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Old 11th June 2006, 06:37 AM   #1
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Default Phono Preamp Design Needed

I'm planning to build a phono preamp. So far I've not found any interesting designs on the intarnub, schematics that look like they were scribbled on the back of a napkin, a few 240 volt models, and things that seem little more than a random collection of components, but nothing I'd actually want to build. Does any one know of a website or book where I could find a decent schematic? Here's what I'm interested in:

A simple phono preamp for moving magnet cartidges that can be assembled quickly, with a minimum of expensive transformers and no more than four or five valves, this is on a budget.

I would greatly prefer one with a volume control that can be directly connected to the power amplifier, but a line level output is also OK. A headphone jack would also be nice, I'll be running this through a MOSFET power amp and on occasion I'd like to be able to listen to just tubes.

Valve rectification is OK, but not necessary. It seems even NOS rectifier valves are fairly inexpensive and I already have a few ancient National Union 35Z5s laying around.

A schematic that can actually be read. I've seen several so far in lousy JPEG compression with missing or illegible component values.

NOS tubes are all well and good, but I'd like to see a design that uses tubes that are still in production and can be found easily at reasonable prices. No really weird components either, the parts list is limited to what I can get from Allied, RadioShack and intarnub tube vendors.
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Old 11th June 2006, 06:40 AM   #2
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Check out the Bottlehead Seduction.

jh
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Old 11th June 2006, 09:08 AM   #3
Giaime is offline Giaime  Italy
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If you are REALLY on budget, like me only 3 tubes and solid state rectification.

http://www.giaime.altervista.org/mypreamp.html

Scroll down please, definitive schematics are on the bottom of the page. Obviously I've built it and there's no problem in helping you if you'll encounter difficulties.

This also is a good project, but a little bit more complex:
http://web.tiscali.it/ampli/ -> select " Preamplificatore Phono RIAA"
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Old 11th June 2006, 10:05 AM   #4
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Here's the one I have been using for years:
http://members.dowco.net/canadageek/geek/scc6.png

I use this SS HV reg:
http://geek.scorpiorising.ca/Geek-0.html

I also use DC on all the filaments.
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Old 11th June 2006, 11:14 AM   #5
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Default Re: Phono Preamp Design Needed

Konnichiwa,

Quote:
Originally posted by AudioFox
Here's what I'm interested in:

A simple phono preamp for moving magnet cartidges that can be assembled quickly, with a minimum of expensive transformers and no more than four or five valves, this is on a budget.

I would greatly prefer one with a volume control that can be directly connected to the power amplifier, but a line level output is also OK.

A headphone jack would also be nice, I'll be running this through a MOSFET power amp and on occasion I'd like to be able to listen to just tubes.

NOS tubes are all well and good, but I'd like to see a design that uses tubes that are still in production and can be found easily at reasonable prices. No really weird components either, the parts list is limited to what I can get from Allied, RadioShack and intarnub tube vendors.
Well, purely for the Phono Part I can recommend this design:

http://www.diyaudio.com/forums/showt...624#post335624

It is meant to drive "passive preamplifiers" of 10K or greater input impedance. Depending on the gain of your amplifier you will only need to add a suitable volume control to the output of the Phono Stage. The Valves needed are 1pcs each 12AX7/ECC83 & 6922/E88CC, both of which are in current production and in fact the circuit was originally voiced for the russian Sovtek 12AX7LP and standard russian 6N23 which are commonly relabelled 6922.

The thread covers the options for powersupplies extensively.

If you want an active line stage I recommend this design:

http://www.diyaudio.com/forums/showt...015#post353015

It may be worthwhile adding gridstopper resistors, evben though I normally find no need for them with the 6SN7 and related valves. My personal favourite for this circuit would be the 5687 which is no longer in production, the JJ/Tesla ECC99 is a close substitute currently made.

I would not really recommend this circuit to directly drive headphones though, if you must have a headphone output simply add a 6AS7 as cathode follower with a 2K2/10W cathode resistor (and gridstopper) and 220uF coupling capacitor after the line stage, this will drive many headphones, though not the ultra low impedance types.

For the PSU, using either valve or solid state recrtification is fine with me. I'd go valve. As the Headphone buffer consumes 100mA HT @ 300V and the Phono STage needs around 25mA @ 250V with the line stage needing another 12-20mA (depending on valves used) I would go for a supply that provides a rectified 330V @ 150mA or thereabouts.

By sheer co-incidence (not bleeding likely with me) a simple 230V:230V isolation transformer can produce the required power supply for the HT, alternatively using a 230V:12V transformer with 160VA power rating could be used with an 80VA one equally rated being used to step the 12V back up to 230V AC to power the whole preamplifier. I would suggest using torroidal transformers, as these allow easily to add windings per hand for more heater voltages.

The 12V winding can be used to power the heaters of the Phono & line stage with rectified and regulated (LM317 et al) DC of 6 or 12V as needed. As the headphone buffer consumes a lot of HT Power you really need to use solid state rectification or you need something like a GZ34 used as hybrid rectifier.

Also, you need to add a 6.3V/2.5A winding to the mains input transformer to supply the heater of the 6AS7, normally around 18 Turns with normal mains code 1mm solid core wire will get you in the ballpark.

If you give up on headphones a EZ80/81 will be okay as rectifier and you can use smaller transformers.

Anyway, that would be all stuff that IMHO answers your requirements well.

Sayonara
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Old 11th June 2006, 11:38 AM   #6
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bottlehead zeduction schematic is here:

http://www.bottlehead.com/et/adobesp...headpreamp.htm

18k load resistors, 470k output resistor, 220 ohm plate stoppers...

160 volt power supply, CRCRC filter

220uf, 1k resistors.

filaments are dc.
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Old 15th June 2006, 10:05 AM   #7
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Thanks for the great designs. I think Giaime's design is the closest thing to what I would need right now. Although I've worked with tubes before, this is the first time I've tried building a tube amp from the ground up, and the simple design seems about right. Just a few questions though, what are the wattage values for the resistors? How many amps does this draw for the signal and heaters respectively? Thanks again.
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Old 15th June 2006, 06:43 PM   #8
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@AudioFox

Mouser 115-230V transformers

RatShack filament xfrmrs

These are the transformers I used in my VTL "Ultimate" clone. The triad model from mouser is a goodin'! The RatShack transformers will buzz a little of course but hey whaddya expect?!

BTW welcome to diyAudio forums!
Good to see another person from WV here!

Cheers
Wayne
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Old 15th June 2006, 06:56 PM   #9
Giaime is offline Giaime  Italy
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Quote:
Originally posted by AudioFox
Just a few questions though, what are the wattage values for the resistors? How many amps does this draw for the signal and heaters respectively? Thanks again.
Hello Audiofox. You're right, in fact you should excuse me if I have proposed a design without specifying in the schematics the wattage of the resistors.

About the schematics attached: you'll need 2W resistors for R9 and R12, 1W one for R4 and all the rest can be 1/4W. Metal film, of course.

This is only about the actual *phono* circuitry, if you have other doubts about the linestage or the power supply let me know.

For the currents, well the whole phonostage takes 1A at 6.3V or 0.5A at 12.6V (connect the two ECC88 heaters in series, then parallel them to the ECC83 heater). High voltage power supply should be capable to handle at least 30mA, just to be conservative. Of course heater currents are the needed one: you should dimension your transformer to take those continuatively, so it's better to use 2A windings for 6.3V operation or 1A winding for 12.6V.

Please note that the design works flawlessy, but can be tweaked to maximize performance in many ways. One of the things I will try is to put a constant current source load for the triodes in the phono section.
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File Type: gif definitivephonostage.gif (6.3 KB, 448 views)
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Old 15th June 2006, 08:11 PM   #10
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Hey, AudioFox, nice avi!
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