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|5th July 2010, 04:03 PM||#1|
Join Date: Dec 2002
Tweaking a TDA7294 Amp
I have rebuilt the amplifier modules in four pairs of Quad active monitors. The circuit is simple, a balanced to SE converter, volume pot, opamp driving active crossover, and then a pair of TDA7294. One is wired directly to a 6 1/2 mid/woofer, the other has a 1.8 ohm resistor and then is connected to the 1 inch tweeter.
Did a lot to the opamp circuits, power supply, and input routing initially. Lately took a look at the actual circuit around the TDA7294.
Already had added local decoupling right on the power pins. Used 22 ufd FM caps. Last pair I changed the ones on the low frequency amp to 56 ufd, and used a 0.033 ufd Siemens stacked film rail to rail on both opamps and 7294's. Replaced the stock bootstrap caps with 22 ufd FM.
The stock circuit uses 10 ufd caps and films to decouple all the chips, but these are a long way from the actual chips. Some are two inches away. I replaced the el cheapos with FM caps, and then added caps to the power pins. Never can have too much local decoupling.
Plan to replace the bootstrap cap for the lf amp with a larger value, maybe 100 ufd. Thinking to leave the hf amp bootstrap cap at 22 ufd. But wondered if there is more to be gotten in the areas tweaking this circuit that is basically straight from the datasheet. The amp with a beefed up power supply, better opamps, and cleaned up input wiring is nice sounding. Just always wanting more.
Cheapest is bestest
|8th July 2010, 12:26 PM||#2|
Join Date: Nov 2007
Tweaking the 7294 can have you "chasing your tail" a bit in that when you get one part of it balanced out then another part needs attention. You're on the right track with the power supply.
TDA7294 datasheet shows 2200uF right at the chip and this will make muddy terrible sound; however if those aren't larger than 220uF (with, of course, some larger caps located at the other end of the power supply cable, at the power supply board as is customary), then that is the first step for cleaner sound from TDA7294. It sounds like you've already done this. And, its about the normal arrangement for most amplifiers anyway.
The datasheet erroneously shows full bandwidth performance with an electrolytic capacitor of 22uF for Bootstrap, but a real life capacitor would have to be a larger size to get at some low bass. About 47uF works fine for me.
The NFB cap forms a filter, which is a bass blocking filter as you must block 0hz, which is DC; however, you probably don't want any effect at 40hz. Most electrolytic caps when used with the datasheet design will be 9db down by 40hz. If the input impedance is in the range of 10k~22k AND if the rest of the circuit mirrors at the nfb with a feedback resistor in the range of 10k~33k, then its highly likely that your NFB cap is far too small. This is fine if your speakers have small woofers, but it might be lacking in low bass that could be used by larger speakers (if the amp wasn't blocked from amplifying low bass as the datasheet design will do).
In the above scenario, the "large enough" cap will probably be so large that it will achieve HF failure mode (muddly treble) which is an error that will get magnified in proportion to the amplifier's gain.
Solution to NFB cap is:
Big enough cap in series with (an additional) 3.3 ohm 1/2w carbon resistor to put it at disadvantage to a smaller cap (for clean treble) that has no additional series resistor. In this way, you're sure that the smaller cap will carry the treble and the larger will not function much until it is needed. Its Big cap+resistor in parallel with small cap.
Check with filter calculator to see if your NFB cap is blocking some of your bass and you can remedy that easily.
If using the datasheet design, you're running the input of the amp at about line level. It may be 20k pot // 22k resistor and that is the most common. In these cases, the output op-amp of the source device may produce plenty of power with that 10k line level load but it may also make a hard sound. That's because the line level spec itself is for the sound to carry a long, long distance from your speakers (as in prosound) and that may not be your preference with point blank range at home use.
The concept "in home" is similar to a 1000w radio transmitter at point blank range to the radio receiver.
To adjust this hard long-range sound to something more pleasurable for home use, you'll need a slight loss. Yes, it really is a loss, but never advertised and hardly ever named as such. You can do this with a preamplifier. The inherent losses will soften and slightly widen the soundfield for a better fit at home. Other options include input transformer, capdiv, ldr, and sometimes you'll see a simple output resistor on the source side. All of these methods, including preamplifier, will make the sound more seemly for in home use. However an overdo would so limit the range that it may be muddy for other rooms in the house and/or decreased bass note impact. So, this could be considered "in moderation."
In the "grand scheme of things" the power supply fine tuning is more important than the nfb circuit fine tuning, which is more important the the input circuit fine tuning. That's listed in order of effectiveness; however, these areas are all important to the sound of your TDA7294.
Notes on options follow. . .
An option that you may want to consider is a 100k LDR type volume control employed as a passive preamp, along with a TDA7294 that uses 100k for input load and about 127k for nfb. You can match this up handily (because Litespeed LDR design is 100k) and the will be using much smaller size capacitors in the small signal areas for, perhaps, an easier time at getting a more realistic sound. . . but that depends on the source device, speakers, and your personal tastes as well. Sure a difference is probable, but whether that is better or not is up to your judgment. An amplifier with 100k input impedance may need a preamplifier to accommodate some sources in order to achieve sufficient bass (example: Creative Labs X-Fi), while other sources may perform very well indeed.
Given the lack of drive strength caused by the smaller workload at the amplifiers input, then the amplifier would probably need higher gain, and in this case dynamics may be larger.
A treble only or mid-n-treble amplifier for multi way use, would use nfb, input or power caps smaller than a full bandwidth amplifier. The decreased bandwidth of the treble only amplifier would allow you to concentrate your efforts on that part of the bandwidth for a lesser amount of compromise. That can make an easier job at getting extraordinarily clean sound.
A full bandwidth amplifier with TDA7294 can benefit from a regulated power supply. This can let you use smaller, 100uF or smaller, local power caps (while the large power parts remain located at a separate power supply board). With TDA7294's sensitivity to power supply differences, a regulated power source can be quite noticeable. How valuable this difference can be, is reliant on the input (your source) the output (your speakers) and the most important part of the audio chain, which is the reception of its transmission (your ears/judgment).
Well, there are many different options; but, from your description, it sounds as if you have already chosen a very good option.
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