First Lm3886

[PJPro]

Originally posted by AndrewT
even more reason to build it up as a single channel prototype on an insulating flat panel.
Start with the transformer. Add a rectifier. Add some smoothing Add the SIMPLE amplifier. Add the input filtering. Add the mains filtering. Add the DC detect. Add the output relay. Add the .... etc.

Once you have one channel working, then start all over again on the second channel and prove it.

Now work out how to package that 3foot long assembly inside your box.
Only then do you spend the time to get the final look that you want.

Yes. Absolutely. And I'll be using my deluxe light bulb tester all the way .

[PJPro]

Originally posted by Greg Erskine
hi PJPro,

I'm not sure if it has been mentioned but always measure Vdc offset whenever you do a change to your amp. It should be below 100mV most of the time with chipamps. After that, connect a cheap test speaker and have a listen (and smell).

BTW: It is possible to polish a turd, I saw it on Myth Busters.

regards

Thanks Greg. Will do.

I agree that it is possible to polish a turd. But however shiny, it's still a turd.

[akis]

Regarding the earth discussion:

"a direct mechanical link from the supply earth wire to the metal chassis".

The above, which is what was recommended above is not near good enough in my opinion. I am not trying to be awkward. The above should be something like:

"a direct mechanical link from the supply earth wire to the metal chassis AND to all other conductive elements on the chassis which the user may touch or work with, eg shielded wires, cables, probes, your electric guitar's lead, the electric guitar's strings, bridge and buckles, your amp's metal knobs and so on".

The advice to keep the RCA plugs and other interconnects separated from the earth (to avoid ground loops) kind of breaks it for me, I can imagine plugging a guitar into your amp and getting electrocuted.

Regrding the slow start circuit, would you like to be a guinea pig ? I have designed one and amabout to build it, the PCB should be arriving soon, it looks like this (I am sure I have posted this before somewhere here). What it does is provides a 12 V on-off switch, so you do not run 240V wires inside the enclosure and onto the front panel (imagine spilling your pint of beer) and additionally a slow start by the means of two relays.

[akis]

By the way, remember that workshop you found near you to make you an aluminium panel? Do you think they'd be able to construct an alumunium enclosure for my guitar amp? I need an encosure to measure 85 cm across, 40 cm depth, and 7cm height - and such things are hard to find :-)

[PJPro]

Originally posted by akis
Regarding the earth discussion:
The advice to keep the RCA plugs and other interconnects separated from the earth (to avoid ground loops) kind of breaks it for me, I can imagine plugging a guitar into your amp and getting electrocuted.

I thought the network provided this....via the resistor and cap (when all is well) and via the bridge in times of trouble?

Originally posted by akis
Regrding the slow start circuit, would you like to be a guinea pig ? I have designed one and amabout to build it, the PCB should be arriving soon, it looks like this (I am sure I have posted this before somewhere here). What it does is provides a 12 V on-off switch, so you do not run 240V wires inside the enclosure and onto the front panel (imagine spilling your pint of beer) and additionally a slow start by the means of two relays.

If I hadn't already bought all the boards, etc, then I'd take you up on your offer. Moreover, I tend to take my time......

BTW, the E24 provides 12 VDC for the switch and allows use of momentary-contact switches (see here).

Incidentally, Headwise appears to be back online.

Thanks.

[PJPro]

The E24 also provides thermal auto-shutoff. See the quote below from AMB taken from the Headwise forum......

Originally posted by AMB on Headwise
I must confess that sometimes I am as guilty as anyone when it comes to creeping featurism, but having a relay switching the power provides another opportunity that would integrate well into this circuit without much additional complexity. Namely, amplifier overheat auto-shutoff. Many high-end and pro-grade power amps have this feature, and I thought it would be useful to have it as an option here. Using Stancor disc thermostats (i.e., thermal breakers) mounted on the main heatsinks, they could enable the amp to shut itself off if the temperature exceeds a certain threshold. Thus, I modified the circuit and worked this into the design. It needs the "normally-closed" thermal breakers (those with part numbers beginning with STO-). For my build, I chose the STO-170 which will open when the temperature reaches 170°F (77°C). The breaker will auto-reset when the temperature drops to 30°F (17°C) below the rated threshold.

Given the SPiKe protection incorporated into the LM3886, should I bother with this capability or is a belt and braces approach better?

If I do go ahead with the thermal auto-shutoff and, given the operating temperature range of the LM3886 being -20 to 85 degC, should I go for the STO-180 (which opens at 79 to 85 degC) or the STO-170 (which opens at 74 to 79 deg C). My view is to go with the STO-170. Why? Well I'm assuming I would mount the sensor on the heatsink (or chip clamping bar) as close the the chip as possible but not actually on the chip. The lower temperature rating would allow for this distancing from the chip itself.

Thanks for any advice/opinion.

[akis]

Originally posted by PJPro
[B]
I thought the network provided this....via the resistor and cap (when all is well) and via the bridge in times of trouble?

Yes except it is not the same* as a simple, mechanical connection to earth which should cover every bit of metal that protrudes or can be touched during normal operation, and that includes things like instrument leads or interconnects.

* this means there is a whole bunch of electronics in the way, additional PCBs, wires etc, there is additional complication and more things to go wrong, eg imagine if you have a bad solder connection, or the bridge is defective and so on. You'd never know until it was too late.

Also, did you see my question about the aluminium enclosure? :-)

[akis]

Originally posted by PJPro
The E24 also provides thermal auto-shutoff. See the quote below from AMB taken from the Headwise forum......



Given the SPiKe protection incorporated into the LM3886, should I bother with this capability or is a belt and braces approach better?

If I do go ahead with the thermal auto-shutoff and, given the operating temperature range of the LM3886 being -20 to 85 degC, should I go for the STO-180 (which opens at 79 to 85 degC) or the STO-170 (which opens at 74 to 79 deg C). My view is to go with the STO-170. Why? Well I'm assuming I would mount the sensor on the heatsink (or chip clamping bar) as close the the chip as possible but not actually on the chip. The lower temperature rating would allow for this distancing from the chip itself.

Thanks for any advice/opinion.

That is a cool feature to have, the ability to sense that something has gone wrong and as a last ditch effort we are switchign off. Except - if you have a proper heatsink, is it theoretically possible to heat it to 75 C ? Do some analysis to see what sort of power you'd need for the given heatsink and whether the chip would have shutdown already before that. Heatsinks have a C/W rating if I remember correctly, and the LM3886 will say how many watts it will allow before one of the trigger conditions makes it switch off, eg shorting the output or driving it with a pure square wave etc. Nah, I think it is not needed, you'd be better off using my board. :-)

[PJPro]

Originally posted by akis
By the way, remember that workshop you found near you to make you an aluminium panel? Do you think they'd be able to construct an alumunium enclosure for my guitar amp? I need an encosure to measure 85 cm across, 40 cm depth, and 7cm height - and such things are hard to find :-)

I did speak to them about listing aluminium cases on their website. The guy did consider it. He took a look at mine and said it was simply two lengths of extruded aluminium (the sides) with cut plate for the rest........ but I suspect he won't bother. Having said that, I am sure they will fabricate whatever you like. Their workshop is littered with big machinery. Why not give them a ring and see? Their website is here.

I suspect you'll have to sort out the finishing yourself although anodizers aren't hard to find.

I'd be interested to know what they quote you for the work. I'm considering using them in the future.

[PJPro]

Originally posted by akis
That is a cool feature to have, the ability to sense that something has gone wrong and as a last ditch effort we are switchign off. Except - if you have a proper heatsink, is it theoretically possible to heat it to 75 C ? Do some analysis to see what sort of power you'd need for the given heatsink and whether the chip would have shutdown already before that. Heatsinks have a C/W rating if I remember correctly, and the LM3886 will say how many watts it will allow before one of the trigger conditions makes it switch off, eg shorting the output or driving it with a pure square wave etc. Nah, I think it is not needed, you'd be better off using my board. :-)

Of course you are right. My heatsink is massive and unlikely to reach the trigger temperature before the chip does. That's why I thought I'd place the sensor on the small aluminium bar which I'm going to use to clamp the chip to the heatsink.