Tappin' a few Ma., Gnd. to V+, a '-/gnd/+' supply

[petemoore]

  I've gotten the LM3886 amp working !!
  I would' like to put an 18v preamp at the input, tapping between the center tap [gnd.] and V+.
  I know it'd 'work' but are there noise issues I need worry about, and although the current draw will be slight in comparison to the amp...
  any suggestions for adding preamp power supply to the project welcome.
 

[petemoore]

  Nope, verified doesn't work, then I tried another booster just won't bias off the transformer for some reason...a basic mosfet w/bias adjust on gate, 150 ohm source R 1k5 drain R, works fine otherwise, same as the last one except cap values.
  Yupp...real good waste of time.  Amp don't work right no more, real loud 'n crispy, nothing much then normal gatey sound, must be the thermal runaway got fried and keeps kicking in or something.
I can't imagine what else.
Voltmeter battery
Voltmeter lead
Voltmeter changing voltages...who knows
Booster dont work
Rocket intermittent
Soldering iron is POS
Testamp '386' is DOA
Speaker fried
Other speaker fried
Chip probably fried
Power supply voltage readings moved up
Caps hold voltage a long time on pin 4 [others also, takes a good 20 seconds of DMM R between leads to get a non shorted out reading on the PS's]
  Pin 8 thing
  Used to work
  Whole buncha other stuff like that, great day for learning the hard way.

[darwin_deathcat]

Hi petemoore, I know this is an old thread, but I'm just tackeling the same issue now (I just posted something identical to your post over at the SSguitar amp board), and I was wondering if you ever figured out a way to get the single supply rail out for you pre? I want to power a professor tweed as the pre for my lm3886, but do not want to create groundloop issues or oscillation from uneven current draw...

[petemoore]

  IIRC as long as the load is 'nominal'...I was using a mosfet, and it didn't seem to upset bias or anything...
  It was a simple tap from one side of the PS to ground, using a filter cap [though..a little noisey]...I would put a voltage regulator in for the 'preamp'.

[darwin_deathcat]

Thx, I was thinking about using a voltage regulator... I will probably do that then and see!

[dschwartz]

Thx, I was thinking about using a voltage regulator... I will probably do that then and see!

i agree with that.. use a voltage regulator (7918) between the pre B+ and the amp B+, that should "isolate" both sources..

did you block dc before entering the amp input?

[petemoore]

did you block dc before entering the amp input?

  I was thinking about that this morning.
  AIUit, since blocking DC takes care of ... DC as far as the signal is concerned, the main priority is to see that a voltage potential the amplifier can work with is supplied.
  I used 18v single ended..1/2 of the supply, and a Mosfet amplifier.
  Noise wasn't too bad.
  I was looking at regulating and physical separation of the pre-amp  [and shielding] from the amp to further reduce the noise.
  I used them..Mic/board/Xover/Mosfet/Lm3886 input for PA.
  Or Pickup/boost-fuzz-etc/Mosfet/Lm3886.
  Worked fine until overheated July 3rd. before last July 3rd.
  @@Rate, I just 'grabbed' a voltage for the pre-amp, IIRC from the middle to the - side.
  But I would go studying at Elliot Amps [correct?], and elsewhere to see proven designs and layouts, even though working with the LM3886's...well they're fairly easy to work with.