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

[abbey road d enfer]

Yeah say 3.3k resistors, but I was curious what drop in db that would be at center.

That's for 3dB drop at center. It doesn't mean it's the "right" value. There's a lot of debate about what is the correct value; and there are good arguments for anything between -3 and -6. Certainly more than 6dB is objectively "wrong".

As far as the mod, the value components it comes with is based on the mix bus resistors being 47k:
http://classicapi.com/catalog/product_info.php?cPath=22_46&products_id=81

The only advantage of using 47k is that it allows loading a typical output with many sends, in particular if there is a passive pan-pot. In terms of noise it has almost no penalty with a 2520 DOA(or most opamp except the VLN types). But it has some drawbacks, in particular it makes the bus more sensitive to capacitive-coupled interference and x-talk due to stray capacitance of switches.
There are good reasons why most console mfgrs have opted for 10k-ish bus-injection resistors.



I pulled a leg on each cap, measuring 470uf for one and. 450uf for the other
[/quote]

 

[Fuccimain]

ok, I ws hoping for a little less of a drop at center,. like 2.3db,.. but I'm sure 3db is better than the 12db i got going on now.  I'd like t place a digikey order today but I'm having trouble finding the right resistors in stock/  i wanted to use presision for everything, 10k mix buss resistors, 3.3k shunt pan , and the 12k7 feedback.. with a trim resistor to even out the gains.. can you make any recommendations that wouldn't be a bulk buy? the Vishay/BC 's i was looking at were out of stock (at least the 10k)

according to the API2520 spec sheet a 100ohm trim pot btwn the V+ and the "trim" leg on the DOA can adjust output offset to  zero, (but that that is usually not necessary for audio applications) on my circuit there is a 47K resistor there... wondering what that's about..
if you could recommend any resistors i'd be grateful,. the image below is what I was looking at on DIgikey(mouser is out too except for SMD)
i wanted a .1% tolerance.... especiallfor feedback.. is a larger ok for mix buss resistors?

decided to replace the Caps with 470uf 16v, unless you object,

thanks again

sean

 

[abbey road d enfer]

ok, I ws hoping for a little less of a drop at center,. like 2.3db,.. but I'm sure 3db is better than the 12db i got going on now.  I'd like t place a digikey order today but I'm having trouble finding the right resistors in stock/  i wanted to use presision for everything, 10k mix buss resistors, 3.3k shunt pan , and the 12k7 feedback.. with a trim resistor to even out the gains.. can you make any recommendations that wouldn't be a bulk buy? the Vishay/BC 's i was looking at were out of stock (at least the 10k)

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%.
And if you want to be anal about gain, use trimmers in series with fixed resistors.

decided to replace the Caps with 470uf 16v, unless you object,

I don't object, but it's worthless IMO, for two reasons:
Primo: coupling capacitors need to be much higher than what the should strictly be, so who cares it it's 10x or 10.5x the minimum value
Secundo: electrolytic caps have rather large tolerances, typically +/-20%. With 470 and 450, your actual caps are within +0%, -5%

 

[Fuccimain]

ok, I ws hoping for a little less of a drop at center,. like 2.3db,.. but I'm sure 3db is better than the 12db i got going on now.  I'd like t place a digikey order today but I'm having trouble finding the right resistors in stock/  i wanted to use presision for everything, 10k mix buss resistors, 3.3k shunt pan , and the 12k7 feedback.. with a trim resistor to even out the gains.. can you make any recommendations that wouldn't be a bulk buy? the Vishay/BC 's i was looking at were out of stock (at least the 10k)

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%.
And if you want to be anal about gain, use trimmers in series with fixed resistors.

so for the trimmers, I like that Idea... what value would you recommend if in series with the feedback resistors.. ?  let's say Im using 14k to compensate for the 3db loss from the pan pots... would something like 13k5 plus a 500ohm trim be good?

decided to replace the Caps with 470uf 16v, unless you object,

I don't object, but it's worthless IMO, for two reasons:
Primo: coupling capacitors need to be much higher than what the should strictly be, so who cares it it's 10x or 10.5x the minimum value
Secundo: electrolytic caps have rather large tolerances, typically +/-20%. With 470 and 450, your actual caps are within +0%, -5%

i'm a little confused.. the original caps were 350uf.. but they are reading as 495 and 503uf ( i re-did the measurements ).. so replacing them is a good idea correct? as they are not behaving as they should.. I read that the Vishay BC axials are good for  sound quality

 

[Fuccimain]

Oh, 2 questions that I've been stumped on.

1. Which is the hot/hi output? (The phase is getting swapped and inverted so many times I'm a bit confused.) hot input gets swapped at input transformer, ( hot into lo in on tx, then lo out from tx  is used to feed the level pot to pan to mix bus. Then swapped again at the inverting DOA. I'm assuming the DOA output to the 350uf cap is the hi out?

2. If I want to wire in a master level pot, dual log potentiometer is correct? Would it be a 10k? Or other value....?

Thanks

 

[Fuccimain]

oh and I just tested the 47k resistors fro V+ to the "trim" pin on the DOA, oneis measuring 47k5 and the other is 51k.. i should prob change those too as the might be causing the offset between the levels from the  LR outputs...

s

 

[abbey road d enfer]

i'm a little confused.. the original caps were 350uf.. but they are reading as 495 and 503uf ( i re-did the measurements ).. so replacing them is a good idea correct? as they are not behaving as they should.. I read that the Vishay BC axials are good for  sound quality

Suspicious; 350uF is not a normalized value. 330 or 470 are.

 

[Fuccimain]

My bad. Was. A 320uf.. Must be old. And considering the cream color bar on the feedback resistors, nothing is really surprising me with odd components.. 

 

[Fuccimain]

so I'm starting to get my order together for pars to attempt fixing this thing up..

I've pretty much decided to go with the 10k buss resistors, as you suggested, but then I'll need to modify the classic API card if  I do.. curious though about using 47k buss resistors.. how will that effect my pan slugging resistors? the 3k3 you recommended kinda go out the window with 47k bus resistors?  is there a formula you would share with me to help me calculate the relationships btwn the buss resistors and or buss source impedance and my 10k panner and what slugging resistors I need to choose?  lots of learning.. and I'm stoked to be learning so much here.

thanks

SEan

 

[Fuccimain]

ok just about got my parts list together for my order to start to fix up this unit... I'm still a bit unsure about the slugging resistors.. 3.3k vs 2.2k.. on 10k panners to 10k buss injection.... curious about the difference in level at center btwn 3.3k and 2.2k.. but I'll experiment with both parts when they arrive.

one thing, I want to add a master trim potentiometer, after the Melcor DOAs from the coupling caps (going to order 470uf 16v) to the master level control

I should get a Dual log 10k pot for this?

or perhaps an attenuator for the master, something like this: http://www.ebay.co.uk/itm/Valab-23-Step-Attenuator-Potentiometer-10K-Stereo-Log-/300868936568?pt=US_Amplifier_Parts_Components&hash=item460d2fa378

I always was under the assumption that resistor attenuators were only good for when you needed to switch between "gain set resistors" externally, (like on neve or api pre amp etc), didn't know they could be used in a more general setting like a master volume etc.. unless i'm totally mistaken..

anyways, I'll try to wait to hear back from abby road before I put in my digikey/mouser order... really appreciate the help

thanks!

 

[Fuccimain]

hyea,

ok I have my order together for replacement parts.. gonna experement with different slug res values for pan and feedback resistor trimmers etc..

just 1 thing i'm not sure what to order is the dual pot for the master level control... should I keep 10k or move up to 50k dual audio pot?  or perhaps go with a switching attenuator..
any thoughts would be appreciated,

thanks!

 

[abbey road d enfer]

What is the intended use for that?

 

[Fuccimain]

for what?

 

[abbey road d enfer]

for what?

For the "master trim attenuator".

 

[Fuccimain]

Oh. Well, just to have a level control over the master level, I'm guessing a 10k dual linear with a slug resistor from wiper to the bottom is something I might try, or just a regular dual 10k audio... I'm not quite sure how to determine what resistance I actually need though, wether it be a 10k or 50k or 100k etc for the pot values. I'd live to learn how to figure that out.

As far as that 10k stepped attenuator, I was interested in something like that for fixed repeatability

 

[abbey road d enfer]

Oh. Well, just to have a level control over the master level, I'm guessing a 10k dual linear with a slug resistor from wiper to the bottom is something I might try, or just a regular dual 10k audio... I'm not quite sure how to determine what resistance I actually need though, wether it be a 10k or 50k or 100k etc for the pot values. I'd live to learn how to figure that out.

As far as that 10k stepped attenuator, I was interested in something like that for fixed repeatability

Do you want a Master fader or is it something in addition to the Master fader?

 

[Fuccimain]

A master fader. Right now only level control is on individual channels ...

 

[abbey road d enfer]

A master fader. Right now only level control is on individual channels ...

If I understand correctly, there is only the summing amp, no separate output stage. In that case, the only proper way of fitting a Master fader is to connect it in place of the NFB resistors.
Wiper goes to the inverting input, bottom goes to output of DOA. Make sure you completely disconnect the NFB resistors. The problem is that the gain structure is utterly dependant on the fader's value. With 10k (the most common value) and 10 bus-injection res, the channel gain will be -3dB with the pan-pot centered. Beware that some faders just don't work that way (P&G series 8000 won't, series 3000 will).

 

[Fuccimain]

Thanks for this.

There is no way for me to hook up a dual trim pot after the DOA's? Keep the NFB resistors, and connect a dual pot to trim output level?

 

[abbey road d enfer]

Thanks for this.

There is no way for me to hook up a dual trim pot after the DOA's? Keep the NFB resistors, and connect a dual pot to trim output level?

If you want to use a rotary pot instead of a fader, you have much more choice. Many possibilities there, depending what range of trim you want. You could even have some gain. You have to make up your mind about what you want.