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Old 21st September 2014, 01:33 AM   #1
tomchr is offline tomchr  United States
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Default Modulus-86: Composite amplifier achieving <0.0004 % THD+N.

Update 24 FEB 2015: Modulus-86 Rev. 2.0 is now available offering improved performance. See Post #838.
To order PCBs, please visit the Modulus-86 website: Modulus-86 Rev. 2.0: Composite amplifier achieving 0.00014 % THD.

Update 28 MAR 2015: For higher output power, the Parallel-86 is now available. Skip to the launch of the Parallel-86 here: Post #955.
To order PCBs, please visit the Parallel-86 website: Parallel-86: High-power composite amplifier achieving 0.00014 % THD.


This spring I set out to design the Mother of All LM3886 Amplifiers. An amplifier so good that it would knock your socks off. But also an amplifier that someone with decent soldering skills could build if provided a well-designed circuit board. This design effort took a while and led to an interesting investigation of the capabilities of the LM3886. It also led me to produce an LM3886 design guide: Taming the LM3886 Chip Amplifier, which I will keep adding material to as it becomes available.

The Modulus-86 is my latest amplifier design. It is a composite amplifier using an LME49710 precision opamp to control an LM3886 power amp. The performance of the resulting composite amp, both in terms of measured performance and in terms of sound quality, is dominated by the performance of the LME49710. Hence, the performance of the Modulus-86 is absolutely stellar. One of the many benefits of the composite topology is that the power supply rejection is very high. This means the Modulus-86 performs as well on a real power supply (I tested with a toroidal transformer, rectifier, and 2x22000 uF) as it does on a regulated, $1200, lab supply (I used an Agilent E3632A). The excellent supply rejection is also one of the main contributors to the stellar sound quality of this amp.
In addition to the LME49710, an OPA277 precision opamp is used for the DC servo. The use of a DC servo avoids the use of large capacitors in the main signal path.
The input to the amplifier is provided by a THAT1200 differential receiver. This makes it possible to use the XLR connections commonly found in pro audio. The THAT1200 provides performance rivaling that of an input transformer. When using differential connections, this minimizes the induction of hum and EMI, resulting in dead quiet during quiet passages in the music. The differential input can also be configured for use with the single-ended RCA connectors commonly used on prosumer audio gear.

The key features of the Modulus-86 are:
  • 35 W output power (8 Ω) using a ±28 V power supply.
  • Easily bridged and/or paralleled for output powers exceeding 200 W.
  • Ideally suited for multi-channel amplification.
  • Ultra-low 0.00020 % THD (35 W, 8 Ω, 1 kHz).
  • Ultra-low 0.00035 % THD+N (35 W, 8 Ω, 1 kHz).
  • Ultra-low 0.00066 % IMD (35 W, 8 Ω, SMPTE 60 Hz + 7 kHz @ 4:1).
  • 0.002 Hz ~ 140 kHz bandwidth.
  • 14 V/µs slew rate (fully symmetric).
  • 84 kHz full-power bandwidth.
  • -125 dBV noise floor.
  • Differential input with 90 dB CMRR eliminating ground loops in the signal path.
  • Phenomenal power supply rejection ensuring consistent, high performance even using unregulated power supplies.
  • Elaborate use of planes and copper pours to maximize circuit performance by minimizing supply and ground impedances.
  • Low-inductance signal ground connects to power ground at one point only for maximum performance.
  • On-board Zobel and Thiele networks for maximum stability even with capacitive loads.
  • On-board EMI/RFI input filter and ESD protection.
  • On-board low noise voltage regulators for the driver op-amp and DC servo.
  • Power and output terminal blocks accept wire sizes up to AWG 10 (5.2 mm2).
  • All leaded. Easy to solder. 90 × 70 mm board footprint.

Circuit boards for this design are available for purchase through my website: Neurochrome.com : : Audio : Modulus-86

The THD+N is constant versus frequency throughout the majority of the audio range. The THD+N does rise slightly towards the high end as the loop gain of the amplifier rolls off. At normal listening levels (<1 W), the THD+N is perfectly flat versus frequency, resulting in incredible sound quality.

The differential input makes it super easy to use multiple boards in a bridge, parallel, or bridge/parallel configuration. All that is required to turn a stereo amp into a mono, bridged amp is that two wires are swapped. No component substitutions necessary.

The Modulus-86 has a gain of +20 dB (10x) for better gain structure in the end system. Should a higher gain be desired, the amp can be configured for +26 dB (20x) by changing a resistor. The circuit support the use of the THAT1203 and THAT1206 for gains of +17 dB (7x) and +14 dB (5x), respectively, should lower gains be desired for further optimization of the gain structure.

"But, but ... how does it sound?", I hear you ask. It sounds fantastic! The first time I turned on the Modulus-86 and started the music, I went “WOW!” before even making it to my listening chair. It was immediately obvious that this amp was something special. What struck me was the level of clarity of the reproduction and the deep quiet during quiet passages in the music. Talk about a huge dynamic range! I have played a few instruments in my life – including the trumpet and a brief stint with a drum set. It is especially important to me that metallic instruments (brass wind and cymbal for example) sound metallic and natural. This is an area that challenges many amplifiers and where the Modulus-86 really shines. The midrange is open and natural. The bass is precise and tight. What can I say? I really like it… The detail reproduced from Dire Straits, “Brothers in Arms” and “On Every Street”, for example, is out of this world. I am certain the incredible sonic performance is due to the stellar supply rejection and flat THD+N vs. frequency of the Modulus-86.

For those interested in more background information on composite amplifiers, I suggest reading the article, "Composite Audio Power Amplifiers" in Electronics Now, Nov. 1992 (pp. 38-44). This DIY Audio thread contains a few pictures from the article: composite amplifiers

For the impact of layout on the performance of an LM3886, I suggest looking at this DIY Audio Thread: LM3886 PCB vs Point-to-Point (with data)

Finally, for a more complete picture, I'll direct you to my Taming the LM3886 page: Neurochrome.com : : Audio : Taming the LM3886 Chip Amplifier

Thanks,

~Tom
Attached Images
File Type: png MOD86_BlockDiagram.png (30.6 KB, 4555 views)
File Type: jpg MOD86_R1p0_PCB.jpg (404.8 KB, 4497 views)
File Type: png MOD86_THDvsFREQ_25W_1W_8R.png (168.8 KB, 4270 views)
File Type: png MOD86_THDvsPOWER_1kHz_8R.png (98.1 KB, 4118 views)
File Type: png MOD86vsLM3886_THD_25W_8R.png (83.4 KB, 4076 views)
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Modulus-86: Composite amp achieving 0.00014 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
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Last edited by tomchr; 28th March 2015 at 05:25 PM. Reason: Parallel-86 available
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Old 21st September 2014, 01:47 AM   #2
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Excellent, Exactly the direction I was heading as well !!!

You had put in lots of hard work on it, and, I thank you for it, as well as sharing your knowledge !!!

Cheers!!!!

jer
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Old 21st September 2014, 06:36 AM   #3
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Congratulations - it looks like a continuation of the evolution of nested topologies like the MyRef, MiniRef, etc. We've just started discussion of a DC Servo for the MyRef this year, so it's good to see that you've already done a working implementation here. I hope I can borrow ideas from the DC Servo, if needed.

A few quick questions:

a) Is it possible to omit the differential input and use a single-ended input fed directly to the input of the LME49710 stage without too much surgery?

b) Are alternative opamps like the LME49990 usable without tinkering with the compensation?

c) Is the schematic free for perusal (mainly for the compensation schema)?
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Old 21st September 2014, 01:25 PM   #4
udailey is offline udailey  United States
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Also see composite opamp articles by Walt Jung but the one listed above is for lm1875 and obviously the most appropriate reference. Opamp rolling should be easy with no resistor changes.
THAT1200 has SE output so removal should be trivial.
Just my guesses as circuit is not in front of me but composite amps are pretty straightforward and well documented.
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Old 21st September 2014, 01:57 PM   #5
Mihkus is offline Mihkus  Estonia
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Holy crap, this design has impressive performance
I must try thing thing out
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Old 21st September 2014, 02:14 PM   #6
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Quote:
Originally Posted by udailey View Post
Also see composite opamp articles by Walt Jung but the one listed above is for lm1875 and obviously the most appropriate reference. Opamp rolling should be easy with no resistor changes.
THAT1200 has SE output so removal should be trivial.
Just my guesses as circuit is not in front of me but composite amps are pretty straightforward and well documented.
Thanks, I've seen some of Walt Jung's composite opamp schematics, and I've also borrowed some ideas from them a few years ago while designing the MiniRef - which uses a Jung-style nested voltage-series feedback loop, but with a Howland current-pump (transconductance block) like the MyRef for the inner high-power chipamp block. It works great, with excellent audible sonics, much like the MyRef, but simpler.

I missed reading the Electronics Now article (referenced above) then, but a quick perusal of the AD711-LM1875 schematic posted at these forums from that article shows that it is significantly sub-optimal - as jcx points out, it throws away a lot of loop gain in a resistive divider in an effort to gain stability (both the MyRef and Miniref are way more sophisticated in applying loop gain for better performance, both locally and globally).

It's possible to implement simple nested voltage/voltage composite opamp/chipamps with fairly trivial (though possibly heavy-handed) dominant-pole compensation - I found one such simulation schematic while digging through some 3-year old simulation files today, which appears to be stable as shown below:

Edit: IIRC, R5 was an attempt to bias the outer opamp into current-sourcing Class-A, which could be omitted or modified with a JFET CCS as needed.
Attached Images
File Type: jpg lm1875_composite_expt.jpg (89.5 KB, 1485 views)
File Type: jpg lm1875_composite_acresp.jpg (229.5 KB, 1092 views)

Last edited by linuxguru; 21st September 2014 at 02:19 PM. Reason: addendum
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Old 21st September 2014, 07:38 PM   #7
udailey is offline udailey  United States
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Yeah I knew you knew how they work. Wondered why you were asking such simple questions as I was pretty sure you knew the answers to those questions. If you're inclined to answer a few of my questions I have two I'll ask on the miniref thread.
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Old 21st September 2014, 08:07 PM   #8
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Quote:
Originally Posted by tomchr View Post
This spring I set out to design the Mother of All LM3886 Amplifiers. An amplifier so good that it would knock your socks off. But also an amplifier that someone with decent soldering skills could build if provided a well-designed circuit board. This design effort took a while and led to an interesting investigation of the capabilities of the LM3886. It also led me to produce an LM3886 design guide: Taming the LM3886 Chip Amplifier, which I will keep adding material to as it becomes available.

The Modulus-86 is my latest amplifier design. It is a composite amplifier using an LME49710 precision opamp to control an LM3886 power amp. The performance of the resulting composite amp, both in terms of measured performance and in terms of sound quality, is dominated by the performance of the LME49710. Hence, the performance of the Modulus-86 is absolutely stellar. One of the many benefits of the composite topology is that the power supply rejection is very high. This means the Modulus-86 performs as well on a real power supply (I tested with a toroidal transformer, rectifier, and 2x22000 uF) as it does on a regulated, $1200, lab supply (I used an Agilent E3632A). The excellent supply rejection is also one of the main contributors to the stellar sound quality of this amp.
In addition to the LME49710, an OPA277 precision opamp is used for the DC servo. The use of a DC servo avoids the use of large capacitors in the main signal path.
The input to the amplifier is provided by a THAT1200 differential receiver. This makes it possible to use the XLR connections commonly found in pro audio. The THAT1200 provides performance rivaling that of an input transformer. When using differential connections, this minimizes the induction of hum and EMI, resulting in dead quiet during quiet passages in the music. The differential input can also be configured for use with the single-ended RCA connectors commonly used on prosumer audio gear.

The key features of the Modulus-86 are:
  • Ultra-low <0.0004 % THD (25 W, 8 Ω, 1 kHz).
  • 28 W output power (8 Ω) using a ±28 V power supply.
  • Easily bridged and/or paralleled for output powers exceeding 200 W (4 Ω).
  • Differential input with 90 dB CMRR allows for the elimination of ground loops and hum.
  • Phenomenal power supply rejection ensuring consistent, high performance even using unregulated power supplies.
  • Elaborate use of planes and copper pours to maximize circuit performance by minimizing supply and ground impedances.
  • Low-inductance signal ground connects to power ground at one point only for maximum performance.
  • On-board Zobel and Thiele networks for maximum stability even with capacitive loads.
  • On-board EMI/RFI input filter and ESD protection.
  • On-board low noise voltage regulators for the driver op-amp and DC servo.
  • Power and output terminal blocks accept wire sizes up to AWG 10 (5.2 mm2).
  • All leaded. Easy to solder. 90 × 70 mm board footprint.

The THD+N is constant versus frequency throughout the majority of the audio range. The THD+N does rise slightly towards the high end as the loop gain of the amplifier rolls off. At normal listening levels (<1 W), the THD+N is perfectly flat versus frequency, resulting in incredible sound quality.

The differential input makes it super easy to use multiple boards in a bridge, parallel, or bridge/parallel configuration. All that is required to turn a stereo amp into a mono, bridged amp is that two wires are swapped. No component substitutions necessary.

The Modulus-86 has a gain of +20 dB (10x) for better gain structure in the end system. Should a higher gain be desired, the amp can be configured for +26 dB (20x) by changing a resistor. The circuit support the use of the THAT1203 and THAT1206 for gains of +17 dB (7x) and +14 dB (5x), respectively, should lower gains be desired for further optimization of the gain structure.

"But, but ... how does it sound?", I hear you ask. It sounds fantastic! The first time I turned on the Modulus-86 and started the music, I went “WOW!” before even making it to my listening chair. It was immediately obvious that this amp was something special. What struck me was the level of clarity of the reproduction and the deep quiet during quiet passages in the music. Talk about a huge dynamic range! I have played a few instruments in my life – including the trumpet and a brief stint with a drum set. It is especially important to me that metallic instruments (brass wind and cymbal for example) sound metallic and natural. This is an area that challenges many amplifiers and where the Modulus-86 really shines. The midrange is open and natural. The bass is precise and tight. What can I say? I really like it… The detail reproduced from Dire Straits, “Brothers in Arms” and “On Every Street”, for example, is out of this world. I am certain the incredible sonic performance is due to the stellar supply rejection and flat THD+N vs. frequency of the Modulus-86.

For those interested in more background information on composite amplifiers, I suggest reading the article, "Composite Audio Power Amplifiers" in Electronics Now, Nov. 1992 (pp. 38-44). This DIY Audio thread contains a few pictures from the article: composite amplifiers

For the impact of layout on the performance of an LM3886, I suggest looking at this DIY Audio Thread: LM3886 PCB vs Point-to-Point (with data)

Finally, for a more complete picture, I'll direct you to my Taming the LM3886 page: Neurochrome.com : : Audio : Taming the LM3886 Chip Amplifier

Should you want to skip the design phase and go straight to building a Modulus-86 amplifier, I will be happy to sell you a circuit board via my website: Neurochrome.com : : Audio : Modulus-86

Thanks,

~Tom

Nice ... So, price for 2 90x70mm PCBs is $120 ?
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Old 21st September 2014, 09:12 PM   #9
tomchr is offline tomchr  United States
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Quote:
Originally Posted by linuxguru View Post
Congratulations - it looks like a continuation of the evolution of nested topologies like the MyRef, MiniRef, etc.
The MyRef series is a Howland current pump, which comes with its own unique set of challenges, in particular when it comes to stability. Bob Pease wrote a nice analysis of the circuit in the National Semiconductor Application Note AN-1515 (available as a .pdf from TI): AN-1515 A Comprehensive Study of the Howland Current Pump. It's a pretty easy read, actually.

The MyRef series has several threads dedicated to it. Discussion about the MyRef (including the airing of grievances with its designer) are best served in those threads. I would prefer that this thread remains dedicated to the Modulus-86, though, I welcome general discussion about composite amplifiers and their design challenges here as well.

My goal with the Modulus-86 was to design a circuit, which
  1. Provides the best performance possible.
  2. Uses parts that are current state of the art and commonly available.
  3. Gives the builder the highest possible confidence that he/she will be able to build the circuit and reproduce the performance I measured.

Quote:
Originally Posted by geraldfryjr View Post
Excellent, Exactly the direction I was heading as well !!! You had put in lots of hard work on it, and, I thank you for it, as well as sharing your knowledge !!!
Thank you very much. It's a route that's well worth taking. This is by far the best semiconductor amp I have designed to date. I've been designing and building amps since the late 1980ies, so that says a bit...

Quote:
Originally Posted by linuxguru View Post
a) Is it possible to omit the differential input and use a single-ended input fed directly to the input of the LME49710 stage without too much surgery?
Yep. You can do that. Pull the THAT1200 and connect two pins on the IC footprint with a piece of wire and you have an amp with a single-ended input. You will lose all the benefits of having a differential input, of course. Tradeoffs, tradeoffs...

Quote:
Originally Posted by linuxguru View Post
b) Are alternative opamps like the LME49990 usable without tinkering with the compensation?
The LME49990 is SMD only and I decided early on to develop the circuit using leaded parts for ease of assembly.

The LME49990 has twice the bandwidth of the LME49710, so you will have to tweak the compensation to compensate. In general, I don't recommend opamp "rolling" with composite amplifiers.

Quote:
Originally Posted by linuxguru View Post
c) Is the schematic free for perusal (mainly for the compensation schema)?
In the past, I gave my work away for free. I have decided not to do that this time around.

If you prefer the DIY approach, I suggest starting with the Electronics Now article linked to in Post #1 and my Taming the LM3886 page.

In these types of ultra-high precision circuits, the schematic is only a small part of the circuit. The majority of the circuit performance comes from the PCB itself. Don't believe me? Do what I did: Build an LM3886 amp using a "connect-the-dots" PCB layout. You'll get horrible performance. Optimize the layout and you can beat the data sheet numbers (not by much, but every bit counts). See my LM3886 P2P vs PCB thread for more detail. I'll add some of these observations on my Taming the LM3886 page as time allows as well.

The biggest difference between the Modulus-86 and other composite LM3886 amps is that I operate the LM3886 in closed loop and use the LME49710 for the outer loop controlling the LM3886. I prefer this approach over the more commonly used topology where the LM3886 operates in open loop. My Modulus-86 has much higher phase margin and much better transient response than the more common composite amplifiers where the LM3886 operates in open loop. The clipping behavior is better controlled in the Modulus-86 as well.

Quote:
Originally Posted by komigenie View Post
Nice ... So, price for 2 90x70mm PCBs is $120 ?
When you buy my boards, you are buying the engineering that went into them. You are buying the confidence that if you assemble the board according to the bill-of-materials, you will be able to match the performance I am measuring. You are buying the piece of mind that comes with knowing that you will receive a well-engineered, well-designed, and fully supported ultra high-end design that delivers state of the art performance. You are also buying the hours of prototyping and testing that went into the board to ensure a successful build once in your hands. The 90x70 mm piece of board material is merely a token representing all the intellectual property of the design.

Also note that the performance of my Modulus-86 is about 100x better than many amplifiers in the >$1k class (comparing THD+N at full power). That's nothing to sneeze at...

~Tom
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Modulus-86: Composite amp achieving 0.00014 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
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Old 21st September 2014, 09:15 PM   #10
tomchr is offline tomchr  United States
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Originally Posted by udailey View Post
Just my guesses as circuit is not in front of me but composite amps are pretty straightforward and well documented.
Well documented, yes. Straight forward, no... Once you start scratching the surface, you'll find many pitfalls in composite amps. Stability is the obvious one. That one is pretty straight forward to deal with for someone skilled in the art. Transient behavior, clipping behavior, and graceful recovery from clipping is something quite different. In my previous post, I gave a few clues to how I designed around those pitfalls.

~Tom
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