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Old 3rd August 2010, 11:36 AM   #4301
RCruz is offline RCruz  Switzerland
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L1 in series between C1 and C2 right ?

It must be much better than my RC circuit
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Old 3rd August 2010, 11:47 AM   #4302
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Yes, L1 is in series between C1 and C2. I forgot to mention that I also bypass C2 with a 10uF polyprop cap and a .47uF polyfilm cap under the board.
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Old 3rd August 2010, 11:51 AM   #4303
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Thr R=0.75R does not include the secondary resitance. The resistance of the inductor is .3R if you trust the DMM. Shorted leads were 0.1R and the actual read was 0.4R so I just subtracted it.

I Set R1 to yield the required voltage. I was lucky that I had a 3W resistor of the value in my box. I started with 1 ohm which yielded too low of a voltage, then piggybacked another one ohm ontop and the voltage was too high so I went between and used 0.75R and it came out just about right.
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Old 3rd August 2010, 12:34 PM   #4304
Salas is offline Salas  Greece
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Quote:
Originally Posted by SGregory View Post
How about if I make the box from steel? The AC mains will come from the back corner next to the RIAA boards. The idea of a piece of steel between the heat sinks is appealing as it is very simple but I am afraid it may not shield it from the power line noise.
The AC mains is dangerous for inducing hum. Even if its near enough to the TT cables outside the box. Use steel plates for the sub box to enclose the phono preamp, even at the bottom. You can also mount the toroid perpendicularly if the height of the box allows.
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Old 3rd August 2010, 01:30 PM   #4305
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Steel box it is then with bottom. The toriod is too tall unfortunatley to fit perpindicular. I thought about putting it into a steel can as well if hum is a problem.
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Old 3rd August 2010, 01:46 PM   #4306
Salas is offline Salas  Greece
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The safest approach by my experience is 2 half width boxes, even one next to another on the rack looking like a nice full width combo. One receives the mains and has Tx, rectification, filter. A DC cable* goes to the main phono box that has the regs and Riaa preamp. The AC power all the way far from the side that TT input cables come in.

*Using XLR cable, shield connects the 2 chassis, the Tx & filter chassis is mains grounded.

** I use single chassis too, but I arrange across width, the mains are coming in as far aside as possible and an R-Core is tolerated. A toroid is offensive. Even with such measures always at least one channel will pick 5-10dB mains on FFT. Don't know if not if there was a steel sub box too. Me I don't mind a -90dB or -80dB single hum band even on my 95dB speakers, I know it will pick up probably when in the rack with the TT cables coming in near other mains feeds and equipment anyway. But FFT is your friend, just don't carve the box design in stone before you got all your parts and you are sure about the Riaa PCBs are dead quiet also, leave slack for changing your practices before final assembly. Low noise-high gain, is a bugger.
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Old 3rd August 2010, 02:18 PM   #4307
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Ok. Thanks for the picture. I have a little room to play to keep the AC more seperated from the RIAA. At some point I will build a second enclosure for the Tx and PS board. As you suggested, I want to get the circuit up and running first, learn from it and then move on to better optimization. Ultimatley I would like to have seperate tx and PS boards for each channel if this really does sound as good as it claims to be;-).

In the meantime I can make the internal steel enclosure large enough to give room to play and as a secondary precaution twist and shield the AC mains in "conduit". The IEC connection is already shielded so I just have to be mindful of the mains wiring itself and properly bonding the shields. The RCA in and out can be kept on the other side of the chassis to keep it as far from AC as possible.
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Old 3rd August 2010, 02:23 PM   #4308
Salas is offline Salas  Greece
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Its very possible you succeed with a single box right away, especially with steel conduit for AC and phono steel sub box. I have to check some shielded filter integrated AC inlet too.
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Old 3rd August 2010, 02:29 PM   #4309
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I even thought about making a shield AC cord.:-)
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Old 3rd August 2010, 06:43 PM   #4310
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HI Salas, the R-core work fine ? sound better ?
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