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Old 1st June 2005, 05:19 AM   #21
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Bill,

You are starting to lose me now. If you can put numbers on your R1,R2 and detail upstream stages in the signal path I might be in a position to offer something of value.

Your aim to optimise U1 by balancing R's may be swamped by the sheer number of stages you're introducing here. I'm not sure that the R's balance is sooo important to the low THD functioning of the 134? Are you sure you're not getting confused with DC conditions and remember it is a FET op amp with 5pA bias currents . Also you appear to have very low R values for so high up in signal level. Value for feedback R2 (in your last diag) could be up to 5K without drama and would lighten the load on the 134.

I need the numbers to see where your gains are and upstream function/optimisation.

Greg
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Old 1st June 2005, 05:39 AM   #22
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Hi,

Do you know what is the average current requirement for one opamp (OPA314)? I am redesinging the PSU for the active circuit.

One may say of course it depends on the load, etc, or need to work out the impedance of the stages...

The datasheet says the quiencent current is 5mA, typical 35mA and current limited to 45mA.

There are 9 opamps in John K's active circuit (the previous version) so for a stereo there are 16. If I design a PSU of +-15V with 35mA x 18 = 630mA capability would it be sufficient? Anyone has tried to run it at +-17V? the distortion will be lower.

Regards,
Bill
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Old 1st June 2005, 05:55 AM   #23
jcx is online now jcx  United States
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has anyone pointed you towards

http://www.linkwitzlab.com/filters.htm

Linkwitz circuit boards include extra open baffle dipole correction stages but you should be able to see how the pieces can be put together
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Old 1st June 2005, 05:56 AM   #24
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Amplifierguru,

Here is the schematic. I changed the original values (in black) to the new values (in green) because I don't want to parallel multiple caps to get 1.2uF and I so happened to have 1uF MKPs on hands. The filter response should remain the same.


Click the image to open in full size.


Here is an extract from the 314 application note:

* * * * * *

SOURCE IMPEDANCE AND DISTORTION

For lowest distortion with a source or feedback network
which has an impedance greater than 2k, the impedance
seen by the positive and negative inputs in noninverting
applications should be matched. The p-channel JFETs in the
FET input stage exhibit a varying input capacitance with
applied common-mode input voltage. In inverting configurations
the input does not vary with input voltage since the
inverting input is held at virtual ground. However, in
noninverting applications the inputs do vary, and the gateto-
source voltage is not constant. The effect is increased
distortion due to the varying capacitance for unmatched
source impedances greater than 2k.

To maintain low distortion, match unbalanced source impedance
with appropriate values in the feedback network as
shown in Figure 3. Of course, the unbalanced impedance
may be from gain-setting resistors in the feedback path. If
the parallel combination of R1 and R2 is greater than 2k, a
matching impedance on the noninverting input should be
used. As always, resistor values should be minimized to
reduce the effects of thermal noise.

* * * * * *


Regards,
Bill
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Old 1st June 2005, 05:56 AM   #25
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Bill,

I don't know the design - you'll need to point me to it. The OPA134, is that what you're using draws 4mA but maybe capable of 35mA but this may be irrelevant under local conditions.

You will achieve more THD optimisation by lightening the load i.e. scaling up your R's - I think, in the absence of details.

Greg
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Old 1st June 2005, 06:25 AM   #26
sam9 is offline sam9  United States
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Quote:
. . .an audio nut once said it was very impressive, was made from the cheapest electric cable and it is being the only speaker cable I am using . . .
ROFL! You may have also have noticed that the actual electronic components (the little things that do nearly all the work) cost less than than front pannel of many "High-End" products. If you have friends who fall into the category of "audio nut" it's a good idea to have expensive looking speaker cables (I always keep an eye out for impressive lamp cord) , otherwise he'll get so obsessed with that shortcomming and never even bother to listen to the rest of the stuff!
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Old 1st June 2005, 06:32 AM   #27
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Agreed, I bought a roll (100m) of 1.5mm single core insulated and plaid 3 for each conductor. It looked great, it sounded great and the ******* on the periphery of the industry (one Australian comes to mind but I'm sure there are plenty more) missed out!

Many have commented on it's superiority.
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Old 1st June 2005, 06:43 AM   #28
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Hi Bill

Your figure of 630 mA is way high. You probably don't want to run anywhere near the max current of the op amps, which means you don't want any of your loads below say, 700 ohms conservatively. You'll want plenty of headroom, so your average program content might be, say 5 v at max drive, i.e. almost clipping. Then,

5v/700 x 18 = 129 mA

Quescient currents will add only slightly to this, and remember that each rail need only supply half of that!
But this is just a figure - You'll want several times this capability for good regulation, etc. And after all, it's easy enough to build a half-amp supply.

(edit: I forgot this is not full-range, so your average program content could be a slightly higher percentage of the peak)
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Old 1st June 2005, 06:50 AM   #29
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Hi Bill,


I think you'll find that input capacitance effects are fairly minimal, verging on non-existent for the output of a bass channel on an active xover using a OPA134. other considerations are of more concern - like the passive xover component C values!

This last cct is a 10dB peaking filter with no subsonic rolloff. Without support from other filters it's pretty ineffectual and a bit of a waste of a stage.
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Old 1st June 2005, 07:19 AM   #30
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Greg,

It is for the EQ of the Peerless XLS 12" in a U-frame. It is John K's circuit and being the last opamp in the LP chain of filters.

JCX,

With all respects to Mr. SL and Mr. John K I have certainly spent much more time on SL's site than any other sites. I chose John K's based on my choice of drivers and my belief that it was cheaper at the time (but it is probably equally expensive!). The decision was made about 9 months ago and unfortunately and fortunately, before finishing John K's speaker I had re-designed and tuned my own speaker to a level that it certainly reaches to high-end class. Still needs fine-tuning hence has not got John K's speaker finished. I am moving on so now refining my own active circuit and designing the board for John K's circuit while making necessary modifications to suit my application.

Regards,
Bill
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