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Old 26th February 2018, 06:38 PM   #291
Antoinel is offline Antoinel  United States
diyAudio Member
Join Date: Jan 2012
Location: Pennsylvania
Default 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.

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Old 10th March 2018, 08:27 PM   #292
Antoinel is offline Antoinel  United States
diyAudio Member
Join Date: Jan 2012
Location: Pennsylvania
Default 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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File Type: png DEFcgLikeRoomAcoustic.png (37.0 KB, 179 views)
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Old 12th March 2018, 07:08 PM   #293
Antoinel is offline Antoinel  United States
diyAudio Member
Join Date: Jan 2012
Location: Pennsylvania
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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File Type: png PowerBuffer.png (39.7 KB, 97 views)
File Type: png DEFcgLikeRoomAcoustic1.png (48.8 KB, 97 views)
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