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In Reply to: Re: Which Lundahl transformer(s) to use for MC step-up? posted by KevinC on October 27, 2002 at 05:44:55:
Thanks.> The 1678 is like the 9206, but has a higher step-up ratio, 4dB to be exact, so it might be necessary for one of those 100uV MC cartridges in combintation with an active phono stage with relatively low gain.
My phono stage is around 40dB which is fairly normal, but my preamp is a passive unit and my current speakers are fairly low sensitivity, so some extra gain wouldn't hurt. I'll do the math once I settle on a cartridge.
> The impedance requirement os your cartridge are met by either adjusting hte step-up ratio (gain) or the loading resistor at the input to the active stage, which is typically 47K.
I understand the math here, I was just wondering if the transformers were "happier" when used in a certain configuration.
> For most MC step-up purposes the 9206 is the unit of choice.
That answers my question :)
Thank you for the quick response.
Follow Ups:
Hi,I've been looking at the various hookup schemes in the PDF for the 9206. If I understand this correctly, it is recommended to keep the secondary windings in series (which will keep the secondary resistance at 790 ohms) and never to connect them in parallel (giving 200 ohms secondary resistance). The gain should be adjusted by series/parallel connections of the primary windings. Is that correct?
Also, even though there are two sets of coils on the transformer, I can't use them as left and right channels, I need a pair of 9206 transformers for stereo, right? I'm sure this is a pretty silly question, but this is my first foray onto low output MC cartridges and DIY phono stages, so please bear with me :)
And the last question for tonight :) How can I mount these transformers? I see the PCBs on your website, so putting the transformers on those and then putting them on standoffs is one simple solution. Is there any way to mount the transformers without using PCBs? Is the pin spacing standard, i.e. can I use a generic Radio Shack PCB or perfboard to mount them? I think I'd prefer to wire them point-to-point instead of having my weakest audio signal go through a PCB trace, so I'd use the PCB/perfboard only for mechanical support.
Yes, you are correct not to parallel the secondaries. The resulting capacitance will cause significant high frequncy roll-off. Only the primaries should be manipulated for changes in gain/impedance.I'm afraid you must use two transformers for stereo. No good way around that.
Perfboard with holes spaced on 0.1" centers works fine. It is tedious to wire and shorts between pins must be watched carefully, but I've done dozens that way myself prior to having the PC boards.
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