My Ruby has a DC voltage on the output - do others have this?

[mojotron]

This is interesting, I built a Ruby, it works great - sounds great for what it is....

But, I noticed that there is a DC voltage across the output to the speaker. I'm using the GGG layout - this was a super easy build BTW, switched output (220uF - rated for 25V+) caps, checked everything... I am using a 16VDC supply and I get 7.7VDC on one end of the 220uF cap and 7.5VDC on the other side - with .2VAC (the AC could be noise depending on the input...).

Anyone else noticed this on theirs, I'm a little baffled as to where the DC voltage is being developed and I'm guessing this would be bad for speaker coils.

The sound is awesome, extremely quiet; and, being such an easy build, and having checked everything I am pretty sure that everyone might see there on theirs too - but thought to check that out.

Could just be the Hi Z of the meter completing a circuit with a huge cap and if I held my probes there for 3.4 days... a bit of a head-scratcher. Ideas?

[bancika]

you shouldn't have any DC after cap because it should block DC. Try replacing it with new one

[mojotron]

you shouldn't have any DC after cap because it should block DC. Try replacing it with new one

Yep, I had the same thought... I tried that, got the same result - that's why I was wondering if anyone had actually measured it on theirs

[Alex C]

This was the case for me as well, and I tried several caps from different batches.  I can't remember if I ever figured it out or just left it as it was.

-Alex

[brett]

Hi
there's a small chance that the amp is oscillating at high frequency.
Do you also read an AC voltage when there's no input?  If so, there's probably an oscillation.
If so, use two power supply bypass caps (100uF and 0.1uF), placed as close as you can get to the 386.
cheers

[GibsonGM]

Hi mojo,

I've had the same problem 1 or 2 or 5 times, reading like .5VDC  to even 7VDC!  I screwed something up in my amp once, believe it was due to this (blew a phase inverter tube, really weird, could be unrelated but...).   Now, as a practice, I test for DC on my effect outputs before throwing them into the amp for testing (or I use my little Marshall SS >;o)  I'm not sure how much DC an amp input can handle before things get nasty...I'd expect there's a cap on the input that can take it, and block it, but you never know.  My standard is allowing only a few mV DC max into my input.  The .2V AC should be tracked down, it is a high level for noise (guitar being about 120mV at peak).

I can usually tell it's happening, the speaker breaks up and it sounds very ugly.  The cause??  Every single time, a missing cap on my part...the Big Muff tone stack is notorious for this for me - I add it last to a ckt, take out my output cap, and forget that 1 resistor in the signal path rather than a cap!      So, that's the place to look first, or a short somewhere would cause this too.  And oscillations...anyway, something to correct/avoid, in my opinion.  Hope this helps explain things!   

[Ronsonic]

Most likely the output is simply drifting up when there isn't a speaker or other load attached. While a cap does block DC current, a voltage will impress itself on the output over time. Try measuring with a resistor in place, say a couple hundred ohms or so and see if you've still got a voltage on the output. If that handles it, all's well. If not then yes, a short or bad cap.  But, if that was the case your speaker would be visably pushed out whenever it was plugged into your Ruby.

Ron

[phil]

Most likely the output is simply drifting up when there isn't a speaker or other load attached. While a cap does block DC current, a voltage will impress itself on the output over time.

Ron is right - I just measured the voltages on my Ruby with NO speaker attached and with the battery at 9.31V - Measured before the capacitor it's 4.38 - measured after the capacitor it's 55mv, but as I leave the dmm connected, the voltage ticks up. I let it climb to 85 mv, plugged in a speaker and now the measured voltage is 0 after the capacitor ...

[grapefruit]

Yep, this is normal for an amp that has a capacitor coupled output. Even  a 10k resistor should be enough to pull it down.

Stew.