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Old 21st December 2011, 05:49 PM   #11
adason is offline adason  United States
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so where is the schematics?
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Old 21st December 2011, 08:33 PM   #12
tweakk is offline tweakk  United States
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Could I use these transformers or do I need a single with dual secondaries.
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Old 21st December 2011, 09:17 PM   #13
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Originally Posted by AndrewT View Post
Dan + Ein,
put your numbers in and check what to expect for current capability and current demand.
Dan look again, I said Normal quiescent supply voltage of +27Vdc.
You are complaining that is too high and yet suggest +-27.5Vdc !
Can't you read english?
You said +-30 abs max. I said +-27.5 abs max.
Running steady at more than approximately +-26 is an amplifier that won't last for a decade, because given a long enough time, there is sure to be a power surge during operation. Meanwhile, back to that question on the R in the CRC for some safety current limiting?
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Old 22nd December 2011, 02:44 AM   #14
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Default Transformer

Originally Posted by tweakk View Post
Could I use these transformers or do I need a single with dual secondaries.
Save your 2 of 24vac transformers for using as a 24+24 transformer with stereo TDA7294 or stereo LM3886 MyRef.
For LM1875, the output of these is either too little or too much.
So, here are some possible options. . .

I could write a gigantic answer as to what transformer, smps, capmulti or regulator options we could use, but in constraining the answer down to our 5 minutes per section assembly time goal, plus excellent performance, I have this abridged list for your consideration:

KBPC2504 or KBPC1604 rectifier snubbed as shown in the Mark Houston Synergy amplifier--just look that up. This high quality rectifier has a steady voltage drop that I find more suited, quieter, and safer than the more variable fluctuation of a fast diode marketing gimmick. Although opinions may differ, the high quality KBPC2504, snubbed with inexpensive polyester, meets all of our goals, including low noise performance, lower fluctuations, better thermal management, higher durability, and short assembly time.

Conservative choice transformer
A 15+15VAC 4+ ampere transformer, while a couple of volts low also promotes easier voicing due to lack of overrun condition, lower heat, and promotion of longevity, without the need to fuss or waste time over push for power design. This one meets all of our goals, including 5 minutes or less hookup. It still meets the goals if you slightly unwrap/rewrap a toroid to change it to a 16+16VAC.

Push for Power choice transformer
An 18+18VAC 4+ ampere transformer is what you would use for push for power design with this amplifier, at the possible expense of some longevity. We would be fussing over adding some means of current drop, possibly the retail ribbon cable trick or PI filter overdo, and in either case cannot meet assembly time goal.
However, you can adapt an 18+18VAC toroid transformer with the unwrap/rewrap trick to get 16.7vac+16.7vac or less, and in this case it meets all of the goals.

Linear unregulated power board
For the 15+15VAC to 16.7+16.7VAC choices, a simple pack of 5 or so paralleled 2,200uF 35v inexpensive caps per each rail (a 10 pack for a split rail board) plugged into a scrap of copperless (no pads) Phenolic board.
For the 18+18VAC push for power option, 4,700uF 50v at the rectifier, 10w resistor and 10,000uF 35v, per each rail in a standard CRC design. It can also be put together on a scrap of copperless phenolic perfboard in just a few minutes.

Transformer amperage
The linear unregulated supply has an weird transformer requirement. If you are using a speaker with your amplifier then your transformer needs to be at least 4 amperes, no matter if dual mono or stereo. Just like woofers and bells, you need a big one if you want bass. Of course a 6 ampere transformer is fine too, because like the 4 ampere transformer, it is also big enough.
Theory: The transformer plus rectifier of the linear unregulated supply are the same thing as fastest possible battery charger, whereby the higher amperage models charge the caps faster and those caps must stay always brim full during playback if you want good performance.
Maths: VA figures 15+15VAC*4a=120va is volts times amperes. Likewise 4a with 18+18vac transformer = 144va, which is 4 times (18+18) = 144VA.
Using a minimum VA figure is not required, you can use a higher amperage model if you wish.

SMPS Alternative:
It is also possible to use a pair of adjustable 24vdc SMPS; however, SMPS, being switchmode technology spends half of its time asleep. Exactly like the computer with the 200 watt demand needing a 430 watt power supply, an SMPS always needs "double-rated" meaning you buy at least twice as much amperage as you need. If there is availability of a pair of sturdy 15a or better SMPS, they are quite useful as amplifier power sources. It takes two units to make a split rail power source for a stereo amplifier, and they must be isolated. Further documentation is at Decibel Dungeon, just look that up.

FAKE (generic) LM1875
Transformer 12.5+12.5 (the common 24vct industrial center tap)
Flyback diodes as seen in the TDA2030 datasheet
About a step smaller capacitance at the chip than for LM1875
Maximum feedback resistor is 68k for fake, 115k for authentic
Generic LM1875 uses Datasheet for TDA2030
Likewise, with such accommodation a real TDA2030/2040/2050 is usable.

Dual mono
Dual mono refers to 1 pair of monophonic, aka monobloc amplifiers put into a single enclosure, and wired exactly like monoblocs, the dual mono amplifier gives a somewhat wider presentation for double the power supply expense of a stereo amplifier.
Conversely, a stereo amplifier is two amplifiers in a single enclosure with only one power supply, and this arrangement is a bit more normal and costs less.

More information
Decibel Dungeon DIY hi-fi index.
There is much more information than this, plus helpful examples at Decibel Dungeon.

Last edited by danielwritesbac; 22nd December 2011 at 02:53 AM.
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Old 22nd December 2011, 05:11 AM   #15
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Default Designs / Schematics

Originally Posted by adason View Post
so where is the schematics?
LM1875 PCB, Which To Use
All over that thread is LM1875 optimizations, discussions, schematics and opinions.

It is my intention to show an assembly method. You can use any schematic you like. Some of the charms of the LM1875 is that you can build your own thing, with quick success, high resolution, and minimal cost.

For parallel, add 0.22 ohm 3W or better resistors series with the speaker output of each chip. This small resistor loss is common to most parallel amplifiers so that they don't fight.

Resistor matching is necessary for parallel amplifiers.
Measure all resistors with digital ohmmeter to be sure that:
all feedback resistors are identical
all feedback-shunt resistors are identical
all stopper resistors are identical
all input load resistors are identical

Last edited by danielwritesbac; 22nd December 2011 at 05:28 AM.
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Old 22nd December 2011, 05:15 AM   #16
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Default NFB Cap for durability and for fun

I will promote using an NFB cap and show how to do a nice job of it. An NFB cap gives you speaker safety, a more durable amplifier, a longer lasting amplifier, and MUCH MORE FUN DYNAMICS.

One aspect that nobody talks about is that NFB caps are much easier to apply and more successful for audio if using a design that is supposed to be AC coupled.
These all have generally higher resistor values for gain setting, such as:
(feedback resistor / feedback shunt resistor with NFB cap)
68k/2.7k with 220uF (general purpose)
100k/3.3k with 100uF//22uF or 150uF (super clean treble)
100k/2.7k with 220uF (high gain)
56k/2.2k with 220uF//22uF (television amplifier)
47k/2.2k with 220uF//22uF
47k/2.7k with 220uF (low gain)
47k/1.5k with 330uF//22uF
*in the above cases you can use larger cap values as it only matters that they are big enough and also clear. You can also use parallel capacitors, such as big electrolytic with small electrolytic, or big electrolytic with small polyester, or any combination thereof as long as it is big enough and also clear.

Selecting an NFB cap or Input cap:
Capacitor quality varies regardless of price, but the solution isn't difficult.
Take five likely candidates of the correct value, try all five and keep the nicest. Also attempt adding very small value (10nF?) polyester for more entertaining treble. I'm not so sure about just five minutes with that method but it is quick inexpensive and higher fidelity. Price doesn't equate to performance with capacitors. Enjoy your powerful clean dynamics.

Credit to AndrewT for fixing MY amplifier by up-sizing my NFB cap until the amplifier is capable of clean extra low bass. That was a great learning experience. Thank you.

Last edited by danielwritesbac; 22nd December 2011 at 05:22 AM.
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Old 22nd December 2011, 10:38 AM   #17
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The resistor values for using with an output cap or a 3rd party speaker protector are:
27k feedback, 820R feedback-shunt, 10k input load, 470R stopper, 4.7uF//10nF input cap, 0.22R output ballast.
Check for DC offset individually. That version is neither as durable nor as dynamic as amplifiers with an NFB cap. I'm not using that version.
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Old 22nd December 2011, 11:21 AM   #18
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Default Photo of the rails

Compare to the photo in post #1
This is a minimal parts and minimal effort way to put rails on a miniature parallel amplifier. A circuit board alternative to this would be to simply put the power caps midpoint between the two chips.

Possible values for this location:
100nF or 47nF for the small caps.
220uF or 330uF or 470uF for the 35v large caps.
330uF is a standard value for this chip--larger is more laid back, smaller is brighter.
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Old 22nd December 2011, 12:40 PM   #19
Einric is offline Einric  United States
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That solution looks great.
It is a little hard to see what goes where though.
A little more info would be greatly appreciated.

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Old 22nd December 2011, 12:55 PM   #20
tweakk is offline tweakk  United States
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Thanks for the reply Daniel. The transformers are actually for your LM3886 kit that I plan on building soon. I've built your 1875 amp and thought the parallel implimentation would be something that would use your PCBs.

It's interesting that you mention using an array of smaller caps for the power-supply. I was trying to figure out a way to do that with my 1875 amp yesterday!

Got a good link for building your LM3886 kit?
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