| sek |
Hi,
while all the threads about the 3886, 4780 and LM338 go in interesting directions, but take their time, I decided to come up with a "sketch" of the good old LM3875T(F) in non-inverting configuration, together with a simple regulated dual power supply.
The intended application is: powering a single speaker in an active 2-way box, yet I'd need two separate circuits with a common transformer but separated rectifier bridges.
Watch this ;)
It features the LT108x/LT108x three-terminal regulators (LDOs), anything between 3A and 7.5A will fit into the board :) I plan to make a combined footprint for TO220 and TO247 regulator cases.
The LM3875 is implemented in it's standard non-inverted configuration (I'm open to experiment with inverted, btw.). The only difference being the "snubber" instead of the usual zobel network, as I have very short speaker leads but want to be able to experiment with different connections. Btw., the connection between signal and power ground is done on the input side.
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I'm very interested in your opinions ;) as this shall become implemented at least four times during this summer. Many of you have different experiences (and knowledge) about the different configurations, please let the comments roll.
As soon as the schematic is discussed and confirmed by - let's say - enough experienced guys for me to feel comfortable, I'll come up with prototypes of the board layout (everything fits on 2" * 3", that's 5cm * 7.5cm).
Cheers,
Sebastian. |
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| squadra |
Hello Sebastian,
Only one remark, but I'm no expert :)
It looks you have misplaced the 10uF cap near the lt1083?
It should be connected to the output, now it is connected to the 'adj' pin.
This way it will probably prevent the regulator trom working correctly.
Peter |
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| sek |
Hi Peter,
those 10u capacitors C5 and C10 are in place to improve the regulator's ripple rejection (as explained in the datasheet here). They "buffer" the regulator's reference voltage and stabilize it when an output voltage change appears at the voltage divider (due to current change in the load). C11 and C13 are the caps "behind" the regs.
Thanks for looking at it. ;) |
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| squadra |
| quote: | Originally posted by sek
Hi Peter,
those 10u capacitors C5 and C10 are in place to improve the regulator's ripple rejection (as explained in the datasheet here). |
Hi Sebastian,
I looked at the same datesheet, and in the typical application the only cap was connected to the output :)
Pj |
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| sek |
Peter,
Have a look at the circuit on page 8. Page 9 has a paragraph "Ripple Rejection".
This is true for actually any three-pin linear regulator IC. These caps aren't really needed, but they don't hurt anyway (as ripple rejection is what the regulators shall be there for). ;)
It's true that there has to be a cap at the output for stability reasons. Those are provided with the 1mF caps near the LM3875.
BTW, I could imagine substituting the LM3875 with the LM3876 in this schematic, as I've read about the turn on problems over in your thread. Can you tell wether the turn-on noise was related to the input buffers or the supply regulators?
Sebastian, |
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| sek |
Okay, maybe the difference between the above schematic (and board) and all the other GCs out there isn't clear enough...
No problem, let me explain:
1. I can live without having the choice between different kinds of rectifier diodes on board. Thus the recifier is kept external, e.g. a metal bridge mounted on the heatsink.
2. I don't feel the need to put many millifarads in front of a voltage regulator, thus, the circuit (and board) suggest only some millifarads ;)
3. I don't want to build a testbed for a multitude of "hyped" components. I want to develop a working circuit. That's why the schematic doesn't feature many different film caps or oversized electrolytics etc.
4. I want the whole circuit on a single board. That's why it's kept to a minimum count of components, yet cared for high sound quality, reliability and usability (one can adapt the regulator to any sensible output voltage).
5. I don't claim it to be HIGH-END in the thread title, nor in the post itself. Because I don't claim to make something better than anyone could. ;)
6. I already made a first attempt on a PCB. It's 5cm by 7.5cm (2" by 3"), double sided, features SMD components (to keep the board layout tight and current paths easily separable) and has a "local" star ground which can be referenced to a "global" star ground and thus the case/heatsinks if more boards shall be combined. I for myself will use it with an active crossover circuit, that's why the above circuit has no buffer.
And finally, I don't plan on doing a group offer, as I don't have the time and resources at the moment. But once the circuit is approved, the board developed and the prototype state behind us, I'll give away the schematics and board layouts for free. At least! :D
I appreciate your opinions.
Sebastian.
PS: and it should have been "whether" in the above post to Peter, of course. :D |
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| squadra |
| quote: | Originally posted by sek
Peter,
Have a look at the circuit on page 8. Page 9 has a paragraph "Ripple Rejection".
This is true for actually any three-pin linear regulator IC. These caps aren't really needed, but they don't hurt anyway (as ripple rejection is what the regulators shall be there for). ;)
It's true that there has to be a cap at the output for stability reasons. Those are provided with the 1mF caps near the LM3875.
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Sebastian,
If it is for stability, shouldn't the output caps be close to the regulator?
| quote: |
BTW, I could imagine substituting the LM3875 with the LM3876 in this schematic, as I've read about the turn on problems over in your thread. Can you tell wether the turn-on noise was related to the input buffers or the supply regulators?
Sebastian, |
The thread you're referring to is not started by me, I have just started with designing a pcb for the lm3886 and it is my first try :)
Maybe you could ask in the thread this refers to?
Peter |
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| sek |
| quote: | | If it is for stability, shouldn't the output caps be close to the regulator? |
They don't look close in the schematic. But they will be on the board. That's all that counts, isn't it? Do you think I should actually draw them nearby, just to comfort?
| quote: | | Maybe you could ask in the thread this refers to? |
I don't want to hijack it. :D
I've read Sheldon and Pedja talking about the fact that turn-on noise occurs in conjunction with the buffer (startup). But as I haven't tried it, I would like to hear some comments from someone who built a regulated GC ;)
Sebastian. |
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| Nuuk |
| quote: | | Can you tell wether the turn-on noise was related to the input buffers or the supply regulators? |
BOTH! But the noises are different.
With my 3875 amps there is a small buzz for about 2 seconds on powering up. IF I power the buffers up at the same time as the amps, there is a loud click/pop.
Solutions:
1) have the amp output switched on after the regulators/buffers are powered up.
2) power up buffers, wait, and then power up amp as noise from regulators is not too bad.
3) use something like the Velleman 4700 speaker protection module which will connect speakers approximately 10 seconds after power up.
4) use a circuit to delay powering up the amp until the buffer is powered up and settled.
5) use a chip with a mute function. |
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| sek |
Hi Nuuk,
thanks for the reply.
Sounds like this project should go into the direction of number 5.
That means adapting the schematic for the LM3876/86. It's a passive mute delay that needs just a few parts, board space requirements won't increase.
I'll do that asap and post the resulting schematic ;)
Question: What about power-off noise???
Sebastian.
PS: A speaker protection circuit shouldn't be introduced, if possible. After all, it's a GC, the protection circuit would have more parts than the whole amp board itself... |
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| Circlotron |
| quote: | Originally posted by sek
The only difference being the "snubber" instead of the usual zobel network, *snip* |
Perhaps the resistor value is a bit low. I would start off with something in the range of 5-10 ohms to set the minimum impedance of this RC combination at high frequencies. |
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| Nuuk |
| quote: | | PS: A speaker protection circuit shouldn't be introduced, if possible. After all, it's a GC, the protection circuit would have more parts than the whole amp board itself... |
This is a debate that has been raised before and must be an individual choice based on how much you love your speakers against how much you are obsessed with that last 1% of sound quality.
When the protection device also provides the bonus of avoiding power up problems, it is not to be dismissed lightly. ;) |
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| sek |
Hi Circlotron,
thanks for the pointer, I'll correct the network with the next schematic revision (the capacitor value is a little off, too). ;)
Sebastian. |
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| sek |
Hi Nuuk,
yes, that's right they way you put it. ;)
My thought went more into the direction of implementing a power-up and power-down control with a mutable LM3876/86. The mute function could be utilized to prevent a power-off noise too (using the same principle like the classical protection units), while keeping the overall complexity low.
Sebastian. |
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| sek |
Okay, here is a quick update on the current schematic.
I've settled for the LM3876/86. The LM3875 might make turn-on/-off problems with the regulators (it does even without).
The large reservoir capacitors have gone where they belong: on the board with the rectifier diodes. ;) They are replaced with small but sufficient bypass caps near the regulator IC pins.
The local supply capacitors are still indicated to be big, because I currently want to keep the mounting space available (can't let them go for now). Mounting holes for small capacitors will be available.
As I changed the chip, I changed the accompanying circuit, too: It now features an input filter network (optional, for EMI suppression due to long connection leads from the input buffers), a feedback HF bypass for the same reason (optional), and mounting space for ac coupling the input ground connection (optional), as this may be needed by different implementations/uses of the circuit. And finally there is a mute pin connection as to the usual recommendations. Thanks to DD, ESP and SL for inspiration. ;)
Note that there's no Zobel output filter (RL series or RC shunt). I'm currently evaluating the influence of such networks on the given schematic, depending on the use of none, some or all optional components. I'd expect the Zobel network to be recommendable with the minimal circuit and to be "optional" with every component populated.
Please remember, this circuit is not intended as a general purpose amplifier, but as a bi- or multi-amping module system for active speakers. However, with all optional components and an additional Zobel network included, this one should make a pretty good standalone amplifier, though!
Here you are:
I'm thinking of including (smd) trimpots for fine tuning:
1. the regulator output voltage according to low voltage drop and/or power needs,
2. the negative input ground resistor in order to adjust dc offset and noise.
And a couple of questions are still unanswered (or have contradicting answers on the forums):
1. Should the input GND connection really go to the board (that's what R* is for)?
2. Should the speaker (-) connection really return to the board (that's what the GND terminal is for)?
3. Do I really have to avoid ground planes at all? I'm thinking of wide traces for current (return) paths where needed and ground planes everywhere else.
What do You think so far?
I'll go buy some components to start prototyping the circuit today. I'll appreciate any answers. I really need some comfort for my decisions. :D
Sebastian.
PS: Board space should increase a little, in order to feature bold connection holes for the power supply. The board is currently 75mm (3") by 30mm (1.2") wide. |
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| tbla |
c2 and c6 could be 100uF......!
and c3 and c7 could be 470uF and should be as close to the reg out as posible.
c4 and c8 should be as close to the lm3886 as posible.
a 0,1uF cap as close to the reg input as posible.....+ 10.000uF also....... |
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| Nuuk |
Sebastian, I believe it is best to keep your main power supply caps close to the regulator circuit.
Also I would recommend using the second protection diodes on the regulators. ;) |
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| sek |
Thanks for the replies. :)
| quote: | | c3 and c7 could be 470uF and should be as close to the reg out as posible. |
Yes. As I said, I thought about this, too. The problem is this: a 50V low ESR type of 470uF can be nearly as big as a 50V 1mF standard type. That's why it makes no difference in occupied board space, only in the schematic value. But I changed it, because it always leads to misunderstandings about my intention. ;)
| quote: | | c4 and c8 should be as close to the lm3886 as posible. |
Okay, I introduced two new components C13 and C14, film caps close to the regs. Because of this, I increased the values of C4 and C8 to 1uF (they are available in the same footprint).
| quote: | | a 0,1uF cap as close to the reg input as posible.....+ 10.000uF also....... |
Let me comment this together with the next recommendation.
| quote: | | I believe it is best to keep your main power supply caps close to the regulator circuit. |
Well, I wasn't clear about that. The rectifiers are sitting on a board directly under the amplifier board, distance is lower than 5cm (2"). Because of this, I can move the main supply bypass caps there without losing benefit for the regulator. For still compensating this little distance, C1 and C5 are in place.
Now, C1 and C5 can probably use some 0.1uF bypassing, I'll include that, if it's neccessary. But I don't think so, as the main supply caps will have appropriate bypassing, already.
| quote: | | Also I would recommend using the second protection diodes on the regulators. |
Hmm, I've wondered about it, too. Okay, if You think so, I'll include them.
That's the updated version 0.2a now:
May I point to the questions in my above posts, regarding the current version 0.2a in this post? :D
Sebastian. |
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| janneman |
| quote: | Originally posted by sek
[snip]1. Should the input GND connection really go to the board (that's what R* is for)?
2. Should the speaker (-) connection really return to the board (that's what the GND terminal is for)?[snip] |
My 2 eurocents worth:
1 Yes.
2 Yes.
Jan Didden |
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| sek |
Great! :D
And how about the trimpots, would that make use for someone besides me? I don't think so, as a better solution would be to experiment with the values and solder in a reliable, stable, simple resistor... |
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| cocolino |
| quote: | originally posted by sek
I've settled for the LM3876/86. The LM3875 might make turn-on/-off problems with the regulators (it does even without). |
Hello sek,
could You please elaborate a little more detailed about power-up/down problems of the LM3875`s?
Do they make a big noise (maybe a loud low frequency bump) while turning on/off?
As I plan to LM-3875 power a tweeter in an active system, I`m a bit worried about this. Not only that I hate such noise when powering up a system, I fear it could actually damage a tweeter when directly coupled to the amp output. |
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| tbla |
| quote: | | Okay, I introduced two new components C13 and C14, film caps close to the regs. Because of this, I increased the values of C4 and C8 to 1uF (they are available in the same footprint). |
ok fine - then skip the c4 and c8 - you don't want too low impedance caps right at the reg output.......but the film or ceramic right at the reg input is very usefull - don't just use it to bypass the big cap.......:) |
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| sek |
Hi Cocolino,
| quote: | | could You please elaborate a little more detailed about power-up/down problems of the LM3875`s? |
I've done this in this thread - in theory. I've studied datasheets, read application notes and asked people on the forums.
The difference between LM3875 and LM3876 is explained in some of those documents.
From both the LM3875 and the LM3876 datasheet: | quote: | | Under-Voltage Protection: Upon system power-up the under-voltage protection circuitry allows the power supplies and their corresponding caps to come up close to their full values before turning on the LM387x such that no DC output spikes occur. Upon turn-off, the output of the LM387x is brought to ground before the power supplies such that no transients occur at power-down. |
From National's application note AN-898 "Audio Amplifiers Utilizing SPiKe Protection":| quote: | | For the LM2876, LM3876, and LM3886, the control signal will not allow the current sources to function until 1) the total supply voltage, from the positive rail to the negative rail, is greater than 14V and 2) the negative voltage rail exceeds 9V. The LM3875 is undervoltage protected with the relative 14V total supply voltage condition only. Thus for the 6 -series, the amplifiers will not amplify audio signals until both of these conditions are met. It is this 9V protection that causes the undervoltage protection scheme to disable the output up to 18V between the positive and negative rail, assuming that both supply rails come up simultaneously. |
So they are different in that regard.
I haven't tested the LM3875, so I just can't help you with that decision. But it is said that the common parts in the internal circuit of LM3875 and LM3876 are identical, so I decided for the '76. Besides that, it's pin compatible to the LM3886, so the more powerful one would also fit in the board. ;)
I want to use the boards for tweeters, too, so I want to be somewhat sure. And guess what, my parts dealer sells the '76 even some cents cheaper: :cool:
Sebastian.
PS: Last weekend I set up a prototype with an OPA541. This IC has no such protection circuitry, and the effect is quite awfull: Spike on turn on, distorted fade out on turn off (depending on the power supply regulaton, e.g. capacitor size). |
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| sek |
Hi tbla,
| quote: | | ok fine - then skip the c4 and c8 - you don't want too low impedance caps right at the reg output.......but the film or ceramic right at the reg input is very usefull - don't just use it to bypass the big cap....... |
Yes, only a couple of days ago I've read an application note about controlling the feedback loop's phase margin via the output capacitor's ESR. I just seem to loose PDFs after I read them, I can't find this one any more! :mad: ;)
Okay, I'll replace the 1uF caps near the elkos with bleeder resistors. That's an increase in stability and safety. :D
20k should be fine here, drawing between 1.25mA and 1.5mA at 25V to 30V. That's below 0.05W, so a standard resistor will do fine. I can even imagine dedicated bleeder resistors for the purpose of draining the caps so that the undervoltage-protection circuit switches off quickly, but I don't want to waste power (component size, heat and cost).
Thank you for all the insight, that's clearly knowledge and experience speaking here. :cool: |
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| sek |
Good, this is version 0.3 of the schematic. An attempt to the power supply bridge rectifier and reservoir capacitors.
It features two different types of diodes in the rectifier bridge: single and dual types, a.k.a. the MUR860 or the MBR20200 style in TO220 packages. Both "soft recovery" and Schottky types will work. I'll try to arrange them so that axial diodes will fit too, but I can't promise...
The bridges are bypassed with 100nF ceramics. Film is not neccessary here, right? ;)
I've drawn in three reservoir capacitors per rail. This is no final decision, just an approach on how to handle board space and parts arrangement (it would be two larger or four smaller caps, then). Each big cap is bypassed with a small ceramic, too. You could argue about capacitance here. Then there are separate bleeder resistors for each cap, just to distribute power.
I've thought about putting in an (optional) smd LED anywhere between the bridge and the regulators. Should I? :whazzat: ;)
Here You are again:
What did I do wrong? Or what could be done better?
Thanks,
Sebastian. |
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| carlosfm |
| quote: | Originally posted by sek
I want to use the boards for tweeters, too, so I want to be somewhat sure. And guess what, my parts dealer sells the '76 even some cents cheaper: :cool: |
For tweeters you don't need more than the LM1875.;)
And for that role it's f-a-n-t-a-s-t-i-c. :cool: |
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| tbla |
| quote: | | Okay, I'll replace the 1uF caps near the elkos with bleeder resistors. That's an increase in stability and safety |
....don't think they will be needed as the lm3886 draws enough power at idle.......;) |
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| sek |
| quote: | | the lm3886 draws enough power at idle |
Talking about regulator stability? Yes, You're probably right. But besides that the circuit is intended for both LM3876/86, those resitors have a different function.
R9 and R10 are in place to drain C3 and C4 after power-off. The internal undervoltage-protection circuit "mutes" the output at below +-9V supply voltage, probably reducing current consumption.
Now, when power is returned very quickly, the chip turns on the output again while surrounding circuitry might not be stable already...
You might be right that R9 and R10 aren't needed, as page 1 of National's AN-898 doesn't indicate that in the internal circuit (including protection blocks). The circuit seems to simply mute the input stage, but doesn't turn off the power supply of the bias and output stages.
According to the datasheet, the LM3876 draws at least 30mA muted and with no signal applied. But how much does it draw below it's +-9V threshold? Does anyone have experiences with periodically switching the power of a regulated GC?
I'm just a little concerned, because when I asked about turn-on noise in another thread, everybody reported some pops sometimes...
Sebastian. |
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| aHobbit |
I experience that. In one instance, I put off the amp, and unknowingly switched it on quite immediately, and I heard a pop and a little smoke in my regulator. Good, my speaker is not connected during this time.
Upon visual inspection and retesting, the regulator was not damaged.
Thus, I thought that time, that the amp needs to have quite a time to discharge cap charge before turning it on again.
Then I noted STK recommended power supply, putting 500R each in the power rail caps for gracious discharging away from the amp package.
But in your case, r9 and r10 may not be needed at all. You see, you already provided the 2 protection diodes (D1 & D2, and D3 & D4), which directs discharging of C3 & C7 to those resistors in the power caps away from the regulator and the chip amps.
Further, when it starts discharging, the mute resistance will not have enough voltage to switch-on the amp out, so your output will just be muted (I think which is why you have to at least configure the Rm as close to the rail voltage, like in national datasheet, if you use +-16V, use 16K ohm, so that it can immediately mute the OUT when voltage drops during discharge).
I am still looking for some other explanation just to see if I have overlooked something else. |
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| sek |
aHobbit,
thanks for sharing this experience. It's very much appreciated, as I now know that my assumption and suspecting wasn't without reason.
As for the parts values in the schematic: the bleeder resistors, the regulator voltage dividers, the feedback network calculation and the input impedance setting are not final values. Those are first attempts from recommendations (mostly datasheets and the mentioned sources) and will be verified and fine tuned in a first prototype "series", where I want to mutually compare two circuits with different combinations and settings.
In particular, the bleeder values seem too high, the time constant t = R*C of 20kOhm and 1mF is too long, I guess (it would take some time to discharge deeply, not enough safety when fiddling with the circuit). :(
The regulator output voltage is currently set to ca. 25V. This has to be adjusted for the desired value and/or low loss.
The gain and impedance settings are just employments of parts I had available. They are not optimized for low offset etc. ;)
Sebastian. |
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| sek |
Okay, version 0.4 so far, please collect complaints for the next couple of days.
I'll be on vacation for a week or two. ;)
Good news is: due to the reduction of regulator input capacitor size, I gained back the needed board space for both an RL Zobel and an RC snubber network (while both are optional)!
When I'm back, I'll post the board layout, the first version is nearly finnished.
To make this project interresting enough for you to stay in mind while I'm away (:D),
here are some features that the final version will very likely have:
- One board per rectifier bridge with capacitors, one board per amplifier with regulators,
- board sizes are 75mm x 30mm (ca. 3" by 1.18"),
- there will be an additional board for more reservoir capacitance, which may also be used in conjunction with a separate bridge rectifier in order to save the rectifier board,
- boards can be stacked to form a module of N amplifier channels with 75mm x 30mm x N*40mm (ca. 3" by 1.18" by N*1.57") volume, e.g. two channels and a rectifier in 75x30x120mm (or a little less in height, depending on max. component height, mainly capacitors),
- the module is connected via bolts/screws and only needs a transformer (approx. 2x25V, >3A per wanted amplifer "channel"), an audio source connection (one per channel), a speaker connection (one per channel) and a heatsink (common to the whole module, big enough to mount whatever module size (channel count and capacitance options) you want, low thermal resistance according to LM3876/86 and regulator needs)
- very variable in component count (see schematic for optional components)
- not so variable in exotic component choice (tight layout due to size constraints),
- not a beginners project (due to two sided mixed thru-hole and SMD component mounting), but everybody who knows the own soldering station well enough should be able to build it.
I would say that this actually fits the specification that I've had in the beginning, a compact design that fits in almost any back side compartment of an active speaker box system.
Thanks to everybody who helped so far, that thing is going on,
Sebastian. :) |
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| carlosfm |
| On the PSU, you don't need one bleeder resistor per cap... |
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| sek |
That's for power handling. With one resistor per capacitor, I can use standard 1/4W resistors. As I already wrote, I probably want to decrease the resistor value for a shorter time constant. A single bleeder resistor would then have to handle more power and thus I had to use smd power resistors, which are more expensive and more difficult to find.
The resistor really mounts under the cap, and eaven if you leave it out, you don't waste unpopulated board space. ;)
Thanks for the hint. Other things to mention?
Cheers,
Sebastian.
But now I'm away... :D |
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| carlosfm |
| quote: | Originally posted by sek
Thanks for the hint. Other things to mention? |
No.
I didn't test the LT1083 regs, I can't help you.
But I trust tbla's advice.:angel: |
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| sek |
Hi there,
here's an updated version 0.5 of the PSU schematic.
Not much changed, I settled on using a single (bigger) reservoir capacitor and provided an alternative for a smaller one. ;)
Sebastian. |
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| sek |
And this is version 0.5 of the amp's schematic.
Some minor changes here, too.
C12* is now connected to power ground instead of signal ground, as pin 4 sources current into the supply, not the signal source (and it is therefore easier to route on the board).
The output filter network now consists of multiple standard resistors. This is smaller, easier to get and looks better (and makes the layout smaller and more convenient). Thanks to Per-Anders and Pedja for the inspisration.
Sebastian. |
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| sek |
And now something new :bigeyes:
The first version of the board, which I will call 0.5 too.
As it's typical for Eagle, red shows the upper and blue the lower side, while 'lila' just indicates the presence of copper on both sides.
To make it usefully readable, I have included the component placement. It makes 'reading' the picture a little difficult at first, but helps a lot compared to four views of two layouted sides and two matching component placement graphs.
Some components overlap, those are the alternatives in the schematic.
Some component names are mirrored, this indicates that the component is placed on the other board side.
The board is cut in half horizontally. The upper half is the amp circuit, the lower half is it's rectifier and reservoir capacitor board.
On the amp board one can see the regulators (with all their adjacent circuitry) in the upper left and right corners.
In between them sits the amp chip with input filter, feedback and output filter circuitry.
On the rectifier board we have sixteen diodes (!) as two alternative sets of eight pieces. The large capacitors can be combined by mounting the bigger under the board, while putting the smaller one as usual.
Sizes: length 75mm (3"), width 30mm (1.18"), mounting heigth determined by capacitors and ICs (max. 5mm SMD height on the bottom side).
It's not optimized, but fulfills the usual design rules of board manufacturers, has no (!) vias and only needs two-sided soldering where it's possible (e.g. on the large caps and the ICs).
What does everybody think of it?
Sebastian. |
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| carlmart |
On the regulator, why don't you try swapping the capacitors at output and adjust pin when you assemble the boards? Listen to both options and see if you find any difference.
The inductor at the amp's output has an air core, right? It should.
Carlos |
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| sek |
Hi,
| quote: | | On the regulator, why don't you try swapping the capacitors at output and adjust pin when you assemble the boards? |
I don't really understand the question, sorry!
The ADJ bypass capacitors are solid tantalums right in front of the regulator ICs.
The output capacitors are the smaller thru-hole electrolytics between regulators and amp.
The bigger electrolytics at the lower edge of the amp boards are the regulator's input capacitors C111 and C115.
Do you mean I should try different types accordingly? I will certainly do that, but as time and budget permits, the first step will be to get the amp to work at all ;)
I decided for tantalums as ADJ bypass capacitors because they are small and inexpensive. The main goal for regulating the LM3876/86's power supply is not ripple rejection, but line regulation. As the PSRR of both the regulator (min. 60dB) and the amp (mind. 85dB) are way good enough, I paid more attention to proper bypassing and left more room for different regulators as well as different input and output caps.
| quote: | | The inductor at the amp's output has an air core, right? |
Shure, it's the typical wire-wound 0.7uH coil, just mounted vertically.
Does the grounding look allright so far?
Or does anyone need a better view on the board? Which?
Sebastian. |
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| carlmart |
| quote: | Originally posted by sek
I don't really understand the question, sorry!
The ADJ bypass capacitors are solid tantalums right in front of the regulator ICs.
The output capacitors are the smaller thru-hole electrolytics between regulators and amp.
The bigger electrolytics at the lower edge of the amp boards are the regulator's input capacitors C111 and C115.
Do you mean I should try different types accordingly? I will certainly do that, but as time and budget permits, the first step will be to get the amp to work at all ;) |
Sorry my comment wasn't clear enough. I think you should try using the 470uF to bypass the adj pin and the smaller cap at the output.
My experience and other's on this forum may show that using small capacitors at the regulator's output pays off in better sound. Perhaps not as small (10uF), but something around 47/100uF.
What I am suggesting will not affect your budget, as it only involves swapping the capacitors you already have and listening to the results.
| quote: |
I decided for tantalums as ADJ bypass capacitors because they are small and inexpensive. The main goal for regulating the LM3876/86's power supply is not ripple rejection, but line regulation. As the PSRR of both the regulator (min. 60dB) and the amp (mind. 85dB) are way good enough, I paid more attention to proper bypassing and left more room for different regulators as well as different input and output caps.
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Only use those tantalums if you have already tried them in another project. I had many tantalums working bad or even catching fire on some of my projects, even on a digital Turtle Beach I had. Don't trust them too much, particularly if those tantalums are inexpensive. A good Panasonic FC should provide similar results and costs cents, with zero risk.
Carlos |
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| sek |
| quote: | | My experience and other's on this forum may show that using small capacitors at the regulator's output pays off in better sound. Perhaps not as small (10uF), but something around 47/100uF. |
Okay, that was the second meaning I was thinking of. ;)
It's just that I don't want to swap the placements of the two components, which I had to do in order to use the large one at the ADJ pin. Not because I'm lazy, but because I don't think it really pays off (It would really be neccessary to change nearly everything, then).
I'll experiment with larger values for the ADJ bypass cap when I have the board working.
| quote: | | and the smaller cap at the output |
That's no problem anyway, as I could just put any smaller value in there. There is room for experiments with anything from 0 to >470uF. ;)
| quote: | | I had many tantalums working bad or even catching fire on some of my projects |
Whoo, that's bad!
I've never experienced such things (once again: I'm not concerned about the sound of a tantalum at the ADJ pin at the amp's output, I wouldn't even expect it to be measureable).
Actually, it should only depend on the regulator's output voltage, as the voltage across the cap is Vcap = Vout - Vref, which would certainly be ca. 25V - 1.25V or something alike.
I've choosen 35V types, which should be save, as the currents involved are in the sub-milliampere range and the cap is 'overvoltage-protected' by the diodes.
Anyway, I appreciate this opinion, Carlos. I'll test it out. ;)
Any further opinion about schematic and board are welcome! :D
Sebastian. |
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