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Old 27th January 2013, 05:00 PM   #11
Mooly is offline Mooly  United Kingdom
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If you just want to cut the level by 50% and the attenuator has to be non inverting then you need the attenuator resistors at the + input of the opamp with the opamp configured as a buffer. You cant get "less than a gain of +1" from an opamp by feedback alone. You can if its configured as "inverting" but I'm guessing you don't want that.
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Old 27th January 2013, 05:16 PM   #12
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Quote:
Originally Posted by Speedskater View Post
On the "X1" connector, neither the pin 1 nor the shell should be connected to the circuit "GND" they should only be connected to the chassis with very short wires. Of course the circuit "GND" should also be connected to the chassis but with only one wire.

Added thoughts
The "WBTOR1" connector should be the circuit connection to the chassis. It's "S" terminal should go to circuit "GND" not to X1. pin 1.

Don't know how I skipped by AndrewT's post! I never saw it on my first read.
Hi Speedskater,

thanx for your help. Just made the changes.....


Cheers,
Chris
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Old 27th January 2013, 05:24 PM   #13
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Originally Posted by Mooly View Post
If you just want to cut the level by 50% and the attenuator has to be non inverting then you need the attenuator resistors at the + input of the opamp with the opamp configured as a buffer. You cant get "less than a gain of +1" from an opamp by feedback alone. You can if its configured as "inverting" but I'm guessing you don't want that.
Hello, yes you are right this was my intention. I just wanted to cut the input signal in half.

Btw. do u by chance know what the lower gain limit is for an opa 134, i didn't see anything in the data sheet?

Thanx again,
Chris
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Old 27th January 2013, 05:26 PM   #14
Mooly is offline Mooly  United Kingdom
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It's unity gain stable so can be used as a buffer.
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Old 27th January 2013, 05:32 PM   #15
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That's as low as any opamp would be specified. As mentioned, you can't have a gain of less than 1.

The OPA134 being FET input has no issues with input bias currents and DC offset (as I see you mentioned something about "equal resistances" earlier). You need to be aware that you might need to AC couple the input to the attenuator. The two resistors alone would allow DC from any source component to pass through.
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Old 27th January 2013, 05:59 PM   #16
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The OPA1632/THS4130 is also another good solution for single to balanced conversion. Requires a few more parts, but well worth it. I made a board for bridging two stereo amps and was very pleased with its performance.
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Old 27th January 2013, 05:59 PM   #17
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Originally Posted by Mooly View Post
That's as low as any opamp would be specified. As mentioned, you can't have a gain of less than 1.


The OPA134 being FET input has no issues with input bias currents and DC offset (as I see you mentioned something about "equal resistances" earlier). You need to be aware that you might need to AC couple the input to the attenuator. The two resistors alone would allow DC from any source component to pass through.
Hi Mooly, so to cut the input signal in half, i would basically use a voltage divider on the input. What would be good values here for line level type signals? I read somewhere that they aren't fully standardized, e.g. 0.7 rms or 0.3...

See schematic.

Why do the resistors change anything, If my input signal had dc to begin with?
What upper bound freq. would usually be considered dc in such an analog circuit?

Thank you very much,
Chris
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Old 27th January 2013, 06:31 PM   #18
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That's it

The resistor values are always a compromise. The lower in value and the "better" that is for noise contribution, however that effect is tiny at the values we are talking. Make them to high and any stray circuit capacitance and the input capacitance of the chip starts to become a factor. The effect is that the capacitance starts to form a low pass filter and so the high frequency performance is affected. Also, lower values place more of a load on the source. So we compromise and here 10K should be ideal.

If your input signal had DC present then this would be attenuated exactly the same as the AC. Normally DC is considered a bad thing in ausio signal processing (and it never appears in music). If you applied a music signal that did have a DC offset to a speaker then the speaker cone would displace depending on the level of DC. So normally we don't want it. If you can be 100% certain that any source you connect will have no DC then you can just use the resistors. Good practice would AC couple to avoid any possibility of problems. The cap size is chosen according to the lowest frequencies to be passed. Two 10k resistors give a 20k input impedance (the opamp loading is negligable) so we could say that 5Hz would be our lower (-3db) limit and that would mean a cap of 1.6uf. In practice you could use a 2.2uf electroylitic or a couple of 1uf films in parallel.
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Old 28th January 2013, 08:28 AM   #19
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Thanx alot for your efforts mooly, you are most generous to spend
your time looking at my stuff!

I will add some audio cap maybe a 2,2uF Mundorf Polypropylen. This
would make a corner freq. of approx. 3.62 Hz.

I will eventually update the schematic with all these insights and repost it.

Cheers,
Chris
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Old 1st February 2013, 07:17 PM   #20
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Default -6db / 0db gain selection circuit

Hi everyone,

here's how i plan to switch the gain on my line driver circuit. Seems pretty
reasonable to me.

I thought about adding an additional +6db gain too but its bit tricky for me to devise a simple circuit with only one 3 pos. switch to do it. I guess an analog switch good be used or some kind of programmable set of three switches but that all adds a bit more complexity to my orig. circuit and i will prob. never need it.
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