ADA4177-4

This is a simple circuit intended to replace any dual op-amp in dip 8 pkg.

Short question is , is there any tendency on this part (ADA4177-4) to self oscillate? Probably not but wanted to ask.

Trapi-ValveopD14.pdf

I have a new layout that is self oscillating usually around 1Mhz.

I did read the layout notes in data sheet.

Test conditions are in a known good purpose built fixture strapped in unity mode dc.

The vacuum tube is removed and a jumper placed across pins 7-8 and 2-3 which isolates the op-amp stages. No other components in signal chain, CC regulators removed...

An external 10k resistor load is across both outputs. Comp caps are 68pf. Previous Rev of board worked perfect. This rev simply adds a buffer to each to each output.

Omission of a build out resistor is intentional as the target board would supply if required. Test voltage is +- 15 VDC.

R1 and R2 may be 1k or 10k depending on which vacuum tube is used. Oscillation is with either value without a change in frequency.

The circuits (both sides ) oscillate with inputs shorted.

Injected audio range signals pass through correctly but with added osc impressed on output.

My question is, with attached schematic in a good layout do you see any condition that would cause such oscillation?

Thanks

Parents
  • VanPay,

      Why not use an ADA4177-2 and solder it directly on the DIP?  Many people have done this.

    Any op amp will oscillate if the noise gain is low and there is capacitive loading on the output.

    The faster the op amp,  the harder to mount things on a DIP adapter and not have oscillation.

    The bypass cap should be right across the 4177 package.

    Why add vacuum tubes when you don't need them??

    Harry

Reply
  • VanPay,

      Why not use an ADA4177-2 and solder it directly on the DIP?  Many people have done this.

    Any op amp will oscillate if the noise gain is low and there is capacitive loading on the output.

    The faster the op amp,  the harder to mount things on a DIP adapter and not have oscillation.

    The bypass cap should be right across the 4177 package.

    Why add vacuum tubes when you don't need them??

    Harry

Children
  • The customers love a tube amplifier. Its a marketing decision.

    Measured differences are primarily increased second and reduced third harmonics.

    Hence subjective/objective differences aside, true or not everyone I know owns and uses some sort of line amplifier based on tubes.

    I picked the ADA4177 for its input protection(s), 125C operation and no phase inversion.

    The board is only 1"x1" two layer.