Understanding and redesigning the input stage of LM1875 - Page 5 - diyAudio
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Old 4th September 2012, 01:10 PM   #41
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Quote:
Originally Posted by AndrewT View Post
Does the reply in post39 reference anything in post38?
Yes, an output cap implementation (old-school C//R) to help the Parallel LM1875 drive the small size 4 ohm speaker, yet not hinder when driving large size 8 ohm speaker. The overcurrent protection is very modest, but working.

Question: Does Parallel LM1875 with CRC and 18+18vac transformer, have enough current tolerances to drive the 4 ohm speaker without adding additional protections?
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Old 4th September 2012, 04:25 PM   #42
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Are you referring to the typical R//L or R+C between the amp output and the amp terminals?

I cannot understand your R//C
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Old 4th September 2012, 05:19 PM   #43
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Quote:
Originally Posted by AndrewT View Post
Are you referring to the typical R//L or R+C between the amp output and the amp terminals?
The normal parts are a helpful prerequisite that needs installed prior to driving the capacitive load.

Would I be right in assuming that an output cap sized to roll off a 4 ohm speaker, can play an octave lower when an 8 ohm speaker is used?
Well, he has a small 4 ohm speaker and a large 8 ohm speaker.
"Output Cap" is part #C6 in LM1875.pdf
Quote:
Originally Posted by AndrewT View Post
I cannot understand your R//C
That's a resistor set parallel with the output cap. The resistor is optional--it is only for fine tuning, which may be unnecessary.

P.S.
No need to run the signal for the 8 ohm speaker through the cap (or cap//resistor) filter. Although the old document that I was studying did have the filter in the amp enclosure (as a cheat to run 4 ohm speakers without exploding the amp during subharmonics), perhaps the filter would look more sensible within the speaker enclosure of the 4 ohm bookshelf speakers. They'd drive easier and x-max less often.
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Old 5th September 2012, 09:32 AM   #44
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Are you referring to the DC blocking capacitor that must be fitted to an AC coupled single polarity amplifier?

If so then one never fits a parallel R here.

If you are referring to a dual polarity amplifier, then that should never need an output DC blocking capacitor. Unless the amp is broken.

You are completely mixed up judging by what you have posted so far.
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Old 5th September 2012, 12:31 PM   #45
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Quote:
Originally Posted by AndrewT View Post
Are you referring to the DC blocking capacitor that must be fitted to an AC coupled single polarity amplifier? If so then one never fits a parallel R here. If you are referring to a dual polarity amplifier, then that should never need an output DC blocking capacitor. Unless the amp is broken. You are completely mixed up judging by what you have posted so far.
Yes, that is the part. An output cap can be fitted to a split rail amp if your desire is bass roll off. In this case, bass roll off of a series cap is desirable for the small size 4 ohm midbass/woofer so they will drive easier and x-max less often. Since Steinway doesn't make pianos the size of toasters, there's likewise no need to pretend that compact speakers run full bandwidth. Therefore, driving 4 ohm compact speakers with a full bandwidth signal from a current strangled amplifier is totally bad design. A cap will help it, in the case of compact speakers.

Remember, it is a compact 4 ohm speaker run from a current limited chip amplifier--a recipe for disaster. Like flushing your last coin, there's no need for abusing a current limited chip amplifier. So, of course it is helpful to roll off that little woofer with a series cap, for obviously better performance.

And remember that we're having this efficiency tuning discussion because the LM1875's inbuilt protection is utterly insufficient in practice. Therefore please omit the paralleled resistor from my series cap example. An ordinary output cap implementation is totally suitable as inexpensive speaker protection, and I think that is useful for longevity.

P.S.
With the creative discussion of overcurrent protection, perhaps Noddy won't desire the dramatically increased load of bridging the amplifier that would, because of redoubling the protection efforts, mean that bridging would simply increase the noise without increasing the output power significantly. Perhaps we should reserve the bridging ideas for driving 16 ohm speakers.
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Old 5th September 2012, 06:06 PM   #46
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Filter the excess or unwanted bandwidth at the input.
Don't mu** about with a corrupted R//C impedance altering component between the amp and the speaker. Leave the crossover to the speaker designer.
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Old 6th September 2012, 10:34 AM   #47
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Quote:
Originally Posted by AndrewT View Post
Filter the excess or unwanted bandwidth at the input. Don't mu** about with a corrupted R//C impedance altering component between the amp and the speaker. Leave the crossover to the speaker designer.
Filter at the amp input? Noddy said that there are selections of Small 4 ohm speaker and Large 8 ohm speaker that may be used. So, the amp input filter must be sized to support the Large 8 ohm speaker.

P.S.
Speaker designers who make 4 ohm speakers are unlikely to have considered current noise and related consequences in audio amplifiers.
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Old 6th September 2012, 02:25 PM   #48
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Hey Guys,

Can we please, keep this speaker impedance and amplifier bandwidth matching issue for later, Please? Let me build something and reach to that level first. I will sure come back to you for such other issues, later. I still reserve my choices for speakers. These choices are still flexible. I myself is a 4ohms bookself speaker designer. I used a crossover inside it which i will show you later for sure. They contains a 4ohms 8inch full ranger and a 4inch tweeter inside it. And, they are not so small. I designed and build them more than 10 years before (in my teenage) so now, I am planning to change their design as according to LM1875. Keep cool and be Happy until.

Thanks.

Last edited by noddy55; 6th September 2012 at 02:36 PM.
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Old 6th September 2012, 06:13 PM   #49
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Sure. Just aim for Parallel LM1875 to 8 ohm speakers. That works wonderfully.
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