| psychocow21 |
Hi guys,
I was just going over some amp ideas in my head and one in particular stood out. It would be a long way off (not much money at the moment), but I have been thinking of something like this for a while.
I am a bassist and I would like a more powerful amp. I'm looking at getting either 2 (4 pcbs) or 4 (8pcbs) of the 'LM4780 Dual Mono Kits' from audiosector and bridging them into one channel. Assuming I calculated this correctly (that being a big assumption at this time of night), I believe those combinations should offer ~450wrms and ~900wrms, respectively.
My first question being: is that correct?
The second being: what sort of Toroid am I going to need for this (assumeing I go for 2 kits = 4pcbs). The biggest I can source are 500VA and I am assumeing I will need two of these.
Also, heat dissipation: I was thinking of using two 300mm Die Cast Heatsinks (0.37C/W each) with 2 or so fans in the case. Would that be enough?
Thanks for any input guys.
Cheers, Nic.
And one last thing: I have two LM4780 chips sitting around, but not the pcbs, how would I turn these into a working amp? I read that soldering the components directly to the chips would be very difficult, if not impossible, but I don't think I would be able to find a generic pcb with holes close enough together. Any suggestions? |
|
|
| SpittinLLama |
On the amp design, with chip amps you are always either limited by voltage or thermals. It looks like the boards are either run in parallel or stereo mode. To make them run in a bridge mode you will either need to modify the circuit or invert one input before the board. So if you get 2 boards and set them both to run in parallel then you will need to invert the input to one of the boards to create a bridge with each board being one side of the bridge. Or you need to modify one board to run non-inverting. Once you have the bridge set up with each LM4780 running it's two outputs in parallel you need to be sure to drive a speaker of 4 ohms for maximum power or higher but you won't get as much power because you will be voltage limited. Below 4 ohms you will be thermally limted. So if your speaker is 4 ohms then you have the potential for about 225W, give or take depending on supply voltage. IF you want to add more PCBs in parallel then your speaker load needs to drop to 2 ohms (two 4 ohms in parallel would give you 2 ohms). With the addition of another 2 boards you could get about double the power.
Check the application note AN-1192 from National to get a better feel for how all this works.
Sounds like your heat sink size might be pretty big, fans may not be necessary. Guess it all depends on the final voltage and design.
-SL |
|
|
| psychocow21 |
I was planning to get two kits (that would be 4 boards/chips in total).
I would run the boards in parallel (each bridged board would then give just over 100wrms, I believe).
I would then bridge/parallel the 4 boards into 2 sets of 2 boards.
Lastly, I would bridge/parallel the 2 sets of boards into one output.
The attached picture might make it more obvious.
I checked out the AN-1192 application notes and I couldn't make much sense of it.
I would prefer a bit of overkill for the heatsink, as if I was running it as a bass guitar amp it would be used pretty hard.
The circuit boards from audiosector are made so you can run them in stereo or bridged, I would be running them bridged.
You said I would need to invert the input to one of the boards: how would I do this? Or how would I make it run non inverting.
I will build the speaker cabinet after the amp, so I can make it whatever ohm rating I want. I would either go for 4ohm or 2ohm.
Much thanks.
Cheers, Nic. |
|
|
| Fenris |
Nic,
I built a bass amp for a friend a few weeks ago using this method. What you will need is called a splitter. What it does is take the input signal in and output both the input signal and an inverted input signal. Two of the parallel amps then amplify the normal signal and two parallel amps amplify the inverted signal. When you sum up the two signals, they are twice as large as the original, giving you four times as much power.
You don't want to run driver(s) below 4 ohms. Bridging 3886s means each sees half the overall impedance, so you're limited to 8 ohms. Putting them in parallel brings the minimum back to 4 ohms.
For a splitter, you're going to need a separate small power supply, and either a chip like the DRV134 or an op-amp pair to invert the signal. You'll probably also need a pre-amp to increase the gain of the instrument up to a usable level. I have PCB board designs for this if you want them. |
|
|
| juan_caliente |
Hello Fenris,
I am sure many of us would like to see your boards, would you mind posting them for us.
Thanks,
John |
|
|
| Variac |
Instead of making one huge amp with the attendent risks of fire, expensive loss of parts, death etc., is it possible that you are interested in running one of those cabinets with 4 smaller (10"?) drivers? If so, you could have a paralleled board for each driver or run two drivers from one module at first , then you could double the number for more power later as funds permit. I have seen stacks of two of these 4 speaker cabinets, which would again give you easy expansion as required.
If you have a amp that could drive a single 15" then you could combine the 15" cabinet with with one of the new 4 driver chip amp cabinets
If you mount the amps in the case next to each driver and they would have great control over the cone and if one blows up then the rest keep playing. Also you could make one extra module and if the modules could be easily removed, then you could quickly swap out a broken one -even at a gig!
You can figure out how this might work for you, but it would be a lot more reliable and pretty cool IMHO. |
|
|
| AndrewT |
Hi,
do your amps a favour and use one amp to drive one speaker.
Variac is right about the commercial need for the sound to continue despite "live" problems.
If you decide that 300W+ is really required then parallel/bridged has gone beyond complex and certainly no longer cheap.
Bite the bullit and go discrete for big power. |
|
|
| psychocow21 |
Hi,
Fenris, I would be very interested in those PCB designs, thanks.
I am also looking at preamps. There is a company in Australia called Jaycar which has a guitar preamp with equalizer and what not, so I'll probably go for that. Although I am considering a valve preamp.
In ther end, I am aiming for having to cabinets - a 4x10 or 4x12 and a 1x15.
I would then drive the two cabinets separately, with two of these circuits in one enclosure.
Although having four circuits going to the four drivers separately sounds good in thoery, the wiring between the boards is where I screw up the most, and this would create quite a bit more. Also, for compatability with other cabinets and such it would be easier to have it driving one channel.
I would do rigorous testing of this before taking it to a gig, believe me.
And trust me 300+ watts is required, as a general rule of thumb, a bassist will need at least 4 times the wattage of an electric guitarist's rig, and most of them are 100wrms.
Cheers, Nic. |
|
|
| FastEddy |
... and there is always just getting a single 800 watt amp, kit or module. I like the NX500 but aussieamplifiers.com has 'em up to a 1000 watts.
My current project, on the kitchen table as I speak, is dual NX500's from a switching mode supply in a "one up" rack mount chassis. The intention is to use a tube guitar pre-amp to drive it ( maybe something like this: http://www.mesaboogie.com/Product_I...ectoPreAmp.html ). The overall weight of the two channel rack mounted NX500 amp above should be under 15 pounds, so it can go on the road without it's own airline ticket ... :D
I'm calling it the One Horse, cause that's about what the output is ... For a bass amp, you might consider one dump truck instead of four compact cars ... |
|
|
| psychocow21 |
FastEddy, the aussieamplifiers amp you're making sounds interesting. Could I see some pictures?
Also, what size transformer and heatsink are you using for it?
Did you buy just the board or the full kit? What type of chips does it use?
Cheers, Nic. |
|
|
| FastEddy |
| basic layout: two aussieamplifiers.com NX500 prototypes connected with #12 AWG fine strand teflon coated silver wire |
|
|
| FastEddy |
| in chassis layout ... (Note "special sauce" is hum bucking torridal ferrite lug, violating "right hand rule" to cancel any and all EMF and other noise from switching supplies) |
|
|
| FastEddy |
| another view inside ... 100 to 240 VAC through w/ 6 amp switch/circuit breaker to dual switching "hospital grade" +/- 48 VDC PSUs ... no XLR input boards yet ... |
|
|
| FastEddy |
Outside view ... "gopher ears" for rack mount not shown. (Chassis is from Isreal for speciality digital modem used in broadcast picked up surplus.)
The reason for using +/- 48 VDC and +/- 60 VDC power in either version is that although the NX-500 can easily be run at +/- 90 VDC, there is the concern that when driven hard there are thermal questions, especially if the amp(s) are mounted in a rack with other equipment that also generates heat = these are meant to be 24/7/365 power amps, thus the "derated" power = no heat at all even in the "road" version. The distortion numbers are great down to +/- 35 VDC rails ... there will shortly be kit versions of these boards ;) |
|
|
|
