A Valve Preamp with Phono Stage and Tone Controls.

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Cressy Snr
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#31 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Paul Barker wrote: Thu Sep 17, 2020 7:17 am My work van with a crap audio that the young lads love gets treble turned down and bass up so I can tolerate the awful quality of the radio and the reproduction. But peace and quiet sounds better.
I like peace and quiet in the car and rarely use the radio these days.
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Ali Tait
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#32 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Ali Tait »

FM is generally crap for content other than R4, at least with DAB there is a reasonable choice of genres.

I tend to listen to audiobooks mostly now, helps pass the time on long journeys.
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#33 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Here's the setup:
PhonopreRobinson.jpg
Circuits, courtesy of Max Robinson's Angelfire (Fun With Tubes) website.
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izzy wizzy
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#34 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by izzy wizzy »

If I may put my 2p worth in. I would stay away from active EQ. For all kinds of reasons, I think it's second rate compared to passive.
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Nick
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#35 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Nick »

Not sure that phono stage will be much to write home about.
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#36 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by JamesD »

Along with the tilt control and the loudness switch how about adding an Harmonic restorer too? Here is JRB's take on one

https://tubecad.com/2013/06/blog0265.htm

Seriously JRB's suggestion that the tilt control could be incorporated into his tetra phono design is a really good one...

ciao

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#37 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Thanks chaps.
Now that the verdict is in on my proposals :) it looks like I need to complicate the phono section and simplify the line stage section. To that end, I’ll just use a tilt control in the tone stack. TBH, I think that using tilt, will be a perfectly adequate method of making fine adjustments.
The phono will be using ECC88s as I have loads of NOS courtesy of Nigel (Shaman of this parish). It is a Broskie circuit, that uses 5 dual triodes - four of them for amplification and the fifth one as the cathode follower output buffer; one section per channel.

I’ll try again with the schematics sometime this weekend.
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#38 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Here's another set of schematics. You'll have to excuse my naivety, but I'm new to preamps, I'm trying really hard and these diagrams take an age to draw.

I need telling about any howlers.

The setup below is put together from a Broskie phono stage design, but with the cathode follower removed from the output. This is because there is a cathode follower driving the tone stack, and the phono stage can use that instead.

The line stage is by Merlin Blencowe, from his high-fidelity preamp book, and uses an active tilt control. I have modified the tone stack myself with a pair of 1M6 resistors in parallel with the 100K resistors in each half. This was because 500K Lin pots are more easily obtainable than the 470K ones Blencowe used, so the 100K series resistors needed recalculating. 93K8 is the new resistance to maintain the same, symmetrical boost and cut as the 470K pot gives.

The cathode follower on the line stage input, which drives the tone stack itself, is from a post by Neal on the WD forum FAQ - upgrades section and is a direct lift from the cathode follower conversion of the old WAD Pre II.
Six valves in all: 4 for the phono and 2 for the line stage.
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Nick
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#39 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Nick »

You will have 3.6v on the output of that phono stage without the cathode follower.
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#40 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Nick wrote: Sat Sep 19, 2020 3:27 pm You will have 3.6v on the output of that phono stage without the cathode follower.
Cheers Nick, I’ll put it back on.
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izzy wizzy
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#41 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by izzy wizzy »

Not sure about Brooksie CCT. But CCDA is a block with gain stage and cathode follower drawing the same current with an option of a bit of feedback or not. Not quite sure what you have going on there with cascaded 88s and DC on the outputs of those stages as Nick says. Can you link to the Tubecd stages you copied?
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#42 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Oh poo!
I see what I did wrong. The CF is needed to keep DC off the linestage input pot! Broskie even says that in his write-up..which of course I didn’t bother to read properly..Stupid boy! :oops:
Told you I was a bit thick about these things.

No problem to put the CF back. Seven valves then.

Using a bought phono stage and just building the linestage, might have been better for a beginner; you know..ease in gradually (cue DTB) but I won’t give up just yet.

Here’s the page in question:
http://www.tubecad.com/articles_2002/RI ... page9.html

I’m steeling myself right now for the verdict.
Why do I start these things? :lol:
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izzy wizzy
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#43 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by izzy wizzy »

You're using a phono at the moment. What is it and do you know the gain? If it is in the right area, this is the gain you should be aiming for with this design.
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#44 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

izzy wizzy wrote: Sat Sep 19, 2020 5:17 pm You're using a phono at the moment. What is it and do you know the gain? If it is in the right area, this is the gain you should be aiming for with this design.
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#45 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by izzy wizzy »

I have a prejudice that phono stages should have a high gain first stage. And with this, a two stage design is enough. There are a number of ways to do that. I don't know for sure, but I would imagine a number of cascaded low gain stages will result in quite high noise. A high gain first stage could be a cascode 88, a 12AX7 in say a CCDA stage or a pentode. I think an 88 as stage 1 will sound a bit wimpy; lacking drive and voom so a bit dynamically flat. But that's a guess. This is why you see a lot of 12AX7 in commercial offerings.
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