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I for myself have the ultimate OpAmp based phono equaliser in mind. But I know: A ridiculous idea. After all I've seen so many good designs over the last months that it is hardly realistic to assume that my third project with opamps will bring me in the hall of fame.
Still I have a few design criteria I want to stick to:
Therefore, a current to voltage amplifier (Transimpedance amp) looks like the way to go. Especially since we have a low-Z cartridge and we want to avoid noise from resistors at the input. in other words, we use a inverted amplifier for the input stage, and since we do not want the signal to invert at the output I'm going to make the second stage of the amp inverted as well.
The advantage of an inverted design is, that the exact impedance of the cartridge is not critical to the RIAA filter in the feedback loop, only for the gain itself.
Originally I planned to make an amplifier with inverted stages and passive RIAA filter in between the sections. However, I was told by people with more experience than myself with OpAmps (thanks Thorsten) that active filters do sound better with OpAmps (which is other way around for tube amps). And since I have time at the moment to build one design, I decided to build a RIAA amp with inverted Opamp sections and active filter in the feedback loop of the OpAmps.
The following schematic is the inverted and active version of
PhonoCard (look on page
5 of the RIAA background article for in-depth discussion of the schema).
Initially I planned to have a coupling capacitor between the stages (avoiding DC offset at the output). However, the calculated value of less than 1uF would not work in an inverted amp as the cap is in series with the negative input and thus is part of the amplification loop (Z_feedback / Z_input). I such a case, it has a much stronger negative effect on the lower frequencies as one would initially expect. To be fair, even with 20uF the interstage capacitor would still be loosing on the lowest frequencies.
Therefore I placed the DC blocking capacitor on the output. Should your amp have an input with its own capacitor, it is possible to omit the output cap as long as you are sure never to connect the phonopre to "unprotected" amplifiers.
This design, in contrary to the phonoclone design, will be specific to my MC setup with a Dynavector 17D2 cartridge with a DC resistance of 38 Ohms. I might change a little in the final setup of the second stage in order to get higher gain (currently 58dB in total @ 1kHz) but for the moment this will be enough.
In the schema is shown how the output of the Opamp is fed back to the negative input. The resistors R2 and R5 make the "zeros" for the RIAA curve. It may therefore be necessary to further tune the values of both resistors based on the Opamp that is used in the specific position. After all, the output impedance of the Opamp itself contributes to the effective value that the input "sees" for R2 and R5. For an AD797 the output impedance is VERY low (about 1 Ohms), but for a OPA637 or LT1115 it's in the order of 50 to 70 Ohms. Although the resulting upswing in the RIAA curve will be so low that you probably won't hear it, for sake of perfection I already adjusted the values of R2 and R5 such that the output impedance of the Opamps is taken into account.
All resistors are 1% or better metalfilm (or better) types, and I try to match components for both both channels since most of the time I have several resistors of same value in house. The value of Ri is not fixed but in the inverted design determined by the load of the cartridge itself.
C3 needs to be a very good audio capacitor. Using Audyn Super Caps is a good idea just as Auricap or similar on this position. I wouldn't use paper in Oils here. Capacitors C1 and C2 may be styroflex or Polypropylene.
For the first amp section I will give the LT1115 a try, for the second stage I still have a pair of OPA637 around that need a good design.
In order to quickly build a working prototype I will stick with the same power supply (and connector) setup as for the PhonoClone. Once the amp is finished I'll have to decide on the Power Supply, or maybe sooner if I find the time.
In any case, it is my objective to make power supply and PhonoClone/PhonoCard amps interchangeable.
On the other hand, should I ever want to build a phono clone that fits in a similar housing as the original, then I need to build this amp in a tight (maybe) 3-dimensional way. This project is part of the less-is-more sequel, and therefore we want to keep the amount of components used limited to the ones that are absolutely necessary. Should I want to build a clone of the original Phonocard, than I need some power regulation inside the amp box (knowing that the original is also powered from an unregulated Power Humpty).
In my tube amps I like to have some room to work in, and this little amp needed to be built in just a few square centimeters. I still think it fits in the same box as the Cyclone. But in order to be comparable with the original and share the same power supply, some additional voltage regulation is needed which I did not include yet.
Above the layout of the first prototype of Phonocard, excluding output capacitor. Shown are both channels one above the other and identical. As with the original, I would very much be able to build the complete amp in a set of tubes we used for the Cyclone.
It took me several months to finish the amp. I do have more hobbies than building HiFi components, and I do have a job as well that keeps me off the streets.
Anyway, slowly this project progressed and neared its completion. I had some discussions with other people on diyaudio.com and decided to remove R0 and R6 in order to keep the noise low. So this is the "final" setup I'm testing right now and plan to use. I've still not decided whether to go for a pure clone or for a battery powered clone which will obviously not fit in the small boxes used for Cyclone.
I received a battery power supply from Sodejuu which I will probably include at some stage once the amp itself is finished. I must make a point here: I spent some effort in getting the power supply right. Based on a separate power supply of PhonoClone I still put several capacitors on the board close to the opamps. As a result I found it very difficult to build the amp on just a few square centimeters: The caps and power lines just make a simple setup nearly impossible.
So should you also build this amp and decide to go for battery power, I do think you can save space by leaving out the capacitor stuff.
Also I changed the Audyn caps in the picture above for Auricaps and made cinch connectors in the chassis. I still use the crappy power connection shown above until I find a way of charging the two big batteries. From that moment on life will be wonderful (I guess). At this moment the circuit is powered from the PhonoClone power supply, but the batteries need less power than the 15V supplied by this PS. The charging circuit delivers something like 12-13 V which is enough for the amp. So I probably need some sort of connector to allow charging of the batteries without lifting the hood every two weeks. And of-course I want a couple of those nice LEDs telling me whether I'm charging or in normal operation.
Ok, following things are left to be done:
Much later than the amp I also finished the battery charging circuit thanks to Sodejuu who not only donated the stuff but also remembered nearly half a year later how to connect it.
At first everything was loosely coupled (just a eufemism for telling that I did not have a box at hand). So I ordered one of the well known boxes from Conrad which took another month to be delivered (Conrad is not exactly in control of their delivery now and then).
The picture above shows the final result. In the right box a power supply with battery charger circuit is placed. I also moved the XLR power input for the amp itself from the front plate to the backplate.
And below you find the latest pictures of the interiors of both boxes. As shown on the right, the toroid is mount to the front plate of the power supply and connected using tie-wraps to a aluminum L-profile that isolates the toroid from the board. The umbillical cord is shielded 4-wire cable which I may or may not upgrade at a later stage.
On the left the amp is shown in its final stage. The same aluminum L-shape strip shields the batteries from the amp board and keeps them in place.
The last thing rest to do is exchange the LT1037 Opamp to a
OPA627 type I received from Dirk, a friendly reader of hear.nl. In my view the
sound of the OPA 627 is superior to the LT1037 although the first is not a very
low noise type. But since my supply of expensive OPA627 Opamps is limited, I'll
first let the amp break-in for a while so that we're sure that nothing unexpected
will happen to the chips.
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