LM3886/Vox SS Amp Thread(w/ Samples!)

[RDV]

Since I'm driving my sub-amp with the aux out of my Mackie mixer shouldn't I just be able to accomplish my sub filter needs by just putting a twin T low-pass filter in-line from the aux out to the amp input tuned to the appropriate freq?

What if I put this in-line between the mixer and amp:


Wouldn't that give me a low-pass at 160Hz with a 12db cut of everything above that? I'm just lost when it comes to simple R-C circuits. It's one of the many reasons I didn't pursue an electrical engineering career. Sad but true. I never really did grasp ohm's law and it was & is very frustrating. I don't even know what the reason for putting two filters(like this) together is. If someone knows of a basic RC filtering article on-line I would be grateful for the info.

Unless someone knows the answer please don't reply as I want someone who does know to see this.

No offense meant.

TIA!

RDV

[R.G.]

No, it won't. It will be a lowpass filter, and it will eventually get to a two-time-constant slope, but it won't be what you think you're getting.

The problem is that the first filter adds source impedance to what the second filter sees, and the second filter loads the first one, so the frequencies and rolloffs are not what you expect. It works if you feed each filter with a low impedance source and buffer after it.

RC filters see whatever drives them and whatever loads them as part of the filter.

[RDV]

I wish you had a filter tutorial on GEOFEX.

Not that I don't appreciate everything that is there. I do.

I'm just a bit frustrated with my brain these days.

I didn't feed and care for it properly in my youth and I'm paying for it dearly now.

RDV

[RDV]

I guess I'll burn some parts and experiment tonight. I've got quite a few pre-wired buffers and what-not. I might come up with something decent by accident.

Who knows?

RDV

[wui223]

i am having the same thought in my mind now.

[RDV]

Tried this when I got home to see what would happen.


It sort of works but lowers the level a bunch.

I'm going to use this until I build my sub amp and cabinet though because along with the graphic in my old Peavey Monitor head I'm using to power my temporary sub it's filtering enough above 225Hz to give me my stereo image back to my near-field monitors. This is DIY-rigging at it's best!

I'm hearing that my old Peavey monitor cabinet is no sub cabinet. The highs and mids that were coming through before were masking the fact that it wants to blow apart!

RDV

[Joecool85]

I would use a smaller resistor.  Something on the order of 8-10 ohms.  But this is from theories bouncing around in my head...

[RDV]

I would use a smaller resistor.  Something on the order of 8-10 ohms.  But this is from theories bouncing around in my head...

I was fresh out.

My plan was a 10 ohm, 100µF combo.

RDV

[Joecool85]

Thats about what I was thinking.

**edit**
Actually, do you have another 15ohm?  You could run it in parallel for 7.5ohms.

[RDV]

Thats about what I was thinking.

**edit**
Actually, do you have another 15ohm?  You could run it in parallel for 7.5ohms.

I used this to calculate the cut-off. I'm not too unhappy with the result.

RDV

[Joecool85]

Cool, good to hear it works pretty well.

[RDV]

I'm very intrigued by the concept of a passive in-line crossover. Mine seems to work a bit, and here's a company that sells them.
I'm interested of course in how this would work(or perhaps if) without any active components.
Mine seems to work because it's just a simple 6db RC filter, but this guy is claiming a 12db filter without added components.

Any theories?

RDV

[Joecool85]

I don't know how they work, but I know they do.  Crutchfield sells similar passive crossovers.

[RDV]

I don't know how they work, but I know they do.  Crutchfield sells similar passive crossovers.

My guess is they work like the one I just built. They filter well enough but they must attenuate the signal a great deal. Mine does. Even with a buffer. I guess I'm gonna run a gain stage 1st.

RDV

[Joecool85]

I dunno...I don't think they cut too much signal strength, at least not in car audio use.  For car audio you put it inline with the low signal level.

[RDV]

There's some trick I don't know then.

*Sigh*

RDV

[Joecool85]

Well, I suppose it might be losing some signal strength, but since it is right before the amp you wouldn't notice because you would just turn the amp's level up more.

[RDV]

Cost of Stereo LM3886 amp

This is a stereo amp I'm using to run my small monitor speakers.

2 ea. LM3886TF - $13.00
1 ea. Dual Mono LM3886 PCB from chipamp.com - $26.00*w/shipping
1 ea AC Receptacle - $.50
1 ea Computer AC Cable - $0.00
1 ea. AVEL Y236503 160VA 22V+22V TOROIDAL TRANSFORMER - $46.99 w/shipping
1 ea Chassis 2 rack space aluminum enclosure - $0.00**
2 ea 10,000µF 50v Electrolytic Capacitor - $7.34
6 ea 100µF 50v Electrolytic Capacitor - $.60
2 ea 47µF 50v Electrolytic Capacitor - $.14
1 ea 10µF 50v Electrolytic Capacitor - $.07
6 ea .1µF 50v Poly Capacitor -  $1.44
8 ea 8.0 Amp 600 Volt Diodes - $10.48
1 ea 1N4004 Diode - $.05
3 ea 10k Metal Film Resistor - $.30
4 ea 22k Metal Film Resistor - $.40
4 ea 1k Metal Film Resistor - $.40
2 ea 680ohm Metal Film Resistor - $.20
2 ea 2.7ohm 2watt Metal Film Resistor - $.64
2 ea 2.2ohm 2watt Metal Film Resistor - $.64
1 ea Green LED - $0.00***


Total: $109.19




*Came with 2 PS PCBs, only used 1, saved 2nd for next project.
**Broken(and unfixable IMO) Carver CD Player from the 80's had 2 space rack mount chassis. A comparable chassis from Par-Metal.com would be around $50.00
***Freebie from the wonderful Brian Marshall. LEDs are cheap.

RDV

[RDV]

I've changed the name of this thread because it has evolved a bit and this may make it easier to find with the search.

RDV

[R.G.]

That's a good illustration of something I keep saying - a power amp is mostly power supply and other things that are NOT the power amp itself.
Here's what I think we can credit to the power amp circuit:

2 ea. LM3886TF - $13.00
1 ea. Dual Mono LM3886 PCB from chipamp.com - $26.00*w/shipping
6 ea 100µF 50v Electrolytic Capacitor - $.60
2 ea 47µF 50v Electrolytic Capacitor - $.14
1 ea 10µF 50v Electrolytic Capacitor - $.07
6 ea .1µF 50v Poly Capacitor -  $1.44
1 ea 1N4004 Diode - $.05
3 ea 10k Metal Film Resistor - $.30
4 ea 22k Metal Film Resistor - $.40
4 ea 1k Metal Film Resistor - $.40
2 ea 680ohm Metal Film Resistor - $.20
2 ea 2.7ohm 2watt Metal Film Resistor - $.64
2 ea 2.2ohm 2watt Metal Film Resistor - $.64

Which adds up to $43.88 if I added right.

And here's the non-power-amp-circuit stuff:

1 ea AC Receptacle - $.50
1 ea. AVEL Y236503 160VA 22V+22V TOROIDAL TRANSFORMER - $46.99 w/shipping
2 ea 10,000µF 50v Electrolytic Capacitor - $7.34
8 ea 8.0 Amp 600 Volt Diodes - $10.48
1 ea Chassis 2 rack space aluminum enclosure - $0.00**
1 ea Computer AC Cable - $0.00
1 ea Green LED - $0.00***

Which adds to $65.31.

The power transformer itself cost more than all the power amp circuitry and purchased PCBs.

The imbalance gets even worse when we notice that all the free stuff was non-poweramp, and that the cabinet could have added more cost than the power amp circuitry; even worse when we note that the PCBs could have been made at home for under $10.

This is the kind of stuff that people forget when they decide to build a big power amp.

By the way - you did a great job of holding costs down. Getting to under $1 per watt is a tricky goal in any kind of home built power amp that looks presentable.