| BWRX |
Hi all. I don't normally visit this forum since the class d forum is just so much fun :) but I came over here because I'm thinking about building a gainclone to compare my own little creations to. Anyway, I was reading through the mauro penasa LM3886 thread (holy jeez it's a long thread) and checking out all the great info in there when mauro's ref schematic jumped out at me. It looked like an intrumentation amplifier configuration but it isn't. My next thought was "would it be possible and has anybody tried a configuration like that?"
The instrumentation configuration would have some nice advantages like high common mode rejection, very high input impedance, a differential input, and a good low impedance buffer to drive the 3886 inputs. My question to the chip amp experts is whether or not a chip like the 3886 used as the differential amp in an instrumentation configuration would work or not? Check out the attached schematic to see what I'm talking about. R5 and R4 would have to be chosen so that the 3886 is stable, then you can fiddle with the gain of the buffers to get the overall gain you'd want. |
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| BWRX |
I've been searching this whole time and finally found a thread (started earlier this year actually) with almost exactly the same schematic - right down to the use of the 2134 which just happens to be the package I use for dual opamps in a DIP8 package. But there's nothing saying if it was ever implemented. Anyone know if it was?
http://www.diyaudio.com/forums/show...5986#post815986 |
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| BWRX |
| Nothing eh? Either the answer to my inquiry is very obvious and I've missed something in my searches or no one has tried this! Either way, I'll be placing a monster digikey order soon so I might as well add a few LM3886 chips to the heap so I can start experimenting. |
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| halo0925 |
I used a 3875 as a stand alone differential amp, takes a little resistor fiddleling to get frequency and stability correct.
I think you should have good luck, I thought about useing a precision op amp in a feed forward error correction mode also.
Good Luck.... |
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| neutron7 |
I think the reason the mauro penasa LM3886 circuit works well is because the op-amp is inside the feedback loop.
it looks like what you have is a balanced driver, but its right in front of the amp instead of in the preamp.
still it looks like it might be worth trying. |
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| BWRX |
I've changed the previous schematic around so that not only the instrumentation configuration could be built, but also many other basic configurations as well. The resulting circuit is fairly simple and lends itself to easy routing with an LM3875 chip, which is the one I have selected to use for these boards. Introducing the highly configurable gainclone...
Would anyone be interested in a surface mount version of this board? The layout is all but finished and it uses 0805 and 1206 size parts and a DIP8 dual opamp package. Very simple, straightforward, and gives you many options as to what to build. If you don't want to do the intstrumentation configuration you can leave R2 open and use the opamp as a buffer. You can configure the opamp to be inverting with unity gain or higher or non-inverting and a gain of 2 or higher. Depending on which input you route the signal to you can have a non-inverting buffer/non-inverting 3875, inverting buffer/non-inverting 3875, or inverting buffer/inverting 3875. You can also choose to bypass the opamp completely and do an non-buffered inverting or non-inverting topology.
Lastly, is there anything anyone would like to see added? As I said, the layout is all but finished and the only thing I'm thinking of adding is an on board regulator for the opamp supply rails.
How necessary is the ~0.1ohm resistor in series with the output of the 3875? I was hoping that the series resistance of speaker cables alone would be enough... If not I'm sure it would be ok to connect a leaded resistor in series from the board output to a binding post (if necessary). |
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| peranders |
| quote: | Originally posted by BWRX
How necessary is the ~0.1ohm resistor in series with the output of the 3875? I was hoping that the series resistance of speaker cables alone would be enough... If not I'm sure it would be ok to connect a leaded resistor in series from the board output to a binding post (if necessary). |
It seems that you haven't read the datasheet and the application note AN-1192 carefully. I strongly advise you to do that.
My opinion is that it is wise to have something between the amp and the speaker cable. Don't forget that 5-10 meters of cable is NOT a pure resistance! You have also 50-200 pF per meter also! |
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| pinkmouse |
| In my experience, the LM1875/3875 seem to be happy without the output resistor, but the LM3886/4780 and OPA 548/9 need it to run well. |
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| BWRX |
| quote: | Originally posted by peranders
It seems that you haven't read the datasheet and the application note AN-1192 carefully. I strongly advise you to do that.
My opinion is that it is wise to have something between the amp and the speaker cable. Don't forget that 5-10 meters of cable is NOT a pure resistance! You have also 50-200 pF per meter also! |
Hi Peranders. Thank you for your concern. I've read both very carefully and more than once now, honest! :)
So they're really that sensitive to what kind of cables are used? If I were to use a single strand of cat5 cable for the conductors and have them spaced an inch or two apart over the length of the cable I could have higher resistance and lower capacitance per meter than what you would normally expect. Do you think a series output resistor would still be needed with cables like that?
If I did build this and the 3875 didn't like whatever amount of capacitance there is on the output I could just as easily put a 5W resistor (and maybe an air coil connected parallel) directly between the output of the board and the binding post (assuming I'd ever get this is in a case!). I don't really see a problem with wiring it like that, do you?
| quote: | Originally posted by pinkmouse
In my experience, the LM1875/3875 seem to be happy without the output resistor, but the LM3886/4780 and OPA 548/9 need it to run well. |
Hi Al. Do you remember what kind of speaker cable and load you have used with your 3875 based amps? Thanks for sharing your experiences with the different chips. |
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| Minion |
I now have built 2 LM3886 Guitar amps useing 2 seperate Guitar preamp designs, One Totally discreet useing Fets and NPN Transistors and one useing a OPA2134 (or any dual fet opamp)....
The LM3886 Power amp stage is the same with both amps and is basicly what is in the datasheet....
The discreet one sounds best to me for guitar but it doesn"t have much of a Clean sound , it has a pretty awesome distorted Crunchy sound which sounds great for Guitar and Bass....
If you want me to post the schematics for either preamp design let me know, They are both very simple and both use the same overdrive circuit which is a 4 stage cascadeing NPN Transistor over drive circuit....
Cheers |
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| BWRX |
I've been thinking about the stability issues without the resistor in series with the output and the fact that I don't want to take any room on the board, so I was thinking that I could implement the low impedance resistor by just using a narrow trace instead. Any thoughts on this idea? I can't really see any drawback to doing this except for not being able to remove it :)
What do you guys think?
Attached is a shot of the layout so far. I also have another version that uses 3 terminal regulators instead of discrete regulators. |
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| filholder |
| any chance i could get details of your guitar preamp front end? |
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| pinkmouse |
Brian
Sorry I missed your earlier question about cables. I used various normal (79 strand) type cables, as well as 2.5mm2 pro speaker cable and cat5.
| quote: | Originally posted by BWRX
... so I was thinking that I could implement the low impedance resistor by just using a narrow trace instead. |
You could always just leave it off, and if required, you could just tack it on to the back of the binding posts. |
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| head_spaz |
I'm interested in seeing how this project progresses. This is something I've been hankerin' to build myself, after reading AD's Inst Amp Guide. Makes sense to me.
BTW... Nice job on the PCB. I like it.
-David |
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| BWRX |
| quote: | Originally posted by head_spaz
I'm interested in seeing how this project progresses. |
For how simple this circuit is, it took me longer to finalize the design than I expected. They do say the :devily: is in the details...
Attached is the schematic. The design is flexible can be implemented with any chip amp provided you tweak the feedback resistor values to obtain stability and the gain you want. IC1A and IC1B are part of a dual DIP8 opamp package so that many different and readily available opamps can be auditioned. R1, C15, R2, C14, and C16 are optional (R1 and R2 would need to be jumpered if no filter is desired) and can be used for RF filtering. R3 and R4 are also optional and can be used to lower the input impedance.
R5, R6, R7, R8, R9, R10, and R11 are used in the instrumentation configuration. C17 is optional and can be used for further RF filtering, although it most likely won't be needed.
R12 is included for configuration flexibility. If you do away with the opamps you can configure the board to work as a simple inverting or non-inverting chip amp.
The opamp will of course need power, so a simple +/- discrete regulator (emitter follower with zener reference) is tightly integrated into the layout. It has the option of drawing power from the high voltage rails of the chip amp or from another separate external supply. Resistors between the emitters of the transistors and ground allow the opamp supply to be biased at a certain current if desired. |
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| BWRX |
The board I designed is 2" by 1.75" and uses the LM3875. It is a double sided board that uses all surface mount components (0805 size or larger) except for IC1 (DIP8 dual opamp), IC2 (LM3875), C1 (5mm lead spacing, 13mm diameter electrolytic), C2 (5mm lead spacing, 13mm diameter electrolytic), Q1 (TO92 NPN), and Q2 (TO92 PNP).
I chose the LM3875 because it has a good reputation, medium power capability, and a good pin out.
I have 6 LM3875 chips and will eventually be ordering at least 10 boards (2 layer, 2 oz copper, green solder mask, and silk screen on both sides). If anyone is interested in an excellent quality PCB for an LM3875 please let me know. The cost would be about $20 per board for 10 boards, $18 per board for 20 boards, and $15 per board for 50 boards. |
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| BWRX |
| quote: | Originally posted by BWRX
It is a double sided board that uses all surface mount components (0805 size or larger) except for... |
I forgot to include C5 and C6. They are also through hole electrolytic caps (2mm lead spacing and 5mm diameter). |
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| juergenk |
| quote: | | My next thought was "would it be possible and has anybody tried a configuration like that?" |
Iīve been running LM3886/LM3876 for several years as differential amplifier.
Square wave response with reactive load was improved over the typical circuit. I donīt know why. This may be because both opamp-inputs see equal impedance or just because of different layout.
Last year I rewired the circuit back to noninverting operation because I wanted to use a preamp with varying output impedance (a simple potentiometer!).
In this configuration I canīt stand the hum because the simple differential amplifier needs matched source impedance for good common mode rejection.
An input buffer transforming the diff. amplifier to an instrumentation amplifier would have been another option.
But I had no time to do that.
Regards
Juergen |
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| BWRX |
| quote: | Originally posted by juergenk
Iīve been running LM3886/LM3876 for several years as differential amplifier.
Square wave response with reactive load was improved over the typical circuit. I donīt know why. This may be because both opamp-inputs see equal impedance or just because of different layout. |
Square wave response may have improved but did it sound any better to you? The difference amp gives you the option to use the one side of the output feedback resistor to sense your reference voltage. If your previous circuit had grounding issues (sound like it might have since you had audible hum) that could be part of the reason for the improved response.
I guess no one is interested in a board then? I'll be placing the order this week so you've got a day or so to let me know. |
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| juergenk |
I can hardly tell about better sound. At this time I changed the whole enviroment.
But it sounded ok. :)
Btw why did you dropped the output inductor/resistor combination?
It may be useful with long cabling.
Regards
Juergen |
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| BWRX |
| I left out the resistor and inductor because they would take up a lot of space on the board and can be added off the board if they are needed. |
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| jeff mai |
| quote: | Originally posted by BWRX
I guess no one is interested in a board then? I'll be placing the order this week so you've got a day or so to let me know. |
I've been following this thread with interest. Good work!
Is the price $20 for a stereo pair or per mono channel? You say "per board" below so I assume it's the latter. Anyway, I'd like to have 3 or 4 or 5 mono channels depending on price.
The odd one would be for my mono speaker testing amp. I just happen to be making it up right now. Let me know.
Jeff |
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| BWRX |
| quote: | Originally posted by jeff mai
I've been following this thread with interest. Good work! |
Thanks :)
| quote: | Originally posted by jeff mai
Is the price $20 for a stereo pair or per mono channel? |
The price is per board and each board will give you a mono LM3875 amp. The pricing is just preliminary and may be a little lower if I can get the board house to give me a better deal. If you'd be interested in 5 boards then I would order a total of 20 and the price would be $18 per board. These will be very good quality boards which is why I don't mind paying a little more for them.
When I get home I'll post a picture of the layout to give a general idea of how the components are arranged. |
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| head_spaz |
I am still very interested in your project.
But as THEY say, timing is everything, and this is NOT a good time for me.
I wish for you great success.
-D |
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| BWRX |
Thanks headspaz.
Attached is a small shot of the layout. It is slightly larger than the actual size of the board. Surface mount components make for a very compact layout :) |
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| BWRX |
Here's a bare bones implementation of the instrumentation configuration. Lovingly cobbled together on high quality Radio Shack perfboard using only the finest bits and pieces lying around my workbench ;)
It's wired it up for 2 dual supplies so I can independently adjust the rail voltages for the opamp and LM3875. |
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| BWRX |
And the underside. Layout and wiring is straightforward since there aren't many parts. Removing the unused pins on the LM3875 helped make some space too. I opted not to include any bypass caps on either the opamp or LM3875 simply because I'm lazy and wanted to see how it works/sounds.
The blue body resistors on the top are 10k and 20k 1% metal film types. The two resistors on the bottom are 2k 5% carbon film types but I matched them to 1%. Matched resistor values are critical for proper operation of the instrumentation amp configuration. |
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| BWRX |
The prototype is up and running. I only have one amp so I hooked up my 8ohm speakers in parallel to at least get a front and center soundstage and used an old ipod as a source. To be honest I didn't expect it to sound this good at all! With no local decoupling and wires all over the place I expected at least audible hum from my speakers (90dB @ 1W/1m)... but there is none. Only if I put my ear right next to my speakers can I hear a faint white noise. DC output offset with the input shorted was about 10mV :up: The OPA2134 opamp supply is a regulated +/-12V (~100mA current capability) and the LM3875 supply is also a regulated +/-12V (dual LT1085 regulators with 23-24V unregulated but very stable input) with a lot more current capability.
As you can see below my setup isn't even close to being optimal :hot: |
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| BWRX |
| I've put in an order for some PCBs, so in a few weeks I'll let you know how they work. If all goes well, I'll have about 20 extra PCBs and will put them up for sale in the trading post. At the quantity I ordered, pricing will be $15 per board. The boards will be 2 layer, 2oz. copper, green solder mask, double sided silk screen, with outer dimensions of 2" x 1.75" (nice and compact), mounting holes for 4-40 screws, and mounting hole center spacing of 1.75" x 1.5". |
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| theAnonymous1 |
Ohnoes.:eek:
I hope you don't go changing "Class D Disciple" to "Chipamp Disciple".:cannotbe: |
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| BWRX |
| The sheer simplicity, ease of implementation, low cost, and moderate level of power that these chip amps offer was enough to pull me away from the class d stuff for a little bit :darkside: |
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| AndrewT |
Hi,
a nice layout, well done.
If I wanted to fit wired ended resistors, can the smd pads be drilled to insert the resistors in stand up mode? or are there traces on the other side? are the pads big enough for 0.7mm holes?
Matching of resistors for balanced mode cancellations should be better than 0.1%.
Why did you choose carbon for those two resistors? |
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| BWRX |
Thanks Andrew.
There are traces and parts on both sides of the board and the pads are only 1.3mm x 1.5mm so you can't really drill through the surface mount pads. You could solder through hole resistors to the pads without drilling through them but it really is easier just to use surface mount resistors. I would have made a through hole part version but I'm used to working with SMDs and there's no way I could have made it as compact as the surface mount layout.
The feedback resistors should be matched as closely as possible, as should the input resistors and capacitors (if used). I plan to order 1% resistors and 5% or 2% C0G/NP0 caps and hand match them.
Those two carbon resistors on the bottom of the board are carbon simply because I didn't have 2kohm metal films. |
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| BWRX |
| quote: | Originally posted by BWRX
I've put in an order for some PCBs, so in a few weeks I'll let you know how they work. |
They were delivered earlier than expected and they gave me a couple extra boards for free. They usually do that when they want to fill a whole panel :) Fit and finish are top notch as expected. Time to order parts... |
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| AndrewT |
Hi Bwrx,
If I were to order/buy a pair of your PCBs, can you supply the SMD components with them?
I don't want to have to buy 100s for just a few. |
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| BWRX |
If you're interested in parts as well, I could probably do that. However, the parts I plan on getting will be good quality and may end up costing more than you'd be willing to pay. For example, the resistors I have in mind (for the feedback, filtering, and gain setting positions) are 0.1% metal film types that are about $0.70 each at a quantity of 10 or more. The important ceramic caps will be C0G 1 or 5% types (others are X7R 10%) which are also around $0.80 each for 10 or more. Looking at my current BOM the cost of parts will be in the neighborhood of $30 (at quantities for about 10-30 amps), including the LM3875TF chip. If you were to get all the parts from my BOM for two boards by yourself the cost would end up being about $40 for parts alone.
edit: sorry for all the edits, I was looking at the wrong parts on the BOM. |
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| AndrewT |
Hi,
you can cut your resistor costs by a factor of ten by matching 1% metal film to better than 0.1% without expensive equipment.
Not sure how you do that with SMD caps though.
Which are the ceramic caps that need accuracy?
I would be thinking NP0=C0G for bypass and maybe RF filtering. These would not need supreme matching. |
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| BWRX |
| quote: | Originally posted by AndrewT
you can cut your resistor costs by a factor of ten by matching 1% metal film to better than 0.1% without expensive equipment. |
That's what I had planned to do initially, but quickly realized that my meter isn't capable of matching to within 1% let alone 0.1%. The 0.1% metal film resistors from Panasonic are reasonably priced (less than a dollar isn't too bad) so I just decided I'd try those out. Using them will only hurt the wallet a little bit ;)
| quote: | Originally posted by AndrewT
Not sure how you do that with SMD caps though. |
You could either do it with a very expensive capacitance meter or build your own measuring circuit based on something like a 555 timer. The main problem is how to properly connect the tiny caps in the circuit without actually soldering them.
| quote: | Originally posted by AndrewT
Which are the ceramic caps that need accuracy?
I would be thinking NP0=C0G for bypass and maybe RF filtering. These would not need supreme matching. |
The input common mode RF filter caps need to be matched or they will throw off the CMR. Think of the input resistors and caps as a bridge circuit. If the bridge becomes unbalanced then CMR is reduced. Luckily there are only two of these caps. The other way to cut costs is to not use them at all, in which case I would populate resistors in their place. In most/some cases they may not even be needed. I would still use the differential filter caps across the input pins of the opamps and the input pins of the 3875 to ensure that RF signals are common mode. |
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| AndrewT |
Hi,
if your input resistors are matched to better than +-0.1%, does that mean the input RF caps should be matched to the same accuracy? Or,
is the bandwidth where the caps become effective so high that a wider tolerance is acceptable? |
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| BWRX |
Chip manufacturers usually recommended to use 2% tolerance caps if possible (5% max), because anything less will degrade the CMR. This really is more critical when amplifying very low level signals than it is for higher voltage audio signals, but it's still good to match them as closely as possible. The whole purpose of the input filtering is to prevent any RF from being amplified. Say there were no filtering. You'd be relying on the RF to be radiated equally into both signal wires so that it is amplified equally by the input opamp amp and then rejected by the diff amp. Filtering makes any RF appear as a small DC offset, which translates to a slightly higher DC offset at the output instead of noise.
If your circuit will be housed in it's own enclosure then it might not even be needed. I have my test circuit (no decoupling on either the input opamp or LM3875 and no filtering on the input) on my desk next to my computer, halogen light, and cd player and the noise really is very low, all things considered. If I turn the halogen light up I can hear some hum caused by noise being radiated into the wiring. |
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| BWRX |
The parts came in and I made some time to build up a board which you can see in the photo below. The fake credit card is under the amp just to give you an idea of the actual size. Bulk caps are 680uF Panasonic FM, op amp caps are Elna SilmicII, op amp is OPA2134 (but I'll eventually give the LM4562 a try too). Supply rails are regulated +/-20V for the LM3875, which is regulated down to +/-17.9 for the op amp using 2N3904/6 transistors and 18V zeners. The 3875 rails can obviously go higher but I don't want to push it with the small heatsink you see in the picture. It doesn't get too hot to touch but it is nice and toasty playing into a 4 ohm nominal load (my two 8ohm speakers in parallel).
In it's current form I just routed the source ground to the inverting input and the source signal to the non-inverting input (both inputs have a nominal 100kohm input impedance set by the 100kohm resistors to ground). It sounds very nice and I can't wait to get another channel up and running. Using balanced inputs should be another step up in terms of noise performance but I don't have a balanced source/preamp.
It's worth noting that neither a zobel or parallel resistor/inductor are used because they don't seem to be needed with my setup. I'll be putting about 20 boards up for sale in the trading post if anyone is interested (details will be included in that thread). |
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| AndrewT |
Hi,
what size is that heatsink?
Looks about 10 to 14C/W to me.
You must be close to shut down driving a 100r resistive load and you go and connect a parallel pair of 8ohm speakers giving a reactive 4ohm load.:hot:
I thinks you're bonkers to rely on the chipamps self protection circuits to save your amp from destruction. :dead: |
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| BWRX |
Honestly, the heat sink is only toasty, I can hold my hand on it indefinitely no problem. I've felt the heatslug on some of my Tripath based class d amps get warmer! Besides, I want to test this thing out, make sure it works and all.
My speakers are 90dB @ 1W/1m and I'm sitting 3ft away so the amp really isn't being pushed that hard. The case they are going in will provide more adequate heatsinking ;)
If I had to guess I'd say the heat sink shown in the photo is probably around 10 degC/W (maybe a little less). |
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| sek |
Hi Brian,
congratulations on the good work and the beautiful PCB. ;)
But one question doesnt't leave me alone: What's the actual use? I mean I of course understand that it has
| quote: | | some nice advantages like high common mode rejection, very high input impedance, a differential input, and a good low impedance buffer |
but why an instrumentation configuration for a line level circuit? Wouldn't hou have had nearly all of it by simply wiring a single opamp buffer in a differential fashion? Wouldn't you have gained all the benefits you need and only missed out the two least needed: extreme impedance and open-loop gain? Wouldn't you achieve the same CMMR with a single amp with a lot less tight-toleranced passive components? But it looks like a lot of fun, though! :cool:
Sebastian. |
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| BWRX |
Hi Sebastian. Thank you for the compliments and questions.
I should preface the following by saying this is the first time I built an amp based on a high power op amp, so I don't know how a minimalist implementation sounds. Therefore, my reason for choosing to try an instrumentation configuration was solely due to it's attractiveness in theory :) Having listened to the end result for a few hours now (dual mono) I can say that it does in fact work quite well in practice.
A differential reciever using one opamp in front of the LM3875 would work fine, but would be more sensitive to impedance mismatch from the source due to the lower input impedance of the differential stage. It would also still require precise resistor matching but you could just as easily get chips with laser trimmed resistors already on the die.
I guess there are three main reasons I liked the two op amp buffer better. The first reason is, as you mentioned, the very high input impedance which is less sensitive to impedance mismatch (the OPA2134 has input bias currents about 2 orders of magnitude lower than the LM3875; 5pA typical vs 0.2uA typical). While it's not optimal to use a single ended source you can do so and still get very good results. This is what I'm doing and it sounds very good. The second reason is that two op amps provide a nice low output impedance to drive each input of the high power op amp. Many people have used a buffer in front of just one input and obtained good results so why not buffer both inputs? Put that second op amp in the dual package to good use ;) The third reason has to do with DC output offset voltage. There's a lot of talk about high DC output offsets with the standard Gainclone configurations and ways to cure it, but the instrumentation configuration will have low DC output offset simply due to the way it works. The first two op amps buffer (or amplify if desired) the input signal while the third op amp rejects the common mode signal and amplifies the differential mode signal with respect to a reference voltage that you specify. For the application of an audio amplifier you simply use the speaker ground as the reference. Viola, the output voltage of the high power op amp is now referenced directly to the speaker ground instead of an input ground. DC offset now becomes more a function of the chip's specs and the quality of the PCB layout.
Other nice features of the instrumentation configuration include the option to use either balanced or unbalanced sources and less susceptibility to noise (radiated EMI, ground loop hum, etc.). Multiple amplifiers on one power supply shouldn't be a problem since both inputs are high impedance which should break any ground loops that would normally form with the standard implementations. |
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| sek |
Hi Brian,
thanks for the detailed response. :cool:
| quote: | | my reason for choosing to try an instrumentation configuration was solely due to it's attractiveness in theory |
Which I perfectly understand! ;)
I'd say, e.g., an INA134/137 provides most of it with a single 'device', but it:
- has been tried before,
- costs more,
- is less DIY,
- depends on availability of a proprietary TI device (or it's proprietary AD or THAT counterparts),
- probably needs a servo for DC compensation,
- is less scientific,
- doesn't feature the high input impedance, large common mode voltage range, low output impedance, etc.
- ... ;)
And - you can really use it for instrumentation! It would work as a measurement amplifier for in-circuit measurements and could directly drive a low-impedance measurement instrument (channel), such as driving:
- a speaker without influencing the measured signal (like a detector amplifier),
- a CRT deflection coil to visualize signals (how about a 30" correlation meter ;)),
- a motor that needs current to work precisely (like a servo in a control loop with a sensor),
- ...
And i'd say it's a proof of concept! Just a little unfortunate that the board doesn't cope with LM3876/86 (more current, lower impedance drive, more headroom/dynamic range), but hey! :D
My opinion: good that someone has tried it and that a board could be available for DIY.
Cheers. |
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| BWRX |
| quote: | Originally posted by sek
Just a little unfortunate that the board doesn't cope with LM3876/86 (more current, lower impedance drive, more headroom/dynamic range) |
I originally wanted to use the 3886 but the pinout of the 3875 lended itself to a nicer layout. In reality the 3875 doesn't seem to have any problem driving a 4 ohm load, but I didn't push it hard at all. That will be something to try! It also doesn't seem to need a zobel or parallel resistor/inductor on the output like most people say the 3886 requires. It wouldn't be too hard to change the layout to work with the 3886 (it's unfortunate National made their pinouts so different) but an even better solution might be to use one of the TI power op amps that is stable at lower gains. That way you can use the buffers for most of the voltage gain and the high power chip for mainly just current gain.
I tried to stay away from the special chip solutions like the INA134 because they limit part choices and generally cost more. |
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| owen |
A while ago, I designed a 10* gain instrumentation gainclone for use with a DCX2496.. It was feature on gainclone dot com, but the site is now permenantly down.
The amp builder was very satisfied. A route to unusual pleasure with gainclones, especially if you have a differential source :D
Owen |
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| BWRX |
| Hi Owen. Did your schematic look like the one posted a few pages back? What op amp and power op amp did you use? And finally, you made it sound like you never got to actually hear it. If you did, how would you rate it against a standard configuration? I'm slowly working my towards a differential signal chain :snail: |
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| BWRX |
Upon request, here is the schematic with all part values. These are what I am using on my boards with very good results.
C14, C15, C16, and C17 are optional EMI/RFI filtering caps. I would recommend at least using C16 or C14, C15, and C16 if possible. C14 and C15 should be 5% tolerance or better.
C9 is an optional rail to rail bypass cap. 100V rating minimum.
R13, R14 and R15, R16 may need to be recalculated if you plan on changing the zener voltages.
R17, R18 are optional loading resistors. Be careful of the rated power dissipation if you decide to lower their values. I would recommend using these to help ensure a symmetrical supply voltage and to help discharge the supply caps when the power is turned off. This configuration only has a small turn on click and a small turn off click accompanied by some other rather quiet noises due to the op amp supply rails discharging.
Q1, Q2 can be changed to any suitable NPN, PNP devices. 2N3904 and 2N3906 are cheap and widely available.
IC1 is the dual op amp. I'm using the OPA2134 but other devices may be used. I plan on trying the LM4562 when I get the itch to swap out the 2134.
IC2 is the chip amp. I'm using the LM3875 but other devices may be used as well. If you use a different chip you may change the feedback resistors to change the gain of the chip amp. Be careful with stability issues and lower gain settings.
The gain of this configuration can be changed with only one resistor - R11. 20k gives an overall gain of 20V/V. 10k gives an overall gain of 30V/V. Set your gain based on your source and power supply voltages. The gain formula is on the schematic. |
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| owen |
LM3875, in differential input mode - it reduced the parts count alot (no need for the additional buffers, as it was being driven differentially in the first place, from the DCX2496.
Sound quality reported was very good - I'm slowly getting the parts together to do a transformer input gainclone... story of my life really, no time to build much at all :bawling: :bawling: :bawling:
Owen |
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| AndrewT |
| quote: | Originally posted by owen
LM3875, in differential input mode - it reduced the parts count alot (no need for the additional buffers, as it was being driven differentially in the first place, from the DCX2496. |
It falls into place now.
Bwrx,
how low can the overall gain be set without stability problems in the chipamp?
Varying R11 from 50k to 5k changes the gain from +23db to +34db.
Could a range of +6db to +26db be achieved? |
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| owen |
Lowest gain allowable depends on the chip, and is detailed in the manufacturors spec sheet.
The 3875 has a minimum gain of 10 - ideal for pro audio balanced input usage...
Owen |
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| BWRX |
A transformer coupled input would be ideal, but those little things are expensive for most of us. The active instrumentation method really doesn't add too many parts and can be cheaper depending on what parts you use.
| quote: | Originally posted by AndrewT
how low can the overall gain be set without stability problems in the chipamp? |
Owen answered that nicely in his post above. If you want lower gains you'll need to use a chip that is stable at those gains. I believe the OPAs (the high power ones from TI) are unity gain stable. The 3875's specified minimum gain is 10, but it might be able to go a bit lower without any issues. So far the 3875 has shown no problems with oscillation when used as a differential amplifier with a gain of 10 with no parallel resistor-inductor or zobel on the output. |
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| owen |
The transformer input does not have to be too expensive... If the stage before has little dc offset ( <10 mV), you can comfortably use small VA mains torroids - high inductance, extremely good coupling, low core artifacts and great bandwidth - or use a parafeed style input (it would still then have the capacitor input) if DC offset of the previous stage was higher.
From Susan Parkers site
15VA Input Transformer Update - 27th Oct 04
Low level bandwidth is 10Hz to 25.5 kHz (-3 dB), 31.5 kHz (-6dB).
And you could use:
http://www.oep.co.uk/audio_transfor...erformance.html
These are available for not a huge outlay. The key to remember is that this is a low signal level, low current transformer, and doesnt need to be a huge lump of iron unless it is a classic value pre with output transformer needed to handle large DC Voltages at moderate currents.... then you do need an airgap...
Have fun
Owen |
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| owen |
The OEP transformers are around Ģ10, with low THD, and a flat frequency response from 40Hz to 35KHz.
A bit of a bargain!
Owen |
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| owen |
I've had contact from Mr Squeegs.... He's looking for the schematic as we speak....
Owen |
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| BWRX |
| Thanks owen. Those OEP transformers look pretty decent and are on the lower end of the price spectrum. Do they happen to have a US distributor? It doesn't really say so even though arrow and the other places are technically worldwide distributors. |
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| owen |
They dont weigh alot, so shipping direct shouldnt be a problem :D
Owen |
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| AndrewT |
Hi,
this instrumentation amplifier has many attractions.
Could the front end be strapped onto a different power amp?
What DC stability would result?
Would a DC servo be required?
Could the integrity of the balanced function be preserved if a DC blocking cap were inserted into the NFB loop to reduce DC gain to 1?
Any better way to extrapolate the inherent advantages of BWRX's creation to a mainstream amplifier? |
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| juergenk |
dc is of no concern, the LM chips have a low offset voltage.
Just make shure that there is no "external" dc at the input, which could be amplified.
Regards |
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| owen |
| quote: | | the LM chips have a low offset voltage. |
With certain provisos. The layout must be good to minimise the possibility of oscillation, RF pickup, and other nasties.
Good tight layout, should provide good sound with average parts (and exceptional with great parts), and minimise the DC offset.
If you use a differential input pair of LM3975s as a balanced gainclone, you can trim the DC offset to produce a net zero easily, just beware that you may up the johnson noise as a result, if you have to use a higher resistance...
Have fun
Owen |
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| juergenk |
| hm yes, building amplifiers is not completely foolproof. :D |
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| BWRX |
| quote: | Originally posted by AndrewT
Could the front end be strapped onto a different power amp?
What DC stability would result?
Would a DC servo be required? |
It certainly could, but would most likely require modifications to the amp. DC stability would probably be good, thus a servo wouldn't be required.
| quote: | Originally posted by AndrewT
Could the integrity of the balanced function be preserved if a DC blocking cap were inserted into the NFB loop to reduce DC gain to 1? |
I don't think this can be done. DC output offset is low and the source has the potential to be the main contributor. Any DC offset is amplified by the gain of the amp. 1mV at the input wlll be 20mV at the output with a gain of 20. Blocking DC at the input is the best option. Unfortunately that means using a coupling cap in the signal path.
| quote: | Originally posted by AndrewT
Any better way to extrapolate the inherent advantages of BWRX's creation to a mainstream amplifier? |
The simple solution would be to just put an IC instrumentation amp in front of an amp's input stage. I took this one step further with my design and combined the power output stage with the differential amp portion. As was mentioned recently, an input transformer will passively provide a similar solution with the added advantage of galvanic isolation.
| quote: | Originally posted by juergenk
dc is of no concern, the LM chips have a low offset voltage.
Just make shure that there is no "external" dc at the input, which could be amplified. |
The LM chips have low DC offset, but poor layout and grounding can easily ruin this. DC coupling definitely requires low to no DC on the source signal. On my design with the inputs shorted to each other, the DC offset on each output is -8mV and -13.3mV. This will vary depending on resistor tolerances, op amp selection, and power op amp selection. With my computer's audio output connected as the source the DC offset increases to 50mV and 100mV. |
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| BWRX |
It took a while but the other boards got built, managed to find their way onto some heatsinks, and get wired up to a suitable power supply. I was going to fit 4 amps per heatsink but the 680uF caps I used were just barely too tall so 2 per sink was the decision. Quite overkill and as a consequence the aluminum chunks don't even get warm with my 8 ohm speakers. They could each be used separately for 4 channels but I've only got 2 speakers so why not bridge them? Since each board is an instrumentation amplifier bridging them is as simple as wiring the inputs in reverse polarity for each half of the bridge. The input impedance (inverting and noninverting) for each board is 100k, so using two in parallel gives 50k.
Currently the left channel uses OPA2134s for the input op amps and the right uses LM4562s. The OPA2134 channel has a small pop at turn on and plays a little music through the left speaker about 5 seconds after powering down the supply. The LM4562 channel has no pop at turn on and no noise when the supply powers down. So far the LM4562 sounds more precise than the OPA2134. The OPA2134 seems to have a slightly looser bottom end and slightly more shimmery top end, for lack of a better way to describe it. |
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| BWRX |
| Regardless, both op amps are very good and have an enjoyable sound - as do the LM3875s! |
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