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Old 17th May 2015, 08:36 AM   #1
max1979 is offline max1979  Italy
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Default First pre phono design [newbie]

Hi,

What follow is my very first attempt to design a MM pre phono based on tubes and I'm looking for an advice. It's just few weeks I'm studying valves and I've tried to collect as many info as possible.

My requirements were:
1) a bit more gain than the standard 42dB (I'd like to have an output voltage closer to the one provided by a CD reader - let's say 1V)
2) as less distortion as possible
3) not expensive tubes (and easy to find)

So I chose SRPP configuration and ECC83 as tubes to have:
- 45-46dB (including a balance control to fine tune left/right channel)
- 70dB between 1KHz and the second harmonic with max gain (HARM.jpg)
- a pretty good RIAA curve (GAINCURVE.jpg)

Now, I'm just a newbie and as any beginner I copied part of the project from other projects, so I'd like to understand if according to you I'm missing something or there is any error or any big problem I will have to face using this design.
Also, I'm going to finalize the power supply and I'd like to know what could be good voltages to biasing the heaters of the upper and lower valves.
Attached Images
File Type: jpg GAINCURVE.jpg (165.4 KB, 473 views)
File Type: jpg HARM.jpg (169.4 KB, 446 views)
File Type: jpg SCHEMA.jpg (83.6 KB, 471 views)
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Old 17th May 2015, 09:59 AM   #2
SY is offline SY  United States
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Advice: don't. A phono stage is the single hardest thing to get right and should only be tackled after you have several other successful designs (line amps, power amps, crossovers) under your belt. Build a proven design to get you by until then. At a minimum, you should understand what an SRPP is, why what you're showing isn't actually an SRPP, why a phono preamp is a totally inappropriate place to use it, why an ECC83 can be a very problematic choice, how to properly model a cartridge, and how to prevent heater-cathode noise transmission and breakdown.

Crawl first, then you can worry about sprinting. And don't feel bad, the VAST majority of diy tube phono stage designs out there are absolutely awful.

At a very minimum, I would read and understand (including working through the math) the phono section of "Valve Amplifiers" 4th edition.
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Old 17th May 2015, 11:37 AM   #3
max1979 is offline max1979  Italy
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Not so happy to hear this from you
Thanks for your advice, I'll get the book!
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Old 17th May 2015, 01:57 PM   #4
SY is offline SY  United States
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Just being honest- and trying to be helpful. It's a real challenge to get that kind of stage designed correctly, then getting the layout and grounding right. People also tend to overlook what the loading on the cartridge looks like and how that affects performance. Noise, gain, overload, all are very tricky when you're dealing with a signal that can be in the microvolt range, is strongly affected by loading, and needs 50-60dB worth of equalization!

Morgan Jones's book will take you through some good design examples (he considers the engineering in some detail), and you could do well by starting out building one of his simpler ones.
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Old 17th May 2015, 05:13 PM   #5
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I did one 20 years ago with a single transistor front end and an LF353 op amp performing the RIAA function. I tuned it using Brothers in arms ( Dire Straights) on LP and CD for frequency response. Still sounds fantastic but I am going to try a 12ax7 in place of the op amp and use 100v B+.
Go For It. A pre amp is much easier then an amp as all you have to tune in the RIAA curve capacitors.
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Old 17th May 2015, 06:15 PM   #6
kevinkr is offline kevinkr  United States
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Have a look in the analog source forum for some well vetted phono stage designs you can build.

I agree with SY, as another prolific designer and builder of phono stages I still design the occasional real stinker. It takes a lot of practice and understanding all of the tradeoffs involved in the design, and a great deal of attention to detail - an area where SY really excels.

I see a lot of issues with the design you posted including that you are very unlikely to get anywhere close to the gain you are predicting with this tube and topology. Noting that the ECC88 is nonetheless one of several good choices.
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Old 17th May 2015, 06:35 PM   #7
Merlinb is offline Merlinb  United Kingdom
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It's not the best design but it is functional, and I've seen worse. It's a simple enough circuit that you might as well lash it up roughly and give it a try for yourself. You'll learn a lot more from a practical experience than from soliciting idle opinions. Just don't spend all your money on putting it into a nice box yet -build it in a biscuit tin with cheap components. Then you can ask questions and improve. Get your hands dirty!
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Old 17th May 2015, 07:13 PM   #8
kevinkr is offline kevinkr  United States
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A good point I think.. Can't hurt to try it.
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Old 17th May 2015, 11:17 PM   #9
piano3 is offline piano3  United Kingdom
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Merlin, great to see another fan of using biscuit tins. My quietest mic amp is built in a "McVitie's Classic Selection" tin. I had it in the girl's school where I teach to record a concert and was just setting up the mics when I saw a couple of hungry looking girls circling the tin like vultures. When I shooed them away their complaining chorus was "how were we supposed to know?" so I explained carefully that the XLR connectors and various wires coming out of the box should perhaps have been a clue that there was nothing delicious inside. I would add that this is a highly selective school..........
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Old 17th May 2015, 11:32 PM   #10
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Quote:
Originally Posted by piano3 View Post
I would add that this is a highly selective school..........
For girls that don't get regular meals?

jeff
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