Electrodyne ACN Summing Mixer

and the output amp.

Try that again....

OK, the reason the Spere has a resistor in series with the pot wiper is that the op amp is configured as an inverting amplifier but you schematic has a non-inverting configuration. This is possibly because the previous summer stage inverts the signal so an inverting output stage restores the phase.

Cheers

Ian

I think this was the confusion I had with the positive feed back. I didn't refer to the actual pinout of the 62.

I've tried researching this but to no real avail. Would there be  benefit of using an inverting opamp vs a non-inverting opamp in this situation?  (Other than polarity flip)

I am going to proto it as inverting. I'm just curious if a non-inverting could be better.

Thanks!

I've attached half of the updated schematic. I'm hoping this is really close.

Thanks again for all the help, Ian and Lance!

Looks goo to me. The only thing you might change is to add a coupling between the first op amp output and the 10k pot. There are two reasons for this. First it means any dc offset at the output of the first op amp is not fed into the second op amp. Secondly, de flowing through a pot wiper is a recipe for scratchy noises.

Cheers

Ian

ruffrecords said:

Looks goo to me. The only thing you might change is to add a coupling between the first op amp output and the 10k pot. There are two reasons for this. First it means any dc offset at the output of the first op amp is not fed into the second op amp. Secondly, de flowing through a pot wiper is a recipe for scratchy noises.

Cheers

Ian

Click to expand...

What would be a good cap value for this? 1uf Film cap?

RSRecords said:

 

What would be a good cap value for this? 1uf Film cap?

Click to expand...

1uF and 10K has a -3dB point at 16Hz. It really needs to be ten times lower so I would recomend at least a 10uF electrolytic.

Cheers

Ian

Started ordering parts today.

How would using a 25k pot instead of a 10k change things? I have a healthy supply of nice 25k pots but no decent 10k unfortunately.

  Any recommendations for decent 10K  Audio pots? Seems like I can only find super cheapies or $10 stuff.  I'd prefer plain 1/4 shaft for the knobs I've chosen but that can change as well.

Thanks

RSRecords said:

Started ordering parts today.

How would using a 25k pot instead of a 10k change things? I have a healthy supply of nice 25k pots but no decent 10k unfortunately.

Click to expand...

The value is not critical provided you use the non-inverting op amp topology so the pot is not heavily loaded.

  Any recommendations for decent 10K  Audio pots? Seems like I can only find super cheapies or $10 stuff.  I'd prefer plain 1/4 shaft for the knobs I've chosen but that can change as well.

Thanks

Click to expand...

Alpha (Taiwan) do a very nice range of 9mm pots with 1/4 inch metal shafts.

https://www.thonk.co.uk/shop/alpha-9mm-pots-right-angle/

also available from Small Bear in the US.

Cheers

Ian

ruffrecords said:

The value is not critical provided you use the non-inverting op amp topology so the pot is not heavily loaded.Alpha (Taiwan) do a very nice range of 9mm pots with 1/4 inch metal shafts.

https://www.thonk.co.uk/shop/alpha-9mm-pots-right-angle/

also available from Small Bear in the US.

Cheers

Ian

Click to expand...

Thanks, I will check it out.

RSRecords said:

Thanks, I will check it out.

Click to expand...

They also do a rather nice vertical mounting version. This is great for PCBs because it has extra soldered tags for mechanical strength which means you can use the pots to mount the PCB to  panel.

Cheers

Ian

hitchhiker said:

The gain set on my SPA-62 output amps are 56k with a 22pf cap.  Not unity but is unity gain what you need?

Click to expand...

Just a quick note here...  All Sphere Eclipse Fader Amps and BST Amps are set up for 15db of gain.

GARY

gar381 said:

Just a quick note here...  All Sphere Eclipse Fader Amps and BST Amps are set up for 15db of gain.

GARY

Click to expand...

That's good to know. I plan on starting with the 56k in the feedback for the plus 15dB to see where that gets me.

Here's a look at what I'm thinking for the front panel.  Meters are some cool NOS Simpson Edgewise style.

Also, I'm looking into adding a mono/stereo switch. I'm not sure yet what that entails so if you have any input towards that I'd be much obliged. 

Thanks!

And here's the full schematic as of now. I'm not sure where to place the mono switch.  Before or after the output amp? Any thoughts? suggestions?

The build is going to be pretty much p2p so I should be too difficult to reroute certain things for experimentation.

edit: I removed the schematic because I realized the switch wouldn't work. I was just tying the ouputs together...not enough coffee..

Front Panel is ordered!

Now I'm dealing with the notion of unbalancing the inputs.
I've been reading about ground schemes from the Rane website and Ian's Mixer Grounding 101 .
Currently, I have the screen tied to the shield and bussed together with 16 ga copper bar. I realize that ideally the power, audio, and chassis will only connect at one point in a system.  Being an unbalanced input I know this cannot be achieved. Is tying the cold and the shield together at the input of the mixer a bad idea?

Should I try to float the power supply pcb (five fish)? It's ground is connected to the chassis by its mounting screws (which I am also using as the star ground.
Would it be a better idea to use the mains safety input as the star ground, rather than the mounting screw of the pcb? DMM reads zero ohms between the two.

Does it even matter for an unbalanced system?

Initial test haven't shown any major noise or hum issues but the cable lengths have been short from the battery powered tone generator.

Wired everything up and signal is passing. Sounds good actually. But there is a good bit of hum and buzz.  I haven't been able to track it down yet.  The hum got significantly better by rotating the toroidal power transformer. The buzz is still there.

Anybody have a guess as to what's causing the buzz and noise? I'm still not sure  the best way to handle the unblanced inputs. Any advice is very welcome.

So, I replaced the grounding buss bar with some heavy gauge copper I took out of a roll of romex. That reduced the noise and buzzing a bit. I also gathered some information using Rightmark audio analyzer...Overall pretty good but I need to lower the input level to keep the ACNs from clipping and I have pretty underwhelming noise specs. Maybe someone can help me make sense of them?

Im am thinking of reducing this to an 8 ch. Mixer and adding input transformers. This could accomplish two things, 1st, balancing of the inputs which would in theory help reduce noise and hum.  2nd, reducing the level into the acns to keep them from clipping. I was thinking 10k:10k input but maybe 10k:600 would be better?

So, a little update on this piece. I added some balanced inputs with GAR381's spa6220 based cards.. Very cool and very effective. To make room for the pcb's, I remoted the psu which further reduced noise. Its very quiet now in regards to noise floor and only has the slightest bit of 60 cycle with everything wide open. In application it hasn't been an issue yet.

I did want to toy with wiring the ouput transformer 2:1. The output gain is a little less than ideal and a few extra dB's might go a long way.

I know this is achieved by wiring the primary in parallel [1--3, 2 -- 4 on the JT-123] and the secondary in series.

Is there anything I should look out for when doing this? How will wiring the OT 2:1 affect the output amp? What might have to be done to compensate?

Thanks!