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RIAA and Linestage for life time

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I am not getting any younger and slowly it is dawning on me that I should discard the assorted riff-raffs of the past and build a gear that will see me through rest of my life. So here we are. The aim is to build a RIAA and Line stage that will not make me want to change it ever. Can we discuss possibilities with following broad parameters?

- No feedback. As far as possible.
- No interstage/output transformers
- Minimum stages
- Only use any or a combination of following tubes
- 6SN7
- 6SL7
- 6DJ8
- 12AX7
- 6267

My options are limited to the above set of tubes. So esoteric types will not be of much help. In past I’ve built several ‘generic’ types 12AX7’s and 6DJ8 topologies and somehow felt all of them lacking in transparency, detail and LF response.

Thanks in advance.
 
Ex-Moderator
Joined 2003
So you're looking at passive RIAA equalisation with valves biased for optimum linearity, and a cathode follower line stage to drive cable capacitance. 6DJ8 can make a quiet disc input stage, and 6SN7 would make good succeeding stages. Are moving coil input transformers forbidden, or is your cartridge moving magnet? What sensitivity is the power amplifier?

Edit: Ah, so you actually require 400-500pF of input capacitance. What is the capacitance of your arm and cable?
 
Konnichiwa,

corbato said:
The aim is to build a RIAA and Line stage that will not make me want to change it ever. Can we discuss possibilities with following broad parameters?

- No feedback. As far as possible.
- No interstage/output transformers
- Minimum stages
- Only use any or a combination of following tubes
- 6SN7
- 6SL7
- 6DJ8
- 12AX7
- 6267

I would say that the above is limiting enough that no matter what, you will be wanting to change.

You have eliminated input/output transformers, that presumably also includes Transformer Volume controls?

For a "limited" Phonostage I'd recommend this one:

http://diyaudio.com/forums/showthread.php?postid=335624#post335624

Linestages are more difficult. How about a parallel 6SN7 or ECC88/6DJ8 with sone output to grid feedback to get the gain low, the output impedance downs and all? Take a 22k Anode load and individual 1k Cathode resistors plus bypass Cap, 1uF ouput cap and a 220k .... 470k Resistor from output to grid. Then a 100k Resistor from the grid to the 100k Log pot or directly attach the wiper of 470/500k or even better 1M Carbon Track linear Pot.

I have drawn this out in the attached piccie, this stage has around 6db gain << 1K Output impedance. You can pretty much do the same thing also with a ECC88/6DJ8 in the linestage of course, simply reduce the cathode resistors to 750R each.

I think if you slightly scale the Powersupply I suggested for the Phonostage you could use it for the linestage too. Not really my kettle of fish, but you set the groundrules to preclude almost all with any seriously good sonic potential, so what can I do? The results should be okay enough I guess.

Sayonara
 

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diyAudio Senior Member
Joined 2002
Hi,

Given the restrictions of the tubelist I personally favour the 6DJ8, both for phono as linestage.
You don't need the gain in a linestage but its high transconductance is always a good thing to have.

While we're talking linestages, I always wondered why no one bothers to design a linestage and headphone amp all in one box?

Instead of using a local NFB loop why not use a good CF and be done with it?

Just my twopence,;)
 
Hi, Ashok -

I would ditch the 6DJ8 and definitely not use any silicon anywhere near the signal path. Even one transistor and you've lost a lot of potential on your very best analog recordings, IMO. Maybe 6SN7's 6SL7's and 6BX7's or 6BL7's if you want low impedance/high transconductance perhaps with an initial gain stage followed by passive RIAA eq, then a second gain stage/buffer driving a passive gain control with an otherwise passive architecture would seem most satisfactory. I did something somewhat similar about ten years ago and there's no comparison with anything commercial I've heard when playing LPs.
 
Konnichiwa,

fdegrove said:
Given the restrictions of the tubelist I personally favour the 6DJ8, both for phono as linestage.

I find all else being equal actually mixing different valves is a good idea. The ECC83/ECC88 Phonostage (BTW, for hopeless Octalists - yes, you can use a 6SL7 in the first stage) tends to work quite good because the two valves have pretty much "opposite sonic personalities". Combine that with the moderatly clean, but normally rather boring sounding 6SN7 in the linestage with a little feedback to get the measured and audible performance up and you have a "nice combo" for a full function preamp.

fdegrove said:
Instead of using a local NFB loop why not use a good CF and be done with it?

Because even the Allan Wright / Joe Rasmussen "SLCF" sounds less that ideal. If an active linestage at all I'd avoid the follower. Quite an easy job to have a listen. Just my sixpence of course and views/tastes differ.

Sayonara
 
diyAudio Senior Member
Joined 2002
Hi,

I find all else being equal actually mixing different valves is a good idea.

In principle I'm in favour of that philosophy as well provided you know your valves and their sonic fingerprint as well as you seem to do.

For a MM phono stage the ECC83-88 combo looks a good idea.
If it were a MM phono stage I'd approach it in an entirely different way.

Because even the Allan Wright / Joe Rasmussen "SLCF" sounds less that ideal.

Never had the chance to listen to one so far which is why I'm still very much in favour of the WCF which comes as close to neutral as its passive components allow for to my ears.

Just my sixpence of course and views/tastes differ.

No arguing there...
Maybe time for us to open a thread revealing what we expect our gear to do sonically.

Cheers,;)
 
Ex-Moderator
Joined 2003
I can't help feel that there's been a lot of fuss and bother before full requirements have been determined.

Does the pre-amplifier need to drive significant capacitance?
Is the arm capacitance confirmed as 175pF?
Is only LP required?

(The input requirements might allow a 6SL7.)
(The output requirements might allow a common cathode 6SN7.)

Let's find out what it needs to do. Then we can argue about the best way of doing it.
 
diyAudio Senior Member
Joined 2002
Hi,

Drrrrrrrr....scrolling back to post # 1 I read:

I am not getting any younger and slowly it is dawning on me

that I should definitely try out that latest HGH hormone that promises me eternal youth...Ooops, wrong forum.

Seriously, given the Shure as an healthy MM with good track record (pun intended)...
Yup, a 6SL7 and a 6SN7 would make a nice preamp capable of phono and line.
Naturally, I'd stick more valves in the PSU than in the circuit proper...That's just me though...

Errrrr....why is it so important to know cable capacitance before we even put pencil to paper?

Cheers, ;)
 
Ex-Moderator
Joined 2003
fdegrove said:
Errrrr....why is it so important to know cable capacitance before we even put pencil to paper?

Early Shures are critical on load capacitance, and need 400-500pF to get their electrical equalisation of the transducer correct. Later cartidges (post CD-4) tend towards 250pF. Oddly, the significance is that an early Shure allows you to get away with something that a later cartridge wouldn't allow. I suspect that Corbato's Shures could be driven very nicely into a 6SL7, but we need to know the cable capacitance precisely to see if the cartridge will tolerate the Miller capacitance plus that cable capacitance.
 
diyAudio Senior Member
Joined 2002
Hi,

I suspect that Corbato's Shures could be driven very nicely into a 6SL7, but we need to know the cable capacitance precisely to see if the cartridge will tolerate the Miller capacitance plus that cable capacitance.

Okidoki...
I usually consider it good practice to keep phono leads a short as feasable and use as low capacitance cable as I can get my hands on even if that means a little added inductance.

Some SL7s can be quite extraordinary as you may know already...

Thanks for the 'splainin',;)
 
but we need to know the cable capacitance precisely to see if the cartridge will tolerate the Miller capacitance plus that cable capacitance.

An alternative is to use feedback around the input stage, feedback connected to the cathode of a cathode grounded stage lowers the miller input capacitance with the same amount as the feedback ratio, this fact is often overlooked when discussing RIAA phono stages.

Regards Hans
 
diyAudio Senior Member
Joined 2002
Hi,

The thing that surprised me was that insufficient capacitance was actually worse than too much (hence the Ortofon add-on capacitor).

Oh, yes...Absolutely. Not enough C and you'll end up with a rising top end wondering who stole the bass.

Hans,

You're absolutely correct but Corbato expressed a wish for as little FB as need be.

Cheers,;)
 
You're absolutely correct but Corbato expressed a wish for as little FB as need be.

Ouch! I didn't see that, I just wanted to point out the fact that also input capacitance is reduced by feedback as many people doesn't seem to remember that, fore instance even Morgan Jones in his book "Valve amplifiers" doesn't mention this fact and is discarding 12AX7's due to high Miller capacitance.

Regards Hans
 
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