DBX 286A Modifications

ecc83 said:
I am not qualified to judge but all I have read in the last 60 years or so tells me you are more in the Russ Andrew's camp than Ethan Winners.

I respectfully urge other readers to take this stuff carefully with a handful of salt and check other, reliable sources.

Dave.
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I'm in the camp of Rupert Neve, Ron Dolby, Jim Williams, Brian Calrstrom, and Rob Zombie.

Not only I hold a ISCET cert, I have a Bachelors in Electronics, plus other certifications along the way in the past 30 years (TV/VCR repair and various Telcom and FCC certifications)

So my electronics theory background is strong, and its not cluttered with the misconceptions that seem to get fed to the diy audio engineers.

I know who Ethan Winner is, and I rocked him before on a 40KW PA system ;)
 
drtechno said:
I'm in the camp of Rupert Neve, Ron Dolby, Jim Williams, Brian Calrstrom, and Rob Zombie.

Not only I hold a ISCET cert, I have a Bachelors in Electronics, plus other certifications along the way in the past 30 years (TV/VCR repair and various Telcom and FCC certifications)

So my electronics theory background is strong, and its not cluttered with the misconceptions that seem to get fed to the diy audio engineers.

I know who Ethan Winner is, and I rocked him before on a 40KW PA system ;)
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Ok, so I shall respect your qualifications. Maybe it is the way you put things over that causes me concern? For instance, your last sentence, were you applauding Ethan or knocking him? (there is another guy here that SEEMS ok but says weird things!)

I have to wonder what the guys at www.soundonsound forum would make of you?

Dave.
 
ecc83 said:
Ok, so I shall respect your qualifications. Maybe it is the way you put things over that causes me concern? For instance, your last sentence, were you applauding Ethan or knocking him? (there is another guy here that SEEMS ok but says weird things!)

I have to wonder what the guys at www.soundonsound forum would make of you?

Dave.
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Well I'm trying to explain without going into great detail and just looking at what I'm concerned with. When someone analyse this input coupling stage, not only you look from the microphone connector going in (referred as Looking in), but you also look at what goes on from the input of the amp back to the input connector(referred as looking out).

What you explained earlier is the "looking in" point of view and what I explained is the "looking out" point of view. Both views are valid and should be taken in consideration when designing.

I will point out the design compromise with this coupling mechanism, which is the source's series DC resistance at finite impedance changes when the microphone is changed. If the DC resistance is lower than the Xc (the resistance at the frequency) of the phantom blocking caps, attenuation of the source signal occurs.

This whole thing might be different to what you are used to when analysing a circuit. Because this methodology takes apart the circuit and analyses the DC and AC return currents it can be used to predict the outcome of gain, phase, and frequency response of the circuit.

I haven't ventured onto the soundonsound forum, but to let you know 99% of those review sites are paid by the manufacturer, so I can see their reviews can be biased. I take most things that come off the internet "with a grain of salt" you might say.
 
Well maybe I really am too old and dim to get it but it still seems to me that you are complicating a very common piece of circuitry that has been around since the 80s (D.Self, Balanced Feed back Microphone Amplifier).

I find the comment about SoS reviews insulting and if reproduced in their forum probably libellous.

I think I shall leave this thread before I write something I regret.

I find Google does not find the mic amp. You will have to buy his book!

D.
 
ecc83 said:
I find the comment about SoS reviews insulting and if reproduced in their forum probably libellous.

D.
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I'm not saying they participate in such activity, but its quite the norm no matter which sector of electronics I've worked in when the end customer is the consumer.
 
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All Jokes aside,

I'm still working on my schematics. One not so nice thing about this that I noticed was the underside of the input board is sticky with flux. They did an ok job on top cleaning it, but the underside is very nasty-sticky and over time, it might cause a short when this residue gets contaminated. I'll also inspect the main board if it has too much flux residue.

Looking at the input board it looks like that is the board I should redo. I personally don't need the phantom power because I run phantom power supplies on the other end of a mic snake that is routed to the tracking room. Also, other thoughts about this is I could add transformers on the inserts to Improve the inserts. Also an output transformer could be added to give this unit more personality.

I'm going to dig out and make a new i/o board. But I should add features to this I would like to see. So, I'm going to take the phantom and run a couple of relays to control. One relay will do phase invert, and the other will pad the input. I could use the phantom power (p48) button for the phase invert relay, and the other switch I'll add on the front and peel it off at the existing p48 power switch.
 
I hooked up the sticky i/o board and measured 8Vdc on the insert jack when phantom power is applied. Cleaned the board and the stray voltage went away.

I told DBX about this and they were happy to provide me with the schematics of this unit. Of course I've seen this mic preamp design used in several units including midas, soundcraft, behringer, presonus, art and many others.

View attachment DBX286S.pdf
 
As far as improving the compressor, the tle074 would be lower noise and its built in emi suppression would be welcomed to that circuit.
the LM4580 in the mic preamp is ok(u3) , but I would prefer to use a lm4562 on the output (u6). I'll be trying that after the input transformer adventure is over.
 
i think i figured out whats broken on the 286S unit, and I've got the part on the way to fix it.

I'm pretty much done with my initial mods and repairs on the 286A unit.
Seriously someone at harman needs a good talking to. some of the implementation flaws are just horrendous.
and it isn't a cost thing.

Sadly some of those flaws carried over to the S model too.

once i get the S model fixed i'll do some initial side by side testing. hopefully mid week next week.
 
innkeeper said:
i think i figured out whats broken on the 286S unit, and I've got the part on the way to fix it.

I'm pretty much done with my initial mods and repairs on the 286A unit.
Seriously someone at harman needs a good talking to. some of the implementation flaws are just horrendous.
and it isn't a cost thing.

Sadly some of those flaws carried over to the S model too.

once i get the S model fixed i'll do some initial side by side testing. hopefully mid week next week.
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There is a few things I would have to say/question about the S model I have.

1. What is the real purpose of R11 and R12? It seems like they are noting more than rfi filter, but their presence attenuates the signal going in.

2. the inserts are not ground Isolated. Even though this is a common theme for budget mixers, I find keeping the these inserts isolated lowers the overall noise. Most of the desirable high end mixers used for recording have transformer isolation on its inserts.

3. The insert return has no metering, nor any trim adjustment. Even though its not a necessity, its nice to be able to control the gain structure going into the compressor after the inserts.

4. The output is not a balanced connection. No matter how you look at that.

I still think transformers are the way to fix this design. I would have to compare and contrast the different ways I've seen balanced insert transformer setups ( like the Api set up of a 2623 or 2503 as an insert send and a 2622 on the return vs. a pair of Carnhill vtb9071, like what is installed as the insert transformers on a Neve 8816). I would definitely do some sort of iron like the 2503 on the output.
 
"1. What is the real purpose of R11 and R12? It seems like they are noting more than rfi filter, but their presence attenuates the signal going in."

They are not resistors, from the schematic they are RF chokes and you remove* them at your peril.

Certainly a good mic transformer will have an inter-winding shield that will all but kill RF but I would keep the chokes then check the HF response at 20kHz. If it is within a dB or so, keep them. The transformer will likely need a 'Zobel' CR network to dampen the HF resonance so that could be tweaked to get a decently flat response.

Always be cautious throwing in traffs. HF and LF resonances can occur, hum induced and, unless driven from a very low (best zero) impedance source, distortion rises with level and inversely with frequency.

*I live within 50clicks of both Rugby and Daventry and grew up with the first transistorised audio gear. I have had a LOT of experience of keeping radio out of audio gear! Valves, especially triodes are fairly immune. 'Bout the only good thing about them really, especially modern ones!

Dave.
 
ecc83 said:
They are not resistors, from the schematic they are RF chokes and you remove* them at your peril.

Certainly a good mic transformer will have an inter-winding shield that will all but kill RF but I would keep the chokes then check the HF response at 20kHz. If it is within a dB or so, keep them. The transformer will likely need a 'Zobel' CR network to dampen the HF resonance so that could be tweaked to get a decently flat response.

Always be cautious throwing in traffs. HF and LF resonances can occur, hum induced and, unless driven from a very low (best zero) impedance source, distortion rises with level and inversely with frequency.

Dave.
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I wish they included the BOM for the jackboard, because it looks like a #603 surface mount resistor. but comparing this to what is typically there, 8 turns of wire is not going to have 2.5K of resistance.

I'm not afraid of transformers, and I know how to set them up in the circuit. As far as response, I'll focus on 50Khz and below, even though with injecting signal and the front jack board removed, the mic preamp section is flat to about 130Khz.

I am going to get a set of the carnhill transformers for the inserts, even though they don't put specifications on them that I look at when I buy transformers (DC resistance and inductance (but will settle for bandwidth at rated source impedance if inductance is not available)) I'm sure they are ok since Rupert Neve used them in his designs. Other ones I know that would work really well, are way out of the budget range ($350/ea) and I'm not trying to take it to the "mastering class" level with it. Just trying to get it into "higher end recording class" instead of "general purpose sound enforcement and recording class".
 
well I got it working really well with the jensen transformer, however, I still had to have one instance of a capacitor in the path to isolate the two bias paths of the differential pair.

So now I have decided to order a LL1530 because it has two separate windings on the secondary, and I should be able to get dc coupled operation with that transformer. As each secondary winding will be part of the dc bias and signal path.
 
drtechno said:
well I got it working really well with the jensen transformer, however, I still had to have one instance of a capacitor in the path to isolate the two bias paths of the differential pair.

So now I have decided to order a LL1530 because it has two separate windings on the secondary, and I should be able to get dc coupled operation with that transformer. As each secondary winding will be part of the dc bias and signal path.
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really well? that is a bit ambiguous :) what difference did it make?
how did the LL1530 work out?
 
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