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Old 3rd October 2012, 07:30 PM   #51
bcmbob is offline bcmbob  United States
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My apologies Daniel, I saw this when you posted it and never got back till now.

You are getting me very excited about the EVO2. I talked to Siva this morning on another matter and he is planning to solidify the MiniRef first.

I had been thinking about doing a bi-amp with the FE on top, but from your information, it appears that won't be necessary. Can't wait till the amp shows and has it's own dedicated thread.

To shamelessly display my lack of knowledge to the world, with your usual clarity could you give a layman's description of "decoupling"? It's one of those things that comes up so often but I haven't asked my friend Mr. Google about it yet.
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Old 4th October 2012, 03:25 PM   #52
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Quote:
Originally Posted by bcmbob View Post
My apologies Daniel, I saw this when you posted it and never got back till now. You are getting me very excited about the EVO2. I talked to Siva this morning on another matter and he is planning to solidify the MiniRef first. I had been thinking about doing a bi-amp with the FE on top, but from your information, it appears that won't be necessary. Can't wait till the amp shows and has it's own dedicated thread. To shamelessly display my lack of knowledge to the world, with your usual clarity could you give a layman's description of "decoupling"? It's one of those things that comes up so often but I haven't asked my friend Mr. Google about it yet.
Neither I nor Google would make any sense of it.
So, here's Tom on power decoupling:
Quote:
Originally Posted by gootee View Post
Edit: The 0.1 uF (or other small value, probably X7R ceramic) that are at each opamp power pin are probably there for BOTH decoupling and bypassing. Bypassing is to bypass the high frequencies to ground, to defeat the positive feedback loop through the power rails for high frequencies that exists for most topologies of transistor-based amplifiers. Decoupling is for supplying fast transient currents when needed, so that the rail voltage is not disturbed too much, i.e. "decoupling" the effects of transient current draws from the rest of the power supply circuitry.
This example infers that there may be an umbilical cable, diode drop, or resistor in-between the 10,000u tank and 1000u decoupling caps on the MyRef board, so that large signal demand comes from the 10,000u and small signal demand comes from the 1000u, but that is not the case if the umbilical cable has been replaced with a 1cm trace. On these integrated amp+power boards, the amp decoupling caps lose charge every time the tank caps need charge. This may be pretty for tone but it is terrible for clarity. I would try a resistor or diode drop instead of shorting out the decoupling with a short length trace. That applies to EVO2, since it is less needy on tone but could use a boost in clarity. To further level out the differences between the FE versus the EVO2, it is highly likely that the amp power decoupling caps need stepped down to 680u or in any case somewhat smaller size than the single chip amp.

Let's look at power supply design for a moment.
Transformer, Bridge Rectifier, Smoothing caps, loss, Tank caps, loss, Decoupling caps, trace, bypass caps, amp. The above mentioned loss may be a cable, light bulb, fuse, transistor, long trace, diode, or resistor. They're all different, but the most important difference is that a ballast loss exists. For each loss that is shorted/omitted, then TWO power supply features are hindered.

So, when I look at the MyRef boards, I think that something needs installed series between the 10,000u and 1000u. On first guess, I'd try a fast silicon diode. Now, the power won't get stolen from the 1000u until the rails droop 1/2v. A 0.22R resistor may also work since the power would come out of the transformer and into the 10,000u more readily than coming out of the 1000u through 0.22R and into the 10,000u. SO, there are some compact options that don't have to involve an umbilical cable.

Crazy trick: A 1 ohm 1/2w resistor parallel to a fast silicon diode will give you a tiny amount of inexpensive reverb and a lot of noise filtering capacity. This exchange of decreasing disharmonic noise for increasing harmonic noise is generally attractive and worth an interview because it is also low loss. However, I wanted to mention it isn't totally noise free--just a lot more likely to be attractive. The resistor part gives the feature of sending noise away from the amp--a feature you wouldn't get with the diode alone (except circuitously); however, the speed of the diode is not the same as the speed of the resistor and you may get a tiny dose of reverb. I'd be sorely tempted to try that on a MyRef, since it is quite likely to outperform an umbilical cable, and the total amount of noise does not increase, and disharmonic noise will decrease. That's a decent feature.
Edit: The D//R unit goes between the 10,000u and 1000u, instead of an umbilical cable.
Edit2: That method is unorthodox--The orthodox solution for clarifying integrated power+amp boards is to install regulators.

Unfortunately, the answer I have to give for the MyRef board, on the topic of power decoupling, is that you didn't install it yet. EDIT3: To install, you'd put in something that looks like CRC.

A possible exception (to missing power decoupling) is if using Standard 10,000u in competition for charge with Low ESR 1000u and then possibly, the more demanding Low ESR 1000u can hold some charge whilst the 10,000u are charging. But, CSD (component specific design) is time consuming and it is problematic if a specified part is discontinued or otherwise hindered in availability. A bit more comprehensive design could make component selection much easier.
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Last edited by danielwritesbac; 4th October 2012 at 03:42 PM.
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Old 4th October 2012, 04:05 PM   #53
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Meanwhile, back to Tom's quote above and also referencing National Semiconductor AN1192 specs: 470u decoupling with 100n bypass. They also list a power supply board, meaning that an umbilical cable is used. If it had been all in one board, it would be 470u//100n bypass since the decoupling and the cable that does it have come up absent. The sound is far different.

Probably the solo chip LM3886's benefit from the dark voice and noise of the integrated power+amp combination board so as to tame the mids for which that chip has a surplus. Maybe all of this typing would have been better if applied to a different amplifier, such as the TDA7294 with 220u decoupling and 4.7n bypass? I do not know.

The only thing I actually know is that you do have options to explore and that power circuit is of equal importance to amplifier circuitry because they are totally interrelated. Any audio feature or effect that you can think of can be installed at either a power board or an amplifier board, because whatever you do to the reference is also done to the signal.
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Old 25th December 2012, 01:20 AM   #54
bcmbob is offline bcmbob  United States
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Hi All,

Four weeks to ship and a few hours to build and I finally can hear a TDA7294. The kit is quite simple compered to the LM3886 MyRefs I'm used to. However, I am very impressed with the sound it produces. A short description would be warm, powerful (in my build), accurate and a bit relaxed. The bottom is cleaner and better defined at this "stock" level than the standard LM builds I have done, but not as powerful. I can clearly understand why so many like the character of this chip.

So let me do a few "IMHO" comparisons with the MyRefs and in particular thr Fremen Edition. What stands out the most is the difference in headroom. Below a particular volume level the TDA really shines. Anything above that shows a lot of inter-modulation distortion, harshness and strain. The last few versions of the MyRef stay clean all the way to max. I'm sure there is a lot more sophistication in the power supply technology for the MR than what this TDA build has now. It would be interesting to hear this amp with something specifically designed to power it.

Unfortunately, there are no props for the TDA's top end when compared to the MyRefs. Again, that may not be a fair comparison as the kit I built is bare bones from my understanding. The MR FE is silky, smooth, defined, airy and shimmering on all types of music. If you are familiar with it, Ravel's Daphnis & Chloe has a wide range of dynamics, instrument combinations and volume levels. As long as everything is soft to moderate the TDA sounds beautiful. It just doesn't take long/much for louder and higher range portions to get muddy and a touch blaring. Bells and chimes break through brightly during crescendos and loud finales, but just below that it gets pretty mushy. Some of that may be resolved with higher grade components on the build, which I hope to do soon.

What I hear in the dynamics in percussion and general mid-rang regesters is very impressive. Without any specific knowledge or technical background, I would suspect the TDA is simply "faster" than the LM3886. If anyone has that knowledge please pass it along. There is a real element of genuine excitement produced by this chip, particularly with wild classical tracks.

So the few hours I've owned a TDA7294 have been very enjoyable to say the least. I did run several passes of the Isotek burn-in cd that cleaned away some fuzz that was there at first power-up. This kit is capable of a BTL configuration and I am very interested in trying that as well as some of the other dual chip setups Daniel W. has suggested. At $30 for a two board kit with all the necessary do-dads, one can't go wrong and a second set won't break the budget.

Thumbs Up for the TDA7294

And....Happy Holidays Y'all.
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Last edited by bcmbob; 25th December 2012 at 01:24 AM.
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Old 25th December 2012, 04:04 AM   #55
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Well, that's actually a really good result for a basic kit in stock condition. Both the gain and the feedback current may be a bit high and the NFB cap and bootstrap may be a bit small. That combination causes it to miss the highest highs and lowest lows, but that is what the datasheet example schematic does.

Try the bootstrap cap directly at 47u for slightly more low bass.
(some newer schematics show 47u)

And, try the NFB cap at least as large as 220u (or bigger)--Also try some tiny value polyester caps and/or electrolytic of 0.47uf or less, as bypass caps parallel with your NFB cap.
Then the lowest lows and highest highs should be working.
(the schematic shows 22u, which is a bass blocker)

Also for low bass this amplifier appreciates a large power supply reservoir capacitance, not located on same board as amplifier.

If it still does some midfi behavior, the gain might be set too high, since it doesn't really have enough gain capacity for a 70w amplifier. The quality drops if the gain is too high. If you want hi-fi, I think you want low gain power amp pushed by a preamp.

The feedback current might be too high due to feedback-shunt and feedback resistor values too small, which also causes fragility and current noise of overloading the small signal section beyond linearity.

To make a reasonable compromise, you could set the feedback-shunt resistor to 2k2 and the feedback resistor to 56k (or even 68k if going without a preamp). Nearby values make really similar results. This ballpark is a good spot to start exploring. That example is off a bit, but not nearly as far as the datasheet schematic.

Before using the amplifier in bridge mode, I'd like to see it running quite happily clear and definitely running cool temps. The operating voltage for bridge mode is with a 20+20vac transformer (max). So, if you're going higher than that, do consider output caps or speaker protector kit. I did test those baby latfets to 127 watts before I ran out of transformer, but I was using speaker protection, just in case.
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Old 25th December 2012, 12:52 PM   #56
bcmbob is offline bcmbob  United States
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Thanks Daniel, this chip contains a lot of potential if what I'm hearing is truly stock performance. It definitely fits well within the definition stated in the thread title. Even with the shortcomings I perceived, I was enticed to listen to some major productions all the way through with its warm and dynamic presentation.

For the benefit of myself and others, hopefully we can go through the adjustments you list one by one on the thread after the first of the year. Though I am gaining a degree of understanding of terminology and function, I still need a little "paint by the numbers" direction to clarify the process.

I did purchase a pair of the Peter Daniels Universal Power Supply boards but won't use them for what I had planed. Don't know if they are capable of what you propose for this chip, but they are available.

Looking forward to drilling down on this project.
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Old 25th December 2012, 02:50 PM   #57
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How come no one mentioned the LM1875. Only about 20 watts but sounds good if implemented well. Chip is very cheap ( about 2% of total cost !) but peripherals cost a bit more !
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Old 25th December 2012, 06:35 PM   #58
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Quote:
Originally Posted by bcmbob View Post
Thanks Daniel, this chip contains a lot of potential if what I'm hearing is truly stock performance. It definitely fits well within the definition stated in the thread title. Even with the shortcomings I perceived, I was enticed to listen to some major productions all the way through with its warm and dynamic presentation.

For the benefit of myself and others, hopefully we can go through the adjustments you list one by one on the thread after the first of the year. Though I am gaining a degree of understanding of terminology and function, I still need a little "paint by the numbers" direction to clarify the process.

I did purchase a pair of the Peter Daniels Universal Power Supply boards but won't use them for what I had planed. Don't know if they are capable of what you propose for this chip, but they are available.

Looking forward to drilling down on this project.
Sure, you can give it its own thread, and introduce it, along with the schematic. I've attached the schematic so that it doesn't disappear. Four of the caps are undersized and two of the resistors have too much current, and the funny thing is that the caps are too small to control where the current is supposed to go. There's a little traffic jam, which is the source of the "muddy" that happens when you try to play loud. It just needs some really basic/generic audiometric tuning to get it in the ballpark (of playing both loud and well) before you begin other fine tuning.

For power supply, we could do the "ladderback" style which is both basic and excellent. That is built with multiple parallel 3300u (at least 8 of them--used as 4 per each rail) on ordinary copperless phenolic perfboard. It would be a great introduction and works well with that amp. This is just recifier+tank, a simple power supply, but these are not dull. Well, it is a good place to start.

Attached is your amp schematic in "stock condition":
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Old 26th December 2012, 12:48 PM   #59
Jay is offline Jay  Indonesia
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Originally Posted by ashok View Post
How come no one mentioned the LM1875. Only about 20 watts but sounds good if implemented well. Chip is very cheap ( about 2% of total cost !) but peripherals cost a bit more !
Hmmm... I'm not so sure, but I guess LM1875 sound is terribly wrong. I like TDA2030 more. It is much more accurate imo. They do sound different, but I think preferring LM1875 is wrong Just kidding
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Old 26th December 2012, 12:57 PM   #60
bcmbob is offline bcmbob  United States
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Please take a look here.
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