[ajames]

I have a cmoy that i've upgraded a bit, and thrown inside of a cigar box (which nicely fits upright with books on my desk ...), but i am having some grounding issues. I have a working Diode OR Bridge to allow the use of both a 9volt and a barrel-connector from a wall source. Using battery alone, there is little to no hum, but using the wall source, i get a small, but noticeable hum.

Is there a way to better ground my amp despite the wooden box?
Or is this just a result of the wall source not being up to par?

 

[tangent]

Hum doesn't always mean grounding problems. It can also mean noisy power. Is this a regulated power supply? Is it well regulated? The op-amp will reject much power supply noise, but the CMoy's resistive virtual ground defeats some of this, so it's best to go regulated.

Also, you suggest there is still some hum with the battery. If so, it's not a grounding problem, or there is a power problem elsewhere, too, like in your source.

 

[ajames]

The supply I'm using was recycled from a powered set of computer speakers i had lying around -- which is most likely not regulated I'm guessing.

Also, I'm confused at where you're saying hum could come from with only battery use -- my "source" is a battery, which inherently doesn't produce hum. Provided I'm only using a battery, what could result in hum other than grounding?

 

[Clutz]

Source is your CD player, or iPod, or turntable, etc.

 

[breakfastchef]

Did you line the cigar box with foil or other material to prevent RF interference?

 

[Paragon]

Probably didn't shield. Especially if you are near a computer, you will get noise from it.

 

[ajames]

Thanks for the clarification. My brain was in "power" mode and i was thinking "power source". My source is mainly a 5.5 gen. ipod, though i haven't had any noise issues from it in the past.

The base of the box is lined, but the lid is not. I found a heavy-duty foil that i'll try re-lining the inside with.

Thanks.

 

[tangent]

Quote:

Originally Posted by ajames /img/forum/go_quote.gif The supply I'm using was recycled from a powered set of computer speakers i had lying around -- which is most likely not regulated I'm guessing.

Prime culprit. Secondary culprit is the interference brought up by others.

I suggest you toddle on down to Radio Shack and pick up a simple 78xx linear regulator and solder tack it to the inside of the DC power jack, so the power goes through it. If your supply is at least 18V, RS carries a 7815, which is easy to apply. If this helps, you may then want to try something more permanent and performant, like my TREAD.

If the regulator doesn't help, it could all be interference. If the regulator helps some but not completely, it could just be too noisy to bother with.

Quote:

My source is mainly a 5.5 gen. ipod

My 5.0G iPod is quiet, so we can probably rule out the source.

 

[ajames]

I re-lined the entire box with a thicker layer of foil, and though it helped a little, I'm convinced that my sub-par power source is the weak link. There is no audible noise using battery power, however. I just hate eating up 9Volts.

And funny that you mention the TREAD. This afternoon i got through my roughest (i.e. boring) classes by browsing the TREAD and STEPS pages on your wonderful website. If i find that i can cross over from the Mechanical to the Electrical lab here at school at some point, that will be my next little project to keep me sane.

At this point, i'm using a wall wart (thanks for the term) rated at 12V. Is a voltage this low compatible with anything, or even worth using in conjunction with your TREAD, for instance? On one hand, I'd love a project, on the other, buying a regulated supply might be a way out as well.

 

[Paragon]

A cheap PS from a cheap pair of comp speakers will be a switching PS.. not regulated. You will get noise from it.

Keep your eye out for a PS from an old printer or other major piece of equipment. it should be a little better but not fully regulated. Will likely have a higher voltage to work with for a regulation circiuit.

 

[infinitesymphony]

Quote:

Originally Posted by Paragon /img/forum/go_quote.gif Keep your eye out for a PS from an old printer or other major piece of equipment. it should be a little better but not fully regulated. Will likely have a higher voltage to work with for a regulation circiuit.

Thanks for this information... I've been saving power bricks from junked equipment for exactly this purpose, and one of them happens to be from an old printer. Sweet!

 

[tangent]

Quote:

Originally Posted by ajames /img/forum/go_quote.gif ...rated at 12V. Is a voltage this low compatible with anything, or even worth using in conjunction with your TREAD, for instance?

If it's regulated -- i.e. really 12V under all conditions, not just 12V under full load -- that's too low to get away with a 78xx regulator. With anything based on the LM317, like the TREAD, you'll have to be very careful about voltage drops, because you can only afford to drop about 2 of those volts. You need at least 0.7V higher than the battery for the OR bridge to function correctly, and a "9V" battery is more like 9.5V when fully charged.

If your printer supply is higher voltage, definitely use it. Not only will it avoid this problem, the stock CMoy design is pretty wasteful of supply voltage, due to the virtual ground shift. It's good to give it plenty of voltage to play with.

 

[ajames]

Could i solve my problem temporarily by building a trickle charger into the system, perhaps on a switch? I could used a NiMH 9-volt to power the amp normally, which gives me no noise, and is quite silent, but use the unregulated source to charge while, i'm say, in class?

Using a simple trickle charge model i found on the forum, it would use the LM317 Regulator, which could work with my 12V power source, no?

 

[tangent]

It's possible, but you do risk running out of supply voltage if you use a true 9.6V battery (8 cell). A 7-cell battery should work fine, though. (8.4V) There are also 6-cell "9V" rechargeables, but that's not going to be enough to run the amp reliably.

You might have to switch to an active virtual ground circuit of some sort to make this practical.