Summin mixer design

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Deepdark said:
Another thing, is it possible, if my input wire from the secondary input transformer, or even the primary output transformer cables, are too close from the DC power cable connecting each rails,  to pick some sort of oscillation? Or the proximity of the dc cables make them oscillate??

Probably not, but the connection from the input transformer secondary to the op amp should be nice and short and keep the wires away from the output pin and negative feedback components.. I am still puzzled why it only oscillates with the input transformer connected.

Cheers

Ian
 
ruffrecords said:
Deepdark said:
Another thing, is it possible, if my input wire from the secondary input transformer, or even the primary output transformer cables, are too close from the DC power cable connecting each rails,  to pick some sort of oscillation? Or the proximity of the dc cables make them oscillate??

Probably not, but the connection from the input transformer secondary to the op amp should be nice and short and keep the wires away from the output pin and negative feedback components.. I am still puzzled why it only oscillates with the input transformer connected.

Cheers

Ian

Maybe a mismatch between the my overall summing impedance and the transformer impedance??? I don't tried it yet with the input transformer disconnected, honnestly, Not since I changed the input transformer. First, when I tried the new input transformer, I had still the bass roll off. I then disconnected the summing card and inject a white noise straight from the output of my apogee into the input transformer. and it worked, I had the full bass response. I then reconnected the summing card and just removed the 100r shunt resistor I had put in // to the 220r initially there (to drop the overall impedance). It had the effect of getting back a little bit of teh bass response. So, I suspected the trouble was there. I decided to test it without any shunt resistor, so full 10K before the input transformer. White noise shown a full bass response but once testing with a track, it was unusable, too much gain maybe, I then played around the summing cards, put an 600r shunt resistor, Put the 150K resistor across the transformer secondary (which I forgot to put first), drop the voltage gain resistor to ground from 470r to about 600r IIRC,  made continuity test on teh summing boards to be sure there was no short, and each board depict the exact same value between hot and cold (first there was a trouble there, one shown higher value than the other) I play around and finally get them to be the same value. So I tried it back, and it was really better. Still, at full gain, there was first this king of oscillation/distortion around 10K, I shut the unit off, then some time later (maybe 30 minutes) turned it on and try again and it looked to work (exept for the crackling/scratching noise, maybe due to some static damage into one of the opamp). 
 
There is one possibility we have not looked at. The bias current for the first stage flows through the input transformer secondary winding. Normally, modern op amps have an extremely low bias current but perhaps your vintage op amps have a higher bias current. If this is the case then this dc current though the transformer could explain the loss of bass and distortion and it would also explain why there is no bass loss when the Apogee drives the +ve input directly. It would also explain why changing the input transformer did not improve the bass. I cannot find a schematic for the original Melcor so we may never know, but we can test the idea.

I suggest you go back to the original 600:2K4 transformer. First fit a 10K resistor directly from the +ve op amp input to 0V. As this is the same value as the feedback resistor is should equalise the bias currents. Then connect the transformer secondary to the +ve input via a 2.2uF plastic film capacitor. It must be plastic not electrolytic because we want very low leakage. Set the proper slug resistor on the bus and test for bass response and distortion. If you don't have 2.2uF try 1uF. This will have a -3dB point at 16Hz so the extreme bass will not be brilliant but it should be better than before.

Cheers

Ian
 
allright, I made some progress. I put the 4,7r before the coupling cap, and I didn't noticed any more oscillation. Next, I replaced the opamp with some new GAR2520 and the scratching noise disappeared. Soe now I have a clean signal. But, it is still shy on bass. I hadn't tried yet the tricks you mentioned. I tried with an 100r slug resistor, I will drop it to 47r, so my summing impedance will driop to about 45ohm. with the input at 150ohm, maybe it will do the tricks
 
Ok, just tried the 47ohm slug resistor but didn't change anything. It's better than it was at the beginning of it all, though. Since we are talking about dc current flowing through the input transformer, I guess it's a measurable value, no? Since we already have an 150K resistor between +ve and ground, if we put a capacitor between +-ve and secondary transformer, then it will make an kind of high pass filter, right? we would have to calculate the corner frequency with our 150K resistor, if we want to test the idea with the Jensen transformer? So fcutoff would be = 1/2*pi*R*C,

If we want a frequency cut off of around, let's say 15hz, then our C would be about, what, 700pf??? Doest my maths look right?
 
Deepdark said:
If we want a frequency cut off of around, let's say 15hz, then our C would be about, what, 700pf??? Does my maths look right?
I get 70nF so use 100nF which gives a 10Hz -3dB point.

You should not need to slug the the bus with as low a value as 47 ohms. There must be somthing else going on somewhere.

At least you got rid of the oscillation

Cheers

Ian

Cheers

Ian
 
ruffrecords said:
Deepdark said:
If we want a frequency cut off of around, let's say 15hz, then our C would be about, what, 700pf??? Does my maths look right?
I get 70nF so use 100nF which gives a 10Hz -3dB point.

You should not need to slug the the bus with as low a value as 47 ohms. There must be somthing else going on somewhere.

At least you got rid of the oscillation

Cheers

Ian

Cheers

Ian

Allright. Will get back to 220ohm slug and will try 100nf cap, i have a bunch of those ceramic 0,1uf caps.  If it dont work, i will try again to disconnect the summing board and inject a white noise straight into the input transformer primary. Maybe there is some phase cancelation?

In exemple, if I inject a white noise straight into the input transformer, without passing by the summing board, my apogee output is 100ohm. If I got full bass response, then it would means the trouble is in teh summing cards? What can go wrong here?
 
Finding bugs like this needs to be a process of elimination. If injection of white noise into the transformer works then the problem must be with the mixer PCBs or the wiring to them.

Cheers

ian
 
I'm thinking about something right now. The way my friend wanted it's summing to be hooked, is one side is the right channel (so channel 1-3-5-7-so on, on one channel, and 2-4-6-8. etc. on the other channel). So, while summing it up, in exemple, an bass track in stereo, one side of the track is sent to one summing circuit, and the other side, to ther other suming circuit. Is it possible my trouble lie there? I mean, maybe there is a kind of phase cancellation or something like that, who know. I know the classis way to do it is to make channel 1-8 on a db25 and 9-16 on the other, so you always keep the entire stereo track on the same db25 connector, on the same summing circuit
 
In fact, the way i did it: i strap all resistors on the ODD channel of eache board. Then each borad go to its dedicated makeup gain. So if i have a drum track on channel 1/2, channel 1 go to one db25 board and channel 2 to the other. And i think this i where the trouble lies. Probably a kind of phase cancellation, just like when you copy/paste, ie, a guitar track and pan it hard left/right. There will be some ohase cancelation.
 
Deepdark said:
In fact, the way i did it: i strap all resistors on the ODD channel of eache board. Then each borad go to its dedicated makeup gain. So if i have a drum track on channel 1/2, channel 1 go to one db25 board and channel 2 to the other. And i think this i where the trouble lies. Probably a kind of phase cancellation, just like when you copy/paste, ie, a guitar track and pan it hard left/right. There will be some ohase cancelation.

f you use one  board for the keft and the other for the right it sould be OK. It is possible you have a phase inversion at the transformer on one channel and not on the other which would certainly lead to a loss of bass. You could try swapping the leads on the primary of ONE transformer and see if that makes a difference.

Cheers

Ian
 
Allright. I'll inject à white noise straight at the summing resistor and on several places to find out if thé signal get lose somewhere. Maybe there is à phase inversion somewhete else in the wiring, maybe  at the switched. Will try before and after the summing board to test out the signal.
 
ok, I've just tried the summing board and it's not the culprit. I tried injecting a white noise before the summing resistor, big loss of bass on both channel, then, I tried to disconnect the transformer from the summing board and injecting the white noise straight at the input, same thing, big bass loss. I'll try the tricks with the cap and see what happen
 
hmm, found some short in the cables my friend sent to me loool. For now, I got channel 1 and 2 to have the right bass response, channel 3 and 4 have the bass loss and one has more gain on one side than the other, so its looks to have some short else where. But I think the trouble is the cable
 
Deepdark said:
hmm, found some short in the cables my friend sent to me loool. For now, I got channel 1 and 2 to have the right bass response, channel 3 and 4 have the bass loss and one has more gain on one side than the other, so its looks to have some short else where. But I think the trouble is the cable

I think you are are the right track!

Cheers

Ian
 
Hi there. I just repaired the short in the cables and I still have a lack of bass. In fact, it's better than before, for shire. The only place where I found it to be more ''problematic'' in certain condition, is on the drum. Depending on the kit, the kick sometimes sound too tin, but bass tracks are cool. I don't know if the input transformer is still too shy, inductance wise. I put an 1uf cap between 150Kohm and the secondary input transformer, and have the big CMOQ-2S output transformer. IIRC, I put some 120ohm slug resistors. Other than that, nothing special.
 
Out of curiosity, is it possible to, ie, complely remove the input transformer, the slug resistor, and connect the hot signal to one input of the opamp (ie, the non inverting input) and the cold to the other input (inverting) or it will not wirk?

Someting like what we found here: http://m.electronicdesign.com/ideas-design/efficiently-design-op-amp-summer-circuit

 
Ok, I just tried the common mode strapping. Basically, I keep all 10k hot and cold resistors, unsoldered the slug resistor and strapped the hot to the non inverting input, with an 10k resistor from the input + to ground, too. And, I strapped the cold to the inverting input, and keep my 10K/120pf between inverting input and output. But it remains the ams, bass wise. Which probably eliminate the input transformer from the equation.  I tried with a trs cable and some croc clip, to inject th signal straight from the output of the apogee to the inputs of the opamp, but no success here.

 
I just came back to the original setup with the input transformer, but removed the output cap, still the lack of bass, and there is probably some remaining dcv at the output, because there is a little distortion when cranking gain. So I'll just put back new 220uf cap and 20k to ground. the trouble seems to indicate the summing board. Don't know how
 

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