Audible Hum in Ruby Amp

[mpuckett]

I've just completed my first Ruby Amp on a perf board and it is working though it has a (very) audible hum. I built a first prototype on a breadboard and it was very clean so I know the circuit is a good one and it must be my first soldered up version that is the issue.

I presume that the hum is related to a poor ground somewhere, is that a reasonable assumption? Which is the more likely section to induce the hum, on the input side or the output side? Any suggestions on how best to debug the problem?

Other than the hum, the amp is working fine.

[phatt]

Q?
     Is this one powered by a Battery or are you using a Plugpak?
Phil.

[mpuckett]

Phil,
Nope, haven't added the wall wart yet. It's powered by a 9V battery.

It seems that the hum disappears when the guitar volume is very low or at maximum. If the guitar volume is somewhere in the middle, say in the 4-8 range the hum is the loudest.

[mpuckett]

Ok, found it. Turns out I had the leads to the input jack reversed. Switched them around and bingo! There is still a tiny hum at full amp volume but nothing I am worried about now. Things sound great!

I got the parts today so on to the wall wart circuit next!

[mpuckett]

So the input side hum makes me wonder, would a twisted pair input line (like a cat 5 network cable pair) be useful to reduce or eliminate hum?

[phatt]

Hi sounds like you doing well,
Trouble shooting your own problems is the best way to learn.

Re CAT5 cable is lightyears away from audio cable it will have very little effect at reducing Audio noise.

If you wish to reduce noise then shielded audio cable is what you want to use,,
not to be confused with 75 ohm TV coax.

I just trash old VCR's and other electronic equipment to get a lot of parts like that.
I'm a pensioner and have very little funds so every little piece of unwanted equipment is gutted for parts.

Try the audio cables that those old CD Roms came with,, the ones that went to the old sound cards,,, they where shielded Audio and they are very fine and flexable.

When you use the Aux power input don't be suprised if there's an increase in background hum,, t'will depend on how good the regulation of the DC supply is.
Some are good and some,,, not so good.

A battery has Zero ripple so darn hard to beat for noise free PSU.
Cheers, Phil.

[mpuckett]

Hi sounds like you doing well,
Trouble shooting your own problems is the best way to learn.

Yep! I quickly built up another perfboard since they are so simple to make, rerouting the input and output lines and being very careful with the soldering. But I reused the already wired pots and input jack and when I connected the battery same hum. So I knew it wasn't in the circuit. Turns out I soldered the wires to the wrong lugs on the input jack so the problem moved to the new circuit. And now I have 2 working circuits!

Re CAT5 cable is lightyears away from audio cable it will have very little effect at reducing Audio noise.

That is good to know! But unfortunate because I am recycling cat5 wire and used it everywhere, darn it!! And I thought I was being very clever to untwist it all and just use the individual wires. I am going to give the twisted cat5 a shot until I can find some recycled shielded audio cable.

If you wish to reduce noise then shielded audio cable is what you want to use,,
not to be confused with 75 ohm TV coax.

I just trash old VCR's and other electronic equipment to get a lot of parts like that.
I'm a pensioner and have very little funds so every little piece of unwanted equipment is gutted for parts.

Try the audio cables that those old CD Roms came with,, the ones that went to the old sound cards,,, they where shielded Audio and they are very fine and flexable.

Thanks for the pointers. I am trying to use recycled parts as much as possible myself too. We have a recycled electronics store that has tons of old used electronic weird stuff and old bulk resistors, caps and everything you can think of that I am using instead of the local Radio Shack. It's where I found my center ground wall wart yesterday in the bins of recycled wall warts. I'm pretty sure I can find something to dismantle there with shielded audio cables. Now that I know what to look for!

When you use the Aux power input don't be suprised if there's an increase in background hum,, t'will depend on how good the regulation of the DC supply is.
Some are good and some,,, not so good.

A battery has Zero ripple so darn hard to beat for noise free PSU.
Cheers, Phil.

Yes, I figured as much about the battery being a noise free PSU. That's why I was a touch confused about the hum I was getting. I am expecting the wall wart to be noisier. I don't think this circuit will be real taxing on the PSU though.

Thanks for the encouragement and great suggestions. I'll let you know how the wall wart circuit turns out.

-mikep

[phatt]

Great,
         Just to be clear you only need shielded cable for the sensitive things like the *Input* and maybe long wires inside cases that might pickup hash and hum.

But sounds like your catching onto it all so keep up the good work.
Cheers, Phil.

[mpuckett]

Just to be clear you only need shielded cable for the sensitive things like the *Input* and maybe long wires inside cases that might pickup hash and hum.

Ok, I cut apart an audio cable with RCA jacks on the end and found a nice shielded audio cable inside. I replaced the cat5 wire on the input side with this and soldered the outer shield to ground and the inner core to the input. This made a nice difference, but there is still a slight hum. When the guitar jack is disconnected it becomes a quite loud hum. When the jack is plugged into the socket it becomes almost silent. But not quite. The input and ground on the perfboard are about 1.5 inches apart, so I stripped the shield back and ran the unshielded (though insulated) input the 1.5 inches and made a short connection of the shield to ground. This seems like it defeats the purpose of the shielded input though as the input signal is unshielded as it runs around the perfboard. Is there a better way of doing this so that the input line stays shielded the entire path?

I am still using the cat5 wire for the power supply, the volume and gain pots and the speaker out. At the moment, these wires are all about 12-14 inches since I haven't mounted the amp into anything yet. Everything is still open on the bench. Would anything else benefit from the shielded wire to eliminate the hum?

Next, I went ahead and added the wall wart circuit. Phil, the design works perfectly. It is exactly what I needed. Now the guitar jack functions as an on/off switch and the wall wart disconnects the battery when it gets plugged in. And the (slight) hum is still there for both the battery and the wall wart, but it is certainly no worse when powered by the wall wart than it is when powered by the battery.

So.... the shielded audio input cable has reduced but not eliminated the hum and the wall wart circuit works perfectly!

[mpuckett]

To answer my own question.... I rearranged the shielded audio cable so that the input line remained shielded right up to the connection and ran a ground up to the shield right at the input connection. MUCH improvement. With full volume and full gain at the amp there is still a faint hint of a hum but this is such an improvement that I am satisfied now. It's not dead silent but close enough. It sounds great, drives a 12 inch speaker with no problem and is plenty loud. I think it's time to find the right speaker now and package it all up.