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Silicon Chip (Australia) 2006 phono MM preamp
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Author:  mwhouston [ 10 Dec 2016, 18:02 ]
Post subject:  Silicon Chip (Australia) 2006 phono MM preamp

Here is a blast from the past - a Silicon Chip phono stage (I have two other kits from 2006). Nice part is you can switch between three different EQ stages if you really wanted. Comes with a PCB and all the parts required. It uses the LM833 OPAMP but it can be swapped out for a OPA2134.
 
Apart from leaving a few parts off and installing the wrong cap in the EQ stage and having to play with the gain, all went well. The original idea was to have variable gain but a $2 pot in the gain stage seemed a bad idea. It is a two stage preamp where Mimic is a single stage.

Solid core hook-up wire was used in a Hammond Al enclosure and WBT silver based solder. Original to be powered by an AC wall wart now running on batteries.
 
Sound is bright and immediate where my DIY Mimic is more about tone this one is about detail.
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PC073552 (800x600).jpg
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Author:  mwhouston [ 14 Dec 2016, 16:55 ]
Post subject:  Re: Silicon Chip (Australia) 2006 phono MM preamp

More tweaks. Moving the preamp to a battery powered device ensures the preamp will always be dead quiet. And it is. With the vol at listening level and all other gear on there is nothing. On first listen I felt the preamp a bit bass shy. On comparison with my tube MM stage there didn't appear to be much in it.

I borrowed another set of ears. His first comment was it may be bass shy but then added he felt bass was right on other tracks. My thoughts too. The second stage is a buffer stage but explained in the accompanying paper work as a high pass filter. The filter rolls off frequencies under 20Hz. Yes a rumble filter. Problem is, because it is simply a 1uf cap and 7.5K resistor (first order attenuation) is probably starting to roll of the low notes from about 40Hz. My Beymas -3db point is 40Hz.

An online calculator for capacitive impedance calcs. shows if I increase the inter-stage cap to 2.2uf the roll off will start at 10Hz. Perfect. So I replaced the 1uf caps with 2.2uf NP caps. What a difference.

Just this simple and inexpensive tweak has lifted the whole MM stage. Bass is back but more than that the whole presentation is punchier and richer and very dynamic. Much, much better and now I feel nothing is missing. The smaller cap was strangling the whole unit. Not any more!

Author:  Gio [ 14 Dec 2016, 21:59 ]
Post subject:  Re: Silicon Chip (Australia) 2006 phono MM preamp

Nice work Mark.

What is the RCA input on the front?

Author:  mwhouston [ 15 Dec 2016, 00:29 ]
Post subject:  Re: Silicon Chip (Australia) 2006 phono MM preamp

Gio wrote:
Nice work Mark.

What is the RCA input on the front?

Battery check point. Put your positive probe in the RCA and the black lead on the outside for a voltage check.

Author:  Gio [ 15 Dec 2016, 19:51 ]
Post subject:  Re: Silicon Chip (Australia) 2006 phono MM preamp

Good idea!

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