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Old 15th Feb 2009, 9:55 pm   #1
Tractorfan
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Default A simple low-fi phono pre amp?

Hi all,
I want to build a simple mono low-fi valve pre amp to raise the measly output from a magnetic cartridge up to the equivalent of a crystal mono cartridge so I can play my decent vinyl through my 50's KB radiogram and keep its Garrard RC80M for 78's.
I'm not interested in audiophool quality, RIAA curves, NFB, complicated HT supplies (I can get power from the 'gram), a flat frequency response that would impress any passing bat, components that were hand made by dusky Amazonian maidens or any of that fancy nonsense.
I would just like to know if a single EF86 would do the job or might an ECC83 2 stage pre-amp be more suitable? The 'gram has an ECC83 pre-amp/phase splitter and 2 x 6BW6 push pull o/p bottles.
I've read the thread on here about hi-fi stereo phono pre-amps but they're all way OTT for what I want as neither my radiogram nor my ear'oles are anywhere near hi-fi.
Thanks in advance.
Cheers de Pete
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Old 16th Feb 2009, 12:38 am   #2
Ed_Dinning
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Default Re: A simple low-fi phono pre amp?

hi Pete, try the Mullard book on audio amps amp pinch the circuit from the front end of the 5-10 etc. If you want something slightly more exotic a cascode ECC** will also be simple and work well. PM me if you need more details but note that I wi8ll not be back in the UK for another 2 weeks

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Old 16th Feb 2009, 1:09 am   #3
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Default Re: A simple low-fi phono pre amp?

I think a simple ECC83 preamp will do. Maybe even a single triode like a 6C4 (EC90). Have a look around the web or if you have Pauls data DVD, that has Mullard circuits for preamps but they may be OTT. If you have an ECCxx something or other just try one section with a 220K connected anode to HT, cathode to chassis and a 470k grid to chassis. Finish off by connecting signal to grid via a .01uF and the output via a .01uF from anode to input of the main amp......dead simple and you can play about with the values. The voltage rating of the capacitors should be 300v or higher.

You should have proper equalisation to RIAA standard for LP's so you could have a look at the Mullard circuits.


Rich.
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Old 16th Feb 2009, 1:43 pm   #4
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Default Re: A simple low-fi phono pre amp?

Here's a circuit I designed a few years ago.

You may want to dispense with the RC coupling at the input - I only featured it as a bit of rumble filtering.

Built up in a diecast box (to minimise noise pickup), it's quite decent!
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File Type: pdf RIAAamp.pdf (37.0 KB, 202 views)
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Old 16th Feb 2009, 10:58 pm   #5
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Default Re: A simple low-fi phono pre amp?

Hi all and thanks for your prompt replies.
Rich, Thanks for the circuit description, that sounds about the right level of complexity for me! You say I should have RIAA equalisation though. If the radiogram played LP's OK with a crystal cart, what difference would an RIAA (or the lack of it) make to the sound to my second rate hearing? This is all very mysterious to me
Kalee, Thanks for posting the circuit. I'm assuming that the circuitery between the valves is this RIAA thingy.
'Course before I left the UK I had TWO paperback copies of a Practical Wireless book from the 60's illustrating various circuits from the magazine (in those days you had to wear a shirt & tie to build an amp according to the cover picture ) and needless to say BOTH of them went missing during the move . But I've got a few old PW's somewhere so there might be something in those.
Cheers de Pete
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Old 16th Feb 2009, 11:10 pm   #6
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Default Re: A simple low-fi phono pre amp?

I know you're thinking about a valve circuit, but you can build a very good very simple RIAA compliant preamp with a couple of op-amps. A huge variety of chips can be used, even dirt cheap ones not designed for audio use (though there will be a bit more noise with these). Have a look at this excellent project page: http://www.geocities.com/rjm003.geo/.../diy_pho5.html

(Just ignore the audiophool stuff about the fancy PSU and run it from a couple of Poundland PP3 batteries or knock up a PSU using an old wall wart. You could even derive the power from the valve HT line.)

I suppose you'd rather use bottles though...

Paul
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Old 17th Feb 2009, 10:19 am   #7
peter_sol
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Default Re: A simple low-fi phono pre amp?

Hi
You do not state the type of cartridge you need the preamp for
If it is the standard modern moving magnet type you will need RIAA compensation otherwise all you will get is a very bass light sound
the one above looks ideal

Regards

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Old 17th Feb 2009, 1:59 pm   #8
kalee20
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Default Re: A simple low-fi phono pre amp?

Quote:
Originally Posted by Tractorfan View Post
Kalee, Thanks for posting the circuit. I'm assuming that the circuitery between the valves is this RIAA thingy.
Yes it is - if you leave this out completely, the records will sound as though you have the treble screwed right up and the bass turned right down.

Crystal or ceramic cartridges need hardly any equalisation, because the output voltage is proportional to the amplitude of stylus movement, reasonably independent of frequency. For magnetic cartridges however, the output voltage is proportional to the stylus velocity, which is amplitude times frequency. That's why you need equalisation.

RIAA equalisation is a tweak to this equalisation curve, and is incorporated in most magnetic equalisation circuits simply by a slightly more complex circuit (2 capacitors and 2 resistors, rather than just 1 of each). The audiophools go to extreme lengths to choose precise values, so that the RIAA equalisation is achieved to 10µdb or so (or should that be just 1 microbel??), but you need not worry about that fanaticism. Many ceramic and crystal cartridges input circuits just forget about any response curve tweaks entirely, and the listener just adjusts their tone control!
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Old 17th Feb 2009, 7:50 pm   #9
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Default Re: A simple low-fi phono pre amp?

Nice and quick - how about a pre-amp pulled from a cassette deck? Somewhat similar EQ I should have thought.
Graham
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Old 18th Feb 2009, 10:59 am   #10
kalee20
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Default Re: A simple low-fi phono pre amp?

But it won't be valve! As per Tractorfan's original request! (Also, although the equalisation won't be a million miles out, there may be too much treble as there'll be compensation for gap-width loss).
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Old 18th Feb 2009, 9:59 pm   #11
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Default Re: A simple low-fi phono pre amp?

Hi All,
To be honest I would prefer a valvey pre-amp (not for any perceived improvement in "warmth" or "soundstage" - I just love 'em and understand them, and they're relatively idiot-proof - Oh, and they're nicer to look at too). But I might consider a tiny black square circuit, although chippy things tend to have a notoriously short lifespan when tractorfan gets his great mitts on them . I never thought of filleting a dead cassette deck, though. Hmm.
Thanks, Kalee, for explaining why I need an RIAA. The mysterious magical mists are clearing and it's all starting to make a bit of sense to me now.
The cart in the KB is something like a Garrard GC-2 and the magnetic cart is, I think, a Shure M55-E type of thing. Don't quote me but you get some idea of the relative in/outputs I seek to match.
Thanks to all.
Cheers de Pete
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