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amp input levels?

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Neil Groves

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Hi Guys....

Well i just finished my Syntom circuit on stripboard after debugging it for quite a time, 2 components in the wrong place, 3 breaks in the copper strips that hadn't been made and 5 breaks that i thought i'd made but the drill hadn't gone right through the strip (i must have been half asleep when i put this together :rolleyes:) and am ready to hear what the beast sounds like, my concern though is although i have a healthy 1V signal bouncing around on the output, it sits on a 4V DC level, is this safe to send into an amplifier input?

Neil.
 
Only if the amp input blocks DC. IE it is AC cuppled with a cap.
Can you post a circuit diagram of your Syntom circuit? so that we may try and help you fix your "DC offset" on the output.
 
Thanks 4pyros, i have to go out for a bit but may find time later to study this. seems these CA3080's are abundant little guys in the music industry, i bought 3 when i made my order and have 2 left, so will consider changing it, i must order another 10 of these while they are still available.

thanks again

Neil.
 
These are in my workshop as my audio test set up, i'm trying to find out if they are AC coupled at the input, can i AC couple the output of my circuit just in case with say a 4u7 cap to send the signal AC coupled? will i damage my speakers if that is too much AC coupling if they are already AC coupled at the input?

https://www.amazon.com/gp/product/B000062VUO

Neil.
 
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The electrolytics facing each other make a non-polarized cap out of two polarized ones.

If you can't adjust that offset down to zero, I think it would be worth putting a DC blocking cap on it with a ground referencing resistor on the output side. That 4V offset on the signal line is just too much - it will slam the output of the amp if you jack it in while the amp is running. Even though your particular amp has a blocking cap, some other amps use an input isolation transformer which could saturate with that 4Vdc, and if you do an image search on "tube amp circuit" and you will find an awful lot of those amps don't have any input isolation at all.
 
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You should be fine with the Klipsch ProMedia 2.1 computer speaker system, it has a nice input ciruit. But as said you should still fix the DC offset incase it is ever used on something else.
But why are the Electrolyics facing each other at the input?
It makes a non polarized electrolytic capacitor out of cheep caps.
 
The input impedance of the Klipsch amplifier is very low. The synth circuit is designed to feed an amp with a high input impedance.
All the resistors at the output of the synth circuit should be removed.
 
if i AC couple the output from the synth circuit with 4u7, why do i need a resistor to ground from the output jack?

Neil.
 
To stop float voltages. It can be 10k, doesn't need much, just enough to bleed off the charge. Without a ground reference there, the cap can float to any voltage - we had a problem one time where we had a DC blocking cap on an output hooked to a DC blocking cap on an input. Every so often, the amplifiers would make a "pop" for no apparent reason. I traced it down with a scope, what was happening was the unreferenced junction between the caps was floating well up over the 16V rating on one of the caps, breaking over the dielectric, causing the "pop" and discharging, then repeating a few minutes later. A 10K bleeder resistor to ground fixed it.

With your setup that wouldn't matter because Klipsch put a 100k ground reference resistor there, for this reason. Some other amp you plug it into might not have that.

Also, like I was saying before, if you are plugging things in "live" you want these inputs close to ground when you connect them. Without that bleeder, the cap will discharge through the amp's input line. Instead of a little "snap" you can get a "ka-THUMP".
 
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why do i need a resistor to ground from the output jack?
That would be after the cap and pulls the output to 0 volts with no output. So it cann't float or make noise.


Opps sorry Duffy didnt mean to stump on you.
 
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I'm sure you guys are sick of me prattling on about this but here's what i did.

I plugged a 3.5mm plug into my cell phone and measured the output resistance, it was 68Ω, i then measured the output of the syntom and it was 63KΩ sooooo on the advice of Audioguru i whipped R28 out of the circuit and replaced it with a 1u Electrolytic, on scoping the output i found the output with it's attendant signal slowely sank to ground on power up, then i had my AC coupled signal bouncing around the 0V rail out, i added the 10K resistor after the 1u cap to ground and then on power up i had my signal immediately sit at ground, no more floating.

Then i got brave and thought ok here goes, i plugged the output of the syntom into the amp, and switched the amp on, i then held my breath and hit the power to the syntom, i nearly fell out of my chair when i heard a slow falling pitch, on tapping the piezo transducer i now get a variety of falling pitch sounds....i am absolutely over the moon!!!!!!!!!

thankyou all so much for your help, i am delighted!!!!

Now i just gotta box the beast up and build my disco light modulator and then i am going to get a good book or find a good site online and learn the art of analogue electronics, a thoroughly fascinating and intrigueing subject!

thankyou again.

Neil.
 
I plugged a 3.5mm plug into my cell phone and measured the output resistance, it was 68Ω
How'd you measure that? You can't use straight DC resistance, it's an active driver....
 
oh....ummm....i measured with an analogue ohms meter.

that sounded great on the amp so i lowered the output resistance of my circuit.

Neil.
 
You can't measure the output impedance of an active driver with an ohm meter, digital or analog.
 
The antique and obsolete CA3080 IC was a transconductance amp with an active output that was a very high impedance for AC. It had a very low resistance for the DC of an ohm-meter.
Maybe the "CA3080" ICs that are available today are counterfeit copies that are completely different.
 
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