18V bipolar power supply for LM3886 amp

[stringsthings]

I wanna build this amp from GGG:

http://www.generalguitargadgets.com/pdf/ggg_lm3886_amp.pdf

But it needs an 18v power source. There's this one:

http://www.generalguitargadgets.com/pdf/ggg_bipolar_ps.pdf

now, how are you going to go about finishing your amp?

[petemoore]

  suggestions:
  Follow a known working diagram, one with explicit and detailed instructions/photos/schematic, etc., followed to the T.
  This generally produces a hum free amplifier, however node counts and extensive cross-DMM-ing the board while comparing it to the schematic.
  Be certain V+ from PS goes to V+ node and nowhere else, same for all grounds, clamp the black lead to ground, test every shiny surface, be sure the schematic allows Gnd. only where marked.
  Every possible node-count cross-check involves most of the settings on the DMM, first and last is to test for non-short across the PS and circuit rails, center ground.
 

[El Heisenberg]

Well great. I got this huge expensice clunkin thick heatsink for my LM3886. I'd be ready to start building but...


The freakin heatsink has no way to mount the chip amp!! No screw hole at all! I gotta like, drill my own or something!! Dammit!

[El Heisenberg]

DANGIT DANGIT DANGIT DANGIT! I'm so MAD! I never saw this comming! A heatsink without any means to attatch an IC or something??? GRRRRRRRRRRRRRRRRRRRRRRRRRR


Here's the heatsink I bought:

http://www.jaycar.us/productResults.asp?keywords=HH8555&keyform=KEYWORD&SUBMIT.x=23&SUBMIT.y=7






Is that was "no flange" means?

[El Heisenberg]

I don't suppose a battery powered hand drill or a dremmill will solve this problem/...

[petemoore]

  Most of these ''brick walls'' have work-around threads at DIY Audio.
  These methods are boiled down, the recipe's make it quicker, less expensive, more compact, less frustrating...totally worth the hours and hours of observations.
  HS Metal is generally soft as far as drilling goes, through bolt and nut is what I'd choose if using the 'TF' suffix chip[s?...that heat sink looks Huge.
  Since it's tricky to mount the chip/heatsink on the same board when they're not pre-fitted, keeping the stress from the pins/solder joints will take some consideration. I ended up populating the board with the 'lighter' parts and the chip, putting the HS on the chip for a pre-fit, removing HS, drill 'precision' holes for dowel/columns to which the HS is connected. Since the dowels are relatively short, warping/contracting amounts probably won't pull on the solders/pins beyond flexing. This was on the amp without the "TF" suffix [insulated body] chips, dry wood is a fair insulator...

[El Heisenberg]

Yea, I was gunna use rubber stuff to prop the heat sink up. The circuit board would be hanging horizontaly from the heatink off the chassi like an inch or something. But whatever. I'm at a brick wall here.


I've been posting at diyaudio.com. That's where I found the heatsink. And it arrived without a hole!

You think a dremel or a hand drill would do the job?


Hey could you direct me to one of those work arund threads if you know or kind find one off the top of your head? heh.

[petemoore]

  They go in depth with layout/schematics/pictures...for various topologies.
  First, find a FWR/capacitor smoothing that fits your needs, then measure the DC voltage that it converts from AC.

[petemoore]

You think a dremel or a hand drill would do the job?
  Dremel's kinda fast, but if the bit's sharp, should go right through, hand-drill too...careful with the smaller bits cause they break easy.
  As far as power supplies, FWR's have arrows, they point 'up' toward + [that's how I remember it, some schematics show them differently]...the secondary connects [AC has no polarity].
 Anyway lets say you get a volt of DC from each FWR/capacitor smoothing...this is still floating voltage, it works like this, and/or can be assigned to a voltage [like Gnd.].
 I think what you want has "Gnd." as a center voltage, between the _ and + power rails. This can be created by connecting the two floating voltages, one providing a + and one providing a - at this connection makes rails twice the voltage, with a center point. Still floating voltages though, none of the nodes have been assigned to a voltage, take the +/- node [the center tap] and assign it to 'Gnd.' or 1/2v or whatever it is they're notating/calling it.
  Get the volt meter out [actually should be testing each and every move like the 12 days of christmas, get the the Partridge in the Pear tree and sign it again !]...volt meter should be finding a _ potential and a + potential from the center tap.
  The data sheet shows these power supply nodes a Vcc+ and Vcc- or V+ and V-, and shows the pin assignments to which these voltages connect to the chip.
  Before you apply the power test for non-shorted conditions everywhere except where there are 'commons'...ie 'they're supposed to be connected'.
  There's a sticky thread IIRC about power supplies, also threading through the Chipamp forum may show additional power supply information, the basic power supply formations don't really change much, but the filtering./voltages and wiring styles may.

[R.G.]

DANGIT DANGIT DANGIT DANGIT! I'm so MAD! I never saw this comming! A heatsink without any means to attatch an IC or something??? GRRRRRRRRRRRRRRRRRRRRRRRRRR
...
Here's the heatsink I bought: Is that was "no flange" means?

Easy, easy. Welcome to home audio building 101.

Of all the heatsink styles, that one is my **preferred** one. If your heatsink has flanges, you're largely stuck with what they wanted you to do.

First off, you'll probably want to put that on the back side of the metal box you'll be putting your amplifier into. So do it the easy way. Figure out where you want the heat sink to go, leaving the fins running vertically, on the back of the box. Leave room to get your speaker jack, AC power cord, fuse holder, etc on the box too. When you figure out where you want it to go, draw the outline of the heat sink on the box. We'll come back to that.

But next, we figure out how the IC will fit on the heat sink. Fortunately the manufacturer has done this for you. The LM3886 mounts with a single screw.

You figure out where you want it to go on that heat sink on the flat side. Then you scribe the inside of the mounting hole with a sharp pencil, remove the IC, and drill a hole where the marks are. Ideally, you will drill a tapping sized hole and tap the hole so the mounting screw will go right into the heat sink's flat back, and no nuts or washers will be needed. If you can't do that, drill a clearance hole for the IC, **being sure that the other side of the hole comes out in the middle between fins**. This lets you get a nut on the back side to hold it.

A tap wrench seems like an advanced technique, but I love them. You have to be careful tapping into aluminum, using oil in the hole and being very careful not to bend the tap, or they will break off. But what you get is a custom mounting.

When you've figured out where the IC bolts to the heat sink, you cut a big opening in the back of the box where the heat sink will attach exposing the heat sink when it's bolted to the box. With the opening in the box in place, drill mounting holes for the heat sink through the back of the box and into the heat sink. Again, either drill them tapping hole sized, or if you drill through-holes, make sure the holes come out between fins so you can get nuts on them.

What's left is mounting the electronics. There are two ways to do this, parallel to the flat back of the heat sink, or perpendicular to it. I personally like to mount the PCB parallel to the heat sink, and put it where the power IC or power transistors can have their leads bent up to come into the PCB from the bottom. This lets me mount the PCB on the heatsink on spacers (again, there's that drill-and-tap thing!)

If you want to mount the PCB perpendicular to the flat back of the heat sink, is there a hardware store near you? Do they stock aluminum "L" angle stock? This is a stick of aluminum with a cross section like an "L" with equal legs. Around here, it's as common as a bin of screws. Get a section of L angle stock, perhaps 18mmx18mm and 3mm thick. This stuff cuts with a hacksaw like butter, so you can make little L brackets with holes which will attach to the back of the heat sink with ... yep, drilled and tapped holes into the heat sink.

Trust me, this is so much better than running wires to a heat sink with other attachment methods that it's not even close.

Questions? I can probably find pictures on the net somewhere.

[El Heisenberg]

Ah I can see why you like these type of "blank canvas" heat sinks. I really got ripped off. I paid 20 bucks for this thing!  I guess I'll head down to Lowes or Home Depot tomorrow to try and get the tools for this.



Anyways, R.G. I can understand what you're saying, but pictures would be even better if you wouldn't mind. Since the heatsink will cunduct the V-, I gotta insulate it. Or someone suggested I mount the chip amp with a thermal pad. On my old Epiphone S10 amp that I took apart, when i took the TDA2030 off, there was a layer of stuff. I thought it was like thermal paste that had hardened or something. It looked like old duct tape residue. Anyways, I gotta get some sorta thermal pad for this thing too, so it doesn't conduct.

[stringsthings]

.... I really got ripped off. I paid 20 bucks for this thing!  ....

how did you get ripped off?

[El Heisenberg]

cheap heatsinks:

http://www.surplussales.com/Heatsinks/HeatSink6.html

[R.G.]

http://www.surplussales.com/Heatsinks/HeatSink6.html

Just to help your composure a bit: First of all, the "$20" you mention corresponds to Australian currency, near as I can guess. The Jaycar price in US dollars is $10. The surplus sales heat sinks are all in US$.  That cuts the difference a lot. Correct me if I'm wrong about you paying in AUD$. It's a guess.

Then there's size. Your heat sink is very close to what I'd pick myself. It's a very good heat sink for an amplifier, being 150mm (abt 6") by 75 (abt 3") by 46mm (abt 1.8") and with a thermal resistance of 0.78 C/W.  The closest heat sink in the SSN list is (HSK) HSIFES50295, 4-9/16" x 2-3/8" x 15/16"H. This is notably smaller, and will not be as good thermally. And it costs US$6.00.

You didn't do badly at all on buying the heat sink.

As for insulation, if you can find Kapton polyimide tape in 20mm or wider sizes, that's ideal for an insulating wafer. You'll want to clean off any adhesive and replace it with heat sink goop. Try a motor or alternator rewinding shop. They likely have small scraps of kapton they'll give you. Don't use other kinds of generic tape. If you can find a silicon rubber insulating wafer, that's even better - no goop to mess with. And a shoulder washer in the mounting hole insulates the (single!) screw from the LM3886 metal tab.

I know this looks like a huge hill at the moment, but trust me - it's a small bump in the road.

[El Heisenberg]

I went ahead and drilled the hole with ym hand rill. It worked great, and I used a bolt and nut insrtead of a screw. No insulating so the heatsink conduct 28- v. But I jsut wanted to build the circuit already and see if it worked. SO I built the amp Point to point cos the chip didn't fit in perf board. It went alright, but I couldn't use the mute switch cos there was no room. Anyway it sounds great but it's too quiet. It's not as loud as my TDA2003 amp running at +12vDC!

I took measurements and the thing is measuring 28-vDC and 28+vDC. But it's almost half as quiet as my TDA2003 amp which is sposed to only be putting out 8-12 watts. The LM3886 sounds like it's only doing like 5. I'm running it through four 4 ohm speakers. Two pairs wired in series then in parallel to get a 4 ohm load. I dunno what's wrong. The chip isn't even getting hot. It's like i did't even need to use the freakin heatsink. I thought this big honking transformer would make the chip go crazy.




Here:



I'll post a pic of the circuit tomorrow I guess.

[El Heisenberg]

I replaced the 10k trimmer with a 100k pot and it's way louder. I think I can get it louder. THe chip is hot too. But with this heatsnk it could handle way way more. I'll see how loud I can get it before it starts sputtering out. Now I gotta get rid of the hum that's in the preamp stage now that I'm amplifying it so much.

[stringsthings]

I went ahead and drilled the hole with ym hand rill. It worked great, and I used a bolt and nut insrtead of a screw. No insulating so the heatsink conduct 28- v. But I jsut wanted to build the circuit already and see if it worked. SO I built the amp Point to point cos the chip didn't fit in perf board. It went alright, but I couldn't use the mute switch cos there was no room. Anyway it sounds great but it's too quiet. It's not as loud as my TDA2003 amp running at +12vDC!

I took measurements and the thing is measuring 28-vDC and 28+vDC. But it's almost half as quiet as my TDA2003 amp which is sposed to only be putting out 8-12 watts. The LM3886 sounds like it's only doing like 5. I'm running it through four 4 ohm speakers. Two pairs wired in series then in parallel to get a 4 ohm load. I dunno what's wrong. The chip isn't even getting hot. It's like i did't even need to use the freakin heatsink. I thought this big honking transformer would make the chip go crazy.

I don't mean to offend you ... but do you have trouble spelling words ? .... or are you spelling words wrong for a purpose ? 

[R.G.]

I took measurements and the thing is measuring 28-vDC and 28+vDC. But it's almost half as quiet as my TDA2003 amp which is sposed to only be putting out 8-12 watts. The LM3886 sounds like it's only doing like 5. I'm running it through four 4 ohm speakers. Two pairs wired in series then in parallel to get a 4 ohm load. I dunno what's wrong. The chip isn't even getting hot. It's like i did't even need to use the freakin heatsink. I thought this big honking transformer would make the chip go crazy.

I replaced the 10k trimmer with a 100k pot and it's way louder. I think I can get it louder. THe chip is hot too. But with this heatsnk it could handle way way more. I'll see how loud I can get it before it starts sputtering out. Now I gotta get rid of the hum that's in the preamp stage now that I'm amplifying it so much.

I have a couple of suggestions now that I know where you're going.
 - The only reason to have that 12V regulator there is to keep the 12V supply to the preamp quiet. The original preamp circuit may have used 12V, but the tubes will happily work up to a few hundred. +18Vdc and +28Vdc will work fine for the tubes if you can keep it quiet. To uncomplicate your circuit, you could ditch the 12V reg and put in a stage or two of 1K/100uF RC filtering on the power going to the tubes.

- alternatively, you could  run the tube plates from +28Vdc and some RC filtering sections, leave the 12Vdc regulator, and run the tube filament/heaters from 12Vdc and eliminate a lot of hum (potentially). As the circuit sits, I'm concerned about the hum you're getting from the way the heaters are wired.

- It would be good to use a 1M pot for the volume pot and then buffer this for use in the power amp. The volume was low because the 10K volume pot was loading down the plate of the second tube section, as you found out. This buffer could have a gain of one, or could even add some gain to drive the power amp to full output.

- As it's set up now, the power amp has a nominal gain of 34 (1+33K/1K), and it's going to drive the output pin to maybe 24Vdc before clipping with +/-28V supplies. The nominal output power will be 24V/1.414 = 17Vrms, and the power about P= (17*17)/8 = 36Wrms into eight ohms. That will be loud. To get there you have to drive it with 24Vpk/34 = 0.705V peak or 0.5Vrms of preamp signal. This 0.5Vrms from the preamp must not load the preamp down in getting it.

[Brymus]

I give you kudos for sticking it out 
How about some pics when you get a chance ?

[El Heisenberg]

I went ahead and drilled the hole with ym hand rill. It worked great, and I used a bolt and nut insrtead of a screw. No insulating so the heatsink conduct 28- v. But I jsut wanted to build the circuit already and see if it worked. SO I built the amp Point to point cos the chip didn't fit in perf board. It went alright, but I couldn't use the mute switch cos there was no room. Anyway it sounds great but it's too quiet. It's not as loud as my TDA2003 amp running at +12vDC!

I took measurements and the thing is measuring 28-vDC and 28+vDC. But it's almost half as quiet as my TDA2003 amp which is sposed to only be putting out 8-12 watts. The LM3886 sounds like it's only doing like 5. I'm running it through four 4 ohm speakers. Two pairs wired in series then in parallel to get a 4 ohm load. I dunno what's wrong. The chip isn't even getting hot. It's like i did't even need to use the freakin heatsink. I thought this big honking transformer would make the chip go crazy.

I don't mean to offend you ... but do you have trouble spelling words ? .... or are you spelling words wrong for a purpose ? 

What do you mean you're not trying to offend me? You could've just jumped to your own conclusion and let it be. But nah, I just type real fast I guess. It was late, and I was tired, and I do have several chemical vices.