Check Out MY El Cheapo Phono Preamp! - diyAudio
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Old 11th January 2004, 03:34 PM   #1
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Wink Check Out MY El Cheapo Phono Preamp!

Hello to all!

I have been reading the posts about low cost phono preamps,
and thought that it might be interesting to post a picture of my
variation on the El Cheapo phono preamp. It is a variation with
MM only, and uses regulated AC power supplies (I like very long
warm-ups for listening). It has the following features:

1. 4.2 Amp toroidal transformer
2. rectification bridge with Harris superfast/soft recovery diodes
with capacitor snubbers
3. 4700 uF Panasonic HFQ electrolytics with 2.2/.01 bypasses
4. JRC +/- 12 VDC regulators (1.5A) with very low noise levels
5. bleeder resistors on regulator outputs to bias regulators
to a higher current output level
6. 3 paralleled Black Gate 100uF electrolytics on regulator
outputs, plus poly bypasses at chip pins
7. Class A output bias on OPA 637 units
8. outputs are direct coupled to RCA jacks (no problems here)
9. all circuit board connections hard-wired point to point
10. seperated ground returns where possible
11. all aluminum case with hardwood end panels

I spent just $ 115.00 (with help from my junk box) to build this
preamp. Even though the transformer is just 6.5" from the
active circuit board, the hum level is inauduible from my own
listening position (I have a subwoofer) at -73dB relative to
a 0dB level on a vinyl record. I broke in the piece for 300 hours
and did 2-3 day warmups for listening. In my opinion, the sound
quality from this design (even without the battery power supply)
is simply stunning for the money. Very fine transparancy, sound
staging, resolution, and a fairly neutral overall sound that does
not at all seem very "solid state". Perhaps just a bit dark in
nature. I have listened to preamps costing much, much more
that did not do as well. Many thanks to Thorsten for his RIAA
network design for this circuit!

Fastcat
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Old 11th January 2004, 03:39 PM   #2
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Very elegant
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Old 11th January 2004, 04:27 PM   #3
mlloyd1 is offline mlloyd1  United States
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VERY nice!

I like the inset jacks on the rear panel. Nice touch.

Did you do the mechanical work and lettering yourself? Looks like a pro did it?

Any hum at all (audible or measured with scope, etc.) with that transformer in the box?

Good job!

mlloyd1
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Old 11th January 2004, 05:41 PM   #4
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Very nice looking
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Old 11th January 2004, 06:26 PM   #5
andy_c is offline andy_c  United States
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Very nice, fastcat! I really like the way you've combined woodworking and metalworking in the chassis design. A very creative and attractive design.
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Old 11th January 2004, 06:42 PM   #6
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Pretty impressive indeed. The sides are milled out from one piece?
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Old 11th January 2004, 09:45 PM   #7
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To All:

The case for this preamp took more time than building the
active circuitry. The front and rear panels are milled from 1/4"
aluminum that were sandblasted & painted. I also did hand
lettering with white Press Type Letters, and overcoated the
panels with Krylon matt spray for protection. the hardwood
end panels are indeed 1 piece millings. This case took so
much time that I may never do this again!

As to the transformer inside the case, there is a residual hum
that is about 73 dB below what would be considered a 0 dB
level (about 4 mV signal @ 1 KHz) on a record. I suspect that having the transformer a bit farther away (mine was within 6.5") from the active circuit board would help with the 60 Hz hum, although I never heard any interference from it unless I was
up very close to my subwoofer.

I am curious if anyone has built this design and listened to it with a very high end turntable/cartridge combo to evaluate the sonics.
I use a Shure V15 series cartridge with a modified Dual belt drive
turntable.

I have attached a detail shot of the circuitry.

fastcat
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Old 11th January 2004, 10:30 PM   #8
KlausB is offline KlausB  Germany
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Congratulation, really nice work.

Did you ever try the battery option ?

Cheers
KlausB
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Old 11th January 2004, 10:37 PM   #9
moamps is offline moamps  Croatia
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Hi,

very nice and clean, but....I don't like unscreened-open mains parts-wires.

Regards
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Old 11th January 2004, 11:27 PM   #10
Chris is offline Chris  Norway
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Thumbs up NICE !!!!

Very nice work fastcat......

I also finished an El cheapo based phono pre recently(MM only). It`s been playing for a few days only, so I think it needs more time to "burn in" Compared to some other riias I have tried (Rega, NAD pp1 and an other diy riia) it sounds very promising. I find it very detailed, dynamic and it has a very good (3D) sounstage. May be bit bright/"over-analytical" with some recordings, but this might have nothing to do with the design (bad recording/record etc.). I use an alkaline battery powersupply so there is very little noise/hum.

I also switched cartridge after finishing the riia. At first I used an ortfon OM 10 (MM), then I mounted a Clear Audio Aurum beta and there was a clearly audile difference between the two cartridges (also in price). Better "everything", more dynamics, controlled bass and sibliance is very reduced with the new cartridge. The new cartridge also need some more groovetime so there might be more good sound heading my way....
My turntable is a Project 2 (not high end), but I must say I`am quite suprised that the new phono pre and cartridge make such a big positve difference.

HAPPY DIY
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