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Discussion Starter · #1 ·
It has been quite some time since I was able to work on my car. Life got kind of hectic and as such I was unable to put any time towards my audio install. Now that the holiday season is finally coming to a close, I once again have a little bit of spare time to devote to my car.

I still have not quite figured out the voodoo that is a custom SMPS, but that is a tale for another thread. In the mean time I have grown quite fond of chip-amps and have used them in several test circuits. I have been trying to find an intermediary project that would allow me to combine and test the various bits of my project.

Given my recent discovery of a pre-built small boost converter, I decided it was time to make my first foray into active amplification. I am still working on my crossovers and I will start a separate post for those when the time comes. For now I am using this thread to discuss the design and assembly of my little amplifier.

I make no claims to have a great understanding of audio amp design. I'm just a young guy with a passion for building stuff. Please feel free to offer insight and advice as you see fit. I also welcome questions and comments.

Design goals
  • Compact
  • Simple BOM
  • Full Range
  • ~30 Watts RMS per channel
  • Capable of single ended or differential input
  • Modular (in groups of 4 channels)

That is a rough outline of what I have in mind for the amp, in my own case I plan on scaling it up to a 12 channel amp so I can go fully 3 way active around my entire car (not pure SQ oriented, just rich cabin filling sound for myself and all of my passengers).

I have to run for now, but I will post more details tonight including links to several of the components I intend to use.
 

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Discussion Starter · #2 ·
The core of the amp revolves around 2 very basic components:

The first is a small SMPS boost converter that takes in a fixed voltage and boosts it up to another voltage. The module I intend to use is this one from Dealextreme. It is capable of handling ~150 watts of power. I plan on adding an additional choke and larger tank circuit (capacitors) so that the power supply is relatively clean and free of noise. I haven't decided if I will separate the supply for each grouping of 4 channels, or share the load across the PSUs.

The second part is the TDA7396 by STMicroelectronics. It is a bridged class AB amplifier capable of driving 2 ohm loads, and was originally designed for automotive use. It has a very low BOM for support components and is capable of being driven by single ended or differential inputs. It also has thermal foldback, and thermal shutdown, in addition to protections against all sorts of shorts. All in all it is a very capable chip for running off of a single rail with a voltage range of 8 to 18v.

I have built a few chipamps using the TDA7396 before and am rather fond of the sound they produce. Also since the support circuitry is so simple I figure the are good candidates for my first car amp. Honestly there isn't that much design work involved so this should be a fairly simple project. Well I'm off for now, I'll post up some progress after Christmas.

As always: Questions, and comments are welcome.
 

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Discussion Starter · #4 ·
Hello everyone! It took me quite a bit longer than I expected to route the board, but here is where I am at so far:



The current circuit board design is still lacking power hookups and it needs to have a few more traces routed, but it is somewhere near 90% complete.

The current design is 2 inches x 7 inches and the capacitors are the tallest part bringing the circuit to less than 1.5 inches tall. Granted that will all change drastically depending on how they are mounted to a heatsink, but that is to be decided later. I largely followed the Application circuit diagram for the IC with a few tweaks here and there as I saw fit. I added in a simple diagnostic LED for each channel so that you can tell when a channel is clipping (>.5%) or in an error state; it won't tell you what type of error but at least should be able to alert you to which IC is having trouble. There is also a sizable tank circuit for each chip, to the tune of 2200uF per channel.

So far I look to be on track to keep all of my design goals, though I still need to finish laying out the ground plane and setting up the star grounding in order to make the amplifier as resilient as possible to humming. Once I do that, I'm going to toss the design up on the DIYaudio forums to ask the electrical wizards over there for advice on how I can further improve the design.
 

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Nice project indeed!! I think the power bus looks like 100 mils wide. If we can have it wider it can carry more current or has a lower temperature rise. Or maybe flood the layer later after finalizing the other traces and add some thermal relief pads. Have fun! Best wishes!!
 

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Discussion Starter · #7 ·
Very cool, keep up the good work!
Nice project indeed!! I think the power bus looks like 100 mils wide. If we can have it wider it can carry more current or has a lower temperature rise. Or maybe flood the layer later after finalizing the other traces and add some thermal relief pads. Have fun! Best wishes!!
Thanks for the well wishes. You are correct, the GND power bus is currently 100 mil and the VSS is 150, but you already guessed what I am up to with the top layer. I just have the current bus tied together on the bottom layer so my design program doesn't whine at me when I do my design rule checks, but I actually intend to use the top layer for my power and ground supply with lots of vias to connect each channel to their respective supplies. Each layer will use 2oz coopper.

I'll have to be careful with my prototype so that I don't drop anything on the PCB and short it out, but after I get my prototype done I am thinking about sending it off to a board house to get a couple PCB made with resist and silkscreens.
 

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Discussion Starter · #8 ·
Ok so before I begin posting more details, let me just say that:

This work is licensed under a Creative Commons License


That license applies to this entire project. If you want to make your own for personal use, feel free to do so. I will be posting the finished design files when I have my amp built and tested. But this design is published "as is" with no guarantees expressed or implied, etc.

IE build at your own risk, not my fault if you manage to blow up an electrical system, toast some speakers, or create a singularity, etc.

If a moderator could copy the above part of this post into the top of my original post, I would greatly appreciate it.



The narrowest trace width on the power bus is on the ground plane, linking 3 of the tank caps to the rest of the board, and also helping to shield the signal traces from ambient noise. The narrow point is 175 mils wide and should be good for 11 Amps sustained, granted it is part of the tank circuit, so in all reality the amp should be able to handle way more current than that, with the limiting factor being the voltage supplied.

It should be able to run off anything between 8 and 18 volts (DC), and can drive down to a 2ohm load. Obviously the voltage supplied will determine your power output. That is why I am going to be using that boost module to bump the voltage from my car up to ~18 volts. But in all practicality, supplying 18v vs 12v will only provide 2x the power IE 30 watts RMS vs 15 watts RMS (3dB) with a 4 ohm load.

It is worth noting that the tabs on the TDA7396 are not isolate and are tied to ground. So whatever heatsink you bolt these chips to will also be at ground potential unless you isolate it with a thermal pad of some sort.

Also the Analog ground is tied to the GND pin directly and not via the ground plane, hopefully that reduces the noise in the circuit when the amp is supplied with single ended sources. It is also provided as a pin breakout for the input stage on the input pin-header as pin 1 (the far left).

Pin 2 is the standby switch and is used to connect to the remote line. When powered this will bring the amp chips up to an operational state, when the pin is low it will bring the amp into standby mode, which should be ~.5 mA but that is without the boost power circuit. I have no idea what the quiescent current is for that module as it has not arrived yet.

Pins 3-10 are the signal pins for the amp, these are paired off (+-) for each channel.

I still need to tweak a few more things and fix some typographical errors in the silk screen, but in essence that is a simple 4 channel amp. I'm going to toss this design before the wizards of DIYaudio and see what they think before I go ahead and order the parts to make a prototype. Once I get it working, I will post the complete design files along with schematic and component values.
 

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That's very nice and a great job so far!! I think it looks like it should work. Yeah the VSS on silkscreen should be Vs or Vcc.

I think there're 3 minor items which may not be an issue at all...
- The drill holes and bottom-side pads for Vcc and GND (and maybe 4 pairs of speakers) may need to be slightly larger to accommodate thicker wires or have more circular/annular space for solder, or I could be wrong as these dots represent the locations and not the actual drill sizes(?)
- Maybe it's good to have some high-frequency bypass caps. There's a 100nf in the datasheet/circuit but i think i did not see it here, or maybe it's hiding somewhere(?)
- Also nice if we have an inline coil right at the incoming Vcc location to filter noise too (optional).

Are you going to make/DIY the PCB, or order it? (I used to DIY PCBs but later on just went straight to the PCB house... theirs are way more pretty looking than what I can make/etch in the sink ha ha.)

Best wishes and have fun!!
 

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Discussion Starter · #10 ·
Thanks!

Yeah I realized after I cropped the pics together for the board layout that I had mislabeled that power rail. I have since corrected that label.

You are correct the holes need to be a lot larger, all of the holes need to be tweaked to fit their respective components. I'm going to try to get the board down to 3 unique sizes.

I completely forgot the bypass caps! I remember looking at them in the datasheet and thinking I should point to point solder those to the pins, but I forgot to put that in my own design notes. Thank you for the reminder! I will be adding 2 bypass caps per chip (I have piles of small value caps lying around so I figure why not).

As for the inductor on the input rail, that will be soldered in separately with the boost circuit. That is in my design notes but since I haven't gotten to finishing up my schematic that has not been published yet. What I have posted so far is the modular core to the amplifier.

I will be DIY'ing the prototypes of my board. After I fix any design issues and get the bugs worked out, I will be sending them off to a board house. I have had a couple friends express interest in putting together their own DIY amps so I will likely be running a small batch of boards.
 

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Etching with ink jet or laser?
 

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Discussion Starter · #14 ·
Sort of, I was attempting to use a 50 year old lathe to turn down an old heatsink from a Sun Fire server I had dismantled when I became acutely aware of how bad of an idea it was...


Needless to say, I stopped trying to prep heatsinks for my project. Instead I am using this as a gross excuse to buy an overpriced tool for the task. I'm about 1/4 of the way there.... I hope to purchase the tool sometime in June.

So future updates will not be coming until I get the new tool, at which point I will promptly resume work on this project.
 

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Bumping for possible updates from the OP. June?

I've had a similar amp idea for several years with respect to power converter separate from amp module(s). For several reasons:
a) Smaller space since you can have the converter somewhere else than the chip amps.
b) Potential for less heat when in different locations and using separate heat sinks for each module/chip/converter.

I've thought about getting some old or broken amps for the case/heat sinks and perhaps finding one that's power supply/converter section is still working and using it just to power the chips in another box (or boxes) somewhere else. I really like the potential for a real stealth setup.

Also, like the OP, I've also thought about repurposing some of my now obsolete CPU heat sinks including maybe using a quiet case fan.

I'm guessing from the OPs original thread that he would use multiple of these SMPS units since each is only rated for something like 150W and look pretty small. Especially compared to what standard car amp converters look like. But at less than $12, multiples wouldn't break the project budget.
 

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Discussion Starter · #16 ·
Bumping for possible updates from the OP. June?

I've had a similar amp idea for several years with respect to power converter separate from amp module(s). For several reasons:
a) Smaller space since you can have the converter somewhere else than the chip amps.
b) Potential for less heat when in different locations and using separate heat sinks for each module/chip/converter.
Very true on both points, also by physically separating the PSU and amplifier, you can reduce EMI by moving the SMPS into its own shielded enclosure.

So it looks like my plans are getting pushed back a bit more, but I might as well talk about why I have such a delay.

I have long been after my own CNC router. I have a small DIY 3D printer, and that became the start of a very expensive hobby. Now I am in the process of designing a full size CNC that can accept 4'x8' media. I was originally planning on just buying a small kit for a 2'x4' router but then I realized that for about double the price I could build a MUCH larger router.

So as far as rough details go, it should have an X,Y travel of ~4.5' and 9' with a Z travel of 1'. I already have the linear rails for the Z axis. They are 18" long and made by THK with 2 carriages per rail. They have a combined dynamic capacity of 3 tons! Granted the gantry isn't going to be able to hold up to that, but it should hold almost 1000 pounds of force at any given part of the frame. I plan on starting out with a cheap Chinese spindle, but eventually will upgrade to an ATC spindle.
 

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I kind of took Sea Shadow's idea and took a broken Sony Xplode amp I had sitting around from which I know that it had a good SMPS. I sliced it in half leaving a +- 40v SMPS. I'm planning on modifying slightly to give a more chip amp friendly +- 30v. I then bought two pre made chip amp TDA7294 boards from ebay. I used a couple of big old surplus heat pipe style CPU cooling towers that dwarf the chip and board. I very carefully drilled the mounting hole for the TDA7294s into the copper heat sink as not to hit the heat pipes. The chip amp boards came with a nice thermal pad and screw/insert which made it easier.

My write up on this is over in the car section of diyaudio: DIY Cheap Car Amp: SMPS from broken amp + TDA7294 Chip amp boards - diyAudio. Once I was complete, I was going to post it here as well.

My current hold up is that I sliced the board just where the feedback circuit for voltage adjustment was. So I need to do a little jumping prior to putting in voltage control.

Yea, Sea Shadow, I hear you on the CNC router. Been wanting to do one myself. Plus I know that sometimes when a project hits a road block, you sometimes switch to another (more interesting) project. Like I just ordered a couple of the 2x4 miniDSPs to replace the MS-8 with its flakey left channel (going in and out). I'm also working on my wife's HHR SS and soon to be third phase of audio upgrade (speakers/deadening, than amp/wiring, soon sub). That will be this weekends audio fun. :)
 
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