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Old 3rd February 2013, 10:33 AM   #31
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Studying the opa/drv134 data sheets and assuming a +- 15 rail supply voltage, e.g.:

OPA 134:
input: (V-) + 2.5 volts, (V+) - 2.5 volts => -13volts - +13 volts
output: (V-) + 2.2 volts, (V+) - 2.5 volts => -12.8 volts - +12.5 volts, load = 600 ohms

DRV134:
input: ? didn't find anything, well this isn't a pure op amp but rather contains 2 + other circuitry
output: (V-) + 2 volts, (V+) - 3 volts => -13 volts - +12 volts


Even with the max 9 volt rms output spec of my preamp for the subwoofer output, e.g. +- 4.5 volts,
I see no problems for my proposed circuit.

Cheers,
Chris
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Old 3rd February 2013, 10:52 AM   #32
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Hi Mooly,

about the load impedance bit...

Since the DRV134 has an input impedance of 10k Ohms and as you said, the OPA134 can happily drive 600 ohms, my circuit should be able to swing all the voltages that my preamp could ever offer, no?


Cheers,
Chris
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Old 3rd February 2013, 11:22 AM   #33
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Default version 6 of the -6db/0db opa amp circuit

Here's the latest greatest version of my -6db or 0db attenuation circuit incorporating all of the great feedback I've gotten so far.


Cheers,
Chris
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Old 3rd February 2013, 11:58 AM   #34
Mooly is offline Mooly  United Kingdom
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Your latest and greatest is fine... but you don't need C2 C1 does it all and can be kept as a small film/poly type.

Your preamp I'm sure will be just fine too.

(Try and understand the output impedance/input impedance/voltage drive thing If you take an opamp and configure it with feedback as here then the output impedance is always really low, perhaps fractions of an ohm. Working alongside that is the fact that the opamp also has a limited maximum output current. Whether that value is 1 milliamp or 100 milliamps, it doesn't affect the output impedance as such. What that current limitation does affect is how much voltage the opamp can put across a given resistance. When that current limit is reached, the voltage output from the opamp levels off... it can't increase anymore)

So as far as driving the DVR134, any normal opamp will easily manage the 10K impedance. The 68 ohm actually raises the output impedance of the circuit as a whole from <<1 ohm to 68 ohm. That means that for 10 volts output from the opamp, you are losing around 67 millivolts across the 68 ohm. So the DVR 134 sees only 9.9325 volts across its 10K input impedance. That loss is negligable and its linear (certainly over the audio band and way way beyond). The benefit of including the 68 ohm is the guaranteed stability it brings.

As to driving the 10K (worst case) input impedance of the attenuator you preamp has to be able to maintain its maximum output voltage (whatever that may be) across 10K. As its solid state it will probably use -/+ rails of an absolute max of -/+22 volts more probably -/+ 15 or 18 volts. Lets say 22 volts and that it can swing to 22 volts. That means it has to be able deliver around 2.2 milliamps into 10K. Yes Its sure to be able to manage that.

9 Volts RMS that you mentioned is 12.8 volts peak (25.6 peak to peak) so that implies rails of 15 volt. Its not -/+ 4.5 volts That level will be OK with your OPA134 running on 15 volt rails.
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Old 3rd February 2013, 06:26 PM   #35
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Default DRV134 with OPA134 Buffer

Hi everyone,

ok this is i hope the final circuit i will be going for...

As always any and all feedback is greatly appreciated.


Thanx,
Chris
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Old 3rd February 2013, 06:30 PM   #36
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Final answer then... yay
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Old 3rd February 2013, 06:39 PM   #37
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If you can accept 10 kohm input impedance I have made a pcb for this. We had a group buy a while back. More info here
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Old 3rd February 2013, 06:42 PM   #38
Mooly is offline Mooly  United Kingdom
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That looks a great PCB Just the ticket I'd say...
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Old 4th February 2013, 01:06 PM   #39
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Default Board Layout, 1st Try

Thanx Mooly for your great help and motivation!

Now I'd like to begin with the board layout...

This is my first attempt at making a PCB, so pls. bare with me.
I'm sure there's lot's of room for improvement.

I'm using eagle to do it and the board has two layers as can be seen by the blue and red tracks.

The thick tracs for power and Gnd. are:
0.066 wide

The thin tracs on the amp / drv. side are:
0.032 wide

I haven't added any ground plane(s) yet. I will post the first version here w/o any. I would think that after adding the ground plane, some of the ground tracks would disappear and the resulting layout would become simpler.

Should i add two, e.g. top and bottom?

Cheers,
Chris
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File Type: jpg DRV134_OPA134_BRD.jpg (91.1 KB, 129 views)
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Old 4th February 2013, 01:54 PM   #40
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Default Board Layout, 1st Try with GND Plane

Here's one with a single ground plane on top.


Cheers,
Chris
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