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Old 5th January 2013, 12:46 PM   #141
bcmbob is offline bcmbob  United States
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This is the kit I bought which contains all the mounting stuff. I'm sure they will sell a single if asked.

TDA7294 monoBTL/stereo 100W amplifier kit two separarate boards !
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Old 5th January 2013, 12:52 PM   #142
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Where is the PCB?
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Old 5th January 2013, 01:48 PM   #143
KSTR is offline KSTR  Germany
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Quote:
Originally Posted by danielwritesbac View Post
Question: I do mute, pin10 with only a 10k resistor but I'm curious about an alternative: If pin10 is not powered, then does pin4 become the non-inverting input as the datasheet suggests, or is there a caveat? I didn't try it--longevity worries.
Daniel,

I did, in fact pin4 becomes a secondary lower gain and lower quality non-inverting input in the mute state. It should tie to GND, it doesn't matter much where provided it's not polluted with supply ripple.

As for the supply section questions, please wait and see my upcoming post on an alternate approach to local decoupling.
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Old 5th January 2013, 02:49 PM   #144
bcmbob is offline bcmbob  United States
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Quote:
Originally Posted by SERGIODS View Post
Where is the PCB?
Sergio, Sorry but still not understanding your question. Here are two choices.

1. Q. Where is the PBC that is the base of the kits discussed on the thread?
A. One of the members is currently developing a group buy at a great price. That hasn't started yet.
A. You can contact any of the eBay vendors listed in post #140 and request a bare board. Other members might have a source if they know what you are asking for.

2. Q. Where is the PCB for the advanced design being discussed?
A. I doesn't exist yet.

Please let us know which you are asking about.
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Old 5th January 2013, 03:07 PM   #145
KSTR is offline KSTR  Germany
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Alternate Local Decoupling.

Instead of using two caps from either rail to GND one can also use two caps differently, one rail-to-rail (C+ below) and one from the neg rail to GND (C-) -- this also has been proposed by other DIYA members in the past. The common point of the caps is the negative supply, and all three feeds to the supply sport series elements (Z1 etc) shown as resistors here for simplicity.

Let's have a look of the current loop for a negative output current in the load :
Click the image to open in full size.
Shown by the red loop, current flows into the output pin of the opamp and it leaves the chip at the neg supply pin. Then it returns back to the output though cap C-, GND attach and the load in counter-clockwise direction, the cap acting as a level shifter.
The green traces show current paths when the local caps become ineffective at LF down to DC, the slim green trace sees no DC, though.

Key points are that the caps (which are composite/arrayed caps) are close to chip (to benefit from their low ESR and ESL) and that the loops have low area for low inductance to neither emit nor reveice magnetic fields. Finally, nodes are located such that any I*R drop along the wires/traces have no consequences.

Assumed that Z1 can be made rather large (as we will see) as well as C-, there is almost signal current alone and very little AC supply recharge/balancing current. The rail-to-rail cap C+ is out of the picture mostly.


Now going to the postive load current :
Click the image to open in full size.
The current leaves the chip via the output pin along the red path, goes through the load back to GND. From there the return path to the V+ supply pin is first through C- in opposite shifting level to V-, clockwise direction and then through C+, level-shifting voltage again to V+.
Again, the green paths show current at LF and LF/DC (thick green only).

The important thing now is the notion that cap C-, the GND-to-V- cap, is always in the loop, plain series to the load and will effectively see no balancing current to/from the supply at MF/HF. This means the V- pin (subtrate) is tightly bonded to GND and any variation of its voltage is small and basically linear, fully correlated to the signal output current (but with a different freq response). This leads to the lowest possible impact of supply rejection issues of the chip including nonlinear distortion.
C+, the rail-to-rail decoupler, feels only the half-rectified positive load current and therefore is a most delicate point where lowest loop area is paramount.


Finally looking at the feedback/input scheme :
Click the image to open in full size.
Other than bias currents and some capacitive stray currents no currents flow in the opamp input pins IN+ and IN-. The arrangement is inverting, Rf/Ri determining the gain. Ci block DC (optional), Cf, Rng, Cng provide means for stability. Rofs minimizes input bias current offsets and Cofs shorts it for low noise. Rz and Cz is the output snubber.
Blue is the input current from the source and gets routed to the ouput pin while the signal current in the yellow GND line is basically zero until it meets star GND from where the return current finds its path back to the source. The yellow GND plane (or better, a shielded cage) must find a short and low inductance path back to the V- pin (and GND). This is not fully reflected in the drawing, also the exposed node area of the IN- network should be physically smaller than shown.
When several amp get connected (on the same PCB or elsewhere), a solid GND buss bar is going though the big yellow trace left of the star GND, and supplies lines would be fed with their own series elements, at least in parts of.

As some may have noted, the chip's frontend positive supply has its own, higher voltage supply but thats not strictly necessary, resp. not an option for chip which don't have seperate supplies.


I'm about to breadboard this asap...
Attached Images
File Type: gif conceptual_neg_current.gif (13.7 KB, 683 views)
File Type: gif conceptual_pos_current.gif (13.8 KB, 679 views)
File Type: gif conceptual_signal_and_out_current.gif (16.4 KB, 388 views)

Last edited by KSTR; 5th January 2013 at 03:16 PM.
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Old 5th January 2013, 03:23 PM   #146
gootee is offline gootee  United States
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GENIUS!!!

Klaus, I am so extremely happy to finally see someone approach a design with the right priorities and sequence of thinking, and who has the knowledge, skills and abilites to do proper analysis and design! I have been waiting for this for a very long time!

Just from your last few posts I have learned so much that I am giddy with excitement! I can hardly wait to see what comes next!

It's even better, knowing that you now have sophisticated test equipment that will enable getting real answers to questions that usually go unanswered, or result in many conflicting opinions and guesses. This could become one of the first truly-well-designed projects that I have ever seen, here.

Highest regards,

Tom

Last edited by gootee; 5th January 2013 at 03:49 PM.
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Old 5th January 2013, 04:13 PM   #147
gootee is offline gootee  United States
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Quote:
Originally Posted by KSTR View Post
Alternate Local Decoupling.

<snipped>

I'm about to breadboard this asap...
Klaus,

Do you have a way to quickly etch and drill PCBs, at home?

If not, you might find it to be a very useful capability. Back when I did a lot of prototyping, I eventually quit using all other methods and just made a PCB every time I wanted to test something. It was much more reliable, and I think that in the long term, it was easier and saved a lot of time.

I could have a board completed in less than an hour, from the time when it was only on my computer screen. So I could build many iterations in one day, if necessary.

I used the "toner tranfer" type of method, usually with two-sided PCBs. I eventually found out how to do it using locally-available supplies, as much as possible, so that I only had to ensure that I had PCB blanks on hand, because I never wanted to have to wait for anything to be shipped to me (and it was much cheaper). The only other real problem was making sure that I had the correct paper to use for the transfer. I found a very good-working one that was carried by the local Staples office-supply store. But they kept changing it, almost yearly.

Regards,

Tom
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Old 5th January 2013, 04:15 PM   #148
redjr is offline redjr  United States
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Quote:
Originally Posted by danielwritesbac View Post
...
Does it come the chip, thermal pad, and shoulder washer so that the board can be used by anyone who has a capacitor and resistor collection?
Board+ChipAmp is interesting, but I have noticed a lack of interest in just the board by itself...
No. The price I quoted earlier was simply for the PCBs themselves. No chip or other mounting hardware. This was investigative pricing for those interested in taking a stock PCB ($4USD for the pair), and using some selective components. Whether or not there's sufficient interest to pursue at this point is unclear. My interest has changed a little, because if Klaus can come up with a new board relatively inexpensive, and the parts($$) are not to bad, I may go that route. However, I'm still questioning the overall value relative to the sonic gains of a total redesign, over the stock boards which I've already built. I know it is very subjective and will vary from member to member. But I may be willing to give it a go.
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Old 5th January 2013, 06:06 PM   #149
bcmbob is offline bcmbob  United States
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Quote:
Originally Posted by redjr View Post
.... However, I'm still questioning the overall value relative to the sonic gains of a total redesign, over the stock boards which I've already built.....
May I ask what speakers you are using?
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Old 5th January 2013, 07:13 PM   #150
redjr is offline redjr  United States
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Originally Posted by bcmbob View Post
May I ask what speakers you are using?
Hi Bob,

At the moment I'm using some near field Definitive StudioMonitor 350s with the 7294. In full disclosure, let me also say, that even given my age, I never had any tube gear, so I can't compare that 'sound' to my modest transistor/IC based amps and receivers I've used over the past 40 years. I also have a pair of Reference Connoisseur RC-30s that I intend to hook up and give a listen to also. Just haven't gotten around to it yet. The 350's sound pretty great though.

In all honesty, the sound with my current 7294 build amp has a lot of overall presence - which I like. While over the long haul it can be a bit fatiguing and somewhat harsh to listen to, I find I enjoy the dynamics of it's range over that objection. Music and vocals are crisp and clear across the range with a modest amount of bass from the 350's passive 8" woofer/baffle. I am NOT using a dedicated sub-woofer yet. I like it when I can hear the fingers slide on a guitar, or the vocalist take a breath! So, having said that, I may find it hard to distinguish a sonic signature with the mods that are being discussed here. But that doesn't mean I won't give it try. Nor does it mean there won't be any (improvement). I might (and hope) be pleasantly surprised! I would like to do some serious A/B comparison between the stock amp sound and a redesigned amp. I don't have any equipment that can measure any of it, so I'll just have to trust my ears!

Rick
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