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#21 | |
diyAudio Member
Join Date: Feb 2012
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Quote:
What if i wanna go beyond 56K? I want to have 100k as the input impedance so as for sure always that i can use any of the input sources which can have 10K output impedance at max. Offcourse, no problem if the output impedance of the source is lower than 10k. Source's output impedance allowable up to 10K is the safest and peace of mind value for me. 1. Will using the 100K fix input impedance resistor have any bad effect on the circuit performance or sound quality, except, other than being acting as the high pass filter along with the input capacitor(1uF) (whose -3db cut off frequency i will manage accordingly by changing the capacitor value and keeping the 100K constant) ? 2. Using and fixing 56K maximum range is just because of the reason that hardly any source's output impedance can be greater than 5.6K (56K is 10 times of 5.6K) or does it implies other important reasons also? 3. Input resistance(1K) along with 1M resistor also forms an input voltage divider network. Will it be reasonable to omit either or both of them? Will either of these resistors have any effect on the high pass filter (1uf+22K)? Thanks. Last edited by noddy55; 22nd August 2012 at 11:58 PM. |
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#22 |
diyAudio Member
Join Date: Nov 2007
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LM1875's have a broad range of tolerances and this makes them excellent for exploring amplifier fine tuning. What we can do with the chip amp is change the design by altering the support circuit. We can't really change a design that is sealed shut inside a chip, so that is much different from a discrete parts amplifier. Even so, many different designs can be done with the chip and we can personalize it for system symmetry.
The prospect needs adjustable prototypes. And we would need to know about the ranges of tolerances and what audible effects they can do. Explanations are seriously hindered by the fact that all of audio is interrelated compromises that you balance towards best effect and thus any document will both help and hinder. It is the extent that matters. Please constrain problems by double-checking with other sources.
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#23 | ||||
diyAudio Member
Join Date: Feb 2012
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Thanks! for such a long response.
1. Yes, I forgot to mention, R5(180K) will offcourse can go upto 100K with R3 being 100k as they both are related to each other. 2. I am targeting an initial gain of 16 and would like to go upto a gain of 25 if the sound quality still remains preserved. Mr.madisonears has already build one such project with k50 kit and he changed the R5(180k) to 150K to jump from a gain of 19 to a gain of 16. He felt some harshness in high frequency tones with a gain of 19 (K50 kit), which diappeared with a gain of 16 (can we think of going beyond the gain of 16 in such a scenario?). So, it seems that a gain of 16 is just fine. Or, may be it is due to the kind of source he is using with his project. I would rather initially like to test the amplifier with a sony dvd player line out with original cd soundtracks and also with an hp laptop audio output with winamp being a player for a .wav and .mp3 files. However, i seek your advise regarding the preferred gain setting value for general playback (but never at the cost of sound quality compromise) so i can accomodate most of the input sources without changing or setting the gain each time for every source. My possible input sources would be dvd player, laptop audio, desktop audio, apple ipod and my collection of vintage stereo boomboxes, cassette decks line out along with a vinyl record player. The amplifier will reside on a desktop computer table, where any source could be attached.So, the desktop PC and a dvd player will always for sure be a alltime source input. Speakers would be either the bookself or a floor standing with a tweeter and a twoway full range speakers with 8ohm impeadance (i am also working on this project) so, that i can later shift to a parallel build of LM1875, if i found the sound quality interesting with a stereo build. Quote:
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This clearly means that if we increase the gain, bandwidth will automatically be decreased and vice-versa. So, the sound quality seems to be the function of the bandwidth which is the function of the gain, which in turn is the function of the ratio of two (feedback and shunt) resistors (Not there values). If the ratio is same we will achieve the same gain and hence the same bandwidth and the same general sound quality (as far as other conditions remains the same). Now, this bandwidth can also be manipulated further by external, high/low pass filters and capacitor values (increasing or decreasing -3db points) which in turn again changes the bandwidth and hence the gain and hence the sound quality. It is not until we change either the bandwidth or the gain we get the different sound quality (with all other things being the same in the setup). So, why should we so worry only about changing/using the mid range values for shunt and feedback resistors rather then about there ratio? Infact, We should be worry about the ratio of the gain and the bandwidth rather then being worry about the single feedback or shunt resistor values. It is the overall setting of this ratio (gain/bandwidth) which gives us the different sound qualities, GBP being always remaining constant. There should be some perfect ratio (of gain and bandwidth) and different for everyone (because we all have different setups like speakers and sources) no matter how or with what values we achieve this ratio. GBP for LM1875 is 5.5Mhz. Please, anyone, correct me to lift/support/help, for anything or if i am confused and wrong (i am still in the learning phase) with any point or words above in this last and important point. Thanks. Last edited by noddy55; 26th August 2012 at 12:22 AM. |
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#24 | ||
diyAudio Member
Join Date: Nov 2007
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It is possible to boost either treble or bass dynamic by the noise added with extreme feedback resistor values like the K50 kits have a treble effect at detriment to resolution, but I don't suggest doing that. Quote:
This time we can look at feedback resistor (FB) versus feedback-shunt-resistor (FBS) as a voltage divider. FB, FBS, Gain 25K, 680R, 37 27K, 750R, 37 33K, 910R, 37 39K, 1K , 40 47K, 1.2K, 40 51K, 1.5K, 35 56K, 1.5K, 38 60K, 1.6K, 39 62K, 1.8K, 35 68K, 1.8K, 39 75K, 2.2K, 35 82K, 2.2K, 38 In the selections above, notice how the feedback current (the current/force of negative feedback) changes over a fairly broad range but the voltage output (gain) can remain nearly the same. 68K/1.8K will give you enough gain and it will give you some relief on NFB cap size due to the 1.8K feedback-shunt resistor. That selection is the most convenient for quick success. The record player will need a Riaa preamp or Turntable preamp added because that is a frequency response leveling necessity for turntable response curves, and that is a need of the turntable not related to any given power amp. All of your other sources have one thing in common and that's a 10k line level load (those headphone sources go from ~16R to ~10K, those line level sources go from ~9k to 100k), so the input of your amp will be 10k and that's perfectly acceptable since non-inverting LM1875 can correct its 10k input load versus 68k feedback resistor without No audible problem whatsoever. P.S. There is one difficulty and it is from high gain. A high gain amplifier is necessarily more sensitive to all of its inputs, every pin is an input and that includes power. Instead of relying on the excuse of power noise rejection specs, we'll need a bit of assurance. You'll need some high efficiency 330uF's (level) or 470uF's (attractively laid back) for amplifier board power caps. You'll also need a CRC power supply, but the good news is that it is very easy to do.
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Last edited by danielwritesbac; 27th August 2012 at 04:30 PM. |
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#25 | |||||
diyAudio Member
Join Date: Feb 2012
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Thanks a Lot! You convinced me for the higher feedback or shunt resistor values are in no way better for good performance.
1.What about lower values(below 56K for feedback)? I still have a doubt about them. Offcourse, a higher feedback current will flow with too lower values as explained by you. What problem do we face with higher feedback current? Just check out the LM4780 datasheet (Page 17, bottom left corner). It says: Quote:
Accordingly, this way if i want a gain of 28, best combination of Rf would be 27K and Shunt resistor would be 1K. Please, comment on this. Quote:
2. What value of input impedance resistor (R3 22K) you suggest with the above conditions? Quote:
No, i don't want to run amplifier at its max volume and source at its lowest. And, i also assume that even at its max or nearly 80% volume, none of my source will clip. So, instead i want to share the load between both (source and the Amplifier). Also, when source at its lowest volume and amplifier at its max, we experience somewhat lower Signal/Noise ratio, and the sound quality seems dull with lower S/N ratio.Hence i need a lower gain with LM1875 as compared. I am not sure but i think 20 to 25 (V/V) gain would be enough. I think this is purely a matter of experimentation. 3. Well, what lower or middle range gain you found the most exciting, perfect and giving the best, widest, sweet soundfield with most of the general input sources, during your experimentations with LM1875? Quote:
4. Can u suggest some best names for high efficiency and online sources to buy from? We only have "PHILIPS" or "KELTRON" here. Quote:
5. What is CRC power supply? Thanks.
Last edited by noddy55; 27th August 2012 at 11:33 PM. |
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#26 | |||||||
diyAudio Member
Join Date: Nov 2007
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I've seen some of your calculations. Apparently, your skills are greater than mine in this area. Perhaps I should be asking you the questions? Notes: In general, higher gain is somewhat easier for stability, so if you're flying blind (building without scope) then keep the gain a bit high. A bigger transformer for your power amp does not make your iphone, digiplayer, or computer more powerful; therefore the bigger the amplifier, the more gain you'll need. Quote:
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For a non inverting amplifier, a strong input load like 10k is excellent for noise reduction and high resolution results. One of your music sources is a computer. Anyone who thinks a computer power circuit is clean may vote "yes" by doing a headstand. See how important it is to have a strong input load on the amplifier to block noise so we don't have to do unnecessary headstands? Quote:
Low ESR types are also useful on amplifier boards. However, ordinary standard types can be more useful on power supply boards. Quote:
The Diyaudio.com store has a CRC power board. Check it out: P-PSU-1V20 - PSU Cap Diode Board (NEW STOCK EXPECTED MID AUGUST) Also scroll down the page and click on schematics and other documents. The next logical accessories (filters) to add are RC filters across the transformer primary and secondary windings to block HF pollution. If you want a more technical description, see this page on the Lenard Audio Institute. Scroll down the topic of ripple and you'll see a CRC has fixed it. I propose to use power noise rejection as a defense against sporadic noise, but use power supply filtering as a defense against nonstop noise. See the difference? P.S. Your questions that endlessly go around in circles directly relate to this: Everything is interrelated and what we need to do is simply avoid any sort of large one fell swoop effect from any one given area. P.P.S. The real job of fine tuning an amplifier into balance with itself is indeed like your questions and going around in circles for at least a day really does happen when building a new amplifier. For safety during fine tuning I suggest bleeder resistors and LED's on your power board. Got light, don't touch.
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Last edited by danielwritesbac; 28th August 2012 at 03:09 PM. |
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#27 | |||
diyAudio Member
Join Date: Feb 2012
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Be sure that the similar power supply may later be used for a parrallel build, if required. Thanks. |
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#28 | ||
diyAudio Member
Join Date: Nov 2007
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Thank you for the compliment.
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While considering buffers preamps, etc. . . I just hatched an alternative idea: Have the 47K resistor (and RF filter) at the amp board. Rig a switch on the front panel to engage a parallel resistor of 12k because 47k//12k=10k. The amplifier is now switchable between 47k input load versus 10k input load. In this case, your input cap also mounts near the front panel. The switch at the 10k position will act as a noise damper, but if source current noise is too much from driving that load, you could flip the switch for 47k for the choice of an easier load to drive into a slightly more noisy amplifier. You will see that the audio differences in practice are very tiny except that the 10k load gets you better bass from your computer. I find it very weird that a cassette deck is made incompatible with 10k line level spec, but the cure for such poor design is a buffer. Here is a JLH buffer that you can add to sources that need a buffer when/if the manufacturer was too cheap to install a buffer. I believe that you could use OPA2132 or OPA627 op amp for buffer, powered by OnSemi MC78**, MC79** regulators, and good quality caps, since fine parts run from clean power is a recipe for fast success. There's also some nice BC560C/BC550C and jFet Diamond Buffer projects documented here at diyaudio.com--just use the search feature and you can find many. You actually don't need to add active stages when using LM1875 power amp. But if you have a personal preference to have everything accommodate theoretically better despite a zoo variety of sources, then you will want a buffer. If you had LM675, TDA7294, I would say that you want a buffered preamp; however, LM1875's have plentiful gain capacity so in this case you want a simple buffer. The next step up in complexity is to add either a buffer or a buffered preamp to every source, individualized for the needs of each particular source. But, do you really need such complexity? I cannot answer this one for you. However, your turntable does require its own turntable-specific preamp. There are also many Computer preamp projects, and again do please make use of the search feature of this forum. Quote:
Current: Watts times 1.5 = VA transformer requirement Question: Are you going to design a Stereo (or VDM) Parallel LM1875 to drive 4 ohm speakers (implying the necessity of both large transient delivery and overcurrent protection in one power supply design)? That would result in the maximum size power supply. However, due to the wattage difference, a Monobloc (or Dual Mono) for an 8 ohm speaker will result in the smallest size power supplies. I need some more input prior to estimating your power supply needs. Are you going to use one power supply or two? Is transformer availability Center Tap or Dual Secondaries? Are you going to use 8 ohm speakers or 4 ohm speakers?
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Last edited by danielwritesbac; 29th August 2012 at 08:28 AM. |
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#29 |
R.I.P.
Join Date: Jul 2004
Location: Scottish Borders
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The 0r47 value depends on how much voltage you can afford to drop.
With low voltage rails used in chipamp implementations expect to drop an absolute maximum of 1V, but typically this resistor will drop far less when playing music. Maybe just 50mV to 100mV. If you guess the value too high, just add another resistor in parallel. Start with a 1r0 600mW. Keep adding 1r0 until it seems about right. eg. 3off 1r0 600mW in parallel is equivalent to 0r33 1.8W. Able to pass a continuous current of 1.5A all day (small ClassA amplifier territory). The cap before the R is loaded very heavily with ripple. You must buy a cap or bank of small caps that can accept this ripple without overheating. Read TNT and ESP for more detail. If you adopt 10mF (10,000uF) for the second C then about 5mF is about right for the first C. Use 5off 1mF caps in parallel for high ripple capacity. If you want your amp to reproduce the lowest of deep bass well, then expect to double all these capacitance values. Where have all your other questions gone? Or have you found the answers elsewhere?
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#30 | |
diyAudio Member
Join Date: Nov 2007
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That explanation is so nicely concise. Thank you sir.
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Catch diodes for the R can assure the voltage drop doesn't go beyond 1v, and a series pair of TO220 fast silicon is ~1v total and can be heatsinked. But I think the catch diode concept, costs and caveats, is unnecessary for lower power amplifiers. So, is there an easier way to heatsink the resistor? Do we just rely on the PCB to heatsink the resistor? Or what ways are there to deal with that heat and keep the caps cool?
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