Matching impedance 800ohm from audio line output etc...

[Audioguru]

The end with the printed bar is pin 1 and pin 8.

 

[Freehawk]

Hi ,

Yes i also tought so , but wanted to be absolute sure of it.
I soldered it on an 'interface' board , doesn't matter at that point were the 1 pin is , it still has to be connected to the circuit.

I finally will try tomorrow to make te rest of the circuit and hopefully i will not ruin it

Warm regards and many thnx for all the help

 

[Audioguru]

Warm regards and many thnx for all the help

Thanks for your warm regards that helped increase the temperature around here from extremely cold yesterday and this morning to not bad and close to freezing now.

 

[Freehawk]

Hello ,

I can't say it's a succes :-(

I really don't know what i did wrong , but i have following issues

1) I have the same 'feedback' problem as when i used passive mixing

2) If i do not provide the opamp of suplly power, the signal is still mixed :-(

I do really not understand ...

Anyone any suggestions of what i could do to test what i did wrong??

Warm regards

 

[Audioguru]

If your opamp mixer is powered then it cannot feed the input from the amplified spring reverb back into the input of the spring reverb.
Of course if the opamp making the mixer is not powered then its resistors make an ordinary passive mixer that feeds its output back to the input of the spring reverb causing feedback.
Please show a sketched schematic of your mixer, its powering, its bias and its connections.

 

[Freehawk]

If the opamp is deffective , would that also result in the same issue ?

I am experimenting with deconnecting the audio input (opamp) and it makes no difference in the result

So really i do not know what to search for in the first place

Schematics is not a strong point for me

especially because i use diy prebuild pre amps before mixing the signals , i do not have a general circuit scheme


But in general , i spilt the audio signal , one is going to preamp - spring reverb - preamp... the other one is after that stage again mixed to one signal (now via the active mixer scheme)

i first used a passive solution , got a lot of feedback ...
Tried to make it an active one with the opamp and scheme you provided me

But apparently i am doing something completely wrong , i do not really know what to look for

can i test the opamp itself? if it really works?

because there are not soo many components other than the opamp , it looked also a simple thing to me

But maybe i am verry bad in these things



I try to give you some more info about what i did or how i connected things together soon

Maybe a picture of my circuit board???

would that be of use?

Warm regards



PS: today it's also freezing cold over here

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I tried a quick scheme forgive me if its not good




Warm regards

 

[Audioguru]

I use Microsoft Paint program to draw my schematics. I use the PrtScn (print screen) button to copy something then paste it into the Paint program. I don't draw resistors, capacitors or transistors, instead I copy them from another schematic then paste them onto my new schematic.
Straight lines are drawn with the shift key held down. Text can be put anywhere on the image.
Please post a photo of how you connected your circuits.

EDIT: your schematic is correct but has no resistor or capacitor values and no opamp part number. The opamp does not show its pin numbers.
Where does IN1 and IN2 come from?

 

[Freehawk]

In1 = original input signal
In2 = splitted signal that was fed trough the preamps and springreverb (result of spring reverb)

Resistors are all 10Kohm
capacitors are all 1uF

In fact the drawing was based on my ciruit board , the layout is almost the same as i put my components on the board

The pin numbers of my opamp is pin 1 is the out
pin 2 is the input -
pin 3 is the input +
pin 4 is the V-
pin 8 is the V+

I put the opamp with the printred bar up and left is pin 1 right is pin 8 (upper pins)

Warm regards

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V- is grounded V+ is fed with +12V

 

[Audioguru]

Your OPA1642 opamp is connected correctly and the 10k resistors and 1uF capacitors are fine. The polarity of the output capacitor is backwards because the output of the opamp is at about +6V. Can you measure it with a voltmeter?
You can test the opamp by disconnecting IN1 then the sound level of the opamp output from IN2 should not change. Then connect IN1 to ground and again the sound level of the opamp output from IN2 should not change.
Then connect IN1 to the opamp and try disconnecting then grounding IN2 and the sound level from the opamp output from IN1 should not change.

Since you are mixing the guitar output with its reverb output then the mixed output should not have any feedback if there is no speaker playing it near the guitar or near the spring reverb.
The spring reverb should not be mounted inside the guitar because then the sound of its spring might vibrate the strings of the guitar and cause feedback.

 

[Freehawk]

I can add a general drawing of what i did



Warm regards

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Hi , thnx for this answer

I am not mixing a guitar ouput , it's a line level output +4db (synth) , the spring reverb is not installed in a case , not in a cabinet
I have also the feedback if i only listen with headphones

The output of the opamp must be at +6V DC or AC? I try to measure it , it's not at all in that range , it's only in millivolts

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Ah i think you mean V- is 6VDC ? that's correct .. it is

 

[Audioguru]

The (+) input pin 3 of the opamp is biased at +6V (measure it) by the two resistors in series and the negative feedback through the resistor from its output pin 1 to its (-) input pin 2 causes its gain to be 1 so its output should also be at +6VDC. Its output will swing up and down with the audio signal.

Here is your schematic:

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V+ is +12V and ground is 0V. The V- pin 4 of the opamp is connected to the circuit ground. The (+) input pin 3 of the opamp is set to +6V by the two series resistors which should cause the output of the opamp pin 1 also to be at +6VDC and modulated up and down with audio.

 

[Freehawk]

pin 1 = 6V
pin 2 = varies from 2.5v to .08V and back
pin 3 = 6V
pin 4 =0V

 

[Audioguru]

The voltage at pin 2 might vary no more than from +5.9999V to +6.0001V if the output of the opamp is not so high that it is clipping. Because the very high gain of the opamp with the negative feedback cancels most of the input signal at pin 2.

 

[Freehawk]

Ok , en what could be the cause if this?

I checked a bit soldering (contact points on the interface board)
And now i am measuring other values

pin 1 = 9v
pin 2 = 9v
pin 3= 8.21v
pin 8= 12.07v

 

[Audioguru]

Contact points? Did you build the circuit on a solderless breadboard that has intermittent contacts and jumper wires all over the place?
Your input and output pins have voltages that are completely wrong.

 

[Freehawk]

No ,
I did use a small general circuit interface board to put the ioc ic on , this board has contact points to integrate it in a circuit
see below



I don't understand a clue of it really...i know it's not correct , but i don't know why it could be

I remeasured also the resistors, they give me a good 10k value

i am really helpless in this

 

[Audioguru]

The inputs of the opamp are Jfets that draw almost no current so they should not affect the +6V bias voltage.
I guess you carefully soldered the tiny IC into the interface board then soldered wires to the contact points and the other ends of the wires were soldered on the resistors somewhere?
Where are the resistors and how did you connect to them?

 

[Freehawk]

Hi ,

I did carefully solder the tiny ic (it worked out quit well) , i tested also the connections between pin and contact points to make sure contacts were ok and there were no shorts
I indeed dit solder then wires to the contact points.
I soldered a circuit on a matrixboard with the components in the scheme and i connected them , then i connected the wires from the 'interface board' to the places in the scheme on the circuit board...

I am so sorry i cannot explain it better that this , i include a picture of what i did :-(

Warm regards

 

[betwixt]

:shock: Over to you Audioguru....

Brian.

 

[Freehawk]

Yes I know betwixt , you can laugh at me and be shocked no prob ...
Thnx for the help


Warm regards

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Meanwhile i made another attempt ...
I just soldered a new opamp to an interfaceboard .. tested connections no shorts
And connected it to power, ground and resistors



pin 4 to ground 0V
pin 1 not connected (out A) 6.3V if i am measuring right on the pin (ic) but not allways the same when i measure on the contact point :-(
pin 2 not connected (no input signal) (- in A) 0.6V when mesured right on the pin but also not allways same on the contact point
pin 3 to the resistors (10kOhm -- pin 3 -- 10kOhm) (+ in A) 6,03V
pin 8 : 12V measuring : 12,07V


Is this better?? (i don't know)



Warm regards