Modding the DEQ2496 - bit of help?

[Tenson]

Hi,

I am trying to mod my DEQ2496. I am going to build some circuits for separately regulated 5V lines. ..at least I will do when the other components arrive tomorrow. If I use S&B transformers for the input and outputs then 5V is all I need I believe.

So, presuming I get the power-supply up and running, I need to connect that to the DEQ board! This is where I become a bit stuck.

I am very new to electronics so it is very hard for me to trace the circuit. I plan on having a separate regulated line for DAC, ADC, clock and input receiver (AK4114?) then running the display/controls and the DSP from another power supply to separate out the dirty stuff.

What I am not sure of is where to physically connect the power lines to!

I am thinking of connecting directly to the DAC, ADC and input receiver chips for the clean supply (not sure about the clock, is it a single component that needs power?) and cutting the supply from the board, but how can I run all the other bits off the dirty supply? Would it be possible to pull out one of the standard regulators and just put in the regulated 5V line in its place? If so, where should the line go?

Thanks for any help! Please remember I am new to all this so very clear instructions please

[Tenson]

To follow up on my last message, can someone point on here where to 'hack in' the new 5V line, please?

Thanks

[Tenson]

Okay scrap all that lol!! (sorry)

I just had a look at the SHARK processor datasheet and it says it needs 3.3V so I can't run completely off 5V.

Thus my new plan...

The DEQ has the following voltages from the PSU before the regulators on the DEQ board.

1 Red = +15V
2 = -15V
3 = 9V
4 = analogue ground
5 = digital ground
6 = 5V
7 = 3V

Now as I said before I am having a completely separate PSU for the clean components and the dirty ones.

Because of this I was thinking of connecting the 'dirty' PSU, after three separate pre-reg’s to get those voltages (9V, 5V, 3V), to the standard DEQ regulators to power the DSP, Display and uP. Do you think this would work?

I would then connect the new 'clean' supply directly to the chips for DAC, ADC, Input Receiver and clock.

I think this would pretty much separate the DSP and display as I think the DSP is the only 3.3V component left after separately powering the other bits from the clean line.

What are the voltages listed there actually for? 9V gets regulated to 5V? 5V gets regulated to 3.3V? Whats is the 3V for? I know the +/-15V is for the op-amps which I am replacing with S&B transformers.

[Tenson]

Hello again!

Alright, I made a 5V regulated supply with an LM7805. It worked perfectly. I then sniped the DEQ's leads from the PSU that supply the ground and 5V and connected my 5V supply instead.

It doesn't work!

What am I doing wrong? Surely replacing 5V with 5V should work?!

I tried connecting their ground back on and it reacted differently but still didn't work. Tried just their ground and my 5V, still no luck.

Please advise!

I have attached a picture of the leads I snipped and you can just about see the new 5V supply.

[Tenson]

Okay I tried connecting it to the Analogue ground and the 5V line. It booted up but after about 30seconds its relay (the one that clicks on power up) started flickering and the screen went blank. Closer, but no cigar!

[RobHolt]

Measure the voltage under load just in case the relay is causing the supply to sag. Seems odd that it runs for 30 secs.

[martin clark]

HI Simon,

Starting form the very basic: identify +ve and 0v rails for the chips you plan to supply anew - find the supply pins, and follow the circuit backwards until you get to a voltage regulator or raw supply. Feed in +/0v pairs to match. Well, that's it in principle..the 'match' can be incredibly specific:

Because of this I was thinking of connecting the 'dirty' PSU, after three separate pre-reg’s to get those voltages (9V, 5V, 3V), to the standard DEQ regulators to power the DSP, Display and uP. Do you think this would work?

Should do, though check you get the appropriate pairs of + and 0 connected, and no new 0v loops. With any new suppplies you add you'll need to ensure that the appropriate return/0v connection is made (analogue and digital ground will connect at one place, and one place only - that's how it should be done anyway...) Final point (and sorry if it's telling you how to suck eggs) don't forget about drop-out voltages. If you're using a new stabilised PSU to feed existing regs then the regulated input to a Vreg needs to be about 3v higher than the output voltage you want. If you have a 5v supply feeding a 5v regulator for instance, you will get between maybe only 3.7v out - what you see dropped 'across' the reg is simply the saturated pass transistor voltage(s), and the output will be simply the input minus these. Not enough voltage headroom to power -up the affected circuit properly. The input nees to be 8.0v+ to get the outcome you want.

Assuming you've got all this correct, then what comes to mind is PSU phasing. It is highly likely the PSU rails need to start-up in a predictable manner/order, and track each other fairly closely until the rails stabilise. For example, it's a common requirement for a bitstream DAC's digital and analogue supply pins (typically +5v both) to need to stay within 0.6v of each other during start-up until stabilised. If these voltages are both the same, from the same raw supply ( ie transformer) it's trivial - connect a pair of diodes in anti-parallel between the rails, so that the higher pulls-up the lower. Once within one diode voltage drop (~0.7v) the rails subsequnetly act as separately-regulated. If this start-up condition is not fulfilled (eg because you've added a huge amount of capacitance to the analogue rail, or it's on a different transformer switched separately) then the DAC can 'lock-up' until next started-up correctly. Read those datasheets very, very carefully...

One final thing: it's typical that the clock PSU rail has to start very early in the sequence- the clock signal needs to be establihed early so that all dependant parts lock to it as they start up.

Hope this helps - Rob Holt has my phone no. if you want to talk any of it through, though I have no DEQ experience(!)

Cheers,
Martin Clark

[martin clark]

One more thing from the pic - if that's a 7805 reg with that axial film cap on the utput, swap it out for a cheapo electrolytic..anything... until you've got things working - the film cap will provoke oscillation of the reg which may not be helping.

[Tenson]

Thanks Martin,

At the moment I am just connecting the new 5V supply to the DEQ's pins where there was 5.3V before, that went on to be regulated (I guess down to 3.3V). So that should have been okay.

I guess that just leaves the PSU phasing. As my new 5V supply is from a physically different transformer from the other 15V, 9V and 3V supplies, this is not easy to fix.

I may have a toy around with it but will probably leave it for now until I have the fully built up supply with all the other voltages the DEQ needs. Then fixing phasing would be a matter of adding the diodes as you say.

Thanks for your help, as I said I am very new to all this so it is appreciated!

Oh, what is a good component for regulating from 12V to 3V? I can do the 9V and 5V with LM7809 and 05 but not sure about the 3V?

Ahh the axial cap.. okay that could be it then. It is sable for a bit then goes into ossilation?

[Tenson]

Ahh now interesting...

I swapped out the film cap for a electroliting and it did just the same BUT like Rob suggested I measured the votage under load this time.

It was slowly getting lower and lower! It started at 4.99V then bit by bit dropped down to 4.85V and I turned it off there as obviously it was going to continue dropping.

Why is it doing that?!

The supply I am using is a 50VA 12V torroidal with a 6A bridge rect then a 10,000uF cap and a LM7805 with a cap across the output. Surely it doesn’t need a bigger cap?