Need to drive 100w-ish into a 16ohm load, bridged LM4780?

[Badside]

Hello everyone,

First a little background, I've built a few tube amps and my main gigging amp is a 50W Marshall style amp.
I'm currently carrying around a smaller 8W (6K6 power tubes) backup amp, which sounds really fine but it's far from being powerful enough for my needs. Monitoring is often deficient and I need my amp to be loud enough to get a nice stage balance with the drums pounding, not to mention the occasionnal gig where the PA can only provide vocal support.

So my plan is to build (cause i love building!) a SS amp that can rival with the 50W tube amp (which is really about 75W clean!) volume-wise (note that I rarely get to go into power amp overdrive territory). The 2nd part of the problem is that the cabinet is 16 ohm. This is not a problem with a tube amp, but it is with a SS amp.

Now, I discovered this forum and I fell in love with it! After much reading, it seems that cascading a pair of LM3886 with a 35V+/- supply would get me about 100W RMS (200W peak) into 16ohms. To be fair, I used to have a Line6 Flextone XL which had 2 LM3886 (stereo) and found it rather weak, but I'll be using a closed-back cabinet, which helps a lot.

I notice tech-diy.com sells a bridged LM4780 PCB, this would make the power amp build a piece of cake! But extracting this much power from a single chip seems maybe a bit risky. We're talking an amp could be running at full tilt for hours at a time (once played a 4h30 straight gig!). We're talking a big heatsink and probably a fan.

Another option is a stereo LM3886 kit from chipamp.com, then I run them bridged by feeding one an inverted signal and hooking up their outputs appropriately. By having the chips farther appart, or even on separate heatsinks, seems it could run cooler. But I still have the same power to dissipate.

There are also a couple discrete circuits circulating on the internet, but a chip amp seems like a safe birst SS build.

For the PSU, I like Hammond cause they are cheap to me being in Canada, they have a 225VA toro PT that would do a good job. Big fat bridge rectifiers and caps are cheap and plentiful at the local surplus store.

Does that seems like a good plan? Double 3886 or single 4780?

Thanks

[Badside]

Seems like I answer my own question
Used the XL sheet provided by National to do the math. With 35W on a bridged 4870 driving a 16ohm load, it works with a 25C ambient temperature (albeit with a huge heatsink), but anything hotter (which is to be expected on stage) and it's beyond the limit of the chip package.

So how easy is it to configure chipamp.com's stereo kit to bridged operation?
I understand that I'll have to provide an inverted signal to one of the amps since it's not exactly possible to configured one as inverting, being fixed in the PCB design.

Could I use a pair of JFET to simulate a tube amp's phase inverter? Sounds like fun...

Sure I can use an op-amp to do it, but where's the fun?

[Minion]

You might be better of Bridging 2 LM3886 chips instead of Bridging a LM4870 , Because the LM4870 is 2 LM3886 chips in the same package the Heat dissapation isn"t nearly as good so the thermal protection will kick in far earlier with a LM4780 than with 2 LM3886 chips spaced Appart .....

To create a Positive and negitive Audio signal for Bridging there are a couple options , the easiest but probably more expensive is to use an audio transformer , a 1:1 with a center tapped secondary would be good .....

the next option would to use a Ballanced line driver to create a ballanced signal from a Unballanced signal , there are Ballanced line driver chips like the DRV134 , of you can make one from a Dual opamp configureing them with one opamp at Unity gain Non-inverted and one at Unity gain Inverted ......

There are also some higher power chips out there , like the TDA7293/94 which can put out close to 100w into 8 ohms with a +/-42v supply or close to 200w into 16 ohms bridged ......

You can also rewire your cab to run at a lower ohms .....



Cheers

[Badside]

Hi, thanks for jumping in!

Thr bridged 4780 solution was appealing because of an inexpensive PCB available on the internet that would make the project a breeze.
But as you say, thermal management would be much more complicated, as confirmed by the calculator provided by National.

So, a pair of bridged 3886 is a better idea considering the higher temperatures that the amp could be subjected to.

I've noticed the TDA7293/94 too, but further reading here and other sites seem to imply the LM serie is a better contender. Also, a pair of 3886 is just the right power point I'm looking for. And there are plenty of sample circuits circulating around, including a ready to print PCB design on GGG.

I've thought about rewiring the cab too, but laws of electricity being what they are, I can only choose between 4 and 16 ohms (2 8ohms driver in parallel or serie). And at 4 ohms, that means paralleling 2 chips, which is not much different from bridging them (except for the lack of a "phase inverter").
If I keep it optimized for 16 ohms, that means I can also use my bandmate's cab if need be. And most Marshall cabs are 16 ohms, so it's a pretty common load.

It would be great though to have the option to run an 8 ohms cab. I understand this is possible but would be safer with a lower voltage (which would make for a less power at 16 ohms).

[J M Fahey]

Hi Badside.
*I* would rewire the cabinet to 4 ohms, in fact that's what I always do, so my clients can use my SS 100W/4 ohms heads *and* use their Marshalls, if they wish, simply selecting that on their heads, or you can even put a 4/16 ohms switch in your Marshall cabinet.(By the way, what model and speakers?)
1960s already include those, they can be used 1x4/16 ohms mono or 2x8 ohms stereo.
Besides, I agree, most Marshall cabinets are (or can be switched to) 16 ohms; but *most* other cabinets in the world are 8 or 4 ohms.
Going back to your idea: 2xLM3886 bridged fed +/- 35V DC will provide around 100W RMS to your 16 ohm cabinet.
It's very easy to get them bridged (1 Op Amp or even 1 transistor) but if your LM3886 Hi Fi kits are or can be wired as "inverting" (many are), you'll only need two resistors. !!!!
Yes, you can simulate the 12AX7 PI with Fets and yes, it *is* fun.
TDA7294 can be used to get around 120W but the PCB is trickier and mounting becomes complicated.
The pinout is plain stupid, pins that have to be joined are separated by others doing other things, what were they thinking?
Check those LM3886 kits being offered here.

[Badside]

My personal cab is actually not a Marshall, as I don't like quad boxes, and the Marshall 2X12 cabs are way overpriced.
It's actually a Traynor YCX212 that I've installed old Celestion 75W in them.
The kicker? It was actually a 4 ohms cab that I rewired to 16 ohms!
Why did I do this? So I can play with two cabs if need be (happens once in a while).

Problem is there are occasions where I may have to use a provided cabinet, and 2 times out of 3, it's a 16 ohm Marshall with no impedance switch. I can't rewire all my friend's cabs.

However, I may have changed my plans... I'll most likely build a combo amp, as it will also make an easy amp to carry to rehearsals, and will provide a spare speaker at the same time.
I'm thinking a single LM3875 into a single 8 ohm Celestion V30 or similar. It's rated at 56W RMS and the V30 has a very high sensitivity, so it will be equivalent to 100W into a single Celestion G12T-75 (what I'm using right now).
I'll leave it sealed to get the maximum output (I hate open back cabs), something that's easier to do with SS as you can still have the heat dissipating part (the heat sink) on the outside without building the amp in a separate part (and end up with a big and heavy combo).

Still undecided about the preamp, and I may experiment a bit. A Thor/Thunderchief sort of circuit with a full tone stack would make for an easy transition from my main amp (a JCM800/Plexi hybrid), but I'm curious about using an LM386 as a preamp. Worst case I'll just use my Marshall Guv'nor pedal to drive the power amp until I settle!

When you say "those LM3886 kits being offered here". Are you referring to Brian's kit?
I've been trying to access his website for the last week and it's offline.
On another board, he keeps saying it should be back within a day or two. I may very much order from him.

[J M Fahey]

OK, keep us informed about your experiments (schematics, pictures, MP3s, etc.)
A Guvnor or similar pedal driving an LM3886 *must* sound very good and very JCM800.
A simple LM3886 , +/- 35V driving a V30 must be plenty enough volume.
You can mount another one in the same chassis, (your power supply must be the stereo kit one) , you can use it to drive an external cabinet when needed.
Yes, I mean these chipamp kits, of course you can use anyone you like, but these have been used by many here, with good results.

[joecool85]

I have two LM3886 chipamp.com boards that I want to bridge for my brother.  Single PSU, 250VA 25v transformer, so power shouldn't be an issue.  This will be for a bass amp, 8ohm 15" speaker. 

How do I go about doing this?  I know these boards can be inverted, but don't remember how.  Isn't it a resistor that has to be removed or something?

[J M Fahey]

Hi joecool.
I'll draw the schematic and the little board needed (uses 1 TL072) and post it here for anybody who needs to bridge a couple chipamps (any of them, from LM386 to TDA7294)
Check tomorrow night.
PS: I'll include it's own mini PSU fed from the main +/-B

[Minion]

There are a couple ways to Bridge chips , have one of the Chips set up as Inverting and the other as Non-inverting, I don"t like this method for the simple fact That it is Harder to gain match each channel with available Resistor values ......

The other method is either useing a Ballanced line driver (DRV134 , Or opamp/Transistor based) or to use an audio transformer ......

For a Bass amp I would go for an opamp based one as Ive had noise issues with DRV134 and it might be hard to get a audio transformer will low enough frequency responce for a Bass amp .....

I would also suggest doing some resistor matching to get the gain of each chip as close as possible .....

Cheers

[joecool85]

Hi joecool.
I'll draw the schematic and the little board needed (uses 1 TL072) and post it here for anybody who needs to bridge a couple chipamps (any of them, from LM386 to TDA7294)
Check tomorrow night.
PS: I'll include it's own mini PSU fed from the main +/-B

Hey JM, any luck drawing that up?  I'm real curious to see it.

[J M Fahey]

Hi joecool.
Had a busy weekend but now I'll do it.
I've answered your other post with the "official" self-inverting conexion for 2x LM383/TDA2002 but mine is "universal", for bridging any amp, including both channels of a stereo receiver, whatever.
Check tonight.
PS: I usually draw in DOS Autotrax, Tango or EasyTrax (he he, oldtimer) but I'll convert the output to a .gif and include a ruler so you check correct printed size.

[joecool85]

Hi joecool.
Had a busy weekend but now I'll do it.
I've answered your other post with the "official" self-inverting conexion for 2x LM383/TDA2002 but mine is "universal", for bridging any amp, including both channels of a stereo receiver, whatever.
Check tonight.
PS: I usually draw in DOS Autotrax, Tango or EasyTrax (he he, oldtimer) but I'll convert the output to a .gif and include a ruler so you check correct printed size.

Thanks!

[J M Fahey]

Component overlay:

Tracks/Board solder side:


Bill of Materials (commented)
-------------------------------------------------------------------------------
BRIDGER.PCB  13:05  24-2-2010    Bill of Material Summary   Page : 1
-------------------------------------------------------------------------------
DESCRIPTION          QUAN.    COMPONENT DESIGNATOR(S)

+B                  1      W5   Power supply positive rail, +12V to +50V     
+OUT              1      W3   Signal in phase for amp #1     
-OUT               1      W4   Signal out of phase for amp #2     
1M                  1      R5         
1UF                2      C1     C2         
2K2 1W           1      R4     Voltage dropping resistor. Size to pass                  around 10 mA.   
10N                 1      C6         
10UF               1      C3         
22K                 4      R1     R2     R3     R6         
100UF             1      C4         
101                 1      C5   100pF ceramic     
GND               1      W1         
IN                   1      W2         
TL072             1      IC1       
Z15V              1      DZ1  Zener 15 or 16V 1/2W     
NOTES:
1)W1 to W5 show wiring pads. They have two holes each, I usually insert a U bent piece of wire (just cut component legs are OK) from the component side.
They allow easy wire solder points both above and below board, whichever is handier.
2) This bridger is universal: for single or dual rail powered amp, it uses the positive one, can be fed from +12V up.
For +12 to +16 V +B replace the 2K2 resistor with a 470r and omit the Zener.
For +B higher than that use the 2k2 and zener to limit voltage to 15V.
3) all electrolytics 16V or 25V; all ceramics 50V or more.
4) All resistors 1/4W; except 2k2 1W.
Optimum value: size to pass around 10mA.
Example: +B=35V Zener=15V Difference= 20V
So R4=20V/10mA=2000 ohms (use closest standard value, in this case= 2200 ohms)
If +B = 12 to 16V, just replace R4 with a 220 to 470 ohm one, simply for supply filtering, omit the Zener or leave it there, "just in case".
5) The two large 1/4" pads are perforated for 1/8" mounting screws,separators, etc.
6) You can turn any old junked stereo home amplifier (which can drive 4 ohm loads, most do) into a fire breathing dragon driving full power into an 8 ohm Guitar speaker. (well, sort of, definitely *much* more than you thought possible)
Have fun.
NOTE1: when printing on photo paper for ironing transfer, print as-is without inversion, tracks are already seen "through the board".
NOTE2: when printing check that the ruler actually measures 2", scale your print size as needed; in theory printing at 300DPI or 1:1 size should be OK but often it's not.

[joecool85]

Is it possible you could give me just the schematic for this as opposed to the layout?  I don't make my own PCBs, and I'm thinking I might be able to do this on strip board.