A Valve Preamp with Phono Stage and Tone Controls.

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Cressy Snr
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#1 A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Yep, that’s the plan. :)
Two boxes: power supply separate. Audio cct to be common cathode stage into cathode follower buffer front end, driving passive James/Baxandall circuitry, then another cathode follower for the output. DC heated, It’ll have selector switch, volume, bass, treble and balance controls. Pure analogue old skool and something interesting to build.
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#2 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by pre65 »

Interesting Steve. :)

Would it drive a MoFo ?
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#3 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Don’t see why not.
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Ray P
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#4 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Ray P »

pre65 wrote: Sun Sep 13, 2020 1:23 pm Would it drive a MoFo ?
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#5 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by pre65 »

Will you be using octal valves Steve, or have you not got that far yet ?
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#6 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Not got that far yet, but It’ll probably be casaded ECC88/6922s in the phono stage and a single one as the cathode follower drivers for the tone stacks, plus an ECC82 for the initial gain stage following the line level inputs.

I may yet go Baxandall active with feedback, for the tone control section, meaning an ECC81/12AT7 or even my prized GEC A2900, to give enough gain for the feedback to do it’s toney stuff and create the unity gain for that section. The tone stack should give 6dB of bass/treble boost and cut, which should be quite sufficient.

There’ll be none of the 20dB malarkey of the original Baxandall circuit. That would need a pentode to muster up enough gain for those sorts of feedback levels. :shock: Besides, those levels of boost and cut though they might be fun, are completely unnecessary these days.

I've also got plenty of NOS in for the valves I mentioned above.
Last edited by Cressy Snr on Sun Sep 13, 2020 7:13 pm, edited 3 times in total.
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#7 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by rowuk »

Dynaco PAS3?
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#8 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Tone stack and preamp output stage will likely be a unity gain variation of the old Brimar tone control cct, so not the Dynaco.
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#9 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Tone cct schematic, from the Angelfire website. I’'m not about to start reinventing the wheel:
6A72192D-DF3C-4C28-8865-6E44E8E7CF92.gif

1136DCA5-17E1-4E9C-BCAD-2D9A4B62F0F9.gif
It’s a nice variation of the Brimar circuit and the bass and treble turnover frequencies can be moved further out from the centre, if desired, by changing the values of the cap across the bass pot and the caps either side of the treble pot.

EDIT. Nay saying bollocks removed. I don’t need to justify what I want to do. I’m among friends. :)
Last edited by Cressy Snr on Mon Sep 14, 2020 5:46 pm, edited 5 times in total.
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#10 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Phono section (Morgan Jones)
90FD21B2-0227-421D-A6D6-2A88282A0686.jpeg
Yep, I know full well that the whole preamp concept is a mishmash of bits from different sources, but I’m not at all experienced in small signal circuit design and acknowledging that limitation, why would think I could do better?
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#11 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Here’s a simple and practical active Baxandall tone circuit, courtesy of Merlin Blencowe’s “Designing High Fidelity Tube Preamps” book.
E45987DE-BDEE-4EAA-BE6F-49441C700F1B.jpeg
There’s plenty of circuits out there to choose from, so a decision will have to be made eventually, but I thought I would illustrate a few of them.

Here’s an active tilt control from the same source:
1D62B841-10BC-481C-B56A-DE1DA494EDEE.jpeg
This might be very useful indeed.

Thin and bright, like much of my old 60s and 80s stuff? Tilt the response to a bass up treble down setting.

Heavy and dull, like some 70s stuff? Tilt the response to a bass down, treble up setting.

QUAD used a tilt control on their solid state preamps.
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Paul Barker
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#12 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Paul Barker »

I used to like “loudness” boosted upper and lower when playing at low volume, just a switch not an adjustment.
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#13 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Paul Barker »

Paul Barker wrote: Mon Sep 14, 2020 7:07 pm I used to like “loudness” boosted upper and lower when playing at low volume, just a switch not an adjustment.
Does Blencowe cover it? Think the Goodmans module 80 had one. Maybe the values are available on a module 80 circuit diagram?
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#14 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by Cressy Snr »

Paul Barker wrote: Mon Sep 14, 2020 7:09 pm
Paul Barker wrote: Mon Sep 14, 2020 7:07 pm I used to like “loudness” boosted upper and lower when playing at low volume, just a switch not an adjustment.
Does Blencowe cover it? Think the Goodmans module 80 had one. Maybe the values are available on a module 80 circuit diagram?
Hi Paul,
I can't see anything in my Blencowe book about a loudness circuit, but I too liked the effect at low volumes. I think that over the last 40 years or so, a lot of quite useful and helpful stuff has been thrown away in the name of purity.

Cartridge/speaker sounding a bit bright? Buy a different cart/speaker, change the cables, buy new stands/equipment supports, or turn down the treble?
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pre65
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#15 Re: A Valve Preamp with Phono Stage and Tone Controls.

Post by pre65 »

In a hush whisper, what about a graphic equaliser ? :lol:
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