A problem with a line amp

[jrmintz]

Hi all,

I have an enclosure and power supply wired for two API 325 cards. The enclosure and voltages check out fine when there are no cards in the slots. The two cards check out fine outside of the enclosure. When I put the cards in and turn it on the power transformer immediately starts to hum and the voltage gets pulled down to +/-2V. I've checked my wiring and it's correct as far as my intentions were concerned, but obviously something is wrong.

I have two unbalanced inputs with the shield and the white conductor feeding the low side, though the shield is only connected at the input connector, not at the edge card connector, and the red conductor feeding the high side. The output is through a 25 ohm 10 watt resistor, each card feeding the tip or ring of a TRS output and both output low sides feeding the shield. Gain set is a 100k reverse log pot between pins 7+8.

Any ideas?

Schemo

Thanks,

 

[wilebee]

what is the resistor for ? and you don't have the card inverted Ie dc voltages feeding the secondaries of your output transformer? Wil

 

[NewYorkDave]

I agree with Wil: if one of the transformer output windings was across the power supply, that'd definitely cause the trouble you're having!

With the power off, take a resistance reading across the power supply pins with and without the card plugged in.

 

[jrmintz]

Doohhhh!!!!!!

It's all bass ackwards. I wired the edge card connectors totally reversed. How can I look at something for that long and not see it?



Thanks guys.

 

[wilebee]

Seth What are you suing the 25ohm 10 w resistor for ? Wil

 

[jrmintz]

Wil,

It's a headphone amp, and NY Dave and PRR both told me to put a 22 ohm 2 watt resistor to keep from frying the phones. The closest I could find on short notice was 25 ohms at 10 watts. It works now that I've sorted out my left and right.

 

[wilebee]

cool I didn't read that it was for a headphone amp Wil

 

[NewYorkDave]

I'm assuming that the transformer winding that was mistakenly shunted across the supply did NOT go open, right? You're a lucky man :thumb:

Since all that direct current went through the winding, you might have to degauss the core by cranking Outkast through it for several hours, though. :wink:

 

[jrmintz]

Well, I had no power for several hours yeaterday so there wasn't much I could do. I find there is just no work-around for no electricity, it's really the one thing you can't fake. When it came back on I sorted everything out and it seems OK. It was actually the third winding that took the full blast, and I'm not using that, so I guess I lucked out.

The only thing that's still wierd is that I'm using a 100k rev log dual gang pot for volume and it doesn't shut the audio off all the way. The minimum level is almost uncomfortably loud. Why would that happen? Should I reverse the pot bottom and the wiper on the edge card connector? I didn't think to try that, but if the gain control is a variable resistor that shouldn't make any difference, should it?

:thumb:

 

[NewYorkDave]

It's a noninverting amplifier. Varying the feedback ratio can never decrease gain below 1.
For a usable "volume control", wire up an audio-taper pot as a voltage divider at the input. Input signal to high side of pot, pot wiper to pin 5, low side of pot to pin 4. A 10K pot should do.

As for the 100K reverse log pot connected between pins 7 and 8, use that to set your maximum desired gain, and then replace with a fixed resistor if you like.

 

[jrmintz]

Thanks, Dave. You're hooking me up, dude. Did I say that? :shock:

I'll post some pix soon, it looks pretty unique -

 

[NewYorkDave]

So, are you happily esconced with some Bach right about now? :guinness:

 

[jrmintz]

[quote author="NewYorkDave"]So, are you happily esconced with some Bach right about now? :guinness:[/quote]

I wish.... I'm ensconced on the phone with lawyers and managers. The Bach comes much later, after many deep breaths, some hard exercise and a Jack Daniels.

:guinness: :sam: :green: