It's always exciting to hear from our good friend, Yazaki-san, and in this article Yazaki-san brings us more illuminating insights as he discusses the GEC PX25A (DA30) monaural SETs that he built for Alan-san.

Yazaki-san has dedicated this Part 4 article to his friends Alan-san and Harald-san, who have enthusiastically pursued and supported the builds of these beautiful artisanal single-ended-triode monaural amplifiers!

A big thank you to Yazaki-san for taking the time to write about this audio adventure with Alan-san’s GEC PX25A (DA30) monaural SETs for us, and sharing his design insights for achieving Real Sound!

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Confucius Teachings

Jun-san, who I introduced to you in my last article (HERE) - who developed a truly innovative Class-D amplifier solution full of musicality - always says to me these cherished words of his in Japanese, 温故知新.

This old four-character ideogram comes from Confucius' teachings in ancient China. This teaching is so famous that there might be a lot of English translations of it, but I’d like to translate the words for you in my own way:

“Even when you learn a new discipline, do not forget the old knowledge, and when you revisit the old ways, please do not lose the spirit of progress that allows you to rediscover the true goodness of the old ways.”

Like Jun-san, I also like these words and believe them to be true.

Especially for analog technology in audio, we should learn humbly from the past wisdom, as that history reaches back almost a century, and contains vast riches of technology that have been built up over time.

In this Part 4, I wish to share with you some of these vanishing old achievements, which are important to remember.

P-K feedback from WE300B to WE310A

Just 24 years ago, while I was a general manager in charge of one of the engineering departments at Pioneer, one day I was casually reading through an audio magazine from the bookshelf of my office.

An attractive photo of a WE310A-WE300B-WE274A monaural amplifier in kit-form by P&C Electronics caught my eye, which is a vintage parts and tube amplifier kit shop in Tokyo.     

The author of this assembly article was Mr. Tadaatsu Atarashi, who had long been a main writer and reviewer for Tube Kingdom, a specialized tube amplifier magazine in Japan that was an authority on vintage USA vacuum tubes, and especially Western Electric electronics.

Well, in the January 1995 issue of Radio Technology Atarashi-san described this wonderful kit in detail, and how to improve its sound quality by his own way of modification, P-K feedback from the WE300B to the WE310A.

Here are my main points from his article:

This was a very splendid kit, which included not only genuine Western Electric tubes, but also reliable vintage parts that were made in the USA.

After assembling the kit he found the amplifier's non-feedback circuitry had an unsatisfactory sound, being not so clear and dull. However, the result may not be surprising, as the higher output resistance of the WE310A pentode driver, and the stray capacitance between the grid and the cathode of the WE300B, surely led to diminished output from the midrange on up into the high-frequencies. The frequency response data showed that at 10 kHz it was down -1.5 dB, at 15 kHz it was down  -3 dB, and 20 kHz it was down -4.4dB.

He recalled that in the legendary original Western Electric 91-B amplifier that it had a feedback circuit from the plate of WE300B to the screen-grid of the first stage tube, the WE310A. He also remembered that when Western Electric used a pentode power tube for his amplifier from the 1940’s P-K feedback circuitry had been applied. So then he installed P-K feedback parts to try for both 9 dB and 13 dB.

The P-K feedback had a remarkable effect by not only improving the frequency response of the high range, but also during actual subjective listening. For example, the frequency response truly improved, with measurements showing at 10 kHz it was down only -0.2 dB, at 15 kHz it was down only -0.5 dB, and at 20 kHz it was down only -1 dB, using 13 dB of P-K feedback. The subjective sound quality was changed and become extremely clear and real sounding. Atarashi-san was truly convinced that this might be the originally intended sound of a WE310A driving a WE300B.

He also said that this sound quality improvement by P-K feedback was not merely due to the improvement of frequency characteristics, as he had tried the usual global feedback from the secondary winding of the output transformer to the cathode of WE310A, which surely improved the measured characteristics, but it was not useful for improving the subjective sound quality during listening.

He provided the total harmonic distortion (THD) measurement data in the article, and pointed to the fact that with the THD values for non-feedback, 9 dB of P-K feedback, and 13 dB of P-K feedback, there were no visible differences in particular, but that only the third-harmonic distortion characteristic was improved by the P-K feedback. When I reviewed the measured value of the THD in detail, its distortion curves and the values of THD and the second-harmonic distortion were almost overlapped and similar for non-feedback, and 9 dB and 13 dB of PK-feedback. That means that the THD of this WE310A-WE300B single-ended amplifier is composed of mainly second-harmonic distortion, and compared to the second-harmonic distortion the third-harmonic distortion was at a far lower revel. But despite its low level, this third-harmonic distortion could be quite harmful to subjective sound quality. The improvement of the high-frequency’s characteristics and the lowering of its third-harmonic distortion by using P-K feedback also brought about the upgrading the sound quality, I also supposed.

Atarashi-san concluded that the use of WE310A-WE300B non-feedback circuitry was not the correct solution for the best sound quality, and he was able to address the deficiency of the circuitry with -9.5 dB of P-K feedback, which not only improved the frequency response, but also the subjective sound quality.

A lot of writers had written articles about building WE310A-WE300B SET amplifiers using non-feedback circuits in the magazines MJ and Radio Technology from the early 1970’s. Surely, the use of non-feedback circuitry among some of the single-ended-triode amplifiers enthusiasts in Japan was a matter of faith, I suppose.

I was so impressed with this article, and felt it highly suggestive for me as my own WE310A-DA30 stereo SET of Anzai-san’s design has just the same type of non-feedback circuitry as did the P&C WE310A–WE300B monaural SET.

Anyways, Atarashi-san’s adventure with P-K feedback would be truly an illustration of 温故知新 for me, as his ideas inspired me to build this P-K feedback circuitry into Alan-san’s and Harald-san’s PX25A (DA30) monaural SETs, more than 20 years after the publication of that issue of Radio Technology. 

2nd Harmonic Distortion and 3rd Harmonic Distortion and the British Hearing Sense

Well, I’d like to share with you my personal experiences here.

During the best season in Europe, from the end of May to the middle of June, just 30 years ago, before the collapse of the Berlin Wall in that year, I was engaged as a member of a three week long Pioneer publicity tour.

At that time, I had taken charge of managing the cassette deck engineering team at Pioneer, and fortunately, the previous year’s models, the CT-737 and CT-939, had already gained a high reputation and were best sellers in the European market, especially in West Germany.

I was responsible for the sales promotion activities for the CT-737 Mk2 as 1989’s new model during this tour. We visited major magazine companies in West Germany and had meetings with reviewers in each country, Sweden, Italy, England, Netherland and Belgium.

I remember well, we visited and met Mr. Alvin Gold when we stayed in London, and you might have once heard of him as an audio reviewer in England. I couldn’t understand him very well because of my poor English, but I do remember him saying, “No matter how much 2nd harmonic distortion would be allowed, but 3rd harmonic distortion could not be permitted even with a small amount”.

These words were his advice to us and our amplifier engineer, who accompanied us on our publicity tour. When I think about what he said, I think it had to do with the difference in the review grade points for the CT-737 between West Germany and England.

As I have said before, from 1988 to 1989 the CT-737 had surely acquired high grade points in each audio magazine in West Germany, but on the other hand in England, the reviews in magazines were not so remarkable compared to the reviews in the German audio magazines.

The CT-939 had gained the highest of reputations not only in West Germany but also in England. It was curious that the electric features were almost and basically the same between the two models, but the major difference was that the CT-737 had a single-capstan mechanism, where the CT-939 had an exceptional dual-capstan mechanism.

The weighted RMS wow & flutter level - which was almost equal to the wow component - of each model had already reached 0.02 % in actual measurement, which would be an ultimate level according to the limit of ultra-precision machining.

The CT-737’s flutter value was one of the best (lowest) among a lot of single-capstan decks in the market, but there was a very clear reason why the flutter component, not only the level, but also the spectrum, was completely different between the single-capstan deck and the dual-capstan deck.

Yes, CT-737’s flutter level was not so good in comparison with CT-939, and this flutter component caused the original signal to be FM-modulated at the time of recording and reproducing, and generated modulation noise, which appeared as a side band wave of original signal.

We could easily hear and distinguish the noise when we recorded and reproduced only one single tone. I do not mind if it is said that this modulation noise would be an irregular distortion, and this distortion would be nearly close to 3rd Harmonic distortion in the sense of harsh sounds.

It would be a matter of course that I do not need to say, but 2nd harmonic exists in nature and in the sound of an acoustic instruments, so that when we hear the distortion in reproduced music, we do not feel anything unpleasant.

What I want to say is that the reviewers in England, including Alvin-san, might had a very sensitive hearing sense for such an irregular distortion such as the moderation noise, which is caused by the flutter on the cassette decks, and also by the 3rd harmonic distortion of the amplifiers.

I also realized that the British had an exceptional inherent aesthetic feeling or sense for sound and tone, which would lead them to be seeking for more natural and finer tone, and curiously enough, I have felt and experienced that British orientation for natural and fine tone in the sound character of UK made vacuum tubes.

I have surely experienced such sounding tubes in my system with the GEC EF86 with a mesh shield, the Mullard ECC83 made in early 1960’s, the GEC and Marconi DA30 new dome type, the Mullard GZ37, the STC 5R4GY, and the Marconi U52.

Such lofty sounding tubes were also my true preferences. It is also a fact that the sophisticated British sense for sound and fine tone have resulted in a lot of excellent speakers being sent out into the world over a long period of time. You will be able to easily think of some examples, I suppose.

Kato-san - who has been a best friend to me throughout the so long years - and I talked about 2nd harmonic distortion on the phone today and he said, “2nd harmonic distortion only increases the number of tone colors, and so the music would be more beautiful. Well-built single-ended tube amplifier’s THD contains mainly 2nd harmonic distortion, which allow us to fully enjoy and be immersed in music. Searching for beautiful tone and harmony is one of the very important factors for enjoying the audio hobby”.

Needless to say, every kind of music signal would be composed of fundamental frequency and harmonic overtone components in complicated patterns over a wide frequency band. If you think about this harmonic content as being like 2nd harmonic distortion, if that so, then is this 2nd harmonic distortion truly harmful for us?

Instead, Alan-san and I might have been fully fascinated with and enjoyed the beautiful 2nd Harmonic tone of this PX-25A (DA30) SET, but I believe now that Alvin-san’s comment that day thirty yeas ago, “No matter how much 2nd harmonic distortion would be allowed, but 3rd harmonic distortion could not be permitted even with a small amount,” would be the expression that captured well the essence of the music and our human organic hearing sense.

The Merits of P-K Feedback

Well, I've done a qualitative discussion on harmonic distortion, but I’d like to look at the merits of P-K feedback from another point of view.

We should also consider the load of the power amplifiers, which is of course the loudspeakers. Needless to say, loudspeakers are the transducer which converts electric energy to kinetic motion, and I frankly tell you that is very difficult for power amplifiers to accurately control the loudspeakers because of how fluctuant the impedance frequency characteristic is. 

Yes, the loudspeaker is quite simply the frequency dependent load of the power amplifiers, but there is also the counter electromotive force of the speakers on the power amplifier to consider.

The motion of speaker starts in the magnetic circuit with the voice-coil of the speaker unit by the current flowing from the amplifiers by Fleming's left-hand rule, and at the same time this motion raises up a counter-electromotive force in the voice-coil by Fleming's right-hand rule.

It is true that the amplifier drives the loudspeakers, but also the loudspeaker constantly affects the amplifiers by this counter-electromotive force as we listen to the music. This counter-electromotive current returns back to the amplifiers via the speaker cables, making it difficult to drive the speakers precisely in accordance with the input signal.

Now, we should recall the negative-feedback circuitry, installed into almost all the power amplifiers, not only tube amplifiers but also semi-conductor amplifiers. The negative-feedback technology for the tube amplifiers, including skillful phase correction, had been already reached sophistication by the end of 1950’s or the early 1960’s.

We can see this in the Marantz Model 8B and 9 power amplifiers, for example. In order to meet the needs of the new era of high-fidelity, as there was a decline in the sensitivity of loudspeakers for home use, that is, that amplifiers not only aimed for higher wattage output, but also required wider frequency characteristic and a simultaneously lower distortion figure based on objective evaluation criteria, and I suppose that the advancement of negative feedback technology had overcome those issues.

However, I have some doubt about the way of negative-feedback, which is called “overall negative-feedback” in Japan, which comes from the output of the transformer’s secondary winding, which is connected to the output of the amplifier, and to the cathode of the first stage tube.

Well, for power amplifiers, the fluctuant impedance characteristic, and also the counter-electromotive current of the speakers, are just disturbances.

When we measure the static characteristic of power amplifiers, we have no other choice but to install fixed resistors - 4 ohm, 8 ohm or 16 ohm - as the load, but we must consider the differences between the load of loudspeakers' actual impedances and that of the fixed resistors.

The fixed resistor’s load is really the resistance and not the impedance, but the load of the loudspeaker is composed of mainly the inductance component, the resistance component, and the capacitance component in some cases. You could easily imagine the load of these two would be completely different.

What I mean is that these complex and complicated disturbances created by the speakers are flowed direct into the input of the first stage tube. When we consider a power amplifier as a system, the transfer function from input to output is, of course, expressed by the gain and the phase characteristics, and we could easily see the gain characteristic by the measurement data of frequency response and its gain.

But it would be hard to find out these phase characteristics when the amplifier is connected with a real loudspeaker speaker instead of a fixed resistor. In my understanding, this phase would be a critically important factor for reproducing true musicality or real sound-stage, because the phase characteristic is exactly the expression of the delay or fluctuation on a time axis.

When we experience true and real stereophonic three-dimensional sound-stage, we could easily distinguish the stage, expanding not only from left to right but also the depth with front-back direction. There is a huge amount of the phase information in music sources, and so we should search for ways to reproduce or recreate this exact phase information when we enjoy the music from our audio system.

Furthermore, consider the rhythm or the swing, which is a key component of our music, and the exact and correct time axis or phase characteristics in and audio reproducing system allows us true vigorous and dynamic feeling. Don’t you think so?

Anyways, controlling the disturbances by the loudspeakers on the amplifiers would be a hard matter, but the overall negative-feedback might have such literally negative factors or weaknesses as well, I suppose. 

But on the contrary, it could be easily supposed that P-K feedback has barriers for the disturbances with the secondary winding, and also the primary winding of the output transformer, and so compared to the overall feedback, the adverse effects by the complicated phase delay could be far mitigated.

The extreme way would be a non-feedback circuit design, and at one time, there were the writers on specialized magazine in Japan, including my master, Anzai-san, who loved the sound of non–feedback tube amplifiers even though the static characteristics were not so good or poor.

I think they might have looked for aspects of sound that bursts forward and is also dynamic, rather than the sound of higher resolution or wider range. Personally, I have to admit I haven’t liked the lack of sophistication and liveliness of the usual high-powered semiconductor amplifiers that have strong negative-feedback applied, so I could agree with our senior’s finding the sound of a non-feedback circuit to their liking.

Well, I hope you realize what a complicated relationship exists between the real load of the speakers on the power amplifier, and the role of the feedback circuitry of the power amplifier for reproducing music.

I believe a potent answer for this problem would be an “impedance compensator” like our Real-Sound Processor, the PSP-AZ9EX. I have often heard from almost all of the users of our Real-Sound Processor around the world, that their power amplifiers have an easier time driving and controlling their loudspeakers after they are installed.

The time axis of reproducing music becomes more precise and correct, and the sound stage expands not only to either side, but also back to front, with the Real Sound Processor. At some time in the future I hope to have more time to describe for you the details of this effective and essential audio accessory.

P-K feedback for Alan-san’s and Harald-san’s PX25A (DA30) SETs

Sorry for these long preceding introductory paragraphs, but now I’d like to get into the subject of Part 4.

As I mentioned in Part 1, I have so long loved the sound of the new dome type DA30, which is installed Anzai-san’s design PX25A (DA30) stereo power amplifier, without any help from feedback circuitry.

My DA30 SET drives my simple high-efficiency Onken 500MT mid-high range driver and 5000T Esprit tweeter over 500 Hz, and I have never felt any dissatisfaction about its high-range until very recently.

Just 10 years ago when I had a pair of the extremely rare old balloon type GEC DA30 and experienced the sound, I was driven to some kind of despair. It was not so clear, but dull, because of its shortage of the higher-range. Contrarily, the mid to low range was full of magnificence.

The new dome type DA30 is a truly direct-heated triode, but has quadrupole construction, which would surely reduce the stray capacitance of each other’s electrical poles, plate, grid and cathode, and it brought out the superior frequency response of high range.

Meanwhile, the old balloon type DA30 is a genuine or textbook direct-heated triode, just like the WE300B.

At that moment, I remembered the article by Atarashi-san from the January issue of Radio Technology in 1995, and realized that P-K negative-feedback would also be needed for fulfilling the potential of the old type of DA30, just as Atarashi-san taught us with the WE300B.

Well, three years ago, just to start this project, Harald-san and I talked about the circuit design for his DA30 monaural SET. His hope was that he would like to get not only the new dome type tube at first, but also the very rare old balloon type triode someday in the future, and his favorite sound was the non-feedback one, and so that was what he wanted me to do for his amplifier.

I had thought about his request for a few weeks, and then I came up with an idea. I could design and install a very simple and reliable user selectable mechanism for either non-feedback or P-K negative-feedback circuitry, so Harald-san could experience both.

As always, I asked Kato-san to design the circuitry for -3 dB of P-K feedback. I was thinking that getting a collection of the old-type balloon DA30 had become too difficult at present, and so I wanted Harald-san to be able to experience the full potential of the new dome type DA30. So that was my immediate priority, and so a value of -3 dB P-K feedback was decided upon, which gave outstandingly wide frequency response, especially in high range of the new dome type.

Kato-san showed me his simple idea for the circuit diagram for -3 dB of P-K feedback, and so now all I had to do was devise a reliable way to select between the two settings of non-feedback and -3 dB of P-K feedback. You can see my idea for using two jumper leads that actually came to shape in the figures and photos below. 

Fortunately enough, the sound quality of this -3 dB P-K feedback using the new dome type was unexpectedly splendid.

As mentioned above, originally the quality of the non-feedback WE310A driving the PX25A (DA30) with the new type DA30 vacuum tube already reached an exceptionally lofty and organic tone, but I had never heard such a supremely noble and fine sound as I did with the -3 dB of P-K feedback.

It could be said that if the non-feedback sound was like the scenery of an untouched and living nature, the sound with the -3 dB of P-K feedback was more like the truly comfortable life, where the richness of nature and moderate urbanization were beautifully fused.

Of course, both are so fascinating. However, I felt that by reducing the 3rd harmonic distortion in the amplifier using -3 dB of P-K feedback, that even the most minute harshness was eliminated from the amplifier. 

Just one day before the shipping Alan-san’s system to him - the modified original Marantz Model 7 preamplifier, and the completed WE310A-PX25A (DA30) monaural SETs with Hashimoto transformers - Nishikawa-san, who has been my best friend for so long a time, and I, made a final sound check on November 7, 2017.

Nishikawa-san is particularly familiar with classical music, and was once a designer of phonograph cartridges. Also, he was the leader of the design team for the Pioneer P3 turntable system, which is still considered a masterpiece of turntable design in Japan, making Nishikawa-san a really reliable and helpful person for the final listening session before shipping Alan-san's system.

Truly, we were just speechless and overwhelmed by the musicality of the presentation. We heard not only the deepest silence, but also an extremely sharply-defined and really stereoscopic sound, like we had never heard before.

What could we say about the noble and the deep soprano of Elly Ameling on an old vinyl record? Nishikawa-san expressed that this disk was his reference when he had developed the Pioneer P3 turntable system, but he had never experienced such an organic and moving quality with the voice of Elly Ameling until listening to Alan-san's system.

We were convinced that Alan-san would fully enjoy the sound of his system, the modified original Marantz Model 7 preamplifier combined with the PX25A (DA30) monaural SET using -3 dB P-K feedback circuitry, which produced such a natural and realistic reproduction of the human voice, which is such difficult to achieve goal in audio reproduction.

Well, if we ever have a chance to obtain the rarest old balloon type DA30, just like a Holy Grail, I am prepared to setup about -6 dB of P-K feedback for improving its high-frequency response, but sorry, we didn’t have the chance to try it until we find the vacuum tubes. 

The Parts for the P-K Negative-Feedback

The choice of resistors for the negative-feedback circuitry would be extremely important in determining not only the reliability, but also the sound quality.

For example, in the tone amp of fame, the original Marantz Model 7, the custom-ordered deposited carbon resistors, 1/2 W, 2 %, made by CGW were adopted for R49 A/B, 82,5 k ohm as the feed-back resistor, and R81 A/B, 4.7 k ohm as the cathode resistor, received the feedback current.

Yes, these resistors were used in pivotal points where precise values were needed, and in combination with the Allen Bradley solid carbon resistors, it allowed us to feel such a rich and refined sound full of musical atmosphere just like the good old days.

Surely, when I modified Handoko-san and Alan-san’s original Marantz Model 7 preamplifiers, I made sure of the true accuracy of these deposited carbon resistors. Despite the passage of time - more than a half century - and also the moderate tone balance, it would be sufficient for the current quality level, and so I dared not to exchange the feed-back resistor, R49 A/B, 82, 5 k ohm.

Then, almost a year ago, I found the solid carbon resistor, 1/8 W or 1/4 W size 82 k ohm, was used in this position in Roy-san’s Marantz Model 7 Replica preamplifier that was released in the middle of the 1990’s, so I replaced the resistor with a high-precision metal-film resistor, 1 W type, without any hesitation. Anyway, I hope you all know the fact of how important the quality of the feedback resistors are.

For the feedback resistor, 100 kOhm, I installed in Alan-san’s and Harald-san’s PX25A (DA30) monaural SET, custom-ordered Tepro RA 1 W, 47 k ohm, and 56 k ohm, with series connection, as I felt the Tepro RA was the best sounding metal-film resistor I have ever experienced.

The Tepro RA has amazingly wide-range, with clear tone and well-balanced rich timbre. The series connection of multiple resistors reduced the wattage of each resistor, reducing the thermal noise of the resistor.

Actually, we can hear this thermal noise as some kind of cloudy sound, and so minimizing the level of thermal noise brings about out clear-cut sound, and also allowed us to find the deep silence. When using the old balloon type of DA30, you can easily bypass these 47 kOhm or 56 kOhm resistors by using a jumper wire, allowing the use of about -6 dB of P-K feedback. 

About the WE310A’s cathode resistor, we should divide 1 kOhm into 900 Ohm and 100 Ohm for setting up -3 dB P-K feedback. For these resistors, I’d like to strongly recommend the use of the Ohmite WH, 5 W type wire wound resistor. Its sound quality is really unique, and ideal and stands out among all the current resistors available.I already described the detail, how to use the resistor in the modification of the Marantz Model 7 HERE.

But, I made 900 ohm by the parallel connection with 1 kOhm, WHE1K0FET, and 10 kOhm, WHE10KFET. And of course, we should make the choice of the WHE100FET for 100 ohms as well. As a result, 1 kOhm was composed of three resistors that leads to the improvement of sound quality, because of reducing the thermal noise of each resistor.

In addition to the capacitor for direct current prevention, I used a 3.3 micro Farad / 600 VDC, 940C6W3P3K-F, made by CDE. This capacitor is a type of metalized polypropylene capacitor, but it was designed for industrial use, and so its construction is robust, but also the E.S.R. (Equivalent Series Resistance) characteristic and Tanδare are very desirable over the whole range.

I also installed an Arizona Red Cactus capacitor, 0.1 or 0.22 uF, connected in parallel with the CDE capacitor for reducing the E.S.R. value, especially from the mid to the high range. Of course, the AZ Red Cactus capacitor adds some organic warmth to the sound of this polypropylene capacitor.

And one more thing, I’d like to share with you is the bypass capacitor for the WE310A first stage tube, where you have to exchange the usual electrolytic capacitor with a wet tantalum capacitor. The wet tantalum capacitor is really expensive, but it gives really clear imaging, and a fine tone of timbre, which many of you have long searched for.

Well, when I started this Part 4 article about the use of P-K feedback, I never imagined it would be such a long story, so thank you for reading all of this!

I only hope this article will be helpful to all of Jeff’s Place readers who are searching for “Real-Sound”, and who want to escape the harmful and the useless information of the current audio commercialism.

Also I want to take the opportunity to express my heartfelt gratitude for my good friends in Japan, Kato-san, Nishikawa-san and Kajiwara-san, but also Jun-san, who works in Infineon Technologies in LA and has long developed and introduced us a lot of innovative Class-D devices, as they have supported me every time, thank you so very much!

Then finally, I must say, for Jeff-san, thank you very much for giving me such an opportunity to describe my very personal experiences and thought.

I think, I should continue to write the article, Part 5, where I’d like to share more about the power supply for the plate and the heater in Alan-san’s and Harald-san’s PX25A (DA30) monaural SETs.

As I have said before, the power supply can be the key factor for getting a desirable and truly comfortable sound quality.

To be continued. 

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I would like to thank Yazaki-san for sharing his deep insights with us, about the circuitry he incorporated into the circuitry of Alan-san’s and Harald-san’s PX25A (DA30) monaural SETs, and about the optimum components to use, which I found to be a very intriguing and illuminating discussion.

Thank you Yazaki-san, and we will all be looking forward to Part 5!

As always, thanks for stopping by Jeff's Place, and may the tone be with you!