Thinking about building a chipamp, have a few questions.

[joecool85]

Originally I was going to build a lm3886 based chipamp for use as a guitar amp, but then I found chipamp.com and realized I could buy kits. These kits don't do regular mono, so I figured I'd do a stereo and use both channels for one input.

Here is the question:

I know stereo would give me 2 x 50wRMS

But what does dual mono do? Would it combine it so it'd be 1 x 100wRMS? Or is it still 2 x 50w?

 

[Russlk]

If you parallel the two channels, it is still 50W, but if you invert one channel and drive both ends of the speaker, it is 4 times (E^2/R), 200W.

 

[Nigel Goodwin]

If you parallel the two channels, it is still 50W, but if you invert one channel and drive both ends of the speaker, it is 4 times (E^2/R), 200W.

A familiar misleading bridged amplifier statement!.

Power is all about available voltage levels and the impedance of the speaker - two amplifiers bridged provide TWICE the voltage capability, and would (in theory) provide FOUR times the power.

However, this requires that the amplifiers are able to provide the required power into that impeadance (each half of the bridged amplifier effectively feeds HALF of the speaker impedance).

Basically bridging provides exactly the same amount of power, BUT into a different impedance.

So, assuming your amplifier module can each provide 50W into 4 ohms, you have three options:

1) 2x50W into two seperate 4 ohm speakers (total 100W stereo).

2) Amps in parallel - 1x100W into one 2 ohm speaker (or the same two 4 ohm speakers in parallel) - assuming the amps are happy working in parallel! (total 100W mono).

3) Bridged amps - 1x100W into one 8 ohm speaker (or the same two 4 ohm speakers in series) - (total 100W mono).

As a 'possible' fourth option, as suggested in the previous post:

4) Bridged amps - 1 x 200W into one 4 ohm speaker, each amp is trying to provide 100W into 2 ohms, both amps overheat and are destroyed!.

Notice all the working options (1, 2 and 3) give exactly the same power (100W) into exactly the same two 4 ohm speakers), but option 1) is stereo, the others only mono. Where bridging comes in is where you don't have two 4 ohm speakers, but only one 8 ohm speaker - in which case you can still get full power, but only in mono.

 

[joecool85]

Ok...so...can you help with some specifics?

I'm wondering specifically what the dual mono kit here:
**broken link removed**

Would put out into a 8 ohm load, and what it would do with a 4 ohm (if thats even possible)

 

[audioguru]

There must be a spec sheet somewhere for that kit amp. It is two amplifiers in parallel. The spec's would say which transformer voltage and current to use which would be different for a 4 ohm speaker and an 8 ohm one.
The spec sheet would also say what heatsink size is needed.

 

[joecool85]

One would think so, but I can't find one. I emailed the guy to ask him.

 

[audioguru]

The instructions say they are new and incomplete. They say to look in the IC's datasheet but it doesn't talk about parralleling the amp ICs.

 

[Nigel Goodwin]

Actually, the datasheet DOES specify parallel use, and even suggests using more than two in parallel - it also gives the required calculations for heatsink sizes and power output.

See https://www.electro-tech-online.com/custompdfs/2006/01/LM4780.pdf page 16.

Basically it depends on the supply voltage you use, and the speaker impedance you use. But if the supply you use will give 30W into 8 ohms, paralleling the two amps will give 60W into 4 ohms (same voltage output, but twice the current capacity).

 

[joecool85]

Ok, so from what I gather, the kit from chipamp should put out two separate channels of 120w, that would mean roughly 240w peak correct?

Would I need to make any modifications to use this for a bass amp or is it all in the preamp/eq I decide to use? I know the lm3886 kits work great as guitar amps out of the box so to speak.

 

[audioguru]

Ok, so from what I gather, the kit from chipamp should put out two separate channels of 120w, that would mean roughly 240w peak correct?

Yes if they have the appropriate power transformer and big heatsinks, each amp into a 4 ohm speaker.
I think its supply bypass capacitors should have their value increased to 3300uF or 4700uF for use as a bass amp.

Would I need to make any modifications to use this for a bass amp or is it all in the preamp/eq I decide to use? I know the lm3886 kits work great as guitar amps out of the box so to speak.

It is a wideband amplifier that might need a separate crossover circuit if it is used for a bass speaker.

 

[joecool85]

Cool. Now one more question. Is it possible to use the dual mono kit (which if I understand correctly is one lm4780 per channel running in parallel) and bridge the two channels giving me 240w at 8ohms?

 

[chemelec]

Originally I was going to build a lm3886 based chipamp for use as a guitar amp, but then I found chipamp.com and realized I could buy kits. These kits don't do regular mono, so I figured I'd do a stereo and use both channels for one input.

I Wouldn't use Chipamps for a Musical Instrument Amp.
There is typically a lot of square wave and fast rise times in these signals. And I find these chips Not very relialable for that application.

Descrete designs are better for this and If you do blow a transistor, It a Lot Cheaper to repair than replacing the whole IC. That is if he IC is still available.
They Discontinue some of these IC products so quickly today.

 

[audioguru]

In order to bridge the two amps to get 240W into 8 ohms you would need to invert the signal feed to one of them.
Instead of messing-up the kit which isn't designed for it, an added opamp inverter stage is best. The dual supply voltage would need to be reduced to positive and negative 15V with resistors, zener diodes and filter capacitors to feed a standard opamp.

 

[joecool85]

Originally I was going to build a lm3886 based chipamp for use as a guitar amp, but then I found chipamp.com and realized I could buy kits. These kits don't do regular mono, so I figured I'd do a stereo and use both channels for one input.

I Wouldn't use Chipamps for a Musical Instrument Amp.
There is typically a lot of square wave and fast rise times in these signals. And I find these chips Not very relialable for that application.

Descrete designs are better for this and If you do blow a transistor, It a Lot Cheaper to repair than replacing the whole IC. That is if he IC is still available.
They Discontinue some of these IC products so quickly today.

I dunno...I know a lot of people online that have used the lm3886 for guitar amps with no problems...

 

[joecool85]

In order to bridge the two amps to get 240W into 8 ohms you would need to invert the signal feed to one of them.
Instead of messing-up the kit which isn't designed for it, an added opamp inverter stage is best. The dual supply voltage would need to be reduced to positive and negative 15V with resistors, zener diodes and filter capacitors to feed a standard opamp.

Ok. This is going to require some thought. And the key reason of doing an amp this way is simply, to keep it simple. Discrete amps are pretty difficult to build from what I've seen. Someone show me a 200 or so watt discrete amp thats not too hard to build and I'll be all over it, until then, I'll probably plan on doing it IC style.

 

[joecool85]

Heck, any solid state discrete amplifiers for 200w or so?

(See why I plan on doing it via chipamp)

 

[Nigel Goodwin]

Heck, any solid state discrete amplifiers for 200w or so?

(See why I plan on doing it via chipamp)

Have a look at https://www.ampslab.com/projects.htm for a number of examples.

As I mentioned previously, a LOT depends on the speakers you plan using.

What EXACTLY are you wanting to do?, and EXACTLY what speakers are you wanting to drive?.

The small chipamps are commonly used in plenty of medium and small power guitar and bass combos - but larger ones are usually descrete.

 

[joecool85]

Well, the one I'll be building for myself is pretty simple, a guitar amp using the stereo lm3886 kit. Pretty straight forward. Whats starting to be a problem is building a bass amp for my brother. He wants it to drive a 15" 8 ohm speaker. Like this one:
**broken link removed**

He might have others in mind, but basically if it can drive that he would be happy. He wants me to build him a 200wRMS (or more) head that would drive that for when he is gigging with his bass.

It seems I could bridge the two lm4780 chips in the dual mono (that are running parallel) and get 240wRMS but it would be for 4ohms.

 

[Nigel Goodwin]

Well, the one I'll be building for myself is pretty simple, a guitar amp using the stereo lm3886 kit. Pretty straight forward. Whats starting to be a problem is building a bass amp for my brother. He wants it to drive a 15" 8 ohm speaker. Like this one:
**broken link removed**

He might have others in mind, but basically if it can drive that he would be happy. He wants me to build him a 200wRMS (or more) head that would drive that for when he is gigging with his bass.

It seems I could bridge the two lm4780 chips in the dual mono (that are running parallel) and get 240wRMS but it would be for 4ohms.

I should do that! - giving 120W, he could then plug a second 8 ohm cab in to get the full 240W - most guitar/bass amps work like that!.

I bought my daughter a Behringer BX1200 120W bass combo, it gives 120W into it's 12 inch 4 ohm speaker. It matches well with a 60W guitar amp, and acoustic drums (played HARD!).

I normally put them all though the PA as well, but the original balance is pretty good.

 

[Nigel Goodwin]

Just a thought! - have you considered using class-d power modules?, there are various types available - for an example have a look at https://www.hypex.nl/, a 400W (into 4 ohms) module is €100 - whatever a Euro is worth these days?.