Digitally controlled preamp/headphone amp

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DcibeL

DcibeL

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Joined 2003
drive.google.com/drive/folders/1KDyECkAIVAuNEtKVobKmsfvtFczGqOQw?usp=share_link
Hi Russ,
I have the Kookaburra preamp sitting in front of me, and I am wondering why an output buffer is even nessicary? The datasheet for the PGA2311 shows that it has an output buffer built in, and short-circuit output current for that buffer is 50mA, while the short-circuit output current of the OPA2227 is 45mA.

I obtained the output buffer data for the PGA2311 on page 2 of the datasheet, very bottom.
 
Russ White

Russ White

Member
Joined 2005
twistedpearaudio.com
DcibeL said:
Hi Russ,
I have the Kookaburra preamp sitting in front of me, and I am wondering why an output buffer is even nessicary? The datasheet for the PGA2311 shows that it has an output buffer built in, and short-circuit output current for that buffer is 50mA, while the short-circuit output current of the OPA2227 is 45mA.

I obtained the output buffer data for the PGA2311 on page 2 of the datasheet, very bottom.
Click to expand...


Yes it is a very good qustion, but the design choice is deliberate. The reason is the the opamps(buffers) in the PGA2311 quite simply sound aweful driving low impedance loads such as headphones, while it is true they can supply the necessary current, the THD(measured) goes through the roof.

The solution was to allow the PGA2311 to drive a high impedance to keep the THD very good, and use an opamp with greater rail split (+/-12V) to get better performance out of the actual buffer(opa2227 or whatever unity gain stable opamp you choose).

In any case I recognise that some people may never drive a very low impedances(like headphones) so in the kookabura 1.1 (just arrived). I have places to jumper either or both buffers. :)

Give it a try driving headphones with just the PGA2311, I think you will agree it sounds pretty pathetic. :)

Cheers!
Russ
 
troystg

troystg

Member
Joined 2004
I built my Kook Saturday and put it in the case with my Rev_C.

It is ready to go except the power. I need an 8 - 10 position term strip as the 6 position just didn't have enough room to hook up the mains, switch and secondaries to the amps and Kook.

I hope to run to the store tomorrow and get one. The Kook and Rev_C should be up and running again by weeks end..

As a refresher, it was originally a Rev_C (2 x 160VA x-formers) with a passive Noble 50K pot.

It is now a Rev_C (single 400VA x-former) and active Kook.
 
DcibeL

DcibeL

Member
Joined 2003
drive.google.com/drive/folders/1KDyECkAIVAuNEtKVobKmsfvtFczGqOQw?usp=share_link
Russ White said:
the opamps(buffers) in the PGA2311 quite simply sound aweful driving low impedance loads such as headphones, while it is true they can supply the necessary current, the THD(measured) goes through the roof.
Click to expand...
Thanks for the explanation. I had already installed the OPA2227 and was seriously thinking about taking it out.
BrianDonegan said:
Also, the DS says 50ma for the PGA2311 (+-25ma), and +-45ma for the OPA2227 (90ma). Strange that they change the way it is written, both from TI.
Click to expand...
I can't say that I agree with you here. I assumed that the +-45mA in the OPA2227 datasheet meant that the current limit is the same whether it is sourcing or sinking. I don't believe that you can add +45mA and -45mA together to get a total short-circuit current of 90mA. The op-amp won't be sourcing and sinking at the same time, so at any given time the maximum current out is 45mA.

I assumed this also with the PGA2311 that 50mA would be either sourcing or sinking the current. I've soldered the chips in the board already, but if you have a couple lying around it's an easy measurement to verify the datasheet information.

I look forward to finishing the kookaburra preamp, as I will be able to do a direct comparison to the Freebird.
 
anbello

anbello

Member
Joined 2004
hifilab.it
noise on vol change

Hi,
i built my kookaburra last night, i premise that i have used the PCB + preprog. PIC kit + PGA2311PA + OPA2227UA + components i already had (not so good as those shipped with the complete kit but new and of the right value only pot is 50K lin). The pre is working ok and sound very well but i have the following problem:
when the source (cheap pc card) is in pause and i go up and down with the volume near the maximum (from 3/4 to max) i hear the noise of the source in background (similar to pink noise - bad pc card i know) together with a buzz correlated to the movement of the potentiometer. I think this is due to the zero crossing detection function of the PGA not working properly in presence of noise but the really strange thing is that i have this buzz only on one channel.
What do you think about this problem?
Let me know if you need more info.

Thanks.
 
Russ White

Russ White

Member
Joined 2005
twistedpearaudio.com
Anbello, it sound like the input is floating at some point when the music is in pause. Try changing volume with the input shorted to ground and it should be close to silent. The opamps and PGA are very sensitive, and when he inputs are floating it will pick up any little RF/EMI including the PIC itself because the input wires and traces will be acting like antenae.

Edit, the foating input noise is particularly noticable if you are not using the input buffer and the 100K resistors to GND. The sound you are hearing is the PIC sending serial data to the PGA as you change the volume.
 
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