SSI suburban sound inc. remix module(summing mixer) 1731 DOA issues & questions.

i was thinking of replacing the power indicator light with a pot for output trim. as it looks like the same size opening for the bulb and the stock pots.  I'd rather not mess with the gain of the DOA and leave that set with it's feedback resistor (i havent decided btwn 14k or 10k as you suggested earlier), but I would like to be able to have a trim on the output.

could I not connect the signal from the 470uf cap (out from the out of the DOA) to the top of the pot, wiper to the output of the mixer and the bottom connected to the common?

Anyways, I def want to use a rotary pot (or switching attenuator) for master level control, I'd prefer to have it act as a trim for the output, rather than a fader controlling the gain of the DOA, if that makes sense and is possible.

Thanks again.

well, I guess I'll take that as it can't be done, or done properly... I guess I thought I could just connect the output signal to a potentiometer and trim the output signal, much like the input signal is trimmed to the summing bus, with the input signal connected to the top, the wiper to the output to the bus and the bottom to ground.... perhaps I'll experemet anyways since I ordered a couple dual 10k pots, one linear and one log..

Fuccimain said:

well, I guess I'll take that as it can't be done, or done properly... I guess I thought I could just connect the output signal to a potentiometer and trim the output signal, much like the input signal is trimmed to the summing bus, with the input signal connected to the top, the wiper to the output to the bus and the bottom to ground.... perhaps I'll experemet anyways since I ordered a couple dual 10k pots, one linear and one log..

Click to expand...

You may run into noise and HF response problems with 10k. Since the DOA handles 600 r load, you could use a lower value like 2.2k or 1k.

Thanks Abby road..

I'll give it a go with the 10ks I have in the mail and if they both sound funky I'll order some lower value pots.

As far as the + output, it's what's coming from the out of the DOA I'm assuming, even though the phase of the signal is getting reversed so many times (input tranny in and out and inverting DOA).. Correct?

hey abby road,

so I changed all the buss resistors to 10k, added 2.2k slugging resistors to both sides of all the panners,... sounds great, the balance btwn LR and center is great, thanks so much.  I also changed the 47k resistors btwn the V+ and the TRIM input on the DOA..

abbey road d enfer said:

26 cents for a resistor is outrageous. You don't get much benefit using 1% resistors. You'd better buy 100 of the 5% type and select them manually with a DMM. The feedbak resistors don't need to be 12.7k; in fact they should be either 14k in order to compensate for the 3dB loss in the pan-pot o 10k for OdB gain with the PP at full left or full right. Anyway, you will find that the pan-pots themselves have a much higher deviation than +/-5%.

Click to expand...

before I attempt my master trim modification, I wanted to make sure everything is functioning as it should, so I went with your 2nd suggestion of 10k feedback resistors,( I added 1k bourns trimmer resistors in series with the feedbackRs for gain leveling) but the output is very low now and recording into Protools, the max I'm getting is around -12db... if I change the feedback resistor to 14k as you 1st suggested, will that increase that gain of the opamps or lower them?  I'm still a little confused about increasing impedance will raise gain or lower it.. I have 14k resistors as well as 13k5 ones (that i can put in series with the 1k trimmers)

thanks again man, really been a hugh help// let me know what you thin k about increasing the gain of my opamps..

sean

Fuccimain said:

the output is very low now and recording into Protools, the max I'm getting is around -12db...

Click to expand...

This is due to the minimal design of the unit. There's only one thing making gain, the DOA, all the rest is just attenuating. I would say that recording at -12 (I surmise it's -12 dBfs) is quite a good practice with 24 bit converters.

if I change the feedback resistor to 14k as you 1st suggested, will that increase that gain of the opamps or lower them? 

Click to expand...

That would increase the gain, but only by 3dB. Increasing the NFB res increases gain. If you really want seriously more gain, you may try 39k, that would give you about 12dB more. The drawback is that the DOA may run out of open-loop gain and the frequency response and THD may suffer. I wouldn't do it. Maybe I'll go up to 22k, for about 7dB more.

Thanks man. Much appreciated. So if I keep the 10k I have and add a 13k5 in series , that would bring me up to 23k5, (plus the 1k trim)... What do you think about that?

Fuccimain said:

Thanks man. Much appreciated. So if I keep the 10k I have and add a 13k5 in series , that would bring me up to 23k5, (plus the 1k trim)... What do you think about that?

Click to expand...

That would work, but the range of adjustment would be about half a dB. That may just be what the doctor ordered...

Cool thanks,

Ill give it a go. The level difference btwn the channels got much better after I did the work. , perhaps it was changing those 47k trim resistors.

On another note, if I wanted to add a pair of output transformers,  thinking a pair of these EA ApI repros: http://www.classicapi.com/catalog/images/gallery/2503/2503-specs.pdf

What you think?

Fuccimain said:

The level difference btwn the channels got much better after I did the work. , perhaps it was changing those 47k trim resistors.

Click to expand...

I very much doubt it. This resistor is either a DC offset adjustment or, more likely a quiescent current setting. It has no influence whatsoever in regard to gain. In a DOA, gain is strictly controlled by the input and NFB resistors. I thnink the better balance is due to the correct value of the bus-injection resistors respective to the value of the pan-pots.

On another note, if I wanted to add a pair of output transformers,  thinking a pair of these EA ApI repros: http://www.classicapi.com/catalog/images/gallery/2503/2503-specs.pdf

What you think?

Click to expand...

Use the unit as it is now.
If you have hum, you may chose to be a big guy and find the solution by rearranging the wiring, or you want to be a kiddie and put transformers.
In you case, output transformers have only one advantage: galavanic isolation, and they have many disavantages in terms of frequency response and distortion. Don't let be fooled by those claims of "warmth", "colour", "grit" and whatnot.

No hum problems thankfully. 1 thing I did notice is that when I have all the channel level pots all the way down I can still hear very very low signal at the outputs... Wondering what that's all about.

Here's something.. I wasn't really paying attention to the aux in, but I did just notice the aux's L&r are hitting the mix busses via 5k1 buss resistors... Perhaps the value was different because the auxs aren't going thru the UTC input transformers? .. Should I change these to 10k buss resistors as well? (Aux is a dual 10k type AB same as panners pots )

Fuccimain said:

Here's something.. I wasn't really paying attention to the aux in, but I did just notice the aux's L&r are hitting the mix busses via 5k1 buss resistors... Perhaps the value was different because the auxs aren't going thru the UTC input transformers?

Click to expand...

No. I think it's because the original 1k resistors gave too much center attenuation, and in order to give the auxiliaries similar gain, they found that 5.1k was correct. Now I suspect the auxes have too much gain compared to the "normal" channels.

  Should I change these to 10k buss resistors as well? (Aux is a dual 10k type AB same as panners pots )

Click to expand...

That wouldn't change much the overall performance. You can do it (or not).

Guess ill just leave the aux as is for now as I'm not planning on using.

I did hook up the dual 1k for the output trim and it seems to work perfectly.

Thanks again for all yr help.

Next I'm gonna ponder adding another 8 channels to it. 1st step to that is finding similar line transformers for the next 8 channels.. 4k:600r  pri:sec

Long shot on this, but do any of you have (or know of)  one of these little SSI mixers that's  for sale? Thanks!