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M³ amplifier build discussions thread - Page 50  

post #736 of 828
8/31/05 at 12:12pm
Thread Starter 
Quote:
Originally Posted by GregVDS
Is a simple twisted pair coming from RCA connectors on the backside to the front switch - crossfeed is sufficient (+- 1 foot, 30cm), or should I better opt for the shield-braided cable (like a microphone cable). Is here silver wire interesting, or normal copper is ok?
Simple twisted pairs are good for less than just a few inches. If you go longer than that (such as with crossfeed), then I suggest using shielded cables. I don't really believe in fancy silver wiring inside the case, especially when the distances are so short. Copper is fine.

Quote:
Again, form the output R, L and G, should I use two wires from ground to output R and L in order to have twisted pairs from pcb to neutrik jack?
It is not necessary to twist the output wires, but if you want to, you can just twist the three together.

Quote:
And also, the output ground is ok for shielding? the input ground is ok for shielding too (since they are not the same)?
You don't need to shield any of the output wiring. For the inputs, if you shield it, just use two shielded cables, one for each channel, and connect the shields together for the signal ground.

Quote:
And last, Is there a minimal thickness for internal separation between M³ and PSU pcbs, in terms of aluminium foil I mean. I plan to build a very tight enclosure, to be able to have it with me at work, and come back home with. Hence, the psu and M³ will be very close. I was planning to separate them by an aluminium plyed foil (0.8mm thick), shielding the psu. The outer box will be in aluminium Alodined internally.
Aluminum does not shield against magnetic interference from the power transformer. A ferrous metal may work better, but only if you completely enclose the PSU in a box. There is really no substitute for sheer distance. Assuming you use a low-leakage toroidal transformer, you should still allow at least 4 inches of space between the transformer and the amp board if you want a quiet noise floor. I can't emphasize this enough: It doesn't make sense to build a "maxxed" amp with fancy boutique parts, only to spoil it with hum and noise induced from the transformer. The best solution is to put the PSU in a separate case than the amp and use an umbilical DC power cord, or put it in the same case with a lot of space between the two.
post #737 of 828
8/31/05 at 12:24pm
Thread Starter 
Quote:
Originally Posted by Dougigs
That's pretty persuasive. In fact, it seems tempting to try a completely unregulated (but filtered) supply, since between the capacitance multipliers, the rail splitter, the various current sources and the intrinsically low PSRR of the op amp front end, we've got vast amounts of regulation already... An unregulated supply (with, say, a good-sized transformer with 4% regulation, followed by a rectifier and a string of 1000 uf low-esr caps) would have a much lower impedance and maybe give the output stage more bang.
While the PSRR of the amp might be good, I do not recommend an unregulated supply. If you look at the output of such a supply on the scope you'll see sawtooth-shaped ripples that even very large filter capacitances won't cure effectively. Perhaps more important, an unregulated supply will be at the mercy of your AC line's voltage stability. The resultant DC voltage could vary significantly.
post #738 of 828
8/31/05 at 12:34pm
Quote:
Is a simple twisted pair coming from RCA connectors on the backside to the front switch - crossfeed is sufficient (+- 1 foot, 30cm), or should I better opt for the shield-braided cable (like a microphone cable).
You should definitely use coaxial cable for the input leads. 1 foot is way too long to go with twisted pair with the added hum and noise source of the power supply inside the amp case. Be sure to position the power supply on the output side of the amp, not the input side, and either space it several inches away, or use a ferromagnetic shield.

Quote:
And also, the output ground is ok for shielding?
If you meant, "can I use output ground to shield other signals?", the answer is no, output ground is only for the headphone jack and nothing else.

Quote:
Looking at the dielectric constant of material, I saw Cotton is even better than teflon.
Cotton is a poor choice for insulation because it degrades over time. Stick with teflon or PVC.
post #739 of 828
8/31/05 at 3:24pm
Ok,

So I should let down my one box design and begin to plan a two boxes one. I already made so much different kind of boxes, but only on plan. Ok, let's go again. But you surely are right.

Thanks again.

GregVDS
post #740 of 828
8/31/05 at 3:35pm
Thanks Morsel for these, but as AMB and you and all in fact want me to position the psu so remotely from the M³ pcb, this would need a really too big enclosure (4 inches minimum?! sure but the box is nearly 40 cm wide then), so I will now try to plan and build a two boxes enclosure. That's the best way to go it seems.

Ok, coaxial so, does the kind of mono jack to jack electric guitar cable does the trick, usually, this is a central conductor, in teflon, in ground braid around and pvc out. I was wondering if multi-wires was not as good as mono-wire, regarding skin effect and things like that? does a 22 to 24 gauge mono wire shielded cable exist, apart from stock TV coax antenna cable?

Thanks for any suggestion,

All the best

GregVDS
post #741 of 828
8/31/05 at 3:49pm
Either a mono coax cable or a stereo coax cable will work. Some people find it easier to use stereo coax cables as wires are easier to attach to the pcb and/or panel jacks than the shield, which can be tied to the ground wire independently.

As for positioning the psu, if it is on the output side of the pcb rather than the input side, that is 3 inches from the sensitive inputs already. You can try it out that way before committing to a specific casing choice. There is no way we can tell you in advance how many inches of distance will be required.
post #742 of 828
9/3/05 at 8:35am

Basic knowledge required in M³ building

I'm so sorry to ask this, for this is barely acceptable not to know this, but I'm uncertain, and don't want to make any mistakes, so laugh at me, banish me, I really deserve it!

First (so ashamed):

I usually use the plastic sheet markings on capacitors to know which leg is the + one, but I know also that one leg is longer than the other. The longer is the + one, right? (recorded laughters here, tomato shots allowed, no: required!)

Second (idem but a little bit more, if possible):

After putting a component through the pcb, should I cut the legs before or after soldering them to the pcb? In both case, what lenght is useful, 1mm, 2mm, far less, even longer? (Ok, I'm leaving now, I'm sorry ;-)

Do accept my apologies for these, ok?

GregVDS
post #743 of 828
9/3/05 at 8:42am

power cord between psu and amp needs shielding?

Ok, AMB,

another one then:

what about the noise collected by this umbilical cable between the psu box and the amp box, this cable should be shielded too, no? The shield should be connected to V-, or ground of the psu box, right?

And inside the amp box, the signal shielded cable could be stereo one with shield, using the stereo pair for signal + and signal ground, the shield being connected to the ground at the input connector? Is this ok?

Is a shielding braid which is not connected to ground still shield something, or nothing?
post #744 of 828
9/3/05 at 8:45am
Quote:
Originally Posted by GregVDS
I usually use the plastic sheet markings on capacitors to know which leg is the + one, but I know also that one leg is longer than the other. The longer is the + one, right? (recorded laughters here, tomato shots allowed, no: required!)
Correct on the caps i've looked at- but always double-check the caps markings


Quote:
Originally Posted by GregVDS
After putting a component through the pcb, should I cut the legs before or after soldering them to the pcb? In both case, what lenght is useful, 1mm, 2mm, far less, even longer? (Ok, I'm leaving now, I'm sorry ;-)
I always trim them flush after soldering- but hey thats just me

No apologies needed- we're all new once (i still am )
post #745 of 828
9/3/05 at 9:05am
Thread Starter 
Quote:
Originally Posted by GregVDS
I usually use the plastic sheet markings on capacitors to know which leg is the + one, but I know also that one leg is longer than the other. The longer is the + one, right?
Right.

Quote:
After putting a component through the pcb, should I cut the legs before or after soldering them to the pcb? In both case, what lenght is useful, 1mm, 2mm, far less, even longer?
First solder, then cut off the leads flush at the solder joint, no need to leave any excess length of the lead.

The fact that you are asking this worries me. It's apparent you have not done any PCB work before. I did say on the M³ site that this project is not recommended for the first time DIY'er.

Before you start building this amp maybe you should practice working on something a little simpler?

Quote:
what about the noise collected by this umbilical cable between the psu box and the amp box, this cable should be shielded too, no?
No, there is no need to shield this. I just use a pair of 18-gauge stranded wires, tightly twisted together, and then wrap them in a techflex sleeve, and terminate both ends with barrel-type 2.5mm DC power plugs.

Quote:
And inside the amp box, the signal shielded cable could be stereo one with shield, using the stereo pair for signal + and signal ground, the shield being connected to the ground at the input connector? Is this ok?
If you have a wire with two conductors (plus the shield), you could do it one of two ways:
1. Use one wire to carry everything: the two inner conductors for a L and R signals, respectively, and the shield for the ground connection from the input jacks to the board.
2. Use two separate wires, one per channel: use one inner conductor for the signal and the other as ground, and connect the shield to the ground wire on one end only.

The first option has the benefit of less wiring, the second has the advantage of less stereo crosstalk. But since you are going to be artificially adding crosstalk with crossfeed, the "less crosstalk" argument is moot.

Quote:
Is a shielding braid which is not connected to ground still shield something, or nothing?
An unconnected shield does not shield against anything.
post #746 of 828
9/3/05 at 9:36am

I already made some, don't worry!

Quote:
Originally Posted by amb
The fact that you are asking this worries me. It's apparent you have not done any PCB work before. I did say on the M³ site that this project is not recommended for the first time DIY'er.

Before you start building this amp maybe you should practice working on something a little simpler?
Thank you to be worried, but I already soldered some stuffs: two modified cmoys with crossfeed (meier, ohman and Linkwitz), on protoboard, doing all traces work with legs, everything works perfectly. No, I know how to solder, don't need to be worried by my capabilities here, but I never soldered on a real pcb like the M³ one, so I just wanted to know precisely the way to do it. That was actually how I made for my former amps, put components, bend legs for trace, solder (the less the better) and then cut the legs just near the solder joints.

For the caps, I just wanted to be sure. Please, be confident, all will be fine!

Just another question, but on an entire different domain:

If this is not confidential, how many M³ pcbs did you already sent, just to know if I will have a very common unit or a more rare one. Could it be possible to know our "serial number", just for the fun. I would be pleased to know I have the 23rd, or the 458rd, or the 1.005.587rd, you see? (Mad guy, I know!)

And how do you all manage to post big pictures here, I tried, but mines are all the time too big or too heavy, and I only manage to attach images through a link, not directly in the message. Explanations welcome (and no, I don't have any html skills, but I can understand it quite easily I think)

All the best,

GregVDS
post #747 of 828
9/3/05 at 1:00pm
Greg, consider studying some electronics basics before plunging ahead with your build. Trial and error on protoboard does not teach good soldering practices or electronics theory. Using Google thusly:

search Google for soldering tutorial
great pdf on soldering
soldering stations
basic soldering guide

Searching Head-Fi, this site was recommended:

Lessons In Electric Circuits

Some of the more popular books on electronics include:

The Art of Electronics by Paul Horowitz and Winfield Hill
Troubleshooting Analog Circuits by Robert Pease
Practical Electronics for Inventors by Paul Scherz
Forrest Mims Engineer's Notebook by Forrest Mims

There are no M³ serial numbers. Last I asked, about 150 boards had been shipped. The exact count is not tracked regularly. M³ board distribution is a community service, not a business.

There is a 25K limit on attachments here. You have to host your own large images and link to them.
post #748 of 828
9/3/05 at 1:48pm
Sorry to be contrary here, but I prefer to cut before soldering, as

a) you get a much neater finish on the board
b) you don't need to heat the component as long (as you have a smaller mass to heat)
c) and the solder doesn't wick up the leg. (basically why "a" is the case)

You need to apply a little pressure from the other side to hold it flush, but by no means hard. Most components will actually stay in by themselves, as the leads go into the sockets quite tightly
post #749 of 828
9/3/05 at 1:55pm
Quote:
Originally Posted by guzzler
Sorry to be contrary here, but I prefer to cut before soldering, as

a) you get a much neater finish on the board
b) you don't need to heat the component as long (as you have a smaller mass to heat)
c) and the solder doesn't wick up the leg. (basically why "a" is the case)

You need to apply a little pressure from the other side to hold it flush, but by no means hard. Most components will actually stay in by themselves, as the leads go into the sockets quite tightly
I second what guzzler wrote.
Especially with smaller, temperature sensitive components, it doesn't make sense to cut after soldering.
post #750 of 828
9/4/05 at 3:45pm
Ok, thanks for the contradictory advices, it means everybody has its own technique (no offense here to nobody, ok? I'm just more relax! ;-).

Regarding the temperature sensitive components, I read somewhere over the Headwize project pages that these particularly need theirs legs as some small heatsink while being soldered. But I understand also the need to heat correctly the legs, and this will be achieved quicker if one cuts the leg before soldering. I think it depends of the point of view.

Really thanks for the comments, I will try my best, both techniques seem to have advantages, and I don't fix my mind at the moment, but, as Mouser takes a long time to come here, I have the time to think about it.

Morsel, thanks for the very useful links, I read some , specially the first pdf, and though I knew a lot, everything is useful to be known.

The head of my last solder iron has been disolved it seems, due to poor cleaning, but I have now the correct equipment, and a new solder iron, with fresh pin, that I will take care of ;-)

All the best,

GregVDS
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