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TLC271: programming or not to

Part Number: TLC271
Other Parts Discussed in Thread: OPA170

 this IC's in this configuration-  can it be programmed or is met to be  just a drop in part that should work as is 

if it can be programmed how do I do it  ? 

it works like it is but I am getting  a little DCmv  on the out put of the power amplifier it is adjusting for 

OH there is a -6.8v on pin 4 and + 6.8v on pin 7 and 115mv on pin 6,8 there is a 2.2 meg on pin 2 going to speaker outputs 



  • Hi Mike,

    The TLC271 is advertised to have a programmable power consumption based on the setting of the "Bias Select" pin (pin 6).  

    It sounds you're asking if there's a way to program the input offset voltage so you can trim the output voltage to 0V.  The TLC271 and most modern op amps without trim pins do not have a way to program the input offset voltage to a particular level other than using an external trim voltage which depending on it's accuracy may be better or worse than the TLC271 is already providing.

    Without seeing the full circuit configuration it's hard to debug if there may be an issue but a first recommendation would be to migrate to a newer device such as the OPA170 which has better input offset voltage and input offset voltage drift specifications.

  • thanks Collin first this is the older plug in IC style type in use also i have seen other  schematics pin 3 is open and pin 1 is grounded

    I was just wondering if it holds a program if momentarily   touching a pin ether grounding or a voltage  with out adding external parts ?

    of coarse if I could add a control to get 0v dc in the output ckt that would be great.

    this problem I am having may be from other parts of the ckt that drifts over time it is just that it happened after changing the IC

    I guess I am suppose to get a stable 115mv  on pins 6&8 for it to work right?  or what would most likely  give me ov DC on the speaker out

    thanks Mike


  • Hi Mike,

    the TLC271 is working like a DC servo here. As the circuit looks a bit old I guess that in an earlier version another OPAmp was used, an OPAmp with a different pinning.

    The DC servo works as follows:
    Pin 3 of TLC271 is at GND potential. By the help of negative feedback pin 2 is held on virtual GND. When the output of power amplifier shows a potential which differs from GND, a current is flowing through R25 and C3 is charged. This circuit is just an integrator. As consequence the output voltage of TLC271 rises and shifts the operating point of input stage of power amplifier, T1, until the output voltage of power amplifier also shows GND potential.

    So, the output voltage of TLC271 has not to be trimmed to -115mV, but is tuned to this voltage automatically when the DC servo loop stabilizes and the regulation of output voltage comes to rest.

    The only error the TLC271 introduces is its own input offset voltage, which is 12mV worst case. I don't think, that this rather low input offset voltage must be trimmed to zero?

    Pin 8 is the bias select input of TLC271. This is no output! To which potential is pin 8 connected? Can you tell us?


  • no -well at the moment on pin 8 I am getting -.031 from ground I don't know where else I could measure from suggestions?
    also I am getting 106mv DC on the speaker output I was told under 10mv might be ok there
  • Hi Mike,

    from the layout of quad606 (which can be found in the www) it can be seen that pin 6 and pin 8 are intended to be connected to each other. So, the output of TLC271 provides a bias select of TLC271 by itself.

    The -31mV you measure is directly the output signal of TLC271. It should be -115mV as shown in the schematic.

    Can you measure the voltages at pin 2 and pin3 of TLC271? They should be at 0V, or at least very close to 0V.

    Are the other measuring points of quad606 ok?

  • T1 e= -.64 b= .04 c= -6.7
    T2) e=6.03v b=6.6v c= -56v
    T3 c= 5.3v b= .64v e=.015v
    T6 e= -.62v b= -1.2v c-56v or 1.6v from Ground
    T5 b=-1.7v e=1.23v
    r16=-28v other side r16 D6= -.6v
    IC1 pin 6= -.033 pin2 & 3 are 0v   pin 7=6.7v pin 4= -6.7

    T15 B.=5v e= 0v c= +64v

    T16  B=.5v e=0v  C= -52v

    R 39 1 side  -52v & -59v  -- looks a little burnt)

    R38  1 side +64v  & + 64v  looks ok

  • Hi Mike,

    thank you for your measurements.

    Well, I think that the supply voltages are much higher than intended? 58V and 54V is usual for a quad606. But your measurements show 64V and 59V. This could shift some of the operation points of the circuit so much that the DC servo is no longer able to regulate the output voltage of power amplifier to 0V.

  • I am having a B+ imbalance problem with one side of the amp the DC mains voltage goes down to +32v and neg goes up to -65v also R12 and R39 looks burnt but check ok ,would T15 and T16 cause this they check good but I don't know if they are matched,
    or could C12 or C13 cause it if one cap is new and the other is original they are 15000 mf at 65v. I replace 1 it vented and split ? 
  • Hi Mike,

    when I have an old amplifier to repair I usually change all the electrolytics by fresh parts. Even if they look right they can have lost all their capacitance. So, replacing C12 and C13 by fresh ones is a very good idea.

    I don't think that T15 and T16 have to be matched parts. As the bases and emitters of both transistors are connected to each other, only one transistor can be open at a time.

  • I switched the card to the other side with the good caps and D11 It does the same thing and left on long enough

    the B+ goes past 65v  up to 77v while the other side goes down to 50v what else could cause this big imbalance ?


  • Hi Mike,

    have you replaced C9 and C11 with fresh caps?

  • yes on both cards even C7

    UP date I had a bad conection on R39 & t6 causing the offset

    also R5 on the schematic shows 120k  but some one did a mod & used a 62k resistor 

    I tested a 100k the voltage went up to -77 mv on pin 6 so if i go up 20k more that might put me at the -115mv  Is there anything wrong with the mod being at 30mv on pin 6

    if it is not hurting anything I could leave the 62k in there what do you think?

    I have only 1 more problem   one side is good at 1mv dc on the output   the other is at 17mv  and is slower

    getting to that voltage   I thank you very much for the help also i does chip regulate this DC bias ?

  • Congrats, Mike!

    the 62k repair could have to do with the aging of C12 and C13. Now with fresh caps again R5 should become 120k again. Also, R5 directly influences the corner frequency of high pass at input of quad606.

    1mV and 17mV is absolutely ok for the offset voltage at the output of a power amplifier. Nevertheless, 17mV is a bit high for the TLC271. What shows the voltage at pin 2 of this TLC271?

  • the voltage at pin 2 was 0v .

    I don't know what happen after checking all parts again all were good 

    so I cleaned the boards with brake cleaner dried them and re installed

    now both boards work  and I get 0v DC off set on both outputs they works great!

    I thank you very much for your help.