Makeup Gain Stage for Passive Summing Mixer Using ETI 430 Line Amplifier

[cpsmusic]

Hi Folks,

I'm planning on building a basic summing mixer. Originally I was going to make it completely passive, however I think that having a final make-up gain stage would be a better option.

In my search for a suitable makeup gain stage I came across the ETI 430 Line Amplifier (see attached).

Would this be a suitable amp for the mixer?

BTW, the project is from a book called "30 Audio Projects" which I'm going to scan and upload to the technical documents section.

Cheers,

Chris

 

[JohnRoberts]

Hi Folks,

I'm planning on building a basic summing mixer. Originally I was going to make it completely passive, however I think that having a final make-up gain stage would be a better option.

In my search for a suitable makeup gain stage I came across the ETI 430 Line Amplifier (see attached).

Would this be a suitable amp for the mixer?

BTW, the project is from a book called "30 Audio Projects" which I'm going to scan and upload to the technical documents section.

Cheers,

Chris

It looks a little dated.

General practice is to use a low noise mic preamp, and scale the passive summing resistors to a low Z source impedance  (hundreds of ohms).

You can ignore the  phantom power blocking capacitors from the typical mic pre front end.

For a modest lesser number of channels being summed a decent low noise op amp could cleanly deliver 20 dB or so.

JR

 

[user 37518]

So are you planning on doing the summing passively and then add that amplifier? The best way to sum is current summing like and inverting amplifier, not voltage summing like when doing it passively which actually is voltage averaging rather than summing.

 

[abbey road d enfer]

So are you planning on doing the summing passively and then add that amplifier? The best way to sum is current summing like and inverting amplifier, not voltage summing like when doing it passively which actually is voltage averaging rather than summing.

Pandora's box is open! 

 

[user 37518]

Pandora's box is open! 

Well, yeah, if you are going to add an amplifier after the passive summing, why not let the amplifier do the summing? I don't see the point.

Yeah I guess I just opened a can of worms

 

[JohnRoberts]

For objectively "best" (linearity, accuracy, etc) combine signal inside the digital domain.  8)

Perhaps the OP can share his design goals.

JR

 

[user 37518]

For objectively "best" (linearity, accuracy, etc) combine signal inside the digital domain.  8)

Perhaps the OP can share his design goals.

JR

Agreed, if there is one thing computers can do is add.

 

[ruffrecords]

Perhaps the OP can share his design goals.

JR

That would help keep the lid on Ms. P's box.

Cheers

Ian

 

[abbey road d enfer]

Well, yeah, if you are going to add an amplifier after the passive summing, why not let the amplifier do the summing? I don't see the point.

Actually, it can be debated the resistors are doing the summing. 
I believe Thevenin and Norton would have lengthy discussions about it. 

 

[ruffrecords]

Well, yeah, if you are going to add an amplifier after the passive summing, why not let the amplifier do the summing? I don't see the point.

Depending on the application there may be several points.

1. Using an external mic pre allows choice in the tone of the final mix if that is your thing.
2. A passive mix box is completely self contained and needs no power supply
3. If you  do decide to include the amplifier in the box. a passive mixer followed by an amplifier has identical noise performance to a virtual earth one but without the latter's potential instability problems.

Cheers

Ian

 

[JohnRoberts]

Actually, it can be debated the resistors are doing the summing. 
I believe Thevenin and Norton would have lengthy discussions about it.

That reminds me... 

Back in the 1980s when I applied for a patent on my current source summing, The wet behind the ears patent examiner cited a textbook description of a simple op amp summing circuit, that unfortunately said it was summing "currents".  :

I didn't have enough cash laying around to educate the examiner about the difference between summing current sources and voltages fed through resistors into a virtual earth, at the hourly rate my patent lawyer was getting. First I'd have to educate my lawyer and that would be like playing the children's game of telephone, getting him to put it into understandable legalese to school the examiner..  I briefly considered traveling down to VA to meet with the examiner in person, I was sure I could explain things in a matter of minutes face to face, but that was not in my budget either. 

We need to realize that simplified analysis techniques are not always literally representative of what the electrons are doing, but close enough to design with.

JR

 

[efinque]

I think summing resistors are mainly used for reducing crosstalk between channels, and to some extent matching the mix bus level to the output (ie. your output stage expects a +3dB signal max and you sum two or more 0dB signal sources, say 0,7V and +3dB being 1V which is somewhere around line level.. with correct summing resistors the mix bus never exceeds 1V even if you max out the volume.. the downside is that a single channel is limited to that 0dB)

You don't necessarily *need* them but in my case it helped a bit as I built a dedicated summing bus which made the rest of the circuit a bit easier to troubleshoot (think of it as a resistor network/bridge which are sold in DIL packages, the one I built was custom with switchable resistors though and I included a ground bus/rail too, although it was significantly larger than a standard-sized, say 24pin DIP)

 

[user 37518]

Depending on the application there may be several points.

1. Using an external mic pre allows choice in the tone of the final mix if that is your thing.
2. A passive mix box is completely self contained and needs no power supply
3. If you  do decide to include the amplifier in the box. a passive mixer followed by an amplifier has identical noise performance to a virtual earth one but without the latter's potential instability problems.

Cheers

Ian

Correct me if I'm wrong, but with passive summing your gain will vary according to the sources you have connected to the inputs, unless you terminate the unused inputs to ground, that doesn't happen with active summing. To me that is a big no no.

 

[JohnRoberts]

Passive combining became popular for reasons other than objective performance (IMO).

Ian hit the two high points, 

1) using an external mic pre for makeup gain provides the coloration of that particular preamp, for better or worse but people don't generally use bad sounding mic preamps in their passive mixers.  :

2) the passive mix box is passive (no power supply), and not scary for beginner DIY projects.

I can't embrace Ian's third caution about stability of active combining (especially if using modern op amps) but he is completely correct that there is absolutely no noise benefit from passive + make up gain, vs the noise gain of active combining.
==
Indeed with passive combining the attenuation is a simple arithmetic function of the number of stems being summed and impedances all around. In my judgement this (gain/attenuation) changing is not a serious problem since using a stand alone mic preamp for make up gain will have its own gain trim.

The larger issue is if you desire panning or other features of more conventional modern mixers.

JR

 

[user 37518]

==
Indeed with passive combining the attenuation is a simple arithmetic function of the number of stems being summed and impedances all around. In my judgement this (gain/attenuation) changing is not a serious problem since using a stand alone mic preamp for make up gain will have its own gain trim.

The larger issue is if you desire panning or other features of more conventional modern mixers.

JR

IIRC Douglas Self in his book mentions that you can leave un-used inputs floating and get a noise advantage. You can leave the inputs open in a passive mixer, but again, it will change the level depending on what is connected, it doesn't seem very practical to me to keep adjusting the level every time you plug or un-plug anything.

 

[ruffrecords]

I wrote a long post about the possible change in attenuation with unterminated inputs in a passive mixer but it got lost in the group breakdown.

To summarize, in cases where the number of inputs may vary it is common to slug the bus to minimise these variations. In many cases it is necessary to do this anyway to ensure the bus impedance is low enough for the mic pre to handle. Hence it is common to slug a typical passive bus with something around 200 ohms. Since a DB25 connector is commonly used lets assume inputs get removed in blocks of 8. Changing from 12 driven inputs to 16 driven and 8 open results in about 0.6dB increase in level. Not a no no in my book.

I don't know about the instability in physically small mixers. I take my lead from Steve Dove's excellent article on c(large) onsole design. He says, and I quote:

"Bus capacitances, from all the cabling and PCB tracks between the channel amplifiers and the virtual earth amplifier, appears across the input of the virtual earth amplifier. This has the effect of eroding the phase margin and can lead to instability and even oscillation. This capacitance reacts against the feedback impedance to to increase the closed loop gain at high frequencies. Even a few pF is enough to tilt up the closed loop response well within the open loop parameters, threatening instability. In a real mixer with cables from many channels, hundreds of pF may be present. This makes ensuring the required phase and response characteristics very difficult. Sometimes a small series limiting resistor can be added to to define just how much this unwanted gain can rise, but this is at the expense of the ‘virtual earth’ now being determined by this resistor (which rather defeats the object)."

This is unlikely to occur I think in a compact VE mixer but as even a few pF can have an effect it is clear that layout and lead dress is important in a DIY version.

Edit: As I said in the lost post, I have noth.ing against either method. Both are engineering solutions and have their respective pros and cons

Cheers

Ian

 

[JohnRoberts]

This is following the trajectory of most WWW discussions, have we scared the OP yet?

JR

PS: Speaking of Steve Dove's console design series in Studio Sound, I got a mention in the credits at the end of the last installment because I pointed out an error in one of the earlier installments.  Yes, Steve is the real deal console designer, I ran into him at a trade show after my console design article ran in 1980, not only was he one of the few in the industry who understood my current source summing concept but he told me he already copied it in a small broadcast console design. In high RF environments a virtual earth sum amp with high noise gain can experience rectification. The reduced noise gain from using current source summing reduces that sensitivity to RF also, an unexpected benefit besides the regular benefits from reduced noise gain (lower noise, lower phase shift, lower distortion). 

 

[ruffrecords]

This is following the trajectory of most WWW discussions, have we scared the OP yet?

JR

Possibly.

To answer the OP directly, the amp he has chosen will work but it is certainly not ideal. How close it comes to meeting his needs depends on those needs.

Itis a very simple design, but its noise and distortion performance will not be the best. Also it's output drive capability is very limited, but that may not be a problem if all he wants to do id drive a 10K load.

If he wants a discrete design then there are better three or four transistor design around.

Almost any half decent op amp will do the job.

Basically there are lots of potential answers but which one is best for the OP depends on what he wants.

Cheers

Ian

 

[JohnRoberts]

IIRC Douglas Self in his book mentions that you can leave un-used inputs floating and get a noise advantage.

What topology...?  In a passive mixer leaving inputs floating will results in slightly less attenuation (more like less noise disadvantage, than an advantage).  In an active combining mixer, leaving feeds floating can compromise the differential noise floor cancellation. 

You can leave the inputs open in a passive mixer, but again, it will change the level depending on what is connected, it doesn't seem very practical to me to keep adjusting the level every time you plug or un-plug anything.

I would advise against you using a passive mixer, for other peeps their mileage may  vary.

JR

PS: I never used one... but don't be like me....

 

[user 37518]

What topology...?  In a passive mixer leaving inputs floating will results in slightly less attenuation (more like less noise disadvantage, than an advantage).  In an active combining mixer, leaving feeds floating can compromise the differential noise floor cancellation. 
I would advise against you using a passive mixer, for other peeps their mileage may  vary.

JR

PS: I never used one... but don't be like me....

The floating un-used inputs was meant for active summing, you can do the same thing with passive summing but the level will change, I still think that after all that Ian has commented, its more about "patching up " the circuit so it works as good as possible  and the level differences are less noticeable, whilst doing active summing is pretty straight forward. All in all I see more disadvantages with passive summing than active summing, but to each his own.