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Old 24th August 2003, 04:54 PM   #1
Freddie is offline Freddie  Sweden
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Default PGA2310 output buffer?

I've just started playing with an Atmel microcontroller and a PGA2310. The PGA2310 is capable of driving loads down to 600Ohm. So a buffer on the output of PGA2310 is perhaps not really needed. But is it recommended to place a buffer there anyway? I have some 2SK170BL which I was planning to use to make some borbely buffers, is that a good solution? Perhaps I should use another buffer or maybe no buffer at all? Suggestions?

One more thing, I have two pretty nice powersupplies (discrete regulators) that I want to use. One powersupply per channel. The PGA2310 have two channels, I just want to use one. Should I ground the unused channel on the PGA2310, or just leave it?

/Freddie
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Old 24th August 2003, 05:21 PM   #2
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Good questions Freddie. You should probably let your ears decide about the buffer. Surely in most situations an external buffer will just reduce the transparency. Why, if i may ask are you using a single ic per channel? Do you reckon they will work better as dual mono? This is probably also worth auditioning. You should make sure the unused input is grounded and the volume byte for that channel set to zero.
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Old 24th August 2003, 05:41 PM   #3
Freddie is offline Freddie  Sweden
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analog_sa,

Quote:
Why, if i may ask are you using a single ic per channel? Do you reckon they will work better as dual mono?
Well, I usually think that it sounds better when I use separate powersupplies for each channel. And I have already built two powersupplies (designed by peranders). See the attached picture, the two powersupplies are on the left side of the picture.
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Old 25th August 2003, 05:31 AM   #4
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Default differential ?

Why don't you make the design differential ?!

grtz

Simon
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Old 25th August 2003, 07:36 AM   #5
Freddie is offline Freddie  Sweden
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Quote:
Why don't you make the design differential ?!
I could make it differential (balanced). But none of my sources is balanced. But perhaps I make it balanced anyway, as I already have the parts.

/Freddie
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Old 25th August 2003, 11:12 AM   #6
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Actually, this seems to be the better way to use those chips (one chip per channel). Audionet are said to do this, and their amps are getting some of the best reviews (for those who want to believe in reviews).

Most DACs are differential anyway, you just have to take the signal before the last stage which is ususally a single opamp configured to work both as difference amp and low pass. It may make sense to incorporate the filter function of this stage separately into the + and - channels.

For non-digital signals like radio, you can use an inverter to symmetrize the signal.


Using an output buffer might be a smart idea. I am a little wary of the op amp used inside the PGA. It might be fine if the load is 10 k, but below or with a long cable, I'd probably use a good op amp for a buffer.

Greetings,

Eric
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Old 25th August 2003, 11:30 AM   #7
Freddie is offline Freddie  Sweden
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Default Output buffer

Quote:
Most DACs are differential anyway, you just have to take the signal before the last stage which is ususally a single opamp configured to work both as difference amp and low pass. It may make sense to incorporate the filter function of this stage separately into the + and - channels
Yes, you're right my DAC is differential. It is easy to take out the balanced signal before it gets buffered, filtered and converted to a singel ended signal.

Quote:
For non-digital signals like radio, you can use an inverter to symmetrize the signal.
That could be nice, any suggestions for a simple but good inverter? An OP-amp configured for gain -1 perhaps?



Quote:
Using an output buffer might be a smart idea. I am a little wary of the op amp used inside the PGA. It might be fine if the load is 10 k, but below or with a long cable, I'd probably use a good op amp for a buffer.
I have some OPA627 that I could use. Otherwise a simple buffer with bipolar transistors, or maybe the borbely jfet buffer based on the white cathode follower might be good. But the jfet buffer is probably not very good driving low impedance loads.

Thanks for the suggestions

/Freddie
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Old 25th August 2003, 12:07 PM   #8
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Yes, an opamp is the best and easiest way to make an inverter. As there is no common mode voltage, even mediocre op amps will do nicely.

I have been using an OPA-627 (with a four transistor unity gain buffer INSIDE the feedback loop) in my amp for years and years. There might be better and cheaper op amps by now. I believe the AD797 (of the "old" league) might be a better choice here because
a) being a bipolar cascoded device, it might have even less common mode distortion
b) you'd be driving it from near zero virtual impedance, so its excessive current noise does not hurt


All in all, DAC op amps have to work much harder because of all the HF junk, so selection is much more critical there than for a volume control buffer.
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Old 25th August 2003, 02:45 PM   #9
Freddie is offline Freddie  Sweden
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Default BJT Buffer 1

A discrete buffer after the PGA2310 seems like a good idea. Here are some different buffers. What do you think about the buffers, which one is preffered?

/Freddie
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File Type: jpg buffer1.jpg (24.5 KB, 1077 views)
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Old 25th August 2003, 02:47 PM   #10
Freddie is offline Freddie  Sweden
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Default BJT Buffer 2

Here's another one..
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File Type: gif buffer2.gif (17.8 KB, 1051 views)
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