I have been putting this off. This is the part I normally dread, not because I have nasty things to say about it but because putting together my thoughts coherently is an uphill task. But I have accumulated more than a 100 hours of pleasurable listening experience on the DIY AD 797 phono preamp. So here's my formal listening impression. What follows is a series of long and meandering ramblings of my analog discovery, accompanied by unsolicited/unwarranted rants. But if you're in a hurry, here's the executive summary: go build yourself one while the kits are still available.
First, some - or rather lots of - background. Prior to this phono pre, I was using the Hypnotoad MC Phono Stage (HTMCPS), a wonderful low output MC-only pickup amp designed by Hypnotoad. The HTMCPS is built around the OPA2134 dual opamp. One can substitute that opamp with dual LME49990 opamps using appropriate adapters, or any other dual opamp with same pinout. Building the HTMCPS was a smooth affair, with absolutely no hitch. It played at first try. Mine came with the OPA2134. TO try a different flavour, I requested Sachin to send me the LME49990. After fairly extended comparisons, I preferred the OPA2134 in my setup. To backtrack even further, I tried the LME49990 on the (DIY) CNC MM phono stage too, and liked the OPA2134 more even in this MM phono stage. I know there are lots of folks out there who prefer the LME but please remember your mileage may vary depending on your room, TT-arm combo, cartridge, taste of music, and the rest of the chain. MM cartridges I used for testing were Shure M97xE cartridge and Decca London Super Gold (actually a moving iron cartridge which outputs similar signal levels as an MM cartridge and uses 47kOhms loading). With either cartridge, the treble gets hot for my liking when using the LME49990. Too much bite in the highs. OPA 2134 consistently presented a more balanced tone in my setup.
Building the AD 797 phono preamp ended up being a fairly frustrating affair due to mistakes in using the wrong adapter for the LME 49990. I had wrongly used a dual SOIC 8-pin SMD to 8-pin DIP adapter (same as the one used in HTMCPS or CNC MM PS) but Hypnotoad had designed the new AD 797 phono pre to be used with a single LME49990. Even his audiokarma thread is clear about this. I made my own mistakes but in the process learned that the LME49990 opamp can be used as single or dual, and that they are not interchangeable. Good that I didn't blow up my dual LME49990 as this is a fairly costly opamp. Thanks are due to Hypnotoad and Sachin for helping me troubleshoot this problem. Also, in the process, I ended up learning to solder the very tiny LME49990 SMD chip, and finally had a chance to use a fancy iron a friend gifted me.
After I received the correct opamp adapter from Sachin, it worked on first try! which at least proved that I didn't mess up the rest of the circuitry. And the sound - out of the box - was very promising. It was apparent from the first drop of the needle that this phono stage has indeed very low noise. This brings along a good thing - one starts to hear more details being extracted from the grooves of the records. It also came with a negative - some records which were less than stellar were, unfortunately, rendered less listenable, if not outright unlistenable. But the AD 797 phono pre has been so good that it has literally egged me to improve the rest of my analog chain. It set me off on many tangents but I had come away with benefits and invaluable learnings.
I started out by questioning whether my cartridge-arm combo had the correct resonance frequency and made me experiment with heavy headshell spacers (heavier than the 3.3 gram spacer I had at hand, but did not use). It is great fun making one's own spacer, and any one with some spare brass plates and a set of small files and a table vise can also do it. There is a caveat in here though - putting more weight at the headshell assumes you have enough mass (or enough length of the stub) at the counterweight end to balance things out. In this quest I ended up acquiring a test record that enables one to check the resonant frequency of one's arm-cartridge combo. This is an invaluable tool in any analog guy's toolkit.
I also reluctantly questioned the almost inaudible hum coming from my Thorens TD124/I. It has been there for a long time, and it has almost stopped bothering me because it is low, and does not intrude into my listening, but I am glad that the solution was as simple as re-routing the cables from tonearm to phono stage. A takeaway from this episode was that shielded tonearm cables may not be sufficient to block electrical pickup from nearby devices or power lines, so route your tonearm cable as far away as possible from them.
It also prompted me to try this phono stage on a work-in-progress Lenco L75 rebuild, to hear how different it might sound. The results of this test has been nothing less than epiphanous. My long-dead thread on the L75 rebuild probably deserves to be raised from the dead and updated. Like many others, I have read about how good this idler wheel-driven vintage 'table is with high-mass plinth and decent arm and cartridge, and actually believed it enough to try it for myself. But, the presence of another turntable in good working condition in the house, a much more famous one to boot, has not been exactly strong encouragement to speed up this build. And so it has been lying unloved and unfinished. Now I am advising whoever will hear to just go grab the first Lenco L75 that comes along. And forget about trying to acquire the much costlier exotics.
But I have digressed enough; to get back to the AD 797, I initially struggled to dial in the correct gain setting. In this phono stage, Hypnotoad has cleverly incorporated adjustable Bourns potentiometers for both impedance matching and gain setting. This is excellent thinking, as it is much more convenient than having to insert a resistor of discrete and fixed value (which may not be the exact value needed) into a terminal block each time one changes a cartridge. And more so because the Bourns allows precise adjustment of impedance loading value, down to an ohm, and will hold that value. Currently, the gain and impedance are set for ZYX RS30-02 which has fairly low output voltage of 0.24 mV and 100 Ohms of recommended loading. This setting is very convenient as I can use it interchangeably with a Denon DL 103 cartridge which has identical values (0.3 mV, 100 Ohms). So the impressions that follow are a composite of listening impressions from both cartridges on two turntables.
Tonality:
My personal benchmark for tonality is my CD player as I feel it produces beautiful and near-neutral tone, albeit on the slightly warmer side of neutral. The AD 797 phono stage brings out the tonality of instruments very nicely. It is a pleasure to listen to piano and violin concertos (rant begins: I know of concertos for horns and trumpets but why can't someone compose a full bore concerto the saxaphone? or for the guitar? rant ends - someone please point me to such concertos). Both pianos and violins are difficult to get right. Especially violins - they can sound strident and jarring in the wrong setup. Massed brass section (like those in Sibelius' Symphonic Poem Finlandia) have just the right bite. It is also a pleasure to listen to small ensemble jazz with either piano or saxaphone leads. The CD player is finally equalled, if not humbled. Here I must add that correct VTA also contributes to the correctness of the tonality.
Resolution:
By virtue of its lower noise floor, this phono stage is very resolving. A review by another builder mentioned hearing the dryness of the pea in the police whistle in the Steely Dan track "Aja" here:
AD797 Phono Stage Build and Help Desk Thread - Page 2 - AudioKarma.org Home Audio Stereo Discussion Forums. I am very familiar with this track and do have an LP of this album, but alas I don't have a chain resolving enough to hear the dryness of the pea (which in any case is very far back in the mix). But suffice it to say the level of details has gone up substantially from the HTMCPS.
Imaging and separation:
Voices are much easier to follow as separate events while being very much recognisable as part of a larger event. But I still can't follow most of what Sir Elton Hercules John mumbles. There has been improvement too in left-right lateral imaging (making it easier to place voices/instruments in the left-right space) and front-back image depth and layering (like backing vocals singing from the "second row").
Dynamics:
Is certainly not wanting. Especially when the music crescendoes and cymbals and tympani come down a'crashin on orchestral tracks. Due to the limitation of the rest of my chain, the low level dynamics are not as discernible at regular listening volumes, but the phono stage is not to blame here. My power amp, and/or perhaps the speakers are not able to bring out more clearly lower level details. But there is a solution here which I usually resort to for more serious listening - crank up the volume by 3-4 dBs and lower level details become more discernible (caveat: louder passages will also be that much louder, and may be beyond what is comfortable for you).
Extension:
The intro to the Olivia Newton John track "Queen of the Publication" (from the album Soul Kiss) has a typewriter chattering and clacking away. The carriage return (or what is suposed to be the sound of the carriage return) is a very sweet sounding, metallic tinkle. It definitely sparkles with the ZYX RS30-02 cartridge. The Denon DL103 doesn't produce as sweet a tone and is grainy by comparison. The AD 797 phono stage is transparent enough to highlight the limitations of the legendary workhorse from the house of Denon. The air around the slide guitar on Dire Straits' "You and Your Friend" (from the album On Every Street) is light, open and airy. It also pushes back the vocals on this track a couple of rows behind, making him sound almost like Ozzy Osbourne singing from a deep well. The title track on the first album mentioned is real bass-heavy, with a hefty weight to it. One can feel the air being pushed when listening to this song on the right set of speakers. With my bookshelves, I can hear a "glimpse" of that weight, enough to know it is there for the picking with the right pair of speakers.
If I had not already bought the board for the Pass Pearl II phono preamp, this would have been my last DIY effort at phono stage (I have a feeling I could suffer from foot in the mouth syndrome here:lol
. Now I will have to find out which is the better one, especially since the Pearl II supports MMs, and I need a good MM phono stage for the Decca moving iron.
On the power supply: initially I ran the AD 797 phono stage with two 9V batteries (+9V, 0, -9V). My old (factory made)phono stage runs off a wall wart supply. So an idea I had been toying with for a long time, but didn't have the courage to take the plunge, was to try two 12 Volt AC-DC adapters. I borrowed a couple of 12V, 2 Amp adapters and the results was nothing less than satisfactory. There is absolutely no loss of dynamics, which I was afraid might happen; nor any grit in the highs that might show up if the power was unclean. Also, 3 volts (12V v/s 9V) made an unexpected difference - volume was higher, meaning gain was better. Which is a good thing as we are dealing here with LOMCs. If any one wants to try this instead of building a much costlier dedicated PS, look for reputed 12V AC-DC adapter (some shopkeepers still use the quaint but charming term "battery eliminator") of 2 Amps rating. If 2 Amps is difficult to come by, you can try 1.5 Amp. Also, since the opamps work across a range of supply voltages, even 15V should work, BUT I have not tried this and cannot say for sure. Of course all this does not mean that I will not complete the build of the OPS, which is half finished.
