Mixing 4 audio channels to 3?

[DaveC]

What is wrong with using an op-amp to "amplify" DC? Surely they are

"meant" to do that.

Isaac

I think he means that better audio results can be achieved by providing true
dual-voltage supplies and eliminating all coupling caps.

The "DC amplifier" is a fine design, but will result in inferior audio
performance due to the caps.

Now, if someone would volunteer such a negative voltage generator circuit...
;-)

Thanks.

 

[[email protected]]

Yeah, I wasn't sure about that. How should I have indicated 2 separate
grounds?

It's not ground. It's Vcc/2. I generally call it Vref, or some such thing.

 

[Don Pearce]

KISS

Exactly. Much easier than all these halved supplies, coupling caps and
multiple grounds.

d

 

[DaveC]

Exactly. Much easier than all these halved supplies, coupling caps and

multiple grounds.

d

So, how -- exactly -- would you create that negative voltage?

Dave

 

[DaveC]

Here's the whole thing; DC coupled mixer with a charge pump for a

negative supply:
[J. Fields]

Thanks John. Nice design.

What part is used for SW1-3? Purpose? If these are "anti-thump" turn-on
suppression switches, I think the main amp is similarly turn-on delayed. Of
course it's important to know the timing of these to avoid any window through
which the sub's cone could launch (or lunch?)...

Thanks,
Dave

 

[DaveC]

Here's the whole thing; DC coupled mixer with a charge pump for a

negative supply:

John, what's the reason behind choosing 6.2K for the feedback R on U3 (sub op
amp)? And if that is to be a pot (the original idea), is 10K appropriate?

There should be some kind of sub volume control...

Thanks,
Dave

 

[DaveC]

C2, C3 are polarized, so I presume electrolytic?

What type should C1 be?

Thanks.

 

[tuinkabouter]

. E1 -0.621V
. | 0.621V R3 /
. [Ra] / +-[10k]-+
. | / R2 | |
. +-+--[10k]--+--|-\ |
. | / |>-+
. [Rb] / +--|+/
. | 0V |
. GND GND

"OK", you may say, "but what on Earth does that have to do with a 50k
pot feeding a 10k load?"


If we make a table of changes in output voltage as a function of
successive 5kohm changes in pot resistance, we'll have:

. R V dV
.--------------------
. 50k 1.000 0.379
. 45k 0.621 0.177
. 40k 0.444 0.103
. 35k 0.341 0.069
. 30k 0.272 0.050
. 25k 0.222 0.040
. 20k 0.182 0.036
. 15k 0.146 0.035
. 10k 0.111 0.042
. 5k 0.069 0.069
. 0k 0.000 -----

You can see from dV that the change in voltage isn't very linear.

Our hearing is more or less logarithmic, so it might be about right.

Further more most volume pots in audio are logarithmic.

 

[DaveC]

Here's the whole thing; DC coupled mixer with a charge pump for a

negative supply:

"mF" means microfarad?

Thanks.

 

[DaveC]

Thanks for the great explanation JF.

news/[email protected]

If that link doesn't work it's over on abse as:

"Pots, loads, and linearity."

JF

The link doesn't work, and my service doesn't carry binaries.

How about putting it here? ::

<http://www.tinypic.com>

Thanks!

 

[DaveC]

Here's the latest:

...
JF

Almost!

I need some absolute attenuation for the sub's output and some relative to
the input signal.

In other words: I need to be able to change the input level to the mixer
(each of the 2 sound card's level will remain most of the time at max) so I
can change volume for each stereo input (from each sound card) and for the
*relative* level of the sub. The way it is now, the sub is outputting full
level (no pots in that circuit) and the L and R channels are variable.

So, the input to the sub mixer circuit needs to come from the input pots'
wipers, not directly from the input. And U3 needs variable gain.

The power amp (that this mixer is supplying the audio to) has a mute pin that
has a slow R-C tc on it, so we can eliminate S1-S3 and assoc. components, I
think.

Thanks!

 

[DaveC]

With R12 cranked down to minimum R the output of U3 goes down to about

600µVPP out with 2VPP out of the sound card(s), and up to about 6VPP
with R12 at 10k so, if you use a 10k pot for R12, there's your center
channel volume control.

Yeah, R12 needs to be a pot. That does it.

Thanks!