LM3876 power supply

[GloBug]

The LM3875/6 is a 56W device.
If you look at page 10 of the Datasheet it has the Output Power vs. Supply Voltage chart.

56w is achieved using ~ +/- 35v into a 8ohm resistive load.

The chart only goes to 40v and if you run it at that assume extra problems when the speaker impedance varies from spec during use.

You will see in the chart that if 40v is used it cannot practically handle anything lower then 8ohm. Take a look how the output power drops of steeply using +/- 34 into a 6ohm load.

So to run a 8 ohm without the issues keep it under +/-35v.

-At 40v the chip is at 75W!

Running the chip that hot will give you problems with the Spike protection kicking in.
Running the chip approaching that temperature actually lowers your power compared to the cold 56W spec.

So because you are using this in a Church and as a bass amp, keeping it cool and reliable is the name of the game, unless you want a nasty sounding distortion box.
You need to keep it cooled to get the maximum clean power.

75w is barely louder then a 56w. having said that, if the woofers in your 4 x XX" cabinets are inefficient, you may have to look at the other chips like the LM4780 or the LM3886 for a little more umph. That can wait though, you will know after you try it.

So to get +/-35v rails, you would need a 25v transformer. 25 x 1.4 =35v minus diode losses. Even 35v is pushing it when you consider a "8ohm" speaker measures as ~6.5ohm resistive load. I myself would feel a little more comfortable with a few less volts with my 25V transformer, I will see what happens.

Trimming a few volts off is easier then dumping 15v. That little regulator is going to be some hot. From my understanding they drop the voltage by converting it into heat.

-15v drop x the 1.5A max of the device is ~22.5W That's enough to solder with. Plus you need more amperage then that.

[discrete]

Originally Posted by happy hippy

I am posting the latest schematic of my proposed power supply, two versions. One is 48v and one is 56v. Notice that I have dropped the voltage regulators for the mains, but I have kept the voltage regulators powering the op amps in the preamp

I think you are all set.

Use the 48 V center-tap transformer, it puts you right at 50 W into 8 ohms after you subtract the rectifier and chip-amp voltage drops.
It seems to be a 96 VA transformer, good match also to the 50 W requirement.

Optional: You may want to add 10 to 100 uF capacitors across the '1320' ohm resistors, this will lower the output ripple (which is already pretty low) significantly.
(I would use 2200 and 220 ohm resistors, instead of the 1320 and 120 units, to set the regulators.
These are bog standard resistance values, giving 13.75 V. The TL072s will not know the difference.)

[discrete]

Originally Posted by GloBug

So to get +/-35v rails, you would need a 25v transformer. 25 x 1.4 =35v minus diode losses. Even 35v is pushing it when you consider a "8ohm" speaker measures as ~6.5ohm resistive load. I myself would feel a little more comfortable with a few less volts with my 25V transformer, I will see what happens.

The Hammond '48 V' transformer shown is actually a 48 V center tap unit, so 24-0-24.
I think it is a good choice if an order to Antek is not possible.

Originally Posted by GloBug

Trimming a few volts off is easier then dumping 15v. That little regulator is going to be some hot. From my understanding they drop the voltage by converting it into heat.

-15v drop x the 1.5A max of the device is ~22.5W That's enough to solder with. Plus you need more amperage then that.

It seems the power amp will tie in before the regulators, as recommended several times.
The regulators will only feed TL072 op-amp(s).
Anything less than ~50 mA will not even require heat-sinking on the regulators.

So I think we are all good to go!

[GloBug]

I gotcha, I missed that in my rant!

[6L6]

Here is a great article worth reading - Roman Black's gainclone amp

The regulator is of specific interest, he uses a 317 with a pass transistor, so the current limits of the 317 are not exceeded.

[happy hippy]

Discrete wrote,
"I think it is a good choice if an order to Antek is not possible"

What is Antec? And should I be interested?

Thanx
The Happy Hippy

[discrete]

Originally Posted by happy hippy

What is Antec? And should I be interested?

Thanx
The Happy Hippy

'Antek', with a 'k': An online seller of a large range of cheap and well made toroid transformers:

Your 100 VA unit would be this one:

Antek - AS-1224

(Shipping could make it expensive if you only buy one though).

[AndrewT]

Originally Posted by happy hippy

.................please try to help me understand the terminology. ...............

I'll be only too happy to help where I can.

[Puffin]

Originally Posted by happy hippy

Quoting Globug and Puffin
"gainclone amp"
Most gainclone amps that I have seen are for PA systems. This is for a bass amp. I'm not sure it qualifies as a gainclone anymore.
Quoting Puffin
"You can use these in a split rail supply. I am using 45v per rail, just on the limits of the LM3875. The 338's are heatsinked, but they break no sweat at all and remain cool. I don't think that you will find the same with the 317/337 you intend to use as you will be dropping 15 - 18v per rail (depending on which version you build)"
I'm using an LM3876, not an LM3875.

Thanx
The Happy Hippy

A Gainclone is a generic term for most amplifiers built using the NS range of op-amps (LM3875, LM3886 and stereo variants and LM3876) The term evolved as these were a "clone" of the original 47Labs Gaincard. Clever marketing meant that these very simple amps were sold at very high prices. When the "secret" got out, others made the same or similar amps for very low cost. Diyers were the most interested as they could tweak them at will. Whether your amp drives a full range or multi-way speaker, I think that you will find that you are building a gainclone (unless seomone tells me I am wrong)

6moons audio reviews: 47 Laboratory Model 4706 GainCard

[6L6]

Re: Antek transformer -

Save a few bucks and buy the unshielded version - Antek - AN-1224

I have bought a bunch of stuff from them and shipping is very fair, single transformers are usually sent in a USPS flat rate box.