Yarg: yet another regulated gainclone

[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.

[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

[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.

[squadra]

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

[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,

[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!

I appreciate your opinions.

Sebastian.

PS: and it should have been "whether" in the above post to Peter, of course.

[squadra]

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.

Sebastian,

If it is for stability, shouldn't the output caps be close to the regulator?

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

[sek]

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?

Maybe you could ask in the thread this refers to?

I don't want to hijack it.

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.

[Nuuk]

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.

[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...