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Old 28th October 2011, 11:57 AM   #11
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Danielwritesbac, regardig questions in post #8 you are correct on all counts. When you're done building these, don't forget to check each amp individually for DC offset.
AndrewT, +inputs are AC coupled to outside world, they are only DC connected to each other, that's why I specified .1% resistors to keep currents equal there, I've never had any problem doing it that way.

Mike
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Old 28th October 2011, 08:20 PM   #12
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Thanks AndrewT. Those comments are valuable options to explore.

Whatever is good for the - input is just about as good for the + input. Between the two different ideas simultaneously shown in the schematic, there are four different options to explore. A triple parallel chip amplifier is well able to vote and to veto such options and triple LM1875 doesn't hesitate to veto. Thanks again. I'm grateful for the problem solving options.

RF attenuation is on the list in post 1. I didn't forget. What value do you suggest to put at input - to input + directly on the chip pins?

I was hoping there wouldn't be an error in the specified components, but I have made a miscalculation. With 3.3k series to it, the 220uF NFB cap(s) is too large, which less efficient than necessary for this project. I'd like to aim filters to take effect at just lower than 44hz, but without seriously reducing 44hz (typical computer source floor frequency).
We need smaller signal caps.
May I use 122.01uF for NFB cap(s)?

The next question after (caused by) reducing the NFB cap(s) value has to be the value of the signal Input cap(s) which also have to decrease in order to avoid harmonic distortion (foldback) of the bass. What is the resulting (smaller) correct value for input cap(s)?

Yet another question: If, perchance, individual signal input caps are used, then can the 1k series at input be decreased to maybe 270R or less? Those 1k are there to decrease current. If we can use a less lossy method, I'd like to know. Thanks again!
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Old 28th October 2011, 09:17 PM   #13
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I would avoid putting a cap across In+ and In- unless absolutely necessary, as it reduces open-loop gain bandwidth and thus increases distortion. RF at the input is best drained away to ground using RC/RLC or R(L)CR filtering.
Make sure you have adequate supply rail bypassing. You should have power ground next to each chip, so run the recommended 100n X7R ceramics/films straight from Vcc and Vee to there.

As a general rule, don't use electrolytics for frequency response shaping. In a highpass involving one, aim for a cutoff frequency of about 1/10 amplifier fc.

Where did you get that ominous 44 Hz number from? Smells fishy.

If you need a low-frequency rolloff, you can use an appropriately sized film cap for the input coupling cap.

LM1875s shouldn't be so low-noise as to require less than 1k input series resistors, but I don't see why you shouldn't be able to use 470 ohms. The chips' input resistance is plenty high anyway.
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Old 29th October 2011, 07:24 AM   #14
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Quote:
Originally Posted by sgrossklass View Post
. . . Where did you get that ominous 44 Hz number from? Smells fishy. If you need a low-frequency rolloff, you can use an appropriately sized film cap for the input coupling cap. . . .
Thanks for the informative post.
I'll try to make an answer. That fishy odor is chip amp design. What we have here is a quality op amp set into the weirdest place imaginable, sealed into the same package as miniature output transistors.

A slight compromise at signal lower than 44hz will buy us increased current handling capacity for lifelike dynamics, thereby providing better support for average speakers. So, there's no plans to support infrequent subsonic emissions into huge speakers at the expense of durability, because the chip failure mode doesn't benefit any size of speaker. There are future plans to do bridging later, so maintaining safe operating area will necessitate planning to decrease unnecessary current surges.

For small signal capacitors, I'll be attempting an old fashioned approach for defeating variances of brand or model--There is advantage in having the main concentration on correct component values.
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Old 31st October 2011, 07:05 AM   #15
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So far, 13,200uF per rail "dually" power supply done, and checking in with a fun photograph of it.

This seems like enough capacitance. Is that true?

Do you think that I should add some bleeder resistors or not?
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Old 31st October 2011, 10:25 AM   #16
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by danielwritesbac View Post
This seems like enough capacitance. Is that true?
if I were asking that question, I would state the frequency bandwidth and the speaker impedance that applies.
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Old 31st October 2011, 12:01 PM   #17
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-0db @ 44hz and a 4 ohm speaker.

In theory, disconnecting the transformer, disconnecting the rectifiers, and then using the DC + and - taps as if it were an output cap, is 6,600uF and with a 4 ohm speaker that's -0db at 24hz. Is that right?

But, does triple parallel change the floor frequency on that? /3?
And, then later bridging change the floor frequency again? *2?

P.S.
I sure would like your view on a bleeder resistor value. I don't want to make it significantly more difficult to charge the caps, but would like, at least a small measure of added safety, considering the terrible whallop that pretty little thing could put out.

Thank you!
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Old 31st October 2011, 12:08 PM   #18
AndrewT is offline AndrewT  Scotland
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Permanent bleeders increase PSU ripple.

I never use them.

Switchable bleeders or auto adjusting bleeders may be useful in some circumstances, probably not all.
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Old 3rd November 2011, 04:24 AM   #19
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Default Deadly exchanged for "less hazardous"

I've been working on the amp a bit every night. The cute but deadly power supply has now had a redo for better power line filtering and mainly for safety reasons. I would like the opportunity to survive while tinkering with the amplifier.

Redo! Sorry, I couldn't imagine up any reliable "Auto Adjust Bleeder" so I ditched modern design entirely. Meanwhile, back at the 1970's. . .

Although visually very similar, the power supply has been rebuilt to a standard Center Tap design with One KBPC2504 rectifier snubbed by 4 of 10nF inexpensive lossy polyester dip caps. As seen previously, there are 6 of decent 2200uF and 1 of 100nF lossy polyester dip cap per each rail.

Close to the rectifier there are two of 2.7k 1/2w resistors hooked to 0v center tap at the input side of the power supply. Both of the 2.7k resistors has its own 30ma rated Bright Amber LED from resistor to rail. Of course there is an LED attached to both rails. That is 290mw per each rail (counting LEDs and resistors). With amplifier (and its speaker load) attached, the power supply drains instantly, as conveniently verified by the amber LED's for safety. If speaker load is omitted, the LED's stay on so that fingers stay away.

Benefit:
This is less hazardous than the previous attempt. After this redo, the output voltage holds steady and same for both rails. The new EI Core transformer and the redone power supply both stay cool and silent while running.

New question:
Should I add an additional "tank section" close to the amplifier, perhaps 35v 10,000uF caps on both rails?
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Old 3rd November 2011, 09:14 AM   #20
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by danielwritesbac View Post
I couldn't imagine up any reliable "Auto Adjust Bleeder"
the very simplest I can imagine is a low voltage light bulb. Cold filament resistance is ~ 1/3rd of hot filament resistance. Just match the bulb/s voltage to the full PSU output voltage.
When the PSU is running at full voltage the bulb filament is hot and bleed current is set by the hot bulb parameters.
after shutdown the voltage falls and the bulb cools and the resistance drops and the bleeder tries to pull a more constant current rather than an exponentially reducing current. When down to just a volt or two the bulb will be drawing ~3times the current that a fixed resistor bleeder would be able to.

More complicated methods, adjusting the bleed current to flow only after loss of mains power. This could use FET switches to bring in stepped resistances controlled by window comparators.

A simple mechanical relay could just return to normally closed on loss of power and the bleeders drain the caps.
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Last edited by AndrewT; 3rd November 2011 at 09:17 AM.
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