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MOSFET power amplifier with error correction

pma

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MOSFET power amplifier with error correction

When designing a linear power amplifier, we have a choice of bipolar or MOSFET power transistors at the position of output devices. Bipolar (BJT) transistors are used much more frequently, but please let me also discuss and show a design with MOSFET power transistors. I would start with a quote from excellent classic design book by Bob Cordell: Designing Audio Power Amplifiers:
-----------------------------------------------------------------------------------------------------------
“MOSFET class AB biasing tends to be simpler and less critical. MOSFETs do not have an optimum class AB bias current. Instead, they operate better with greater bias as long as thermal objectives are met. For this reason, typical MOSFET power amplifiers operate at higher bias current per output pair and have a larger class A region of operation for small signals. However, their lower transconductance tends to result in higher measured values of static crossover distortion. Because of their high speed, MOSFET amplifiers are less prone to dynamic crossover distortion as a result of switch-off characteristics. MOSFETs do not suffer from beta droop and fT droop at high currents and are generally able to handle high peak currents better than BJTs. The ease with which MOSFETs are driven also means that there is less stress on driver transistors when the amplifier is delivering high current to the load.
MOSFETs are a bit more prone to high-frequency parasitic oscillations as a result of their inherently higher speed. For this reason, circuit design and layout can require more care than for BJT designs.
Biasing power MOSFETs for class AB operation is different from that for BJTs in two ways. First, MOSFETs require greater forward bias at the gate than BJTs do at the base. Vertical MOSFETs can require up to 4 V. Some vertical MOSFETs like the 2SK1530 require only about 1.7 V, however. If MOSFETs requiring 4 V forward bias are combined with emitter follower drivers, the total bias spreading voltage will be on the order of 9.2 V. This compares to a typical bias spreading voltage of about 4.0 V for a BJT output Triple. This difference in required bias spreading voltage is not a problem for the traditional Vbe multiplier or minor variants of it, but it does imply that the driver circuitry will require more voltage headroom in a MOSFET design. This is sometimes dealt with through the use of boosted power supplies for the circuits preceding the output stage.

Crossover distortion is one of the most insidious distortions in class AB power amplifiers. It occurs at fairly low signal levels and often contains a high-order distortion spectrum that is more dissonant and difficult to remove with negative feedback. It is a result of the changing gain of the output stage as the signal current delivered to the load goes through zero (the crossover). MOSFET output stages are also subject to crossover distortion.
Static crossover distortion in BJT output stages is a result of the output impedance changing as the output current goes through zero. The output impedance forms a voltage divider with the load impedance; as a result the gain of the output stage changes. The lower the value of the output impedance, the smaller the crossover distortion will be for a given percentage change in the output impedance.
The same is true for power MOSFET output stages, but the output impedance is generally quite a bit higher for a given amount of bias current. This is because the transconductance of a MOSFET is much smaller than that of a BJT. The transconductance of a BJT at Ic = 100 mA is about 4 S. The transconductance of an IRFP240 biased at Id = 150 mA is about 1 S. As a result, the sum of the transconductances of the upper and lower MOSFETs dips in the crossover region. This is referred to as transconductance droop.
Conventional negative feedback is not the only way to reduce distortion. Various error-correction techniques can be used in place of, or in connection with, negative feedback.

In virtually any well-designed power amplifier the output stage ultimately limits performance. It is here where both high voltages and large current swings are present, necessitating larger, more rugged devices that tend to be slower and less linear over their required operating range. The performance-limiting nature of the output stage is especially evident in class AB designs where the signals being handled by each half of the output stage have highly nonlinear half-wave-rectified waveforms and where crossover distortion is easily generated. In contrast, it is not difficult or prohibitively expensive
to design front-end circuitry of exceptional linearity.
Overall negative feedback greatly improves amplifier performance, but it becomes progressively less effective as the frequency or speed of the errors being corrected increases. High-frequency crossover distortion is a good example. The philosophy here is based on the observation that only the output stage needs extra error correction and that such local error correction can be less complex and more effective.
While the power MOSFET has many advantages, it was pointed out that the lower transconductance of the MOSFET will result in moderate crossover distortion unless rather high bias currents are chosen.”
-----------------------------------------------------------------------------------------------------------

So, to utilize MOSFET power transistor advantages and keep reasonably low distortion, we need to apply error-correction circuit in the output stage. One of the error-correction circuit attempts is my power amplifier called PM-AB2. The amplifier is very simple, it uses an opamp as a voltage amplifying stage and an output stage with N-MOSFET/P-MOSFET pair, that's all.

Its basic schematics can be seen below.

PM-AB2_413_118_simplsch.png


The error-correction circuit is constituted by Q4 and Q7 transistors, R4-R6 and R15,R16 resistors and C1, C2 capacitors. These parts monitor the output voltage and create the variable bias voltage between gates of M1 and M2, to minimize non-linearity of the M1, M2 output stage with the load used. In the image below, we can see that the bias voltage between gates of M1 and M2 is not constant, due to action of the error-correction circuit, but tries to minimize output stage non-linearity to keep the low distortion. Without this error correction, amplifier distortion is about 10x higher.

Vg_bias_errcorr.png


Complete schematics of the amplifier follows:

PM-AB2_realsch1.png


One of my goals is to have high input impedance, then the amp makes light load for the DAC or preamp and works well even with passive “preamp” or tube preamp. The input impedance is close to R3 value, 100 kohm. To keep this high input impedance, the IC1 opamp must have JFET input stage. This excludes bipolar input opamps from considerations. And, I like JFET input opamps also because of their much higher immunity to EM interference. OPA134, OPA627, LT1122 and OPA445 were tested in this amplifier. OPA445 would allow for higher power supply voltage and thus higher voltage swing/output power, but its noise and high frequency linearity is worse. The results with the other three opamps were almost identical.

I use Hitachi 2SK413/2SJ118 power transistors for the reason that I have a good stock of these parts. So the error-correction resistor network is optimized to these devices. Idle current is set to 80 – 100mA.

2SK413_2SJ118_genuine.JPG


Measurements

Square wave response


This is one of the most important measurements, on power amplifiers, at least to me. At lower repetition frequency, it directly shows the low frequency extension of the frequency response, at higher repetition frequencies it shows rise time (Tr 10%-90%) and is directly related to -3dB high frequency corner (Fc = 0.35/Tr). The flatness of square wave response top and bottom is related to frequency response flatness. Square response also shows the stability of the amplifier and possible high frequency oscillations, that are impossible to disclose by a slow spectrum analysis in the audio band.

PM-AB2_10ksq.png


We can see that rise time/fall time is below 2us, corresponding to at least 170kHz/-3dB bandwidth. The response is aperiodic, free of overshoots or oscillations or ringing.

Harmonic distortion

ASR asks for 5W/4ohm/1kHz SINAD measurement, so here it goes:

PM-AB2_E1DA_5W_4R_1k_90dB.png


THD+N = 0.0032% equals to SINAD = 90dB. This is not a record breaker, but is more than enough to be inaudible. Mains related residuals are below -108dBr. There is no hiss, no hum, no buzz audible even with the ear at the tweeter/midrange/bass drivers of the speaker with 90dB/2.83V/1m sensitivity.

THD and THD+N as a function of output power at 1kHz/4ohm is shown below

PM-AB2_E1DA_thdnpower_4R_1k.png


and THD/THD+N as a function of frequency at 16W/4ohm

PM-AB2_E1DA_thdnfreq_4R_16W.png


CCIF Intermodulation distortion 19kHz+20kHz
is again of of the important measurements to me, as it shows possible issues in high frequency linearity

PM-AB2_E1DA_5W_4R_CCIF.png


As we can see, intermodulation products are below -90dBr.

Amplifier construction

The following images show amplifier construction. It uses a linear power supply and it is built in a standard 3U x 19” case with side heatsinks. It is one of my prototype cases. Though it is a linear class AB amplifier with a linear power supply, its power consumption is 14W at idle or low volume.

P1050158-1.JPG


P1050156-2.jpg


IMG_4077-3.JPG
 
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restorer-john

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1662552402664.png


Boucherot cell/Zobel network on the speaker terminals. Did you need it for stability or was it just left there from the last amplifier you built? :)
 
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DonH56

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@pma Very cool!

Typo: "The error-correction circuit is constituted by Q4 and Q7 transistors, R4-R4 and R15,R16 resistors and C1, C2 capacitors." -- should be R4-R6, yes?

Square-wave response is onto a 4-ohm resistive load?

Question: Not being an audio (or LF) amp designer as my main career, most of my circuits are (were) much simpler (albeit faster) since there simply wasn't the bandwidth to do a lot of the neat stuff possible at lower speeds. I implemented a somewhat similar circuit as a buffer, but I used MOSFETs (or PHEMTs, forget the technology, long ago) for Q4/Q7 in an attempt to match and compensate the gm-modulation and thermal characteristics of the output devices. That was based on the old emitter-follower BJT output stage that was a precursor. It seemed to help compared to using BJTs, but gain was lower, so I was never quite sure if it'd been better to stick with BJTs for the error amp. Curious to hear your thoughts (and from anybody who knows better)?

I also added a tweak circuit to better match the current source and sink values to control offsets, again because at high speed there really wasn't enough feedback to suppress the offset to the desired level, and any offset or crossover glitches in the circuit led to undesirable output jumps (this was an ADC driver and little CM jumps led to settling issues). It was basically a CM feedback loop. Not sure I would do it in an audio amp as it allowed baseline wander, i.e. LF modulation of the output CM level, which I did not care about in my application but could be an issue in an audio design unless the cutoff was set very low (few Hz).

Curious, thanks - Don (showing my ignorance of audio amplifier design here; I need to dig out my old copy of Cordell's book)
 
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kchap

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MOSFET power amplifier with error correction

When designing a linear power amplifier, we have a choice of bipolar or MOSFET power transistors at the position of output devices. Bipolar (BJT) transistors are used much more frequently, but please let me also discuss and show a design with MOSFET power transistors. I would start with a quote from excellent classic design book by Bob Cordell: Designing Audio Power Amplifiers:
-----------------------------------------------------------------------------------------------------------
“MOSFET class AB biasing tends to be simpler and less critical. MOSFETs do not have an optimum class AB bias current. Instead, they operate better with greater bias as long as thermal objectives are met. For this reason, typical MOSFET power amplifiers operate at higher bias current per output pair and have a larger class A region of operation for small signals. However, their lower transconductance tends to result in higher measured values of static crossover distortion. Because of their high speed, MOSFET amplifiers are less prone to dynamic crossover distortion as a result of switch-off characteristics. MOSFETs do not suffer from beta droop and fT droop at high currents and are generally able to handle high peak currents better than BJTs. The ease with which MOSFETs are driven also means that there is less stress on driver transistors when the amplifier is delivering high current to the load.
MOSFETs are a bit more prone to high-frequency parasitic oscillations as a result of their inherently higher speed. For this reason, circuit design and layout can require more care than for BJT designs.
Biasing power MOSFETs for class AB operation is different from that for BJTs in two ways. First, MOSFETs require greater forward bias at the gate than BJTs do at the base. Vertical MOSFETs can require up to 4 V. Some vertical MOSFETs like the 2SK1530 require only about 1.7 V, however. If MOSFETs requiring 4 V forward bias are combined with emitter follower drivers, the total bias spreading voltage will be on the order of 9.2 V. This compares to a typical bias spreading voltage of about 4.0 V for a BJT output Triple. This difference in required bias spreading voltage is not a problem for the traditional Vbe multiplier or minor variants of it, but it does imply that the driver circuitry will require more voltage headroom in a MOSFET design. This is sometimes dealt with through the use of boosted power supplies for the circuits preceding the output stage.

Crossover distortion is one of the most insidious distortions in class AB power amplifiers. It occurs at fairly low signal levels and often contains a high-order distortion spectrum that is more dissonant and difficult to remove with negative feedback. It is a result of the changing gain of the output stage as the signal current delivered to the load goes through zero (the crossover). MOSFET output stages are also subject to crossover distortion.
Static crossover distortion in BJT output stages is a result of the output impedance changing as the output current goes through zero. The output impedance forms a voltage divider with the load impedance; as a result the gain of the output stage changes. The lower the value of the output impedance, the smaller the crossover distortion will be for a given percentage change in the output impedance.
The same is true for power MOSFET output stages, but the output impedance is generally quite a bit higher for a given amount of bias current. This is because the transconductance of a MOSFET is much smaller than that of a BJT. The transconductance of a BJT at Ic = 100 mA is about 4 S. The transconductance of an IRFP240 biased at Id = 150 mA is about 1 S. As a result, the sum of the transconductances of the upper and lower MOSFETs dips in the crossover region. This is referred to as transconductance droop.
Conventional negative feedback is not the only way to reduce distortion. Various error-correction techniques can be used in place of, or in connection with, negative feedback.

In virtually any well-designed power amplifier the output stage ultimately limits performance. It is here where both high voltages and large current swings are present, necessitating larger, more rugged devices that tend to be slower and less linear over their required operating range. The performance-limiting nature of the output stage is especially evident in class AB designs where the signals being handled by each half of the output stage have highly nonlinear half-wave-rectified waveforms and where crossover distortion is easily generated. In contrast, it is not difficult or prohibitively expensive
to design front-end circuitry of exceptional linearity.
Overall negative feedback greatly improves amplifier performance, but it becomes progressively less effective as the frequency or speed of the errors being corrected increases. High-frequency crossover distortion is a good example. The philosophy here is based on the observation that only the output stage needs extra error correction and that such local error correction can be less complex and more effective.
While the power MOSFET has many advantages, it was pointed out that the lower transconductance of the MOSFET will result in moderate crossover distortion unless rather high bias currents are chosen.”
-----------------------------------------------------------------------------------------------------------

So, to utilize MOSFET power transistor advantages and keep reasonably low distortion, we need to apply error-correction circuit in the output stage. One of the error-correction circuit attempts is my power amplifier called PM-AB2. The amplifier is very simple, it uses an opamp as a voltage amplifying stage and an output stage with N-MOSFET/P-MOSFET pair, that's all.

Its basic schematics can be seen below.

View attachment 229252

The error-correction circuit is constituted by Q4 and Q7 transistors, R4-R4 and R15,R16 resistors and C1, C2 capacitors. These parts monitor the output voltage and create the variable bias voltage between gates of M1 and M2, to minimize non-linearity of the M1, M2 output stage with the load used. In the image below, we can see that the bias voltage between gates of M1 and M2 is not constant, due to action of the error-correction circuit, but tries to minimize output stage non-linearity to keep the low distortion. Without this error correction, amplifier distortion is about 10x higher.

View attachment 229263

Complete schematics of the amplifier follows:

View attachment 229253

One of my goals is to have high input impedance, then the amp makes light load for the DAC or preamp and works well even with passive “preamp” or tube preamp. The input impedance is close to R3 value, 100 kohm. To keep this high input impedance, the IC1 opamp must have JFET input stage. This excludes bipolar input opamps from considerations. And, I like JFET input opamps also because of their much higher immunity to EM interference. OPA134, OPA627, LT1122 and OPA445 were tested in this amplifier. OPA445 would allow for higher power supply voltage and thus higher voltage swing/output power, but its noise and high frequency linearity is worse. The results with the other three opamps were almost identical.

I use Hitachi 2SK413/2SJ118 power transistors for the reason that I have a good stock of these parts. So the error-correction resistor network is optimized to these devices. Idle current is set to 80 – 100mA.
Nice.

I built this nearly 40 years ago: ETI-477. The workmanship was terrible compared to yours but it chugged along for nearly 20 years.
 
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voodooless

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That's quite a simple amp, very nice! Seems like this could be built into a much smaller PCB for a high-density design. Those MOSFETs are not very current parts, however.

Any chance a higher-powered version would be feasible with more modern MOSFETs? I'm guessing you'd need a high-voltage op-amp for that? Or a second gain stage?
 
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pma

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Boucherot cell/Zobel network on the speaker terminals. Did you need it for stability or was it just left there from the last amplifier you built? :)
It is not absolutely necessary, but stability is better with this Zobel placement, most probably because of natural inductance of the wire between amp output and speaker terminal :).
 
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pma

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@pma Very cool!

Typo: "The error-correction circuit is constituted by Q4 and Q7 transistors, R4-R4 and R15,R16 resistors and C1, C2 capacitors." -- should be R4-R6, yes?

Square-wave response is onto a 4-ohm resistive load?

Question: Not being an audio (or LF) amp designer as my main career, most of my circuits are (were) much simpler (albeit faster) since there simply wasn't the bandwidth to do a lot of the neat stuff possible at lower speeds. I implemented a somewhat similar circuit as a buffer, but I used MOSFETs (or PHEMTs, forget the technology, long ago) for Q4/Q7 in an attempt to match and compensate the gm-modulation and thermal characteristics of the output devices. That was based on the old emitter-follower BJT output stage that was a precursor. It seemed to help compared to using BJTs, but gain was lower, so I was never quite sure if it'd been better to stick with BJTs for the error amp. Curious to hear your thoughts (and from anybody who knows better)?

I also added a tweak circuit to better match the current source and sink values to control offsets, again because at high speed there really wasn't enough feedback to suppress the offset to the desired level, and any offset or crossover glitches in the circuit led to undesirable output jumps (this was an ADC driver and little CM jumps led to settling issues). It was basically a CM feedback loop. Not sure I would do it in an audio amp as it allowed baseline wander, i.e. LF modulation of the output CM level, which I did not care about in my application but could be an issue in an audio design unless the cutoff was set very low (few Hz).

Curious, thanks - Don (showing my ignorance of audio amplifier design here; I need to dig out my old copy of Cordell's book)

Thank you, Don!

1) the typo corrected, thanks again!

2) yes, square response is into 4ohm resistor, it is mentioned near the bottom line of the oscillo plot, quite small characters, though.

3) frankly I have not thought in deep about MOSFETs at Q4/Q7 position (in the simplified schematics), so I have no meaningful answer at the moment.

I made another MOSFET amplifier with error correction, 18 years ago, please see the thumbnail. It was based on a hint and discussion with Nelson Pass. It had even lower distortion, about 0.001%, but the correction circuit had acted as a current limiter as well, which was a problem with low impedance load, and the quiescent current was much higher, like 350mA, so there was too much heat radiated and high idle consumption.

pm_ab_er1.gif
 
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pma

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That's quite a simple amp, very nice! Seems like this could be built into a much smaller PCB for a high-density design
Sure :). My PCB is "big" for the reason that I repeatedly use the same amplifier cases for the prototypes, because the cases with heatsinks are pretty expensive. So I am trying to use the same drill holes for different circuits.
 

DonH56

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Thank you, Don!

1) the typo corrected, thanks again!

2) yes, square response is into 4ohm resistor, it is mentioned near the bottom line of the oscillo plot, quite small characters, though.

3) frankly I have not thought in deep about MOSFETs at Q4/Q7 position (in the simplified schematics), so I have no meaningful answer at the moment.

I made another MOSFET amplifier with error correction, 18 years ago, please see the thumbnail. It was based on a hint and discussion with Nelson Pass. It had even lower distortion, about 0.001%, but the correction circuit had acted as a current limiter as well, which was a problem with low impedance load, and the quiescent current was much higher, like 350mA, so there was too much heat radiated and high idle consumption.

View attachment 229301
Thanks!

At one time I had analyzed that older circuit. Would be a good brain exercise to do it it again. So easy to get lost chasing the currents around the algebra... IIRC, the current-limiting ability was a feature meant to save amps, but required fiddling if you had low-Z loads (or high output peaks). I have a vague memory of moving the collector connections to bypass the limiting, and an even vaguer memory of trying to add slew enhancement to provide high current peaks under slew without losing the longer-time current limiting the voltage amp stage provided. It all got complicated and after graduating I moved on to high-speed design with it's (usually) simpler circuits...

Ironically, my last couple of years focused on digital circuits, but I fell in love with the analog side. The digital courses helped designing the logic circuits for data converters and bias controllers so not a total waste. But I still think recursive code, let alone when implemented in HW, is the work of the devil. :)
 
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restorer-john

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I built this nearly 40 years ago: ETI-477. The workmanship was terrible compared to yours but it chugged along for nearly 20 years.

You know, there's a shoebox around here someplace with a few Tillbrook designed 477 modules I picked up in a DIY box for a few dollars. The original Hitachi MOSFETs are gold now.
 

restorer-john

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My PCB is "big" for the reason that I repeatedly use the same amplifier cases for the prototypes, because the cases with heatsinks are pretty expensive. So I am trying to use the same drill holes for different circuits.

Personally I think that is brilliant. You have a chassis development 'mule' which any design can go into, look decent, be tested and evaluated in your system without looking like a lethal rat's nest.

I wish I'd thought of that back in the day when I was building amps every other week. No, I had to go the full hog for every build and consequently, they got stalled and some never got finished.
 

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MOSFET gurus
If you remember the Jim Strickland designed Acoustat Trans Nova Twin 200 and 120, how do those compare to the later products branded as Halfler or later Mosfet A/B? I had a TNT200 in '91 and traded it for a '80 Honda Civic. :)
 

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I built this nearly 40 years ago: ETI-477.

I just happened to have the actual magazine I bought back in 1981 sitting next to my bench!

scan669 (Medium).jpg


Back when Louis Challis pioneered speaker testing in Australia, The Apple II was brand new, Nakamichi were on top of the heap and ETI cost $1.75!

Go to worldradiohistory and download both January and February 1981 (parts 1 and 2) for a full description and the entire design of the 477 MOSFET module.


Bonus edit: checkout the review in the January 1981 issue of the DCM Timewindow loudspeakers (page 132 or page 134 in the pdf)
 
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kchap

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I just happened to have the actual magazine I bought back in 1981 sitting next to my bench!

View attachment 229367

Back when Louis Challis pioneered speaker testing in Australia, The Apple II was brand new, Nakamichi were on top of the heap and ETI cost $1.75!

Go to worldradiohistory and download both January and February 1981 (parts 1 and 2) for a full description and the entire design of the 477 MOSFET module.


Bonus edit: checkout the review in the January 1981 issue of the DCM Timewindow loudspeakers (page 132 or page 134 in the pdf)
Are yes, the good old Nakamichi 1000. I had these magazines along a few "Radio TV Hobbies" and most of the "Electronics Australia". When I moved to a smaller unit, I had to make some tough decisions, SIGH!

If anybody was going to have ago at building the amps, I think there was an addendum. I seem to remember that the amps could become unstable and oscillate. If I've not confused this with another project the solution was to replace the original WW resistors R25 to R28 with ceramic types. Less self-inductance.
 

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If I've not confused this with another project the solution was to replace the original WW resistors R25 to R28 with ceramic types. Less self-inductance.

Yes, there was an issue with a bunch of the 'faster' amps of the day. Cheap WW 5W resistors vs Noble or IRH brand low inductance. You're right. Same issue with the 300W ETI-466 modules I built back in the day.

World Radio History used to have all the R TV&H issues and all the EA issues until some clown at Silicon Chip (who now apparently own the copyright to all of it) stuck World Radio History with a DMCA takedown notice! Such a short sighted and plain dumb attitude. At that time, I decided to never, ever buy a magazine from that publishing stable ever again. And I was one the earliest subscribers. But I have all my hundreds of printed copies so they can also go take a flying jump at themselves instead of attempting to charge people $10 for a schematic or a project from the 1980s. Absolute idiots IMO.

Here's (attached) a cross reference of all the (Australian) ETI projects you can use, if you are ever wondering what projects came from what issue, then you can download the issue from World Radio History.
 

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kchap

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Yes, there was an issue with a bunch of the 'faster' amps of the day. Cheap WW 5W resistors vs Noble or IRH brand low inductance. You're right. Same issue with the 300W ETI-466 modules I built back in the day.

World Radio History used to have all the R TV&H issues and all the EA issues until some clown at Silicon Chip (who now apparently own the copyright to all of it) stuck World Radio History with a DMCA takedown notice! Such a short sighted and plain dumb attitude. At that time, I decided to never, ever buy a magazine from that publishing stable ever again. And I was one the earliest subscribers. But I have all my hundreds of printed copies so they can also go take a flying jump at themselves instead of attempting to charge people $10 for a schematic or a project from the 1980s. Absolute idiots IMO.

Here's (attached) a cross reference of all the (Australian) ETI projects you can use, if you are ever wondering what projects came from what issue, then you can download the issue from World Radio History.
Possibly a bit harsh but I agree. SC is not the magazine its predecessors were. I can't see it surviving much longer.

Thanks for the attachment. 30 years ago, I could struggle through a DIY project but not anymore. Buying a RPi and mounting it in a manufactured case is my limit. For more advanced projects I'm very much an armchair expert these days.
 

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I built this amp and it works quite well and seems pretty bomb proof. https://sound-au.com/project101.htm One problem these days is that you can not reliably get the Hitachi MOSFET's anymore. The vast majority found online are fakes. Luckily you can get a very similar new Exicon ECX10N20 and ECX10P20 directly available from Profusion PLC in the UK and they will be drop in replacements in many cases (not sure about the OP's design as it seems to be optimized for the Hitachi parts).
 

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SC is not the magazine its predecessors were. I can't see it surviving much longer.

When Leo Simpson left EA and started SC, that was 1987. SC was great in the early years and especially when EA hit the skids becoming a lifestyle magazine. Soon EA folded and it made sense for SC publishing to buy the rights I guess.

But going after world radio history- a guy who has been cataloguing and carefully preserving magazines from all around the world for all to read was the last straw for me. Millions upon millions of these magazines have been scanned, copied, used in projects, libraries, passed on from father to son for many generations. About as public domain as it gets.

And they try this chit on their busted a#s 1990 website?:

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No thanks, I'd rather buy an entire year or collection on eBay for a few dollars.

Yep, I subscribed, bought the overpriced binders, sent in letters, supported EA, ETI and SC for decades. But when you burn the original high spending hobbyists and professionals, your magazine is doomed. Watch it go the way of EA and good riddance.
 

Jim Shaw

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Interesting topology. Ever production variation-minded, I wonder how this performs with expected variations in device gain, especially over temp variations and manufacturing tolerances. And is there any predicted need to trim component values in final testing? Or, do the cookbook values and economical tolerances provide good performance? (These trims can cost boucou, or not.)

Thanks for the post. Crossover distortion can seem like an eternal enemy.
 
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