-

.

All voltages are relative to some reference 0V****... For a mix bus 0V would be the ground reference (i hate calling it ground, because ground means much more than 0V).

When you take a send from a channel strip that sent audio is relative to the channel 0V, at the master it gets summed relative to the master 0V. In an ideal world they are the same 0V, but we must live in the real world so 0V can vary between here and there.

JR

***** differential audio can exist as an active  + audio leg, and active - audio leg, so that voltage is the + minus the - stem.

80hinhiding said:

PS.  The LM394H and replacements are more costly than I realized.. maybe I won't test drive this amp.

Click to expand...

If you're referring to replacement from Analog Devices or THAT Corp, yeah, they are costly.
You can get some MAT-02 at a slashed price on eBay.
It could be a genuine product, or a counterfeit. A working product, or a dud. 50/50

However, the good people here have found the right replacement for LM394. Apparently there's a company in Latvia that still manufactures it according to the specs.

http://www.ericasynths.lv/en/shop/diy/diy-accessories/npn-matched-transistors-in-dip8-case/

metalb00b00 said:

http://www.ericasynths.lv/en/shop/diy/diy-accessories/npn-matched-transistors-in-dip8-case/

Click to expand...

Note the shifted pin out on this.

80hinhiding said:

Looking at the schematic again,

Click to expand...

Post it here, so we can play along at home.

80hinhiding said:

This is the schematic I was referencing from another thread.

I might setup two different mix amps, and switch between them depending on material.. using dual gang controls on them to affect both mix amps.  Sort of A/B compare the mixdown.  One IC based, one discrete component based.

Adam

Click to expand...

it's pointless. you won't hear any difference.

why???

because the SSL4K mix amp was designed to achieve the lowest noise possible when summing many channels, not to transform the sound magically whatsoever. back then, a one IC solution couldn't do that. ultra low noise, ultra low distortion op-amps didn't exist yet.

nowadays is different. almost everything has a one IC solution that is being manufactured and improved from time to time, so engineers can design less complicated circuits to get the kind of performance they want.

suggested reading material: Small Signal Audio Design.

>   Which could also be achieved with a NE5532

'5532 is hiss-optimized for 1K-10K source impedance.

Sixty-Four 15K mix sources is 234 Ohms, well off the best zone for '4432 hiss.

Yes, eight 10K sources (more your size) is spot-on.

There's also issues of gain. A 64-in mixer has 64:1 of loss to overcome just to reach unity. '5532 needs help to get that much gain dead-clean. 8-in, not so hard.

> What's the idea behind the bias even when they're using dual rails?

The input "should" be Zero volts DC. But never is quite zero. A few mV of DC, to a pot, will "scratch" when knob is moved. Upsets users. You usually *want* to block DC many places through a recording chain.

> Is that the only reason they used an inductor

The mix-bus is a l-o-o-n-g wire draped over groundy chassis and maybe shielded. It is a capacitor. The op-amp rules show that this "operation" is unity gain at DC and INFINITE gain for infinite frequency. That can't happen. The opamp gain declines to about-none long before infinite frequency. The conflict leads to instability. The choke puts a well-chosen bend in the curve. A small resistor works in smaller mixers. At 8 inputs you can often ignore it.

80hinhiding said:

I might setup two different mix amps, and switch between them depending on material.. using dual gang controls on them to affect both mix amps.  Sort of A/B compare the mixdown.  One IC based, one discrete component based.

Click to expand...

Keep in mind that you can't use one set of summing resistors into two separate virtual earth amplifiers at the same time.  The simple explanation is that you can't tell the bus current how to split between the two current to voltage (mix) amplifiers - the two amps will just fight it out on their own, and probably fight it out with each other. If you used two sets of summing resistors (i.e. make two summing buses) you could reliably use two summing amps, but at a certain point, you just have to decide what you want and use that.

Also keep in mind that you'll build a mix differently to compensate for the mix bus since you're mixing using your ears, and as you build the mix, you are always adjusting the channels through that mix bus. This is another reason why different mix buses aren't going to sound all that different. It's similar to the observation that a guitar player's sound is more from their hands and less from the guitar.