Active volume control - Be quiet please.

[Buehgemeiste]

Hello,
i would like to implement an active volume control into my next preamp.
I read those two articles:
http://www.ti.com/lit/ug/tidu034/tidu034.pdf
ESP - A Better Volume Control
and played around on the breadboard.

Now i'm listening to music via this setup:
music from pc
-> breadboard:

-> sure tpa3116 class d amp (with same signal on both channels (mono))
-> low sensitivity monitors from the 80s.
power supply is ~19v from sla batteries.

Opamps are TL072 for the signal and TL071 for the ps.
R9 and R10 represent a B50K pot.

Sound is good.
The volume pot gives a nicer 'response' than a standard log pot at the output of the preamp.

BUT:
This circuit cannot completely mute the signal.
Is there a modification to this circuit to achieve that?

edit:
R15 is not 22k but 100R
R13 is a link

 

[Mooly]

I've used these type of circuits in the past and it should go all the way to zero. If you physically short the pot wiper to R14 (in other words short out R10 in the sim) do you get silence. If so then the pot isn't up to scratch and is leaving some residual resistance in circuit.

 

[Buehgemeiste]

I hoped for you, mooly, to give some advise.
Thanks a lot!

So maybe this is an effect i have recognized a couple of times now in my still short diy carreer:
I hear my sound transducers creating small acustic signals even if no electrical signal can flow through the wires (no connection).
But the signals seem to be induced via magnetic or electric fields from one wire into the other.
Is this possible?
I think this effect might be even stronger on the breadboard with long leads and so on...

So maybe i can only really test it when built on perf-board.

 

[Mooly]

On a breadboard there is significant capacitive coupling between the strips of contacts (many 10's of pf) which is enough to couple AC signals between relatively high impedance circuitry. Wires would have to be very close and over some distance to do the same but yes, it happens.

 

[Buehgemeiste]

one more question about this, that doesn´t seem to be worth an own new thread.

the same preamp should include an baxandall tone control similar to this project:
Universal Preamp/ Mixer

is it better to have the volume control before or after the tone control stage?

 

[AndrewT]

Why is u3 in there?
What is it adding to the performance?

 

[Buehgemeiste]

I quote both sources from the original post:

The buffer (u3) enables the inverting stage (needed so the circuit can work) to have a very high input impedance.

from what i understand gain should be added as 'early' as possible in the signal path.
so what is better at first volume control or tone control?

 

[Mooly]

Any stages after the volume control circuit will have their noise contribution ever present and will also be working with very small signals (when the volume is low).

I quote both sources from the original post:


from what i understand gain should be added as 'early' as possible in the signal path.
so what is better at first volume control or tone control?

If you are dealing a small signal such as a phono cartridge or tape head then yes, amplify that signal to line level first.

Place the tone stage before the volume control and it will be working at normal line levels. The volume control will attenuate both signal and noise as the gain is lowered. That is the preferred option.

 

[Buehgemeiste]

i'll do it like that.

 

[Mooly]

Best way imo

 

[AndrewT]

Why is u3 in there?
What is it adding to the performance?

I quote both sources from the original post:

" The buffer (u3) enables the inverting stage (needed so the circuit can work) to have a very high input impedance. "

With a 50k pot in there I don't see that the first non-inverting stage is achieving anything.
The input impedance of the inverting stage is the first resistor value inserted between the source and the inverting input. That is the pot upper half resistance.

The two seem the same to me. both go to zero ohms when at infinite gain, both go to 50k when at maximum attenuation.

What am I missing?

 

[Mooly]

With a 50k pot in there I don't see that the first non-inverting stage is achieving anything.
The input impedance of the inverting stage is the first resistor value inserted between the source and the inverting input. That is the pot upper half resistance.

The two seem the same to me. both go to zero ohms when at infinite gain, both go to 50k when at maximum attenuation.

What am I missing?

Its perhaps not ideal value wise. The buffer allows R12 to be fully driven which can also be suitably partnered with R14 to set the maximum gain of the stage. A simple active gain stage just uses one opamp.