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avielnah wrote: ↑29 Jun 2021, 08:23 i also dont really get it how come i see such a gain boost, the overall gain should be determined by the feedback network, and not the op amp itself.

I agree. Do you have a scope to measure gain against calculated gain?

Which charge pump IC are you using, the 1044A, 7660A 0r 7660S ?

Manfred wrote: ↑30 Jun 2021, 02:17 Which charge pump IC are you using, the 1044A, 7660A 0r 7660S ?

i use 1044A without a regulator in front of it.

bmxguitarsbmx wrote: ↑29 Jun 2021, 21:58

avielnah wrote: ↑29 Jun 2021, 08:23 i also dont really get it how come i see such a gain boost, the overall gain should be determined by the feedback network, and not the op amp itself.

I agree. Do you have a scope to measure gain against calculated gain?

yup, i attached the schematic,
if i do rough calculation, i would expect gain of ~22 (220k/10k).
results from measuring this with a scope:
1. OPA2134: 500mV pTp input yields 10.2V ptp output => roughly gain of 20 (with high frequency oscilations of 500 pTp)
2. TL072 : 500mV pTp input yields 7V output => gain of 14 (no oscilation)

The maximum undistorted output amplitude depends on the slew rate SR of the operational amplifier which is 13V/us for the Tl072 and 20V/us for the OPA2134.
If the output signal is distorted, the read value is different from the pure sine wave.
What was the measurement frequency.

frequency was 800Hz..
the signal is also undistorted in this stage, its quite clean. i run it on +-12v rail to tail

Thanks, that was just an idea, but for both types the slew rate is so high that for the audio frequency range there is no deformation of the output signal.
I am wondering that the gain is smaller with the TL072, because normally the gain of the op-amp stage at this frequency is determined by the external circuitry independent of the type of op-amp.

Manfred wrote: ↑30 Jun 2021, 19:21 Thanks, that was just an idea, but for both types the slew rate is so high that for the audio frequency range there is no deformation of the output signal.
I am wondering that the gain is smaller with the TL072, because normally the gain of the op-amp stage at this frequency is determined by the external circuitry independent of the type of op-amp.

yes, i am confused as well to be honest, NE5532 and TL072 both show x14 gain vs OPA 2134 that reached 20, but with oscilations.
i ended up using a TL072 and changed the feedback resistor to get that x20 gain boost

That's a head scratcher!

This could be an operational amplifier biasing problem, try R3 = 10k and C3 = 10uF.

bmxguitarsbmx wrote: ↑01 Jul 2021, 06:35 That's a head scratcher!

whats the logic behind that?
wont it just reduce the signal amplitude in the opamp positive input?

avielnah wrote: ↑02 Jul 2021, 06:05

bmxguitarsbmx wrote: ↑01 Jul 2021, 06:35 That's a head scratcher!

whats the logic behind that?
wont it just reduce the signal amplitude in the opamp positive input?

Don't worry, this does not reduce the input voltage of the positive input because the output resistance of the previous buffer is less than 50 ohms. This output resistance is in series with R3, there is no noticeable voltage drop.

The rule for the value of the bias resistor is Rf in parallel to Ri,
so here R3 = R4||R5 = 220k||10k = 9.56 kOhm ~ 10kOhm.
This serves to minimize the offset voltage (voltage between the positive and negative input) of the op-amp.
If this value does not fit or is too high as here, it can lead to a larger offset voltage and thus to an internal operating point shift of the op-amp input stage, the op-amp then no longer functions 100%.

I ran some simulations with the circuit from input to R7 with different opamp types with each R3 = 100k and R3 = 10k.
Unfortunately I don't have an OPA2134 in the library I took an OPA2132.
The circuit worked with all opamp types regardless of the value of R3.
I fed a sine signal of 500 Millivolts peak at 800Hz, the power supply was +/- 12 Volts.
The only noticeable thing was that with the TL072 the output signal was already clipped at pin 7, the clipping started at 465 Millivolt peak.

Really a head scratcher.

well the only thing i can think of is that the oscilation somehow causes this?

what gain did you got for the TL072? to be honest i am suprised i got x14 and not x20 as expected..

avielnah wrote: ↑05 Jul 2021, 05:17 well the only thing i can think of is that the oscilation somehow causes this?

what gain did you got for the TL072? to be honest i am suprised i got x14 and not x20 as expected..

I got a gain of 20.
Did you tried it with R3 = 10k?
Connect a 10K in parallel to the 100k and keep the C3 with 1uF and see if the oscillation stops.
I also remember experiences that i had made with tube amplifiers.
With bad grounding e.g. by alternating currents over a ground line part a voltage drop was produced
which lay over the ground connection of the grid resistor at the input of the tube stage with high gain and thus led to oscillations.
Maybe there is a ground problem here.
Look at the supply voltages with an oscilloscope in the case of oscillation and see if there is a noticeable AC component on the DC voltage.

i tried with R3=10K but no diff.
BUT, by connecting a BIG 470uF cap from the -V of the op2134 to ground, i eliminated the oscilation, but i still get some quite high level brown noise even when input is grounded, i tried also a cap to the +V and it didnt make any diff,
any idea what i can try? it's certinaly comes from the OP2314 second stage of amplification

What is the frequency of the oscillation?
Insert a 1k resistor between pin 7 of the op-amp and ground.
Add 10nF capacitors to pins 4 and 7 directly at the IC, both other leads to ground.

the original oscilation were at ~18Khz, they were solved only by the big 470uF cap from pin 1 to ground.
the noise that i get now is not oscilataion, its sounds like "white noise" in the range of 0 to ~800MHz, this noise is there even if the input is grounded.

I would skip the 2134. They need good layout and proper decoupling. I'm not sure why there was an issue with the 072, but the 072 is very forgiving. At the point where you are modding something, you are probably adding more stray coupling. I know that is a sensitive topic here. Not trying to trigger anybody, just trying to be realistic.

hi guys

anyone who have diy'ed this and NOT squeeling at higher gain settings?

im using an BB OPA2604AP(just a faster variant of 2134) and MAX1044CPA for the inverter, this has almost NO whine when compared to the ICL7660(not the S variant, dont have) and I just limit input supply to 9v battery.

I dont have log250k, only A500k. also tried lowering it using linear 100k.

but still squeels at about maybe 20%

even added ferrite at input, to no avail