racked console channels help (audix 35102)
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#1: racked console channels help (audix 35102) Author: I'm Painting AgainLocation: New York, New York PostPosted: Sun Mar 12, 2017 11:58 pm
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fired these up for the first time in ages and they seem a bit off

my symptoms include

lack of low end and possibly a skewed frequency response overall

seems to be a bit louder in the mids then an obvious lack of bass

also the 18k high cut on one channel is not working - when engaged it cuts the signal completely

both channels seem pretty matched in their sound and level so I'm thinking

recap or old transformers or possibly the rack job itself might be the problem..

sans the 18k thats prob an inductor or resistor or something I'd guess ?

it was racked by boutique audio and design in the early 2000's

anyone know these puppies ?

#2: Re: racked console channels help (audix 35102) Author: Nick SevillaLocation: Los Angeles California USA PostPosted: Fri Mar 17, 2017 9:56 pm
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Hi,

This looks like a weekend job for you, and your oscilloscope.

There could be a ton of reasons, but if the symptoms are on BOTH units, then it follows it can be the rack job / power supply.

Or simply the same component giving out on both units, rare, but not impossible.

Good luck!!!

#3: Re: racked console channels help (audix 35102) Author: I'm Painting AgainLocation: New York, New York PostPosted: Sun Mar 19, 2017 10:36 am
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I heard many of these units pulled out of consoles and racked have been done so incorrectly. I could have sworn i read something on a forum about how doing it incorrectly can result in a skewed frequency response though I can not find that info on the web any longer.

It's a fairly complicated circuit and I'm not confident in being able to DIY without some direct guidance.

Crying or Very sad

#4: Re: racked console channels help (audix 35102) Author: The ScumLocation: Denver, CO PostPosted: Sun Mar 19, 2017 3:33 pm
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Any transformer input or output circuit will be really nasal sounding if one end of the transformer is left floating...like broken AM radio. If you're running them unbalanced, you need to ground the cold side of the transformer.

For the 18K thing, you got a working one and a not working one? Compare & contrast. Start from the rotary switch, and work through the components behind it.

Schematic here:
http://www.proharmonic.com/images/schema/audix/35102%20centre.jpg

It looks like the LP frequency is simply a pair of 910 ohm resistors, on a 2 pole rotary switch. From the little schem below that, if the resistors or switch failed open, you'd get no signal.

#5: Re: racked console channels help (audix 35102) Author: I'm Painting AgainLocation: New York, New York PostPosted: Sun Mar 19, 2017 7:31 pm
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ya the eq on the 18k that cuts the highs n passes the lows there is only one channel where it cuts the signal so I can check the 910 ohm pair

if it's the switch that would mean it's failing to make contact at that one band's switch point ?

The freq response thing does not sound like AM radio

it's more like the lows are there but very under represented and the midrange sounds slightly boosted the top seems OK

for stuff like snares and most vocals it's usable but for an entire mix or bass or a lot of other things it's not happening

#6: Re: racked console channels help (audix 35102) Author: I'm Painting AgainLocation: New York, New York PostPosted: Sun Mar 19, 2017 7:32 pm
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Thanks kindly BTW

#7: Re: racked console channels help (audix 35102) Author: I'm Painting AgainLocation: New York, New York PostPosted: Wed Mar 22, 2017 10:23 am
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checking the 910 ohms behind the switch they seem to read at spec as do all the other resistors in the low pass / high pass group there

I'm thinking it's the switch point..

it's a big complex switch not easy to replace unfortunately..it's sealed and looks like it would be hard to find a replacement Crying or Very sad

#8: Re: racked console channels help (audix 35102) Author: floidLocation: in exile PostPosted: Wed Mar 22, 2017 1:28 pm
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You've probably seen this, it was easy to find:

"The later rev. 35102 did not have an integrated output stage (necessary to drive the output TX if you're racking 'em up) - you will need an additional 3XXXX 'quad amp' board which will provide you with the necessary circuitry."

Maybe your output is coupled to an xformer with too low L?

#9: Re: racked console channels help (audix 35102) Author: I'm Painting AgainLocation: New York, New York PostPosted: Wed Mar 22, 2017 2:22 pm
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yes I've seen that..

would this cause the symptom of an even roll-off from 1000Hz down to 20Hz ??

here is a video of white noise on an analyzer through the unit

(both channels are exactly the same - take them out of the path and the noise is equal amplitude for 20-20k)

http://agatha-christie-fanclub.tumblr.com/post/158710104561

#10: Re: racked console channels help (audix 35102) Author: I'm Painting AgainLocation: New York, New York PostPosted: Wed Mar 22, 2017 2:31 pm
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where da low end go ? Shocked

#11: Re: racked console channels help (audix 35102) Author: I'm Painting AgainLocation: New York, New York PostPosted: Wed Mar 22, 2017 2:45 pm
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the output is wired like this and has the same roll-off on the line in and the mic in

http://agatha-christie-fanclub.tumblr.com/image/158609399752

thanks for yr insight !

#12: Re: racked console channels help (audix 35102) Author: floidLocation: in exile PostPosted: Wed Mar 22, 2017 3:05 pm
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A 600 ohm load paralleled with an inductance of 4.7H (aka output tx primary) forms a 20Hz hi- pass filter. A 30k load paralleled to the same forms a 1kHz hi- pass filter.
I could be way wrong but this might explain it. The missing circuitry may, among other things, convert high impedance to low.

#13: Re: racked console channels help (audix 35102) Author: I'm Painting AgainLocation: New York, New York PostPosted: Wed Mar 22, 2017 5:36 pm
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I tired removing the cap then the cap and the resistor off of the transformer - the result was just a hotter version of the same skewed response

#14: Re: racked console channels help (audix 35102) Author: floidLocation: in exile PostPosted: Wed Mar 22, 2017 8:20 pm
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Found the full schematic on GroupDiy. I think "circuit 5" is the interesting bit. Also, notice the note in the bottom left that this is a -10 dBu circuit. The extra stage probably kicked that up.
What happens when you disconnect the transformer from the circuit completely (the side in your pic labelled "to audix...") and check the unbalanced output at "pin 15 - low z ouput" or "pin 17 - high z output"?
Do "pins 16,18 - output signal return" and "pin 40 - audio switch common" show continuity with pins 4,10,11, which your picture shows being used as tx ground? There is a network composed of c158, c159, r160, and r161 between pins 15/17 & 40 that would be in parallel with whatever actual inductance value of your output tx primary (the other side from the 600 resistor and cap). This would form a filter on your output.


Last edited by floid on Wed Mar 22, 2017 8:36 pm; edited 1 time in total

#15: Re: racked console channels help (audix 35102) Author: I'm Painting AgainLocation: New York, New York PostPosted: Wed Mar 22, 2017 8:33 pm
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yes i believe the extra amp people are using is to alter the headroom..I'm not sure about the frequency response though..no mention of that being altered adding the amp stage...

Quote:
The stock circuit was NEVER intended to drive a signal into "the outside world", but rather into faders, assign switches, etc., hence, the module's output can barely hit +18 dBu at clip because of the limitations of a single-ended 24 VDC power rail.

My client did have a pair of Neve LO-2567 output trannies, however, so I concocted a push-pull driver circuit using NE5534 opamps, and that combo resulted in a second, balanced output that could indeed push +24 dBm (yes, into 600 Ohms) from the balanced output connector. (That's why there is a TERM switch, to add an 820 load R across that balanced output when the preamp is NOT driving a true 600 Ohm destination).


As mentioned, I used a pair of NE5534 chips, both connected as inverters. Stage 1 drove one end of the Neve output transformer's primary *and* the other inverter (running at unity gain), which in turn drove the other end of the primary. Hence, "push pull" since the ends of the primary were driven 180 degrees apart. The opamps were powered from the single-ended 24 VDC supply with a bias circuit to make the output pins float at 6 VDC. Since both opamps floated at 6 VDC, there was no need to use a coupling capacitor to prevent a DC difference across the primary.

Whenever possible, I run opamps as inverters...lower distortion in most cases. But, I have to "keep track" of the signal path's polarity, and thus end up moving transformer inputs or XLR pins around to keep the polarity ("phase") correct.

Biasing the opamps was required in the Audix project since there was only a single-ended 24 VDC supply available, and not the +/- power expected by typical opamps.

For instance, this diagram shows two different methods to power an opamp:

http://www.brianroth.com/library/vu-buffer.pdf

The power connections at the left/bottom supply the "bias" for a single ended supply, like what Audix modules required In fact, in the Audix schemos for version 3, they did that exact same thing for the 5534 used as the mic preamp.



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