Chip amp power supply- a beginners guide

[nige838]

Originally Posted by pacificblue

Yes, a single supply needs two rails, one positive and one negative, where the negative is usually at the same time the ground. A split supply has three rails, positive, negative and ground, where the ground potential is in between positive and negative.



The basic differences are usually that you

• need a virtual ground between the positive rail and the real ground, to which the input signal is referenced. That means usually three more resistors and two more capacitors. An additional transistor or even op amp is sometimes used to improve the stability of that virtual ground.
• need a (big) DC blocking capacitor between the amp output and the speaker. The virtual ground is present at the amp output and the resulting DC signal at half the rail voltage could destroy the connected speakers without that capacitor.

Depending on the chip amp IC you use, some of those components may already be built-in. You need to read the corresponding datasheet. Usually there is a schematic of how the IC you use must be implemented with a single supply.

Thank you very much for your detailed answer, it cleared up several questions.

I was mistaken about the power supply.
It is a center tap transformer, that shows (after rectification):

55+ 0 55-

I have tied the 0 to ground (chassis). I'm building the LM3875 amp, but I am only using this same supply for both channels. From what I gather you said, I in fact have a split supply I believe.

I'm using this schematic by the way.

Thank you for your help, I believe I now understand how to go about it...

[AndrewT]

Don't use +-55Vdc to power any of the National chipamps.

National do the LME498xx series that can be powered from +-55Vdc and even higher.

Are you new to chipamps and simple electronics? If so then don't build that example schematic.
All the optional components to improve reliability have been omitted. This example should only be built by experienced builders who know how the circuit works and how to check it is working properly and know the risks they run if adopted as shown.

[FastEddy]

Originally Posted by AndrewT

Don't use +-55Vdc to power any of the National chipamps. National do the LME498xx series that can be powered from +-55Vdc and even higher.

Are you new to chipamps and simple electronics? If so then don't build that example schematic.

All the optional components to improve reliability have been omitted. This example should only be built by experienced builders who know how the circuit works and how to check it is working properly and know the risks they run if adopted as shown.

There are several things wrong with building from that schemo ...

One might hope that AndrewT would offer something a bit more reliable / cleaner / practical ... Thanks AT

[ArtsyAllen]

Originally Posted by AndrewT

I hope the kit designer/supplier comes in with an opinion.

Thanks Andrew, Simple yet sound advice. It makes sense that what will work for one amp design may not work for another. I have found some other threads that feature Peter Daniel, and I am zeroing in on what I want to do.

[ArtsyAllen]

Thanks Pacific Blue, very informative post. I am going with the "stardard" PSU as this is my first one!

I was not too keen on these Disadvantages either:

May show serious HF instability problems, either alone or in combination with the amplifiers powered. The regulator output impedance is likely to rise with frequency, and this can give rise to some really unpleasant sorts of HF instability. Some of my worst amplifier experiences have involved (very) conditional stability in such amplifiers.
The amplifier can no longer deliver higher power on transient peaks.
The overall power dissipation for a given output is considerably increased, due to the minimum voltage drop through the regulator system.
The response to transient current demands is likely to be slow, affecting slewing behavior.

Like everything in DIY, take the good with the bad!

Allen

[Bicolor]

Hi all!
I was at the local junkyard and salvaged an old pioneer amplifier to check if it contained any useful stuff for my first lm3886 build.
Found a neat little transformer (weighs about 1,7kg which should be around three pounds).

I fired up the amplifer, turned it upside down and started measuring.

Between the secondaries marked 7-8-9 I got 22,8v when I measured 7-8 and 8-9.
I followed these leads to their fuses and they were 3,15A. Can or will this be "ok" for a stereo pair of lm3886? I do not know the VA rating of this transformer, it is probably not all that high.

I am planning on using at least 10000uf per channel in the powersupply, "snubbersupply".
Anyone with experience of this, is it better to use less or more?

I have read about others using 80VA toroids for a stereo amp, but of course they have been limited when it comes to output power.

Thanks in advance for any thoughts.

[FastEddy]

Mmmmm ...

Bicolor: best you find a manual or spec sheet for that transformer before you assume too much about which pin does what ....

[pacificblue]

The findings indicate a 120 VA transformer and the voltage appears to be a nominal 2x20 V. That is comfortable for 8 Ohm speakers. It is pushing it a bit for 4 Ohm speakers, but with generous heatsinking it is worth a try.

Snubbers in the power supply don't make much sense to me.

[FastEddy]

Originally Posted by pacificblue

The findings indicate a 120 VA transformer and the voltage appears to be a nominal 2x20 V. That is comfortable for 8 Ohm speakers. ...

Your comfort zone may be different than mine ...

... Snubbers in the power supply don't make much sense to me.

Your comfort zone may be different than mine ... I like to see the power supply noise floor down at least -110 db = snubbers help.

[Bicolor]

Thanks, I looked around for some specs but I could not find anything.
Maybe I'll give it a try.
It's easy to change to a bigger one later down the line.