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Old 11th November 2014, 11:55 PM   #1
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Default The many parallel-TDA7293 ACE-BASS amp project

I seem to have a lot of time on my hands these days and lots of TDA7293 chip amps (more than 20pcs) staring at me from a defunkt project. As a result, I have been thinking of designing a mono amplifier PCB with up to 6-8 parallel ICs. On paper, this seems to be perfect for subwoofer duty, or even low frequencies up to about 1-2kHz where the 7293 distortion starts to rise.

What's the performance potential of the amp? If we use six 7293s and operate them from +/-40Vdc rails, by my guesstimation the amp could produce on the order of:
80W @ 8R load
160W @ 4R load
320W @ 2R load
425W @ 1.5R load
THD distortion (on paper) would be well below 0.1% (typ. below 0.05%) for all power levels up to clipping. With a 1.5R load, each device will dissipate about 15-20W with 40V rail voltages, so a good size heat sink is required (not to mention the under-power dissipation of a class AB device). The above figures are based entirely on Figure 9,10 and 13 from the datasheet for performance into 8R with +/-40Vdc rails. There are implementation specific details that influence these numbers.

I have some knowledge of the problems that one can have with a 7293 based amp. I believe that these are limited to the following two issues:
  • proper boostrapping caps must be used for the master/slaves
  • loss of negative rail causes IC to explode
That last one can be fun. One workaround is to fuse only the positive rail and ground, and leave the negative rail permanently soldered in place. Judicious choice of the bootstrapping cap for the number of ICs used makes sure that the amp operates properly. If I have missed something here, please let me know.

With that said, let's talk about the design... I'm interested in doing a "modular" design, consisting of a main board plus sets of holes that would receive "cards" that could hold up to two 7293 ICs that would be oriented perpendicular to the main board. These would be separately assembled and then soldered on to the main board. Right-angle-header type pins would connect the main board and cards (with all connections soldered). Assuming a linear power supply, I am leaning towards a separate PS/cap arrangement and local supply caps for each IC on the cards.

One other potential design aspect would be an optionally populated on-board bridging circuit. In case someone wanted higher class-AB power levels or just wants to draw current from the rails symmetrically, it would be easy to populate the bridge circuit on one board and then connect its output to the input of the other amp. To make bridging "brainless" I would use a balanced line driver IC such as DRV134 or THAT1646. This would require the addition of a line-level PS, but a shunt regulator would probably work just fine, or even something less elegant like a LM317/337 in the TO92 package plus a pre-regulator to keep the dissipation down in the regs. One question that I have is whether I should worry about DC offset for a bridged application, and whether the bridge circuitry is worth adding (e.g. maybe this would be better left for a separate board?). Please feel free to comment on this.

I'm happy to take some suggestions, start in on a design, get feedback, and then update the design. If no one has anything to add I will just post my first effort when I get something worked up. This will be a 100% DIY, license free design. I would consider posting the Gerber files when we are all done.

Let's get to it!
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Old 12th November 2014, 03:07 AM   #2
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After thinking a little more about what I am going to do with this project, and wanting to do something a little different, I'm announcing that I will be incorporating Stahl's ACE-BASS circuit (the 4-op-amp version) into the main board design. My concept of parallel TDA7293s is well suited for this kind of application and I have been meaning to build and experiment with ACE-BASS for a long time. Why not now???
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Old 12th November 2014, 06:51 AM   #3
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After thinking even a little bit more... I'm also going to try and incorporate the circuit described by Tim Mellow in his two AES convention papers: "A New Set of Fifth and Sixth-Order Vented-Box Loudspeaker System Alignments using a Loudspeaker-Enclosure Matching Filter (published in Parts I and II).

Mellow also uses feedback around the driver, but primarily to vary the apparent driver Q (both increase and decrease Q). He gives the following perspective on the differences between his approach and Stahl's at the end of the "Part I" paper, as follows:
Quote:
It is interesting to compare this method with the elegant ACE-bass system described by K.E. StŚhl [8], especially as the current feedback loop forms a band-pass filter in both systems (with the mechanical circuit blocked). The ACE-bass system can be used to match a driver to a smaller box than that to which it would normally be suited using conventional alignments and can therefore perform a similar function to the Type 2 LEMF. It has the additional flexibility of allowing theoretically unlimited low frequency extension, albeit by drawing yet more power from the amplifier.

However, the ACE-bass system cannot match the driver to a larger box, as does the Type 1 LEMF, which allows the low-frequency response to be extended with a more moderate increase in power. The reason for this is that the ACE-bass system reduces the effective compliance of the driver's suspension by placing another 'virtual' compliance in parallel with it. The cut-off frequency can then be extended downwards by adding virtual mass to the diaphragm. Re-adjustment of Qts is also allowed for.

By contrast, the LEMF allows use of either larger or smaller boxes, albeit within limits. Also, the design is achieved more directly through use of alignment tables and Thiele-Small parameters without any need to calculate individual driver parameters such as mass or compliance. Furthermore, the LEMF scheme is less complex because it simply involves applying current feedback via an integrator and uses the existing high-pass filter without the need for an additional band-pass filter. Of course, the ultimate in design flexibility could be achieved by combining the two systems.
Both Mellow's LEMF circuit and Stahl's ACE-BASS circuit are interesting ways to make a driver do what you would like it to do. If you don' tneed to decrease the apparent driver Q with Mellow's circuit you don't use positive feedback and so the instability that might occur with Stahl's circuit when RE is "over cancelled" is rendered moot. I think it would not be too difficult to make both of these possible to execute as part of this amplifier design and would set up some possible performance comparisons.
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Last edited by CharlieLaub; 12th November 2014 at 06:53 AM.
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Old 12th November 2014, 06:15 PM   #4
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For starters I will throw up an ACE-BASS circuit design - this has been taken directly from "Baldin's Blog" on the Sensible Audio web site, which you can find right here:
AceBass up and running | Baldin's Blog

Here's his schematic (below). It's essentially Stahl's circuit with the addition of a clipping indicator on the input and a unity-gain inverter (using amplifier U3A) that leads to a connector to the (separate) power amp. Since he is using a commercial power amp and most are non-inverting this addition seems prudent. As part of this project we will be building the circuit around an inverting amp. Stahl's circuit includes the power amp's feedback network itself, and it was intended that these were designed together. Here is Baldin's schematic:
Click the image to open in full size.

Here is the realized circuit board:
Click the image to open in full size.

I know that Baldin used his circuit with success. He also has provided a design spreadsheet for the component values as well as a design tutorial that is very handy. These are found as links on the Baldin's Blog web page I linked to above, just above the comment section.

Next up, I'll present the LEMF circuit from Tim Mellow and compare it to the ACE-BASS implementation.
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Old 12th November 2014, 06:48 PM   #5
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Here we will review the Loudspeaker Enclosure Matching Filter (LEMF) by Tim Mellow. The LEMF has two slightly different implementations, depending on whether you want/need to decrease the effective driver Qts or increase it (see attachments). The difference is around the power amplifier, specifically how and where the feedback network that includes the driver is connected. Note that increasing Qts is not possible with ACE-BASS, and this can be somewhat limiting in terms of system design and choice of alignment.

Mellow's two convention papers on the subject of the LEMF are complex but I highly recommend reading them. They describe how the LEMF circuit can be used to tailor the response of a driver to a prescribed type of alignment. You need all the equations from the papers to work out how to set up the circuit correctly.

I have a couple of ideas for speakers that will use the "increase Q" version of the LEMF circuit, one of these using the Exodus Anarchy drivers now sold by the DIYsoundgroup. I happen to have four of these on hand. These drivers would make for a nice comparison between ACE-BASS and LEMF implementations since they are almost identical to the drivers that Stahl used in his 1981 AES paper, have a very flat Bl vs. excursion curve and relatively high Xmax.

Both the LEMF and ACE-BASS circuit use feedback that includes the driver. Both are designed to tailor the power delivered to the driver to cause it to act differently than its native self in a way that is useful (e.g. to extend response, or change the response shape). The LEMF circuit has the added potential to use low Q drivers as if they were higher Q ones, which can have certain advantages in vented systems, and the circuit in that case would not be prone to instability because positive feedback is not used. The ACE-BASS circuit is intended only to modify the driver and is not limited to 6th order vented systems like the LEMF circuits shown. ACE-BASS, for example, can be used with a driver in a closed box, but a Linkwitz-Transform might provide similar performance without the complexity of the feedback used in the ACE-BASS circuit. Any direct comparisons will need to be done vented alignments with similar frequency response targets.
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File Type: png Figure 5.png (99.1 KB, 260 views)
File Type: png Figure 6.png (95.3 KB, 257 views)
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Old 12th November 2014, 11:56 PM   #6
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Here is a first stab at a circuit topology that can accommodate both the LEMF and ACE-BASS circuits. Depending on which circuit was desired some components or circuit elements are eliminated (left as an open circuit) or shorted/bypassed. As shown in the legend at the top right hand corner, for the LEMF variant the orange portions are used and the blue portions deleted. For the ACE-BASS circuit the blue parts are used and the orange deleted, except for the large central orange block which is simply bypassed.

The LEMF circuit is for the version that is used to increase driver Q. The version to decrease Q only varies by the feedback connection to the power amplifier section, IOP4.
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File Type: jpg first stab.jpg (89.6 KB, 248 views)
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Old 13th November 2014, 02:34 AM   #7
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ebay madness!

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High Power TDA7293 IC in Parallel 555W Mono Power Amplifier Board Assembled | eBay
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Old 13th November 2014, 03:15 AM   #8
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Yeah I bought one of those once just to check it out. What a piece of crap.

...but I do like the concept!

The parallel TDA7293 is more about using what I've got. I'm now thinking that I should separate the project over two boards - one for the amplifier and the other for the ACE-BASS and LEMF circuit. That way each can be built and re-used independently.
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Old 13th November 2014, 12:58 PM   #9
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Still enjoying my JBL K140 in a 57 liter vented enclosure every day !
I use the 3 op amp version well described on this forum .
Had to replace my DC protection relay as the contacts played up recently .
But so far no complains , the bass is amazing ! -3dB at 17hz and plenty of punch !
My mains are 15 inch Tannoy DC's , not bass shy at all , but the sub is "the finishing touch"
Good idea to use multiple parallel'd chipamps , my sub sound improved a lot when I upgraded the power amp to 400 watts .
Don't forget to drive your acebass with a high pass filter with a Q of 2 at fBox as mentioned in the original Stahl paper .

Good luck and eager to see your results !

Cheers ,

Rens
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Last edited by doctordata; 13th November 2014 at 01:23 PM.
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Old 13th November 2014, 04:28 PM   #10
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Quote:
Originally Posted by doctordata View Post
Don't forget to drive your acebass with a high pass filter with a Q of 2 at fBox as mentioned in the original Stahl paper.
That's only true if you plan to use the 6th order Butterworth alignment, although that is certainly a good one.

The Mellow paper presents tables for for 5th and 6th order alignments such as Synchronous, Bessel, B3^2, Butterworth, and Chebychev with ripple 0.1dB, 0.5dB and 1dB. The B^3 looks very interesting in fact.
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