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Old 25th November 2003, 06:36 PM   #21
matt_uk is offline matt_uk  United Kingdom
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IT WORKS!!!

Added 22uF caps (biggest caps I had a pair of) across the 475ohm resistor and got the DC offset down to ~300mV so decided to plug it in and try it.

First listen was to 'Planet Perfecto - Bullet in the Gun', it sounded terrible, very crackly and vocals were really broken up. I think this vinyl is a bit warped though so tried my 'Blur' album instead. Sounded better, I haven't listened to vinyl for a while (hence why I wanted to build the phono stage) but this was listenable to. It all sounded a bit muffled, the instruments or vocals didn't really stand out individually as they usually do.

Very hard to compare as i'm playing this into a Panasonic Midi HiFi system, so it's obviously not comparable to my Arcam playing out through B&W Speakers.

I think I may need a bit more gain, I was having to put the volume right up and slight volume variations at the top end were having a very noticable effect on the sound and tone. Can I adjust the gain (with 1 resistor) without losing the RIAA accuracy? I'm sorry I haven't got around to reading up on RIAA properly yet.

Still need to add the power supply caps that I foolishly missed, and of course the essential blue LED

The case is one of those old fashoined aluminium fronts with black vinyl over the top and sides. But i have used it longways and upside down so it's black on the front and shiny aluminium on the top and sides. Hoping to build a gainclone (or maybe FET amp) to match.

Matt
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Old 25th November 2003, 10:00 PM   #22
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The problem with the gain and also likely to be the cause of the crackle/distortion is because you have added a capacitor across the 475 ohm resistor. This completely messes up the frequency response!

The way to add the cap is to unsolder one end of the resistor and put the cap in series with it, i.e. one end of the cap goes in the hole left by the resistor.
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Old 26th November 2003, 05:28 PM   #23
matt_uk is offline matt_uk  United Kingdom
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SORTED! What a beauty!!

The reason I put the cap across the resistor was because it was the easier way to add it to my circuit and I didn't really understand what it was doing. Maybe that is why the distortion was really bad on vocals but guitar solos played much better, a frequency issue.

Anyway all is fixed now and it sounds great, I was really dissapointed last night when I first heard it and thought I may need to make some big changes. Played a few songs through it now from Paul McCartney to Guns N Roses, although I listen to Guns N Roses so often in the car i'd got used to the sound so it now sounds completely different through a hifi and on vinyl.

Can you explain exactly what the extra capacitor does, now i've finished the circuit I really need to go over it and work it all out properly, I know I should've done that first! I am an electronic engineer but haven't touched analogue stuff for a couple of years, to busy with programmable logic and ASIC cores.

Thanks for the help guys, can't wait now to make a matching amp, although I do have to wait till after christmas as I know I have half of the components already, just at my parents house. Then it will be onto the CD-ROM CD-Transport project, thats much more my domain!!

Matt
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Old 26th November 2003, 06:24 PM   #24
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The impedance of a capacitor increases as frequencies get lower, so by placing it in that part of the circuit that determines the gain, the gain changes with frequency. In this case, the gain is reduced as frequency gets lower, thereby not amplifying the DC offset so much. It will also make your system less sensitive to walking on the floor by the turntable and record warps.
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Old 26th November 2003, 07:30 PM   #25
matt_uk is offline matt_uk  United Kingdom
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DOH!! Should've realised that one. Thanks for the info.

As the signal tends towards DC the 'resistance' between the -v input and ground tends to infinity and hence the gain tends to zero.

It's these simple things that I do know but I just can't see them in the circuit because I only ever did them at Uni around 4 years ago.

Have read Thorstens page about the circuit, I must say that I didn't use any of the specified components (probably cos I didn't read it first!!), just whatever they sent me polyester film caps, box caps and 40-63V radial electrolytics, although I avoided basic ceramics, and 1% Metal Film resistors throughout, resistors were hand matched on a meter. In a metal case (as NOT recommended), cheapo RCA sockets and 'standard gauge' basic wires throughout, including the long battery leads with no supply filter caps

Anyway, sounds good to me, will completely re-wire and upgrade connectors when I get it in my proper hifi setup instead of the midi system I am currently stuck with.

Matt
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Old 26th November 2003, 08:25 PM   #26
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Metal boxes are fine. Some stay steel ones are bad because they are magnetic, however, an aluminium one is not. I would advise strongly to use a metal case as it provides shielding.

Thick wires will make no difference in this application as the currents will be so tiny. The only wires I would advocate you pay any attention to would be the signal leads. These should be screened and ones with the screen and core close together can have more capacitance than more separate ones, which is not great, especially for phono pre-amps, as the cable capacitance can affect the loading of the cartridge.

Supply caps are not essential with battery power as there is no mains hum to filter out Just 1uF or even 100nF caps will probably be fine.

IMO the components you have used are fine and, as you have matched some of them and avoided ceramic caps, there is no need to do any more.
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