Need enlightments on GainClone input impedance and volume control

[Elkaid]

I'm currently trying to build a digitally controlled input selector and volume control using BB/TI PGA2310 IC. For the digital part, no problems. For the analog part, that's another story.

In the datasheet, it is mentionned that the PGA2310 can drive load down to 600ohms...
1. OK but... what is the input impedance of a gainclone ? What will happen if one day I switch to a paralleled or bridged gainclone design ?

2. How can I calculate it ?

3. Do I need a buffer now ? Will I need a buffer later ?

4. Is there any drawbacks when using a buffer when not necessarly needing it ?

5. What about NE5532 or DRV134 ? One better over the other ?

If that makes any difference, I'll be using Non-Inverted topology.

Thanks for your help !

[analog_sa]

In the datasheet, it is mentionned that the PGA2310 can drive load down to 600ohms...

In that case you needn't worry about driving any number of chipamps whether paralleled or bridged.

[tlmadsen]

Elkaid

Sounds good with that PGA2310. Please show us when you are done.

I will try to anwer you question.

1. The input impedance for a non-inverted op-amp is in the mega-ohm range. This basically means that the input-impedance is equal to the resistor that you put between the Vin+ and ground of your op-amp. Typically from 10Kohm up to 100 Kohm.
Even paralled or bridged chips-amps normally don't go belowe 10Kohm.

2. See A to 1.

3. No, you don't neet a buffer now and you will most likely not need a buffer later. If your PGA2310 is only going to drive normal impedance op-amps you are fine. If you are thinking about driving extremly long cabels you might want to think about a buffer, but we are talking seriously long cabels. If you think about drive headphones you need a buffer.

4. Depents very much on the buffer you use, but if I understand you thinking behind the question, then no. Use something like the BUF634 and you will be fine.

5. Ups. NE5532 is a normal dual op-amp and are not much more "buffer" than you already have in your PGA2310. It might be better in very low loads, but in normal use it would not imporve anything to put one after your PGA2310.
The DRV134 is a balanced drive. You only need that if your power-amp have a balanced input. Do you have that ??

Hoep this have been to some help otherwise write again on- or offline.

Have fun

THomas

[Elkaid]

Thank you for taking time to answer my questions !

I should be able to go on for a while

I'll keep you informed the progress of the project

Have an nice day !

[SuppersReady]

Hi Elkaid,

elektor electronics magazine, April 2004, has a preamp project based around the PGA chip. It may be of some assistance.

Regards

Paul

[Elkaid]

Thanks

I'm eagerly waiting for it.

However, we're getting the magazine a bit late here in Canada (still waiting for the march issue...)

Have a nice day !

[Elkaid]

Uhhh... sorry, a little mistake...

I've mixed up source and output... Instead of being able to drive 600 ohms load, the PGA2310 source must be LESS than 600 ohms so... What is the typical source impedance of a CD player or a computer sound card by example ?

Thank you !

Here's a little picture of my progress :

Chip on the left is MAX232 (firmware upload)
Center, PIC16F877A
Right PGA2310

Provided on the PCB : Input for a 2bits encoder and output for an LCD display and 4 outputs to drive a ULN2803 itself driving 4 relays on a separate PCB for input selection.

[tlmadsen]

Elkaid

You are confusing yourself !!

Your preamp need a low output impedance otherwise the output impedance will make a low-pass filter together with the capacitance of your cable. You already have a low output impedance with your PGA2310 otherwise it would not be able to drive the 600 ohm.
For input impedance you should have something like 10-100 kohm. Your PGA2310 have 10 kohm so you are also OK here. Alsmost all sources; CD-players, soundcard and so on should not have any problem driving your 10 kohm input impedance.

[Elkaid]

Thanks !

Here's some lines from the datasheet

Using high performance operational amplifier stages internal to the PGA2310 yields low noise and distortion, while providing the capability to drive 600ohms loads directly without buffering.

And the quote at the source of my questions :

It is important to drive the PGA2310 with a low source impedance. If a source impedance of greater than 600ohms is used, the distortion performance of the PGA2310 will begin to degrade.

I see that the datasheet mentions that the PGA2310 has an input impedance of 10kohms but the previous quote is mixing me up...

Is that an error or I'm simply confused ?

By the way, what about my layout ?

Thanks for your help !

[tlmadsen]

Elkaid

Your source imoedance is typically from around 50 a couple of 100 ohms, so you will be fine.

Your PCB looks good but without a schematic is is hard to tell.......

What is the MAX-IC doing ??

THomas