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Old 23rd April 2013, 05:49 PM   #411
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Nice work, Daniel. I would like to ask a few questions about your resistor selection and make a couple of comments.

You keep talking about using carbon resistors. I have heard this before. I have been looking at 0.1% resistors for my parallel amp. There is a variety and the prices vary quite a bit. Have you looked at the thermal drift specs on the resistors you are using? Some precision resistors have one tenth the thermal drift of ordinary 1% resistors! And there are two sources of heat the resistors are subject to: the ambient environment (which can get quite hot if the resistor is mounted near the chip), and the heat generated by the signal on the resistor. This may seem like hairsplitting, but consider this: a 10K and a 1K resistor in series in the feedback loop essentially carry the same current. The 10K resistor will be subject to 10 times the thermal drift from the signal generated heat! This can be mitigated of course by making the resistors bigger (but not too big ). So these are all things to think about when selecting parts and planning layout.
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Old 23rd April 2013, 10:53 PM   #412
boroson is offline boroson  United States
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Thanks for the helpful reply Daniel. I will await your photos and last changes before ordering parts for the boards and pwr supply so I can incorporate your changes. I am getting antsy to begin the build and will keep my progress posted.
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Old 24th April 2013, 08:40 PM   #413
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Hi Eddie,
The resistors that came with the kits have hair fine wispy leads, so I threw them away--either low end junk or high end zoot would change the component values and compensations, and doesn't promote repeatable results. Therefore, I suggest upgrading the signal path to high quality normal-behaving carbon film resistors from Xicon, Vishay, and Koa Speer, for the simplicity that helps promote a normally behaving amplifier, and a schematic with repeatable results.

The carbon comp or film is ideal for base stopper, but transistor base locale is unannounced with chips, so I thought it convenient to avoid inserting random metal resistors at extra cost. I've already been dodging some metal PCB traces. Probably it was also helpful to route the good quality normal-behaving resistors as directly and simply as possible--a miniscule amount of point to point underneath the board, avoids PCB layout influence on performance.

Other than these factors, I cannot identify if the old school tricks or exactly right value helped more, because it was both.

P.S.
So, I start up the parallel amp today and think "gosh that's good" and then I check to see if the computer EQ was turned on. It wasn't.
Meanwhile, onto making the little solo board do its job. . .
Let's see what happens.
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Old 24th April 2013, 08:56 PM   #414
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Default Antsy TDA7294 single layer board. . . :D

Here's the single sided TDA7294 board available Jim's Audio and elsewhere. I believe this resistor re-route configuration and values are correct; and I hope to test this one fairly soon. All of the resistors are on the board and by shortest route, so if you don't want any trackside work, this is probably the way to go.
Click the image to open in full size.
Quote:
Originally Posted by boroson View Post
Thanks for the helpful reply Daniel. I will await your photos and last changes before ordering parts for the boards and pwr supply so I can incorporate your changes. I am getting antsy to begin the build and will keep my progress posted.
Antsy? That's good news! I have a suggestion. . .
Howabout a trip to the nearest Radio Shack for:

"Micro 1A Diodes" such as 276-1104 or 276-1101. This is to restrict charge build-up in the giant NFB-shunt cap. Assuming 2 boards, you'll need 4 standard silicon diodes if your rails are level, or 8 diodes if your rails are not level.

Excellent flux, such as 64-022. This is to convert non-shiny imperfect solder joint to perfect solder joint--apply drop of flux and re-heat. Perfect, and quickly. Excess washes off with alcohol.

500-Piece 1/4-Watt Carbon-Film Resistor Assortment, 271-312. This is a big collection of very good quality resistors, and exactly what I've used. They're similar to other quality parts like Xicon, Vishay, Koa.

And they've got thermal compound, Arctic Ceramique, which is excellent for chip amplifiers.

Also consider purchasing the three Smallest sizes of tiny green polyester dip cap. That's for use as bypass helpers to larger signal caps. They work very well to bypass an Elna Cerafine of 4.7uF and Smaller sizes for simulating BlackGate sound without the wallet smash effect. Anyway, these little green polyester caps from the Radio Shack can help remove treble droop from larger signal caps. They're slightly lossy, and therefore far less risk of ringing if you install a mismatch; so, they're easy to use for bypass duties. I believe the packages are marked 0.047uF, 0.022uF, 0.010uF

You may also want to experiment with Radio Shack's big blue 1uF as input cap, although not exactly my preference to use HV plastic for signal, it is well reviewed and there is no need to pay more for that approach--although possibly not optimal, at least it does not treble droop, so it is not confusing to use it. The main benefit is that it will give you an option for comparing.

Elsewhere:
When you order your 220u and/or 270u amplifier board power caps, consider getting a small variety. It is quite likely that you will want to do some comparing for quality control. No need to compare everything under the sun--a tiny amount of 220u, 270u, Panasonic FC, Nichicon FW, 50v caps will do.

If the same online vendor, such as Mouser, happens to have Nichicon ES, 0.47uF and 1uF sizes, get those too, because they're extraordinarily easy to use for bypassing larger electrolytic signal caps, such as your Nfb-shunt cap. These two smallest sizes of Nichicon ES bipolar cap are so small that they won't treble droop within the audio band. They're high resolution and generally work well, so that's not confusing to use.

If the kit didn't come with thermal pad/mica or shoulder washer, you'll be wanting those too.
Attached Images
File Type: jpg TDA7294 single layer.jpg (186.8 KB, 220 views)
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Old 24th April 2013, 10:10 PM   #415
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Thank you for your comments, Daniel. I do appreciate your input.

The resistors in question are the feedback resistors. For optimum matching (and thus performance) these need to be very closely matched. I am looking at 0.1% tolerance resistors. After scouring data sheets I'm leaning towards Vishay CMF-RN non inductive metal film resistors, which Vishay claims are suitable for high frequency circuits. I have been using "no-name" surplus non-inductive metal film resistors from a large (600!) assortment I bought, and they have worked very well; but they are only 1% tolerance.

There are no carbon resistors that are that closely matched that I am aware of. I could build a bridge (with precision resistors in the bottom) to match up resistors from a batch, but I am not convinced that that it is the right way to go. Plus carbon resistors can drift, which will ruin this particular circuit.

Do you know exactly what kind of resistor it was that you used that gave such poor results? Remember, even a non-linear resistor can make a circuit sound "better" by serendipity.

Any nonlinearity in the feedback network will translate directly into nonlinear performance of the amplifier, so it is crucial.

As far as coupling caps the RS big blue boy is excellent, but it is big and expensive. Panasonic has non-inductive film caps (type ECWFD) that are 47 cents apiece for 1 uF and are only 15 mm long. Too good to be true? They are on my short list to order.
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Old 25th April 2013, 07:16 AM   #416
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Quote:
Originally Posted by danielwritesbac View Post
Well, that's very strange. My own

Did you try this?:
Click the image to open in full size.

i tried above schematic in 660u + 1k2 and 47k config

it worked nice at input of 15-0-15v
but
at 31-0-31, it smoke off

Pls help...
Any valid hypothesis for that?

I have checked wiring, soldering -all connections are good
also got i3 stock sink on back of ic- so no overheating issues
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Old 25th April 2013, 11:43 AM   #417
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Quote:
Originally Posted by Fast Eddie D View Post
As far as coupling caps the RS big blue boy is excellent, but it is big and expensive. Panasonic has non-inductive film caps (type ECWFD) that are 47 cents apiece for 1 uF and are only 15 mm long. Too good to be true? They are on my short list to order.
Ah, don't forget the Nichicon ES (little green bipolar Muse electrolytic) in 1uF and 0.47uF sizes. They work normally, fit easily and have clean high resolution. Just the thing for prototyping stuff and not have to wonder about your input cap quality. At Mouser they are 26 cents (the 10 spot price is 17 cents per cap), and the canister width is just 5mm. They can also be used to bypass larger electrolytic caps and work easily for that.
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Old 25th April 2013, 11:57 AM   #418
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Quote:
Originally Posted by navZ View Post
i tried above schematic in 660u + 1k2 and 47k config. It worked nice at input of 15-0-15v but at 31-0-31, it smoke off Pls help... Any valid hypothesis for that? I have checked wiring, soldering -all connections are good also got i3 stock sink on back of ic- so no overheating issues
That one was tested at up to 35+35V DC rails (transformer is 25+25vac), with authentic TDA7294.

However, the fake/re-mark chips can't stand that voltage, but they can be run at up to 22+22VDC rails (transformer is 15+15vac).

Unfortunately, 45+45VDC rails (31+31vac transformer) will blow up either (kerblooie!).

See also Bulb Test: Powering Up Your Radio Safely With a Dim-bulb Tester

Also, shoulder washer and thermal pads are necessary because tab has voltage.
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Old 25th April 2013, 12:26 PM   #419
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Quote:
Originally Posted by danielwritesbac View Post
Here's the single sided TDA7294 board available Jim's Audio and elsewhere. I believe this resistor re-route configuration and values are correct; and I hope to test this one fairly soon. All of the resistors are on the board and by shortest route, so if you don't want any trackside work, this is probably the way to go.
Click the image to open in full size.
You Sir are awesome!

Many higher-level DIYers forget that this hobby is confusing and complex for the newbie. Your work provides a perfect entry point.

It's also great for those who are pressed for time, wish to start from a known-good platform and do some experimenting ...

Throw in: cheap, widely available PCB; interesting chip that provides for some learning, expansion and testing; and the Energizer Bunny of hands-on testing ... a perfect storm of DIY!

This sort of thing bring more people into our wonderful hobby than all the clever schematics, shiny PCBs and larger-than-life personalities combined (not that those aren't fun too!)

You are an inspiration, keep up the good work my friend!

Cheers,
Jeff

PS We should have a big sticky on the top of the diyaudio page-- "Wanna try DIY?" --that links to posts like this and maybe a CMOY headamp and a simple USB 1543 DAC.

With a couple clicks and a borrowed soldering iron, we could have people hooked on DIY in an afternoon!

Last edited by AudioLapDance; 25th April 2013 at 12:31 PM.
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Old 25th April 2013, 12:30 PM   #420
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Default Now Playing--Jim's Audio Single Layer TDA7294 board.

It came on playing efficiently while the bulb tester remains dark.
Upper left, the bootstrap is 47u. Standby has a 10u cap, mute does not. The input load is 12k. The gain divider is 27k vs 510R+220R. Resistors shown more clearly at post 414. The NFB-Shunt Cap is 660u with an additional bypass cap under the board (try a 1u or smaller electrolytic as your first guess). Plastic power terminal modified to support the MBR schottky for to block blur and block crosstalk. Picofareds RF filter cap added to small signal input area under board. More elegant small input circuit not yet built. The amp is playing nicely direct driven from my computer.
Click the image to open in full size.
Here is the safety clipper that plugs in right next to the NFB-Shunt Cap. The connection is clipper parallel to large nfb-shunt cap. It doesn't clip audio. This little clipper prevents charge build up in your cap so that it doesn't knock out the inputs like a taser. The unit shown limits the charge to 0.65v with 1n4007 didoes. If your rails are quite lopsided, two such units can be put series for 1.3v limit. This unit shown in the photo below, prevents the destruction of the chip's input.
Click the image to open in full size.
Attached Images
File Type: jpg TDA7294 single layer2.jpg (172.7 KB, 203 views)
File Type: jpg 1n4007 NFB-Shunt Cap Protector.jpg (117.7 KB, 200 views)
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Last edited by danielwritesbac; 25th April 2013 at 12:41 PM.
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