• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

AD797 Phono Stage Build and Help Desk Thread

jkeaft said:
I'm stuffing the new Sachin board and it appears to have 10uf 50V caps connected between the V+ and V- rails, one at each opamp. Is that correct? Never seen this...can anyone answer why? Is there a schematic that shows these...maybe I'm wrong.
Click to expand...
Some opamps have dominant pole compensation where the caps are effectively to the supplies, or have internal feedforward circuits that are essentially supply referenced. TI and others recommend the use of low impedance AC paths (large shunt caps) between the supplies so that these circuits work properly.
In addition, the presence of significant loads on the outputs of the opamps together with the limited near class A region of the output stage can cause the current waveform into the supplies to be somewhat rectified, causing even order harmonics to appear on the supplies and resulting in even order harmonic distortion at the output of the opamp due to the finite impedance at the supplies and the finite power supply rejection of the amp. Having a cap between the supplies helps in the rejection of this effect and can improve the even order harmonic performance of the design.
 
Hi, No idea, he was trying to sell it with the pair od AD811 loaded in the phono. I just enquired why he had not used AD797 & he answer was he was using it for MM.

Cheers
 
tabarddn51 said:
Hi, No idea, he was trying to sell it with the pair od AD811 loaded in the phono. I just enquired why he had not used AD797 & he answer was he was using it for MM.

Cheers
Click to expand...
AD797s are also pretty poor for MMs because the input current noise together with the cartridge impedance makes the noise high, and the offset also high.
There is a better way...
 
Hi there,

Old chap starting all over again !
I just ordered the 3rd set of Hypnotad's AD797 Board and the CNC, the latest board version from Sachin. I was quite happy with the first sets using OPAmps like AD797, LT1115 and OPA627. With this new set I want to go Class A with discrete class A OPamps and in the first stage using OPA1611A Class A modules. Now what I'm not so sure about is the requirements on the Power Supply. I'm using Sachin's / Omishra's one with the LM317/LM337 combo. I have a 15-0-15V 30VA (1A) transformer. The 2 boards will host 6 single Class A OPamps and 1 double OPamp each giving an output of 200mA. So roughly about 1.6 A in total. As the current draw will be much higher I tend to go for a 60VA (2A) transformer instead. The Voltage regulators are rated at 1.5A to 2.2A. Could be just fine, else could I consider the LM1085 regulators with 3A instead ? Is is advisable to change the diodes to Schottky type 3A ?

Somewhere page 50 nor 51 I read from user Rothwellaudio : "30VA is huge for a circuit that's demanding 1 or 2VA. Oversized transformers cause more problems than they solve.",
I agree but is this not applicable for my planned configuration ?

Kind Regards,
Mario
 
Last edited:
DCESpeedy said:
High all. I'm slowly getting back into the preamp builds. I found out the LME49990 op amps a no longer available. Bummer! ;-( I did purchase a few of them though so I will be okay for my projects.
...
I looked at the opamps in the XPS-1 and they are Linear Technology LT1115CSW. At first glance it looks like a quad opamp, but it is actually a single opamp in an over sized package. Interestingly the specifications are very close to the LME49990 with the exception of the corner frequency which is 250Hz instead of 43Hz. From what I have read it should be a very good baseline preamp to try to push the CNC and 797 past. I also received a pair of Klipsh RB-81 II speakers for Christmas to replace my Infinity Alpha 20 pair. They are at least 10 times better in all aspects using the the AD797/DL-103R combo with no subs! So I will be able to hear the subtle differences better.
...

Later :thumbsup:
Click to expand...

The LT1115 are indeed a very nice OPamp on the neutral side, but very dynamic a crisp presentation, the LT1007 are a bit warmer sounding. I also tried the OPA1611 and OPA1622 on other preamps and those are also a good replacement for AD797 and LME49990.

Rgds,
Mario
 
hipine said:
I know this post is a little old, but just in case it helps, I had a similar issue when I started. Connecting the phono stage would draw the - side of the OPS down to low voltage. In my case it was because I used a "mic cord" to connect the two devices that had the shield of the cord wire internally connected to the pin I was using for the - power. Since my cord sockets were connected to the case and the case to ground through some dubious connections, the cord was drawing the - side of the power supply down.

Only yesterday did I discover that I think the - leg 10mH inductors were damaged by overheating at that time. The OPS powers the phono stage ok, but the inductors on the - side only measure 2.7 ohms where the other 4 measure 6.7 which is right at spec. Luckily I found a couple identical inductors so I won't have to replace all 6. I'm running the 'stage on 9-volts for a bit and have to go find the instructions to set up the OPS again. :)

I hope I didn't do other damage running it as it has been. I might have other parts to replace. But it has sounded fine throughout.

Hope you already solved your problem long ago, Mario!

Best wishes,

-Dave G.
Click to expand...

Hi Dave,

I'm reading your post just now since I have another build in project. Yes I solved the problem, the cause was indeed grounding of one of the components, don't remeber which one though. I want to thank you for your feedback !
Best wishes,
Mario
 
Hi, Just thought i would let you guys know that I am currently using my 2nd AD797 phono amp with a MC H/O cartridge. I picked up a NOS for a $100 so I though I could keep as a backup. As I do not have any quality MM phono amps, I though I would try it by setting the impedance up to just under 1000 ohms. And boy I am very impressed with the results I was hearing, I would say a least as good as the 103. MC.

Cheers Derek
 
maco said:
The LT1115 are indeed a very nice OPamp on the neutral side, but very dynamic a crisp presentation, the LT1007 are a bit warmer sounding. I also tried the OPA1611 and OPA1622 on other preamps and those are also a good replacement for AD797 and LME49990.

Rgds,
Mario
Click to expand...

The LT1028 are very nice in the output stage of the board.

Supply voltage ±4.5V to ±18V
Noise 1.1nV/√Hz max. @ 1kHz
Bandwidth 50MHz
Offset voltage 40µV max.
Slew rate 11V/µs min.

Class A discrete OPA on the output stage tend to be very noisy (hum) and unstable. Is it due to the shunt caps 10uF between the V+/V- legs on the OPA socket ?
 
My philosophy is akin to more is often less. More of the correct components produces less power supply interaction, less distortion, less error in the RIAA- and more accuracy in the design.
When I tried to explain, in technical terms, the answer to a prior supply decoupling question it did not receive a single like, so I'm not going to bother explaining beyond this...
 
wyn palmer said:
My philosophy is akin to more is often less. More of the correct components produces less power supply interaction, less distortion, less error in the RIAA- and more accuracy in the design.
When I tried to explain, in technical terms, the answer to a prior supply decoupling question it did not receive a single like, so I'm not going to bother explaining beyond this...
Click to expand...

Have a link to the decoupling post?
 
wyn palmer said:
My philosophy is akin to more is often less. More of the correct components produces less power supply interaction, less distortion, less error in the RIAA- and more accuracy in the design.
When I tried to explain, in technical terms, the answer to a prior supply decoupling question it did not receive a single like, so I'm not going to bother explaining beyond this...
Click to expand...
Please dont let "likes" counts keep you from sharing with the class from the chalk board.
 
maco said:
The LT1028 are very nice in the output stage of the board.

Supply voltage ±4.5V to ±18V
Noise 1.1nV/√Hz max. @ 1kHz
Bandwidth 50MHz
Offset voltage 40µV max.
Slew rate 11V/µs min.

Class A discrete OPA on the output stage tend to be very noisy (hum) and unstable. Is it due to the shunt caps 10uF between the V+/V- legs on the OPA socket ?
Click to expand...
Maybe it's because the OPAs are not stable under the closed loop conditions of the output stage? You just can't randomly exchange opamps and be sure that all is well...

The decoupling comment was post #1601.

A general comment, although I am not always 100% technically correct, even when I'm wrong I research the issue and admit it, supplying correction where needed. However, I am mostly right about such things, and I endeavor to share these insights. However, some people actively dislike this, expressing their displeasure, and there rarely is any kind of acknowledgement when I post such things, despite their accuracy and seemingly especially if they are entirely apropos.
I really don't care what anyone thinks on an internet forum, and I use my name because I am always willing to stand by my reputation, but honestly, I often wonder why I bother. If people really aren't interested in the answer why ask the question?
It seems that most would prefer an entirely empirical answer to their questions and that someone expressing an entirely unsupportable opinion based on no technical basis is accepted without question, while more carefully considered and supportable answers are completely ignored...
SO, if what everyone wants is " I swapped capacitor X with capacitor Y and the sound was elevated to god like levels" or similar, which seems to be all too common, then so be it. You'll get lots of that kind of stuff from (almost) everyone else, but not from me.
 
Last edited:
wyn palmer said:
Maybe it's because the OPAs are not stable under the closed loop conditions of the output stage? You just can't randomly exchange opamps and be sure that all is well...

The decoupling comment was post #1601.

A general comment, although I am not always 100% technically correct, even when I'm wrong I research the issue and admit it, supplying correction where needed. However, I am mostly right about such things, and I endeavor to share these insights. However, some people actively dislike this, expressing their displeasure, and there rarely is any kind of acknowledgement when I post such things, despite their accuracy and seemingly especially if they are entirely apropos.
I really don't care what anyone thinks on an internet forum, and I use my name because I am always willing to stand by my reputation, but honestly, I often wonder why I bother. If people really aren't interested in the answer why ask the question?
It seems that most would prefer an entirely empirical answer to their questions and that someone expressing an entirely unsupportable opinion based on no technical basis is accepted without question, while more carefully considered and supportable answers are completely ignored...
SO, if what everyone wants is " I swapped capacitor X with capacitor Y and the sound was elevated to god like levels" or similar, which seems to be all too common, then so be it. You'll get lots of that kind of stuff from (almost) everyone else, but not from me.
Click to expand...

I appreciate all your posts but don't understand a lot of it. I did read the decoupling post just now though and am happy to report I understand what you're saying. And that is because of the foundation of your previous posts.

So don't give up on us just yet. And ignore the outliers with the boutique cables and caps, etc., I don't think they represent the lot of us.
 
Does anyone have one of these already made to sell? Or that would be willing to build one and sell it to me for a reasonable price?
thanks
p
 
I ended up building these and I am playing it now. It sounds excellent to my ears, but I have no term of comparison as I just got a TT.
Biggest differences in sound: the output caps! I just replaced the wima with the russian caps that took a while to arrive and they indeed transform the whole thing.
Stayed with the AD797 in the first stage, I use 38 ohms load, my pickup is 3.5ohms. Seems fine.
Second stage OPAMPS tried: the LME49990, OP1611 and the LT1028.
I would rate the LME and OP as the best, and LT1028 not up to the level. The LME is more subdue and dark but it maybe more 3D in the image, the OP is more punchy opened and detailed. The OP fits my system better and that's what I am going to stay with for now. Easy build and nice outcome. Thanks.
 
pr1324 said:
I ended up building these and I am playing it now. It sounds excellent to my ears, but I have no term of comparison as I just got a TT.
Biggest differences in sound: the output caps! I just replaced the wima with the russian caps that took a while to arrive and they indeed transform the whole thing.
Stayed with the AD797 in the first stage, I use 38 ohms load, my pickup is 3.5ohms. Seems fine.
Second stage OPAMPS tried: the LME49990, OP1611 and the LT1028.
I would rate the LME and OP as the best, and LT1028 not up to the level. The LME is more subdue and dark but it maybe more 3D in the image, the OP is more punchy opened and detailed. The OP fits my system better and that's what I am going to stay with for now. Easy build and nice outcome. Thanks.
Click to expand...

Would you mind sharing the specific changes? With pics if possible. I have this lying around and would like to bring them into play. I get the OPAMP, but is there a cap similar to the russian ones?

Thanks!
 
I have a pair of Russian caps used in the AD797 build. Far better than rolling OPA’s.

P
Message me if interested. Basically free.
 
You must log in or register to reply here.
Back
Top Bottom