How to best combine two audio signals to feed into an amplifier? (For a new audio cha

[Eric Mack]

For the audio electronics experts, here's what I am trying to accomplish: I want to combine two or more audio signals into one so I can feed an amplifier to create a new audio channel that will have its own speaker.

Background:

At the recent ATOS 2018 Convention last week, I met Jonas Nordwall, and we got to talking about my Rodgers 340 that I am restoring. We discussed speaker channels and he gave me a number of recommendations, which I will be blogging about.

One of his recommendations was that I move the Diapason and String Celeste audio signals into a new separate speaker channel from the rest. I know where to find these two signals and how to disconnect them from the MAIN channel. What I don't know is the proper way to combine these audio signals into a new single channel to send to a dedicated amplifier.

I suppose I could just tie the two audio signals together and then feed that into the amplifier. I'm sure that would work, but something tells me that is not the way to do it. I wonder if perhaps some kind of mixing circuitry is needed?

Before posting this question for the experts here, I did some research into the Rodgers 340 Schematics to see what I could figure out on my own.

Looking at the Rodgers 340 OUTPUT Circuits Schematic, it that shows where the preamps are routed into each channel. Next to each channel, there is an AUX 1, 2 or 3 for auxiliary input via RCA Jack that mixes into the amplifier. This is basically what I want to do, except It will have a few signals.

Referring again to the output circuits schematic, they appear to use a circuit like this to do the mixing: (See screen shot)



Should I just duplicate that circuit for each signal I want to mix in? That is a passive circuit. Will that be sufficient, or do I need to have an active mixer of some sort? Or, is there another simple way to properly accomplish the signal mixing?

Thank you

Eric

[Admin]

What I don't know is the proper way to combine these audio signals into a new single channel to send to a dedicated amplifier.

I suppose I could just tie the two audio signals together and then feed that into the amplifier. I'm sure that would work, but something tells me that is not the way to do it.

No, no, no. That is not the correct way to do it and can potentially damage circuity. There are special Y adaptors available for this purpose that have built in passive mixing circuitry (note these are NOT the common splitters you see everywhere), or you could make your own passive network by inserting suitable resistors in series with the signals. The optimum actual value of the resistors will depend on the output circuitry of the signals and the circuitry of the input, but using resistors between 4.7 k and 10 k would be a good starting point. Like all passive mixers, there will be a reduction in signal level that an active mixer would not have.

[Eric Mack]

Thank you for the prompt response. Would you recommend that I duplicate the circuit from the output mixer board? That used a 10K resister into a 20K pot (see attached screen shot above)

[Admin]

Thank you for the prompt response. Would you recommend that I duplicate the circuit from the output mixer board? That used a 10K resister into a 20K pot (see attached screen shot above)

The diagram is incomplete as it doesn't show the output circuitry of the brass channel. The 10k resistor attached to the pot is the summing resistor for the aux input, so I'm going to assume the brass channel also has 10k resistor somewhere in the signal path before that point. So, you don't need the pot at all. Just try 10k resistors in series with the signals you're combining,

[Eric Mack]

The diagram is incomplete as it doesn't show the output circuitry of the brass channel. The 10k resistor attached to the pot is the summing resistor for the aux input, so I'm going to assume the brass channel also has 10k resistor somewhere in the signal path before that point. So, you don't need the pot at all. Just try 10k resistors in series with the signals you're combining,

Admin, I took some time to reread both of your comments and I think I understand what you are looking for. Let's ignore the brass channel as I was only using that for illustration.

My real objective is to isolate the Diapason and String Celestes signals (before they go into their respective preamps) and combine these into one signal to be fed into a new amplifier/speaker channel.

Based on your feedback, I located the schematics for the Diapason and String Celeste circuits (see attached PDF).
Rodgers 340 Diapason and String Celeste Output Schematics.pdf

As I read it, the outputs of the Diapason and String Celeste circuits each already have a 10k resistor in series with the output. So if I understand your comments correctly, these circuits already have the summing resistor in place within each of their respective circuits. If that is the case, I could then simply tie these to together without any additional resistors.

Do I understand correctly, or am I still missing something?

Thank you for your help.

Eric

[Admin]

As I read it, the outputs of the Diapason and String Celeste circuits each already have a 10k resistor in serious with the output. So if I understand your comments correctly, these circuits already have the summing resistor in place within each of their respective circuits. If that is the case, I could then simply tie these to together without any additional resistors.

Yes, since the outputs have the resistors, you can just tie them together.

[Eric Mack]

Thank you! I tried to research this before I posted the question. Obviously, there were some concepts I did not understand.

As a result of your feedback and continued research on my part, I think I now understand and have learned something. It appears that each of the Rodgers analog instrument circuits already have a 10K (or sometimes a 4.7k) resistor at the output before the signal is connected to the preamp. So that would mean I can isolate these signals or reroute them as needed as without concern for crossover. Thank you for helping me understand.

[Lucien Nunes]

There is a good primer on the subject of mixing audio signals here:
http://sound.whsites.net/articles/audio-mixing.htm

If your sources do not drive any other destinations, e.g. Diapason only feeds this amp channel and nothing else, and the level pot is only adjusted during setting-up, isolation is not critical and passive mixing is adequate. The resistors simply prevent each source acting as a heavy load on the other source (i.e. trying to fight over what the voltage should be on the mixing bus at any instant). The ideal value of resistor for minimum loss of signal level depends on the output impedance of the sources and the input impedance of the amplifier, but a few kilohms to tens of kilohms are typical for solid-state circuitry. Active mixing with a virtual-ground summing amp provides mutual isolation between the sources, but I don't think you need that here.

[Eric Mack]

There is a good primer on the subject of mixing audio signals here:
http://sound.whsites.net/articles/audio-mixing.htm
Active mixing with a virtual-ground summing amp provides mutual isolation between the sources, but I don't think you need that here.

Thank you for the link and explanation!

Eric

[toodles]

If you separate the diapason and string celeste into a separate channel, you'll need to figure out how to put them back under expression control. The 340 inserts the expression control after everything is mixed into the main channel.

[Eric Mack]

If you separate the diapason and string celeste into a separate channel, you'll need to figure out how to put them back under expression control. The 340 inserts the expression control after everything is mixed into the main channel.

Toodles, you are correct. I have my proof of concept working already for the String Celeste. I am repurposing my unused Leslie interface that was previously used on the Flute channel on the 340. This interface has a motor control relay -- which I removed. It also has expression control.

Here's what I did:
1. I tapped off the String Celeste as it leaves the preamp but before it enters the main mixer. (I actually installed a slide switch so I can reverse what I am doing, if needed)
2. I then routed the String Celeste signal into my unused Leslie interface. This way, it is under expression from the Main swell shoe.
3. I then take the output of the Leslie interface and send it to a dedicated amplifier channel and to its own speaker

I have all of this working with the String Celeste now.

What I want to do next, and thus the purpose of this thread, is to pull out the diapason from the Main channel and add it to the channel that has the String Celeste -- per recommendation from Jonas Nordwall. I hope to accomplish this second step tonight. Once I have all of that working, I will try to tidy things up and blog about it and share what I did and why.

What I needed to resolve, and have now done, was the idea of how to combine the signals into one. Since the Rogers designs have each voice exiting its preamp via a 10k resister, I can combine these and feed them into the expression circuit.

In short, what I will have accomplished is to make the 340 a 5 channel rather than a 4 channel organ and I will have mapped the signals based on starting recommendations from Jonas. I plan to share those here, if there is interested. I just need to document this first.

Thank you for responding!

[toodles]

The idea of separating these ideas is a good one, so if you have the expression licked, you should be good to go!

[Eric Mack]

The idea of separating these ideas is a good one, so if you have the expression licked, you should be good to go!

Update: It worked!

I now have managed to isolate the string Celeste and the Diapason into the unused leslie expression circuit which feeds a dedicated amplifier and Rw3 tone cabinet.

Sounds great. I will now need to clean up my mess of test wires and document, but it works.

Thank you to everyone who offered feedback.

[Eric Mack]

Now that I have the speaker channel splitting working, I want to improve the reverb. I have a similar question on splitting outputs. To keep the forum topics clean I have begun a new thread here.