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.
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.
I think that this free, downloadable book will be VERY useful, for you:
ADI - Analog Dialogue | Op Amp Applications Handbook
ADI - Analog Dialogue | Op Amp Applications Handbook
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?
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?
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.
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.
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.
I dont think they were suggesting you buy the kit, just that you might look at the layout and find something worth using.
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
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
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.
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.
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Thank you very much for all responses. I will look into datasheet again.
Juma,
Thanks for the schematic. This is really what I can follow. I am not the person knowledgeable enough to comment your design, but I see you did not use the buffer and you put input capacitor in front of volume control. I went through many schematics and this is not very common design. What do you gain by implementing input cap infront of the pot?
The second question is about lack of buffer. Whenever the AD797 is mentioned use of the buffer is recommended. I think it is character of this op-amp. Should I be careful about used inteconnects and input impedance of the amplifier, which will be driven by this preamp?
The third question is about using your design also as headphone amp.
Thanks again
Tom
Juma,
Thanks for the schematic. This is really what I can follow. I am not the person knowledgeable enough to comment your design, but I see you did not use the buffer and you put input capacitor in front of volume control. I went through many schematics and this is not very common design. What do you gain by implementing input cap infront of the pot?
The second question is about lack of buffer. Whenever the AD797 is mentioned use of the buffer is recommended. I think it is character of this op-amp. Should I be careful about used inteconnects and input impedance of the amplifier, which will be driven by this preamp?
The third question is about using your design also as headphone amp.
Thanks again
Tom
That preamp was made for source that puts out some DC voltage.
IME, AD797 doesn't need a buffer (except if you want to drive headphones with it). Chip's output stage is diamond buffer (see datasheet fig. 31.).
Only if your headphone's impedance is 600 ohms or higher. Otherwise, the use of current buffer is recommended.
I'm not into cable blues (I'm fine with UTP cat. 5), but AD797 very successfully drove the next stage that had a Zin of 1k (SE2BAL convertor with opa1632).
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I was looking for headphone amplifier ideas for AD797 with BUF634 and I have found this circuit:
http://www.vegalab.ru/forum/attachment.php?attachmentid=18837&d=1171824603
Unfortunately, the site is russian. Can anyone translate the the details of the circuit?
I see no volume control, but the input has switch with R1 - R2. can it be step attenuator?
Also, there are two switches at the output: SW4 and 5. I guess they are to bypass two resistors, but what can be the purpose of them?
I also see trimpot VR1 by AD797. What is a purpose of that?
I hope someone with knowledge of Russian can help.
http://www.vegalab.ru/forum/attachment.php?attachmentid=18837&d=1171824603
Unfortunately, the site is russian. Can anyone translate the the details of the circuit?
I see no volume control, but the input has switch with R1 - R2. can it be step attenuator?
Also, there are two switches at the output: SW4 and 5. I guess they are to bypass two resistors, but what can be the purpose of them?
I also see trimpot VR1 by AD797. What is a purpose of that?
I hope someone with knowledge of Russian can help.
Hi,
s1 is the input selector switch
s2 is the ON/OFF switch
s3 is the attenuator
s4 & s5 adjust the output impedance, in steps of 47r.
Vr connects to the LTP collectors. It adjusts the output offset by changing (lowering) the collector resistor loads.
10k is just 5k (when set to middle) in parallel with the integrated collector resistors. This is quite a severe adjustment. Look at adding a pair of resistors between the pot ends and pins 1 & 5. Maybe 20k.
This gives 25k parallel existing r and allows adjustment from 20k to 30k // existing r.
Look at the decoupling. The datasheet details precisely where and what is needed.
C5 seems to be much larger than the datasheet recommends. For gain of 2 (+6db) using R1=R2=1k0 they show 20pF.
C1 & C3 must be X1 or X2 rated. All caps from Line (L and/or N) to PE must be Y1 or Y2 rated.
Scott Wurcer on this Forum designed this opamp.
I'm sure he can give better information.
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Thanks for the comment.
Is the VR1 pot necessary? is it possible to simplify this part?
Also, output of the amp with switches. I would like to get rid off any switches from signal path. The headphones which I have are all 32 - 150 ohms.
Is it possible to use one set resistor value for the output?
Is the VR1 pot necessary? is it possible to simplify this part?
Also, output of the amp with switches. I would like to get rid off any switches from signal path. The headphones which I have are all 32 - 150 ohms.
Is it possible to use one set resistor value for the output?
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