AD797 in preamp: need help - diyAudio
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Old 9th September 2010, 05:18 PM   #1
TB is offline TB  United States
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Default AD797 in preamp: need help

Hello,
I have posted this thread on chip amp forum, but it was suggested to move it here.
Few years ago I was able to audition Norh ACA preamplifier build around AD797 / BUF634. The sound of the preamp was really good. I have decided to get these opamps and try to build my own. Since I am beginner in electronics I searched the forum for available circuits, but with no avail. Instead, I have found many threads with information how difficult this chip is to work with.
Also, quality of the sound, if correctly implemented, was mentioned too. I am not able to design the basic circuit by myself. Can anyone point me to available schematic?
I did check AD797 data sheet, but only one schematic with buffer is shown there ( please look at page 16. They use AD811 buffer ):
Data sheet: http://www.analog.com/static/importe...eets/AD797.pdf
I am not sure this circuit can be used as a preamp..
What I intend to do is to build norh ACA like preamp with discrete diamond buffer or BUF634 (btw... how AD811 buffer compares to BUF634 in quality of sound? did anyone tried to use it?).

Thanks for the help.
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Old 10th September 2010, 12:01 AM   #2
Leon08 is offline Leon08  Germany
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Looks like you need to learn some basics first.
Read here Basic Op Amp Design
and here Audiologica
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Old 10th September 2010, 02:23 AM   #3
gootee is offline gootee  United States
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I think that this free, downloadable book will be VERY useful, for you:

ADI - Analog Dialogue | Op Amp Applications Handbook
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Old 10th September 2010, 04:47 AM   #4
TB is offline TB  United States
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Hello,
Thanks for the response.
Oh well...., it is not what I have asked for. I do have a pleasure to build electronic circuits, but I just do not have time to really study electronics. I like to build them just to listen to music not to experiment with them. Whenever time I have, nowdays is less and less, I try to spend on listening to music. I do have basic knowledge of electronics, but not enough to destroy my only two chips which I have.
I have noticed some members did talk about AD797 sound. I guess someone has used circuit for this purpose. Or even someone might have orginal ACA circuit since the preamp was discontinued a long time ago.
So, can anyone assist with the existing circuit or at least point to one which can be adjusted to accomodate AD797?
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Old 10th September 2010, 06:43 AM   #5
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You may have a look in eBay for item id 330468830263

Franz
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Old 10th September 2010, 03:01 PM   #6
TB is offline TB  United States
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Hello Franz, how are you?
I have seen this amp when I tried to browse the web for ACA circuit.
My junk bin has a lot of parts so spending $100 it is too much.
OK, Pavel Macura buffer. Is it possible to tune it for AD797 instead of OPA627?
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Old 10th September 2010, 03:37 PM   #7
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Opamp basics really aren't that hard to understand. For a preamp all you really need to consider is the gain you want to use. Most of the time unity (a gain of 1) will work fine, as most sources will output a 2volt rms signal, which is more then enough to drive modern amplifiers into clipping.

For a preamp you want to set a high input impedance so your source has an easy time driving it. Then you want to use an output buffer, such as the BUF634, although you might want to consider this as well, so that it can drive anything without breaking a sweat.

Any extra criteria required for stability will, most often then not, be included in the data sheet. These are mentioned in the AD797s data sheet.

Usually they concern layout - having decoupling caps placed as close to the power supply pins as possible - sometimes having the ground plane below input pin traces removed - using a tiny cap (like the AD797) in parallel with the feedback resistor to aid stability in a low gain situation (I can assure that the AD797 will oscillate if you omit this!) and decoupling large capacitors in the feedback path with a low value 10-100ohm series resistor.

Some opamps are not unity gain stable, so watch out for this - it will say so in the data sheet, usually close to the beginning.
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Old 11th September 2010, 03:24 AM   #8
qusp is offline qusp  Australia
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yes, bandwidth limiting for the 797 is probably the most important for a simple preamp layout IMO. I use 797 and stacked BUF634 for my portable headphone amp, basically same as what you would use in a preamp I would imagine except I use more gain (though on the lowest setting its just 2x). its a little tricky to get going at unity gain, but entirely possible and the low noise regulators I use it in for my dac are proof it works at lowish supply voltages (+/-6v in and 3v3 out)

I dont think they were suggesting you buy the kit, just that you might look at the layout and find something worth using.
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Old 11th September 2010, 04:57 AM   #9
gootee is offline gootee  United States
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You will want to look at Figure 6-47 (in Chapter 6), and pages 473-475, from the link I gave you.

It shows recommended connections and component values for distortion cancellation, specifically for the AD797.

If using a buffer, MAKE SURE that you also read the paragraph after Figure 6-48.

The lowest-gain case for which they provide component values is 10. But the distortion-cancellation technique is largely the same for all of the gains shown. (Interestingly, for G=10, Figure 6-48 shows THD+N at ZERO percent up until around 5 kHz and then rising to about 0.0001 percent at 20 kHz! [when the given technique is used])

You will also need to look at Figure 7-111, in Chapter 7.

Cheers,

Tom
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Old 11th September 2010, 11:56 AM   #10
juma is offline juma  Germany
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This is the preamp with AD797 I made a year ago:
Jos jedan preamp - diyaudio.rs

Gain is 6dB. Freq. range at -1dB is 3Hz to 300kHz. Phase is within +/-5 degrees from 10Hz to 100kHz. Zin=10k (value of the volume pot, can go up to 47k). Zout is about 15R (set mainly by R9 which serves to tame capacitive loads).
Sound is very exact, clean and dynamic.

Last edited by juma; 11th September 2010 at 12:10 PM.
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