8PAKAMP & 12PAKAMP KIT BUILD THREAD

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Joe Malone
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Re: 8PAKAMP KIT BUILD THREAD

Postby Joe Malone » Sat Apr 14, 2012 8:58 am

Wow that layout worked out very well. Great huge heatsink that allows the amp IC's to run along the middle.

Also good reuse of a old case and I always like a bit of hammer tone paint :-). Where did you get the round transformer cover from?
Joe :-)
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wizardofoz
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Re: 8PAKAMP KIT BUILD THREAD

Postby wizardofoz » Sun Apr 15, 2012 8:10 pm

Transformer cover is JS standard 6" PVC end cap (Made in Japan) ~A$20 - the JS standard is for high pressure use and is much thicker walled so was able to drill and tap 3mm threaded holes to attach it to to the cover. Needed to be bigger than normal as the top cover slides from front to back and needed the extra size to clear the toroidal. you can get metal covers on the web ... just google for round transformer covers.

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Re: 8PAKAMP KIT BUILD THREAD

Postby wizardofoz » Wed May 09, 2012 2:50 pm

Joe... just a question with respect to the single channel operations of this and the 60watt board too I guess

Is this considered a fully balanced design? as the output is to ground....or would one need 2 channels bridged to effectively claim a fully balanced config?

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Re: 8PAKAMP KIT BUILD THREAD

Postby Joe Malone » Wed May 09, 2012 2:58 pm

wizardofoz wrote:Joe... just a question with respect to the single channel operations of this and the 60watt board too I guess

Is this considered a fully balanced design? as the output is to ground....or would one need 2 channels bridged to effectively claim a fully balanced config?


The normal single amp circuit on the 8 channel or single amp PCB is fully balanced input but drives the speaker unbalanced with 0v ground return.

When running a pair of amps are run out of phase in bridge mode then the amp is fully balanced in and out.
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Re: 8PAKAMP KIT BUILD THREAD

Postby wizardofoz » Wed May 09, 2012 3:29 pm

Thanks Joe... that's what I assumed it would be.

taniknight
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Re: 8PAKAMP KIT BUILD THREAD

Postby taniknight » Tue Jul 17, 2012 9:27 pm

Hi Joe,

I'm looking to build a multi channel amp for a small cafe installation and just stumbled upon this kit. It will just be background music so all I need is low(ish) levels of clear sound.

I wanted to check the LM3886's ability to drive longish cable runs... Do you think that a single channel would be happy driving an 8ohm load at the end of 15-20 metres of 14AWG cable? This is how far the furthest of two outside speakers would be. I'm trying to avoid 70/100v line PA if possible.

I will have only 6 speakers to power in total, though the stereo signal will be mixed to mono first, so could I run the two 8ohm outdoor speakers in parallel from a bridged pair of channels?

I'm keen on this kit as I have all of the other necessary items after gutting a previous custom LM amp: 300VA 18-0-18 toroidal, 35amp bridge, heatsink, chassis with input/output/IEC connectors.

Many thanks,
Tani.

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Re: 8PAKAMP KIT BUILD THREAD

Postby Joe Malone » Wed Jul 18, 2012 10:45 am

taniknight wrote:Hi Joe,

I'm looking to build a multi channel amp for a small cafe installation and just stumbled upon this kit. It will just be background music so all I need is low(ish) levels of clear sound.

I wanted to check the LM3886's ability to drive longish cable runs... Do you think that a single channel would be happy driving an 8ohm load at the end of 15-20 metres of 14AWG cable? This is how far the furthest of two outside speakers would be. I'm trying to avoid 70/100v line PA if possible.

Yes will work fine. We have just used this PCB in a new project we did for Dolby Labs R&D where it runs a 35 metre run easy. 14AWG is fine as well as it be under 0.5ohm (for the total 2 x 20m = 40m of wire)

I will have only 6 speakers to power in total, though the stereo signal will be mixed to mono first, so could I run the two 8ohm outdoor speakers in parallel from a bridged pair of channels?

A single channel will run the two speakers as a parallel 4ohm load fine. When bridging amps the load should be a minimum of 8ohm unless you drop the voltage rails lower but at this low levels it would also work fine but is probably not needed as a single amp driving 2 speakers should be fine level wise for background music. I do prefer each speaker to have it own amp so all speakers can be level adjusted for best balance easily over the different areas.

I'm keen on this kit as I have all of the other necessary items after gutting a previous custom LM amp: 300VA 18-0-18 toroidal, 35amp bridge, heatsink, chassis with input/output/IEC connectors.

Perfect you would only need the 8pakamp kit to complete :-)
Joe :-)

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Re: 8PAKAMP KIT BUILD THREAD

Postby taniknight » Thu Jul 26, 2012 1:31 pm

Joe, thanks for the reply. This looks like the kit for the job... I'd make an 8 channel with 2 unused channels for future expansion of the system.

As mentioned, I'll be summing the stereo input channels to mono and driving all amps with the same signal. This will be done passively with a stereo-to-mono cable so the amp will be wired for stereo (more future proofing). The cable insertion loss shouldn't be a problem at the levels required here and it's driven by an iPod headphone output, so there should be plenty of drive for the resistive summing.

I'll need separate level control for 4 zones with 2 channels in each zone, ie. dual gang pots on each pair. It would be useful, as you mentioned, to also have trim control of all amps independently to adjust balance within each zone during setup. Can I achieve this passively without channel interaction, as I'd like to avoid the complexity of active buffers etc.? Paralleled pots (see attached) are no good, right?

Thanks,
Tani.
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Re: 8PAKAMP KIT BUILD THREAD

Postby Joe Malone » Tue Jul 31, 2012 7:16 pm

taniknight wrote:Joe, thanks for the reply. This looks like the kit for the job... I'd make an 8 channel with 2 unused channels for future expansion of the system.

OK perfect

As mentioned, I'll be summing the stereo input channels to mono and driving all amps with the same signal. This will be done passively with a stereo-to-mono cable so the amp will be wired for stereo (more future proofing). The cable insertion loss shouldn't be a problem at the levels required here and it's driven by an iPod headphone output, so there should be plenty of drive for the resistive summing.
I'll need separate level control for 4 zones with 2 channels in each zone, ie. dual gang pots on each pair. It would be useful, as you mentioned, to also have trim control of all amps independently to adjust balance within each zone during setup. Can I achieve this passively without channel interaction, as I'd like to avoid the complexity of active buffers etc.? Paralleled pots (see attached) are no good, right?

Diagram you have drawn will work fine. See mod to your drawing below on how to add easy mono switch with 2 x 1k resistors if using 10k log pots. With a ipod the 2 resistors could be 100R and pots 1k log.
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taniknight
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Re: 8PAKAMP KIT BUILD THREAD

Postby taniknight » Wed Aug 01, 2012 8:05 pm

Joe, many thanks. It's true what I read in the reviews.... you are very helpful and obliging, particularly towards someone who hasn't yet purchased a product of yours. I'm sold now!

I was sure that I'd have to buffer the pots; so much so that I thought it was silly to post the question with my attachment. Proves: it always pays to ask.

I have just placed an order, along with some VU buffers and an AmpHead (for an ongoing DJ mixer project...). I look forward to the cafe sounding fine :D

Cheers,
Tani.

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Re: 8PAKAMP KIT BUILD THREAD

Postby Joe Malone » Thu Aug 02, 2012 7:08 pm

taniknight wrote:Joe, many thanks. It's true what I read in the reviews.... you are very helpful and obliging, particularly towards someone who hasn't yet purchased a product of yours. I'm sold now!

I was sure that I'd have to buffer the pots; so much so that I thought it was silly to post the question with my attachment. Proves: it always pays to ask.

I have just placed an order, along with some VU buffers and an AmpHead (for an ongoing DJ mixer project...). I look forward to the cafe sounding fine :D

Cheers,
Tani.


Cool :D

Parcel shipped out today overnight so you should have it tomorrow.

Let me know if you have any questions while you building any of the kits
Joe :-)

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taniknight
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Re: 8PAKAMP KIT BUILD THREAD

Postby taniknight » Mon Aug 06, 2012 5:29 pm

Hi Joe,

So I've learnt a lot about passive networks over the last few days. A small change in the design brief for this cafe amp has made me rethink the input/volume section that will proceed the power amps. Basically, the amp now needs 2 stereo inputs (say iPod and Radio), independently selectable per zone and independent mono mixing per zone.

I used the falstad.com circuit simulator to help me come up with the following (the link will take you to a JAVA applet with my animated simulation). I've attached a screenshot of the circuit if you don't have time to play with the app... as shown there is redundant input sources/switching that were useful while experimenting:

http://tinyurl.com/rhc-4zone-input-network

The app lets you flick the switches and watch the effect on the scopes in realtime. As shown, the only issue I can find involves crosstalk between stereo channels in a zone, when all other zones on the same input are mixed to mono. This simulation shows me that the worst case stereo separation is ~20dB (1.75V of signal with 182mV of unwanted signal injected) when all 4 zones are on the same input bus and 3 of them are mixed to mono.

Switching 3 zones on or off a bus only yields a very small change in the remaining zone (50mV worst case), given a 100R source impedance. Have I modelled the source correctly... R in series with an ideal voltage source? Given it is likely that all kinds of interesting sources will be connected to this amp in its life, should I use an input line receiver to present a constant source impedance to the resistor network? If so, what design will happily receive signals from headphone outputs as well as line outs?

Any advice would be most appreciated before I buy components.

Regards,
Tani.
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Re: 8PAKAMP KIT BUILD THREAD

Postby Joe Malone » Tue Aug 07, 2012 12:20 pm

All looks fine. I think the line input will need to be attenuated to match the ipod which can be done by a 2 series resistors to match the ipod level and to keep the output resistance the same make the bottom resistor to ground 0v 100R and vary the top resistor value to get the same level as the ipod. Or use a stereo 1k log pot to drop the line level if it is louder than the ipod.

To get less crosstalk . Do the mono switch at the wipers of the level pots instead of before the level pots. As they would rarely be at full volume setting.
Joe :-)

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taniknight
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Re: 8PAKAMP KIT BUILD THREAD

Postby taniknight » Tue Aug 07, 2012 12:28 pm

Thanks for the tips. I'll get to it!

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4 Zone Amp Build Commenced - Layout

Postby taniknight » Wed Aug 08, 2012 2:29 pm

Hi all,

I thought I'd post my progress on the 8 channel, 4 zone amp. The input/volume network has been laid out, though I forgot to order the 4 SPST switches for the mono mixing :x

I was going to mount these switches under the volume and source selection on the front of the chassis... though I could implement it with jumpers internally as there is little reason for regular switching between mono and stereo once the system is set up.

The labelling on the chassis is from the original application and needs changing. I have access to a friends vinyl cutter, which should do the trick.

IMG_0976.jpg

IMG_0978.jpg

IMG_0982.jpg

Volume pots and input selectors (ON-OFF-ON type DPDT for SOURCE1-MUTE-SOURCE2) for each of the 4 zones.

Utilising the holes that were already in place means that the control layout is not ideal... but neither is mangling the chassis with limited metal working tools :? Clear labelling will remove any ambiguity.

Internal layout to follow.

Tani.
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