Compact desktop line mixer?

[JohnRoberts]

There are modern op amps that drive lower than 600 ohms, of course do the math.

The aux send pots could be somewhat higher impedance without compromising performance, likewise the aux bus send resistors could be higher impedance.

JR

 

[ruffrecords]

The NE5534 is not unity gain stable so you cannot use it as a unity gain non-inverting buffer. Since noise is not really an issue for line level you could just swap the 2 x NE5534 for a single NE5532 which is unity gain stable.

Cheers

Ian

 

[Newmarket]

Isn't 5534 discontinued / obsolete / NRND now ? Or are there still manufacturers turning it out ? Differences (Real or Perceived) between brands notwithstanding.

 

[OneRoomStudio]

Isn't 5534 discontinued / obsolete / NRND now ? Or are there still manufacturers turning it out ? Differences (Real or Perceived) between brands notwithstanding.

Still listed as “active” on TI’s website: https://www.ti.com/product/NE5534 plus, they seem to be in-stock at most major suppliers.

@ruffrecords is right though, in this case, I’m better off with a 5532 to cover both stages and allow for unity gain.

 

[Newmarket]

Makes sense. Although you can make 5534 unity gain stable with a single 22p cap. And it's a tiny bit better. OTOH, depending on application, can parallel up both sections of a 5532 for lower noise. Swings / Roundabouts...

 

[JohnRoberts]

Makes sense. Although you can make 5534 unity gain stable with a single 22p cap. And it's a tiny bit better. OTOH, depending on application, can parallel up both sections of a 5532 for lower noise. Swings / Roundabouts...

I have never tried (heard of) paralleling the inputs of the two 5532 op amps together, to reduce noise. Being on the same die they should be very similar but will they share well?

I have paralleled opamp outputs together for more drive capability but even then I sum the outputs together using low resistances.

JR

 

[ToBSn]

Found this yesterday on YouTube


The hifi guys are a step further
They quadruple 5532 together.
0.8 amps for just one.

 

[ruffrecords]

I think Doug Self covered some of this in his book.

Cheers

Ian

 

[JohnRoberts]

I think Doug Self covered some of this in his book.

Cheers

Ian

That would explain why I never heard of it....

JR

 

[ToBSn]

Doesn't the summing network noise (resistor noise) have much more influence than the opamp itself?

 

[ruffrecords]

Doesn't the summing network noise (resistor noise) have much more influence than the opamp itself?

More influence on what?

Cheers

Ian

 

[Newmarket]

I have never tried (heard of) paralleling the inputs of the two 5532 op amps together, to reduce noise. Being on the same die they should be very similar but will they share well?

I have paralleled opamp outputs together for more drive capability but even then I sum the outputs together using low resistances.

JR

Yes you should have low value shareing resistors to inhibit currents between the outputs. Assuming the voltage noise of two sections is uncorrelated and same signal going to both sectons then statistically you gain basically 3dB SNR.

 

[JohnRoberts]

Doesn't the summing network noise (resistor noise) have much more influence than the opamp itself?

Your question is not concise.

Generally the active devices make the greatest noise contribution while resistor values have a marginal scaling factor (higher resistance generally makes more noise).

There are diminishing returns involved with making resistors smaller, while some modern uber low noise opamps had sturdy low impedance drive capability, perhaps to accommodate some such tweaking.

JR

 

[JohnRoberts]

Yes you should have low value shareing resistors to inhibit currents between the outputs. Assuming the voltage noise of two sections is uncorrelated and same signal going to both sectons then statistically you gain basically 3dB SNR.

I need to soak this in beer tonight. I am still having a little difficulty visualizing this topology configuration wrt feedback network(s).

JR

PS: I think I have seen op amps paralleled together where shorting together internal compensation nodes insured good sharing behavior overall, but I never did that for production either.

 

[Newmarket]

Found this yesterday on YouTube


The hifi guys are a step further
They quadruple 5532 together.
0.8 amps for just one.

Yes you can parallel up for current drive. Wrt noise it's a case of diminishing returns in dB terms as you have to double up for every (not quite) 3dB gain.
And of course you go have to take power dissipation into account. Esp wrt 5532/4 - do maybe better using less power hungry opamps.

 

[Newmarket]

That would explain why I never heard of it....

JR

Works though analogous to paralleling of bjt transistors in mic pre either ic (LM394 IIRC as per SSL) or discrete (as in AMEK 9098). E&OE

 

[Newmarket]

I need to soak this in beer tonight. I am still having a little difficulty visualizing this topology configuration wrt feedback network(s).

JR

PS: I think I have seen op amps paralleled together where shorting together internal compensation nodes insured good sharing behavior overall, but I never did that for production either.

Duplicate the circuits and sharing resistors sort out problems with tolerances etc.
Obvs there's cost / PCB estate / heat dissipation matters to consider.
Might be better off with a lower power consuming opamp since 5532/4 areva bit on the power hungry side.

 

[JohnRoberts]

I haven't consumed enough yet for this one to make sense, but I'm still working on that.
===
I understand the concept of paralleling devices. I read about that back in the 70s (Motchenbacher and Fitchen "low noise design").

JR

 

[OneRoomStudio]

Still making progress. Here's my first swipe at the FX send/return module. As a reminder, there are two mono and one stereo sends, and one mono and two stereo returns. The "2-track return" and "external input" will be on the mix/monitor control board. I realized I needed signal/clip lights on both the send masters AND the returns, so those have been added. I also realised I needed to sum the stereo inputs for the mono solo bus. Hopefully I didn't miss anything major.

Question: I'm guessing my solo bus buffer arrangement might contribute some crosstalk between left and right channels. Will this be a problem?