Simple active summing mixer.

Sorry for yet another summing mixer thread :wink:
Here a schematic of what I want to build. Actually it's going to be more complicated but for now this will do. Input is a THAT 1243 line receiver with a mute switch.
Summing section is derived from this schematic from Fred Forssell only without the DC servo circuit, which I might use but left out for now...
Any comments are welcome.....


I also have a few things that I'm not sure of.
1) will I need a cap after the THAT 1243 to get rid of the DC offset? Maybe meassure this before building right?
2) The 10K volume pot is in parallel to the 49,9K resistor to ground. I know this resistor is there to discharge the caps but is it still needed if I have that 10K pot there?

Thanks!

Just making a few quick comments in passing.

The mute switch disconnects the 10k but not the 510 ohm. This imbalances the bus differential. Either back ground the 10k or disconnect both resistors.

If you are blocking DC at the sum amp output the blocking cap in the input may be redundant unless you are expecting large DC offsets from DOA that will diminish headroom.

I don't have experience or opinions about THAT chips. Sounds like a question for Wayne.

JR

1. Yes you might. It's going to depend wholly on the circuit. Even when designed for zero crossing, the layout and wiring can influence some DC offset. It's always good form to plan for the cap in the layout, you can always place a zero-ohm jumper if you determine that you won't need it.

2. I don't see what you mean by discharging the caps. These caps would be biased on the *positive* side if anything. The 10k pot is shown with 10k to ground. This is less than 49k9, and would "discharge" the caps *better*. I don't really understand what the 49k9 is doing. It seems almost like it would change the law of the pot..

3. You should really think about adding some kind of high value load resistor before your mute switch. I don't think it's good design to leave an output from an active device unterminated, especially when switching in/out another load.

Thanks all!! I'll reconsider the mute switch. I might just ditch it since it's not really needed.

[quote author="Svart"]
2. I don't see what you mean by discharging the caps. These caps would be biased on the *positive* side if anything. The 10k pot is shown with 10k to ground. This is less than 49k9, and would "discharge" the caps *better*. I don't really understand what the 49k9 is doing. It seems almost like it would change the law of the pot..
.[/quote]

That 49,9K resistor is in the schematic from Fred Forssell
> http://www.forsselltech.com/downloads/schematics/Summing%20Buss2.pdf

I read somewhere that it's there to discharge the DC blocking cap.

But yes, now you mention it. I think it will change the pot law :?

As shown in the Forssell schematic, that is a load resistor. You should be able to omit that if you use the voltage dividing 10k pot. Come to think of it, I should have written RATIO instead of LAW (I don't know why I wrote that, it has not been a good day..) it would change the division ratio not the law. If the resistor was on the wiper side it would change the law.

:roll:

Ok, I've made a few changes to the mute switch and I ditched the 49,9K resistor. I also added a 100K resistor (R1) to ground just after the line receiver so it's not floating when the mute switch is engaged.

R1 is entirely pointless. The ICs output is very low impedance and nothings floats.

Samuel

[quote author="Samuel Groner"]R1 is entirely pointless. The ICs output is very low impedance and nothings floats.

Samuel[/quote]

Thanks Samuel, I'll get rid of it then... :thumb:

Also, by floating I ment that the line receiver was not terminated when the mute switch was engaged, as Svart pointed out earlier

[quote author="Svart"]
3. You should really think about adding some kind of high value load resistor before your mute switch. I don't think it's good design to leave an output from an active device unterminated, especially when switching in/out another load.[/quote]

Maybe audio doesn't need it. When switching RF, you can get an offset when floating unterminated and having a slight mismatch on the input differential signal, like when the switch is not connected. Once it contacts you might get an audible *click* from offset change. It's just a thought.

I'll leave a space on the pcb for R1 just in case...

Also, the load presented to the line receiver (with R1 in place) is 1/R1+10K right? But does the load presented to the inverting input from the summing opamp change when R1 is in place? Is it R1 + 10K ??

A minor and esoteric point, the blocking capacitor in series with - bus imbalances the differential at very low frequency. Probably not a big issue if pole is set low enough and hum inside the box contains mostly higher harmonics.

Losing that cap would reduce offset voltage due to input bias and error current, but increase DC gain for offset voltage. Adding a cap in series with + bus and 10k to ground at + input gives first order correction of DC effects but cap needs to be sized for best differential balance.

Using a DOA you may have alternatives to perhaps servo or trim inside the opamp to tweak input stage DC balance.

JR

[quote author="JohnRoberts"]

Using a DOA you may have alternatives to perhaps servo or trim inside the opamp to tweak input stage DC balance.

JR[/quote]

Yes, there are alternatives. I might use the servo circuit which is used in the original circuit from which my master bus is an shameless copy
You can see the schematic of the original circuit here

http://www.forsselltech.com/downloads/schematics/Summing%20Buss2.pdf

I resisted going to any link that spells bus wrong. :roll:

I like the ferrite bead better than big ass caps, for a large physical console. In small closed chassis, RF or hum for that matter should not be huge issue.

The 49.9K of his servo also imbalances the bus but differential balance is academic when driven from pan pots whose source impedance will vary with pan settings. Your approach at least can be in balance.

My point about the DOA is that it is a discrete circuit, not an integrated circuit that can't be hacked into. There are probably sundry ways to trim out input DC voltage or current errors.

YMMV

JR

[quote author="JohnRoberts"]I resisted going to any link that spells bus wrong. :roll:

I like the ferrite bead better than big ass caps, for a large physical console. In small closed chassis, RF or hum for that matter should not be huge issue.
[/quote]

Yeah, it's bus alright :?

This is not goint to be a large console.It will all fit into a 4 unit 19 inch case with a psu in a separate case.
As for the different approaches..I'll leave space for both caps, ferrites and the servo circuit, so I can experiment with them.

An opportunity available with DOA is that you can trim DC offset, or even apply servo correction, inside the opamp loop rather than outside, so your differential values don't get corrupted by imbalances from spare parts.

JR

Ok, I made some changes in the summing stage. I put the master fader on a different location and added an insert point. Everything before the 990 DOA is the same as in previous pics.
Any comments on this approach?
Also, can someone explain the function of R1 and R2? (yeah i know, I put them there but I'm not really sure if thei're needed)
Then there are R3 to R6 which I'm not sure about. I saw this in another schematic but I don't know if thei're needed... :?

R2-R5 are not needed, they just increases noise and distortion. R1 is the source resistor for the inverting configuration (i.e. part of the feedback network for the opamp); the 49k9 (as well as R6 and the other 100k resistor at the switch) is not needed there as is the 49.9 at the output of the DOA.

I think you may want to check out some reading on opamp bias. There is a small current flowing from/into the opamp inputs, which you need to provide a path for. I think that the 49k9 and R6 are meant to provide this, but note that in both cases there is another DC path (the 10k feedback resistor for the first case and the OPA604 output for the second) which allready provides bias.

And please provide designators for all parts in your schematic; written guidance is otherwise horribly cumbersome and error-prone...

Samuel

The 18pF across the 10k in the feedback loop of the inverting buffer OPA604 looks a little small (-3dB lpf at ~825kHz). 68pF or 82pF might be more effective.
The hard to get 91pF (-3dB lpf at ~175kHz) in the feedback loop of the DOA990 might be sized down to 68pF or 82pF (for cutoff at 234kHz or 194kHz) and should keep the amp stable.
A That1246 might fit better than a That1243 line receiver when switching your insert loop.
YMMV.

Thanks for the comments:thumb: :thumb:

[quote author="Samuel Groner"]...... and the OPA604 output for the second) l[/quote]

Sorry, I don't understand this part of your answer. Can you explain?

[quote author="Harpo"]
A That1246 might fit better than a That1243 line receiver when switching your insert loop.
YMMV.[/quote]

Why is that?

[quote author="radiance"]Why is that?[/quote]
There is no level adjust within your insert loop.
As drawn, a That1243 drops level by -3dB, but a 1246 by -6dB.
As the 1646 has +6dB gain, your insert loop will be at unity using the 1246 instead of the 1243.