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Old 17th June 2006, 08:08 PM   #1
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Default HELP with PHONO and MIC preamps.

Hi. First of all, let me apollogize for lacking all the basic electronic knowledge here. I would appretiate any links to pages explaining the really BASIC stuff. I should have payed more attention at the physics classes!!

What is all that impedance matching and capacitance!!!

Well, my question is this: I am trying to digitize some records. I have an acceptable sound card. And also a new turntable.

Would it be possible to use a comercial regular tube MIC PREAMP, and adapt it somewhat so that it could act as a FLAT PHONO PREAMP? I do not care that much about having to correct the signal later,... but apparently the phono outputs are extremely sensitive to anything not being a phono input.

I was thinking of applying the RIAA eq after the A/D process. Maybe a simple circuit can introduce the RIAA before that, but I do not know (I know capacitors are being used for the tone control of electric guitars, so I assume they have "filtering" capabilities). Any suggestions?

Best regards,

P. Lazkao

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Old 17th June 2006, 08:41 PM   #2
phn is offline phn  Sweden
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You can use any phono preamp. Just connect it to the mic input on your computer. You might need an RCA to 3.5 mm phono converter or cable.

Or you could get a TerraTec Phono Pre Amp Studio USB.

You will also need some sound editing program, like Sound Forge.
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Old 17th June 2006, 11:52 PM   #3
EC8010 is offline EC8010  United Kingdom
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You need a decent RIAA pre-amplifier and you need to connect it to the "line in" input, not the microphone input (which it would overload).

I'm afraid that designing a good RIAA pre-amplifier is not trivial - it's probably the hardest "consumer" design, comparable to designing a microphone amplifier for "professional" kit. It's easy to do either badly, but doing them well is much harder. Forget trying to modify something. And in your proposed venture, any failings would be permanent.
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Old 18th June 2006, 09:23 AM   #4
lndm is offline lndm  Australia
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Quote:
Originally posted by P Lazkao
a comercial regular tube MIC PREAMP, and adapt it somewhat so that it could act as a FLAT PHONO PREAMP?
I know this is not typical, but P Lazkao seems to have the drive to try it.

Correct me if I'm wrong, but a phono stage is different from a normal amp as it needs:
1/ Low noise.
2/ Matching.
3/ EQ.

--------
1/ A mic amp is already a low noise amp.

2/
Quote:
What is all that impedance matching and capacitance!!!
At the input of a typical moving magnet amp, the cartidge should see 47k of resistance. It should also see maybe 200-400pF of capacitance. This capacitance is usually a quality of the cable, so nothing to do here (unless you are trying to get it perfect).

3/
Quote:
I was thinking of applying the RIAA eq after the A/D process.
I think maybe yes?
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Old 18th June 2006, 03:26 PM   #5
EC8010 is offline EC8010  United Kingdom
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Agreed, a microphone amplifier is a low-noise amplifier, and it's easy to modify it to be 47k//250pF (add that to the arm cable capacitance and you get the 400-500pF that many MM cartridges need).

It's the RIAA that's the problem. RIAA equalisation has a dynamic range of about 40dB from the 19dB of boost required at 50Hz to the 20dB cut at 20kHz. Implement that digitally and expect convertor issues to surface. You could treat the microphone amplifier as an op-amp and wrap the RIAA equalisation around it. You'd need to know exactly what you were doing to avoid oscillation.
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Old 18th June 2006, 05:36 PM   #6
poobah is offline poobah  United States
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You know guys... the idea of storing emphasized audio is interesting. The limitations of groove width in an LP are directly equivalent to the limitations of a digital window.

I guess my concerns would be the how's and where's of RIAA de-emphasis during playback.

Hmm...

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Old 18th June 2006, 06:52 PM   #7
kevinkr is offline kevinkr  United States
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Here's a simple design that you can build that is intended for what you want to do. I've heard it, and the designer is a close friend. It should be pretty easy to build and deals with all of the stated issues.

Link to site: http://www.triodeguy.com/other_projects.htm

Note if you navigate around the site you will find several other tube/hybrid phono stages you may want to investigate. I have heard several of the others as well. Seems like either the jfet mentioned above or the hybrid mentioned for the calrad mod might be another good choice.

There are also several simple designs on my site you might want to look at under old ots articles. www.kta-hifi.net Personally I don't think any of them are as good as triodeguy's.

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Old 23rd June 2006, 12:34 PM   #8
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Default Thanks a bunch!!!

Thanks a lot for the feedback!!! I can believe so many people got interested in helping this poor electronics-ignorant musician.

By the way, if I did not answer before it is because I do not have internet at home, so I usually need to get to the conservatory in order to access the internet.

Well, I am interested in that post that claims that it is possible to adapt the preamp by modifying the cable.

The preamp I have in mind is a cheap one that uses 12ax7 tubes. I would need a couple. Here is a link to the manufacturer's webpage.

http://www.behringer.com/MIC200/index.cfm?lang=ENG

This is a cheap German brand that makes all their stuff in China. There is also another brand called ART selling a mic preamp for the same price.

Unfortunately I do not know of any Tube-preamp, either finished or as a kit, that sells in Europe, that is specifically a phono preamp, that is why I had this idea of using mic-preamps, and hopefully I would get a better sound (as compared to those from 50 bucks solid-state phono preamps, that I could get a hold of easily).

I have seen wonderful reviews of the bottlehead kit, but I live in Europe, and the overseas shipping is completely out of the question (lucky you that have an American adress to ship at).

I assume the preamp has its own impedance (i might check their specs, and tell you later). So you suggest to solder some electronic compenet at the cable, before going thru the preamp, to create those magical 47KOhm and 200-400 pF (by the way, what does that pF mean? Is a different unit, or is something that you can also express in Ohms, 'cause I am a zero at electronics, believe me).

Well, thank you again all for the posts. It is great to see people helping each other so selflessly. Best regards,

Pau Lazkao.
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Old 23rd June 2006, 12:34 PM   #9
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Well, thanks again, all, poobah, kevinkr for the link, ec8010 and indm.

Now, to this dialogue between ec8010 and indm...

What are those convertor issues that would surface, even after getting the capacitance matching right?

I am intending digitazing at the highest quality available (which is 96KHz, 24 bit). I know that I will be loosing 40db of dynamic range with the bass. So for the bass, I would get less than 24 bits of resolution (20? 16? 12? Any idea?).

Besides this, is there any problem with the A-D convertor? Is it not capable of handling a RIAA emphasised signal properly?

I have also read somewhere about phase issues, but I do not know what that means.

And ec8010, do not panic, I would not even dare to design my own preamp... I can barely follow a kit's instructions...

But I wanted to ask you, the ones that know about this stuff, what would be better, if a cheap phono-preamp, or a modified (and, by the way, also cheap, oops) tube mic preamp and riaa after A-D.

And then, I assume that building a circuit to do JUST the RIAA eq, by leaving all at the bass signal level, is out of bounds, am I right? I was hoping that a couple of electronis components might do the job...

Thanks again, regards,

Pau Lazkao.

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Old 23rd June 2006, 01:02 PM   #10
lndm is offline lndm  Australia
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Quote:
Originally posted by P Lazkao
So you suggest to solder some electronic compenet at the cable, before going thru the preamp, to create those magical 47KOhm and 200-400 pF (by the way, what does that pF mean?
Well, not exactly. A MM phono cartridge works best when 'loaded' with, or connected to a 47k resistance and a capacitance. A little difference from this is not too drastic.

Basically, an amp already has a resistance at it's input. It's really a matter of how much. If it's 47k, do nothing and plug in your phono and use it. If it is higher than 47k, it can be fixed with a component outside the box, if it is lower than 47k it is more difficult to fix.

The capacitance affects the sound and is largely a matter of taste. Phono cables act like a capacitor (all cables do), but with phono this capacitance is useful. With my example, my cables act as if they are 275pF capacitors (I've measured that). I like the effect of this capacitance on my cartridge, so I do not add any external capacitors to adjust the sound. EC8010 mentions liking a higher capacitance and would then add a capacitor across the phono terminals to make up the difference.
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