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

[RDV]

I've decided to buy my amps at the store. I added the first stage from the limiter and also tried a BMP vol recovery stage. Both caused an avalanche of hum, as well as the desired volume. I just don't know what I'm doing. To all those who do, god bless you.

RDV

[RDV]

I just don't know what I'm doing...

Boy, truer words were never spoken; however I decided to not let the thing beat me. I put it to the side about a week or so and just tried to suss it all out in my little pea-brain. Being self-taught at everthing you do can be a painful process(psychic pain that is).

So... on turkeyday eve I decided to apply a few theories.

First... I disconnected the preamp and decided to find the rest of the ground loops in the poweramp section. To do this I looked the PCB on GGG which was a huge help(thanks Dean & JD). The last ground loop was between the 100µF cap on the standby & the PS filters on the negative side. Once that was corrected I fired up the poweramp section and plugged my new $10.00 graphic EQ in and Shazaam!!

No more poweramp hum(@ least from ground loops).

Second... I remembered that the preamp sounded sweetest running at 12v off the PS for the cooling fan. I wired that back along with extra filtering and Shazaam!!

It sounded good again(though not very loud).

Third... I decided to attack the 300lb. gorilla of the volume recovery circuit. I used the one at the front of the limiter circuit with a 10µF coupler cap and Shazaam!!

It was loud(but popped and crackled when I touched the strings).

Fourth... The crackle & pop thing amounts to the bad wiring in my old house. The outlet I'm using is a 3-prong modern type outlet, but the ground is not a real ground but wired to the neutral(white) wire which is a disturbing situation concerning my personal safety. I decided to attack this one the "Hartley Peavey way" which is adding a high voltage .01µF ceramic cap between neutral(white) and the chassis ground and Shazaam!!

No more crackle & pop(well, maybe just a little).

Anyways, this thing now sounds good and is loud and noise is at an acceptable level.

The next amp will include PCBs and proper transformers and a chassis.

AMEN.

Happy Thanksgiving!!

RDV

[Eric H]

Very nice sleuthing, Sherlock.

Sometimes you just have to put it down for awhile.

-Eric

[B Tremblay]

Well done!

I'll be pretty disappointed if you don't decide to name it Shazaam!  Or, how about Shazaamp?

[R.G.]

Have you decided not to do the limiter?
Yeah, cause the preamp as built doesn't have enough gain to make the poweramp clip.

From what I see you now have additional gain. I think you'll like the results if you include that limiter. It turns the clipping behavior of the amp into soft limiting from the hard limiting you'll get from the power amp itself.

[RDV]

Here's a modified schem of the preamp as it stands.


RDV

[RDV]

Have you decided not to do the limiter?
Yeah, cause the preamp as built doesn't have enough gain to make the poweramp clip.

From what I see you now have additional gain. I think you'll like the results if you include that limiter. It turns the clipping behavior of the amp into soft limiting from the hard limiting you'll get from the power amp itself.

There are some things I don't understand about the limiter section. I'm using a 12v supply for the preamp and I don't know if that's going to be sufficient at all the voltage points.

I also don't know what the limiter pot does. Does it act as a headroom level? Or maybe like a Master volume? It scares me a bit; fear of the unknown.

We've also got to remember that I still pretty limited by the 56va transformer I used. It's getting loud, but I don't think I've got the juice to really make the chipamp clip. It's also about a foot and a half from my head where it's set up, so I'm only gonna turn it up so loud LOL.

I'm enjoying this a lot though, and I sure appreciate your help R.G. I don't know if too many guys are really interested in building their own SS amps, but I think it's sorta neat.

RDV

RDV

[RDV]

Oh, also thanks to you other fellows who've been following this protracted thread and offering hints and encouragement. It's all much appreciated.

Ricky

[stm]

Ricky, it sounded sad to me when you decided to give up on this. I can understand the frustation you experienced at that time, 'cause I've seen that too.

Now I'm glad to see how well you kicked the ass of every underlying problem. Certainly some time to cool down things helps a lot. I think issues keep running in background, then, some time later a new idea or inspiration appears. I'm sure this is typical for all humans.

The day of my wedding I was in the shower prior to dressing up and go to the priest. Then suddenly I saw the subroutine and lines of code where the error was--I've been struggling for over a month with a piece of hardware and DSP code but it kept hunging up.  I got married, went to Dominican Republic for honey moon, took an extra week of vacation to setup the new home, and then three weeks later I returned to the office and fixed the damn bug in five minutes. Human brain is an extraordinary machine!

A final word of advice: your words of appreciation are in complete contradiction with your avatar image!!! 

[R.G.]

There are some things I don't understand about the limiter section. I'm using a 12v supply for the preamp and I don't know if that's going to be sufficient at all the voltage points.

That's a reasonable point. I think it can be re-converted down to 12V. I'll take a look.

I also don't know what the limiter pot does. Does it act as a headroom level? Or maybe like a Master volume? It scares me a bit; fear of the unknown.

How the heck do you ever burn up parts with that kind of attitude? 

The limiter is a variable clipper. The limiter pot sets the voltage at which the limiting occurs. I refer you to John Greene's post here for the action of the limiting pot. It is not a master volume, or a headroom level. It's like having a pair of clipping diodes ahead of your power amp that can be set to clip at any of a range of clipping voltages, not just +/- 0.6V.

We've also got to remember that I still pretty limited by the 56va transformer I used. It's getting loud, but I don't think I've got the juice to really make the chipamp clip

Making the chip amp clip is not the point. The idea is to have the limiter limit the signal **before** the chip amp clips. Having the limiter stop the signal at the input from getting large enough for the chip amp to clip means that the chip amp itself is always in its linear region. It never clips. The limiter prevents it from ever having to.

And as to whether you have enough signal to make the chip amp clip: The chip amp is an opamp with a monster output stage. It's only compensated to be stable for gains of 10 or greater, but it's an opamp nevertheless. What would happen if we drive it from +/-30V supplies, and set it up for a gain of 100? The input needed to make the output swing +30V is only 0.3V. You could make it clip driving it with some humbucker pickups directly. If we set if for a gain of 10 (the lowest we can), then a voltage of 3V peak drives it to clipping on the output.

If we have +/-30V supplies and a gain of 10, and also a limiter in front of it set to clip at +/-2.7V, then the limiter prevents the input signal that the amp sees from ever reaching 3V peaks, and so the chipamp never clips - the limiter keeps the input from going that high (or low).

If you put two clipping diode sets in series in front of the chip amp, then the input to the chip amp is limited to +/-1.4V peak by the diodes. The chip amp can then have any gain lower than the power supply divided by 1.4V and not clip.

Does that make sense?

[RDV]

A final word of advice: your words of appreciation are in complete contradiction with your avatar image!!! 

That's Johnny Cash flipping the bird to "the Man". It's not intended to reflect my attitude toward friends. 

RDV

[RDV]

Does that make sense?

Of course. All your posts make sense. I'll probably make another daughter-board(the 4th) and try the limiter for the heck of it. This whole contraption is on that brown RS perf. I'll report back with the results.

RDV

[octafish]

Lokking forward to your results. I'm less than thrilled with the tone stack though, while I like the MRB, the treble and bass control seem to do too little to me. I may replace it with a baxandall/james stack. If I had the nous I'd add the MRB to the james but I'm pretty limited.

[RDV]

I added the limiter to the mix and while subtle I think it's working fine. As I suspected to get it working I had to turn the amp up WAY louder than I ever will just sitting here at my desk which is also in our TV room. I couldn't have it up very long like that but I think as I turned up the limiter level(lowering voltage I think) it seemed to lessen the volume a bit. It still basically sounds the same though which is good cause I like a loud clean amp.

As far as the tone controls, I like them though all the action happens in the 1st half turn. I like the voicing they have. Too wide ranging tone controls have always been hard for me to dial in.

RDV

[R.G.]

I did some reverse engineering on your power. With a 36Vct transformer, you get 36*1.414= 50.9V peak to peak. That's 25V peak, less a couple of volts for sag and amplifier losses, so you could put maybe 22V peak across an 8 ohm load. Thats 15.6Vrms, and into 8 ohms that's 30W.

And it's an honest 30Wrms out of that chip. That's LOUD. The Vox AC30 is 30W rms, and it's LOUD.

If your speaker is 90db and 1W/1M (and that's common for guitar speakers; some are more like 96db), then 20db up at 10Wrms is 110db, and from there to 30W you get another 4.7db - the spl out of that thing can easily be 114db at  one meter.

That's not a small amp.

[RDV]

You're not kiddin' about loud. You could gig with this thing! It's as loud or louder than my old Peavey Bandit 65. It's zonking the holy livin' crap out of the little eminance 10"s I've got it plugged into. I think an all inclusive PCB for this amp would be really cool. You could sure build a worse SS amp(& many have!).

My meter tells me it's producing 27.5v +/-

Whee!!

RDV

[R.G.]

Does your transformer get hot when you run it full blast for an hour or so?

[RDV]

Does your transformer get hot when you run it full blast for an hour or so?

I couldn't take this thing full blast for an hour in my house!
I've not felt much heat from it at all though, even if it stays on all day.
One reason may be the 12v cooling fan that runs over the whole thing as long as it is on.
I used a small heatsink for the chip, so I had to use the fan.

RDV

[R.G.]

OK, special advice.

As I tell people a lot, power amps are essentially a power supply with some other junk on them that lets a little of the power out carefully. That's not a complete description. Power amps are also heat-exchange engines as well as power supplies.

Before you make yourself a sealed up amp, go to ebay and buy one of the flat-back heatsinks that come up there every now and then. You can get on for under $10 that's heavy with lots of fins. 3" along the fins, 6" wide, and maybe 1.5" to 2" fins would be good. Make that heatsink part of your amp. Ditch the fan. You have the beginnings of a very good amp.

[RDV]

I use a 50watt Marshall to gig with, but am retiring to devote a bit more time to my children(@ least for a while) after New Years, but were this amp to be anything but a bench amp I would do many things differently. It's not going to do any traveling in it's present state for certain. A total of 6 seperate pieces of brittle brown perf and two PTs make this one go right now, so I don't really know if it could ever be made exactly road-worthy. I've got another LM3886 that may be going to be made into a real amp with chassis, heatsink, PCBs, proper 300va tranny etc.

I sort of knew going in that this one would be a learning vehicle and perhaps nothing else. The fact that it's turned out so well is just icing on the cake.

I will definitely be on the lookout on eBay for heatsink material and torroidals.

RDV