JLM 60Watt Power AMP Kit

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Joekkel
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Re: JLM 60Watt Power AMP Kit

Postby Joekkel » Sun Oct 09, 2016 8:00 pm

Hi there,

i've finally managed to build a new stereo amp with the amp kits. it has 3 stereo-cinch inputs, selectable via switch. the volume-poti is an alps 10k stereo poti. Attached you'll find a picture of the build.

So far it tested flawlessly and i have sound.

I have two problems: The output is rather low. If i turn up the volume-knob fully clockwise (connected to my mp3-player) i get a decent level, but nothing really loud. I know i loose 6dB with the unballanced inputs, shorting -in to 0v. Which resistor (i guess the 1k one) can i lower, to get more gain? How much can i lower it, without risking self-oscillation of the amp?

Second problem is, as i allready feared, hum. it's not much by now, but it's going be more, if i reduce the 1k-resistor to get more gain. The input-pair closest to the trafo has the loudest hum. do i need to redo the whole wiring as in this tip on the 8Pakamp-hread: "You can shield the RCA input wiring by using balanced cable but only connect 0v ground at 8PAKAMP PCB but not to RCA." (This would mean no bridging -in to 0v). Or should i buy a metal-shield like this: https://www.don-audio.com/trafo-abschir ... chirmblech ?

Thx a lot!
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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Mon Oct 10, 2016 1:25 pm

Joekkel wrote:Hi there,

i've finally managed to build a new stereo amp with the amp kits. it has 3 stereo-cinch inputs, selectable via switch. the volume-poti is an alps 10k stereo poti. Attached you'll find a picture of the build.

So far it tested flawlessly and i have sound.

I have two problems: The output is rather low. If i turn up the volume-knob fully clockwise (connected to my mp3-player) i get a decent level, but nothing really loud. I know i loose 6dB with the unballanced inputs, shorting -in to 0v. Which resistor (i guess the 1k one) can i lower, to get more gain? How much can i lower it, without risking self-oscillation of the amp?

Second problem is, as i allready feared, hum. it's not much by now, but it's going be more, if i reduce the 1k-resistor to get more gain. The input-pair closest to the trafo has the loudest hum. do i need to redo the whole wiring as in this tip on the 8Pakamp-hread: "You can shield the RCA input wiring by using balanced cable but only connect 0v ground at 8PAKAMP PCB but not to RCA." (This would mean no bridging -in to 0v). Or should i buy a metal-shield like this: https://www.don-audio.com/trafo-abschir ... chirmblech ?

Thx a lot!


Ok level with a couple of simple changes will be easy to increase without the need to change the 1k gain resistor since all your inputs are unbalanced.

But the first thing is the hum which is obviously due to the power transformer in the middle of all the low level audio wiring. Good wire twisting is not much different to shielded cable but shielded cable can help a bit.

The wiring from the power transformer must follow the diagram on the side of the transformer. So the 2 wires in the centre of the diagram are 0v ground. One will usually be marked with a voltage like 18 to 24vac and the other 0v. So going to the 0v and ground will be one 0v and one marked 18vac or 24vac. This is due to the need for the 2 winding to be working out of phase. So always remember wires shown in the diagram centre go to 0v and the 2 outside ones go to the AC in the bridge rectifier. This will give you 100Hz refresh on the cap bank and minimize any hum directly on the power rails. If you wire the transformer following its 0v markings you will only have 50Hz refresh and the power rails will have higher ripple and power rail hum. This the main mistake I see from beginners.

The highest flux hum radiated point on a toroidal is from the point the wires come out of the transformer. So often mounting these away from any audio wiring will help or mounting the toroid upside down so these wires are against the metal case can help. But layout is very important. Keep all mains wiring away from the audio wiring. A Mu shield or 2 or 3mm thick steel U angle separator in the metal case forming 2 separate boxes in the case will greatly drop hum.

Moving the power transformer tighter over to the mains power corner and making a L corner steel bracket to box that corner in with no gaps or use a mu shield would be a good start. And then take all the low level wiring around the other side of the case against the side wall of the case would be the next thing.

I will draw a diagram up on how to get the extra gain.
Joe :-)
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Joekkel
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Re: JLM 60Watt Power AMP Kit

Postby Joekkel » Wed Oct 12, 2016 4:20 pm

Hi Joe,

thanks! I will move the trafo to the left side and both ampcards to the right. And wire the trafo correctly...thanks for explaining why wiring it like this: phase. if i know why, i can remember it. looking forward to the diagram.

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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Tue Oct 18, 2016 10:09 am

Joekkel wrote:Hi Joe,

thanks! I will move the trafo to the left side and both ampcards to the right. And wire the trafo correctly...thanks for explaining why wiring it like this: phase. if i know why, i can remember it. looking forward to the diagram.


Higher gain mod image below
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Joekkel
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Re: JLM 60Watt Power AMP Kit

Postby Joekkel » Tue Oct 18, 2016 7:51 pm

Thanks! But what do i connect the rca -in (sleeve) to? Directly to the starground?

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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Tue Oct 18, 2016 9:18 pm

Joekkel wrote:Thanks! But what do i connect the rca -in (sleeve) to? Directly to the starground?


0v on amp pcb like bottom leg of volume pot
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rtchau
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Re: JLM 60Watt Power AMP Kit

Postby rtchau » Mon Nov 14, 2016 11:35 am

Just finished a stereo build - the resistor delete for the gain worked a treat and it's running quite happily with unbalanced input.

The only problem I had was with my little tube preamp - I think it caused a super nasty ground loop as there was a nice low hum with one channel muted, and then both channels unmuted kicked off some very nasty ~2hz motorboating (I could literally watch the driver go to full excursion in each direction - yes, it is a throw-away test speaker!). The preamp runs off a little AC power brick with no ground, and taking it out of the equation (and being judicious about the volume level on my PC) has sorted it out. Amp is dead silent, and power is coming via an umbilical from an external supply with a 225va toroid and a single 35A bridge rectifier (I did that so I could get it tested/tagged for insurance reasons, and keep mucking around with the amp internals). Using 3-pin DIN plugs for the power connectors.

I'll probably just use the volume control capability on the PCBs and just make it an integrated amp (albeit with only one input), but if I were compelled to use the pre-amp (and changing its circuitry isn't an option), would a ground loop isolator on each of the RCA inputs solve the hum/motorboating?

Thanks again for a top notch kit! I might have to get a third one to drive a sub in the near future!

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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Tue Nov 15, 2016 9:56 am

rtchau wrote:Just finished a stereo build - the resistor delete for the gain worked a treat and it's running quite happily with unbalanced input.

The only problem I had was with my little tube preamp - I think it caused a super nasty ground loop as there was a nice low hum with one channel muted, and then both channels unmuted kicked off some very nasty ~2hz motorboating (I could literally watch the driver go to full excursion in each direction - yes, it is a throw-away test speaker!). The preamp runs off a little AC power brick with no ground, and taking it out of the equation (and being judicious about the volume level on my PC) has sorted it out. Amp is dead silent, and power is coming via an umbilical from an external supply with a 225va toroid and a single 35A bridge rectifier (I did that so I could get it tested/tagged for insurance reasons, and keep mucking around with the amp internals). Using 3-pin DIN plugs for the power connectors.

I'll probably just use the volume control capability on the PCBs and just make it an integrated amp (albeit with only one input), but if I were compelled to use the pre-amp (and changing its circuitry isn't an option), would a ground loop isolator on each of the RCA inputs solve the hum/motorboating?

Thanks again for a top notch kit! I might have to get a third one to drive a sub in the near future!


Cool. If using a computer as sound source it is usually hard mains grounded so best to float the power amp 0v with 10R 1W to stop any ground current to chassis causing hum loop. The motorboating can from running the amp at less than a gain of 10 as the LM3886 doesn't like this. But is stable if run balanced in with our normal circuit.
Joe :-)

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rtchau
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Re: JLM 60Watt Power AMP Kit

Postby rtchau » Fri Nov 18, 2016 2:34 pm

One last question (hopefully) - I've done the gain mod from the diagram above, however the 10k logarithmic pot only starts attenuating below about 20% of its range, e.g. amp is full tilt after turning the knob up about 20%, and then the rest of the travel just doesn't make a difference.

Is the 10k logarithmic the wrong pot for the unbalanced mod? I went off the original (balanced) diagram when choosing a dual gang pot.

Sorry if that's a dumb question, but this is my first semi-serious audio project.

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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Mon Nov 21, 2016 10:56 am

rtchau wrote:One last question (hopefully) - I've done the gain mod from the diagram above, however the 10k logarithmic pot only starts attenuating below about 20% of its range, e.g. amp is full tilt after turning the knob up about 20%, and then the rest of the travel just doesn't make a difference.

Is the 10k logarithmic the wrong pot for the unbalanced mod? I went off the original (balanced) diagram when choosing a dual gang pot.

Sorry if that's a dumb question, but this is my first semi-serious audio project.


Have you wired the unbalanced input and pot as shown in the image just up on this page?

No need to do the resistor mods if you have good gain at full level already.
Joe :-)

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gdayj
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Re: JLM 60Watt Power AMP Kit

Postby gdayj » Mon Nov 28, 2016 1:49 pm

Anyone had luck using a switch mode power supply with these?

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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Mon Nov 28, 2016 1:57 pm

gdayj wrote:Anyone had luck using a switch mode power supply with these?


We run them often with 2 x 24vdc 100W smps turned up to 27.5v each and wired as +/-27.5vdc. Works perfectly for a pair of amps.
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gdayj
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Re: JLM 60Watt Power AMP Kit

Postby gdayj » Mon Nov 28, 2016 2:33 pm

Thanks joe!

frazzman
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Re: JLM 60Watt Power AMP Kit

Postby frazzman » Sat Feb 04, 2017 6:05 pm

Hi all,

Just finished building a bridged amp, very nice and loud sounding unit!

I haven't had the chance to use it extensively but I did give it a go yesterday to drive a 4x12 guitar cab fairly hard.

I noticed that on one channel the LM3886 ran considerably hotter than the other. Both have the same sized heat sinks and voltages.

Cheers

James

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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Mon Feb 06, 2017 9:40 am

frazzman wrote:Hi all,

Just finished building a bridged amp, very nice and loud sounding unit!

I haven't had the chance to use it extensively but I did give it a go yesterday to drive a 4x12 guitar cab fairly hard.

I noticed that on one channel the LM3886 ran considerably hotter than the other. Both have the same sized heat sinks and voltages.

Cheers

James


There main heat is usually the chips bias idle current which can be from about 40mA to 85mA. So they can be different by a fair bit temp wise. Just make sure they both have thin amount of heat compound between the heatsink and the back of the LM3886 for good thermal coupling. Also check both amp outputs have no oscillation on them as any oscillation will heat up the LM3886 heaps.
Joe :-)

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