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Chip Amps Amplifiers based on integrated circuits

Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886
Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886
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Old 19th November 2019, 08:57 PM   #1
Mark Johnson is offline Mark Johnson  United States
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Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886
Default Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886

In his book Designing Audio Power Amplifiers (2nd edition), Bob Cordell discusses a chipamp he designed around the National Semiconductor LM3886. Bob calls it the Super Gain Clone, and he summarizes its features as follows:
  • 40 Watts per channel into 8 ohms from LM3886 devices
  • The LM3886 is operated in inverting mode for best performance
  • Inverting input buffer stage provides overall non-inverting amplifier
  • Audio-grade DC servo provides control of output offset voltage
  • No output coupling capacitor to degrade sonics
  • No input stage feedback return capacitor to degrade sonics
  • No electrolytic capacitors in the signal path
  • High input impedance
  • Adaptive soft clipping (Klever Klipper) ahead of the feedback loop
  • The amplifier-proper never clips
  • Extra input-path RFI filtering
  • and more
I contacted Bob and received permission to create a PCB of the Super Gain Clone, for members of diyAudio. The schematic of my PCB, which is merely a straightforward copy of the schematic in Bob's book, is shown in Figure 1 below. Figure 2 is a photo of the board, stuffed and soldered and mounted to the heatsink of a diyAudio Store "Dissipante 2U" chassis. The complete stereo amplifier is shown in Figure 3. It fits nicely and looks quite sharp in the not-very-tall (88 mm) 2U chassis. I used the Dissipante with the smallest available depth (300 mm).

And then a second PCB, for people who want a much smaller amp-channel board, was created. This board began as a copy of the Super Gain Clone board layout, but then discarded several pieces of circuitry. I amputated the inverting input buffer, the DC servo, and the Klever Klipper subcircuits; the resulting small board is called "Compact3886". Its schematic is shown in Figure 4. Figure 5 is a photo of a populated Compact3886 board, and Figure 6 is a complete stereo amplifier using two Compact3886 amp boards.

MOUNTING TO THE HEATSINK

Unlike the PCBoards which follow the diyAudio Universal Mounting Specification ("UMS"), the Super Gain Clone PCB and the Compact3886 PCB are mounted perpendicular to the heatsink. So they are certainly not compatible with UMS. You will have to drill and tap three holes for each amp PCB; one hole to bolt the LM3886 to the heatsink, and two more holes to bolt the PCB support L-Brackets to the heatsink. A drilling template is provided in the pdf documentation.

In fact, drilling and tapping your own holes is inevitable, because the very slick-looking 2U chassis in the Store are only available with undrilled and untapped heatsinks. (2U is not tall enough to accomodate the UMS footprint). Luckily, 3 holes is not a large number to drill and tap, even for a beginner. And remember, if you make a mistake, you can slide the template 10mm left or right and try again. The PCBs are short while the heatsinks are long. The "extra" holes will be inside the chassis and invisible to onlookers.

WHAT POWER TRANSFORMER WILL YOU NEED?

You'll need a power transformer rated at least 200 volt-amperes ("VA"), with dual independent secondaries, and with primaries that are compatible with the AC mains in your country. Using the rule of thumb

DC_supply_voltage =approx= [(Trafo secondary rating in VAC rms) * 1.414] - 3.1V,

and consulting the LM3886 datasheet's Absolute Maximum Ratings, I conclude that you can use any 200VA transformer you like, as long as its secondary's output voltage is between 14.5VAC and 31.5VAC (rms ratings at full load). The LM3886 datasheet provides a helpful graph that I have copied below as Figure 7. Applying the above rule of thumb, we get Figure 8. These predict the maximum output power at the onset of clipping (namely, when THD rises to 0.1%).

WHICH SPECIFIC POWER TRANSFORMER IS RECOMMENDED

I strongly recommend you use a dual-primary, dual-secondary, 200VA toroidal power transformer, whose secondaries are rated 22VAC rms. Consulting Figure 7 we find this gives 38 watts into 8 ohms, 68 watts into 4 ohms. The "AS-2222" from Antek, is the one I prefer. They sell direct from their website and they also sell on eBay; check their listings.

SUPPLYING POWER TO THE SUPER GAIN CLONE

The "Universal Power Supply" PCB sold in the diyAudio Store, works well with a pair of Super Gain Clones and/or Compact3886s. Figure 6 shows a complete amplifier using the Universal Power Supply PCB.

Another option is a new PCB called "PSU For Chipamps". It is smaller than the Universal Power Supply board, because it makes no attempt to be universal. It's just a simple PSU without any options or user configurable features, and it's specifically designed for stereo chipamps plus 200VA transformers. The idea is to allow builders to fit an entire stereo amplifier, including transformer and power supply, into a relatively small box. Figure 3 shows a complete amplifier using the new PSU For Chipamps PCB. I've started another discussion thread which focuses exclusively upon that PCB, you can find it at

PCB available: PSU for Chipamps , such as Super Gain Clone and Compact3886


HOW ABOUT THOSE ZENER DIODES

The Super Gain Clone board includes a pair of 7.5 volt Zener diodes, component IDs "D1" and "D3". These diodes protect the downstream voltage regulators ICs, especially during power-up events. But you need to select the right zener diode which will provide a good match with your power transformer and DC power supply. After you've chosen a transformer, select the Zener diode as follows:
Code:
Secondary volts (AC rms)         Zener diode     
--------------------------------------------     
      Vsec < 21V                 none (jumper)     
21V < Vsec < 28V                 7.5V (1N4737A)     
28V < Vsec                       15V  (1N4744A)
You'll notice that the recommended Antek AS-2222 transformer provides 22V output, which calls for a 7.5V Zener diode. AND that is exactly the Zener voltage and Zener part number, found on the schematic.

You'll also notice that the Compact3886 board doesn't contain any Zener diodes, so you don't need to read this section if building a Compact3886.


READ BOB CORDELL'S BOOK

Bob is the inventor of the Super Gain Clone, read what he has to say about it. You'll discover that Bob recommends a second Zobel network (33nF series 10R), after the output inductor. These components are not present on my PCB layout; if you decide to install them, one good option is to solder them directly across the loudspeaker output terminals.


CHECK OUT THE BUILDER'S NOTES (19 PAGE PDF DOCUMENT)

Attached to post #109 of this thread. Sorry, I am only able to edit the body of post #1, not the attachments. So the builder's notes are further down.


CHECK OUT THE DETAILED PARTS LIST

There's a .zip archive attached to this post, which contains a very detailed Parts List for the Super Gain Clone and another one for the Compact3886. Extra detail has been included to make it easy for you, the builder, to find parts whose electrical performance is acceptably good and whose physical size will fit the PCB. If one or more parts are out of stock or backordered, the Parts List is your savior. It will help you find a replacement that's just as good or better.

Also included in the .zip archive, are web links to Mouser.com preloaded Shopping Carts. So-called "one click" Shopping Carts. BEWARE: don't turn off your brain when using these; check to be sure that every line item is in stock and not backordered. Check to be sure you're not accidentally ordering 1000 of something.


HOW AND WHERE CAN YOU OBTAIN THE BARE BOARDS

Update 18 Dec 2019: Group Buy will close on Tuesday Morning 24 Dec 0600 San Francisco time. These boards are 1oz copper with gold "ENIG" pads.

If the Group Buy does well, I'll probably organize another Group Buy in the GB section of the Forum. And if there is LOTS of demand, I'll show the numbers to the diyAudio Store people, maybe they might want to eventually offer these boards through the Store.

_
Attached Images
File Type: png Figure_1.png (111.4 KB, 6025 views)
File Type: jpg Figure_2.JPG (432.2 KB, 5709 views)
File Type: jpg Figure_3.jpg (544.2 KB, 5333 views)
File Type: png Figure_4.png (58.2 KB, 5631 views)
File Type: jpg Figure_5.jpg (503.6 KB, 5161 views)
File Type: jpg Figure_6.jpg (493.6 KB, 1903 views)
File Type: png Figure_7.png (40.7 KB, 1625 views)
File Type: png Figure_8.png (41.5 KB, 1022 views)
Attached Files
File Type: zip Rev2_Parts_Lists_and_Mouser_Shopping_Carts_3_pcbs.zip (22.4 KB, 288 views)

Last edited by Mark Johnson; 12th May 2020 at 05:13 PM.
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Old 19th November 2019, 08:58 PM   #2
Mark Johnson is offline Mark Johnson  United States
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Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886
This post is a place-holder, for additional pictures and other things.
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Old 19th November 2019, 08:59 PM   #3
Mark Johnson is offline Mark Johnson  United States
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Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886
Yet another place-holder. Right now it's boring but someday, wow.
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Old 19th November 2019, 09:22 PM   #4
rsavas is offline rsavas  Canada
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Hi Mark,

Great work, I see you have shopping carts set up too, I should have looked at all your files, thx, now where are the gerber files just kidding your IP

Last edited by rsavas; 19th November 2019 at 09:30 PM.
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Old 20th November 2019, 12:13 AM   #5
Speedskater is offline Speedskater  United States
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In this day and age, balanced inputs are the only way to go.
(even if the pre-amp's output is unbalanced)
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Old 20th November 2019, 05:39 PM   #6
zbass123 is offline zbass123
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That looks strikingly similar to Neurochrome modulus-86. Any chance you can create this project in easyeda to make it open source?
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Old 20th November 2019, 07:03 PM   #7
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by zbass123 View Post
That looks strikingly similar to Neurochrome modulus-86. Any chance you can create this project in easyeda to make it open source?
The Neurochrome Modulus-86 is a composite amplifier, the Super Gainclone is not. A composite amplifier, such as the Modulus-86, uses a precision opamp to provide error correction on a less precise power amplifier.

Tom
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Modulus-86, 186, 286, & 686: 40-240W (8Ω), <-120dB THD. HP-2 Headphone Amp: 660mW, <-130dB THD. Taming the LM3886.
Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right
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Old 20th November 2019, 11:47 PM   #8
mexmike is offline mexmike  Mexico
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Hey Mark,

Nice to see someone else giving the Cordell SGC a go! I love the sound of mine.

I'm sure your layout is way better than mine. Any chance of some bare board photos? I'd like to see where I could improve on my layout.

How did you find the Klipper? I'd thought of using the SGC as a guitar amp. Bob mentioned to me in an email that the Klipper gave the amp a tube sound when turned on and adjusted. I ended up leaving the KK out in my final build as my primary use was for listening to music but the bigger plus for me, was that it made my layout way simpler; something I'm not the best at!

I noticed that in Bob's original schematics there are two 1k resistors... Fig 27.3, R6, 1k resistor and then on Fig 17.4, R1, 1k. These equate to your R9. I thought it a little strange to have two 1k resistors and did basically the same as you and only used one! My thinking was that having the extra 1k would attenuate the signal into the KK and that really R6 and R1 on Bob's shematics are really one and the same! Is this correct, or did I miss something?
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Old 20th November 2019, 11:59 PM   #9
Mark Johnson is offline Mark Johnson  United States
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Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886
As far as I am aware, this schematic from the 2nd edition of Bob's book, is the final authority for resolving questions about what is, or is not, a Super Gain Clone.

_
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Old 21st November 2019, 12:13 AM   #10
bkercso is offline bkercso  Hungary
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This Klever Klipper is interesting! I'm just wondering how much distortion its produce way before the clipping. Because I see that diodes are involved, and soft clippers are known from their distorion (link).
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