Tonemender bias...

[petemoore]

  Tonemender, ROG.
  PAsses signal, maybe sligthly distorted [my testube amp needs a PS Filter Caps change, I'm not use to the Jfet>386 amp for testing...testing @ low volume].
  I've given it a good thrice over, pots indeed have shown themselves to the DMM and me as connected right, everything else too. Pots work right, it has good gain.
  It's these OA Pin voltages that concern me.
  Battery is good, @ 12.51V.
  I've thus far omitted the 100ohm resistor between +9v and V+.
  pins     V's
1 6.27
2 6.27
3 5.60
4 0
5 5.97
6 6.27
7 6.37
8 12.51
  Midshift Sw's are sockets instead...
  the 470n between 4k7 and Vref...that is two 2.2uf electrolytics paralleled, + from each capacitor to each capacitors -.
  The 1meg measures about 1meg from OA +input to Vref, which is Vref. Vref works out to 6.37v out of 12.51v, a little high, as ROG recommends rail to rail OA's, it would seem biasing to a very close to 1/2v Vref would gain more with less OA distortion...
  But then there is Pin 5 @ 5.97v, and pin 3 @ 5.60v, with measured 1m from Vref to each...they seem a bit low, and this concerns me, however, all the debugging methods have been applied...checks for continuity to other places [Vref is above board except one connect, very easy to see a these 1m's connections, which of course were tested for Resistance and continuity].
  I've been here before, not knowing what one could do [else the doable has been re-verified as done] to bias an opamp pin.
  I don't know whether I should let voltage readings such as this bother me, I use the +/- 10% tolerance rule for what 1/2v 'should be', just glancing at the numbers It looks like they're all within these specifications...but the 'some lower/some higher' is a more complex equation, it looks as though distortion caused by one higher/one lower biasing of seriesed opamp stages would become multiplicative he first offbiased stage throwing throwing the second one nearer the rail it's biase closer to...as they're both non inverting stages...that theory is about 'half grasped' to me.
  Comments...suggestions ??

[petemoore]

Bump

[petemoore]

last bump, if no response I assume it's ok, or there's nothing that can be done about it.

[MartyMart]

Pete, you're using a 386 ?
Dont you have an NE5532 or TL072 ?
Perhaps this is the problem, the pin voltages look "about right" though !
All controls working fine then. so just a touch of breakup  ?

MM.

[stm]

last bump, if no response I assume it's ok, or there's nothing that can be done about it.

Hi Pete.

Sorry for taking so long to respond--had presidential elections here in Chile, so I didn't go online during the whole weekend.

The voltages you mention are a bit funny. You shouldn't have clipping if using 12V, unless you set the gain/volume to max.  Using 12V more than compensates using 9V with a R2R opamp.  Please indicate which type of opamp are you using.  Sometimes the 1Meg biasing resistors may produce bias shifts with bipolar opamps (like RC4558) since they have a rather low (200k) input impedance.

Regards.

[petemoore]

   Sorry for confusion...
  The Test Amp...it has a 386...
  this circuit calls for and I'm using a dual Opamp...
  Don't have the rail to railers, so I used 25v+ rated caps, this way I can just bump the headroom up by raising the PSVoltage.
  Thanks for the responds...I should be able to ferret out the remaining answers !!