Diodes on input

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I have a bass guitar amp that has opamps in the input section of the preamp and tubes in the driver and output section.The first stage is non-inverting and has (2) 1N914 diodes in parellel and reversed from each other in polarity between the non-inverting input and the inverting input terminals. What pray tell does this arrangement of diodes provide for this 1974 amp. Can I remove them to improve sound quality because I have never seen them used in a musical amplifier before.:confused: :confused: :confused: :confused:
 
Those diodes placed anti-parallel are there to protect the amp against too high input signals, and so to avoid the clipping of the amp. When a (sine wave) signal with too high amplitude is applied the sinewave will be flatten instead of cut off (see pic).

HB.
 

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Protection

The diodes are there to prevent the common-mode input range of the op-amp being exceeded.

On many op-amps exceeding the common-mode input range causes permanent degradation of performance, that may not even be noticed at first glance.

Andy.
 
Re: Protection

ALW said:
The diodes are there to prevent the common-mode input range of the op-amp being exceeded.

On many op-amps exceeding the common-mode input range causes permanent degradation of performance, that may not even be noticed at first glance.

Andy.

Al,

The original poster stated that they were connected between pos and neg input terminal. That in my view makes them limit the differential input voltage.

Jan Didden
 
Since the inverting and non-inverting inputs should have exactly the same signal on them, the diodes don't normally conduct. While they're just there for protection, they can also inject noise too. Any ambient room light leaking into the case can be amplified like a photo-diode. If the case is not light-proof, paint the diodes black.
 
Diodes function

:) Thanks for all the input everyone and I get the jist that these diodes were for input protection of an old and obsolete Op-Amp, one of the first good ones of its day,but not needed for a OPA-627 or Analog Devices 843 or 8010. I am assuming because I have not seen any kind of diode protection on the input on current designs. KMcC
 
Thanks for reminding me to to look at the data sheet, I did and found the answer I was looking for. The B.B. 627-637 data sheet states that if the input voltage will exceed +Vs plus 2V. and -Vs plus2V. that the input diodes should be used for protection, although not configured between inverting and non-inverting inputs. That would mean if the input exceeded +17V. or -17V. I would need clamping diodes. Not going to happen with a bass guitar with even active electronics after the magnetic pickups. They did say the added diodes would increase input bias current by a factor of 1000 if 1N4148 diodes were used.They also echoed the response of someone on this forum that light falling on the diodes increased leakage current. It all goes back to the addage that simpler is better.:D
 
Musical instrument gear should be designed to withstand 120V on the inputs, a very common occurance with any PA gear. A few µA of leakage from touching a microphone with its mixer line cord reversed and your fancy new $10 opamp is gone.

I used to lose the input buffer on the FOH crossover every time a guitar player flipped the line reversal switch on his amp.
 
Diode input protection

Wow, I am learning more on this website than I did in school. DJK made a good point about protecting the input from 120V. of accidental input. The singer from a 60's band Stone The Crows found out about grounding and Hot and Neutral the hard way and was electricuted on stage because of a mismatch between his mike stand and ground. Sad thing was the audience thought he was just getting into the moment. I have reread the Burr-Brown application sheet and discovered that a FET can be used as a low leakage diode when configured as a diode-connected FET.The 2N4117A has a leakage of 1pa and it's metal case shields the junction from light,which increases the diodes leakage current. Even though I know my friend will never be on tour with roadies and all kinds of mismatched equiptment, the situation is worth considering just in case. I do not have a scanner or I would post the Burr-Brown schematic for the diodes, they show the two diodes or FET's back biased by the two power supply rails on the non-inverting input only, is this different than in parallel with the inverting and non-inverting inputs. I will probably use the Burr-Brown arrangement because they should know their Op-Amp better than the designers of the original Op-Amp used in this preamp, National Semiconductor. An alternative to the OPA-627, I believe, is the Analog Devices 843 or 8010. I think I will check out their spec sheet and look at their input protection arrangement and report back. KMcC:cool:
 
Could it be a fuzz effect?

Look at the M/M/M Instrument Amplifier here:
Popular Electronics Magazine

There's schematics of tube amps in The Tube Amp Book by Aspen Pittman with a similar fuzz effect built in which uses anti-parallel diodes for clipping the waveform. I built a circuit like that in my tube guitar preamp. I also put a potentiometer arranged as a variable resistor between the diodes and ground to control the amount of hard or soft clipping.

The way anti-parallel diodes work is because silicon diodes are forward biased above about 0.7 volts (germanium is 0.3 volts), so anything out of that range gets clipped.
 
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