Spot on, I'm afraid. I can't believe I overlooked this!
A JFET op-amp is in order here. OPA1644, would seem to be the best choice, all factors (including economic) considered.
Hi Monty,
Have you looked at the PDF in posting #59. The Opa1644 is also listed as dual version opa1642.
It is not the best choice, the OPA1632 is doing better.
Hans
Maybe so, but you've got to know when to stop before things get impractical
. It's a single op-amp that'll cost the best part of $20 or so, as opposed to the quad OPA1644 which will only be about $5 or so, working out to $1.25 per op-amp.Besides, the OPA1644 features other useful features, such as low DC offset, rail to rail operation, and low quiescent current.
Now to redesign it for JFET op-amps!
Maybe so, but you've got to know when to stop before things get impractical
Besides, the OPA1644 features other useful features, such as low DC offset, rail to rail operation, and low quiescent current.
Now to redesign it for JFET op-amps!
O.K perfect choice in that case..
I will withdraw from this thread now, after having having contributed quite substantially in simulating, much more than I had in mind
Wish you all the best with the project.
Hans
Cheers, Hans 
!
I'll be sorry to see you go, but do so if you must. I'm somewhat surprised that people aren't designing MM phonostages with JFET op-amps, having been shown to be superior for dealing with the high impedance of a MM cartridge as frequency increases.
I've re-designed the input stage to use the OPA1644. I've increased the resistor value at the bottom of the RIAA network and adjusted the topology (to reduce the total number of capacitors and capacitance required, and also improve accuracy with the limited range of component values available) to a different configuration. No point in reducing the impedance of the RIAA network too much as we don't have to deal with input current noise anymore. Besides, it helps take a load off the op-amp .
!I'll be sorry to see you go, but do so if you must. I'm somewhat surprised that people aren't designing MM phonostages with JFET op-amps, having been shown to be superior for dealing with the high impedance of a MM cartridge as frequency increases.
I've re-designed the input stage to use the OPA1644. I've increased the resistor value at the bottom of the RIAA network and adjusted the topology (to reduce the total number of capacitors and capacitance required, and also improve accuracy with the limited range of component values available) to a different configuration. No point in reducing the impedance of the RIAA network too much as we don't have to deal with input current noise anymore. Besides, it helps take a load off the op-amp
.Attachments
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O.K. I admit, I deserved your comment.
Hans
OK, i beg your pardon for my offensive wording
Udo
So I've thought about the overload performance of this pre-amp. It is now already improved by 1dB or so by using rail to rail op-amps (OPA1644 is incredibly good value for money as far as audio op-amps go ), but the inverting stage in the active loading circuit has a flat gain of 26dB. This gives an input overload of about 0.5V RMS at 20kHz, which isn't great as the RIAA stage itself is able to cope with 1V RMS, and even though most magnetic cartridges don't have a hope of producing such a high output, this still leaves it as the weak link.
So I've adjusted the values to match the overload level in the active loading stage to the RIAA stage at 20kHz, which means changing the gain from 26dB down to 20dB. However, this shouldn't make more than a fraction of a dB of difference to the noise performance in practice. In fact doubling the value of R1 to adjust the 50k91 of active impedance down to 47k should make up for this difference to the point of actually improving things .
Now to crack on with the PCB layout
.
), but the inverting stage in the active loading circuit has a flat gain of 26dB. This gives an input overload of about 0.5V RMS at 20kHz, which isn't great as the RIAA stage itself is able to cope with 1V RMS, and even though most magnetic cartridges don't have a hope of producing such a high output, this still leaves it as the weak link.So I've adjusted the values to match the overload level in the active loading stage to the RIAA stage at 20kHz, which means changing the gain from 26dB down to 20dB. However, this shouldn't make more than a fraction of a dB of difference to the noise performance in practice. In fact doubling the value of R1 to adjust the 50k91 of active impedance down to 47k should make up for this difference to the point of actually improving things
.Now to crack on with the PCB layout
.Attachments
more load?
Hi!
I like your project very much, it is clever, clear, relatively easy, reasonable cheap, etc...
did you think to load mm cartridge less than 50k, lets say 18k with electronic cooling technique and make 75us constant weaker to get even better noise figures and no resonance peaks from cartrigde loading circuitry? here is the idea (a few posts before and after too)http://www.diyaudio.com/forums/analog-line-level/221298-bob-cordells-vinyltrak-6.html#post4212783
Hi!
I like your project very much, it is clever, clear, relatively easy, reasonable cheap, etc...
did you think to load mm cartridge less than 50k, lets say 18k with electronic cooling technique and make 75us constant weaker to get even better noise figures and no resonance peaks from cartrigde loading circuitry? here is the idea (a few posts before and after too)http://www.diyaudio.com/forums/analog-line-level/221298-bob-cordells-vinyltrak-6.html#post4212783
The electronic loading is there specifically to mitigate the need for other noise reducing measures such as dropping the loading impedance and compensating with EQ. By lowering the value of R2, you could lower the input impedance if you wanted, though .
The best thing is that the op-amp quiescent current is quite low thanks to the joys of OPA1644. Power supply can be fairly simple, based upon the LM7XL15, with a simple split voltage doubler power supply.
.The best thing is that the op-amp quiescent current is quite low thanks to the joys of OPA1644. Power supply can be fairly simple, based upon the LM7XL15, with a simple split voltage doubler power supply.
New Complete Schematic
Just thought I'd upload the full circuit with the changes that I've made with the start of the thread.
The changes so far are, different RIAA feedback network configuration to reduce the total capacitance needed from the relatively expensive polypropylene or polystyrene caps that will be used, change of op-amp to the OPA1644, and adjustment of the active loading stage for best overload performance...
Just thought I'd upload the full circuit with the changes that I've made with the start of the thread.
The changes so far are, different RIAA feedback network configuration to reduce the total capacitance needed from the relatively expensive polypropylene or polystyrene caps that will be used, change of op-amp to the OPA1644, and adjustment of the active loading stage for best overload performance...
Attachments
By lowering the value of R2, you could lower the input impedance if you wanted, though
this is exactly what I will investigate after I buy the pcbs from you
Not really. I don't read any audio magazines, as I'd rather spend the time conjuring up my own topologies and experimenting with my own ideas
.A brief look on the web shows it's some sort of vertical filter with a cut-off in the sub 100Hz range. Fourier analysis of a good sample of transfers I have made in the past shows this can be as high as 150Hz without removing any musical material.
this is exactly what I will investigate after I buy the pcbs from you
Good to know I'll have some support in developing this stage
. As a heads up, I must warn you that the OPA1644 is unfortunately an SOIC package, which requires surface mount soldering. I think it will be worth it, as I can't find a through hole device that can compete, all other factors taken into account .
I saw your interesting disclaimer may be you should read
https://en.wikipedia.org/wiki/Bridgeman_Art_Library_v._Corel_Corp.
Once in the public domain no further derivatives have any copyright protection i.e. restored public domain recordings can not be copyrighted.
https://en.wikipedia.org/wiki/Bridgeman_Art_Library_v._Corel_Corp.
Once in the public domain no further derivatives have any copyright protection i.e. restored public domain recordings can not be copyrighted.
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I suppose you are right, although I do offer a free download . I just don't want people nicking my text and then uploading it onto YouTube.
I don't see any reference to recordings that are out of copyright. As restoration can take a lot of time and a certain amount of artistic input (such as cross-fading sides and blending the noise so as to make the side changes transparent) I would assume that the laws on photographic reproduction do not apply here. I could be wrong, though...
. I just don't want people nicking my text and then uploading it onto YouTube.I don't see any reference to recordings that are out of copyright. As restoration can take a lot of time and a certain amount of artistic input (such as cross-fading sides and blending the noise so as to make the side changes transparent) I would assume that the laws on photographic reproduction do not apply here. I could be wrong, though...