Yet Another Phono Stage Suggestions Thread

[dsavitsk]

I have an old LP12 -- from 1974. A couple years ago I put a "nice" cartridge on it (some Clearaudio thing that cost too much), which broke after about a year of light use. So, after not listening to any of the gems in my record collection in a long time, I am now going to put a cheap cartridge on it, probably a Shure M97xE unless someone has a better $50 option. But I need a phonostage.

Any suggestions? I like the look of Thorsten's (12AX7 into 6922) and it seems simple enough, and I think I have all the parts necessary to build a cheapo version, but people seem to badmouth the 12ax7 for phono input duty. I have a really good stash of 6SL7's and 6SN7's that I am never going to use for anything else, so I wouldn't mind using them here. I also have several lifetime supplies of 6688's and 5879's so a pentode stage would not be out of the question.

Any particular circuit suggestions, or online tools for calculating RIAA curves? Also, this is going to have to push longish (15 foot) cables with a TVC on the other end. I do want to do tubes and stay away from opamps, but a mosfet follower on the output is an option. Cathode follower is probably fine as well, or a grounded cathode if rp is low enough.

[Yvesm]

My two pence:

http://www.dissident-audio.com/RIAA_PC900/Page.html

But build it larger than I've done, each channel dissipates near 10 Watts including heaters

Not to tell about regulators efficiency

Yves.

[koolatron]

My understanding is that the 12AX7 is a bad front-end tube due to its high input capacitance. Given that what you're going for here is total gain, what's the problem with using a 6922 as your input tube and moving the 12AX7 behind it, where it won't load the cartridge excessively?

[Eli Duttman]

Originally posted by dsavitsk
I have an old LP12 -- from 1974. A couple years ago I put a "nice" cartridge on it (some Clearaudio thing that cost too much), which broke after about a year of light use. So, after not listening to any of the gems in my record collection in a long time, I am now going to put a cheap cartridge on it, probably a Shure M97xE unless someone has a better $50 option. But I need a phonostage.

Any suggestions? I like the look of Thorsten's (12AX7 into 6922) and it seems simple enough, and I think I have all the parts necessary to build a cheapo version, but people seem to badmouth the 12ax7 for phono input duty. I have a really good stash of 6SL7's and 6SN7's that I am never going to use for anything else, so I wouldn't mind using them here. I also have several lifetime supplies of 6688's and 5879's so a pentode stage would not be out of the question.

Any particular circuit suggestions, or online tools for calculating RIAA curves? Also, this is going to have to push longish (15 foot) cables with a TVC on the other end. I do want to do tubes and stay away from opamps, but a mosfet follower on the output is an option. Cathode follower is probably fine as well, or a grounded cathode if rp is low enough.

Dude,

The objection to high mu/high R_P types at a phono preamp's I/P is the potential for frequency response trouble, due to cartridge/Miller capacitance interaction. Some MM level carts. are fine working into an 'X7, while other carts. do poorly.

A variation on the RCA passive RIAA EQ theme should do the job for you. To make 100% certain that Miller capacitance is not an issue, use 6GK5s in the 1st gain block positions. Talk to your "neighbor", Jim McShane about 6GK5s.

KAB provides a passive RIAA EQ calculator. Look here.

Thorsten's design lacks the drive necessary for your situation. AAMOF, you have an especially difficult setup. Interconnect cables that are 15 feet long MUST be shielded. You need a phono preamp whose O/P impedance is LOW and whose drive current is HIGH, to deal with cable capacitance and the TVC. I suggest you use 6GK5 1st gain blocks, grid leak biased 6SL7 2nd gain blocks working into DC coupled 6SN7 cathode followers. The 'SN7 CFs have the "sock" needed to drive IRFBC20 source followers. Run the FETs at I_D = 20 mA. Good bye to drive capability issues. Use a 9 V. Lithium battery in each channel to forward bias the gates. Since a high gate resistor value is not problematic, use a highly transparent PTFE dielectric coupling cap. of modest value between the 'SN7 and the FET. My guess is that a bypassed 10 μF. motor run cap. will be needed between the FET and the O/P RCA female.

[Wavebourn]

Never mind, sorry...

[dsavitsk]

Originally posted by Eli Duttman
A variation on the RCA passive RIAA EQ theme should do the job for you. To make 100% certain that Miller capacitance is not an issue, use 6GK5s in the 1st gain block positions. Talk to your "neighbor", Jim McShane about 6GK5s.

Interesting, looks like a good tube to use here. Maybe these in conjunction with 6T4's for an all 7 pin project.

Originally posted by Eli Duttman
KAB provides a passive RIAA EQ calculator. Look here.

Thanks. Very useful. R1 in the equation should be the resistor (R1') used in series with the Zo from the first stage?

[jeffdavison]

an excellent phono stage... I built it and sub'd ecc35 for the 5751's

You could also sub in 6SL7's for the 417a and 5751.

Is a Mu stage WE417a input and a Mu stage 5751 output with a passive RIAA eq in between.

There is no mistake in the schemo.... there is no grid stopper into the 417a..... designer says 417a is very happy without and one less resistor for the signal to pass through.

I used Allen Wrights SuperReg in the power supply

here's his PCB version with designers notes:

http://www.dddac.de/tp06.htm

here's the hardwaire version:

http://www.dddac.de/tp07.htm


JD

[oldeurope]

Originally posted by dsavitsk
...
Thanks. Very useful. R1 in the equation should be the resistor (R1') used in series with the Zo from the first stage?

Hi,
I am sure you don't want to know that A1 gain should exceed 40dB.

Kind regards,
Darius

[kevinkr]

Originally posted by dsavitsk


<snip>

Thanks. Very useful. R1 in the equation should be the resistor (R1') used in series with the Zo from the first stage?

Yes you need to include the preceding stage's rp in the total. Also to reduce sensitivity to variations in rp due to operating point, aging, and initial tube tolerances it is a good idea to make R1 several times larger than rp at minimum - with very low rp tubes you can make it much greater without exacting a huge noise penalty. Don't neglect the johnson noise generated by both the rp of the preceding tube and R1 - try to choose a high mu tube with a low rp for this reason. (Seems like triode connected 6688 might be good in this location as is the 5842/417A, or triode connected D3A or 7788.) FWIW I am a huge fan of the D3A in triode connection for this task.

All of my phono stages use this basic topology and the passive equalization is based on the work of Stanley Lipshitz (U of Ontario) who deserves to be mentioned when this eq network comes up.

I aim for as high mu as I can get in that first stage, and as few active stages overall that I can manage. (Just 2 currently) Gain in my first stage is about 37dB - not quite the 40dB that Darius recommends.

[SY]

I cheat and use an input transformer, so my first stage gain is about 58dB. You need to do a few tricks to prevent overload...