DBX 286A Modifications

[drtechno]

Well I'm decided to bail on it guys. The big strikeout for me is that this is a two stage mic preamp. the gain control is on the second stage with a four transistor comparator/Impedance buffer which wouldn't make the best. Second, there is no trim resistors on the power supply, so I would have to sit there and change out regulators. Btw the + rail is .8V off from the - rail and that is why it has so much common mode noise without even a mic connected! I could modify the input board to install a transformer, but its just not worth the trouble.

If the power supply wasn't so off, I would go ahead and remove the surface mount transistor stage. But since I don't want to correct a brand new one (nor I should have to) , I'm sending this one back....

 

[drtechno]

Those are lm339's or ST's equivalent.
i noticed the changed to using that 4805's for the vca / rms cips.
also changed to a jrc4580

interesting changes.

I would be curious what the rest of the board uses. if you pop the hood again id love to see some more pics as i cant see the chip numbers for the glare... and even be interested in what they did on the "S" version to the power supply and transformer too.

it does looks like there is not enough chips there, id bet they have surface mount chips on the underside.

here is some pics of the gutz.....

 

[innkeeper]

Thanks!

 

[innkeeper]

Well I'm decided to bail on it guys. The big strikeout for me is that this is a two stage mic preamp. the gain control is on the second stage with a four transistor comparator/Impedance buffer which wouldn't make the best. Second, there is no trim resistors on the power supply, so I would have to sit there and change out regulators. Btw the + rail is .8V off from the - rail and that is why it has so much common mode noise without even a mic connected! I could modify the input board to install a transformer, but its just not worth the trouble.

If the power supply wasn't so off, I would go ahead and remove the surface mount transistor stage. But since I don't want to correct a brand new one (nor I should have to) , I'm sending this one back....

Not so fast.....

i would assume they replaced the single instrumentation amplifier with discreet components. this would have several advantages over using a single chip solution like an ina163 / ssm2017 / ssm 2019 / that1510 etc. namely, you can achieve higher bandwidth, lower noise.

instrumentation amplifiers are all two stage, that is how they work and provide good CMRR

on my A model, i had the same issue with the regulators, so i did change those out to make the power fairly symmetrical. also, increased the precision of the resistors on the input of the instrumentation amplifier and the ones in the 48v supply to increase the CMRR.

 

[drtechno]

Not so fast.....

i would assume they replaced the single instrumentation amplifier with discreet components. this would have several advantages over using a single chip solution like an ina163 / ssm2017 / ssm 2019 / that1510 etc. namely, you can achieve higher bandwidth, lower noise.

you've got a point. I went and re-flowed the solder on the regulators and its much better

OK since you put it that way, let's do a facelift on the front end. Say, an API 312 w DI input:


The only thing different with the input circuit I would do, is change it to where the transformer is para-feed and the di input capacitive coupled too.

 

[ecc83]

Not so fast.....

i would assume they replaced the single instrumentation amplifier with discreet components. this would have several advantages over using a single chip solution like an ina163 / ssm2017 / ssm 2019 / that1510 etc. namely, you can achieve higher bandwidth, lower noise.

instrumentation amplifiers are all two stage, that is how they work and provide good CMRR

on my A model, i had the same issue with the regulators, so i did change those out to make the power fairly symmetrical. also, increased the precision of the resistors on the input of the instrumentation amplifier and the ones in the 48v supply to increase the CMRR.

+1 to selected resistors for the spook juice to improve CMMR. Instrumentation amplifiers are not all that useful for audio (ref Self) Their high CMMR is only achieved at very high gains.
If OP is going for input transformers, the NE5534 works well, e.g. F'rite Red.

I would be pissed about that crap, clamp style open frame power traff! I would want a toroid at least, preff' a 'universal' SMPSU.

Dave.

 

[innkeeper]

For me, id love for the unit to be able to operate at high gain with a low noise floor for use with dynamic mics.
So far, i've not had my hands on a high gain (50-60db) and low noise preamp that uses an instrumentation amplifier up front.

not that i am anti instrumentation amps.

for me, on my dbx 286A anything over about 45db gain the noise starts creeping up rapidly. like -98 dBv at 10db gain, -95 dBv at 30db gain, -90 dBv at 45dB gain, and -77 dBv at 60dB gain. if i boost it by ~30DB with an inline amp (pick your poison for inline amp, fethead, cloudlifer et al.) then i can reach a total of 60DB gain at under -95dBv noise.

i know my measurements are dBv not dBu so do the conversion if that's important .

i'm probably fighting an unworthy fight here in tyring to get that in a single unit, and should be happy with the changes i've already made and the results i have. i can always use the inline amp for my dynamic mics for voice over. I could put in a discreet ~30DB amp up front inside the unit, but that would force the lowest gain to about 40DB. kinda makes it a one trick pony. not sure that is a solution.

perhaps a separate input for the higher gain that switches in when plugged in??

oh for those who are interested, i did do response curves for the 286A ... and its pretty flat, and the thd is extremely low. I'll post that if there is any interest in actually seeing it.

 

[arcaxis]

perhaps a separate input for the higher gain that switches in when plugged in??

My separate input for my 286s when I use an SM7b is a Triton Fethead. Pretty quiet with the Fethead and the SM7b takes on a bit more clarity with the Fetheads 22 kohm impedance.

 

[innkeeper]

My separate input for my 286s when I use an SM7b is a Triton Fethead. Pretty quiet with the Fethead and the SM7b takes on a bit more clarity with the Fetheads 22 kohm impedance.

Currently using a NOS AUDIO "mic booster microphone activator" yea i know not well known. got one while they were/are blowing them out. internals is basically the cloudlifter circuit, right down to using sk170's, and hence the reason they are discontinuing manufacturing.

does the job. The dynamic mic i have been using is MXL BCD-1 which has about a -54 dB 1 V/Pa output so jsut a little hotter then the sm7b. rated at 600 ohms. (kinda high) so the higher input impedance of a fethead would suit it better as the cloudlifter is closer to 3k in its design.

but it gets me by for now while i play.

 

[innkeeper]

HA - just scored a DBX286S for 25 bucks - with a non working processor section.
I should be able to fix that, and i can compare the preamp part even without repairing it.

 

[drtechno]

So, getting on with things, I decided to order a sowter 9775 for this. Its not a big step up, plus this mic preamp circuit reminds me of the Langevin AM16. In the mean time, I will make a schematic of the i/o board and the front end of the main board.

 

[drtechno]

So, getting on with things, I decided to order a sowter 9775 for this. Its not a big step up, plus this mic preamp circuit reminds me of the Langevin AM16. In the mean time, I will make a schematic of the i/o board and the front end of the main board.

edit.... just tried to order one. they are out... So, I will try a different transformer, the Jensen JT-13K6-C

the only thing is, I will lose the 600 ohm input that would have like to test with modern condensers.

 

[innkeeper]

edit.... Just tried to order one. They are out... So, i will try a different transformer, the jensen jt-13k6-c

the only thing is, i will lose the 600 ohm input that would have like to test with modern condensers.

lundahl ll1578?

 

[drtechno]

lundahl ll1578?

I was considering that one (actually the LL1578XL), but I decided to go with the Jensen since I have an account with them and don't have to go through a dealer + vat + shipping.

Even though my Jensen rep would like to see me manufacture stuff under my own brand. The engineering sample (jt-13K6-ct) they are sending me will be fine.

 

[innkeeper]

The engineering sample (jt-13K6-ct) they are sending me will be fine.

maybe you can talk him into sending me one too

i saw the XL, but thought the extra headroom was overkill, though, i couldn't see any downsides to it.

 

[innkeeper]

HA - just scored a DBX286S for 25 bucks - with a non working processor section.
I should be able to fix that, and i can compare the preamp part even without repairing it.

It arrived yesterday! I'll get it on the bench tomorrow. i bet someone blew the output op-amp. if so that's a quick and easy fix.
I did pop the hood. i do like the layout changes, short runs from the pot to the amp circuit is a big plus.
also, extra room in ther for mods. surface mount stuff doesn't lend itself well to the average DIY'er doing mods.

For anyone following from the other forum thread, I've now got the ability to now capture the output plots from my DSA to the computer, so no more screenshots .. so any test outputs will look a lot more professional.

i want to work on capturing data points to the computer, this way i can through them into excel if i want to. it's really unnecessary, more just seeing if i can.

 

[innkeeper]

[MENTION=174130]drtechno[/MENTION]
are you able to do have the tools to do some calibrated noise measurements on your unit.
looking at the DBX286S vs the A model the noise appears to be a lot higher with processing bypassed
though, could just be this unit needing some work, as it is looks like the processor section is oscillating, so just could be related, noise leakage and all.
i defiantly see some weird stuff.

 

[drtechno]

[MENTION=174130]drtechno[/MENTION]
are you able to do have the tools to do some calibrated noise measurements on your unit.
looking at the DBX286S vs the A model the noise appears to be a lot higher with processing bypassed
though, could just be this unit needing some work, as it is looks like the processor section is oscillating, so just could be related, noise leakage and all.
i defiantly see some weird stuff.

On the A model I would look to see if you can get the +/- 15 to be exact. But the power supply caps can do that along with other things.

But I do notice that going on inside the S model on a smaller scale. I only have an o-scope but that will be good enough to look at the noise and help determine its origin.

 

[ecc83]

On the A model I would look to see if you can get the +/- 15 to be exact. But the power supply caps can do that along with other things.

But I do notice that going on inside the S model on a smaller scale. I only have an o-scope but that will be good enough to look at the noise and help determine its origin.

Not sure what you mean about the power lines but you can record the 'noise' into a PC then use something like Right Mark Analyser. Virtually any DAW will generate calibration tone/noise.

Dave.

 

[drtechno]

Not sure what you mean about the power lines but you can record the 'noise' into a PC then use something like Right Mark Analyser. Virtually any DAW will generate calibration tone/noise.

Dave.

the problem with measuring with the computer is the variance of adding a converter in the circuit will give inaccurate results. Audio converters are not electronic lab/instrumentation grade in their coupling. So you would get what is capable for that signal chain.