8PAKAMP & 12PAKAMP KIT BUILD THREAD

[Joe Malone]

Hey there,

I am really happy with my 12PAKAMP, powering a pair of LXmini+. The woofers and subs are powered by a pair of bridged amp channels each and the tweeters by a single channel each. It is powered by an 18 V transformer.
The amp is running quite a while now, but I finally put it into a nice case, so I can share some pictures.

Niklas

Wow Niklas. That is a great looking neat build Thanks for taking the time to load up the photos.

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[rrp]

Hi Joe!

I purchased a couple 8pak kits about a year ago, and I’m just now getting around to putting them together. I’m building one kit as 4 bridged amps, and the other as 8 individual amps.

I’d like there to be external volume pots that attenuate from 0 to -infinity at the input of the amp. In the LM3886 datasheet, the example schematic shows an unbalanced 10k pot to ground at the input. Since the 8paks are balanced at the input with a resistor network (I think), I’m wondering how to implement this.

Is the input impedance of the amps high enough that I could hang a balanced attenuator in front of the input? Something like the attached image. Or, would I need to do some kind buffer? If a buffer would be required, maybe I would just get a balanced line receiver like INA134, then use the 10kpot, 1k input resistor combination used in the datasheet. I feel like that might be overkill though.

Any recommendations? Also, would it be possible to get a schematic for the 8pak circuit?

Apologies if this is answered earlier in the thread. I read through it and didn’t quite find what I was looking for.

Thanks!
Raphael


Pad idea:

InputPad.png

Schematic from the LM3886 datasheet that I’m referring to:

LM3886DatasheetSchem.png

[Joe Malone]

Hi Joe!

I purchased a couple 8pak kits about a year ago, and I’m just now getting around to putting them together. I’m building one kit as 4 bridged amps, and the other as 8 individual amps.

I’d like there to be external volume pots that attenuate from 0 to -infinity at the input of the amp. In the LM3886 datasheet, the example schematic shows an unbalanced 10k pot to ground at the input. Since the 8paks are balanced at the input with a resistor network (I think), I’m wondering how to implement this.

Is the input impedance of the amps high enough that I could hang a balanced attenuator in front of the input? Something like the attached image. Or, would I need to do some kind buffer? If a buffer would be required, maybe I would just get a balanced line receiver like INA134, then use the 10kpot, 1k input resistor combination used in the datasheet. I feel like that might be overkill though.

Any recommendations? Also, would it be possible to get a schematic for the 8pak circuit?

Apologies if this is answered earlier in the thread. I read through it and didn’t quite find what I was looking for.

Thanks!
Raphael


Pad idea:

InputPad.png

Schematic from the LM3886 datasheet that I’m referring to:

LM3886DatasheetSchem.png

Our schematic is very different to the standard which is why the LM3886 takes a balanced signal in. But the Inputpad idea works fine and it exactly how the input trimpot circuit works on the 8PAKAMP pcb. You can connect to the outside trimpot legs and parallel in and out of phase channels at this point and replace the trimpot with a volume pot.

[rrp]

Got it. Thank you!

[tpetmed]

Received 12PAKAMP Kits today! Nicely packed and in great condition. Now for the assembly...I need some advice.

Purpose: Home built speakers. I don't have any crossover building knowledge so I use MiniDSP 2x8HD. For this I need multiple amp channels.
Goal: Configure PAKAMP to 6 channels in Bridge mode to drive speakers with 8ohm impedance on each channel.
Request: Schematic/Instruction/Picture or any other information on how to accomplish this. I have reviewed all TI LM3886 papers for typical/bridged and parallel modes but not sure I am seeing what to do. Please dumb it down, I am a general surgeon that likes to tinker with electronics.

Thank You to Joe and others that have been posting on this forum.

[Joe Malone]

Received 12PAKAMP Kits today! Nicely packed and in great condition. Now for the assembly...I need some advice.

Purpose: Home built speakers. I don't have any crossover building knowledge so I use MiniDSP 2x8HD. For this I need multiple amp channels.
Goal: Configure PAKAMP to 6 channels in Bridge mode to drive speakers with 8ohm impedance on each channel.
Request: Schematic/Instruction/Picture or any other information on how to accomplish this. I have reviewed all TI LM3886 papers for typical/bridged and parallel modes but not sure I am seeing what to do. Please dumb it down, I am a general surgeon that likes to tinker with electronics.

Thank You to Joe and others that have been posting on this forum.

Below shows the amp configured for 6 bridge channels. Feel free to ask any questions you have while building here.

If using the level control in the minDSP. Fit 5k1 or 4k7 resistors instead of the 5k trimpots
or turn all 5k trimpots 14 turns fully clockwise for full level so you have 6 bridged identical gain amps.

[tpetmed]

Starting to populate the first board. I have noticed that there are two parts that I need to acquire, ZD1 and R16. Both are labeled FAN and are adjacent to the 3 pin header labeled preamp header. Could you please provide guidance on what parts to get.

I have selected a 600Kva transformer with two 18v secondaries. The plan is to wire the secondaries in series and use the center junction as the 0v based on the diagram you previously sent.

Also pictured are the rectifier and soft start board. I intend to still use the 1000uF caps on the board since they were included.

Lastly, the heatsink will be comprised of 3 of the pictured CPU coolers mounted on 1/2" x 5" aluminum flat bar that I am milling to create a support structure. I haven't settled on what the remainder of the case will look like.

Please let me know if you see any problems with my plans. Thanks.

[Joe Malone]

Starting to populate the first board. I have noticed that there are two parts that I need to acquire, ZD1 and R16. Both are labeled FAN and are adjacent to the 3 pin header labeled preamp header. Could you please provide guidance on what parts to get.

These parts are not normally used. If the LM317 and LM337 are used for +/-V for a preamp no fan parts are fitted. If no preamp +/-V are needed then one of the regs can be used to run fans. The Fan parts allow the fans to be adjusted to lower volts for super quiet operation but if the amps are run loud the voltage is adjusted up to fan max voltage to give extra cooling. As soon as the audio level drops the fan are dropped to lower voltage so make no noise again. So when you have fans adjust them to the lowest speed they start at reliably and let me know there full voltage eg 12v so I can advice Fan parts. Basically for 12vdc fan you should be able to get reliable fan start at 6vdc to 7vdc and ZD will be 12V 1W and R16 820R.

I have selected a 600Kva transformer with two 18v secondaries. The plan is to wire the secondaries in series and use the center junction as the 0v based on the diagram you previously sent.

Yes 600VA with 2 x 18vac centre tap making 36vac centre tap is perfect.

Also pictured are the rectifier and soft start board. I intend to still use the 1000uF caps on the board since they were included.

Ok cool. The local pcb 24 x 1000uF improve the stability of the amp so are still needed.

Lastly, the heatsink will be comprised of 3 of the pictured CPU coolers mounted on 1/2" x 5" aluminum flat bar that I am milling to create a support structure. I haven't settled on what the remainder of the case will look like.

ok 3 fans just running at a set low speed should be heaps of cooling if fans are needed at all. But the block will have to be setup well to get the heat to the cpu heatsinks.

[tpetmed]

I finally have completed the amp and all channels seem to be working properly. Sounds great! Noise floor seems to be completely silent. So far only running low power due to heat issue and the fact that current speakers are 4Ohm. My plan was to run 8Ohm speakers with this configuration. I am surprised how hot it gets even at idle and will need to change the heatsinking unless there is a problem. How much power does each IC dissipate without any input? The current heatsink is aluminum plate 0.5in thick, 6in wide and 14in long. There are two large server CPU coolers attached. The ICs are secured with aluminum barstock milled to accommodate the thinner section of the IC. Everything is attached with thermal paste and heat transfer seems to be good as the entire assembly is hot to touch. I am in the process of acquiring a non-contact temp gun but estimate the assembly to be 35-40 C at idle and 45-50 C running 4 channels into 4Ohm load. Is it possible to have something wired incorrectly and create a lot of heat without any audible signs? Having a lot of fun with this project and appreciate your feedback.

[Joe Malone]

I finally have completed the amp and all channels seem to be working properly. Sounds great! Noise floor seems to be completely silent. So far only running low power due to heat issue and the fact that current speakers are 4Ohm. My plan was to run 8Ohm speakers with this configuration. I am surprised how hot it gets even at idle and will need to change the heatsinking unless there is a problem. How much power does each IC dissipate without any input? The current heatsink is aluminum plate 0.5in thick, 6in wide and 14in long. There are two large server CPU coolers attached. The ICs are secured with aluminum barstock milled to accommodate the thinner section of the IC. Everything is attached with thermal paste and heat transfer seems to be good as the entire assembly is hot to touch. I am in the process of acquiring a non-contact temp gun but estimate the assembly to be 35-40 C at idle and 45-50 C running 4 channels into 4Ohm load. Is it possible to have something wired incorrectly and create a lot of heat without any audible signs? Having a lot of fun with this project and appreciate your feedback.

The amps each idle at 50mA current draw. So roughly 600mA x 50v = 30W of heat without any audio. I actually find that sets the main heat and with normal audio the amps do not get very much hotter. If a amp oscillates it will make a lot more heat so definitely check all amp outputs with scope to make sure they are not oscillating. The CPU radiators are to far away to thermally help. The Aluminium plate has good thermal spread but not a lot of surface area like a normal heatsink close to the amps to get rid of the heat to air quick enough.

[tpetmed]

Thanks Joe. I put scope on each output. All looks fine. Revamped/added heatsinking and added small fan. Working great now.

Next project with remaining kit is to build a 2 channel amp if possible. Can you show me how to configure the Pakamp 12 for two channels please? I am guessing this is a bridged/parallel configuration. I would like to use this to power dual subwoofers. Thank you for all your help so far.

[Joe Malone]

Thanks Joe. I put scope on each output. All looks fine. Revamped/added heatsinking and added small fan. Working great now.

Next project with remaining kit is to build a 2 channel amp if possible. Can you show me how to configure the Pakamp 12 for two channels please? I am guessing this is a bridged/parallel configuration. I would like to use this to power dual subwoofers. Thank you for all your help so far.

See attached on 8PAKAMP pcb same as 12PAKAMP pcb except 3 amps will be paralleled. Parallel same phase amp outputs with 0.22R or 0.33R resistors after setting them to same output level. So they current shared. Use +/-35v power rails

[tpetmed]

OK. Just a bit of clarification needed please. What wattage do I need for ballast resistors? Would 5w 1% work? Also, I previously used 4k7 ohm input resistors to set gain instead of 5k trim pot. It sounds like I still need to do that then connect the chip inputs as you have drawn to parallel three devices in positive and 3 devices negative for each channel. If I am reading this drawing correctly, this would be unbalanced input? Can a balanced input option be done using the 40 pin connector? Thank you.