# Simple OPA627+BUF634 Pre-amp



## MaxW

Hi All,
 I wondered if someone could just give me some advice/improvement about this schematic. I want to make a simple and flexible pre to use with my Gainclone (but not limitted to). The + and - rails will be connected to this PSU and I will use a pot at the input of this design.

 My quesitons are:

 Is this design OK for what I want?
 The values are OK?
 Will this design be fussy with interconnect length or type?

 Design was copied from here.


 I will make and share a PCB design soon


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## mono

About the power supply- See a whole lot of those turquoise G-Luxon capacitors on it! Can't know for certain if it's the same model of cap but I used to have a shoebox full of turquoise G-Luxons that'd failed (came off of motherboards), they aren't such great caps to use to say the least.


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## amb

Looks fine to me... Basically what you have is the MINT amp except you're using a split power supply and different opamp, and you don't have the MINT's class A biasing or Jung multiloop. Your amp is set to a voltage gain of 2, not sure if it's appropriate for your application. Also, since your R2 is 100K, you should use a 10K pot at the input, or increase the value of R2 to > 470K to use a 50K pot.


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## Glassman

http://www.pha.inecnet.cz/macura/buffer_en.html


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## Nisbeth

The design looks OK to me 

 1) I would replace C1-C4 with smaller caps (say 10uF tantalum or electrolytics) for better decoupling.
 2) With R2 = 100k, you should use a 10k pot. If you want to use 50k, increase the value of R2 to minimum 470k.

 I'm not an EE, so don't put too much weight on what I say 
	

	
	
		
		

		
		
	


	





 /U.

 EDIT: I need to type faster


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## MaxW

Quote:


  Originally Posted by *mono* 
_About the power supply- See a whole lot of those turquoise G-Luxon capacitors on it! Can't know for certain if it's the same model of cap but I used to have a shoebox full of turquoise G-Luxons that'd failed (came off of motherboards), they aren't such great caps to use to say the least._

 

They are actually Saxom brand, I dont know much about them other than they are low ESR 
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *amb* 
_Looks fine to me... Basically what you have is the MINT amp except you're using a split power supply and different opamp, and you don't have the MINT's class A biasing or Jung multiloop._

 

Ahh, didn't realise but cool. I will add class A bias then by copying that.



			
				amb said:
			
		

> Your amp is set to a voltage gain of 2, not sure if it's appropriate for your application./QUOTE]
> Well my CDPs volume controlled output is the same at full as the fixed output and its plenty loud going straight into my GC, so I figure a gain of 2 will be fine.
> 
> Quote:
> ...


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## MaxW

Quote:


  Originally Posted by *Glassman* 
_http://www.pha.inecnet.cz/macura/buffer_en.html_

 

Yep, thats where carlos copied it from but he just took a couple of components out. I dunno if you can call such a simple design a "copy" though.....


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## amb

Quote:


  Originally Posted by *MaxW* 
_Hmm, the pot I want to use is 100k, its a relay based one. So maybe I should just leave R2 out? or have it at 1M?_

 

No, you should not leave R2 out because in case the pot wiper loses contact (even if for a brief moment), the opamp will lose its bias at its + input and you'll likely get a large DC offset at the output, loud noise, or both.

 A 100K ohm pot is kind of high, and not real good for noise performance. If you must use 100K then increase R2 to 1M ohms.

 Oh, and you should probably add a small compensation capacitor between pin 6 and pin 2 of the opamp (around 10pF - 22pF). This will tame any ringing in the amp's square wave response.


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## Nisbeth

Max sorry for "borrowing" your thread, but from your post I just remembered that I actually did a similar layout a while ago but never really managed to do anything more about it. It's basically a cross between a MINT and Pavel Macura's buffer and was intended to be a sort of universal preamp/headamp which i find myself needing quite frequently. If you or anyone else could comment on this I'd be grateful 
	

	
	
		
		

		
		
	


	




 (I can post the eagle files as well if anyone's interested)


 /U.

 PCB-layout:





 PCB-layout with unrendered groundplane for easier viewing.





 Schematic:






 EDIT: Layout updated. Input caps moved to make the board smaller. Size is app. 3" x 3"


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## MaxW

Quote:


  Originally Posted by *Nisbeth* 
_Max sorry for "borrowing" your thread, but from your post I just remembered that I actually did a similar layout a while ago but never really managed to do anything more about it. It's basically a cross between a MINT and Pavel Macura's buffer and was intended to be a sort of universal preamp/headamp which i find myself needing quite frequently. If you or anyone else could comment on this I'd be grateful 
	

	
	
		
		

		
		
	


	




 (I can post the eagle files as well if anyone's interested)_

 

No worries at all, go for it 
	

	
	
		
		

		
		
	


	




 Yours has a few extra components than mine, its a pity its on a double layer PCB otherwise I'd be able to print it out and etch it ghetto style 
	

	
	
		
		

		
		
	


	



 Looks nicer though, being able to fit it all on one PCB in all.


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## stadams

Hey guys,

 Just a couple of minor changes, and this design can be made to fit single-sided PCB. Take a look at it again. I do not think that it would be that hard to do.

 Later,


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## ~n00beR

I have also been working on my own similar preamp design.

 With some clever component placement it will be possible to fit it on a single sided board.
  Quote:


 Oh, and you should probably add a small compensation capacitor between pin 6 and pin 2 of the opamp (around 10pF - 22pF). This will tame any ringing in the amp's square wave response. 
 

This is very interesting I have never seen this before, however I have seen this method used to "decouple" the opamp signal from the buffer signal.

 here is what I have for the opamp section so far:


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## MaxW

Quote:


  Originally Posted by *~n00beR* 
_I have also been working on my own similar preamp design.

 With some clever component placement it will be possible to fit it on a single sided board.


 This is very interesting I have never seen this before, however I have seen this method used to "decouple" the opamp signal from the buffer signal.

 here is what I have for the opamp section so far:_

 

Post your work as you go because it will probably end up better than mine!

 This is what I have got, except I will morror it (more or less) horizontally for two channels sharing the same power rails:


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## amb

MaxW, the class A biasing of the opamp is done with a CRD in the MINT amp (CRD = "current regulating diode", basically a JFET wired as a constant current source with a source resistor built into a two-pin package). You labeled it 1N4004 on the board, and that's a regular rectifier diode, not a CRD. Look into the 1N5283 - 1N5314 series of CRDs.


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## MaxW

Quote:


  Originally Posted by *amb* 
_MaxW, the class A biasing of the opamp is done with a CRD in the MINT amp (CRD = "current regulating diode", basically a JFET wired as a constant current source with a source resistor built into a two-pin package). You labeled it 1N4004 on the board, and that's a regular rectifier diode, not a CRD. Look into the 1N5283 - 1N5314 series of CRDs._

 

Yes, I do know this but I couldn't find 1N5283 - 1N5314 in eagle so I chose the 1N4004 because it has the same package


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## Nisbeth

It's not the same package, but nevermind, Tangent has an Eagle library with the CRDs available 
	

	
	
		
		

		
			



http://www.tangentsoft.net/elec/parts/eagle/


 /U.


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## Nisbeth

Allright, did another layout with both OPA and BUF in DIP-packages. I tried to keep it as compact as possible, because (sometime) I'm going to build a multichannel preamp with 3 or 4 of these boards. The supply connections on both edges of the PCB allow for easier wiring if more boards are cascaded 
	

	
	
		
		

		
		
	


	




 The input caps are MKPs with 15 mm. lead spacing.

 PCB with unrendered groundplane:





 By reducing the size of the input caps to 10 mm lead spacing, it is possible to compact the board even further. this version measures app. 1.9" x 2.3" 
	

	
	
		
		

		
		
	


	




 PCB with unrendered groundplane:





 Comments are most welcome 
	

	
	
		
		

		
		
	


	





 /U.

 EDIT: The schematic for the above boards is here:







 EDIT: Board layouts updated. Changes: 
 - Traces widened to 40 mil all round. 
 - Small changes to routing


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## individual6891

Quote:


  Originally Posted by *Nisbeth* 
_PCB with unrendered groundplane:





 /U._

 

This seems preferable, people don't need huge C1 caps, and if they needed audio grade polys, they'd have to mount off board anyways 
	

	
	
		
		

		
		
	


	




 10mm spacing should be sufficient for most standard polys imo. Plus reduce board space is always good


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## Nisbeth

Quote:


  Originally Posted by *individual6891* 
_This seems preferable, people don't need huge C1 caps, and if they needed audio grade polys, they'd have to mount off board anyways 
	

	
	
		
		

		
		
	


	




 10mm spacing should be sufficient for most standard polys imo. Plus reduce board space is always good 
	

	
	
		
		

		
		
	


	


_

 

The biggest MKP-type I can get with 10mm lead spacing is 220nF (Rifa PHE426). They should work fine as long as R2 in the schematic is 150kOhms or more 
	

	
	
		
		

		
		
	


	





 /U.


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## amb

Nisbeth, the Wima MKP2 0.22uF 100V box caps has 5mm lead spacing. You might want to add some pads to allow 5mm LS caps too.


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## MaxW

Nisbeth, if you get some of these manufactured let me know


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## Nisbeth

Will do 
	

	
	
		
		

		
		
	


	





 /U.


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## Nisbeth

Allright, I played a little more with the layout and added a pot for usign the design as a complete preamp. Then it looks like this:






 The pot is a Noble, but an Alps blue fits just as well 
	

	
	
		
		

		
		
	


	





 /U.


 PS: AMB, I'd like to modify the boards to allow other lead spacings for the input cap, but is there an easier way to create the pads in eagle besides modifying the existing library parts to include the extra pads? If anyone has a library of box caps modified to have extra pads, I'd really like to know 
	

	
	
		
		

		
		
	


	




 EDIT: Layout updated with cleaner routing of the input traces


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## amb

Quote:


  Originally Posted by *Nisbeth* 
_AMB, I'd like to modify the boards to allow other lead spacings for the input cap, but is there an easier way to create the pads in eagle besides modifying the existing library parts to include the extra pads?_

 

I am still rusty with Eagle, but is it not possible to just place an extra pad and trace without adding or modifying a part?


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## DaKi][er

Quote:


  Originally Posted by *Nisbeth* 
_I'd like to modify the boards to allow other lead spacings for the input cap, but is there an easier way to create the pads in eagle besides modifying the existing library parts to include the extra pads?_

 

just place more caps of the size you want over the top of the ones already there, so they will be 2 seperate parts in there but you'll only use one in the end


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## Nisbeth

Thanks for the tips, I'll look into it 
	

	
	
		
		

		
		
	


	





 /U.


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## MaxW

Does this look bad for a single layer design?






 I know it still needs a tidy up, some traces will be thicker, a couple of extra holes etc but any obvious problems?


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## individual6891

Quote:


  Originally Posted by *MaxW* 
_I know it still needs a tidy up, some traces will be thicker, a couple of extra holes etc but any obvious problems?_

 


 Fixed gain? Or adding pot later?


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## MaxW

Quote:


  Originally Posted by *individual6891* 
_Fixed gain? Or adding pot later?_

 

I will be adding a pot later as its a relay based one on a separate PCB


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## mirlo

How bad does a BUF634 sound all by itself, no feedback?


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## MaxW

I have finished it.
 EDIT: For some reason the schematic file is 196k so I cant attach it. The brd file is attached so you can see that layout and anyone who wants the schematic file can just ask and I'll email it to them.


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## MaxW

I have finished it but have 2.5V and 0.8V offset 
	

	
	
		
		

		
			




 I have gone through it and all resistors are what they should be according to my original eagle file. All resistors are 1% and are within a few ohms of each other (I matched them). I have measure the voltage at all the ICs power pins and they are all normal.

 I think maybe I have made a really obvious mistake or something??
 Any ideas?

 Here are my eagle files:
http://max8888.orcon.net.nz/temp/preamp.zip

 And some pics.....











 Thanks alot if you can help


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## MaxW

Do the buffers affect the DC offset at all? Can I just take them out and measure the offset at pin 3 of the buffers empty socket to make it more simple?


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## amb

Quote:


  Originally Posted by *MaxW* 
_Do the buffers affect the DC offset at all? Can I just take them out and measure the offset at pin 3 of the buffers empty socket to make it more simple?_

 

The buffers are wrapped within the global feedback loop. If you yank the buffers, you break that loop and the DC offset at the opamp output would most likely go skyhigh.

 Just to rule out a dumb error, when you measured the DC offset, were you using the mV scale on your meter? If so you probably read 2.5mV and 0.8mV, which are quite reasonable.

 If the offset really is 2.5V and 0.8V, then something is definitely wrong. Maybe the circuit is oscillating?


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## MaxW

Quote:


  Originally Posted by *amb* 
_The buffers are wrapped within the global feedback loop. If you yank the buffers, you break that loop and the DC offset at the opamp output would most likely go skyhigh._

 

OK, I wont try that.

  Quote:


  Originally Posted by *amb* 
_Just to rule out a dumb error, when you measured the DC offset, were you using the mV scale on your meter? If so you probably read 2.5mV and 0.8mV, which are quite reasonable._

 

I wish! My meter is auto-ranging 
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *amb* 
_If the offset really is 2.5V and 0.8V, then something is definitely wrong. Maybe the circuit is oscillating?_

 

OK, I'm guessing you have to have an input connected to measure DC-offset? I plug in my CDP (which measures 10k ohm) and offset is 1.2mV and 0.4mV and it sounds all good 
	

	
	
		
		

		
		
	


	



 Still alot for me to learn


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## Nisbeth

Quote:


  Originally Posted by *MaxW* 
_OK, I'm guessing you have to have an input connected to measure DC-offset? I plug in my CDP (which measures 10k ohm) and offset is 1.2mV and 0.4mV and it sounds all good 
	

	
	
		
		

		
		
	


	



 Still alot for me to learn 
	

	
	
		
		

		
		
	


	


_

 

On the contrary, you should have your inputs shorted to ground. If you haven't, then that's probably the source of the problem 
	

	
	
		
		

		
		
	


	





 /U.


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## skyskraper

if ur meter's auto ranging its likely it did go down to mv. at least thats my experience with auto ranging meters. maybe check if it said V on the screen when u took the reading? oh and ive picked up from being on here to have nothing connected when measuring dc offset... just ground to signal out.


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## MaxW

Quote:


  Originally Posted by *Nisbeth* 
_On the contrary, you should have your inputs shorted to ground. If you haven't, then that's probably the source of the problem 
	

	
	
		
		

		
		
	


	



 /U._

 

Yep, sorted, less than 1mV i each channel now 
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *skyskraper* 
_if ur meter's auto ranging its likely it did go down to mv. at least thats my experience with auto ranging meters. maybe check if it said V on the screen when u took the reading? oh and ive picked up from being on here to have nothing connected when measuring dc offset... just ground to signal out._

 

I have mastered my meter, the problem was above 
	

	
	
		
		

		
		
	


	




 The eagle files are also linked above, when I move them I'll update the post so people can get them in the future. Thanks for your help guys!!


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## skyskraper

so. hows it sounding?


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## MaxW

Quote:


  Originally Posted by *skyskraper* 
_so. hows it sounding? 
	

	
	
		
		

		
		
	


	


_

 

I think good


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## tobias_svensk

Became quite inspired when Nisbeth posted post#9 so i also tried to make a layout. Ended up simular to Nisbeth's 
	

	
	
		
		

		
		
	


	




http://www.ettnet.se/~tobias/diy/opabuf/

 On the bottom side i now have some SMD JFets to bias into class-a


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## MaxW

Quote:


  Originally Posted by *tobias_svensk* 
_Became quite inspired when Nisbeth posted post#9 so i also tried to make a layout. Ended up simular to Nisbeth's 
	

	
	
		
		

		
		
	


	




http://www.ettnet.se/~tobias/diy/opabuf/

 On the bottom side i now have some SMD JFets to bias into class-a_

 

Very nice.


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## ~n00beR

Quote:


  Originally Posted by *tobias_svensk* 
_Became quite inspired when Nisbeth posted post#9 so i also tried to make a layout. Ended up simular to Nisbeth's 
	

	
	
		
		

		
		
	


	




http://www.ettnet.se/~tobias/diy/opabuf/

 On the bottom side i now have some SMD JFets to bias into class-a_

 


 I'm still woking on my own single sided version, but its looking very very similar to your own.

 Do we still need to bias the opamp into class a, if we set the wb mode of the buffer without a resistor?, I think the buffer wouldl draw around 15ma.
 Am I on the right track with this, or am I lost 
	

	
	
		
		

		
		
	


	




 Thanks

 Craig


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## tobias_svensk

You still have to bias the opamp. 

 I'm having R2, the step response Cap. and two JFET as SMD components on my layout now.
http://www.ettnet.se/~tobias/diy/opabuf/smd.jpg 

 Was thinking about OnSemis BFR30L & BFR31L in SOT23 package.
 BFR30L - IDSS(min) 4mA / IDSS(max) 10mA
 BFR31L - IDSS(min) 1mA / IDSS(max) 5mA

 What do you think about these?


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## ~n00beR

Well after much reading,working and admiring I have a circuit I'm happy with. Well I was happy with my design until tobias posted his and I thought "but it's so much prettier than mine" :jealous:

 So off I went and rejumbled his layout to come up with my own. 
 Hope you dont mind tobias.

 Can you guys have a quick check over the circuit to make sure its ok, before I go any futher.

 The pigtails joined by the yellow connection lines will be joined by the groundplane 
	

	
	
		
		

		
		
	


	




 ok here it is:


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## ~n00beR

and with ground plane:


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## tobias_svensk

Ohhh now im drunk. noober, if i were you i would go for your own design because the designs that have been posted here are pretty much alike so you would be most satisfied with your own.

 Sorry spelling


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## ~n00beR

Ok here is my oiginal attempt, I know two of the resistors don't go to ground, I will probably just jumper them.

 Any input is much apprciated.

 Sorry only half of the cicuit as image was to large 
	

	
	
		
		

		
			






 Cheers

 Craig


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## meat01

I have seen this design and always wanted to try it out, but never got to it. I believe I have a lot of the parts on hand and with a layout and a schematic, I am going to try to build it on a proto board.

 Can you help me out with some of the values? Are these values correct?

 C1=?
 C5&C9=10uF
 C4&C8=.1uF
 C10=?

 Thanks.


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## MaxW

Quote:


  Originally Posted by *meat01* 
_I have seen this design and always wanted to try it out, but never got to it. I believe I have a lot of the parts on hand and with a layout and a schematic, I am going to try to build it on a proto board.

 Can you help me out with some of the values? Are these values correct?

 C1=?
 C5&C9=10uF
 C4&C8=.1uF
 C10=?

 Thanks._

 

Im not sure where you are getting those cap names but there are only 2x different types used.
http://www.pha.inecnet.cz/macura/buffer_en.html
 C2-C5 = 100nF (0.1uF)
 C6-C9 = anything from 200uF - 2200uF or more if you wish. (I used 330uF)


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## meat01

I was looking at the PCB layout and schematic that Nisbeth posted. Sorry I didn't clarify with there being so many different schematics and PCB layouts being posted.


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## ~n00beR

Here is my new design.

 After a bit of discussion over at diyaudio, I have decided to go for independant channels, and after a coment someone made they are the same size, and of similar layout to BrianGT's gainclones.

 This first version is single sided, for testing, but I am also working on a double sided version.

 Let me know what you guys think

 Cheers

 Craig.


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## ~n00beR

Well after my moment of Brian GT clone hybrid monster insanity, I returned sensibly to my original design.
 I have been discussing my design over at Diyaudio, and this is my latest itteration.

 Dual mono, independant left/right ground planes, and independant power/signal gounds.

 As always your comments ar welcome 
	

	
	
		
		

		
		
	


	




image link


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## MaxW

Good job n00beR 
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *~n00beR* 
_independant left/right ground planes, and independant power/signal gounds.[/URL]_

 

Does anyone have an oppinion on this? Aren't they joined on a Gainclone anyway so they will be joined when you connect them to Gainclone?

 Also, do the people here think it makes a difference to an OPA627 whether the 100nF caps on the power rails are 5mm from the IC or 25mm from the IC?

 Its interesting to hear different people oppinions on this


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## ~n00beR

Quote:


  Originally Posted by *MaxW* 
_Good job n00beR 
	

	
	
		
		

		
		
	


	






 Does anyone have an oppinion on this? Aren't they joined on a Gainclone anyway so they will be joined when you connect them to Gainclone?

 Also, do the people here think it makes a difference to an OPA627 whether the 100nF caps on the power rails are 5mm from the IC or 25mm from the IC?

 Its interesting to hear different people oppinions on this 
	

	
	
		
		

		
		
	


	


_

 

If you buy gainclone yes they will be connected, but if you design your own....

 My idea was if anyone wanted to use my design, it would allow the use of any power supply, if you had a noisy supply ground it wouldn't effect the signal ground. 
 But if you have ripple on the supply pins, well thats a different probleme entirely.

 Craig.


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## ~n00beR

Quote:


  Originally Posted by *MaxW* 
_Good job n00beR 
	

	
	
		
		

		
		
	


	






 Does anyone have an oppinion on this? Aren't they joined on a Gainclone anyway so they will be joined when you connect them to Gainclone?

 Also, do the people here think it makes a difference to an OPA627 whether the 100nF caps on the power rails are 5mm from the IC or 25mm from the IC?

 Its interesting to hear different people oppinions on this 
	

	
	
		
		

		
		
	


	


_

 

Dont know if anyone has read the article that PMA linked to but, it discusses all the points we seek clarity on.

  Quote:


 Problems arise when designers fail to pay attention to some important details that may, at first, seem trivial. A very typical problem occurs when the same grounding point is used for both high- and low-level signals. An extreme example of this is where a low-level signal component is tied to the same grounding point as a power-supply filter capacitor. The high currents flowing in the capacitor modulate the low-level signal and introduce power-supply hum into the signal path. Similar problems occur when high- and low-level signals share a common ground-return line, especially a thin (high-Z) run on a pc board. The high-level signal mixes with and modulates the weaker signal, often causing crosstalk or oscillation. 
 

Hehehe, I knew my idea wasn't so stupid


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## individual6891

Quote:


  Originally Posted by *Nisbeth* 
_



_

 

I was just wondering whether this'll be turned into a GB at all? I know there'd be loads of people interested too over at the "other diy forum" for the gainclones.


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## ~n00beR

I agree things have really ground to a halt every where, lol. Must be something to do with the start of the better weather 
	

	
	
		
		

		
		
	


	




.

 I have finally decided on my own layout and will probably not make anymore changes to it.

 The designs are discussed more in this thread at diyaudio.

 If there is enough interest I will post my eagle files (sorry no schematic)

 Good luck and have fun


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## Nisbeth

Just noticed the bump in this thread (been out of town for a few days). I will be ordering protoypes of the above design when I am no longer bogged down by various other projects, school, work etc. This is most likely not until mid july 
	

	
	
		
		

		
		
	


	




 After that, a GB might be worth looking in to. In the mean time, if anyone is interested in the eagle files just send me a PM! 
	

	
	
		
		

		
		
	


	





 /U.


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## ryssen

Got any PCB´S for sale yet?


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## rsaavedra

Any news on the availability of the BUF634's?


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## ~n00beR

The design really took off over at diyaudio.com, and the enthusiasm was picked up by other users.

 Cant remeber the guys name that was involved in the thread, but he cam up with his own designs, and was selling them.

 The designs were the "freebird" and the "yardbird" do a search over there and you might find he may have a couple left.

 As far as i can remember neither of those designs include a buffer.

 Hope this helps

 n00ber


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## Nisbeth

Quote:


  Originally Posted by *~n00beR* 
_The design really took off over at diyaudio.com, and the enthusiasm was picked up by other users.

 Cant remeber the guys name that was involved in the thread, but he cam up with his own designs, and was selling them.

 The designs were the "freebird" and the "yardbird" do a search over there and you might find he may have a couple left.

 As far as i can remember neither of those designs include a buffer.

 Hope this helps

 n00ber 
	

	
	
		
		

		
		
	


	


_

 

The guy's name is Russ White. He also has a website up at www.twistedpearaudio.com





 /U.


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## DaKi][er

I'm sure if anyone asks nice enough for gerber layouts, then batchpcb will make them for only $2.50 per sq in. +$10 setup
 Doesn't look much over $20 all up from here, bargain for a custom made single pcb order


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