# AD8397-class A?



## MASantos

Would the AD8397 OPAMP benifit from class A biasing or would this cause any oscilation/instability problems? 

 Manuel


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## robzy

Class A biasing is not really possible for opamps driving a headphone load directly. (I assume you dont have a buffer?)

http://tangentsoft.net/audio/opamp-bias.html

 To summarise: The amount of current you would have to draw might be in the order of hundreds of milliamps, which is unrealstic.

 Rob.


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## tangent

Quote:


  Originally Posted by *MASantos* 
_Would the AD8397 OPAMP benifit from class A biasing_

 

Why not try it and see?

  Quote:


 would this cause any oscilation/instability problems? 
 

If that's all it takes to destabilize your configuration, you didn't have enough margin to begin with.

  Quote:


  Originally Posted by *robzy* 
_The amount of current you would have to draw might be in the order of hundreds of milliamps_

 

I can't see why. Even with an unrealisticially high load of 2V peak into 32 ohms, you only need 62 mA. That's well within the 8397's abilities. 

 Now, problems I do see are:

 1. How do you keep the op-amp cool? You'll definitely have to use the EPAD version, and it'll have to be connected to a substantial copper plane.

 2. What do you use for the CCS? A lot of the favorites around here won't scale up that high.


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## robzy

Quote:


  Originally Posted by *tangent* 
_I can't see why. Even with an unrealisticially high load of 2V peak into 32 ohms, you only need 62 mA. That's well within the 8397's abilities._

 

Hrm, good point. 

 A bit off topic then - is the reason the PIMETA/MINT have buffers after the class-A biased opamps because the opamp did not have the current capability for both the load and the CCS?

 Thanks,
 Rob.


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## MASantos

Quote:


  Originally Posted by *tangent* 
_Why not try it and see?

 If that's all it takes to destabilize your configuration, you didn't have enough margin to begin with._

 

I don't understand your point. Could you explain this?

  Quote:


  Originally Posted by *tangent* 
_Now, problems I do see are:

 1. How do you keep the op-amp cool? You'll definitely have to use the EPAD version, and it'll have to be connected to a substantial copper plane.

 2. What do you use for the CCS? A lot of the favorites around here won't scale up that high._

 

Cooling: Using the EPAD version connected to the ground plane and use some sort of heasink on top of the IC.

 CCS: either a CRD or jfet cascode configured for more current output.

 the reason for this thread is that the PINT(mini3fied) has amazing sound for it's size and I was thinking what could be done without size limitations. A home version with beefier PS section using a TLE2426 to create the virtual ground, Maybe isolated power rails a la PPA and M3, Alps RK27 pot etc.

 It would be a killer amp!


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## jcx

Quote:


  Originally Posted by *robzy* 
_Class A biasing is not really possible for opamps driving a headphone load directly. (I assume you dont have a buffer?)

http://tangentsoft.net/audio/opamp-bias.html

 To summarise: The amount of current you would have to draw might be in the order of hundreds of milliamps, which is unrealstic.

 Rob._

 


 Unrealistic but quite doable- isn't that what this hobby is about?

http://www6.head-fi.org/forums/showt...pa#post2082883

 (scroll up page to see pics)

 My proto is about Class A output operation over the full range with op amps - yes it does require "order of hundreds of milliamps" - I used ~220mA push-pull (440mA pk) to get 3 Wrms into 32 Ohms - as per Gilmore's recommendation of required headphone power 


 most "class A op amp biasing" you see in headphone op amp circuits is only a few mA for internal stage use

 it is likely the AD8397 is already working with 3-5 mA pp output stage bias internally to get good MHz distortion specs - so the 1st 5-10mA of output is probably "class A"

 if you really want to go full ouptut swing Class A you're limited in sucking power out thru the pcb with the EPAD to a few W - heatsink on top of the package won't help much - after all the die/leadframe is deliberately set downward to be exposed on the bottom of the package so you have nearly the full package thickness of molding compound in between - this isn't like a large chip area/thin molding memory or cpu thermal path - the op amp die is small and the plastic molding compound thickness is large

 which is why I went "belly up" with the heatsink clamped to the exposed thermal pad - I am sucking 6 W from each package from the 2 most heavily loaded chips in my cascaded/paralleled amp - the bigger TPA6120 op amp packages in my amp have the Cu slug Zalman cpu heatsink clamped to the TPA power pads with a few mil smear of Artic Silver in between - pcb power plane heat sinking is for wimps!


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## MASantos

Could you post some pics of the internals and how you secured the heatsink to the IC?


  Quote:


  Originally Posted by *jcx* 
_Unrealistic but quite doable- isn't that what this hobby is about?

http://www6.head-fi.org/forums/showt...pa#post2082883

 (scroll up page to see pics)

 My proto is about Class A output operation over the full range with op amps - yes it does require "order of hundreds of milliamps" - I used ~220mA push-pull (440mA pk) to get 3 Wrms into 32 Ohms - as per Gilmore's recommendation of required headphone power 


 most "class A op amp biasing" you see in headphone op amp circuits is only a few mA for internal stage use

 it is likely the AD8397 is already working with 3-5 mA pp output stage bias internally to get good MHz distortion specs - so the 1st 5-10mA of output is probably "class A"

 if you really want to go full ouptut swing Class A you're limited in sucking power out thru the pcb with the EPAD to a few W - heatsink on top of the package won't help much - after all the die/leadframe is deliberately set downward to be exposed on the bottom of the package so you have nearly the full package thickness of molding compound in between - this isn't like a large chip area/thin molding memory or cpu thermal path - the op amp die is small and the plastic molding compound thickness is large

 which is why I went "belly up" with the heatsink clamped to the exposed thermal pad - I am sucking 6 W from each package from the 2 most heavily loaded chips in my cascaded/paralleled amp - the bigger TPA6120 op amp packages in my amp have the Cu slug Zalman cpu heatsink clamped to the TPA power pads with a few mil smear of Artic Silver in between - pcb power plane heat sinking is for wimps!_


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## jcx

maybe pics soon -

 if I forget the weeks of all niters required to build it

 if I figure out the digital camera at work

 if I don't play outdoors weekends - its summer in case you hadn't noticed

 I may open it up to rework/optimize some compensation before the Boston meet


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## Andrea

jcx, maybe it's just me but that sounds pretty crazy


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## tangent

Quote:


  Originally Posted by *robzy* 
_is the reason the PIMETA/MINT have buffers after the class-A biased opamps because the opamp did not have the current capability for both the load and the CCS?_

 

Bingo. Not only are most op-amps limited to around 40 mA into a dead short circuit, they'll actually go into current limiting, harming their sound quality, well before that. There's just no good way to push such op-amps into class A if you want them to drive a load, too.

  Quote:


  Originally Posted by *MASantos* 
_CCS: either a CRD or jfet cascode configured for more current output._

 

The biggest CRDs I'm aware of are around 5mA.

 As for raw JFETs, when you cascode JFETs, the total current is some fraction of the Idss of the lower JFET, so you want one somewhere up in the 100mA range to hit my 62mA number, and more like 200-300mA to make jcx happy. I did some searching, and couldn't find such a beast.

 This is exactly why I said what I did above. You're going to have to look beyond the old favorites. As a starter, watch the "Precision Current Source" show here. There are several more that Pease didn't cover there. In fact, I believe I recently saw a book just on this topic.

  Quote:


  Originally Posted by *jcx* 
_I used ~220mA push-pull (440mA pk) to get 3 Wrms into 32 Ohms - as per Gilmore's recommendation of required headphone power_

 

Hmm...I see 125 mA in the standard Gilmore dynamic amp article on Headwize. You need twice my recommended voltage for this, and I thought I had it overspec'd pretty wildly already.

 All three numbers probably give class A, but the sound is no doubt different. The main points are: 1) yes it can be done with this chip; 2) no it can't be done with most other chips.


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## tangent

Quote:


  Originally Posted by *MASantos* 
_ Quote:


  Originally Posted by *tangent* 
If that's all it takes to destabilize your configuration, you didn't have enough margin to begin with.

 

I don't understand your point. Could you explain this?_

 

A CCS presents only two minor problems to its current source: a static (DC) current load, and probably a bit of input capacitance. The 8397 is a cable driver IC, so high currents and a fair bit of capacitive load are what it is designed to drive. You would have to be driving the chip right on the edge of oscillation already for this to send it over the edge.


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## steinchen

Quote:


  Originally Posted by *tangent* 
_As for raw JFETs, when you cascode JFETs, the total current is some fraction of the Idss of the lower JFET, so you want one somewhere up in the 100mA range to hit my 62mA number, and more like 200-300mA to make jcx happy. I did some searching, and couldn't find such a beast._

 

just for info: BF246C is such a beast, followed by BF246B and pn4391. Nevertheless I'd rather pick a good buffer than deal with the all the hassle of this route.


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## jcx

OK a little “moonlite” photo session at work:












 The heat sink is a //Zalman//[edit: Thermaltake] Golden Orb II, drilled and tapped for a ¼-20 ss socket head screw which protrudes from the center of the Cu slug bottom and to which the pcb carrying the TPA6120s is clamped

 The 6 TPA prints of Artic Silver are visible on the Cu slug of the Zalman

 The Al tripod has nylon pcb standoffs cut down to press on the pcb – the conical spring washers maintain clamping force

 The TPA6120 op amps were laid down belly up on the pcb and each leg was bent down and soldered (how? – very carefully, with the aid of optics) – You can see I didn’t bother to bend down all of the legs – some are not connected internally in the TPA

 The pcb turned out to be flat enough that even with the 6 packages there was only a few mil of unevenness – plenty small enough gap given the Artic Silver thermal conductivity (don’t get it on the circuitry – not formulated as a electrical conductor it still can be a problem)

 I’m sure there are simpler solutions possible but I wanted a “universal” proto amp +/-24 V swing for Hi Z cans and +/-440 mA pk Class A for Lo Z cans – these #s give ~ 25 W total power dissipation in Class A for the 2 channels/6 TPA6120s


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## tangent

Quote:


  Originally Posted by *steinchen* 
_just for info: BF246C is such a beast, followed by BF246B and pn4391_

 

Not really. I can't find any sources for the 246C in the US, but I did find the B, and its minimum Idss is 60mA. That means you'll have to cherry-pick parts to get a cascode with >60mA using this JFET. You'd need a minimum Idss somewhere above 100mA to be able to use arbitrary JFETs.

 Look, I'm not trying to say this is impossible. Just impractical.


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## MASantos

I don't think it will be practical to implement this feature acording to the replies I am receiving here. 

 Other questions:
 The datasheet says that the AD8397 benefits much from well regulated power. Since batteries aren't really desired for a home amp, I tought about a LM317 based power supply with a Filtered IEC receptacle. 
 Would there be any other regulator IC that would be a good improvement without increasing the price of PS section a lot?

 As for the amp's power section I want to use a TLE2426 as voltage divider and probably more capacitance. Would there be any benefit of using a buffer(probably opa551) to increase current capability? I believe that using jfet to isolate the power rails would be useful? Or does this only apply if there are two different power sections such as in the PPA? 

 I read in the PPA page that is uses diferential drive topology bt haven't found anything about it in the other documents. Could you explain this better?

 Tangent, apart from your article regarding virtual grounds and related articles, what other documents are worth reading for this subject?

 Manuel


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## jcx

You do seem to be stuck conceptually here on the idea that fets are the only current sources in the world and that you even want a ccs for Class A bias for full range output drive

 A ccs loading a op amp creates a single ended Class A output with only 25% efficiency of peak output power to static dissipation ratio – measure output in rms W and efficiency drops even more – ½ of the static power is wasted heating up the op amp, the op amp sees even worse dynamic current demands as it has to supply 2x current on positive peaks – both the load and the ccs current (sink)

 Class A bias can also be done push-pull – each of 2 amplifying devices see ½ the peak load current at rest, each device current changes linearly as the output moves, one increasing, the other decreasing so that at full swing one device just reaches 0 current as the other supplies all of the load current

 Push-pull Class A is 50% efficient from peak power to static and requires each device only be biased at ½ the peak current and dissipate only ½ of the peak rated power – much easier if heat removal is your limitation

 A op amp push pull class A can be done with the A47 style parallel scheme – just introduce a floating voltage source in series with the “slave” op amp’s input to offset the 2 op amp outputs to get the desired current thru the resistors that sense and add the outputs – the approach I used


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## tyre

Hi jcx, do you have a schematic for your tpa6120 amp?


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## jcx

seems likely doesn't it...

 but I'm not willing to post the full schematic - at 100+ smt parts/channel on both sides of a 4 layer board I doubt many here would get it to work - this is very much an experimental prototype and far from a DIY construction project - and I might prefer presenting the unique multiloop feedback idea in a more formal setting

 I may give more details of the TPA "totem-pole" cascade power output stage later if people are interested - although it has unique stability limitations of its own too that will probably prevent much DIY use

 the above LtSpice "sketch" demonstrates the principle I used for Class A bias in my amp


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## jcx

my TPA6120 multiloop Class A parallel/cascade opened up on bench - how it usually runs for debugging/mods

 side view shows thermal path stackup:






 AD8397 package is smaller but likely it is possible to suck enough power out with a similar approach that at least the push-pull Class A scheme I use could work over some range - someone else has reported using them upside down with Cu sheet(flashing?) "U" glued to their EPADs


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## motherone

Dang JCX, nice work. I should dig around and see if I can find one of my nice Panaflo orb heatsinks (those are the ones that got the whole "orb" heatsink thing started ~10-12 years ago). I keep thinking about using them to cool a gainclone... Never thought I'd see one used on a headphone amp


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## mefistofelez

Dear jcx,

 you wrote: "I may give more details of the TPA "totem-pole" cascade power output stage later if people are interested - although it has unique stability limitations of its own too that will probably prevent much DIY use"

 I am surprised that nobody has taken you on the offer. I for one would be extremely interested in your implementation of the multiloop concept, having read all you have published about it on various forums, especially because you are using amplifiers in both loops instead of the (questionable?) amplifier-follower configuration.

 Thank you,

 M


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## Filburt

Quote:


  Originally Posted by *mefistofelez* 
_Dear jcx,

 you wrote: "I may give more details of the TPA "totem-pole" cascade power output stage later if people are interested - although it has unique stability limitations of its own too that will probably prevent much DIY use"

 I am surprised that nobody has taken you on the offer. I for one would be extremely interested in your implementation of the multiloop concept, having read all you have published about it on various forums, especially because you are using amplifiers in both loops instead of the (questionable?) amplifier-follower configuration.

 Thank you,

 M_

 

I'm surprised as well. I'd have been willing to try it ;D


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## jcx

[a little revisioning - moved this post which really addresses this thread's title to here from the "Designing a Amp" thread]

 This sim shows how you could make a multiple-input differential “instrumentation” op amp circuit to perform the same Class A bias trick with AD8397 op amps which do not have rail-to-rail inputs

 The circuit is designed so that the AD8397 op amp U2 inputs only swing ~ ½ the rail voltage and uses all equal value precision resistors in the critical diff gain setting resistor network

 An added advantage is that the bias setting V source doesn’t have to float and could be common between channels – simultaneously trims both channels and allows a single Class A bias on/off switch

 R1 is the “master” output current sense resistor driven by U1, R2 is the “slave” current sense resistor

 U2’s R network forms a differential input “instrumentation” amp which measures R1’s voltage drop and references it to the output node, applying the diff + the offset from U3 bias source to R2

 The sim uses the LT1128 since it comes with the LtSpice and can output ~ 30 mA – enough to swing 1 V on the 32 Ohm load for demonstration purposes U1,2 of course should be AD8397 in the real circuit and the bias bumped up to ~ 50 mA for +/- 100 mA, 3.2 Vpeak drive with 6 cell battery power








 A cost is the precision required for the diff amp resistors – but what self respecting audiophile doesn’t spend more on resistors than the active components in their amp?

 the attachment is the LtSpice .asc - just rename without the .txt and run in the free Linear Technology SwCAD III Spice



 the "totem-pole" or cascade design I used for my proto's high V swing output should probably go in another thread - maybe I''ll do that sooner now


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## mefistofelez

Dear jcx,

 I have seen this (I told you that I was watching your multi-loop posts like a hawk 
	

	
	
		
		

		
		
	


	




 ).

 Anyway, thank you for considering another post with your _implemented_ circuit.

 M


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## Dreamslacker

Big OT here:

  Quote:


  Originally Posted by *jcx* 
_The heat sink is a Zalman Golden Orb II, drilled and tapped for a ¼-20 ss socket head screw which protrudes from the center of the Cu slug bottom and to which the pcb carrying the TPA6120s is clamped_

 

Actually, that's a Thermaltake Golden Orb II.. Zalman flower heatsinks don't use a slug with aluminium radian-crotch fins like that.. They clamp thin copper and/ or aluminium sheets together and fan them out into a orb design..

  Quote:


  Originally Posted by *motherone* 
_Dang JCX, nice work. I should dig around and see if I can find one of my nice Panaflo orb heatsinks (those are the ones that got the whole "orb" heatsink thing started ~10-12 years ago). I keep thinking about using them to cool a gainclone... Never thought I'd see one used on a headphone amp 
	

	
	
		
		

		
		
	


	


_

 

Yep.. Panaflo 'orbs' on the DEC Alphas which led to Artic Agilents then to the ThermalTake Golden Orb, Blue Orb Dragon Orb & what not... The Tt products were infamous core-crushers back then..


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## jcx

Yes, Thermaltake - my mistake, maybe poor branding? not that I ever followed cpu heatsinking, just saw store display

 in any event I selected this Golden Orb heatsink for the Cu slug to quickly spread the heat from "over clocked" TPA6120 running at ~7.5 W per pkg to widely spaced fins that looked like they would work in convection without the fan - seems to be fine in this app, I measured < 1.5 degree C/W without the fan which is fine for my 25-30 W total heat load

 to return to the thread topic it isn't necessary to go to such extremes if you match supply V and load impedance a little better while running the ad8397 in true Class A for the full output swing

 nominally you only have to dissipate ~ as much quiescent pwr in push-pull Class A as you want to deliver to your load (with peak to rms and V swing issues maybe quiescent * 1/2 => rms pwr to load is a better estimate)

 the ad8397 could handle 2 W pwr diss per pkg with a good EPAD pcb plane heatsinking scheme - this would allow 1 Wrms/channel to the load, enough headroom for many cans in full output Class A operation if you match the supply V to the impedance without my extreme heatsinking measures


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