Saturday, April 27, 2013

PGA2311 based (remote) volume control


There was a question on the forum about a remote controlled volume control, the intention was to place it between a Quad 33 and 303. At that stage we could not help, so we advised to look at a kit or a complete unit based on a PGA2311 chip. On eBay there are plenty of companies who can deliver such a box or kit in various forms. I also bought such a system as a kit, more or less a kit, the electronic modules are already assembled. After a few weeks the kit was delivered. I had to ad a transformer and some mechanical fixings to get the kit operational. Some first remarks, there was no manual provided, so this kit is not for absolute beginners. Secondly there was no provisioning for a fuse holder. I measured the unit, it performed well. It sounded just allright. I found a schematic on the web. I checked the schematic with the unit under test, the schematic matched my unit. I also downloaded the datasheet of the PGA2311. The first problem was there already, the unit was DC in and DC out on the signal in and outputs, for my own equipment I find AC in and DC out acceptable, but for a commercial and/or professional product, AC in and AC out is the norm. Also they added 10K resistors in parallel to the input, probably they thought the input was connected via a buffer stage, but the input of the PGA2311 behaves like a 10K resistor already. In this way they effectively reduced the input impedance to a lowish 5K. This (can) give serious frequency response errors when driving from an AC coupled high impedance source preamp. According to the datasheet the PGA2311 input must be driven from a low impedance source, our friends put 220 Ohm in series with the input, with higher distortion as a result. The 220 Ohm resistors make sense, if you complement them with small caps to form a RF/EMC filter, but without those, you only add distortion. Also they 'forgot' the decoupling capacitors which must be placed as close as possible to all the power supply pins of the PGA2311. In the output they put 330 Ohm resistors, in it self a good thing for stability when driving real cables and inputs of power amps, but 150 Ohm is a better value or compromise if you wish.

So I replaced the 10K input resistors with 220K bleeder resistors. The 220 Ohm series resistors in the input circuit where replaced by 220uF bipolar capacitors, this might look as overkill, but to keep the distortion of the capacitors low, you have to reduce the AC voltage across the cap. And I put all the decoupling capacitors in place, according to the datasheet. The resistors in the output where replaced by 150 Ohm. The unit measured and performed ok, this unit was programmed for 1dB steps, which worked very good. Also you could select three sources. Which makes it a preamp. The only thing which wasn't there was a balance control.

A general remark about the PGA2311 chip, it will only give very low distortion when it is used with high level signals, high levels compared with the Quad standard of 500mV for their power amp inputs and pre amp outputs.

So to get minimum distortion, the levels between the Quad equipment must be raised. So the output of the preamp must increase, but more importantly the input sensitivity of the power amp must be set lower. The PGA2311 based unit performs at his best when the 0dB setting means full living room volume.

Then the last check. I bought a PGA2311 PA chip from our regular supplier and placed it in the unit...........The distortion and noise went slightly up! So the chip supplied was a real PGA2311 PA. Very good indeed!

 

Good things:

  • Very good casing and mechanical work.
  • Electronic modules were perfectly built.
  • Price quality ratio.
  • Remote controled.

Improvements:

  • Ad some form of manual, real or downloadable.
  • Follow the advice in the datasheet from the manufacturer about decoupling.
  • Redesign the input and output circuit to make it a real world preamp.

So with some work, you can improve this kit or unit from good to very good!

 Ps, the display is blue, I changed it with a red filter!

Joost Plugge

DaDa Electronics

3 Comments:

Anonymous Alton said...

This is cool!

7:13 AM  
Blogger Johan said...

Hi! I have this preamp as well. I haven't modified it yet but it have performed well for about a year. The only thing is the volume encoder skips steps unless turned real slow which means to lower volume with an audible difference, i might have to turn several full rotations.
I was wondering if you have the same issues with yours?
Mine also has pretty bad IR reception...

11:12 AM  
Blogger Stefaan said...

No,it works ok. The IR recption can be improved by enlarging the hole in the front and place the IR reciever in the front, instead of behind the front.

Joost

11:36 AM  

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