at this point for DIYers to mull over the strength and weakness of this Compound Power Amplifier. I assembled it a couple of days ago, collected the indicated voltage measurements, and listened to it [one channel]. Here is the pleasant surprise: It sounds great. Hint: Imagine using your choice of amplifier building blocks [VSAs and TCAs] like those found in www.firstwatt.com

[Zen Mod]

from quick look at schm - you are listening difference between these two blocks ?

[Antoinel]

Originally Posted by Zen Mod

from quick look at schm - you are listening difference between these two blocks ?

A closer analysis of the schematic [ATLAmp6A] shows that the measured output voltage to be the sum contribution of both amplifiers; just like in the output stage of simplified F6 which are are in-phase.

Trust me ZM, and I do not use this term loosely, I am listening [and you, other DIYers can easily too] to the sum and not the difference.

[kasey197]

Antoinel, sorry I don't get it.... The upper fet and lower fet in the f6 simplified schematic are both operating as transconductance amps and operate in the same mode if there is no nfb right .... In the classic Lin circuit they have different transfer functions but here I don't find that to be the case....

Notwithstanding that, this is an interesting experiment

[Antoinel]

Originally Posted by kasey197

Antoinel, sorry I don't get it.... The upper fet and lower fet in the f6 simplified schematic are both operating as transconductance amps and operate in the same mode if there is no nfb right .... In the classic Lin circuit they have different transfer functions but here I don't find that to be the case....

Notwithstanding that, this is an interesting experiment

I like these words by Mr. Pass from his recent post in the F6 Amplifier thread :

I just hook these things up and see what I get.

• The VSA [S/150] has a published voltage gain of [Vo/Vi] of ~21-22. From the schematic [Vo] = 0.61 V and Vi = 0.027V. Measured Av = 23; close in value within experimental error.
• The VSA [S/150] has transconductance too. First; its output current = 0.03V/0.5 Ohms = 0.06 A. Its transconductance or gm = 0.06A/0.027 V = 2 Ao/Vi
• Its output signal is in phase with its input signal or the signal phase at its secondary winding [solid triangles].
• The TCA inverts the phase of the signal at the secondary winding. But; the phase of the signal at its secondary winding is already out of phase with that at the upper winding driving S/150. So the phase of output signal from TCA is overall in phase with that emanating from S/150. A consequence of 2 successive phase inversions.
• The transconductance of TCA is calculated like that for S/150. First, output current = 0.033/0.5 = 0.07 A. The value of its tranconductance is 0.07 A /0.07 V = 1Ao/Vi
• The voltage output of TCA = Output current of 0.07 A times 4.5 Ohms [load resistor] = 0.3 V. Its voltage gain is Vo/Vi = 0.61Vo / 0.066Vi = 9; or roughly 1/2 the output voltage of S/150.
• Since the load resistance [4.5 Ohms] is lower in value than that of the TCA's output impedance, the current from S/150 [0.06A] sums with the contribution of TCA = 0.07 A to equal 0.13 A. This is sensed across the load as the output voltage = 0.13 A times 4.5 Ohms = 0.6 V as measured.

Clearly, I chose equal output currents to emanate from S/150 and TCA. I can choose other ratios [more or less from S/150] by tweaking the 25 Ohm potentiometer at the input of S/150. Or manipulate load resitstance [future post].

[Antoinel]

ACA [Amp Camp Amplifier] is a candidate to plug in the circuit of OTLAmp6A.pdf in post #1. ACA with loop feedback is a phase-inverting VSA. By contrast, ACA without loop feedback is a phase inverting TCA. The phase of the input signal to VSA [upper winding] now needs to be aligned with the phase of the input signal to TCA [lower winding]. The overall phase of the output signal is out of phase relative to that at the primary winding of the transformer. This ACA Compound Power Amplifier inverts the phase of its input signal.

[Antoinel]

The schematic which goes with post #6 is attached and is named OTLAmp6B.pdf. The only circuit parameters which are needed to write [and describe] it follow. They are published and/or known for ACA. Use them to understand the circuit.

• The closed loop voltage gain [Av], and transconductance [gm] of ACA operating as VSA.
• The transconductance [gm'] of ACA operating as TCA.
• The value of the load impedance. Use 4, 8, 16, and 32 Ohms. Why this broad range? Possible variation in the impedance of a practical loudspeaker load. Plug in the circuits parameters for each value of load impedance. Notice the relative contribution of the output current from VSA and TCA to the load. At each of the above load values, Vo will [and must] remain constant; because it cannot violate the constant voltage gain equation of VSA.

[Zen Mod]

check phase symbols on each secondary (not triangles , but sines)

[Antoinel]

So, the voltage gain [Av] of VSA is a constant [fact]. The transconductance [gm'] of TCA is simultaneously a constant too [fact]. By contrast, the impedance of the loudspeaker varies as a function of the excitation frequency in a predictable [measured] or not manner. What gives? Does this compound power amplifier named OTLAmp6B have an operational problem? Absolutely not. The answer lies in the transconductance [gm] of VSA. It will decrease as the impedance of the loudspeaker increases. This also means that the contribution of the current from VSA decreases as the loudspeaker impedance increases, and clearly vice versa. The output impedance of VSA is inherently low by design. Thus, VSA operates as a continuous donor/absorber of output current; a dynamic ballast of sorts.

So at a certain high impedance of the loudspeaker, the contribution of the output current by VSA will reach zero. At a still higher value of a loudspeaker impedance, the VSA will shunt the excess current [from TCA] or divert it from the load. Otherwise the resultant voltage drop across the load will violate the constant gain equation of VSA. Throughout this exercise, TCA did not give a damn. Its output current is only dependent on the value of its input signal signal.

[Antoinel]

Originally Posted by Zen Mod

check phase symbols on each secondary (not triangles , but sines)

Thank you ZM for catching my mistake. The corrected schematic is attached as OTLAmp6B1. The signals at the primary and the secondaries of the transformer are in-phase [triangles and sines]. The output signal of the compound amp is out of phase with the primary and the secondary signals [sine]



Noteworthy is that the general circuit format of OTLAmp6B1 has 4 possible variations. Thus, it is versatile. But first, the circuit constants are: a transformer with 2 independent secondaries, a VSA and a TCA. The 4 variations follow. In each variation, the circuit is made operable by manipulating the relative phase of the input signals [to VSA and TCA] at the secondary windings.

1. A non inverting VSA and a non inverting TCA.
2. A non inverting VSA and an inverting TCA [e.g. OTLAmp6A]
3. An inverting VSA and a noninverting TCA
4. An inverting VSA and an inverting TCA [OTLAmp6B1]

Interestingly, the four variations operate in a similar fashion.