Hello everyone,
This is my first attempt at designing a phono stage from scratch, so it might have a few teething problems.
I was rather hoping that some of the more experienced members might cast an eye over the schematic for me and point out any problems, and/or improvements.
I have many of the parts required to build this circuit, hence the tube line up, so I probably won't change them!
Thanks to all respondants in advance.
Bob.
This is my first attempt at designing a phono stage from scratch, so it might have a few teething problems.
I was rather hoping that some of the more experienced members might cast an eye over the schematic for me and point out any problems, and/or improvements.
I have many of the parts required to build this circuit, hence the tube line up, so I probably won't change them!
Thanks to all respondants in advance.
Bob.
Attachments
12AX7 and 12AU7 are not the best choices for linearity or noise, especially with a low plate load. To boot, the input tube will have a stunningly high input capacitance. Gain is also likely to be marginal here- figure the first stage is something like A = 40, the second stage is about A = 12, for a total A(DC) of 480. Now knock off 20dB, and A(1kHz) = 48. For a 5mV nominal output cartridge, that's only 250mV output from the preamp; ideally, you'd like 5 times more output. A 12AT7 as the second tube will get you better linearity and about 3 times more gain.
I haven't rechecked the EQ components- you did annotate the source resistance of the input tube and the Miller capacitance of the second tube, but I'd like to know where the 21k came from. I get a significantly higher number because of the cathode degeneration.
I haven't rechecked the EQ components- you did annotate the source resistance of the input tube and the Miller capacitance of the second tube, but I'd like to know where the 21k came from. I get a significantly higher number because of the cathode degeneration.
Thanks for the expert input Sy.
I forgot to draw the bypass caps onto the legible redraw! There should be 100uF bypasses across both 910 R's. That should add gain to the input, and lower the output resistance somewhat. Sorry for the misinformation.
I calculate 21k by:
Plate resistance of 60k deduced from tangent drawn on curve (from data sheet), at theoretical operating point. Maybe not perfectly accurate, but close enough for first sketch up.
RL in parallel with Rp's (68k//60k//60k).
I know other tubes would be more suited to the application, but I'd like to try this out at little cost, hence the compromises. I can try more suitable tubes later, if this one at least works! It's just a first step on a long road.
In my current phono, I use 12ax7 input, and I don't find the noise too intrusive. The crackle 'n' pop disguise it. (record cleaner is on the cards)
The purpose of the parallel input was to reduce noise.
I suspect my Grado cartridge will benefit from the increased input capacitance.
Do you think I should increase the load (instead of 68K) to increase gain?
I can do the same on both common cathode stages, and re calculate the other values to spec.
Alternatively I could make the final stage common cathode? I don't need a lot of drive current.
One final thought was to use the 12AU7 for input, and the 12AX7 for gain and output. Would this benefit linearity?
If you see no reason to waste your brain power, and my time on this, I will have to bite the bullet and blow the cobwebs off my wallet, in which case I might as well use 6922's, or such.
Thanks Sy.
I forgot to draw the bypass caps onto the legible redraw! There should be 100uF bypasses across both 910 R's. That should add gain to the input, and lower the output resistance somewhat. Sorry for the misinformation.
I calculate 21k by:
Plate resistance of 60k deduced from tangent drawn on curve (from data sheet), at theoretical operating point. Maybe not perfectly accurate, but close enough for first sketch up.
RL in parallel with Rp's (68k//60k//60k).
I know other tubes would be more suited to the application, but I'd like to try this out at little cost, hence the compromises. I can try more suitable tubes later, if this one at least works! It's just a first step on a long road.
In my current phono, I use 12ax7 input, and I don't find the noise too intrusive. The crackle 'n' pop disguise it. (record cleaner is on the cards)
The purpose of the parallel input was to reduce noise.
I suspect my Grado cartridge will benefit from the increased input capacitance.
Do you think I should increase the load (instead of 68K) to increase gain?
I can do the same on both common cathode stages, and re calculate the other values to spec.
Alternatively I could make the final stage common cathode? I don't need a lot of drive current.
One final thought was to use the 12AU7 for input, and the 12AX7 for gain and output. Would this benefit linearity?
If you see no reason to waste your brain power, and my time on this, I will have to bite the bullet and blow the cobwebs off my wallet, in which case I might as well use 6922's, or such.
Thanks Sy.
I designed and built similar phono stages early in my audio career. It will work, but you definitely need all of the gain you can get out of that first stage - I'd increase the plate load resistance to at least 88.7K and recalculate the eq components.
I would either substitute a higher mu tube for the 12AU7A or consider a mu-follower for the second stage instead. Gain will be close the theoretical mu, and output impedance should be under 1K with a 12AU7A - note that you need to run current through this tube to get decent linearity, say something >5mA. Some 12AU7A have significantly better linearity than the ones Morgan Jones wrote about, but most are not as good as other types - at these signal levels I doubt it is going to be a big issue. (I found Mazda, Mullard and Dutch or German Amperex to be exceptionally good sounding compared to anything else - they also measure better too. JJ's do sound ok, most vintage US made ones seem to have poor linearity, and sound mediocre to awful.)
I find that the minimum useful gain for a phono stage is typically about 40dB @ 1kHz so this implies at least 60dB (1000) of gain prior to the application of eq.
I used up to 4 12AX7A/5751 sections in parallel in early designs for noise reduction, and independent cathode bias for each one is absolutely the right thing to do. My cartridges at the time were all moving coil types and not too sensitive to capacitive loading - a possible issue with some induced iron and mm types. These days I use really high transconductance tubes like a triode connected D3A for this task.
I would either substitute a higher mu tube for the 12AU7A or consider a mu-follower for the second stage instead. Gain will be close the theoretical mu, and output impedance should be under 1K with a 12AU7A - note that you need to run current through this tube to get decent linearity, say something >5mA. Some 12AU7A have significantly better linearity than the ones Morgan Jones wrote about, but most are not as good as other types - at these signal levels I doubt it is going to be a big issue. (I found Mazda, Mullard and Dutch or German Amperex to be exceptionally good sounding compared to anything else - they also measure better too. JJ's do sound ok, most vintage US made ones seem to have poor linearity, and sound mediocre to awful.)
I find that the minimum useful gain for a phono stage is typically about 40dB @ 1kHz so this implies at least 60dB (1000) of gain prior to the application of eq.
I used up to 4 12AX7A/5751 sections in parallel in early designs for noise reduction, and independent cathode bias for each one is absolutely the right thing to do. My cartridges at the time were all moving coil types and not too sensitive to capacitive loading - a possible issue with some induced iron and mm types. These days I use really high transconductance tubes like a triode connected D3A for this task.
I suggest (other than the great help you have here) adding the fourth time constant 
Very good results.
Just parallel that 191k resistor (or you could also use the 56k resistor, series resistors in the RIAA correction) with a 50pF-100pF capacitor. The right value could be calculated with much hassle: SPICE gives great help here
http://palimpsest.stanford.edu/byform/mailing-lists/arsclist/2007/03/msg00424.html
Very good results. Just parallel that 191k resistor (or you could also use the 56k resistor, series resistors in the RIAA correction) with a 50pF-100pF capacitor. The right value could be calculated with much hassle: SPICE gives great help here
http://palimpsest.stanford.edu/byform/mailing-lists/arsclist/2007/03/msg00424.html
Thank you all for the great information.
I can't find what the specified input capacitance should be for my grado prestige black cartridge. All I can find is a passage mentioning that the cartridge inductance is low, and it is relatively immune to cable and input capacitance (compared presumably to other similar cartridges).
So the general concensus is that the input capacitance is too high for most cartridges, the gain is too low, and the 12AU7 should be popped in the rubbbish where it belongs?
I will investigate the fourth time constant, but I'll need to do some more reading first.
It looks like it may well be back to the drawing board for this one then, with different tubes. I know this might turn the thread into a battle ground, but does anyone have recommendations for reasonably priced (common) tubes for phono applications? I would at least like to use the 9 pin bases. How about 6922/6DJ8 etc? They aren't too dear.
Thanks again for all the input.
I can't find what the specified input capacitance should be for my grado prestige black cartridge. All I can find is a passage mentioning that the cartridge inductance is low, and it is relatively immune to cable and input capacitance (compared presumably to other similar cartridges).
So the general concensus is that the input capacitance is too high for most cartridges, the gain is too low, and the 12AU7 should be popped in the rubbbish where it belongs?
I will investigate the fourth time constant, but I'll need to do some more reading first.
It looks like it may well be back to the drawing board for this one then, with different tubes. I know this might turn the thread into a battle ground, but does anyone have recommendations for reasonably priced (common) tubes for phono applications? I would at least like to use the 9 pin bases. How about 6922/6DJ8 etc? They aren't too dear.
Thanks again for all the input.
6DJ8 have lower gain than 12AX7 but also a much lower equivalent noise resistance. They also are happy with lower plate voltages. You do need to pump higher current through them to realize these advantages, and you'll probably need three gain stages before the output buffer. I'd look at some of the designs from Allen Wright and Morgan Jones using those tubes.
A phono stage is the single hardest thing in audio to get right, so don't be surprised if you make a few false starts.
A phono stage is the single hardest thing in audio to get right, so don't be surprised if you make a few false starts.
I have seen some Morgan Jones designs in my book, but am not familiar with Allen Wright's designs. Where will I find these?
The general point of the exercise is to 'have a go' at a phono, and see what I can do. I wasn't expecting brilliant performance, just a good learning experience.
Maybe if I can get reasonable results at low cost, I wouldn't mind spending some cash on much nicer tubes (and other parts) for future projects. You gotta start somewhere.
Do you have any idea where I might find design details for 'single stage' passive eq. (Lipshitz), or will I have to fork out on the white paper from AES?
Thanks again.
The general point of the exercise is to 'have a go' at a phono, and see what I can do. I wasn't expecting brilliant performance, just a good learning experience.
Maybe if I can get reasonable results at low cost, I wouldn't mind spending some cash on much nicer tubes (and other parts) for future projects. You gotta start somewhere.
Do you have any idea where I might find design details for 'single stage' passive eq. (Lipshitz), or will I have to fork out on the white paper from AES?
Thanks again.
I've seen the Lipshitz papers around the web; I'm not sure where, but have you done the obvious thing and looked on his website? Lots of good info there.
Allen's designs are in his preamp cookbook and on his site, www.vacuumstate.com under "schematics."
I think my criticisms of your design were directed more toward a preamp you can live with long-term; if this is a learning exercise, then by all means start with what you've got, see what it does well and what it doesn't, then start diddling (you've got nothing there that will explode or anything). When you're done, there won't be much left of your original, but you'll have a good phono amp and a terrific variety of hands-on training.
Allen's designs are in his preamp cookbook and on his site, www.vacuumstate.com under "schematics."
I think my criticisms of your design were directed more toward a preamp you can live with long-term; if this is a learning exercise, then by all means start with what you've got, see what it does well and what it doesn't, then start diddling (you've got nothing there that will explode or anything). When you're done, there won't be much left of your original, but you'll have a good phono amp and a terrific variety of hands-on training.
I can't find S.Lipshitz website anywhere. Do you happan to know the address?
I had a quick look at the vacuum state site. Very nice looking designs (from my limited knowledge of these things).
I have been looking at quite a few designs with FET input stage. Do you know if the sonic results are as good as the technical improvement implies. I have not heard one, so I wouldn't know. The implementation is elegant and simple, and the transistors are so cheap, it might be worth a whirl.
Thanks for taking the time to help.
Bob
I had a quick look at the vacuum state site. Very nice looking designs (from my limited knowledge of these things).
I have been looking at quite a few designs with FET input stage. Do you know if the sonic results are as good as the technical improvement implies. I have not heard one, so I wouldn't know. The implementation is elegant and simple, and the transistors are so cheap, it might be worth a whirl.
Thanks for taking the time to help.
Bob
I just replaced my phono preamp which had given me 25 years of service; it used the same sort of FET/tube cascode that Allen shows. Worked great.
I didn't find Lipshitz's paper on line but you can get the same results with this handy-dandy calculator:
http://www.kabusa.com/riaa.htm
I didn't find Lipshitz's paper on line but you can get the same results with this handy-dandy calculator:
http://www.kabusa.com/riaa.htm
Seller on eBay located in Germany.. I neglected to add the shipping expense, but last time (two months ago) that worked out to about $1.75 per tube. I typically buy them in lots of 4 or more.
They were appealing initially simply because they were so cheap compared to the 5842 my other big favorite in high transconductance tubes.
Vendors selling them here tend to get at least twice what I have been paying for them.
They were appealing initially simply because they were so cheap compared to the 5842 my other big favorite in high transconductance tubes.
Vendors selling them here tend to get at least twice what I have been paying for them.
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