AEA Audio

Well I'm confused by Carver's comments for sure..!!!! Thanks Ken for your comments and I agree with your statement "The concept of using a speaker as a microphone to capture some “echo” while it is playing music cannot be done, for many reasons. The biggest reason is that the sensitivity of the speaker (ability to pick up sound) is almost zero when the cone is extended under force from the voice coil." "he has a way of using terms to describe technical things that can be a bit confusing" Boy you can say that again .... I understand damping and also that feedback effects damping but it seems to me Carver is talking about something different with his Speaker-Mic Reverb comments and what he is trying to accomplish with it.. What I don't believe is all Tube Amps via "speaker-microphone" pickup and amplify the rooms reverb at a perceptible level let alone that we would perceive it as reverb and an enhancement of the acoustical space. I've high lighted in Red and Blue his comments that make me believe he is talking about artificial reverberation enhancement of the recording via the "Speaker-Microphone" reverb effect. http://www.enjoythemusic.com/superioraudio/equipment/0512/bob_carver_cherry_180_tube_monoblock_amplifier.htm Speaker-Microphone Effect It is well known that a loudspeaker can work backwards, converting sound pressure into an electrical signal at its input terminals. Many years ago, it was Roger West who mentioned having used a Sound Lab ESL as a microphone, though I'm not sure with what degree of success. Carver believes that his amps can take advantage of this effect and says that: "The extra cool thing about this amp is that it has the ability to listen to the room via the principal of reciprocity. The loudspeaker the amp is connected to behaves like a microphone and "listens to the room." That signal is then sent back to the input to be mixed with the forward signal to help generate a more delicious sense of acoustic space by including the room in the final sound in a way other amps can't." I expressed my doubts to Carver about the audibility of this effect, noting that the speaker microphone effect couldn't amount to more than a few millivolts. Carver's response was as follows: "You are right - about 3.1 mV at 90 dB SPL at our listening position, then multiplied by the gain of the amp, 32 X = 0.0992 volts rms. With a high quality speaker about 89dB or so sensitive at 14 feet away we have about 8.5 forward volts at the speaker, and 0.0992 "speaker microphone" volts. Finally, 0.00992/8.5 = -38.6 dB, just barely noticeable, but quite significant after we get used to listening to it." "As low as it is. All tubes amps do this a little bit by their nature, transistor amps don't do it at all, and this amp does it a lot by intentional design. It's easy to hear the "oom signal if we hook up a small speaker on the end of about 150 feet of wire and go outside. Hook it across the main speaker in your living room with the amp turned on, but no input. Then have a friend come into your living room and clap his or her hands, stomp around, make noise and sing happy birthday. While you are 150 feet away outdoors with the door closed so you can't hear your friend from the room, hold the small test speaker in your hand and listen to it. You will hear the sound of the room. With a solid state amp, you will not hear a thing, just silence. All tube amps that have a non-zero output source impedance do it, at least a little bit as you surmised. Transistor amps don't do it at all because the almost zero source impedance shorts out any "speaker microphone" signal. Yes, my amps do it a lot by virtue of the current feedback loop. With this amp, lower idle current (as read on the meter), makes the amp do it more. Higher idle current makes it do it less." Yep, I also found it confusing when it was first mentioned in this thread. But after thinking it through, I’m still fairly certain Bob is really just talking about damping factor. The key references in the conversation are when he says, “All tubes amps do this a little bit by their nature…” and “Transistor amps don't do it at all because the almost zero source impedance shorts out any speaker microphone." Bob also says, “Listen… with the amp turned on, but no input…With a solid state amp, you will not hear a thing” These comments all point to damping factor. The microphone affect he mentions is indeed associated with the effects of damping factor because they are both caused by the same phenomena: That speakers produce their own “voltage/current” as the coil moves through the magnet’s field. This “counter current” is “shorted out” by a SS amp’s low output impedance, and acts like a “break” slowing down, or “damping” this effect, which slightly changes the way the speaker cone moves. You are also correct that feedback can change the amp’s "effective" output impedance, and Bob was famous for using inventive feedback methods to shape the way his amps sound.

This is an interesting concept. On occasion I've experienced a reverb, or echo-like, effect in some amplifiers which wrap the feedback loop from the opt secondary to the voltage amp cathode. When removing the global loop and replacing it with local feedback, the effect disappears making me believe that it's caused by phase distortion. Paradoxically though, in my amps which use frequency selective local feedback (from a typical series RC filter), there is no audible effect from the unquestionable phase distortion which is present. I look forward to Ken's thoughts on this. Maynard Sorry to have taken so long in responding. I believe the ”reverb effect” Bob Carver is talking about is caused by what is commonly known as “damping factor.” I’ve known Bob since the Phase Linear days, and he has a way of using terms to describe technical things that can be a bit confusing…( I won’t say misleading ). Its part salesmanship and part trying to use “layman” terms to make things more understandable by the average customer. The concept of using a speaker as a microphone to capture some “echo” while it is playing music cannot be done, for many reasons. The biggest reason is that the sensitivity of the speaker (ability to pick up sound) is almost zero when the cone is extended under force from the voice coil. Think of the effect of putting your hand firmly on the cone while trying to use the speaker as a mic. The pressure will keep the cone from vibrating, so it can’t pick up sound. The same thing happens when the cone is under force (being pushed) by the voice coil, causing any ability to pick up ambient sound to be greatly dampened. What Bob is actually talking about is the effects of damping factor, and more specifically, the ability of feedback to affect an amp’s output impedance, and therefor damping factor. One way to think of the effects of damping factor is its effect on controlling speaker cone “rebound.” Bob used the term “echo effect” to describe the effects of damping factor. The ability of his amp to respond to and control this “rebound” voltage with increased damping factor through the use of feedback is what he was touting. Bob is a very creative and innovative design engineer, and an equally creative salesman.

QFT. Key aspect: tube amps driving transducers. The folks using tube buffers or line stages expressly in pursuit of "tube sound" should take note. I would tend to agree. Just having a tube in the buffer or line stage would, in most cases, not make a great deal of a difference. It’s the driver and output stage where most of the difference between tube and SS is heard, and also some of that difference in sound is from the fundamentally different way in which they interact with the transducer (speaker).

If it were possible to use the old original caps, and get the original sound quality (and reasonable reliability) from a vintage amp, then I would recommend leaving the caps (and other components) in place. However, most (but not all) types of caps used back then have a service life (and/or shelf life) less than the 40 to 60+ years since they were manufactured. So, leaving them in place does not necessarily preserve the original sound at all. In actuality, the original sound quality has been degraded by component aging and by tarnish (I think of tarnish as ‘metal aging’). Certainly all electrolytics, and almost all film types, should be replaced due to their performance being quite different now than what it was when they were new. It’s not just that they become leaky, they also change a lot in the way they perform. That’s because their “characteristic operating parameters” also change over time. An example in the case of a capacitor is that even a new cap’s ESR and effective capacitance reactance changes with both frequency and with applied voltage. These operating parameters are taken into consideration in the original design, but after so many years these operating curves have changed quite a bit, causing the amp’s sonic qualities to change, even if the old cap checks good on a simple cap meter and shows no leakage. All out of tolerance components, including caps, resistors and tubes, etc. need to be changed for these same reasons: They can no longer function as they were originally designed to do. That’s why I think a restoration needs to include replacing all components that have exceeded their operational lifespan (as well as gone out of compliance). Unless you want to restore an old amp using all original vintage components for “historical reference,” you end up listening to something that has less sound quality than it originally had. As for which is better, tube to transistor, my experience is this: neither. They are different, tube distortion is “better tolerated” by the listener, but SS has a lot of other technical advantages that can be used to overcome that. However, the sad fact is that most SS gear (not all) that is aimed at the "mass market" is designed solely based on competitive profitability (price vs specs). And that’s why most (not all) of the mass market SS gear does not sound all that good (as we all know). I bring this up because the ubiquitous presence of SS gear tends to color our opinion of it. In my experience looking into this “difference,” I have concluded that tubes don’t color sound any more than SS does. They both do it to some extent, but they do it with different "pallets." And to continue on that metaphor: A really good “artist” can design an amp that is quite transparent… with either pallet.

I thought it might be good to walk through the whole recording and playback process to get a better understanding of the differences between analog and digital recording sources, as well as understand if analog recordings are better for tube amps. As we all know, acoustic music sources (those not electronically synthesized) start out as sound waves. These are then converted to electrical signals (usually with a mic) so they can be amplified and processed (mixed, equalized, digitized etc.). These original acoustic sound waves are then changed yet again to another medium so they can be recorded, copied and distributed in large numbers. The available recording mediums have expanded over the years, from vinyl to tape to digital. But the objective is always the same: to convert the original analog electrical signals into some form that can be stored, copied, and then converted back into something close to the original electrical signal, allowing us amplify them so they can be converted back into sound waves. The point is that it’s always analog voltage in— analog voltage out, no matter what the recording medium. Of course there is a lot of difference between how these different storage methods work, but they are all used to recreate an analog signal that is (hopefully) very close to the original analog signal from the mic(s). The difference between recording methods, and their merits, is another subject, but you always end up with an analog signal. There is no inherent difference between vinyl and a FLAC file that would favor either tube or SS for playback. No matter if they were recorded on an analog board or a digital board, they all start with analog voltage from a mic and end up with an analog voltage going into your amp. That brings us the question of what the difference between tube and SS amps is, and why they sound different. I don’t want to open that debate too wide, but I do want to say that the difference is irrespective of the recording format itself. The difference in sound between tube amps and SS amps is something I have spent a lot of time analyzing, and so have quite a few other people. The short answer is that it’s not frequency response, or how much distortion they produce, or what source material works best. The difference is mostly in how they produce distortion, and how they interact with speakers. They both do these things in highly different ways. SS amplifiers did not replace tubes because they sound better. It was economics. SS gear is a lot cheaper to make, especially on a watt for watt basis. SS is also more user friendly (no tubes to replace, lighter, smaller and less heat). Tubes don’t “color” the audio any more than SS does. Neither type of amplifier is perfect. A good design engineer can make a great sounding amp with either. However, having designed both, I will say it is generally easier to make a good sounding amp with tubes. On the other hand, it’s also easier to make a good sounding high power amp with transistors… These technical differences between SS and tube amps are at the heart of why vintage tube amps represent such outstanding performance for their relatively low cost. No manufacturer today can build a SS or tube amp that has the quality of components and sound that a properly restored high-end vintage tube amp has for anywhere near what they are going for. (Which is why vintage tube amps are steadily going up in price as more people realize they have a huge bang-for-the-buck advantage.)

It's great to see performance testing after your rebuild to verify the unit is operating within or above specifications. This is clear evidence that your ultrasonic cleaning has done zero harm to the unit as some fear might happen. The test you posted and especially the Video of the dynamic live (input vs output) test is very impressive..!!! Can you elaborate on this particular test and others your using after a rebuild? miketn Thanks! We certainly believe in rigorous testing, as well as critical listening, to be certain things are working correctly after ultrasonic cleaning, restoration, and/or modification. Here are the performance graphs we made for this Fisher X-100B after ultrasonic cleaning and the rest of our restoration/modification process: The technique of comparing the input and output signals while an amplifier is playing actual music was something we pioneered back in the late 70’s. We used what were called mini-computers (forerunners to the PC) to digitally sample the audio signals and run complex mathematical analysis of the difference. I might add that these tests are done while the amp is playing music into speakers and not into a dummy load. This type of computerized testing was unheard of back then. Computers were very crude and couldn’t do the test in real time as you saw in the video. Back then it literally took hours of number crunching to analyze a minute of music. Now A to D conversion is pretty easy to do, and we can do analysis in real time and display it directly on the screen as the music plays. However, what is still unique about our technique are the methods we developed to mathematically take out errors caused by the amplifier’s frequency response and phase shifts, to find the underlying distortion components produced from live music. What we found was quite interesting. And also proved why traditional steady-tone THD and IM tests are next to useless for measuring how an amplifier will actually sound in the real world with music playing through it. Basically we found new types of what we refer to as “transient intermodulation distortion.” It is distortion that changes with conditions in the music signal such as combinations of signal levels, wave forms and spectral content. We also spent a lot of time figuring out what it was about each particular circuit type that caused these transient anomalies. This new insight allowed us to create IC preamps back in the 70’s that could sound as good as the best tube preamps. Something most thought impossible. Unfortunately we became so interested in what could be done with computers that we stopped developing audio gear and went full time into designing computer systems. I say "unfortunately" because I think we could have had a greater positive impact on the development of audio systems if we had stayed in that field… Oh well. But now we are back into audio again, after a long sabbatical, and we have plans to go back into developing systems utilizing computers tor help make analog audio more enjoyable.

Ken I've noticed you included Tone Controls (selectable if desired) in the AEA 520 pre-amp which I consider valuable features due to non-standards in the recording-playback-listening room chain. I actually run the EV DC-One DSP unit for my Jubilee system for bi-amp operation but also I have filters programed to simulate the Cello Palette Tone Control adjustment features which I find invaluable for my enjoyment of many less than ideal recordings. Just curious due to your background have you used or experienced any of the Cello equipment with tone controls and how do you feel about Tone Controls today? miketn Yes, I still feel tone controls are a useful feature, especially if they are by-passable when not needed. Having designed mixing consoles used in many recording studios, I also agree with statements made by Cello that the tonal balance of recordings is "manipulated" by the recording engineer based on both their “taste” and on their monitoring environment. Every mixing board I designed had tone controls (with at least 3 bands) on each input, and often included graphic equalizers on the output channels, and they did get used. The best engineers usually did a good job of trying to use equalization to only correct for recording defects. Some engineers went to great lengths to listed to several different monitors to be sure they were not over correcting based on one particular speaker/amp combination. But not all recording engineers are equal when it comes to their ability and time allowed to correctly compensate. Plus there are things like room acoustics, speaker performance, and personal tastes that can sometimes benefit from tone controls. So I would have to say that if done correctly, they can be quite helpful in recreating the playback tonal quality originally intended by the artist and recording engineer.

Well, nice to meet you Ken, even if it is electronically. You guys made a cool splash in one of the greatest eras in HiFi. Of course, us old guys always think it was better back in the day! Looks like Lloyd did a marvelously detailed job of assembling the tutorial. It's really nicely done and the photos are quite appropos along the way. I avoid antique electronics like the plague, myself, but I know there is an adoring fan base for that old school sound. Personally, I praise the day Stewart Hegeman schooled the world on wide-band tube design with H-K. Now those, I'll listen to! I also admire Bill Johnson for having the cajones to buck the SS trend as long as he did in the era. He made it possible for guys like me to introduce tube stuff "under the glow" which he was creating with a 1000 watt torch! Anyway, good to see the old names pop up here and there. I wish you and your son great success with your new venture in resto-mods. Have fun. Thanks very much! It is nice to be back in audio after spending nearly 40 years designing computer systems. We had a lot of fun, and of course learned a lot, along the road to designing the first generation of SS gear that could give the best tube designs a real run for their money. In a way, the result was an even deeper appreciation for just how well tube gear can perform. My involvement with vintage restorations seems like some sort of karmic full circle: Going from designing SS to beat the best tube equipment to breathing new life into the old tube classics to show that they can still outperform most SS designs. We may have won the battle back then, but it looks like we lost the war…

Thanks everyone. This is a great community and we are happy to be here, sharing and learning along with everyone. After experimenting with ultrasonic cleaning, we decided to share our experience with the community because it seems to us to be a method that can solve a lot of restoration problems associated with tarnish and corrosion. We totally understand the controversy over this method of cleaning. We did a lot of head scratching ourselves as we got into it. And no offense taken at all by the obvious concerns raised. We invite questions about these techniques. Being skeptical is not only healthy, its necessary in the case of new ideas. However, one can be skeptical of new ideas without being paralyzed by their preconceptions. If we let our prejudice limit our exploration of new ideas, we never make progress. ​ As for "poof" that it works, we ​have tried to document our own experience with it, and address the issues around submersion of components. I first learned about these methods years ago in the manufacturing process, and later in restoring vintage tube equipment, including audio and ham radio gear. I personally have many pieces of tube equipment from the 50's and 60's that I restored many years ago by submerging them in hot soapy water in a large sink for up to an hour while they were hand scrubbed. And they are all still "show case'"quality in looks, and function like new with most of the original components still in place. The video we linked to by Bob Sumption is a great resource to learn about restoring old tube gear. Most of the methods he demonstrates translates directly to any tube gear, not just radios. The video linked is only one in a longer series. Bob is an advanced class amateur radio operator and retired electronics engineer, and extremely knowledgeable on these subjects, having actually designed tube gear back in the day. He is very highly respected in the amateur ham radio community. There is no real difference between scrubbing down components and leaving them wet for up to an hour- and immersing them under a few inches of water for a few minutes. To summarize the whole issue: The materials in electronic components are not water soluble, but they can be damaged by moisture that is adsorbed into them when voltage is applied. However... no moisture remains in them after the drying process. That is the key... after proper drying there is no moisture at all left inside. Also, the real benefit of ultrasonic cleaning is that it removes not only dirt, it removes tarnish from years of corrosion. And most importantly, it removes tarnish from wiring, inside tube sockets, jacks, switches and pots. There is no other practical way to do this that doesn't require an inordinate period of hands on work, and the benefits are obvious. We would like to encourage experimentation with ultrasonic cleaning techniques. Our experience has been very positive. This could be a big step towards more complete restorations, giving longer life and more enjoyment for all us of these fantastic pieces of equipment. - Ken

Yes, this is the same AEA, and the same people who designed the 520. My name is Ken Leonard and I was the lead designer. Thanks! Always nice to hear from someone who remembers the 520. And I'm always happy to answer any questions about the old AEA line. There is an interesting story about the 520 preamp and how I came to design it on our site at http://aeaaudio.com/legacy-page/. It was actually very much as you described - a crazy startup challenging the SP3. My son, Lloyd Leonard, who convinced me to recently restart AEA and who now works for the company, originally wrote and posted this thread, sharing the method of cleaning and the restoration process we currently use.

First of all, we would like to thank everyone who chimed in on this subject for their interest. These subjects are all things that interest us also. And we certainly agree with people approaching things they may not be familiar with by using a heavy dose of critical analysis and healthy skepticism. Our hope is that the information we provided will be useful, and we will try to answer anyone’s questions if we can. Contrary to what almost everyone feels when they first hear about immersion cleaning, or using soap and water to clean electronics, this is an old and tried-and-true method of restoration for vintage tube electronics, especially tube gear. It has been used for years by many. One of the best YouTube tutorials about this was done by Bob Sumption. Bob shows how to wash, rinse and bake the chassis of an old Heathkit tube radio, but the same technique can be used on all old tube gear. Buy the way, Bob worked as an engineer for Heathkit, designing tube ham radio gear before he retired. We strongly recommend the techniques Bob demonstrates for restoring old tube amps: What we have done is experiment with adding ultrasonic methods to improve this process. We have used the more traditional soap and water hand scrubbing method on all kinds of vintage tube gear for many years on our own vintage equipment, long before starting a vintage tube amp restoration service. After using this new ultrasonic method with a large number of amps, and seeing its benefits, we decided to share it on this forum in hopes readers would find it useful. We found that using ultrasonic cleaning, with tarnish removing cleaners added to the water, results in an almost miraculous transformation. It’s not only restoring the shine on what you can see, it’s that the tarnish is removed from places that were not possible before, including all the connectors, sockets, controls, component wiring, etc. We do not want to start a debate or argument, but want to reassure everyone who is interested in these cleaning methods that if done correctly and carefully, they do no harm components in any way. The amp we used to document the restoration process was from our own stock of vintage gear we acquire for restoration from time to time, not a customer’s amp. When we got the amp it was not working, with all the tubes and a knob missing. It was quite a mess and probably had not been used in over 20 years. Hopefully the results speak for themselves. To answer some specific questions: Yes, of course we return bad tubes to customers, as well as any parts we replace. And yes capacitors and resistors are manufactured in sealed coatings or containers, and suffer no harm. Any water that might enter a component is evaporated in the drying process. The drying possess should be done directly after the cleaning. We do not recommend ultrasonically cleaning for power and output transformers because it has a possibility of abrading the thin ceramic insulation used on the windings and this could cause troubles due to the high voltages present. Receiver IF transformers, on the other hand, are not effected by ultrasonic cleaning. Yes, we wouldn’t recommend using orange drop capacitors for speaker crossovers either. We use them for what they were designed for, and they work great. Although our company is indeed a recent arrival to the community, it was originally founded by our current CEO and lead designer/tech way back in 1970, designing and manufacturing recording studio equipment and high-end audio equipment. Our team has a lot of experience in audio design, and in other fields of electronics design, along with advanced degrees in physics as well as a long history of founding successful technology companies. We certainly don’t claim to know-it-all by any means, and always welcome constructive criticism and new ideas or suggestions. Our aim is to participate with the community in learning more about audio equipment and the enjoyment of music. Along with everyone here, we want to share information and learn from each other’s experience.

Actually most transformers are quite robust and the insulation and wiring is more often than not not "brittle or crumbling." We have yet to see such a case, but you're right in that some really old amps can have transformer wiring in such a condition. However if the insulation is indeed crumbling, it needs to be repaired for safety and longevity reasons. Such repairs are not so difficult since the leads coming out of a transformer actually terminate inside the transformer where they are connected to the special wire used for the windings. I certainly understand how you feel about water and cleaning detergents. Many people are a bit shocked to hear that soap and water cleaning is actually good for potentiometers, switches and other components, but they soon find that this is an old tried-and true technique that has been used for years, and is highly effective in removing the corrosive buildup that makes old controls noisy and intermittent. In fact, there are many good tutorials and videos online on restoring old tube equipment that document these methods and the terrific results they bring. Yes, I know the size of the capacitors must be shocking. There has been a great deal of technical advancements in capacitor design and manufacturing over the past 50 years. Today's electrolytic capacitors are much smaller, and much better, than what was available back in the 50's and 60s. The caps we use actually have more capacitance (up to 200% more), higher voltage ratings, lower ESR and can tolerate higher temperatures than the original parts. Plus they are a lot smaller and have a much longer life-span rating. We take advantage of the smaller form factor by disconnecting the old-fashioned metal can capacitors and wiring in their replacements under the chassis. That way we preserve the original aesthetics of the vintage amps, while vastly improving their power supply's performance. As for coupling and bypass capacitors, we select the best type of caps for each particular circuit. The caps you are referring to in the photo are CDE polypropylene types, specifically made for top performance in the types of bypass circuits we install them in. As with all the caps we use, they are chosen for superior sonic performance, and are of a much higher rating and quality than the originals. Yes, replacement tubes are often a problem with vintage amps. We only replace tubes that are at or near the end of their service life. As for Russian tubes, the ones we choose to use are of high quality, and are completely tested by us to perform as well as the originals in the particular vintage amp we are installing them in. We do this selection process by detailed tube performance analysis with our curve tracer. This allows us to test the tubes under the exact same operating conditions of the amplifier they will be placed into. By doing curve tracing, we can match the performance characteristics of a replacement tube with the performance curves published for the vintage tube it is replacing. This is the only way to be certain the tubes will perform the same as the originals. Replacing and matching vintage tubes is a really important area that we have done a lot of technical research in. Our website has a lot of information about why and how to use curve tracers for this, and why common "tube testers" don't provide the detailed information required to do this properly. What we strive for in our restorations is to bring the vintage amp back to or surpass its original performance level, while enhancing its physical condition. Removing tarnish is a big part of restoring the original reliability and sonic performance, as well as the appearance. Yes, these amplifiers did have a good amount of 'bling' when they were new, with copper or cadmium plating and lots of shiny metal, especially compared to the modern "sterile box' look of many SS amps. We feel that this is part of their charm. Removing 50 plus years of tarnish not only restores their good looks, it improves their reliability, sonic performance, and their long-term value. As far as upgrades go, we offer a lot of them, including regulated power supplies, improved biasing circuitry, soft-start high voltage, and even custom total gut and rebuild performance mods. We feel many of these modifications are the types of things the original designers would have done if the technology had been available back then. Fact is, that we have been designing and manufacturing high-end audio gear since the 70's. Saul Marantz and Avery Fisher were on our advisory board back then, and we feel a deep commitment to helping keep their legacies alive.

SOLD

Thanks Maynard! Very good question. We were also concerned with the IF/RF transformers in receivers/tuners, but surprisingly haven't had any trouble with them after ultrasonic cleaning. We flush the transformers out thoroughly with water after dunking a receiver/tuner in the ultrasonic cleaner, and make sure the whole unit is completely dry before running and tuning it. We have had great success with this method.

Thanks! Feel free to ask any questions you may have about cleaning / restoring your own amps.