DEF Amp

Part 1. Amp with DEF therein wired in common gate configuration

Positive Current Feedback [PCF] continues to be of interest. Diyer vaskozl asked to understand how positive feedback increases damping factor in post #1115 of the "First Watt F7 review" thread. Pass and juma replied with their understanding in posts #1118 and #1117 respectively. I hope to use their explanations in the next two posts to show the presence/use of PCF [or not] in the two amps containing DEF wired in the common gate configuration. Here's a brief review from this current thread:

1. I speculated about PCF in the attached schematic of Post # 245.
2. The attached schematic in post # 267 does not use PCF.
3. The attached schematic in the recent post # 280 is alleged/expected to use PCF.

The attached and simplified schematic [of an actual proto amp] is like that shown in post#267. It is the "baseline/reference" which does not use PCF. Please note the following:

1. The left of the view shows a power output stage of the voltage source amp [VSA] which drives DEF [right view] that is wired in the common gate configuration.
2.The complementary BJTs of the power out stage or buffer are forward biased with batteries of Vbe= 0.6V. Let's make their idle current or bias = 1 mA for example and simplicity.
3. The PSU of this VSA is a dual polarity +/- 50 Vdc which is used as an example to differentiate it from the independent PSU of DEF.
4. An 8 Vdc input signal is offered to the bases of the output stage BJTs.
5. It follows that ~ 1 Adc flows out of the NPN emitter.
6. This exercise follows this ~1 Adc output current from its PSU origin[ #1] and back to its resting place origin [#19] at the same PSU.
7. So, follow the increasing numbers from the Start[ 1] at the positive pole of the upper PSU to its Finish [19] at the negative pole of the same upper PSU.
8 The flow of current which is indicated in point 7 is the only "route" this current follows to energige both loads [twin loudspeakers] as I've explained in post#267.

Three important connections are observed:

1. This amp [meaning VSA and DEF combined] does not employ PCF as it is formally understood.
2. The lower PSU of DEF works to only "move" the ~1Adc of VSA's NPN output. It is a potential [voltage] driving force" which does not alter the magnitude of the aforementionned ~1Adc.
3. PNP in VAS has the only function to stay or be off during this application. No current flows in/out of its emitter to energize the loads.

Part 2 will follow for the identical amp "re-wired" with PCF.
 

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Part 2. Prototype amp with DEF therein wired in common gate configuration

The attached schematic is for a bench prototype. Its operation had perplexed me for some time; but it became clear by following the route or flow of the "load current". Please note the following:

1. The left side of the schematic shows the OpAmp [LF356N] connected as a buffer. Its [+] input is driven by a battery [Vin signal] so as to get a monopolar flow of load current as an example to grasp the operation of the amp.
2. The output of LF356N drives the joined sources of DEF through the lowered Load = 8 Ohms. The [*] signifies a phase dot when the Loads = 8 Ohms are loudspeaker.
3. The right side of the schematic is the DEF circuit. The gate of each FET in DEF is AC grounded with a 220 uF capacitor.
4. The resistor ladder generates the required -Vgs values for the FETs.
5. This resistor ladder and DEF operate as a DC servo to minimize DC offset at the center point of their PSUs. I find this servo quite effective when DEF is used in a common source or in common gate configurations.

LF356N is considered in this application to be a voltage source micro-power amp.
1. The positive voltage at the output of LF356N increases the value of /-Vgs/ for P-MOS.
2. This turns on [ups the conduction] P-MOS further due this monopolar positive-valued input signal.
3. Follow the route of the resultant "load current" which starts at # 1 of the positive end of the lower PSU.
4. This load current continues its path to energize the upper Load= 8 Ohms, the lower Load = 8 Ohms, enters the source of P-MOS, exits the drain of P-MOS, and finally rests at #12 which is the negative end of the same lower PSU.

The above proto amp plays clean and loud music. DEF was the only source of the "load current" energizing the two loudspeakers.

LF356N micro power amp has a low ouput impedance which affected favorably the damping of the loudspeakers' woofers. The damping of the loudspeakers was further enhanced by using power amps way more powerful than LF356N like Threshold S/150 etc...

These powerful amps driving DEF in CG configuration play the same role like that of LF356N. None of them push current in/out of the joined sources of DEF; meaning DEF is the only source of power [current] output. The power amps weak and powerful manage to their abilities the damping of woofers.

I'll show in the next post the same schematic with an input negative monopolar signal for completeness. Plus additional conclusions regarding the original question of whether the operation of this DEF is a case of Positive Current Feedback [PCF].
 

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The attached schematic shows two objectives.

1. The flow of current or its route when the joined sources of DEF operating in common gate are energized with a negative unipolar signal.

Thus, the bottom left view of the schematic shows that this NJF [R085] becomes more conductive by the negative unipolar input voltage source. Follow the route of the resultant current from #1 at the positive lead of the upper PSU to the same PSU's negative port at #12.

2. The second objective was to get a semi-quantitative read of the music current/voltage [or power] offered by LF356N to the joined sources of DEF.

Thus the circuit at the upper left of the schematic was connected instead of the lower unipolar input source signal. A small signal transformer was used inside the feedbackl loop of LF356N to sense and ~measure [via scope] its output music current granted to the joined sources of DEF. The magnitude of the resultant music current was trivial or minuscule and was in the order of less than of 0.1 mA ACp-p at listenning levels.

The signal transformer is an old-fashioned [milliwatter] used in the hayday push-pull output stages of hand-held/pocket transistor radios. Its 8 Ohm secondary coil [L1] is inside the feedback loop of LF356N, and the loaded L2 winding [1 K Ohm] is connected to the scope. N.B. Its center tap is not shown.

Conluding:
1. Frankly, I don't see evidence of PCF in this application; just the normal currents circulating around.
2. I was surprised at the low music power offer by LF356N to DEF. LF356N "did not do work" anywhere near that of DEF. This reminded me of the requirements of the STASIS [section] by Mr. Pass. Low ouput impedance [at the output of LF356N], its trivial work [power] in the loads, and the preservation of the signal fidelity.

So the subject posts suggest biasing DEF low [Class A/B] or high [a la Pass] , and using a high fidelity music signal driver which can be a low or higher power amp for enhanced woofer damping.
 

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A High Fidelity DEF Amplifier

The attached schematic is for a prototype amp which uses DEF therein. It mimics the performance of Threshold's "STASIS" aka Constant Voltage-Constant Current High Fidelity Amplifier taught by Nelson S. Pass in US 4,107,619.

The schematic is understood as follows:

1. A high fidelity amp is shown on the left side of the view. It is modeled as a generic voltage source amp which uses overall negative feedback.
2. It can be any amp of your choice. Any Class of operation for solid state or vacuum tube. Its output [Vout] can be directly or capacitor, or transformer coupled to the next DEF stage.
3. It also can be a very low power [signal] amplifier or another of higher output power.

4. An idealized DEF stage is depicted for simplicity. Its actual schematic was shown in the previous post.
5. The quiescent conditions for its PSU of +/- 35 V and a bias [idle current] of +/- 0.2 A are actual.
6. DEF operates in the common gate configuration.
7. The output current of this DEF stage appears at Vout which is the center point of its two PSUs, and simultaneously the Vout of the high fidelity driver amp.
8. The output current of DEF bootstraps or pushes back against the output current of the high fidelity amp driving it.
9. Note the mechanical switch at the joined sources of DEF.
10. If/when this switch is open, the high fidelity driver amp is denied injecting any of its output current into the DEF circuit. Because there in not a path for it [current] to return to the PSU of the high fidelity driver. Thus, the input impedance of the DEF stage [at the joint Vout] is extremely high from the perspective of the high fidelity driver. Meaning this high fidelity driver can be low power or signal-style.
11. If/when the mechanical switch at the joined sources of DEF is closed, the high fidelity driver amp is allowed to infuse a low magnitude current into the DEF circuit. The magnitude of this current is small compared to that from DEF. It finds its way back to the PSU of the high fidelity driver via ground.
12. One has a choice to adjust the magnitude of the current infused by the high fidelity driver amp by using lower valued joined-source resistors to ground. The extent of injected current may consequently affect the sonic performance of the overall amp. Probably by imposing the sonic qualities of the high fidelity driver of DEF.

I am using a Threshold S/150 as the high fidelity driver to DEF. It was used strictly as a signal source [switch open], and as a low power driver [switch closed]. Subjective performance was superb either way.

I have used other power amp drivers to DEF which have a high fidelity signature that is different from that of S/150. The resultant performance of the overall amp [driver + DEF current bootstrap] was way better subjectively than that of the high fidelity power amp operating solo.
 

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Positive Current Feedback [PCF] Inside DEF

Thanks to Mr. Pass, and the moderators for allowing me to continue exploring DEF applications which are/maybe outside the scope of the parent subject. Fortunately everything I know that is DEF is together inside this thread.

The attached schematic is for the high fidelity power amplifier which I described in the previous post PLUS added Positive Current Feedback [PCF] inside the DEF current bootstrap stage. Here are the details:

1. The left view shows a high fidelity power amp [HFPA] which drives the DEF stage.
2. HFPA is shown as a signal generator of power output = V3.
3. DEF is a current bootstrap which operates in the common gate configuration [DEFcg].
4. The output current of DEFcg appears at/as V3 and drives the load = 8 Ohms.
5. The joined sources of DEFcg have a finite low "input" resistance.
6. A consequent voltage drop is symbolized as [^] and appears at the joined sources of DEFcg.
7. This voltage drops [^] is proportional to the current [predominantly from DEFcg bootstrap] flowing through the load = 8 Ohms. This is a prerequisite for enabling PCF.
8. The circuit which enables PCF is sandwiched between the joined sources and the joined gates of DEFcg. Focus on P-MOS.
9. The phase of sensed voltage [^] at the joined sources of DEFcg is inverted by OpAmp OPA134 of shown voltage gain ~2 [symbolized by V] , and then fed to the joined gates of DEFcg.
10. Here's the PCF process; the simultaneous out of phase voltages [at the sources and gates] essentially increase the absolute value of [-Vgs] for P-MOS to cause an increase in its current gain relative to the base situation. A gain increase is a second prerequisite/manifest for PCF.
11. The following was observed. The resultant steady state voltage at the joined sources decreases, and its inverted voltage appears/increases at the joined gates of DEFcg.
12. There was a measured 3% net increase in the magnitude of the AC signal between the sources and gates of DEFcg after feedback compared to the base state without feedback.
13. The magnitude of the power output signal [V3] was not affected by this PCF. It should not because of the control imposed by the fixed voltage gain of HFPA.

This amp sounded great, smooth and fast. I'll report in the next post about the HFPA that I'm using in this prototype.
 

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The acronym DEF is a specific and special device as defined by Mr. Pass. The attached schematic shows a look-alike arrangement of DEF which uses a TIP 125 PNP Darlington and an LT317A positive voltage regulator. Here are its details.

1. This is the prototype High Fidelity Power Amp [HFPA] which I mentioned in the previous post.
2. It is the signal generator which drives the DEF bootstrap stage per the previous schematic.
3. The voltage at the [Adj] point of [LT317A] is inherently [ -1.25 V] relative to its output node.
4. The voltage at the base of TIP 125 is inherently ~[ -1.25 V] relative to its emitter.
5. Thus LT317A and TIP 125 were wired in this HFPA like that proposed for DEF by Pass.
6. The performance simulation by LTSPICE ran without difficulty and with excellent results.
7. I show a load = 16 Ohms instead of the traditional 8 Ohms. A hint that this output stage will see a light load which can still be higher than 16 Ohms. The output stage of this HFPA is expected to operate at constant current [or its idle current or bias] and to only use a trickle of it to drive the DEF bootstrap.

An HFPA using a DEF output stage is in the works.

N.B.
1. Circlomanen had suggested in a past post of the diyF6 thread that the voltage regulators 317T/337T can/do behave as depletion devices.
2.The example I used for P-MOS in the schematic of the previous post suggested that P-MOS with Positive Current Feedback [PCF] appeared to simultaneously operate in a common gate PLUS a common source configurations. Both configurations harmoniously working together.
 

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Ah, now the bipolar hybrids appear. I've been sitting on a DN2540-2SA1837 DEF-like circuit for headphones for several months, but have never got around to prototyping it. Simulates fantastically and uses the base current of the PNP to set the bias/offset.

Hello needtubes. The application scope of DEF, and its DEF-like circuits is broad. A testament to DEF's simplicity and robust concept.
 
Another application of a power amp using DEF wired in common gate configuration

I will post a "listening application" for this amp in the thread entitled DIY the device of US Patent 4,899,387 which is found in the current forum.

The salient features of this amp are shown in the attached schematic:

1. Detailed values are shown for resistors, capacitors, and idle operating conditions.
2. The series resistor string which generates Vgs for the FETS, starts at Vout [center of the two ~+/-36 V PSUs] and terminates at a separate regulated PSU [-21 V]. Current flowing through this resistor string is ~1 mA. This biasing arrangements minimizes DC offset at Vout with some help from the 5K pot DC offset adjuster.
3. A dual voice coil [DVC] subwoofer is the load to the overall power amp [OPA]. OPA is a voltage source amp [VSA] which drives DEF common gate amp [DEFcg].
4. VSA drives voice coil #1 of subwoofer to common. VSA is also connected directly to voice coil #2; but it does not actually drive it like it drives voice coil #1; because
5. DEFcg drives voice coil #2 exclusively because it is a current bootstrap which prevents/blocks VSA from independently driving voice coil #2.
6. Voice coils #1 and #2 are wired in phase [note the phase dots on the coils].
8. Another OPA was also wired by using BJTs only instead of DEF FETs in a similar DEFcg-like circuit.
7. Both OPAs are stable and sound excellent in the final listening application.

The subwoofer and VSA will be described in my upcoming post in the thread DIY the device of US Patent 4,899,387.

Best
Anton
 

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Versatile [fresh] applications of DEF and DEF-like amps

I am a bit uneasy [yet apologetic] using DEF-like circuits in this DEF thread. But I'm thankful to Mr. Pass and the moderators for allowing this. The attached circuit is DEF-like. I have it in LTSPICE to model the equivalent [and genuine] DEF circuit. It runs without a problem in LTSPICE. Its components are already in LTSPICE; meaning I do not have to import parameters for the FETs which I frankly find complicated. But I'll learn to do this from the thread of DIYer Mooly.

The DEF-like and the genuine DEF prototype amps of the attached schematic have been assembled and are fully operational. Here is an explanation of the schematic and its versatility of application:

1. The input [Vin] can either be from a Power Follower [Voltage gain =1] and/or from Standing Waves in a cubic listening room. That's versatility!

2. The loads which are inside the schematic are the two voice coils [L1 and L2 each = 0.5 mH plus 8 Ohms] of a hypothetical dual voice coil [DVC] subwoofer.

3. Note the encircled components. This applies to the physical amp. Open the mechanical switch to disconnect the power follower at Vin. Disconnect the 25 Ohm resistor at Vout. Disconnect the Zobel [10 Ohm in series with 0.1uF] at Vin.

4. Slowly increase the PSU's voltage of the power rails. I use a Variac for the PSU, and practice this with every circuit change I am describing below and/or as a matter prudence.

5. This DEF-like and genuine DEF amps oscillate at even a +/-2-3 Vdc power rails. Not good!

6. The oscillation happens when L1 and L2 are wired as indicated [phase dots opposing each other]. And when L1 is wired the other way relative to L2.

7. Closing the switch silences the oscillations which happen as described in point #6. Good; but not enough!

8. Next open the switch to disconnect the power buffer which has a low output impedance [~0 Ohms]. Add the Zobel at Vin and the 25 Ohm resistor at Vout. They silence the oscillations which emanated in the circuits of point#6.

9. The value of this 25 Ohm resistor was the important contributor to quieting oscillations. It has an upper limit. It was determined by starting with low values and increasing it to the point where oscillations began to set in [as in point #6] at full PSU voltage rails.

10 The oscillations in the circuits of point #6 were a consequence of "too much voltage gain" in the actual amps/actual DVC subwoofer used.

I now have the following two great [amp-subwoofer combination] systems to experiment with and application test. Their application and subjective performance will appear in the thread "DIY the device of US Patent 4,899,387".

1. The DEF-like power amp/DVC subwoofer which uses Standing Waves [SAs] only for generating its input source signal Vin. The DVC subwoofer is in a corner of the listening room where SAs are found. SAs are intentionally generated by listening to another sound system [#2 below] in the same room. SAs push in and pull out the cone of the DVC subwoofer which essentially acts as a microphone. Thus, the motion of both coils [attached to the woofer's cone] in the magnetic field of the magnet generates the source signal Vin. Will this system absorb standing waves which are offensive?. TBD.

2 The genuine DEF power amp/another DVC subwoofer with a a Power Follower as the music signal source Vin. The second DVC subwoofer [and satellite] sit in a second corner [same wall] of the listening room. Will the cone of this DVC subwoofer independently also act as microphone towards SAs in its corner too?

I owe this thread the schematic of the Power Follower. A voltage gain of 1 for this power follower is all that was needed to drive the DEFcg proto amp. The output of a SONY CD player was the input to the power follower. DEFcg and DEFcg-like amps have a high voltage and a high power gains. Run the attached LTSPICE to confirm

To be continued..
 

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The schematic on the left is for the Power Follower [Buffer] which I mentioned in the previous post. N-channel mosfet IRFP140N [actual] is in the feedback loop of [OPA134]. The load to this mosfet is LM317T which is wired as a constant current source idling at 300 mA.

I need to emphasize several points in the second attached schematic. It shows two DEF-like circuits in it. Note the title of each:

1. The title to the left schematic says its amp uses Positive Feedback. Because the two voice coils [encircled] of the Dual Voice Coil [DVC] subwoofer [load] are wired as shown. The electrical currents which flow through them are in phase.

2. The title to the right schematic is DEF-like Amp using Negative Feedback. Because the two voice coils [encircled] of the same DVC subwoofer load are alternatively wired as shown. The electrical currents which flow through them are out of phase.

3. Both circuits oscillate in the absence of the Power Follower [Buffer] as I mentioned in the previous post. The high impedance of each voice coil at the resonance frequency is a suspected culprit. Because circuit gain becomes high especially for the [left-side] coil loading the common base [gate] amp; which is a current source. I prefer the genuine DEF and the DEF-like prototype amps which use Positive Feedback only. Each has a clean sound from top to bottom, and will not oscillate in application. Most likely because a part of their operation conforms and/or is like that of the device taught by Mr. Pass in his US Patent 4,899,387.

4. I will add further details about these amps in their application thread " DIY the device of US Patent 4,899,387".
 

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FYI: Re-emphasizing the value of Post # 284


I just posted the attached schematic in the Thread by DIYer DieterK which is entitled "Paralleling a tube-amp with a class d amp? of the Tubes / Valves Forum.

The prototype amp option is feasible and trustworthy. It works very well because it is based on the US Patent 4,107,619 by Nelson S. Pass. It may suit DieterK's diy need. It also sounds great too.

I do not have a vacuum tube voltage source amp [VT-VSA]. So I substituted instead a Class aP prototype amp for now. It is transformer-coupled to the loudspeaker like that of a classical Class AB push-pull VT-VSA. The transformer component was important and needed to confirm the broad versatility of this suggested approach.

Notes:

1. See the recent last two posts #213 & 214 of the Thread "Class aP amplification" [This Forum] for the schematic and works of the prototype amp which I used.

2. The Constant Current inventive component of US 4,107,619 was apparent by measurement [on the scope]. It was manifested as a ~tenfold decrease in the current processed/used by the gain FETs [UJN1208K]; meaning due to bootstrapping by DEF as compared with the same Class aP amp operating without it or solo....to the same sound level by the loudspeaker. Further cascoding [of UJN1208K] refined/elevated the status of the "hybrid" amp to that taught in US 4,107,619.

Best
Anton
 

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New DEF schematic and application thereof

The attached is a simplified schematic for a bench prototype. I am listening to it. It shows the following:

1. Two secondary windings [L1 and L2] of the audio output power transformer [OPT; TRIAD VPT24-4170] are embedded in the DEF amp.. The two secondaries are wired in series. Note their relative phase which enables positive feedback in DEF.

2. The audio OPT is separately driven by a Class aP amp. It can also be the OPT of a classical push-pull Class AB vacuum tube amp. Or other..

3. Secondary #1 simultaneously drives loudspeaker #1 [ADS L730] and the DEF FETs. This loudspeaker sits in the right corner of the listening room.

4. Secondary #2 drives loudspeaker #2 [ADS L730]. Additional current from the joined drains of the DEF FETs also flow through both, and enables the induction of positive feedback in the secondary #1. This second loudspeaker sits in the left corner of the same room wall.

5. This is a mono amplifier. I am using loudspeaker #2 to get more sound pressure because there's power output from secondary #2 of OPT and DEF.

6. The DEF circuit has two feedback loops which are essential. The joined sources of the DEF FETs are the "non-inverting" port; because they operate in common gate configuration. They enable an in-phase current to flow through the FETs and the secondary #2 to induce positive feedback . The joined gates of the DEF FETs are the " phase inverting" port; because the FETs are operating in common source configuration.

7. One must balance the extent of the resultant positive and negative feedback; otherwise the system will spontaneously sing.


What is the value of the resultant sound system [aka the application?]

1. The bass is enormous; deep and articulate. No need for a sub-woofer.

2. What standing waves [SWs]? The loudspeakers' woofers simultaneously play music and wipe out SWs in the corners; because of a net positive feedback in the DEF amp as taught in the Pass US patent 4,899,387.

3. I sit smack in the middle of the sound room. The wall that I face and the loudspeakers is illuminated [a term used by S. Linkwitz] with a clean and detailed sound across the audio spectrum. It can only emphasize the absence or high attenuation of SWs/ room-induced resonances. I do not hear any sound filth from the room!

This system has high value. The resultant solution is a nexus of uncluttered loudspeaker placement, bass enhancement and the cancellation or attenuation of SWs in one package. It sounds superb.

I'll post the full schematic.

N.B. Negative feedback from Vout to the joined gates of the DEF FETs is believed to convert their pentode-like characteristics to that of triode-like VFETs. Meaning they are simulated SITs, and operate accordingly. Another vector to add to the vectors of the corner-placement nexus.

Best

Anton
 

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The complete schematic of the sound system is attached. Please note the color codes of the audio output transformer [OPT; TRIAD] and the following:

1. It is a must to balance the extent of positive relative to negative feedback in the DEF circuit so as to prevent oscillation. And to possibly maximize this ratio [fine tune it] for an optimal cancellation of standing waves in the room corners.

2. The extent of negative feedback from Vout [@ the center of +/-36 V PSU] to the joined gates of the DEF FETs is determined by the value of R4 found at the bottom right of the schematic. Values of R4 lower than the shown [3.7 K] will decrease the extent of negative feedback. The value of R4 will need fine tuning.

3. Overall negative feedback from the secondary#1 winding of [OPT] at the joined sources of the FETs is normally used and sent to the front-end of the vacuum tube amp. It is adopted in the like prototype Class aP amp.

4. The power which is inherently induced in the secondary#2 winding [exactly like that in secondary #1 of OPT] bucks or bootstraps the power [VA] released by DEF. Thus the shown 0.6 A idle bias of each FET in DEF is probably too rich.

5. It follows from [point #4] that DEF adds more in-phase power to secondary#2 winding to induce positive feedback in secondary#1. It is clear that this positive feedback which is internal to DEF is also exported to the front end of the vacuum tube amp in the form of overall negative feedback.

I'll fine tune further the balance of positive/negative feedback. I will post a commentary in the Thread "DIY the device of US Patent 4,899,387" of the Pass Labs Forum. This sound system is a favorable solution or answer to my quest therein.

Best

Anton
 

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Another likable sound system..

The attached schematic is for a mono sound system which I am listening to. Its principal components are:

1. A TRIAD power transformer depicted on the left side of the schematic. It has two [independent] 115 Vac primaries and two [independent] 12 Vac secondaries. A primary is connected in parallel and in-phase with one secondary to generate Coil #1. Ditto for the other primary and secondary to make Coil #2. Both coils are then connected in series as shown and implanted in the power amp using DEF FETs. Note their relative phase [dots]. TRIAD imbibes the "system" with positive feedback.

2. A DEF-style power amp [old faithful] which was posted several times to-date. The right-side of the FETs is their biasing scheme to include the important use of overall negative feedback; from Vout to their gates c/o [R12 +R13]. This resultant DEF amp practices a combination of positive and negative feedback; which is important for stability against oscillation mostly influenced by the positive feedback from the transformer.

3. Your choice voltage source amp [VSA]which drives the joined sources of the DEF FETs.

4. A loudspeaker [ADS L-730] load across Coil #2. It can sit anywhere in the listening room including and preferably in one of its corners.

Please note the encircled voltages on the schematic; namely Vout, Vload, Vnoise, and V[-]fdbk which are the analysis points of the system. I used a 150 Hz signal to drive Vin of VSA, an 8 Ohm resistor load across Coil #2 and measured the said voltage values with an AC multimeter. The extent of overall negative feedback to DEF; meaning V[-]fdbk was decreased by decreasing the value of [R12 +R13] relative to R6. V[-]fdbk was the variable which essentially changed the system's ratio of positive to negative feedback.

I'll post a follow up to detail more techno -nuances to this great sounding system.

Best

Anton
 

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