'no output cap" question

midwayfair · January 22, 2013, 06:31:37 PM

I built an effect recently where I tacked a gain stage on at the very end, built on a little piece of perf. It's just an emiter follower, 1uF cap > MPSA18 with a gain trim & bias > output. For some reason, it works without an output cap. Which is fortunate because it sounded better without one.

Will I damage anything by not including a decoupling cap? And why is it working without one?

R.G. · January 22, 2013, 07:24:19 PM

No way to tell without schematics and voltages.

midwayfair · January 22, 2013, 07:35:20 PM

Okay. It's the Vibratone (your FET/transistor version). I can't figure out a way to link to it because Geofex masks urls and the picture isn't "grabbable." Otherwise I would have linked to it, so I was hoping there was a very general answer to a very general question.

~~8.8v~~ 5v on the collector of the MPSA18 with where I've got the gain pot set. The cap between the last stage of the Vibratone and the base of the MPSA18 I added is 1uF. No cap from there to the output.

R.G. · January 22, 2013, 08:06:32 PM

OK.

How is the bias and gain adjust hooked up to that MPSA18, and what it is driving. It is possible that there is a capacitor at the input of what it's driving. There are a few other possibilities as well, but that's where I'd start. Does the output have meter-measurable DC on it when driving the next thing?

midwayfair · January 22, 2013, 09:00:34 PM

Quote from: R.G. on January 22, 2013, 08:06:32 PM
OK.

How is the bias and gain adjust hooked up to that MPSA18, and what it is driving. It is possible that there is a capacitor at the input of what it's driving. There are a few other possibilities as well, but that's where I'd start. Does the output have meter-measurable DC on it when driving the next thing?

It's on my breadboard, going straight into the amp (Tone King Imperial) from the output of the gain stage.

The output of the effect measures the same as the collector. I don't understand how to answer your last question.

Woops, sorry. The bias is 2.2M collector to base, 2.2K to 9V, 1K trimpot emitter to ground. Except for the transistor and the 1uF cap preceding (from the output of the Vibratone), there's nothing else in the circuit. I didn't draw a schematic or layout for it.

R.G. · January 22, 2013, 09:22:58 PM

Collector to "+9V"?

When it's plugged into the amp following it, is there a measurable voltage across the emitter trimpot?

midwayfair · January 22, 2013, 09:41:35 PM

Quote from: R.G. on January 22, 2013, 09:22:58 PM
Collector to "+9V"?

When it's plugged into the amp following it, is there a measurable voltage across the emitter trimpot?

Sorry, yes. The 2.2K is between the collector and 9v.

Emitter voltage is ~.7v where I've got the trimpot set.

Also, sorry, 4.5v on the collector. Measured the wrong side of the 2.2K

Drew up a thing in DIYLC:

R.G. · January 22, 2013, 09:57:46 PM

That helps.

Connect it up to the amp, and then measure the DC voltage on the output going to the amp.

If it stays 4.5V, that says one thing, if it drops to almost nil, that says another.

midwayfair · January 22, 2013, 10:03:59 PM

It stays the same.

By the way, thanks for sticking with this, RG. I really appreciate the time.

R.G. · January 22, 2013, 11:08:59 PM

I looked and could not find the schematics for that amplifier. I'd be willing to bet that it has an input capacitor that blocks the incoming DC.

If you didn't like it with an output capacitor, then the output capacitor you tried is not big enough.

There are many amps which would immediately go belly up with 4.5V of DC on their input. These are the ones with DC coupling from the input jack to the first amplifying element, usually a 12AX7 grid. A 12AX7 with its grid held above about 2V will not be doing any amplifying.

So my best guess is that your amp is holding the DC out.

midwayfair · January 23, 2013, 08:29:11 AM

Quote from: R.G. on January 22, 2013, 11:08:59 PM
I looked and could not find the schematics for that amplifier. I'd be willing to bet that it has an input capacitor that blocks the incoming DC.

If you didn't like it with an output capacitor, then the output capacitor you tried is not big enough.

There are many amps which would immediately go belly up with 4.5V of DC on their input. These are the ones with DC coupling from the input jack to the first amplifying element, usually a 12AX7 grid. A 12AX7 with its grid held above about 2V will not be doing any amplifying.

So my best guess is that your amp is holding the DC out.

Good to know that I could have blown someone's amp up! I'll increase the output cap to 10uF and hopefully that'll be big enough.

Thanks again, RG.

News:

'no output cap" question