@ OP: if you want to play guitar powered only by solar power, use an acoustic guitar and become a vegan.

Quote[...] The unit runs 2x LM3886 in parallel driving a 4 Ohm woofer and a single LM3886 for tweeter.
[...] but that chip was too hot to touch and measured rail voltage on the output which obviously burnt out the tweeter.
[...]No real heat sink except for a 6mm flange which is also bolted to outer alloy case.

You already know the answer then.
Read this:
https://www.ssguitar.com/index.php?topic=16.0
It contains a lot of my advice to RDV about heatsinking LM3886s. If you simply attach a chip amp to a flange and hope the chassis will take care of it, you;'re almost certain to be disappointed.

Even if an adequate heatsink is used, the mounting is critical. It simply MUST be in good THERMAL contact, so the heat can get to the adequate heat sink. Loosened mounting bolts can undo whatever good a heat sink might do you.

In this case, it is entirely possible that poor operating conditions made the LM3886 oscillate. That's a suspicion based on (1) it was a Behringer, (2) it was attached to a tweeter (3) the heatsinking wsa inadequate. Oscillation would make it dissipate a huge amount of heat, and any inadequate heatsinking could let it fry before the thermal protection could cut in.

There is nothing inherently wrong with the parallel use of LM3886s if you do it right, use the right precision parts, and pay proper attention to thermal issues. But just like the use of fire, you can do a bunch of things wrong before you get smelting metals and internal combustion engines to work.

There is no substitute for knowing what you're doing. Like the song says, "you gotta know when to hold 'em, know when to fold 'em".

Oh, and I forgot to add - I'm funny about just blankly trying this, too. That was one other thing that voted for "why worry - just do one amplifier per speaker".

I have nothing but respect for your technical expertise and experience.

However, I did a look at the numbers, and given:
1) well behaved chipamps, like the LM3886 (perhaps excluding others; I did not do the math there)
2) good heatsinking
3) precision feedback resistors and biasing resistors
paralleled LM3886's can be made to work reliably.

My professional and personal experience up to very recently would agree with your position. However, if TI went to trouble to write up an app note on bridged and paralleled amplifiers with the LM3886, and go so far as to publish their results. I know, I know, lies, d@mned lies, and app notes.

I have read several sendups on the web of people successfully getting the paralled version (and bridged+parallel version) to work.

I calculated the cross currents and the watts they'd add to the chips for the offsets and for the cases of worst case 1% and 0.1% resistors. The math says it's untenable with 1% resistors, but pretty well acceptable for the 0.1% case.

When I was starting, I was told to figure out how to design things with 10% resistors, saving 5% for closer tolerance work. I had to get higher engineering approval for 1%. A few years later they encouraged us to use 1% whenever needed. 0.1% resistors were not in any catalog I knew of (these were on paper back then). But today I can get literally thousands of values and types of 0.1% resistors from my local distributor overnight. So I've had to re-think what it too demanding to even try.

You are correct - this is not a situation for use by one-week-wonder geniuses. But It's not all that far off in the tall weeds, either.

QuoteParalleling *voltage*  sources is a disaster waiting to happen.
The additional condition that now gain must track each other within 1% or less, any volume, any frequency, plus NEEDING cheesy series resistors to somehow help speaks volumes about what a poor Engineering choice it is.

Well... maybe. Pretty much no really high power amplifiers in existence get by without emitter resistors. The emitter resistors are there to wash out the microscopic differences in Vbe. Engineering has pretty well accepted that sometimes you can't get big enough power devices and you're going to have to parallel them sometimes.

In a big high power amp, you're going to have one 0.1 ohm resistor as an emitter ballast per output device. It's only a trivial amount more waste to use one 0.1 ohm resistor per amplifier to be able to gang up N amplifiers.

The other issue is the 0.1% resistors. That used to be an impossible hurdle, but Mouser Electronics (sorry - I know this won't help you much) lists a few thousand 0.1% resistors in stock in just it's film resistor category. The 1K 0.1% and 22K 0.1% resistors that would be used for a common setup on the LM3886 and most discrete power amps in 1/2W are from US$0.25 to US$0.50 each, although you can buy much more expensive ones.

So ballasting and precision resistors are a pain, but it's a pain you can deal with.

As a side note, I am at least daily amazed at the tech items that I thought impossible in my youth that are available in days for reasonable prices. Sometimes I'm stunned at this topic.

QuoteNot sure I follow you: you use 4 x 3886 in parallel to drive a 2 ohm cabinet through a single cable pair or a special connector with 4 individual speaker cables? Not problem in a combo, not so sure in a head+cabinet situation.

Yeah, that's how it looks at first blush. Combo, no problem. Head and cab, you get into funny speaker cables or replacing speaker cables to run many outputs.

Neither of those is what I really foresaw. I actually wanted to use powered speaker cabs. This is beyond the Thomas amps, but it's a decent solution for a gigging musician. For practice, you need only a few watts, maybe only one. For band practice, you only need maybe 30-60W. For a small bar gig, 60W is plenty and 30 may be enough. For bigger halls, and especially bigger spaces, you might need 100+ watts. If your unit of power was one 12" speaker and a 30W amp driving it, you can lift the speaker enclosure and internal amp with one hand, like a small combo. Need 60W? Pick up one powered speaker in each hand. For 120W, you need to make two trips to the van. If you're playing a soccer stadium, you might need six or eight. The nice thing about this idea is that it scales nicely. It's similar to the way that all PAs are set up today. And you don't need to replace your preamp setup. Just glom on more power amps and speakers.

But before I get off into the ether, yes, you would probably have to do special purpose connectors and speaker cables.

As usual, I have a mildly heretical viewpoint.   

Chip amps do have fundamental limitations on their ability to dissipate power. As a practical matter, the TO-220 packages like the LM1875 can only really do a 20W amp out at the edge of reliability. The LM3886 and similar packages can get to about twice that and be reliable without gigantic heat sinks.

My solution to this is to use the LM3886, which is an incredible deal at about US$5 each, and use more of them. There are circuits to parallel them up for higher currents and better power dissipation, and to run them bridged for higher voltage. However, I would not use them that way.

The LM3886 is quite reliable (given a decent heatsink) at 30-40W output. Most guitar amps run one 12" speaker on about 30-50W of amplifier power. When it gets over that most amps run two 12s or four 12s. Why fight the 30W per speaker practice? Why not make one highish quality amplifier per speaker. They're easier and more reliable that way, as well as more adaptable to situations and portability.

I just went through this in designing a replacement/repair power amp for the Thomas Vox amps. I opted for using an LM3886 per 30W speaker load. This covers the Buckingham and Viscount with 8 ohm loads with one amp each. The Royal Guardsman needs two modules to run two 12s at 60W, and the Beatle needs four for 120W.

It is far easier to get a 30W amp to run right than to get a 120W or higher to run right. And replicating several small things can be simpler than making one big one.

I did a PCB that is about 2" by 3" and runs one LM3886. I found a suitable heat sink for under $8 at heatsinksusa.com. Antek offers the AS-0522 toroidal transformer for $17.50, and this is a nearly ideal transformer to power the LM3886. You can fit the whole amplifier on one heatsink serving as a sub-chassis, power supply included.

And if one fails, you've thoughfully made one more power amp module than you really needed, so you swap in the spare while you fix the broken one.

Just sayin' ...

OK then. I didn't mean to talk down to you. It's just that there are hordes of people wanting to work on AC that really don't have any idea about how it can make them dead. Given that you're experienced, let's forge ahead.

Your comment actually set me off on some more grinding it out on a Thomas Vox Guardsman head that's on the bench. Same preamp setup as your Buckingham. Here's some fresh info.

It really is likely that your amp has either a partial connection to the chassis from the AC wiring or a fault in the line reverse switch or cap. It is possible that this is even a short from the primary side of the power transformer to the transformer iron. That's rare, but happens. Your ohmmeter will be your most valuable tool here, as it will just tell you both that there is a low-resistance connection to the chassis. It's also possible that there's some contamination or wire issues in the cable between the preamp and power amp, as the 120Vac wires run through that cable.

I have better info now on the grounding scheme. With the additional sleuthing I did, there are two wires I found that are apparently on every Thomas Vox "big head" amp that I cannot find on any of the factory schematics. These are a wire bringing power ground to the preamp circuits and a wire attaching power ground to the power amp chassis. They pretty much had to be there, but were undocumented (in the technical sense    )

Once you run down that AC fault, here's how to make your Buckingham head very quiet.

It turns out that I was wrong about needing the 12position connector. There are just enough positions in the 9-position connector in the cable to do a decent grounding job. Of course, this is made worse by the fact that I can't find where to get the terminals to modify that particular cable connector anywhere. So changing to third wire ground or any other changes to the cable wiring will need a new connector. Both Molex and Amp make polarized cable connectors suiltable for this. I like the Molex "MLX" series; You can get the full set of plug, socket, and pins for a 9-position connector replacement at Mouser for under $8.00 all told.

The biggest hum reduction is from cutting the input jacks loose from the preamp chassis. Switchcraft and Amphenol both make nylon body jacks that will fit the same holes as the originals, and Amphenols are only $0.80 each. So replace the input jacks with nylon-sleeve jacks, and run a ground wire from each jack to the section of the PCB it goes to. Thomas Organ actually provided a field upgrade kit for this to its service centers. No changes to the cable needed.

Next hum issue is third wire ground. For this, you need to bond the incoming AC power cord safety ground to the preamp sheet metal, then run an additional wire through the cable to the power amp chassis, and bond it to the power chassis. That means you'll need a new connector setup as mentioned if you do this. If you don't run a separate ground wire in the cable, third wire grounding only the preamp chassis will increase the hum you get at the end.

That leaves one position open in the cable connector. To really get lower hum, you can run a ground wire there that carries just the ground return current for the indicators. This removes the sawtooth ripple 120Hz current from the indicators, and cuts some more millivolts of hum out of the speaker output.

I'm still writing and drawing this up for inclusion in the Vox Owner's Safety Net, but you needed the tech info, as you're right at the point of fixing one with these issues.

Somebody call a tech guy?


First - don't mess with that any more if you are not already skilled and experienced enough to work on AC power wiring >> safely <<. It's easy to get a bad shock or get killed if you are not already skilled enough.

As a side thought, it's >possible< to get yourself killed with AC power wiring even if you are already skilled and experienced enough to work on Ac power wiring.

What you describe, a significant spark when connecting a wire between an earth ground and the metal chassis, sounds like more than just trivia, AC leakage. It sounds like there might be a significant fault in the AC wiring. This might be in the line reverse switch or the AC "death cap".

Before you do anything else, get out your digital multimeter, set it to read AC volts and read the voltage between the accessible metal on the preamp (that is, the knobs and jack bushings) and the earth ground connection you're worried about. Test this in both positions of the "line reverse" switch. One position of the line reverse switch should be significantly more voltage, but neither position should be more than maybe 20-30v. If one or both readings are more than that, there are additional tests to run, but those are in the "gotta be skilled" category.

I don't mean to be talking down to you if you already do have AC power wiring skills, I just don't want someone to get in over their head with something dangerous.

Having done the due-dilligence warning, there are some issues with three-wiring the Thomas Vox amps.  They are set up in a funny way compared to most guitar amps, with a cable between the preamp and power amp chassis that carries AC power and DC back and forth, and a somewhat delicate signal ground setup. the more I get into these amps, the more complex the situation appears. I have actually written a significant new set of tech literature on the Thomas Vox amps, both a main overall book, and dedicated per-model supplements, far more than the factory schematics information. The "Vox Owner's Safety Net" is the main book, and it talks a bit about three-wiring. You can find this service stuff at thomas-vox-repair.com if you're interested.

To three-wire them, you need to remove the wiring from the "line reverse" switch, attach the incoming safety ground wire from the line cord to the preamp chassis with a dedicated safety ground screw, then run an additional safety ground wire through the cable to the power amp chassis, and attach it to the power chassis with a dedicated safety ground screw to that chassis as well.

Doing this will make the amp safe, but also induce a certain amount of hum. This is because Thomas had some grounding problems that they solved by floating the preamp chassis on signal ground as carried to it by the RCA signal jack from the power chassis, not the main power ground in the power supply in the power chassis. There are some funny reasons for this, but it's a complex issue to me, and I've been working signal grounding for a few decades.

The best theoretical solution is to change that 9-pin connector between the two chassis to a 12-pin universal make-n-lok type and add additional wires to fix both the grounding and safety issues. This is again an issue for a skilled amp-tech.

The Zobel network is a protection against oscillation caused by an overly inductive load. The damped inductor LR network in series with the output of many amps is protection against overly capacitive loads. Pretty much 100% of hifi amps use both.

It's entirely possible that this is simply a thermal problem, too. If heat sinking is inadequate, even the internal thermal shutdown may not save the chip after zillions of overheat cycles. That can be as simple as incorrectly torqued mounting bolts holding down the semiconductor to the heat sink, or too much force that tilts the metal tab on the heat sink.

Just speculating.

OK then. If it went into your own personal pedals and sounded fine to you, that's all that matters.

As you can tell from the range of answers you got, the pedal world has been filled with a combination of charlatans and the deluded who think some magic somehow attaches itself to sound as it wanders through your resistors. This is of course false, but the advertisers are quick to say *anything* that will make another sale.

Long-timers in this forum and others have seen so very many people spend about one week learning which end of the soldering iron to hold while soldering, build a first pedal, then open up a boutique pedal shop, referring to themselves in the first person plural and describing vintage components lavishly. The first few posts of these people are often worded like yours.

Forgive us our mistake.

Quote from: LJN on March 17, 2016, 02:55:41 AM
[...]1/2 watt carbon composition types

Organ makers used 1/2W carbon comps because they were the *cheapest* resistors back when the organs were made. The organs had signal levels of several to many volts so the internal uglinesses of the carbon comp resistors were not very noticeable.

QuoteI'm planning on using them in my more important fuzz pedal builds.

?? More important fuzz pedal builds??
Really?

QuoteThey seem to work really good in my Tone Bender MkI clone.

How exactly are you measuring "good"?

QuoteIn fact, it sounded better to me after changing over to these old resistors, which have drifted with age.

In formal psychological research, designing experiments to find out what people really like or not requires that the experiment hide what is being tested because people simply cannot be objective about outcomes on tests they make themselves. They will like **whatever** they do, thinking it makes a positive change.

So what this proves is not that carbon comp resistors sound bettter, but that you have the response to your own tinkering that a human would.

QuoteAnyway, I'm wondering what you guy's opinions are on the subject

First, some facts, rather than opinions.
Carbon comp resistors have excess noise above what pure carbon would as a result of their construction. Carbon film resistors have lower noise. Lower yet is metal film, and lowest is wirewound. Carbon comp resistors drift worse than any of these, and have a higher failure rate. Carbon comp got the reputation for "good tone" because they also have a high voltage coefficient of resistance, and so in vacuum tube circuits where the voltage across the resistor varies by something over 100V, they can introduce a little soft asymmetrical distortion, which guitarists have a sweet tooth for.

My opinion is that it's silly to introduce noisier, driftier, more-likely-to-fail parts into a pedal build *unless* what you're really after is advertising bragging rights to sell the pedal to less-well-informed people.

Quote
and if a few hundred ohms difference is really enough to significantly affect the outcome.

No. A few hundred ohms is not enough to significantly affect the outcome.

Being human is, though.   

I just got the first protos of two more PCBs for the Thomas Vox line. These two boards go on the power chassis. One holds a pair of snap-in style electro caps and a bridge rectifier module and replaces the two twist-lock can caps in the original. The other implements the power amp circuit board from the originals, for any of the 30W, 60W or 120W amps. This renews the power chassis on any of the Beatle, Guardsman, Viscount, Buckingham, Westminister, Sovereign, and Scorpion amps.

Quote from: engelheimer on December 18, 2015, 08:41:32 PM
I posted this on the Marshall Forum last week and they gave me the boot,

That's a pretty ominous start.

Quote
Swap the 7815/7915 TO220's with 7818/7918 TO220's, with the +3 and -3v supply boost you get headroom and crunch out the wazoo.

That doesn't make any technical sense. The difference between ±15 and ±18V doesn't make that much difference to the signal an opamp can put out. Even if it were clipping at 15V and not at 18, the difference in signal level is only about 1.6db.

This might be significant if there was something following the opamp that clipped sharply at, say ±15.5V, but that is antithetical to the idea of "increased headroom", and increased headroom is also antithetical to "crunch out the wazoo".

Doesn't make sense as stated. Of course, you go on to say:

Quote
My objective here is to entertain, annoy, confuse and send others off on tangents.

So maybe you're being absolutely straight with us - you're here to cause havoc. Most forums do not take kindly to that. There's quite enough of that with everyone trying to play nice.

You may want to rethink your approach to forums; otherwise many forums may give you the boot. I've seen it happen before, to others with what turned out to be similar agendas.

Quote from: plugger on October 19, 2015, 11:00:51 AM
Can someone explain why substituting one op amp for another in a preamp stage [...] would result in more gain through the preamp stage? I can't really square this with my understanding of how op amps work. [...]

Why would this be?

It probably isn't. At least it isn't until you look more closely at the word "gain".

If by "gain" what you really mean is "distortion", like many people casually and incorrectly use the word, you probably do get more distortion. This can easily happen if the opamps happen to have different degrees to which the outputs can approach the supply rails. An opamp that can only go to within 3V of its power supply rails will distort sooner than one that can approach the rails to within a volt, and be more distorted with the same signal voltage an power supply voltages. And it will sound different to one that can go to within 0.3V of the negative supply but only 3V of the positive supply, all of which exist as standard opamps with compatible pinouts.

Ack - I already got a proofreader on that one.

You want to volunteer for the next booklet? There's going to be several on different parts of the Thomas Vox line.