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Old 14th March 2007, 09:24 AM   #1
impsick is offline impsick  United States
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Default dc output control loop

im sure its a stupid question but i gotta ask cause i cant find my answer anywhere.
for the ina217 they show the optional dcout control loop on the schematic. using an opa237.
so my qwuestion is what pins would be used on the opa237?
and can i use any opamp?
and once i know this i will never some of your brain to to look over my preamp designClick the image to open in full size.
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Old 14th March 2007, 07:18 PM   #2
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ok i got the answer. will someone please look over my layout. its my first one yet so its probably a mess. its a mic pre w/ balanced outs. basically streight from the data sheets of the INA217 and the DVR134. it has 48v phantomk power and hot plug protection. Click the image to open in full size.
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Old 14th March 2007, 09:45 PM   #3
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So ... a balanced input to unbalanced output circuit. I love it. I've been looking for a source for a board like this for months. This will make a nice front end for my aussieamplifiers.com NX500 amps.

FYI: The OPA137 see: http://focus.ti.com/lit/ds/symlink/opa137.pdf ... pin connects are industry standard for single channel op-amp = mini DIP 8 pin = 2, 3, -/+ input, 4 & 7 power, 1, 5 & 8 = NC ... but you already knew that or you would not have been able to complete the board design.

Evaluation:

> schematic = no problems found.
* The use of the opa137 is optional, but will improve the quality of resulting audio in many cases.

* The requirement for phantom power only applies to microphones. This may not be necessary, in which case a lot of the front end parts, etc. , can be left off of the boards during assembly, but it is a good idea to keep the traces & through holes for this ... jus' in case. (My usage would be for interconnect between multi-channel pre-amps and bi-amped speakers, XLR cabling being shielded and better CMRR on longer lines ... the usual reasons for using pro-audio stuff instead of RCA type cabling.)

> Circuit board layout suggestions.
* Open up the gap between the traces somewhat. During DIY assembly, solder can axidently spill across narrow gaps ... not that you are that sloppy, but I am. Gap might be as large as 1/16" or even more. Common practice a couple of decades ago was a gap of 1/8" so that us ham fisted gringos could assembly 'em without too many errors, especially around the solder pads and through holes. (A lot of us old guys prefer British Units (inches = ") rather than the French hoseup ... 'cause we are peculiar and appreciate a True Earth Centric measurement standard than a fictitious one.)

* The mount holes on the four corners should be of a compatible diameter with the computer motherboard plastic standoffs = those cute little plastic push ons with the gummy backs are cheap and plentiful and available at all most all surplus stores. And make sure there is plenty of gap around the mount holes for the same reasons mentioned = sloppy old geeks acidently using metal standoffs and grounding out the back planes & traces, etc.

* It looks like there is room around the DC blocking caps for some extra through holes. This will allow for the use of bigger bi-polar electrolytics or even MKT type blocking caps. (BP1 and BP2)
===0=0=0 0=== instead of =====0 0==== ... or vise versa ... same, same around other parts that may be optional (using jumpers) or of alternate physical sizes or types. (Around E$17 and E$18 = are those coils? ... I wouldn't want 'em and would substitute for jumpers, others might want a different size, etc. ... it is advisable = greater versitality / usefulness of the basic board layout.

* The Ground Plane (connected to pin 1 of XLR) is a little overpowering = too much copper left behind. Keep the gap between the Ground Plane areas and the actual signal paths bigger as well ( Pin 2 & pin 3 of XLR input thru the DC blocking caps and on thru to the imputs of the first stage (INA217 op-amp) ... and then on thru the output and around the balancing op-amp input & output traces (OPA173)

* ?? Is that an extra line driver I see? (DVR134) ... If so, putting this extra set of parts, solder pads & through holes on this board might be a waste, somewhat, for the purposes here = balanced to unbalanced and back to balanced is not the best idea for quality audio "pass through" or daisy chain of XLR connections. Too many op-amps spoil the result as it were = each amp stage compounds the distortion and errors of the previous stage = possibly twice the over all distortion and error even without any gain. ... I see where you want to go with this = an XLR pass through "breakout box" with an unbalanced tap ... Much has been written about these kinds of efforts. All electronic taps have a problem of one kind or another: TV splitters = loss of signal strength, digital splitters = timing consequences & reduced performance, analog audio breakout boxes like this contribute to distortion or depreciation of the noise floor or degradation of some aspect of quality. Leave the holes, prepare to ignore the usage.

* Follow the signal ground connections around the circuits' op-amps and passive components, looking for any topography other than a "star" like grounding (+15 / 0 / -15 ... 0 = signal ground). It appears that you understand the differences between signal ground and the Ground Plane that is attached to the XLR shield (pin #1) ... inattention to this and the topograhpical layout of the signal paths (+, -, gnd, shld = balanced and plus, minus as shield = balanced) ...

Keep the faith. This is a worthwhile project and I will buy some of your boards or help defray some of you costs when you are ready. (I would offer to buy a couple of these, as is, but I would leave a lot of the parts off and maybe even chop it up with a saw ... )
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Old 14th March 2007, 10:48 PM   #4
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fast eddy,
thanks for your input. i'll get back to the drawing board.
What do you think about the option of balanced and unbalanced outputs? I agree as far as two many op-amps will degrade the signal but i like the idea of having balanced outs if needed. and i like what i've heard and read about the drv134 and its ability to boost level 6dB.
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Old 15th March 2007, 09:34 AM   #5
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Fast Eddy
ok i took your advice and i believe i covered most or all aspects but like i said this is my 1st layout so hopefully you or someone would look it over.
im going to work on the power supply and then i will join the boards. I assume this would make it more cost effective. and basically like the Audiosector boards, how they snap apart.

Also because i will use this for a mic i would like to have a VU meter or a clip light. Im not to sure how i should rig this up or where. anyone know of any links.

Also i was thinking after seeing this idea somewhere, to put a patch point somewhere in between the DRV134 and the INA217 so if i decide later to make say a compressor i could put it all in a single RU.
anyways here it is
Click the image to open in full size.
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Old 15th March 2007, 04:51 PM   #6
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" ... What do you think about the option of balanced and unbalanced outputs? I agree as far as two many op-amps will degrade the signal but i like the idea of having balanced outs if needed. ..."

I like it too, but I would take a look at how it is done on a pro amplifier ... usually done with passive elements rather than active op-amps. Checkout a Crown Amp with the XLR pass through ports = usually done with transformers and/or an array of other passive resistros, etc.

It is desirable to have a pass through XLR port for general purposes, but for higher quality audio work, the simpler balanced in to unbalanced out is better. It is possible to add the XLR pass through ports on another board, so having some through holes for this accomodation may be in order if you decide to remove the rest = very optional as I can live with the parts' spaces / holes on the current board design.

As for your latest board design: Consider removing that "tongue" of copper between the trim pot traces = this encourages crosstalk between the signal paths and the Ground Plane ... same, same with the Ground Plane "tongue" that parallels the signal path 'tween the blocking caps' traces, BP1 & 2, and the first stage op-amp inputs ... same, same with the Ground Plane island 'tween and around the clamping diodes, etc. ... and there are several other Ground Plane "tongues" and islands that might or might not degrade the audio signal. It is common practice to have these "tongues" and islands on power supply boards, but modern "audiophile", golden ear practice and theory of design of amplifier boards indicate that this is just another source of "stray capacitance" and possible crosstalk at higher frequencies ... those parallel ground "tongues" and signal path traces acting like tiny capacitors, bleeding off the signal quality ...

I see you opened up the gaps 'tween traces somewhat ... IMOP these could use some more 'tween and around any signal path = reducing the above mentioned stray capacitance, but if you remove and reroute those Ground Plane traces as above, this may not be required or even a possible bother. ... Fat Ground Plane(s) around the perimeter of an audio board = very good = act as shielding. Ground Plane traces in and amoungst the signal traces = not so good and in a few cases, bad ....
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Old 16th March 2007, 07:22 AM   #7
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ok got it. so you think i need transformer outs? ok, one problem for me is good transformers are $50 +. do you know of any good priced transformers for audio? pcb mount preferably.
anyways i already have a better design, but im gonna do some more to it and post it up later. thanks for your time.
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Old 16th March 2007, 06:29 PM   #8
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" ... so you think i need transformer outs? ..."

I did not mean to interfer with your design perameters ... so I would suggest you not significantly change your existing approach and try for transformers as "pass through" output to XLR ... but it would not hurt to have solder pads & thru holes ready as an option for other users of you boards & design. ... In fact leaving the output impenedence matching op-amp connections AND transformer output connections on the board ... both as options ...

Personally, I will just not install those parts on any boards I might use, bypassing that need, since I would be simply interconnecting between balance XLR input connectors to an unbalanced input amplifier module. ( adding them to this type of amp: http://3dotaudio.com/ampics.html )

As for the costs of XLR I/O transformers ... you get what you pay for. There may actually be a very good qaulity balanced to unbalanced PCB mount transformer (I've seen 'em) but I don't have a link or suggestion for this.
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Old 16th March 2007, 07:16 PM   #9
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cool, i got you now. great idea! i will make those available options.
your not interfering with my design. i asked for the input. so thanks. its really nice to have a forum with so many knowlegable people willing help out.
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Old 17th March 2007, 06:53 PM   #10
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fasteddy, here is another revision. tips?
i noticed on the drv134 i had V+,V- pins crossed up. it gave me more of a worry then it should've thinking that i woouldnt be able to keep the board single sided. anyWAYS where should i have the taps for transformer out? is it fine the way i have it set up? im not too familiar with that process i need to do some research.
Click the image to open in full size.
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