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Old 26th December 2012, 08:57 PM   #21
bcmbob is offline bcmbob  United States
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This is the board I'm using now:

Heavy Duty Supply.......

This is available at the DIY audio store.

Each builder could probably chose the format that best matches their skills and interest as long as the needed values are available .

Daniel's nick name should be "The P2P Man"
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Last edited by bcmbob; 26th December 2012 at 09:17 PM.
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Old 27th December 2012, 12:17 AM   #22
bcmbob is offline bcmbob  United States
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Stanton was fine with posting the info on the board I'm using. It is the same standard design as was posted earlier but with english comments and little larger. I'm adding a picture of the board to be a visual aid for folks like me to understand which segments are being discussed.
Attached Images
File Type: jpg JA Kit.jpg (117.1 KB, 1095 views)
File Type: jpg TDA_Big.jpg (118.0 KB, 1094 views)
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Old 27th December 2012, 12:58 AM   #23
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Quote:
Originally Posted by gootee View Post
Daniel,Wouldn't the cap bank be ALMOST as easy to construct using an unetched double-sided PCB for each polarity, a la Terry Given? It should then have much lower impedance, right? Or am I missing something? Tom
Sure, that's fine too.
For the photo in post#15, I just found it fairly easy to use 3 strips of 14ga solid copper, a scrap of phenolic, a bit of flux and a ~35w iron.
Quote:
Originally Posted by bcmbob View Post
I'm following along with this data sheet. Are we all based there? So va is arbitrary and derived - not being limited by a posted spec - correct? Daniel, How did you determine the number and size of the caps used in the ps you posted?
The datasheet has terrible quality schematics but good quality footnotes.

For when the transformer specs are unknown but it is known there's an 8 ohm speaker, then the audiometric value is at least 13,200uF (or more) per each rail of a split rail supply, if you want TDA729X, STK or a discrete amp to hit the low notes as clearly as possible. This capacitance will work best if kept full charged, so it remains true that the transformer must have sufficient amperage to simultaneously charge UP the caps while running the amplifiers. Problem: Using a slightly smaller transformer would require a lot more capacitance.

The transformer is charging the caps while the amplifier is discharging the caps.
We need to keep the caps fully charged if you want high quality clear bass.
That's why needed transformer amperage is decided by amplifier watts.
Quote:
Originally Posted by bcmbob View Post
This is the board I'm using now:
Heavy Duty Supply.......
That's just fine.
You can use 3300u caps for smoothing (C1, C2, C3, C4) and 10,000u caps for reservoir (C5, C6, C7, C8).
It has a lot of capacitance, and, that's probably beneficial with that 200va transformer.

Before parts swapping tinkering/tuning with the amplifier, I'd like to see an LED's, amber or red, per each rail on that power board and a drainer resistor per each rail. Parts: 2 led's and 4 of 5.6k resistors. Each LED needs its own resistor. Each rail also has a 5.6k drainer resistor. Of course you could do 2 led's per rail if you wanted to--they'll drain it down 1.9v per rail, shortly after power off. The combination creates a dual-duty drainer and a safety indicator, useful for. . . waiting for the LED's to go dark before tinkering with the amp. The drain is very mild (doesn't interfere with audio performance) and will take a little time for the LED's to go dark. The LED's are inexpensive and they might save your life.
Quote:
Originally Posted by bcmbob View Post
Daniel's nick name should be "The P2P Man"
P2P? That's doable.
Click the image to open in full size.
After this photo was made, the drainer/indicator leds have been added, they're amber, and the whole thing lights up. Without those additions, and without load, this supply stays fully charged (fully hazardous) for about a week. The white diode snubbing caps shown per each diode are 3.3n polyester. The red DC cable snubbing caps shown are 47n polyester. Those added polyester caps are unnecessary for your CRC power board. The power supply in the photo shows the estimated minimum 13,200u per rail for solid clear low bass and will do it if the transformer has enough amperage to keep the caps fully charged in all conditions. . . else double the capacitance or a replacement transformer may be indicated.
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Last edited by danielwritesbac; 27th December 2012 at 01:12 AM.
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Old 27th December 2012, 01:18 AM   #24
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The TDA7294 works best with a cleaner supply to it's input stage. This has been covered in various threads put up a few years ago.
But it is very important to put in protection diodes in the supply pins to ensure that the chip will not get voltages out of the permissible range which could cause it to blow up ! It does sound cleaner this way and the reduction in distortion in not small ! Check out the FFT .

The last time I tested the 7294, it appeared to have more transient impact than the 3886 ! Even at low volume!It's a good idea to try out both and see for onself. Is this better performance or just more HF trash ? Doesn't look like it on an FFT !
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Old 27th December 2012, 01:23 AM   #25
bcmbob is offline bcmbob  United States
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Thanks, all useful information. I did install two LEDs on the PS boards when they were used with the BPA150s. Nowhere as sophisticated as what you suggest - just a straight connection.
Since I hadn't used them in months I forgot about the retained energy, and got a quick reminder when a short happened. Luckily, my body wasn't included in the circuit, but that shock hazard has to be constantly in mind. That's one reason I would prefer a board approach over P2P as there is a lot of bare metal exposed in that style.

The time the LEDs on this kit stayed up was one of the first things I noticed. That was another reason I started to use my two lead light bulb dis-charger as I finalized the build.

Lets see what other replies come in on this portion of the project.
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Last edited by bcmbob; 27th December 2012 at 01:26 AM.
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Old 27th December 2012, 01:36 AM   #26
bcmbob is offline bcmbob  United States
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ashok, I was being polite when I commented on the other thread using words like "mushy", some IM distortion and low headroom. "Junk" sounds as good as anything I suppose and may be more direct,

The base sound is what attracted me and it's my hope that people more knowledgeable than me can help identify and correct those shortcomings. I am a strong devotee of the LM3886 designs, but this TDA has a character and warmth I feel is unique and presents a real high value potential.
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Old 27th December 2012, 02:04 AM   #27
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Quote:
Originally Posted by bcmbob
"Junk" sounds as good as anything I suppose and may be more direct,
Yes, the datasheet example schematics are dreadful.
However, the footnotes and text is of useful quality.
And the actual component is of excellent quality.

Problem: The datasheet example schematic shows overlarge power caps at the amplifier board, your amplifier's schematic shows undersize 22u power caps at the amplifier board. However, this amp is quite sensitive to small signal power. I think you're going to need to upgrade to some 220u caps (or 330u caps) for the amplifier board power caps. . . before fine tuning anything else.
Quote:
Originally Posted by bcmbob View Post
2. Bootstrap Cap
This is a miller comp amp and that cap is required for function. Your schematic shows 22u from pin6 to pin14. This is slightly undersize. Some of the newer schematics have 47u. According to the datasheet footnotes, upgrading the bootstrap cap to 47u will do slightly better bass clarity.
Quote:
Originally Posted by bcmbob View Post
3. Bypass caps
Adding a tiny cap to an ordinary quality large cap, to create an audiophile quality large cap. Each combination is unique. I suggest trying it at the NFB cap to improve the treble quality.
Quote:
Originally Posted by bcmbob View Post
1. NFB cap
5. Feedback-shunt
6. Feedback resistor
Your schematic shows 47u NFB cap series to 680R feedback-shunt resistor divided with 22K feedback resistor.

First lets figure the gain. Calculator keys 22 / 0.68 +1 = 34x gain.
That's a bit high for this amp.

Next, let's look at some audiometric NFB cap sizes with matching feedback-shunt resistor values.
150u with 3k3, and nearby values work
220u with 2k7, and nearby values work
270u with 2k2, and nearby values work
330u with 1k9, and nearby values work
470u with 1k2, and nearby values work
1000u with 680R and this isn't a good idea because it is hard to find small signal worthy 1000u caps and that much cap discharge can break the input transistors.

Currently, your NFB cap with 47u and 680R, on the schematic is a bass blocker and the source of poorer quality bass. See the datasheet footnotes where they advise that a bigger NFB cap can get you better bass.

Here's some options that don't have a bass blocker effect:
150u with 3k3 fs and 68k fb, 22x gain

220u with 2k7 fs and 75k fb, 29x gain
220u with 2k7 fs and 68k fb, 26x gain
220u with 3k3 fs and 75k fb, 24x gain
220u with 2k7 fs and 56k fb, 22x gain

270u with 2k2 fs and 68k fb, 32x gain
270u with 2k2 fs and 56k fb, 27x gain
270u with 2k7 fs and 68k fb, 26x gain
270u with 2k2 fs and 47k fb, 22x gain

330u with 1k8 fs and 56k fb, 32x gain
330u with 1k8 fs and 47k fb, 27x gain
330u with 2k2 fs and 56k fb, 26x gain
330u with 1k8 fs and 39k fb, 23x gain

470u with 1k2 fs and 33k fb, 29x gain
470u with 1k2 fs and 27k fb, 24x gain
470u with 1k5 fs and 33k fb, 23x gain
Quote:
Originally Posted by bcmbob View Post
8. Output caps
9. Speaker protector kit
Output caps block DC at the speaker jack, about 6600u for an 8 ohm speaker can be made high quality with 3300u||3300u for low loss. This cap simply goes in series with the speaker+, blocks dc, and protects the speaker.
Speaker protector kit from velleman is an alternative and the kit disconnects the speaker if DC is sensed at the speaker jack.
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Last edited by danielwritesbac; 27th December 2012 at 02:08 AM.
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Old 27th December 2012, 02:05 AM   #28
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Quote:
Originally Posted by ashok View Post
The TDA7294 works best with a cleaner supply to it's input stage. This has been covered in various threads put up a few years ago.
But it is very important to put in protection diodes in the supply pins to ensure that the chip will not get voltages out of the permissible range which could cause it to blow up ! It does sound cleaner this way and the reduction in distortion in not small ! Check out the FFT.
Do you have a link or some examples?
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Old 27th December 2012, 06:13 AM   #29
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Quote:
Originally Posted by bcmbob View Post
The base sound is what attracted me and it's my hope that people more knowledgeable than me can help identify and correct those shortcomings.
What you heard can be different from different system (it is very true for mosfet in class-B). The speaker and the preamp is critical. It can sound good and it can sound terrible. If you have a good speaker, almost any amp will sound good. Can you specify your speaker and preamp?

The bass character and "speed" of this amp is close to Hitachi lateral K135/J50. Even with most discrete K135/J50 class-B amps people are divided into loving it or hating it. Most are against this "muddy" sound, that's why in so many blind test, TDA7294 never won against LM3875/3876. And the mosfet amp only shows its most lovely character rarely in rare music such as an orchestra (while what everybody want is that bloody sweet FEMALE vocal )

Quote:
Originally Posted by danielwritesbac View Post
Adding a tiny cap to an ordinary quality large cap, to create an audiophile quality large cap.
I use a bypass cap with 1/1000 of the bigger cap, to avoid "overlap". If bigger than 1/1000, a series resistor with the smaller cap may be needed. OR, as in my TDA7294 amps, I use paralleled caps of the same capacitance.

Last edited by Jay; 27th December 2012 at 06:17 AM.
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Old 27th December 2012, 08:19 AM   #30
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Quote:
Originally Posted by Jay View Post
The speaker and the preamp is critical.
Especially in this case. I'd like to see the gain not up in the 30'x's but rather down near 24x, and then push with a nice preamp. That's an inconvenience, but I think it is worthwhile if one wants real hi-fi from this chip.
Quote:
Originally Posted by Jay View Post
TDA7294 never won against LM3875/3876.
Both the ideal support circuits and the usage for which these excel is so very different that I'm not sure how we could compare these, as the only way to make any of them "lose" is misuse it.
The TDA729X is further disadvantaged by its terrible datasheet schematic examples, its short gain capacity and its bizzare pinout that often leads to unoptimized PCB layout. Nevertheless, the part itself is of good quality.
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Originally Posted by Jay View Post
If bigger than 1/1000, a series resistor with the smaller cap may be needed. OR, as in my TDA7294 amps, I use paralleled caps of the same capacitance.
Earlier I was speaking of the NFB cap, which is really not different from an Input cap (except that the typical sizes are different). Effective and easy bypass does include parallel same value cap; but it also includes 10n and nearby value polyester as well as 0.47u and smaller electrolytic. 4n7 and smaller ceramic can also work. And, combination bypass can be done.

At the NFB cap, series resistor with a larger cap works. Here's a composite approach: 22u NFB cap with 10n bypass cap. Onto that is 220u+3R3, and that resistor is at groundside. And additional 0.47u electrolytic can be added to the 220u. Basically the smaller signals and vocals will go through the 22u, while the larger signals and bass will go through the 220u. The approximately 243u agrees with a 2k7 feedback-shunt resistor. It is a real example. This approach and either noise gain or anti-miller comp can be added to TDA7294 to use it high gain without a preamp. That is quite time consuming because the amp is not suited to high gain use. And, actually, the higher the gain on that chip, the more likely it is to be a mid-fi and/or make the fine tuning difficult.

But, of course if the amp is set to low gain, there's less gain put on cap peculiarities of any cap at the amp, so extremes of complexity would be unnecessary. . . or perhaps merely "relocated" to the preamp on clean regulated power not affected by jolts from driving a speaker, which makes the preamp a more sensible spot to have some gain. And, if at low gain, the TDA7294 is more likely to do hi-fi. There's many other factors, but low-ish gain probably makes them easier.
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Last edited by danielwritesbac; 27th December 2012 at 08:37 AM.
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