post fader amplifier questions

[Analog_Fan]

Just consider that the lower tehenominal operating level, the higher the noise.

Well, I'm not an expert on distortion in circuits, i made synth modules and they all work good.
Line Level should be like 1.0 V.p.p. to 1.5 V.p.p.

Using these tiny logic switches on +2.5V/-2.5V PSU's, should give me like 1.75 to 2V headroom on both sides, right?

I could stick these 74LVC1G3157, 74LVC1G66 anywhere in between components on the PCB and not having to worrie about DIP14 or DIP16 sized bricks, route alot and reduce the pcb size alot.

What page 27?

From this "Steve Dove" document you gave me.

 

[JohnRoberts]

a CMOS analog switch inside the feedback loop?
You don't happen to have a drawing on hand?

nope... I used it inside a (Loft/Phoenix Audio Lab)) console I designed back in the late 70s/ early 80s...
Allow me to try using words.

Visualize a simple inverting op amp with two sets of input and output resistors not hard connected to the op amp - input. Then a CMOS transfer gate connects one set of I/O resistors or the other to the op amp completing the NF circuit.

Of course there is more glue circuitry but that is the basic theory. The transfer gate is only driving the op amp - input so near zero current for near zero distortion.

Back in the early 80s I couldn't measure any distortion using the best test equipment I could get my hands on.

JR

[edit- I'm pretty sure I have described it multiple times here, probably better than this.../edit

 

[Analog_Fan]

nope... I used it inside a (Loft/Phoenix Audio Lab)) console I designed back in the late 70s/ early 80s...
Allow me to try using words.

Visualize a simple inverting op amp with two sets of input and output resistors not hard connected to the op amp - input. Then a CMOS transfer gate connects one set of I/O resistors or the other to the op amp completing the NF circuit.

Of course there is more glue circuitry but that is the basic theory. The transfer gate is only driving the op amp - input so near zero current for near zero distortion.

Back in the early 80s I couldn't measure any distortion using the best test equipment I could get my hands on.

JR

[edit- I'm pretty sure I have described it multiple times here, probably better than this.../edit

Something like this or close?



Link to Falstad simulation: https://tinyurl.com/ywbboecp

I used the mute circuit from the Cirrus logic 4271 with the MMUN2111 transistor.
It seams to leak, the switch of the opamp doesn't seams to cause massive current usage.

 

[Analog_Fan]

Link to Falstad simulation: https://tinyurl.com/ykowhdov

This should work with CD4051, CD4052, CD4053, CD4066.
It's totally silent, remove the NPN transistor and some will leak through and the same happens if you remove the switch and rely only on the NPN.

using CD40106, MMUN2111 (transistor) and a normal (configured) CD4066.
Of course, you have to factor in the 150 Ohm of the analog switch and discount from 1K8 resistor.

 

[JohnRoberts]

View attachment 117545
Something like this or close?

only close in that I see an op amp and some resistors.
===
Instead visualize the wiper of the switch connected directly to the op amp - input. In one throw of the switch it connects the - input to the junction of those input and feedback resistors. In the other switch throw the - input is connected to a different pair of resistors fed from a different input but with the feedback resistor still connected to the op amp output. (of course there is some more housekeeping glue components to manage clicks, kill, etc) .

I used this to electronically switch 16 channels of a console between mix and monitor mode with a single logic voltage. I know I have discussed this before but AFAIK there are no old PAL console schematics out in the world.

JR

 

[abbey road d enfer]

What is this doing?

It looks like it injects DC into the inverting input. Or is it not connected?

 

[Analog_Fan]

Well, the CD4066 is configured as normally open, this inverts that.
Using a CD4053 pretty much remove all the stuff, like transistors and simplifies the circuit, space usage.

I did not add capacitors, that's detailing

i made a new "design" using CD4053, this removes alot of parts.


Falstad simulation: https://tinyurl.com/ykurncuj

 

[Analog_Fan]

using a CD4013 witch has Q and NOT Q and push button solves the problem and you can keep using CD4066.


Link to simulation: https://tinyurl.com/yswnej2b

 

[abbey road d enfer]

That's pretty much what Brian describes in his post #24.
I would suggest you follow zamproject's recommandation in post #4 to look at how the Studer engineers dealt with that.
They had access to CMOS gates, but they opted for discrete FET's because it had a functionality that transfer gates cannot offer: soft-switching.
It means conduction, instead of being almost instantaneous (a fraction of us), which results in an unpleasant thump when commutation happens elsewhere than zero-crossing, is actually temporized over a few dozen ms, which produces a non-audible transient.
I'm not saying that it's what you should do, just suggesting you look at these brilliant designers' take on the subject for enlightenment.

 

[Analog_Fan]

That's pretty much what Brian describes in his post #24.
I would suggest you follow zamproject's recommandation in post #4 to look at how the Studer engineers dealt with that.
They had access to CMOS gates, but they opted for discrete FET's because it had a functionality that transfer gates cannot offer: soft-switching.
It means conduction, instead of being almost instantaneous (a fraction of us), which results in an unpleasant thump when commutation happens elsewhere than zero-crossing, is actually temporized over a few dozen ms, which produces a non-audible transient.
I'm not saying that it's what you should do, just suggesting you look at these brilliant designers' take on the subject for enlightenment.

found the schematic here:
https://ia801301.us.archive.org/23/items/studer_961-962_Op_Serv/961-962_Op_Serv.pdf
There are many variants of mute and also many "lines" in a bus and hard to tell where are heading or come from from.

I took this from page 150:


link to Falstad simulation: https://tinyurl.com/yw9c6z4g

i needed to add the transistor mumble jumble to be bi-polar, the CD40106 in the Struder 961 is powered bi-polar.
the previous circuit from post #48, post fader amplifier questions is totally silent with nano volts, but this circuit still leaks a few micro volt according to simulator.

the "transistor mumble jumble" consumes very little.

edit / update: I turned down the "scope" timing of the Falstad simulator in to the nano second range, rather than the default 5 micro second range and it indeed looks like a soft starter.
Also look at the Headphone output on page:164, but that doesn't work.
Falstad simulation: https://tinyurl.com/yttb7n8m

p.s. I recently discovered the Falstad simulator in a more than 10 year old YouTube video!
somewhat primitive but recognizable.

 

[zamproject]

found the schematic here:

Nope !!
Look at 169 !
7 resistors, 3 transistors (1PNP 2FET), one capacitor, one DPDT switch per channel, and a single -logic- bus

but this circuit still leaks a few micro volt according to simulator.

How many is few ?!? 10uV is -100dBu...
Maybe series/shunt in place of shunt/series ?!?

 

[Analog_Fan]

Nope !!
Look at 169 !
7 resistors, 3 transistors (1PNP 2FET), one capacitor, one DPDT switch per channel, and a single -logic- bus

Thank you, but ...
https://ia801301.us.archive.org/23/items/studer_961-962_Op_Serv/961-962_Op_Serv.pdf
on page 159 (vertical), i see the same as i got currently.
But on page 169, as you indicate shows me flat ribbon connectors for monitoring.

How many is few ?!? 10uV is -100dBu...
Maybe series/shunt in place of shunt/series ?!?

I don't handle a lot with Shunt, pretty much the only i now is that people use a 1 Ohm resistor and measure across it.
I'm happy to learn more.

 

[zamproject]

But on page 169, as you indicate shows me flat ribbon connectors for monitoring.

Not the 169th page from Studer 961 console service manual ....
But Studer 169 console service manual !!! (section 7.4/11)

I don't handle a lot with Shunt, pretty much the only i now is that people use a 1 Ohm resistor and measure across it.
I'm happy to learn more.

I mean first switch in series (signal line) and second across 0v, meaning you get a FET Ron/off voltage divider with 10+digitΩ and 2digitΩ figure.
You do the opposite, can't be sure what it dose but somehow the buffer input become floating ?!?

 

[zamproject]

 

[Analog_Fan]

I mean first switch in series (signal line) and second across 0v, meaning you get a FET Ron/off voltage divider with 10+digitΩ and 2digitΩ figure.
You do the opposite, can't be sure what it dose but somehow the buffer input become floating ?!?

Thank you, i read a title further on the document of the Struder 961 and the fet mute circuit is to be found on many places or a slight different configuration. the FET salesmen must have jumped a hole in a SKY, that many fets are i there!

The suggestion you made improved the circuit, now the (simulator) "leakage" is around 3 micro V compared to about 11.x micro V in the circuit from post: #50



I also added the 6nF8 capacitor.

link to simulation: https://tinyurl.com/2xbrz6bn

 

[Analog_Fan]

The behavior of the circuit is pretty cool, it opens slower, but it kills pretty fast.

Routing a module in Eagle.

edit: the 6nF8 seams to make performance worse if you equal it as the others, the performance is equal or just remove it, than it's more silent.

the FET is no unknown part, but rather a common one.
So i had the datasheet.


How does that translate when being fed either +/-10V?

 

[Dimitree]

I ended up getting some ADG419 and will use them in "series" configuration,
do you think I can use it before the fader, like shown here? I'd like to mute also the signal that goes to the pre fader auxes. In other examples I've seen, the switch was always after the fader and before the post fader amplifier. I'm wondering if placed before the fader would be a problem

 

[abbey road d enfer]

Since the post-fader level is lower (typically 10-12dB less), distortion is lower when the switch is pot-fader. Now when installed pre-fader, the load is lower so cut-off will be better.

 

[Dimitree]

Since the post-fader level is lower (typically 10-12dB less), distortion is lower when the switch is pot-fader. Now when installed pre-fader, the load is lower so cut-off will be better.

Thanks. With better cut-off you mean better offness? And how can I calculate the distortion difference between pre and post, assuming a 10dB level difference? So maybe it will be clear to me if the difference is negligible

 

[abbey road d enfer]

Thanks. With better cut-off you mean better offness?

Yes.

And how can I calculate the distortion difference between pre and post, assuming a 10dB level difference?

Very difficult to calculate, because it depends on several factors that are not specifically documented.

The only thing is that THD will be lower at -10 than at nominal.

So maybe it will be clear to me if the difference is negligible

Very likely.