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Valve phono preamp build


Stewyh
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Hi all, I'm about to have a stereo valve phono pre built for me by a techie mate. He has the power transformer and all components needed besides the valves which he advised me to source myself based on what would be most suitable to my needs. So, all you experienced DIYers, if you were building a valve phono pre for yourself, what configuration ( push pull, single ended) and valves (pentode, triode?) would you use? My knowledge of tubes is very limited so feel free to dumb it down!

My hifi is not exactly what you'd call high end. Technics SADX 940 amp, a fairly basic Technics SLBD20 TT with Audio Technica cart and a pair of Usher S520s. I listen to a very eclectic mix of tunes from country, jazz to grindcore and drum'n'bass, but I suppose I mostly lean towards "classic rock". Not sure if this would have any bearing on tube choice.

Any input is appreciated. Cheers.

Edited by stewyh
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Does your mate have a circuit drawn up for this yet? If so then your answers we be there. If not then finding an appropriate circuit is a priority - you can't choose valves without knowing the circuit. Most phono circuits are SE and use triodes, BTW.

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That's a good question. He has a couple. I'll track them down and post the one we'll go with. I know it's an extremely open question, but you've narrowed it down considerably RoHo. SE & Triodes + circuit diagram = possible valve choices.

Cheers.

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I can't read circuit diagrams, but someone here may have the answer to this question. Ask your friend if the 12AX7s are being used as a pair or in sequence. If in sequence, then you need 12AX7s with "matched halves". If being used as a parallel pair for each channel, then you need to ensure that the pair that you buy is a "matched pair". Good suppliers can do this matching for you.

There are lots of cheap 12AX7s out there, Chinese and NOS (new old stock). But for a phono amp, I'd be tempted to go for a high-quality specialist hi-fi valve manufacturer. For example Psvane (which would have been Pavane but for a printer's error in China) make a 12AX7_T hi-fi-quality unit. I bought mine from the US distributor, because they do a much more careful job of grading and pairing that the Chinese factory does, but that made them about $50 each. Sophia also make them, in the same factory production line as the cheaper Full Music, but I have heard complaints about quality control for Full Music and none for the more expensive Sophia. I doubt there is much between them for sound quality, which is apparently very good (I have Sophia 6SN7s and they are exceptional, but that does not necessarily imply that their 12AX7s are of the same musical quality).

Hope that helps

Cheers

Warren

Edited by warrenmmmmm
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I'm no designer...far from it and i only have a basic understanding, but looking at Patrick's site relating to this design, i found the following...

This is the simplest phono amp I know of. Where someone has a reverse RIAA eq network

and sig gene the values of C5, C6, R8, R9 can be trimmed to get a very good square

wave at 1 kHz and a level response within +/-1dB with sine waves between 50Hz and 16kHz.

This type of phono amp was used routinely in thousands of phono stages fitted to budget

hi-fi gear in the 1950s and 1960s.

The downside is the high output resistance of about 20k at low frequencies because there

is only 6dB of NFB at 20Hz and not much more at 50Hz so the input resistance of anything

connected after this stage needs to be above 200k for unattenuated low bass

That high output resistance makes me wonder a bit about it :confused:

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I imagine the diagram will indicate whether they're a pair or sequenced, but like you I've no idea haha. I wonder if it's a necessity to use 12ax7's or would something entirely different be advantageous in any way?

12AX7s seem to be the triodes of choice for phono preamps due to high amplification capability and fairly low noise. A 12AT7 would not provide the necessary amplification. My MiniMax uses a single 12AX7 as two matching halves for the first amplification stage and two matching 12AX7s for 2nd-stage amplification.

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I'm no designer...far from it and i only have a basic understanding, but looking at Patrick's site relating to this design, i found the following...

This is the simplest phono amp I know of. Where someone has a reverse RIAA eq network

and sig gene the values of C5, C6, R8, R9 can be trimmed to get a very good square

wave at 1 kHz and a level response within +/-1dB with sine waves between 50Hz and 16kHz.

This type of phono amp was used routinely in thousands of phono stages fitted to budget

hi-fi gear in the 1950s and 1960s.

The downside is the high output resistance of about 20k at low frequencies because there

is only 6dB of NFB at 20Hz and not much more at 50Hz so the input resistance of anything

connected after this stage needs to be above 200k for unattenuated low bass

That high output resistance makes me wonder a bit about it :confused:

Interesting. The implication would be that it would need to feed a valve amp, or alternatively, you need a buffer amp to bring the resistance back down to a solid-state-friendly level. Good quality buffer amps do not come cheaply (more expensive than some phono preamps).

Edited by warrenmmmmm
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Feeding my valve amp, would be a mismatch!

Edit: While I'm a novice with this stuff, I think I would prefer the next schematic on his page...passive RIAA and low output impedance.

post-105164-0-45316700-1347722504_thumb.

Edited by datafone
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Feeding my valve amp, would be a mismatch!

Edit: While I'm a novice with this stuff, I think I would prefer the next schematic on his page...passive RIAA and low output impedance.

Ok there's no reason we can't use this design. As I have a solid state amp and don't wish to add a buffer amp, would this design be a better option than the schematic I originally posted?

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Stewy,

I can only emphasise Data and Warren's points.

This (2 stage) circuit is not a good match for an AV receiver due to impedance issues. This primarily comes through using the 12AX7 as the final stage. This is a cost-saving measure as it draws very little current which means a smaller power supply. Also you can use a single tube for the 2 stages - the 12AX7, like most small triodes, is 2 triode sections in the one "bottle". Back in the days of mono, when this circuit was introduced, that meant you only needed one tube for the whole thing. Simple, and cheap to manufacture. There are plenty of newer 2 stage circuits out there using a 6922 or 12AU7 as the final tube giving a much lower output impedance, but still marginal for use with an AV thing.

Ideally you want a circuit with a buffer stage (3 stage circuit) but that means more cost and complexity. The EAR 834P is a classic, simple design here

Look, the original circuit will work. It will just sound flat and tinny. If you just want something really basic it will do, but why bother going to the effort of getting it custom built? Just get an op-amp based NAD PP3 or similar which will actually sound better for less than $200. Or be patient, try to understand the basic tech issues and you'll end up with an excellent phono stage you can be proud of long term for not much more than it would cost to do the original 12AX7 circuit.

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I would head over to diy audio and in particular the valve itch phono by salas or the phono designed by SY. Not only will the designer assist, there is a community of tweakers and improvements all too willing help out.

Edited by LockedoutofCraigC
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Good points. I don't want an impedance mismatch. So that EAR 834P is a three stage circuit with 2 12ax7s and one 12au7. I imagine both the following designs would require a different power transformer than the original schematic I posted. Am I correct?

So looks like these two are good contenders, any opinions on which would suit my SS amp?

RoHo's suggestion

EAR-834PT-1.gif

And found this one on DIY Audio

http://www.flickr.co...tream/lightbox/

You can read about it here http://www.diyaudio....itch-phono.html

Edited by stewyh
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Good points. I don't want an impedance mismatch. So that EAR 834P is a three stage circuit with 2 12ax7s and one 12au7. I imagine both the following designs would require a different power transformer than the original schematic I posted. Am I correct?

Probably not. Both circuits will draw more current but in the overall scheme of things still a relatively small amount. The main power supply voltage (known as "B+" in valve circuits) is broadly similar in all circuits so that shouldn't be a problem.

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Cool, so, which circuit/valve diagram would be the better for my application? I've no idea, I could stare at schematics all day and not know the difference haha. Wouldn't mind seeing some more but if anyone thinks either of these would be perfectly suitable, feel free to say so! My mate says he can build either of these no problem

Cheers.

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Actually, having read up a bit on the "Valve Itch" I might be inclined to go with it. The thread on DIY Audio is a bonus - any problem you may come up against is likely to have been covered. And "Salas" the designer is a vey helpful and clever chap. I built one of his regulator circuits and it works a treat.

The power supply is more complex, though, with the "upper" triode's heaters needing a separate, elevated supply. Nothing really tricky, though.

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My mate is rather keen on the EAR 834P design but it will boil down to it or the valveitch one. Will post the results when we have the finished product.

The 834P is also popular with the modders. Might be worth searching the web to see if you want to incorporate any of these modifications into the build.

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My mate is rather keen on the EAR 834P design but it will boil down to it or the valveitch one. Will post the results when we have the finished product.

Then again, as there is a post on these pages that says an owner of duc's phono stage said it trashed his genuine EAR 834p ... why not buy duc's phono stage?

Or do you prefer the involvement of building something ... over the enjoyment of listening to the result? (IE. DIY is what appeals to you - not the sound of the result?)

Regards,

Andy

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Then again, as there is a post on these pages that says an owner of duc's phono stage said it trashed his genuine EAR 834p ... why not buy duc's phono stage?

Or do you prefer the involvement of building something ... over the enjoyment of listening to the result? (IE. DIY is what appeals to you - not the sound of the result?)

Regards,

Andy

In a word, cost. Stewy hasn't mentioned budget but I'm guessing it is considerably less than what he would need to obtain Duc's unit.

With the other gear Stewy there's no point in spending kilobucks on a phono stage - you won't notice the difference. I'm not suggesting these circuits as being SOTA, just that they are simple and well proven. DIY makes sense 'cos you can end up with the equivalent performance of a "bought one" for about 30% of the price, labour not included.

The real question is "do you really want valves?", not "which is the best circuit?". You could build an op-amp based phono stage for a lot less moola that will probably sound just as good on his system. But would it have the same feel good factor?

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All up the project is gonna cost me about $150 for the EAR 834p type or closer to $200 for the valveitch depending on which one we choose. So it's a combination of DIY satisfaction, cost and sound quality. So long as we obey the circuit diagrams to the letter, and use high quality caps and valves the sound quality shouldn't be an issue anyway. And given that a genuine EAR834p is around $800, I think it's a pretty sensible option. The reason I'm going valve over SS is that my amp already has an SS phono stage, which I've never been happy with. Sure I could add a better SS pre, but hey, I just like valves! I like how they sound and how they look. I've got a little 12watt mono valve amp (resto'd by the same friend) hooked up to a single full range 3" driver and it's awesome (in it's own way) hehe.

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