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Old 24th February 2008, 05:03 PM   #1
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Default DEQ2496 consumer level analog board

Hi Folk!

I started to design a small "consumer level" analog board for the Behringer DEQ2496. I need some advice to go further on this project.

The base idea is to build a small pcb to be connected directly on the HE10 connector on the DEQ2496 DSP board. A second HE10 connector permit to connect the cable going to the original input/output board. Most of the original signals pass true the pcb except the input and output analog signals for main left and right.

Click the image to open in full size.

I plan to use DRV134 and INA134 chips from Texas or one of their equivalent from THAT.

Beta schematics

My questions are the following:

- I'm loose with signal level and not sure what is the right chip to use for input and output (0dB, +3dB or 6dB version). Note that on the DEQ DSP board, the ADC connections are not direct and this circuit can not be removed. I inserted the circuit in my schematic.

- Do I really need to have a LFE circuit at AK4393 DAC output? I've read in the datasheet that the AK4393 already have an internal LFE circuit.

- Are the coupling capacitors well placed in my current beta design?

One important point: I want to stay with a minimum solution with the minimum possible quantity of components.

If you want to help, thank a lot.

Stef...
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Old 25th February 2008, 09:17 AM   #2
Pulse-R is offline Pulse-R  Australia
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Hi Stef,
I'm not saying you're wrong, but the recommendations I have seen is for the decoupling caps on the op amps were always straight across the +15 to -15, without the gnd. connections.

Also, I think the caps on the analog inputs (47uF) wouldn't be needed unless there was excessive DC offset from the DRV134. Perhaps a 1 or 2 uF poly or bipolar would be better only if it's needed?

otherwise, I'd like to see a bit of gain in the output stage - I know the DCX2496 direct output voltage is a bit lower than I like.

just my thoughts. looks like a good idea though.
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Old 25th February 2008, 09:51 AM   #3
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Quote:
Originally posted by Pulse-R
just my thoughts. looks like a good idea though.
Some of your comments are already in beta3 schematic. Output is now +6dB. The PSU is far from the THAT chips, the 100nF/10uF couple is better right to the THAT pins.

DEQ-miniAnalog beta3

I still need to solve the lowpass filter problem. Still not sure for the input gain.

The board will be vertical if possible. Better for heat.

Stef...
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Old 25th February 2008, 11:02 AM   #4
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another suggestion, can you run the inputs as more balanced, such as with + in to THAT +in (J1-1 to IC1.4, and J1-2 to IC1.3), and and say 10pf from IC1.3 to gnd? not sure if that's good, but would allow some lower noise/lower offset perhaps by lifting the input gnd. to cable gnd?

with the decoupling, what I thought was THAT cap pin 6 to 5 with 10uF low ESR? not sure if you knew what I meant.

input gain of standard analog I think is -12dB, same as DCX2496 input.
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Old 25th February 2008, 12:38 PM   #5
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Stephan,

The ADC needs the signal centered on 2.5VDC. So you need to offset the outputs of the 1646 to be at 2.5V and indeed you don't need (don't WANT) the output coupling caps.

That is why in my DCX output board I used the OPA1632 for the analog input channels. This chip is specifically designed to drive balanced ADC's and has a pin for offsetting the output DC level.

You put 2.5V (from a simple reference) on this pin and you are done. I think you can do it with the non-inverting input pin of the 1646 but depending on the gain the reference will need to be lower than 2.5V of course.

Jan Didden
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Old 25th February 2008, 02:29 PM   #6
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HI Jan!

Thanks for your help.

I was not aware of the OPA1632. Pretty nice chip. The difficulty with it seems that I don't have a direct access to AINL+/- (resistor deviser present on the dsp board), Vcom (2.5v) is not present on the X1 connector too, its an SMD chip and it require many more external components. But the chip is really great and also have an Enable pin.

Currently, I don't know how to use it in an easy way with the DEQ but if you have some idea, I'm not sticked on the THAT1646...

Stef...



Quote:
Originally posted by janneman
Stephan,

The ADC needs the signal centered on 2.5VDC. So you need to offset the outputs of the 1646 to be at 2.5V and indeed you don't need (don't WANT) the output coupling caps.

That is why in my DCX output board I used the OPA1632 for the analog input channels. This chip is specifically designed to drive balanced ADC's and has a pin for offsetting the output DC level.

You put 2.5V (from a simple reference) on this pin and you are done. I think you can do it with the non-inverting input pin of the 1646 but depending on the gain the reference will need to be lower than 2.5V of course.

Jan Didden
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Old 25th February 2008, 03:20 PM   #7
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Quote:
Originally posted by stef1777
HI Jan!

Thanks for your help.

I was not aware of the OPA1632. Pretty nice chip. The difficulty with it seems that I don't have a direct access to AINL+/- (resistor deviser present on the dsp board), Vcom (2.5v) is not present on the X1 connector too, its an SMD chip and it require many more external components. But the chip is really great and also have an Enable pin.

Currently, I don't know how to use it in an easy way with the DEQ but if you have some idea, I'm not sticked on the THAT1646...

Stef...




Ahh! Yes, but the resistive divider on the input of the ADC does NOT divide the 2.5V. The 2.5V is present on both outputs so does stay the same. Seems strange, but observe: you have three resistors in series. At each end, there is 2.5V. So, at the junctions, you can not have anything other than 2.5V also. It is only the analog signal that is divided but that can be taken care of with some extra gain in the 1632. You would need that gain with an other solution anyway.

I use the 1632 in my DCX mod, and for the 2.5V I use an LM285Z-G fed from the pos supply. It is a precision 2.5V ref.

I can just manage to solder the 1632; any smaller things like Frank Oettle's digital DCX mod I have to pass .
Indeed, you would need a couple of resistors externally, but it's a choice to make.

Jan Didden
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Old 25th February 2008, 04:21 PM   #8
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SOIC-8 is not a problem to solder. Space is counted on the board (something as 30x80mm). In fact, it's better.

Ok, if I understand, the resistive divider on the DSP board is not a problem. I just need to adjust the gain on the 1632.

I also have +5v (digital) on the pin 40 of the J2 connector. I can may be use it to produce the 2.5V?

I also have something as an analog mute signal on the pin 28 of of the J2 connector. May be it can be use to fit 1632's Enable pins? Is this interesting to ovoid "ploc" at startup?

On Figure 1 of the 1632 datasheet, I can see two 1nF (C1 and C2). Do I really need them?

I will design a new circuit later using the 1632 to see.


Quote:
Originally posted by janneman


Ahh! Yes, but the resistive divider on the input of the ADC does NOT divide the 2.5V. The 2.5V is present on both outputs so does stay the same. Seems strange, but observe: you have three resistors in series. At each end, there is 2.5V. So, at the junctions, you can not have anything other than 2.5V also. It is only the analog signal that is divided but that can be taken care of with some extra gain in the 1632. You would need that gain with an other solution anyway.

I use the 1632 in my DCX mod, and for the 2.5V I use an LM285Z-G fed from the pos supply. It is a precision 2.5V ref.

I can just manage to solder the 1632; any smaller things like Frank Oettle's digital DCX mod I have to pass .
Indeed, you would need a couple of resistors externally, but it's a choice to make.

Jan Didden
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Old 25th February 2008, 04:56 PM   #9
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The caps are used to isolate cap loads but in the DEQ you have large enough series resistors of 470 ohms IIRC, so you don't need them.

Yes you can make the 2.5V from the 5V without any problems; it iss not critical. Even like 2.7 or 2.3 will work OK but your max output level is max with 2.5V. Or you can use a LED if you select one for close to 2.5V.

But if you use the 5V make sure you decouple the 2.5V to the 1632 with an R-C to get rid of any supply hum & noise.

Jan Didden
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Old 25th February 2008, 05:13 PM   #10
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Quote:
Originally posted by janneman
Yes you can make the 2.5V from the 5V without any problems; it iss not critical. Even like 2.7 or 2.3 will work OK but your max output level is max with 2.5V. Or you can use a LED if you select one for close to 2.5V.
Will use +15v to produce the 2.5v. The +5v available on the connector is the digital's one.

Do I really need the 1K resistors at input? Still trying to reduce parts... I may be have to add a caps at input to protect all this stuff.

I've read in the 1632 datasheet that the 1632 input is very sensitive to load inpedance. It need low inpedance input. Can be a problem with tube preamp?

They recommand 0.25 gain value in the datasheet? Seems low?

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