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Old 24th August 2011, 01:36 AM   #1
lnh is offline lnh  United States
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Default First chip amp, please look over my ideas

So, I'm getting ready to start my first chipamp, and well, it's gonna kinda be crazy in the end lol. The current plan is the following:
2 Channel LM1875, 2 chips per channel
Mono LM1875 with 4 chips
NE5532 headphone/preamp (here)
TL072-based volume control (here)
Signal summing for the mono input (haven't figured this part out yet)

The NE5532 will also function as a headphone amp. I'll be using a 3/4PST switch to do this:
Position 1-NE outputs linked to LM inputs, power on to all LM boards
Position 2-NE outputs linked to headphone jack, power off to all LM boards



Now, I chose these boards for two reasons: I can use the regulated 12v supply I already have for now, and then upgrade to a nice 24v one later, and it is a nice tiered setup that allows me to easily work in pieces for minimal cost. I'm starting with the NE and volume control first...total cost is less than $20 to start

The volume control and headphone/preamp looks like they will still work on 12v instead of the specified 15v without any issues, too. Then once I build a better power supply, I can just add an extra regulator to give 24v and 15v.

All of this will go in a single case...1 master power switch, 1 volume control, 1 power cord. 2.1 channels with headphone out. The case will be aluminum and copper...not sure yet how it will be constructed, but that's something to worry about as I move forward. Right now I'm thinking about 16"Wx12"Dx4"T, 1/4" aluminum machined case with copper faceplate. There will be a fan or two in the case to help with cooling since this will be sitting on my desk, and I don't feel like burning myself lol.

So that's what I have so far! Biggest question is how to sum the rca input signal from stereo to mono before the sub amp...got a while before I build that one anyways. The idea is the headphone/preamp w/volume control first, then the 2 channel, then the summing input and sub amp. Ideally I'll add an adjustable LP filter to the sub amp input too, but that's waaayyyyy in the future lol. This is probably going to be a 3-6 month project for me just because I can.


Any input would be greatly appreciated
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Old 24th August 2011, 10:59 PM   #2
lnh is offline lnh  United States
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Nobody has any input?

I've priced out the parts for the headphone amp and volume control and will probably order them next week. I spec'd 5% tolerance 1w resistors, and 20% tolerance 20v or higher caps. All in all will cost ~$20 for the parts.


The 'tiers' I referenced to in the previous post are layed out below in order that I would build/add them on:

Stage 1:
Tl072-based volume control
NE5532-based headphone amp
3.5mm input jack
3.5mm (headphone) output jack
I'll be using 5% tolerance, 1w resistors and 20% tolerance caps



Stage 2:
add on an rca input with source selector switch

add on a 2 channel amplifier utilizing 2 LM1875's per channel, fed by P113 as a preamp
add in a 2 position switch (output selector) that functions as follows:
-Position 1: NE outputs linked to LM inputs, power on to all LM boards (NE outputs paralleled after the switch to the different channels)
-Position 2: NE outputs linked to headphone jack, power off to all LM boards (so no signal or power to LM's to avoid and possible noise introduction)



Stage 3:
add on a mono amplifier utilizing 4 LM1875's, also fed by NE as a preamp
hook into the output selector to function the same as above (just adding one power wire to the switch for the mono amp and paralleling the NE outputs to the new third channel after the switch)


Stage 4:
add in an adjustable HP filter after the output switch, before the L and R channel boards
add in an adjustable LP filter after the output switch, before the mono channel board
most likely add in an adjustable subsonic filter before the mono board, too
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Old 25th August 2011, 01:56 AM   #3
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The plan you've presented is a little short on details. How were you planning to implement a mono amp with 4 LM1875s? If you could post a schematic it would be helpful.

Mike
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Old 25th August 2011, 02:38 AM   #4
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I'm working on drawing up schematics now, for both the 2 channel and the mono amp first. Eventually I will have full schematics drawn up for everything. I'm new to schematic programs, but have lots of experience with CAD, so it should be a probably a day (maybe later tonight) before I post the first one.
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Old 25th August 2011, 06:48 AM   #5
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Here is the dual 1875 schematic...just one channel. This is my first schematic, made in TinyCad, so I hope it's not too difficult to follow. I don't like how big the symbols are in comparison to the text, but I didn't feel like editing them just for the first time out.

The schematic is based off of Rod Elliot's. I originally made two versions...the first was just a straight copy with a second chip added in parallel and all the components after R401 doubled and then paired back to the power inputs and outputs. After I drew this up, it occurred to me that while the caps will be fine paralleled together, I've just halved the total resistance for each pair of resistors in parallel, and that also the caps are redundant. I took all the resistors that were in parallel, doubled the wattage rating on them and kept one from the pair; then took all the caps in parallel, doubled the capacitance and kept just one from the pair. From a basic electronics standpoint this makes perfect sense, but from a functional standpoint, I'm not sure how the system will react and whether or not I'll end up with oscillation, other noise or gain issues.

The only component that is doubled is the 2k resistor right before the input (pin 1) for each IC. They are not paralleled and I didn't want to directly connect the inputs of each IC to each other. The other thing that I'm unsure on is that the outputs are directly connected to each other...should I add a small resistor network to each IC output before they are connected to prevent forced feedback from one IC to the other? I would use the same 3x0.5ohm resistor network on each channel like this. This only adds 1/3Ohm internal resistance on the output channels, which to me is relatively negligible.


The quad chip 1875 board would be the same as this one, except I would modify the wattage and capacitance values to account for 4 chips in parallel.
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Old 25th August 2011, 05:56 PM   #6
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Changed the parts list to use 1%, 1/4W resistors for the volume control and preamp.
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Old 25th August 2011, 11:48 PM   #7
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Found an error in the schematic, C409 should be a polarized cap with the positive side towards ground.


Going to be posting up more schematics most likely later tonight. Mainly source/output switching and a rough power supply schematic.
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Old 26th August 2011, 12:17 AM   #8
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That design won't work. The amps need thier own independant feedback networks, and there needs to be current sharing resistors between the outputs. What you have designed will likely be an oscillator. Take a look at this -

http://www.national.com/an/AN/AN-1192.pdf

Same principles apply to LM1875.

Mike
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Old 26th August 2011, 02:02 AM   #9
lnh is offline lnh  United States
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That's exactly why I'm posting here lol. Thank you!


Would this work then?

Added 2x0.2Ohm resistors to each IC output. If this one will work, do I need both C403/404 and R404/405, since they are connected in the ground path after the feedback loop, or is this still part of the feedback network?
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Old 27th August 2011, 11:25 PM   #10
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Yeah, that will work. You do need to keep those components in there to keep the feedback networks independant. For least output voltage offset and lowest power loss in output current sharing resistors (R409 thru R412), input and feedback resistors (R403 thru R408) should be selected for .1% tolerance. Alternately, you can match to within .1% the coresponding resistors from one amp circuit to the other; R403 with R406, R404 with R405, and R407 with R408, that way the exact value won't matter as much.

Mike
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