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Old 29th August 2012, 12:55 PM   #31
AndrewT is offline AndrewT  Scotland
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Daniel, for a couple of weeks after you came back you were making some sense. I could understand almost all of what you posted.
But you have gone back to your old ways. Using language that makes no sense. As a result the concepts you try to explain go right over my head.
Use plain english and stick to the correct electrical words and definitions that we all should know. Stop the gobbledegook.
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Old 29th August 2012, 01:08 PM   #32
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Perhaps a photo.
Here is catch diode applied to the "R" of the CRC power supply.
If the voltage drop max is 1v, then 0.5v+0.5v meets your 1v spec.
The specified parts are TO220 size and may be heatsinked.
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File Type: gif CRC Catch Diode.gif (1.4 KB, 160 views)
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Old 29th August 2012, 01:09 PM   #33
AndrewT is offline AndrewT  Scotland
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Then you lose the RC filter effect.
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Old 29th August 2012, 01:19 PM   #34
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Quote:
Originally Posted by AndrewT View Post
Then you lose the RC filter effect.
Only for brief moments. And that depends on the resistor value.
In this case, the resistor (and filter) works normally when the voltage drop is less than 1 volt (those diodes are off, not conducting).

I don't see a reason to do this to a low power amplifier. But it does show a way to attach a heatsink (via the heatsink-able TO220 diodes) to those resistors so that the nearby caps can remain as cool as possible.
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Old 3rd September 2012, 10:55 PM   #35
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Thanks,

1. I like your idea about using buffer, infact i can use an Unity gain buffer or a Voltage follower circuit, but would'nt the buffer will have high input impedance again?

2.I will use:
a). 18-0-18, center tap trafo, split supply.
b). Will have both 4ohms bookself and 8ohms floor standing (Planning to use 8ohms floor standing with LM1875)
c). Pure stereo build. No parallel, No bridge initially.
d). Later want to go for a Parallel build,if required or found it worth jumping to (so traffo VA rating should be as such, don't wanna use another traffo for a parallel build).
e). I will use only one power supply. No gainclone.

Please, calculate the values for both 4ohms speaker as well as 8ohms speaker and also, consider the PS for parallel build (at least traffo VA ratings). Will there be any problem if i build the power supply for a Parallel build and use it for a stereo build until i make up my mind to jump to a parallel build or i may also drop the idea (because those large filter capacitors, really comes at heavy price)?

3.And, the parallel build. Is it really needed or is it worth building in terms of experiencing better sound quality, both with 4ohms and 8ohms speakers, individually?

Thanks.

Last edited by noddy55; 3rd September 2012 at 11:02 PM.
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Old 4th September 2012, 12:11 AM   #36
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Quote:
Originally Posted by noddy55 View Post
Thanks,1. I like your idea about using buffer, infact i can use an Unity gain buffer or a Voltage follower circuit, but would'nt the buffer will have high input impedance again?
Buffers and very low gain preamplifiers are likewise less prone to magnifying their own noise. A bit different than a preamp, a buffer can most easily translate for impedance matching. Just pick a project made for that or one that is known to be helpful. Do the research.
Compare: The non-inverting LTP type amplifier has least noise with a very strong input load; however, an ideal buffer does not have this same action. I do not know, but merely guess that you would want to explore the diamond buffer.
You can consider the buffer a separate project, since the majority of your sources are unlikely to need the buffer in practice.
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Originally Posted by noddy55 View Post
2.I will use:
a). 18-0-18, center tap trafo, split supply.
b). Will have both 4ohms bookself and 8ohms floor standing (Planning to use 8ohms floor standing with LM1875)
c). Pure stereo build. No parallel, No bridge initially.
That's a bust. You must use parallel build if running 4 ohm speakers.
And you don't ever want to bridge the LM1875 (except maybe for 16 ohm speakers).
Quote:
Originally Posted by noddy55 View Post
e). I will use only one power supply. No gainclone.
Gainclone refers to a National Semiconductor Overture series amplifier with no caps on the power board but giant caps on the amp board to swamp the noise of the internal hard clipper limiter (Spike system) that is mostly counterproductive and can be greatly annoying for tone.
None of that applies to LM1875 except that National Semiconductor is the manufacturer; but fortunately, LM1875 doesn't contain the noisy Spike system limiter. Of course that means LM1875 has easier to manage tonality; however, the caveat is some fragility for overvoltage/overcurrent for which it has insufficient protection
Quote:
Originally Posted by noddy55 View Post
Please, calculate the values for both 4ohms speaker as well as 8ohms speaker and also, consider the PS for parallel build (at least traffo VA ratings). Will there be any problem if i build the power supply for a Parallel build and use it for a stereo build until i make up my mind to jump to a parallel build or i may also drop the idea (because those large filter capacitors, really comes at heavy price)?
The thing to do is pick the transformer VA large enough so that the required capacitance figure doesn't runaway. I don't have complete data on this right now, but let's estimate anyway. 100 watts (4 chips) x 1.5 = 150va. Well, see that's not expensive. Now do a 200va or greater, then your power supply board probably won't need epic capacitance. See? It is actually less expensive to get the somewhat bigger transformer.
Quote:
Originally Posted by noddy55 View Post
3.And, the parallel build. Is it really needed or is it worth building in terms of experiencing better sound quality, both with 4ohms and 8ohms speakers, individually? Thanks.
No matter what speaker you choose, when a transistor is overloaded for current, the resolution is crap and the frequency response unpleasant. Conversely, the parallel build of LM1875 usually has the lavish tone of an expensive discrete amplifier. This is worth the $4 added cost of two more chips. The parallel build will give you basic 4 ohm support (without instant amplifier fire) and lavish 8 ohm support (no overcurrent noise).
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Old 4th September 2012, 12:42 AM   #37
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Decades ago, C//R was sometimes seen at the amplifier output whereby the resistor will dump excessive current for subharmonic noise; but the cap will maintain higher frequencies (such as 60hz and up) at full force, and this was an odd sort of headroom boost being that the amplifier so equipped wasn't able to do lots of clipping in trade for a single thud and I suppose it mitigated the differences between tracks somewhat. At that timeframe, LP had a tendency to make lots of thuds that were sporadic noise, and radio broadcast didn't go much lower than 60hz.
For more modern use. . .
If you sized the cap for 30hz with an 8 ohm speaker, it gets 60hz with a 4 ohm speaker, and then you set the output cap's paralleled resistor by ear (range 2 to 10 ohms) to outright dump overcurrent conditions from that 4 ohm speaker. That should work transparently. However, you do still want a parallel amplifier. And, you need the full Thiele Small output parts so that the amplifier can gracefully handle capacitive loads.

This should automatically navigate the differences between your 4 ohm bookshelf versus your 8 ohm floorstanders.

This old method of selective current dumping does not cause losses at the power board and thus does not increase the need of capacitance at the power board. Conversely, power board current dumping, which is more popular today, does act just like you bought a smaller transformer and necessarily dramatically increases the need for large capacitance. Apparently, the designers of decades ago knew what they were doing with the amp output filters.

Edit:
Discussing a small amount of current dumping because driving 4 ohm speakers is just a bit over the mark for Parallel LM1875. We have almost enough current handling in the parallel amp. The discrepancy is quite small and so the question is which trick to use to complete the current handling.
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Old 4th September 2012, 09:51 AM   #38
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by danielwritesbac View Post
Decades ago, C//R was sometimes seen at the amplifier output whereby the resistor will dump excessive current for subharmonic noise; but the cap will maintain higher frequencies (such as 60hz and up) at full force, and this was an odd sort of headroom boost being that the amplifier so equipped wasn't able to do lots of clipping in trade for a single thud and I suppose it mitigated the differences between tracks somewhat. At that timeframe, LP had a tendency to make lots of thuds that were sporadic noise, and radio broadcast didn't go much lower than 60hz.
For more modern use. . .
If you sized the cap for 30hz with an 8 ohm speaker, it gets 60hz with a 4 ohm speaker, and then you set the output cap's paralleled resistor by ear (range 2 to 10 ohms) to outright dump overcurrent conditions from that 4 ohm speaker. That should work transparently. However, you do still want a parallel amplifier. And, you need the full Thiele Small output parts so that the amplifier can gracefully handle capacitive loads.
What?
Do you know anything?
Or do you hide your lack of knowledge behind gobbledegook so that we can't see what you don't know?
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Old 4th September 2012, 11:18 AM   #39
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What? Do you know anything? Or do you hide your lack of knowledge behind gobbledegook so that we can't see what you don't know?
I'm not the first person to employ a bass blocker cap for tiny speakers.
It is not useful to have LM1875 to push little 4 ohm bookshelf speakers to x-max for attempts at replaying low pitches beyond the capacities of the speaker. The prospect, both useless and harmful, can be fixed with a cap in series to the little 4 ohm woofer to roll off pitches the speaker couldn't reproduce anyway.

That lightens the amplifier load a bit.
Remember, he has a combination of:
small 4 ohm bookshelf speakers
big 8 ohm floorstander speakers

The big 8 ohm speakers is the reason why we didn't decrease the transformer voltage.

Overview:
SO, I've suggested Parallel amplifier build (with ballast resistors, of course) and a CRC (with yet more resistors). These help the LM1875 wrangle 4 ohm speakers, and those steps were almost enough. The little C//R filter attached to the 4 ohm woofer will help a really modest amount, but by that much, current noise will be decreased and safety margins improved.
Do we need to add more current handling protections?
If so, perhaps the 4A 600v Fairchild Stealth used for bridge rectifier?

P.S.
It seems that if I didn't try to explain it, the design would look totally sensible.
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Old 4th September 2012, 11:21 AM   #40
AndrewT is offline AndrewT  Scotland
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Does the reply in post39 reference anything in post38?
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