Positive Feedback in Tube Power amps, Digital/Vinyl Accuracy

I keep banging on about accuracy. I think it is the most important aspect of hifi reproduction, but it seems not all audiophiles agree with me.

The emphasis on audio quality these days seems to be on a “musical” and “liquid” sound- everyone wants their system to sound “nice” and pleasing.

Audio equipment isn’t judged  so much on reproduction any more but on a gratification process.

Audiophiles are perhaps the only people who don’t want to hear what the musicians actually made, they want to impose their own interpretation of how the music  should sound.

Reviewers rave on about the quantity of bass or the “magical” midrange of equipment  that from a reproduction viewpoint should have as it’s first priority absolute accuracy.

How does a reviewer really discern the difference between two dacs when he’s using a tube amp which erratically swings 3dB either side of a flat response?

I don’t get it, and I can understand why those in the professional side of music making- the recording engineers, masterers etc think all audiophiles are crazy.

The good ones go to a lot of trouble to get the recording as close as possible to the sound of the band as they recorded the songs and then we the audiophiles use equipment that is “nice” sounding but so far from accurate it is embarrassing .

Surely the whole point of high fidelity reproduction is to hear what the musicians recorded with nothing added and nothing taken away.

I hear a lot of systems, but I probably hear 1 or 2 accurate systems a year,  sometimes none.

And I’m considered a heretic, my main system is pretty good for accuracy, but I haven’t gone to the considerable trouble to get the room acoustically perfect, so my absolute accuracy would be lucky to get a rating of 80%.

Virtually all the systems I hear would not get to 50% of a reasonable  “ accuracy” rating

 If there is a typical tube amp in the system it is often worse.  All for the sake of a “musical” sound, the dynamics are compressed, the frequency response is bumped up in the midrange , the bass is more bloom than bass . 

Now we are getting 96Khz and 192KHz sampling rate recordings we can actually listen to, whereas previously the recordings were often done at these higher rates and then “dumbed” down to 44.1Khz for CD.

But to hear the very obvious advantages of these much better quality recordings, we need to use higher quality ACCURATE equipment. As well as the new breed of usb-spdif convertors, 24bit and higher dacs with sampling rate capability up to 352.8 Khz, we need accurate electronic equipment and speakers to resolve this new level of recording quality.

If we are using speakers with 10 dB frequency response aberrations (and they are very common), and preamps and power amps that cant deliver a flat response into their respective loads , then in my opinion we are wasting our time with the new breed of higher resolution studio master recordings.

You may prefer a system that is warm and liquid but please don’t impose your will that it is the “best” , because judged on the only real criteria , accuracy, it is an abject  failure.

I was recently criticised for my statement that state of the art digital recordings, using the highest quality 192Khz sampling rates, well recorded and engineered, and played back on state of the art equipment, both digital and analogue stages,  has actually equalled the quality of the best vinyl.

Now I have a couple of very good vinyl rigs, around the $20-30K mark, have been building phono stages all my adult life,(30+ years) have nearly a thousand records, and love the sound of vinyl.

I have also got around 10,000 hours of critical listening experience, plus many times that of pleasurable listening enjoyment under my belt, so in my maybe not so humble opinion I have a clue about what I’m talking about.

The howls of protest came for two reasons. Firstly the detractors don’t have the equipment to hear how good the high resolution files are, or they haven’t even heard them.

Secondly, vinly playback is affected by many more potentional colourations- tonearm resonance, turntable resonance/colouration, imperfect RIAA filters, impedance interactions problems between cartridge, RIAA preamp,and preamp. Flat frequency response cartridges are very rare, indeed the audiophile world shuns flat response cartridges.

Fortunately for vinyl, all these imperfections result in a pleasing sound that was clearly more preferable to the poor digital sound pre 2011.

Not so now, where the near perfect accuracy of digital is now equally state of the art- vinyl still sounds great but it is no longer the only high quality medium .

But that coloured sound of vinyl is still considered the  “standard” by which the modern recording is judged by audiophiles.

If you define sound quality subjectively  that is, if you are only interested in your interpretation of how music should sound, you will never agree with the above, and you will spend your life searching for the perfect  hifi sound that agrees with your ideal.

Get yourself a vinyl recording of Neil Youngs Archives , and the same in 96Khz digital.

Young hated digital and wouldn’t release any of his material on CD for a long time, and rightly so, but a listen to the 96Khz recording off the master tape, compared to the vinyl convinced him, but the majority of the audiophile world still need to catch up.

Have a listen to “Down by the River”  - the first few bars of the amazing transperancy and openness of the 96Khz recording, compared to the vinyl should convince you ,IF you have equipment capable of resolving it. The digital IS the recording, the vinyl is an interpretation of it, it might even sound better, but it is not as pure and unflawed as the digital.

It is true that compared to vinyl, there are very few hi-rez albums available, but that will change.

I’m not deserting vinyl, as there are many thousands of wonderful sounding vinyl records available, and very few equally wonderful sounding digital, but I’m looking forward to future  hi-rez digital  releases that will  soon surpass the best vinyl sound.

The “best “sound can only be the most accurate, anything else cant be quantified.

Ok enough ranting , let’s get to the point of this article.

Positive feedback has been around for a long time, there were a few American tube amplifiers which used the concept back in the late 50’s (the so called Golden Age of tube design) .

Used properly positive feedback can lower the output impedance, or increase the damping factor of tube power amplifiers.

Even big powerful tube amps, using lots of negative feedback struggle to maintain a flat frequency response into a real load.

The diagram below is frequency response of a 400 watt tube power amp- the black line is the response into a simulated speaker load and  whilst this is actually pretty good for a tube amp, it is far from ideal and partly accounts for “tube” sound.

Using more negative feedback can help to improve the situation, but this is about as good as it can get with negative feedback and using too much negative feedback can make the sound “closed in” and veiled.

Positive feedback on the other hand, can increase the damping factor by a large margin, in theory up to infinity, although not realized in real life.

And positive feedback doesn’t have the sound degrading properties of negative feedback.

With positive feedback , and the resultant higher damping factor, the amplifiers response into a speaker load is much more linear .

The Supratek Mondeuse and Malbec  tube power amplifiers both use adjustable positive and negative feedback, with the option to turn one or both off completely.

This gives a huge range of control over the sound, from typical “warm” tube sound, to highly accurate almost solid-state like sound.

It is interesting to follow the observations and operating points used by new Mondeuse and Malbec owners.

Most people like the midrange bloom of tube amplifiers, but when you can change the sound of an amp to suit your speakers, system, room and taste it does give you a huge advantage over a amp with a “fixed” permanent sound.

Most owners of Mondeuse and Malbecs do seem to end up using a neutral sound that is accurate and musical- not many tube amplifiers can be so versatile.
Kevin Covi is responsible for the design of the positive feedback  amps, and to show the concept here is a Covi Single Ended design with positive feedback. The SE amps really get a significant technological advantage with positive feedback as they have high output impedance to start with. The frequency response typical of the above measurement is greatly improved, becoming much more linear. It's still not flat, but is reasonably accurate.  Prototype monoblock amps very similiar to this  for sale.
We no longer build SE amps however, for your interest only

More Circuit Design from the Archives

Going back through some old circuits I came across this circuit I designed back in 1999, 12 years is a long time in audio design, but I remember this fondly.
I built hundreds of prototypes, some just worked really well from the initial turn on, some tooks months to finalise, and some were just magic . And plenty were duds. This was one of the very good ones- a Directly Heated Triode Preamp, that is transformer coupled output, direct coupled between stages, uses uncommon DHT tubes.

The input tube is a 30, which  is a high gain dht triode, battery biased and directly coupled to the output tube which is a 33, a dht pentode, triode connected. It makes quite a respectable triode with low plate R - this gives the circuit low output impedance and will drive any power amp with excellent linearality.
The disadvantage with this preamp is both tubes are microphonic, something which has never worried me, I suspect it contributes to the euphonic sound of these circuits,but it freaks a lot of audiophiles when they hear the tinkle of a microphonic tube.
This design has aspects which are common to the DHT pres I build now, but they are much more evolved with different loading for the tubes, parafeed outputs, different biasing, fully shunt regulated power supply, etc, etc.
This circuit also works well with 71A DHT output tube, with adjustment for bias.
Still , most people would be very surprised by the sound, and I'm going to have to build another just for nostalgia.

Computer Audio for beginners.

I'm a big fan of computer audio, and I listen almost exclusively to digital files off computers now.
The improvement in audio quality has been downright amazing over the last 6 months and I think even the most diehard CD user realizes that computer audio (CA) has now overtaken CD transports in terms of quality.
However there is no denying that computer audio can be very confusing, and daunting to the inexperienced .
There are a myriad of systems, and more than a little computer expertise is needed to be successful at achieving the fantastic sound available now.
If you'r a baby boomer or older it can be mind-boggling, but you can be assured that computers are not going to go away , you may as well get stuck into it, as you are going to need these skills in the future for more than just listening to music.
I'm not going to attempt to describe all the possibilities of CA, or even try to demonstrate the "best" but simply to show what works for me. This may or may not suit you, but hopefully if your yet to take the plunge into CA , it may give you a few ideas.
I have 2 systems- one based on Windows 7 using a Acer Laptop, and the other OS (Operating System) is Mac Snow Leopard using a MacMini with seperate display screen.
Both have advantages and disadvantages-if I was going to just have one i would use the Mac- I think it sounds slightly more natural and musical, all things being equal, but others will disagree. Use whatever you are comfortable with.
Lets start with the Mac system.
The MacMini is a small footprint computer about 150mm square and around 40 mm thick. It has no display, so you have to connect an external display screen (cheap) or use a TV screen or other video device to set up your system and view the artwork, files etc of your music.
We need a program to play music files- obviously the easiest to use is iTunes, and you really shouldn't go any further into CA until you can competently use iTunes to burn discs, download music and most important play music with your computer. Using lossless files with iTunes gives surprisingly good sound.  Lossless files to use with iTunes are ALAC and m4a, or just look for apple lossless files. Lossless basically means they are uncompressed audio quality files.
Once you get the hang of iTunes you can download one of the music programs that claim to give better sound- PureMusic, Audivana, Decibel are the 3 I use, the latter two because they can easily play flac files which is a common format for downloaded files from the internet. I like PureMusic because it integrates with iTunes to make a very accessable platform, although it is a bit cloggy with flac files.
 PureMusic basically takes over the audio playing side and leaves iTunes to manage file selection, playback etc.
 I think it is the best bet for beginners wanting to test the waters of hi-rez files. It is a bit tricky initially setting it up but so easy to use once it's going.
The macmini has usb outputs so we need either a dac capable of usb input or a usb-spdif convertor, I prefer the latter , the Audiophilleo2 and the M2Tech Evo I use sound excellent, although I'm sure there are some great usb capable dacs out there that dont need the convertors.
For a dac you really need to choose something that has a minimum capability of 24 bit/192KHz - there aren't a lot of 192Khz files now, but that will change, so you may as well future proof yourself.
I use 32bit/192Khz Sabre dac and a 24bit/96Khz Lavrey DA10 dac.
Dont even consider any of the old 16 bit dacs- despite what some will say they are so off the pace you have to doubt the hearing ability, or the equipment of their proponents. I've built quite a few 16 bit dacs using the 'legendary" TDA1541 chip, with very evolved circuits and output stages, solid state and tube, but they are no match for a good modern 2011 dac with a good spdif convertor.
So you just have to download some music files and in theory your away. It wont be that easy though as there will be programs to set up and computers to frustrate you. Dive in, keep calm, you'll get it sorted in time and enjoy the music and the accessability that CA gives.
I use an external hard drive with nearly 2Tb of music files on it- that's about 6000 CD's. I can sit down in the sweet spot with my iPad and remotely choose whatever track i want to listen to from my seat, uninterrupted, no need to change CD's , find CD's- I can view artwork, look ahead while listening , it is simply enjoying the musical experience!
Windows is basically the same, except for different programs for playback, you can use windows own Windows Media Player, but I find it frustrating- I persevered with JRiver Media Centre  till I figured it out.
I think the windows audio quality is very "hi-fi', to my ears mac is a bit more natural, but there is no denying that the resolution and detail of a good 96Khz studio master through a highly tuned windows CA system is very exciting. I only have experience with vista and 7 , cant comment on older platforms.
Any of my customers using a system like either of the above is welcome to email me for any advice setting up etc, but please only with equipment listed, I dont have experience with other brands or systems.

Circuit Design from the Archives

Over the years that Kevin Covi and myself have worked together , not all the designs we have finalised have gone into production.
This particular circuit is perhaps one of the most complete designs we have done, however Single Ended circuits dont really sit well with either of our audio philophosies , as we are both mostly electrostatic speaker users and these type of speakers just dont work with Single Ended amplifiers.
Over a period of more than 20 years I've built many SE amps using all the classic output tubes- 300B,211,845,2A3,45,212E, and some with rarer tubes like the 10,71A, DA41, 813,811,805.
My favourites were the 211 and 212E and there's no denying that a well designed SE amp working into an ideal load, can be very nice sounding, however the high output impedance of all SE amps make them less than ideal in terms of accuracy.
Have a look at this frequency response measurement of a very expensive SE amp- the black line is the response into a simulated speaker load- hardly what you could call accurate!

Even the response into an easy resistive load is just barely ok.The problem is the high plate resistance of the triode output tubes doesn't allow for a low output impedance.
Plate resistances are usually over 1000 ohms, some many times that.
One tube that has found a lot of favour with SE designers is the Russian 6C33C-B . This tube has a plate resistance around 100 ohms and with a good output transformer the performance , while still not ideal, can be much improved.
The tube does have some quirky features though that makes it a little difficult to work with- the characteristics vary quite a lot from tube to tube, making them difficult to bias consistently, they also change with operating temperature. Using a combination of cathode and fixed bias helps a lot (100R on the cathode and negative bias supply on the grid) but owners need to keep a close eye on the operating points to ensure consistent performance.
Kevin Covi came up with a circuit that gives excellent performance and maintains consistent bias , and operation regardless of the tube characteristics and temperature. The servo circuit in the amplifier has worked flawlessly for the 5 or 6 years I have had this amplifier. Once set to 250ma current , it has sat there consistently- this extends tube life, and ensures the amp is always working optimally.
The actual amplifier circuit design is nothing groundbreaking or revolutionary- just very good solid design engineering- notice the use of a 300B triode to drive the difficult 6C33C-B, the very solid power supply with shunt regulator. Many variations of this circuit were tried, choke loads, different driver tubes etc, but this circuit ended up being the most pleasing.
I believe this amplifier is the equal of any SE amp available now, including the ridiculously priced exotica marketed for those with too much money.
Kevin has come up with many fantastic designs over the years, and I this is one of his design high-points.
Like all SE amps it has to be very carefully matched with speakers, and no SE amp can be regarded as "accurate", however with the right choice, something like a Tannoy Gold Monitor , the combination can be very rewarding .
Keep an eye out for other circuit designs to be showcased here.

.

 Completed 6EM7-300B-6C33C-B SE monoblock (2 for stereo)

A hi-rez digital system

My previous post was all about getting optimal performance using a tube preamp with hi-rez (96KHz or higher sampling rate)digital files.
My definition of optimal performance hinges primarily around accuracy. I dont want "nice" and I dont want "musical" - I want to hear EXACTLY what is on the recording. If we finally have a chance to hear the studio  masters that the recording engineers hear before they get dumbed down to 16 bit/44.1Khz CD quality, why would you want to use equipment that doesn't let you hear the difference?
If your equipment cant easily discern the difference between 16 bit and 24 bit, dont bother with hi-rez files.
Similiarly, if you have a closed ideal how music should sound, and you want all recordings, good bad or whatever to sound "musical" or "nice" then hi-rez digital is wasted on you.
Accuracy is everything, if you dont know the truth, you have no base to interpret on.
Having said that, a Tannoy Gold Monitor driven by a well designed tube system can sound very nice, but for the purposes of this blog we are looking at high resolution, free of coloration.
(just to confirm I'm not an audio fundamentalist I do own a vintage pair of Tannoy Golds)
The system shown here is one of 4 systems I regularly use, but this is the one I use most for critical listening of digital files, and audio equipment , using both digital and analog sources.
This system takes no prisoners, it is ruthlessly accurate, as dynamic as any horn system, can play at near concert levels and stay clean and controlled. It is very low distortion ,  and easily discerns the difference in sound quality of recording mediums.

The speakers are based on Martin-Logan Request electrostatics, but with extensive modifications. The passive crossovers are removed, and line-array multi ribbon drivers take over the lower midrange duties. I think electrostatics are wonderful, accurate transducers but imo they tend to have a "plasticicy" sound  as they get down to the bass ranges, whereas ribbons have an outstanding  response from the upper bass regions all the way up to upper mid where they are overtaken by the electrostats for linear response.
The speakers dont sound like "ribbons" nor do they sound like "stats"- they reproduce the music with vitality, precision and accuracy. 
The ribbons and stats combined have an amazingly accurate naturalness, that is very low distortion, free of any grain. The transition at the crossover points is seamless and undistinguishable . The bass driver is the stock 12" Martin-Logan in a large  sealed cabinet with sub bass EQ applied via the remarkable DEQX crossover.
This is the version with optional Jensen output transformers and is completely free of any digital  "sound" .
Crossovers are all 24 dB , linear phase .  Power amplifiers are a Ramsa 100 watt for the bass drivers, chosen simply for it's sound. For the 4 channels of ribbon and electrostatics, a Carver Sunfire multi-channel amp is used. In a just world this amplifier would be considered as one of the greatest amplifiers ever built- it's main attribute is that it is "load invariant" - it can deliver prodigious power into virtually any load, and do it with a degree of accuracy rarely found in any other design. Capable of delivering 1000 watts into 1 ohm it is the perfect amplifier for ribbons and electrostatics.  It has a neutral tone and simply amplifies the signals from the DEQX crossover ,nothing added, nothing taken away- what more could you want from an amplifier?
One of the greatest classics in amplifier design ,incredible design, incredibly reliable , my upmost respect Mr Carver. The Martin Logans are the only electrostatics that can handle the power of the Carvers to play at concert level, classical or Led Zeppelin, as loud as the real thing yet still clean and full of realism.
The DEQX is driven by one of my tube preamps, either a 6SN7 or 101D DHT version, see a previous post on why I consider a tube preamp a necessity in all systems.
Digital sources at June 2011 are :(1) windows 7, JRiver Media Centre, M2Tech Evo usb-spdif, Sabre ESS32 dac, and an exaU2I usb-I2S interface capable of 32bit/352.8 KHz that can provide direct I2S into the Sabre dac.
(2)MacMini, PureMusic,Audivana,Decibel players , Audiophilleo2 usb-spdif ,LavreyDA10 dac (24bit/96Khz)
Analog source is a Luxman PD444 direct drive turntable with Dynavector 505 arm, SPU cartridge, FR-64 arm with Denon DL-103 cartridge (both cartridges modified by Expert Stylus in UK)
Also have a Micro-Seiki RX5000 turntable but with digital files improving so quickly I dont have much time for vinyl at the present.
This system is the result of more than 30 years of audio experience, and while not suited to everyone's taste is a source of daily joy. It's only fault is that it beams , and is a one person"sweet spot" speaker .

Hi-Rez digital and Tubes??

Computer based digital audio just keeps getting better and better. A year ago I would never have believed that so much progress would happen in such a short time.
I haven't used a CD for a long time now, and most of my listening is done with computer based files. Even CD quality 16 bit/44.1KHz sounds better off the computer than using a CD transport IMO.
The difference is even more apparent when we start using higher sampling -96KHz and 192Khz files are getting scarily close to the best vinyl sound. Of course the proviso is that the recording technique, engineering , mastering etc has to be top class to take advantage of the superior resolution of the higher sampling rates. A badly mastered 96Khz recording can be beaten in sound quality by a a well engineered mp3, but when all things are equal the best high sampling recordings are simply awesome.
But don't expect miracles from any digital gear more than a couple of years old, sure some of the old classic dacs can sound "nice", but the real advances are recent.
Like wise , the rest of your equipment must be capable of resolving the extra resolution of hi-rez files.
Some of the digital gear I am using at June 2011 is: M2Tech Evo and Audiophilleo2 USB-SPDIF convertors, Sabre 32bit/192KHz and Lavrey DA10 24 bit/96KHz dacs. An Exadevices ExaU2I USB-I2S 32 bit convertor is capable of playing sampling rates up to 352.8 KHz, hopefully i'll find a dac that can keep up with it soon!
I'm using both Mac OS and Windows7 with PureMusic, Audirvana and Decibel players with the Mac and JRiver MediaCentre with Windows7.
So where does tube based audio equipment fit in with this new direction?
I can't see the sense of using equipment that is inherently colored or inaccurate with studio master recordings-surely if we can finally hear exactly what the recording engineers heard, then why color the sound with a slow warm "tube" sound.
Similarly , if a preamplifier, or any other piece of equipment  is low on resolution, then it seems pointless to be using such high resolution digital files.
Of course, not all tube equipment is deficient in these areas, but a lot of it is.
It is important for all tube preamps and power amps to have low output impedance, or the ability to drive the following power amplifier, for a preamplifier, and the speakers for a tube power amp. If the equipment is well designed and has low output impedance, it will be able to drive it's load ACCURATELY , without the dips and roll-offs experienced with higher output impedance devices.
As an example look at this diagram of the response of a very expensive Single-Ended tube amplifier into a real world speaker load. The black line is the response into a speaker load.

I find it fascinating to read a review of a highly accurate and resolving piece of digital equipment and then discover the reviewer is using an amplifier much like this to review it. The ear is capable or hearing as little as 0.25dB difference in frequency amplitude response, but surely it is expecting too much to be genuinely critical about a piece of equipment using a tool like this. There's a 3dB variation in that response!
The next diagram is of a big powerful solid-state amp - the green line is the response into a simulated speaker load, same as above.

Now if your trying to decide which of two dacs is the "better" one, which amplifier do you think would give you a truer indication?
Now I suppose it sounds like I'm saying not to use tubes with digital, but not at all, I'm suggesting  you have to be aware of how the equipment interacts with each other. Indeed, I think a really good quality tube preamplifier is a necessity with ANY system, whether it is digital or analog based. When the interacting impedances are right a good quality tube preamp (and the really good ones are rare) kicks everything into place. These days it's easy to remove the preamp as most devices have enough gain to do without a preamp, but they rarely work optimally, dynamics are the first thing to go and pretty soon you'll be spending less time listening to music as you wonder how it got so boring.
I haven't heard a single system of any type that hasn't been improved by the addition of a well designed preamp, that is capable of high gain into ANY load, flat response from a minimum of 20Hz to 20 KHz, again into any load and a minimum of coloration and distortion.  Preamps can be designed to sound  "warm" or "tubey" but the bad ones sound like this because they cant maintain a flat response into any power amp load.
A good tube preamp will bring out the very best of your hi-rez digital files, especially when the sampling rate equals or exceeds 96 KHz, but make sure it is a good one.