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 on: June 21, 2018, 03:54:42 PM 
Started by edvard - Last post by R.G.
[...] The unit runs 2x LM3886 in parallel driving a 4 Ohm woofer and a single LM3886 for tweeter.
[...] but that chip was too hot to touch and measured rail voltage on the output which obviously burnt out the tweeter.
[...]No real heat sink except for a 6mm flange which is also bolted to outer alloy case.
You already know the answer then.
Read this:
It contains a lot of my advice to RDV about heatsinking LM3886s. If you simply attach a chip amp to a flange and hope the chassis will take care of it, you;'re almost certain to be disappointed.

Even if an adequate heatsink is used, the mounting is critical. It simply MUST be in good THERMAL contact, so the heat can get to the adequate heat sink. Loosened mounting bolts can undo whatever good a heat sink might do you.

In this case, it is entirely possible that poor operating conditions made the LM3886 oscillate. That's a suspicion based on (1) it was a Behringer, (2) it was attached to a tweeter (3) the heatsinking wsa inadequate. Oscillation would make it dissipate a huge amount of heat, and any inadequate heatsinking could let it fry before the thermal protection could cut in.

There is nothing inherently wrong with the parallel use of LM3886s if you do it right, use the right precision parts, and pay proper attention to thermal issues. But just like the use of fire, you can do a bunch of things wrong before you get smelting metals and internal combustion engines to work.

There is no substitute for knowing what you're doing. Like the song says, "you gotta know when to hold 'em, know when to fold 'em".

 on: June 21, 2018, 09:13:15 AM 
Started by edvard - Last post by phatt
Great reading and helpful info :dbtu:
I don't know if it helps but a while back I posted a blab on a Behringer monitor (Truth 2031) which had a blown tweeter. The unit runs 2x LM3886 in parallel driving a 4 Ohm woofer and a single LM3886 for tweeter.

I was surprised that the rails were +/-40 and seems to work without issue,, for the low freq at least. I have no idea why the hi freq blew but that chip was too hot to touch and measured rail voltage on the output which obviously burnt out the tweeter.

No real heat sink except for a 6mm flange which is also bolted to outer alloy case.
These were obvioulsly a pair and last I spoke to the owner the other unit is still working 3 years later, being used as a local monitor.

I'm not keen on building things that rely on super critical parts to run. Better to over build it. 8|

Adding schematic of the truth speaker and gut shot.
It just bolts onto the back of the speaker box.

 on: June 20, 2018, 06:28:51 PM 
Started by luteski - Last post by Bill Moore
Thanks for the memories, Juan! As a kid on the ranch, we would spend a week or more at a time at a couple of "camps". Really decent places, (us boys had a bed on the screened porch). Had a propane refrigerator, and stove, usually hung a quarter of beef in the shade in the porch covered with a tarp. Not old Lister power, but WWII Navy 4 cyl generators, usually run for an hour or two, and then lights out, they too were pretty efficient! The neighbor west of there had a Wincharger, and batteries, had lights as long as you wanted at night, even in the bunkhouse, (and a radio!) The kitchen even had a toaster! I don't remember staying up too late at night, as morning, (and breakfast) came before full sunrise! We took turns wrangling the horses, and since it was dark, we would look for old man Tom's white mule, to find the stock.

 on: June 19, 2018, 11:59:32 AM 
Started by Psabin8951 - Last post by J M Fahey
OK, let´s separate power supply and amplifier and measure.

With a sharp boxcutter cut tracks leading from +V and -V to amplifier at points 01 and 02.
Cut a narrow 1mm strip or so and remove copper , later you can scratch soldering mask on both sides and rejoin with a drop of solder or a little bit of wire.

1) Measure +V and -V to ground at points 04 and 06 .
If normal (35/40V ) , also measure +/- 15V rails at ungrounded ends of ZD1 and ZD2 .

2) with amp OFF measure resistance to ground from points 03 and 05.
Both ways and using resistance (say 200 ohm or 2k) and diode scales.
Open circuit is no problem, I´m trying to find some continuity to ground which shorts raill voltage.

I also suspected some open track or connection, but you confirmed
There is continuity back to -V on C61 and DB1
so that seems to be working.

 on: June 19, 2018, 11:39:03 AM 
Started by JB074 - Last post by J M Fahey
It´s designed to grab attention  :cheesy:

Now that he actually sells it, is completely different; price is outrageous, it could command $20/25 TOPS if it were in good working order, which it is not.
I have never seen such RUSTY jacks and handle, not sure it even works and certainly any control there must be scratchier than somebody rolling naked in Poison Ivy.

 on: June 18, 2018, 10:14:02 PM 
Started by edvard - Last post by R.G.
Oh, and I forgot to add - I'm funny about just blankly trying this, too. That was one other thing that voted for "why worry - just do one amplifier per speaker".

 on: June 18, 2018, 10:11:32 PM 
Started by edvard - Last post by R.G.
I have nothing but respect for your technical expertise and experience.

However, I did a look at the numbers, and given:
1) well behaved chipamps, like the LM3886 (perhaps excluding others; I did not do the math there)
2) good heatsinking
3) precision feedback resistors and biasing resistors
paralleled LM3886's can be made to work reliably.

My professional and personal experience up to very recently would agree with your position. However, if TI went to trouble to write up an app note on bridged and paralleled amplifiers with the LM3886, and go so far as to publish their results. I know, I know, lies, d@mned lies, and app notes.

I have read several sendups on the web of people successfully getting the paralled version (and bridged+parallel version) to work.

I calculated the cross currents and the watts they'd add to the chips for the offsets and for the cases of worst case 1% and 0.1% resistors. The math says it's untenable with 1% resistors, but pretty well acceptable for the 0.1% case.

When I was starting, I was told to figure out how to design things with 10% resistors, saving 5% for closer tolerance work. I had to get higher engineering approval for 1%. A few years later they encouraged us to use 1% whenever needed. 0.1% resistors were not in any catalog I knew of (these were on paper back then). But today I can get literally thousands of values and types of 0.1% resistors from my local distributor overnight. So I've had to re-think what it too demanding to even try.

You are correct - this is not a situation for use by one-week-wonder geniuses. But It's not all that far off in the tall weeds, either.

 on: June 18, 2018, 11:40:21 AM 
Started by Psabin8951 - Last post by Psabin8951
Thanks for the fast reply.
I used a mica sheet plus grease.
Collector shows no connection to heatsink or chassis on meter.
There is continuity back to -V on C61 and DB1

 on: June 18, 2018, 02:32:56 AM 
Started by JB074 - Last post by JB074
This ad got my attention (attached)

 on: June 17, 2018, 11:38:45 PM 
Started by JB074 - Last post by JB074
I was unashamedly asking for *opinion* on heads you may have used that you like the sound of or conversely do not like.

My apologies, but I plead the hive-mind of the internet.  Ask opinions on a ham sandwich and by the second page it's a full-blown smackdown on whether French Crullers are better than Danishes.

What kind of music do you play?  I am assuming something Rock-oriented but I don't like to assume.  If you lean towards the harder side, I don't think you can go too wrong with a Randall RG series.  I can personally vouch for pretty much any Peavey heads for clean tone, and some TransTube heads have switchable loads. Beyond that, not sure.

I play a lot of varied stuff, from surf rock to slayer. I play a bit of country, a bit of 80's but most of my love is 70's hard rock. So you're all wondering why I don't have a plexi....
I mostly have 'clean' amps - blackface twin reverb, JC-120, Vox AC head, thr100 yam, katana 100 1x12. for dirt a heavily modded fm100H fender head, which is a one-trick pony.

I'm looking for a slightly crunchy head for this vintage marshall 4x12 1960ahw cab. If it's a 50-100w head I will need to attenuate it, or it will need to have a low-power mode. And at least at this stage I'd prefer to drive the 16Ω cab without rewiring - not that it matters terribly for an SS output stage.
I've spent many years repairing rackmount power amps and I don't need or want a huge gig setup.

I have thought about reworking a small SS amp and putting a tube preamp channel in, but I can buy stuff like that off the floor in a shop too -

Just throw some ideas out there - I don't care if it's a head for Jazz or a modded high gain unit - I'm just out of ideas.

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