DBX FS900 Power supply

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thomasdf

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Hi all !

I am in the process of racking a pair of DBX902 DeEssers. The original FS900 is hard to come by here in France, and racking is fun and will be cheaper, so that's what I'll do, I guess.
The power supply I need is regulated +/- 15V and unregulated 24V.

I am having a hard time finding at single power transformer that would give me +/- 15V and +/-24V so I was looking at the original FS900 power supply schematics (joined here)
Do you thing it is a good idea to clone it ? If yes, here comes my questions :

- what power transformer should I use ? I cannot figure this out. My only clue is that the regulators accept max 60v at their input, and they're located between a diode so I'd say the main PSU output can't be over 40 to 50v. I'd bet 30. But is there a way to know ?

- Can you guys help identify the semi conductors : TIP128, TIP125, MPS8590 (I'd need to find an suitable replacement, it looks like it hard to find), Q2 : mistery

If no, I'll probably stick to my idea of making two generic PSUs, and I'll probably use two transformers.

Thanks a lot in advance,

thomas
 

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Newmarket

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That design looks a bit 'light' around the LM317/337 to me. Look to design in the 'usual' noise reducing capacitor Cadj and protection diodes. See datasheet for details.
 

Lerok

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Are you looking to go linear for the PSU? I'm not familiar with the french market at all, but I imagine a SMPS could be fairly easy to implement...

Side note, I have a few 902s in a full 900 rack, and I must say, best de-esser I've ever used :)
 

thomasdf

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Thanks for your answers !

I'd like to go linear indeed. I'm not super familiar with switching power supplies... I have many components at home and an access to a CNC machine to etch my PCBs.

I'll think I'll go with something like this, with 7815 and 7915 of course. I've sourced small and cheap power transformer that I can fit on the PCB. They are not toroidal so I hope the magnetic radiation will be OK, otherwise I'll build some shield around it, or I'll figure something out.
Should I add diodes around the regulators?

PS: and yes, I've always been stunned by the sound of 902s so I got myself a pair of them, that I need to rack and wire now.
 

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Lerok

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That looks solid, very similar to something I built to drive preamps - my only thought would be upping the capacitance on C1 and C2 to account for however many amps your transformer is outputting - I forget who, but someone around here recommended around 2000uF/amp - same with the capacitors after the regulators, just depends on how much current the 902s draw.
 

Newmarket

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thomasdf said:
I've sourced small and cheap power transformer that I can fit on the PCB. They are not toroidal so I hope the magnetic radiation will be OK, otherwise I'll build some shield around it, or I'll figure something out.
Should I add diodes around the regulators?

Bear in mind that for mains transformers an effective shield would ideally be made from a suitable grade of mumetal (£££) and be handled appropriately. Or you would need to use steel at a minimum. Geometry/spacing may be of more use for H field.
Diodes ? - yes - cost pennies and could save your regulators in case of 'reverse discharge'.
 

abbey road d enfer

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If I were you, I would use a pair of 24V Meanwell smps and LM317/337 for the regulated rails. That would take care of the magnetic radiation issue.
The 24V rails would need additional CLC filtering. See attachment.
I have PCB's for that. PM me if interested.
 

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thomasdf

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The thing is I don't need regulated +/-24V so I'm not super worried about that part. I need a steady and clean +/- 15V but I need only 120mA so I don't think its necessary to bring out the big (and expensive) guns.

Thanks for your input, though
 

Lerok

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I think what abbey road d enfer was suggesting was using a pair of 24v meanwell SMPS, and taking the +24v unregulated off of them, then using the LM317/337 to step down from 24v to +/- 15v. I'm not sure how available meanwell smps are in France, but assuming you can get your hands on them, this is probably the cheapest/easiest option without loss in quality.
 

mjrippe

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thomasdf said:
The thing is I don't need regulated +/-24V so I'm not super worried about that part. I need a steady and clean +/- 15V but I need only 120mA so I don't think its necessary to bring out the big (and expensive) guns.

Thanks for your input, though

Having the 24v UNregulated is not a requirement either.  By starting with the Meanwell supplies (not very expensive) you get 24v reg and an easy way to generate the 15v reg.

EDIT: Basically, what Lerok said!
 

emrr

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I like abbey’s plan too. Those supplies are as cheap as you can do anything, and noise will be vanishingly low.  I used their +/-15 for some 2520 preamps and the noise measurements mainly show external environmental spikes, stuff you’ll never see under typical power transformer spikes. 
 

abbey road d enfer

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thomasdf said:
The thing is I don't need regulated +/-24V so I'm not super worried about that part. I need a steady and clean +/- 15V but I need only 120mA so I don't think its necessary to bring out the big (and expensive) guns.

Thanks for your input, though
I believe you don't understand what I meant. In my scheme the LM317/337 are for dropping the +/-24V rails to +/-15. You get relatively clean +/-24V in addition for less than the cost of a toroidal xfmr. These Meanwell smps cost about 12€.
 

Audio1Man

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I find that a few things about the AKG power supply should be added for improved reliability. And if some other unseen thing happens you don’t have to replace so many parts.
1. Add BASE resistor to Q2 & Q4 like 1k. This will limit the EB current flow upon a short if R3 or R6 opens.
2. Add reverse diodes across VR1 & VR2. This will protect VR1 & VR2 from becoming reversed bias upon power turn OFF.
3. Add Capacitor from ADJUST to GND on VR1 & VR2. This may improve any HUM or NOISE floor.
 

abbey road d enfer

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I think the whole schematic should be reconsidered in view of current resources and decent design rules. The "AKG" PSU could not have been designed by a real AKG engineer (nor by any dbx engineer). Those were troubled times, with the earthquake and the culture shock.
 

thomasdf

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I'll add that 110 to 230 VAC input seems like a dream for post covid international touring and recording !  8)
 

thomasdf

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Hello everyone

First of all, I wanted to thanks Abbey Road d enfer because the PSUs are perfect !

Also, I have something weird with the 902s, and I don't know what to blame. They seem to work OK, but the LEDS are on when it passes no audio. They'll show a certain amount of dB GR depending on the range knob. When the unit passes audio they'll just blink randomly on and off. (Being mostly off when applying GR, it seems)
I am wondering if this has to do with a bad power hookup, or if both of my units have the same problem and need some care. I have double checked the wiring and tensions and everything seems fine.
My only twist is linking pin 3 (chassis ground / earth) to pin 4 (circuit ground)

I am driving the units with impedance balanced audio, and when I tried "true" transformer balanced, nothing changes.

Schematics are available here : https://coreyeng.tripod.com/dbx1.htm

Any help / pointers welcome :) Have a good day !
 

abbey road d enfer

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thomasdf said:
They seem to work OK, but the LEDS are on when it passes no audio. They'll show a certain amount of dB GR depending on the range knob. When the unit passes audio they'll just blink randomly on and off. (Being mostly off when applying GR, it seems)
I am wondering if this has to do with a bad power hookup, or if both of my units have the same problem and need some care. They double checked the wiring and tensions and everything seems fine.
The LED board needs the +24V rail to work.
 

thomasdf

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+24 and -24 are spot on... -24 on pin 1, 24 on pin 2

The GR led turn off (normal display at rest) when nothing is plugged in the input.

???
 

abbey road d enfer

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thomasdf said:
+24 and -24 are spot on... -24 on pin 1, 24 on pin 2

The GR led turn off (normal display at rest) when nothing is plugged in the input.

???
Didn't you write "the LEDS are on when it passes no audio. " ? I'm confused...
 
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