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I was always fascinated by the Unity Coupled output stage in old McIntosh tube amps. In many ways it is an incredibly elegant solution to many design problems in output transformers. Ever since I first studied the circuit I thought it would be really neat to build one myself, my way.
- Mcintosh Unity Coupled Transformer 3
- Mcintosh Unity Coupled Transformers 2
- Mcintosh Unity Coupled Transformer Series
- Mcintosh Unity Coupled Transformer 1
Obviously, the most difficult design challenge to overcome is the large voltage drive requirement of the output stage. I got to wondering if it would be possible to build a Unity Coupled amp without a bootstrapped driver and if it would be possible to do that well enough that a global feedback loop wouldn't be necessary. Not that I have anything against a feedback loop, but negative feedback tends to linearize things at the cost of making overload behavior worse. If I can make an amp good enough without feedback I will do without it. So a few years ago I challenged myself to see if I could build such an amp without feedback, but I left my options open in case it was needed. Here was what I built:
I got the output transformers from Plitron and they cost a pretty penny but are really outstanding transformers. The impedance ratio is 4k:5 Ohms or 6.4k:8 Ohms. I was a little concerned about this at first. I don't like driving pentodes into saturation since that is really hard on the screen grid. I thought the impedance ratio was a bit high and actually corresponded a bit with the transformer designer and told him so. He thanked me for my input but stated that he would not be doing another design at a lower impedance ratio.Apr 19, 2019 Frank McIntosh and his company got off to a very strong start in 1949, in Silver Spring, Maryland, with their first product, the tubed Unity Coupled 50W1 Power Amplifier. A patent was granted that same year for the Unity Coupled circuit and transformer. In 1951, McIntosh Laboratory moved to Binghamton, New York, where they've been ever since. The McIntosh Unity coupled circuit. Is the only creation Tim de Pavaricini says he wished he had inventednT, posted on August 31, 2013 at 19:33:39 Cleantimestream Audiophile. Aug 09, 2020 This is thanks to McIntosh’s patented Unity Coupled Circuit output transformer technology – the same technology McIntosh was founded on in 1949. Two Unity Coupled Circuit output transformers flank the power transformer which is positioned between them. Each transformer is housed in a glass topped extruded enclosure that displays the wiring. The MC901’s valve amp section uses McIntosh’s patented Unity Coupled Circuit output transformer (the same technology McIntosh was founded on in 1949) to deliver its full 300 watts into almost any speaker, regardless of impedance; similarly, the solid-state section delivers its full 600 watts using McIntosh’s Autoformer™ technology.
I eventually settled on making a floating screen regulator from a MOSFET source follower to reduce the screen voltage and make it so that the load line for the KT88s hits the knee of the pentode curves. I think that this is just a really healthy choice for the KT88s and will probably make them last a lot longer without any real sacrifice in output power. As far as I know I'm the first to use this solution on an amp like this. The simplified schematic doesn't show it but I added a current limit circuit on the source follower that will make the screen voltage sag if the screen draws too much current.I have a source follower driver for the KT88s, not so much because they need it but because I wanted to present a very high input impedance to the 841s that develop the high voltage drive.
![Coupled Coupled](https://1.bp.blogspot.com/-QzzokP4xvFQ/WjwpmGLSG3I/AAAAAAAAMIg/ijw6ASbUimM5QQPKTMH3SzOLIU-dZ6d5gCLcBGAs/s1600/_blog-McIntosh%2BMC3500_f.jpg)
For the driver, I had some very unique requirements. I needed about 200Vrms (560Vpk-pk) of drive to take the output stage to clipping. That means I had to have a driver capable of dealing with an 800V or so B+ and be able to idle at 400V if I wanted some headroom. Most small tubes can't idle at 400V since it takes special cathode materials to withstand the ion bombardment that happens at these higher voltages. Looking at oxide coated cathode tubes only power tubes and a few selected types like 6SN7, 6BL7, could handle high idle voltages. I also discovered that thoriated tungsten filament tubes typically have higher idle voltage ratings since thoriated tungsten filaments are fairly resistant to ion bombardment.
At some point I came across the 841 which is a high impedance triode with mu = 30 that is meant for zero bias operation and needs to be operated in A2 to swing significant voltage. It has absurdly linear curves over a very wide voltage range, though:
I thought if it were driven with a source follower and loaded with a CCS it would do very well. I was not disappointed. This is the distortion performance at 230Vrms output running at 10mA and 400V idle:
Now you will notice a hump in the noise floor. I suspect that might be caused by the white hot filament in a high impedance tube but I really don't know that for sure.
Last of all is the input stage, which is pretty unremarkable. It is just a 6SN7 LTP with CCS tail operated in a very linear range. You may be wondering what the deal is with the pot. I knew that I would have to count on balance between 841s, which are made of pure unobtanium (I was able to acquire three sets for this amplifier with great effort). I added a pot so that I could attempt to introduce some imbalance in this stage to counteract any imbalance in the gains of the 841s.
Mcintosh Unity Coupled Transformer 3
All high voltage supplies are implemented as Maida-style regulators.
Anyway, it is time for pictures of the completed amp:
Kinda ugly inside. Oh well. I really love the glow of those 841s. They kind of steal the show from the KT88s.
Measurements:
I measured output impedance at ~1 Ohm. I'll look at doing some distortion measurements this weekend and update this page (hopefully).
As far as listening tests go, the amp just does everything effortlessly. It sounds great. The bass is strong. I can crank it to ear bleeding levels and it doesn't break a sweat. I'm pretty happy with the sound.
If I put my ear right in the tweeter I can hear a slight hiss, maybe something to do with that noise hump in the output of the 841s. I was hoping it would be dead quiet but apparently not. There is no hum or buzz though.
Oh, and I should thank Tom Christiansen for his excellent filament regulator boards and Guido Tent for the automatic bias board. I love that my unobtanium 841s have a soft start on the filaments and that I always have perfect balance on my KT88s. Both boards work well and are highly recommended.
Addendum 3/1/15:
More Measurements:
When I initially got this amp together, I flipped it right-side-up a bit prematurely. I was excited to have it finished and thought I would test distortion with it right-side-up.
I realized later that I had not adjusted the pots in the input stage plate loads to null out the gain imbalance in the driver tubes. The problem was that the amp was so heavy that every time I thought about flipping it over to make the adjustments, I just felt this overwhelming lazy feeling and flipped on the music instead. Besides, it sounded quite good despite making almost 1% distortion at 10W.
Anyway, this weekend I finally got back to it and I'm really excited with the distortion measurements with everything tuned up.
I measured:
0.025% @ 1W
0.15% @ 10W
0.22% @ 20W
0.24% @ 30W
clipping occurs at ~ 40W.
All were 2nd harmonic dominant, despite this being a push-pull amp. I could really get that 2nd really low by introducing imbalance in the phase splitter, but I never could get it under the 3rd.
All in all, I think these are pretty good figures for an open-loop amp.
Anyway, this weekend I finally got back to it and I'm really excited with the distortion measurements with everything tuned up.
I measured:
0.025% @ 1W
0.15% @ 10W
0.22% @ 20W
0.24% @ 30W
clipping occurs at ~ 40W.
All were 2nd harmonic dominant, despite this being a push-pull amp. I could really get that 2nd really low by introducing imbalance in the phase splitter, but I never could get it under the 3rd.
All in all, I think these are pretty good figures for an open-loop amp.
Mcintosh Unity Coupled Transformers 2
Then I tested bandwidth. These output transformers are supposed to have ~500 kHz of total bandwidth per the manufacturer.
Mcintosh Unity Coupled Transformer Series
The amp was down 3dB at 120kHz. I'm guessing that the limiting factor in the amp is the 841 driver stage. I did some back-of-the-envelope calculations and it looks like since the 841 has such a high plate resistance, it only takes a 22pF load to put it 3dB down at 120kHz. An 801A would probably perform much better here in terms of bandwidth but they are too expensive and I don't really need to make an amp with half a megahertz of bandwidth.I knew that would be the price of using a high-impedance triode as the driver, but the 841's curves sure are pretty. The other thing about the 841s is that I think they are the dominant noise source in the amplifier. I think those white-hot filaments combined with the high plate resistance aren't particularly good for low-noise design. This amp has a tiny bit of background hiss (with ear close to the tweeter), whereas other amps I have built have had none, and I used the same input stage I always use.
That said, I spent a bit of time listening after making the adjustments, which mostly just took out a bunch of 2nd harmonic distortion, and I think that modern pop or heavily electronic music definitely sounds better now, but simpler songs sound slightly less alive. I would say that songs with lots of electronic sounds and loud bass lines are now subjectively clearer.
My conclusion: The 2nd harmonic distortion can add a pleasant effect to sounds, but any non-linear transfer function, even if it only generates even harmonics, will generate IM products all over the place when you start mixing a lot of sounds together. Those IM products don't sound good and will muddy the sounds if you start mixing enough together.
Upgrade your analog audio experience with McIntosh's new MC1502 Vacuum Tube Amplifier, a replacement for the now-discontinued MC2152 Limited Edition 70th Anniversary. Sharing the same specs as the MC2152, the new MC1502 uses eight KT88 output vacuum tubes protected by Air-Pipe cooling to deliver 150 watts per channel. The new amp can be paired with most speakers and retails for $11,000.
Additional Resources
• How Mcintosh Powered Woodstock at HomeTheaterReview.com
• McIntosh Returning to Luxury Audio Biz with Alps Alpineat HomeTheaterReview.com
• McIntosh Announces New Five-Channel Amp and AV Preamps at HomeTheaterReview.com https://oeowi.over-blog.com/2020/10/motorola-gm300-radio-doctor-download.html.
• How Mcintosh Powered Woodstock at HomeTheaterReview.com
• McIntosh Returning to Luxury Audio Biz with Alps Alpineat HomeTheaterReview.com
• McIntosh Announces New Five-Channel Amp and AV Preamps at HomeTheaterReview.com https://oeowi.over-blog.com/2020/10/motorola-gm300-radio-doctor-download.html.
Read more about McIntosh's new amp below:
McIntosh, the global leader in prestigious home entertainment and ultimate-quality audio for 70 years, is proud to announce the MC1502 Vacuum Tube Amplifier. Logilink usb 2.0 serial adapter driver.
The new McIntosh MC1502 Vacuum Tube Amplifier is a stereo amplifier designed for home audio and home music systems that produces 150 Watts per channel. It is the direct replacement to McIntosh's award winning MC2152 Limited Edition 70th Anniversary Vacuum Tube Amplifier, which had a limited production run in celebration of their 70th Anniversary that occurred in 2019. The new MC1502 has identical audio specifications and sound reproduction performance of the heralded MC2152 but in a more customary McIntosh design.
The MC1502 shares the timeless styling of their classic MC275 Vacuum Tube Amplifier. As the MC1502 has twice the stereo power rating of the MC275 (150 Watts vs. 75 Watts) and is physically larger, in many ways it can be considered the MC275's big brother. And while a pair of MC275s can be run in mono to achieve the 150 Watts per channel output of the MC1502, the MC1502 boasts a better signal-to-noise ratio of 112dB vs. 105dB for the MC275 pairing - producing an even quieter performance than the already quiet MC275.
The MC1502 is powered by eight KT88 output vacuum tubes, with four assigned to each stereo channel. It also has eight small signal tubes, consisting of four 12AX7A and four 12AT7 vacuum tubes; two of each of these vacuum tubes are dedicated to each audio channel. The 12AX7A vacuum tubes are for the Balanced and Input amps, while the 12AT7 tubes are for the Voltage and Driver amps. All the vacuum tubes are covered by a 12-gauge stainless steel wire cage with a black powder coated finish.
The KT88 vacuum tube sockets include Air-Pipe cooling at their bases to help facilitate long service life. https://synology-surveillance-station-license-crack0.peatix.com. The eight signal tube sockets have ceramic base construction with gold plated contacts, providing protection from atmospheric contamination. All the vacuum tubes sit atop the precision formed stainless steel chassis polished to a mirror finish. McIntosh's Sentry Monitor technology monitors the output current and shuts the amplifier off if it ever exceeds safe limits; the signal tubes will flash red if Sentry Monitor is ever activated.
Unlike many other amplifiers, the MC1502's full 150 Watts can be delivered to almost any pair of speakers regardless if they have 2, 4 or 8 Ohm impedance. This is thanks to McIntosh's patented Unity Coupled Circuit output transformer technology - the same technology McIntosh was founded on in 1949. Two Unity Coupled Circuit output transformers flank the power transformer which is positioned between them. Each transformer is housed in a glass topped extruded enclosure that displays the wiring diagram for each transformer. Two fully spelled out vintage die cast aluminum name badges adorn two sides of the MC1502, while an abbreviated badge is centered between the Tube Lights and Power knobs.
Both balanced and unbalanced inputs are included to connect the MC1502 to your preamplifier and the rest of your home audio system. McIntosh's patented Solid Cinch speaker binding posts will securely connect your speaker cables to your speakers. An input and output for McIntosh's Power Control technology allows other connected McIntosh components, such as preamplifiers, CD players, turntables, or media steamers, to automatically turn on and off.
Pricing and Availability
Orders for the MC1502 can now be placed with Authorized McIntosh dealers with shipping expected to begin in August 2020.
Mcintosh Unity Coupled Transformer 1
Suggested retail price (VAT, shipping and any customs duties related to current standards of individual countries are excluded): $11,000 USD