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Old 17th February 2017, 04:00 AM   #1
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Default moving coil transormer with low turns ratio

FET and other transistor moving coil pre-preamps have a bit of noise and distortion increases with rising frequency. Most transformers with a 1:30 winding ratio have more distortion as frequency goes down and limited bandwidth, but I think they can make the unshielded tonearm wiring effectively balanced differential and they don't add to background noise.

I have an Emotiva preamp with a moving-coil input that seems to have plenty of headroom and switchable input impedance. Jensen Transformers makes a 1:4 turns ratio transformer model JT-346-AXT (can be wired multiple ways) that has incredible bandwidth and extremely low distortion, mostly because it has that moderate turns ratio. These transformers cannot be used to drive a standard moving-magnet preamp input like the more typical 1:30 ratio transformers. This would be used with an Audio Technica AT33Sa cartridge. I figure it would bump up the cartridge voltage just a little and let me turn down the preamp volume to reduce the background noise, and also might reduce any noise from the turntable arm wiring. The down side, of course, is the cost of quality transformers. So what's that in decibels, like less than a half a db? I think the preamp circuit has that much headroom to spare and I won't need any pad, but that's a pretty small increase in signal which probably means a pretty small improvement in signal to noise ratio. But it might be about as good as it gets short of absurdly expensive transformers with amorphous iron and silver wire etc.

I see Lundahl also has a 1:5 ratio transformer. I don't think Sowter does. Should I be looking at microphone transformers, or can I wire a mic splitter transformer for 1:2 and expect decent results? Is there a more affordable option, as these Jensen transformers are rather pricey.

The Emotive has switch-selectable input impedance. But sometimes transformers like to drive into a custom capacitor and resistor network to get a real square wave out. I could drive the transformer input with my signal generator and see how a square wave looks farther down the signal path...

Guidance is welcome and appreciated.

Last edited by cyclecamper; 17th February 2017 at 04:14 AM.
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Old 17th February 2017, 06:28 AM   #2
Zoran is offline Zoran  Serbia
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I think Yu are right. I tryed once ballanced input with MC head with good results. Note to drive trans. whith generator with same or close impedance. I think they have 600ohms Output resistance. for MC load You need 12 to 18 ohms. Maybe to build some bufeer?
Eventual ringing of square at the output You can solve with RC net... Happy building experiments.
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Old 17th February 2017, 02:44 PM   #3
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But I was wrong about one thing, this is about 11.7 db of gain, which would provide a pretty significant reduction of electronic preamp hiss. It might be too much gain. I wish I could find a 1:3 transformer.
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Old 17th February 2017, 03:00 PM   #4
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20log(4) is 12dB yes. Why would you think otherwise?

A good MC stage should not have audible hiss with a 0.4mV cartridge. Are you sure you don't have a problem somewhere else?
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Old 17th February 2017, 04:31 PM   #5
kevinkr is offline kevinkr  United States
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You could just go with a transformer having a 1:8 or even 1:16 step up ratio and use the MM input.

I've used up to 32x on MM input and find them to be pretty poor performers. (Poor HF bandwidth mostly)

A 16x with a 300uV LOMC is going to give you 4.8mV at the MM input excluding some minor loading effect, and probably will be a quieter combination than the Emotiva MC input and a low ratio SUT.

Lundahl makes a number of interesting SUTs, I prefer the LL1931 and LL1941 myself, but there are other choices.
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Old 17th February 2017, 04:33 PM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by billshurv View Post
<snip>

A good MC stage should not have audible hiss with a 0.4mV cartridge.<snip>
I've heard elsewhere that the Emotiva MC input is not particularly quiet.
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Old 17th February 2017, 06:33 PM   #7
gofar99 is offline gofar99  United States
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I do not have experience with your particular preamp but have found with several other preamps that using a SUT with the MM inputs always has had lower noise and better sound. I have a stable of 4 different ones now and as you might expect all are slightly different. I generally stick to ratios in the 1:8 to 1:10 range as the bandwidth is good there. My current favorite for use with an AT33PTG/ll and Hana EL cartridges is a PLT1 from Softone. For a Dynavector Karat 23RS-MR one from Silk Audio. Others here are Lundahl 1678 and 9226. Nice, but not up to the others. Differential modes are possible on all of them but I find the single ended way quieter in my system. YMMV.
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Old 24th February 2017, 08:59 PM   #8
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I guess the issue of differential vs single-ended has more to do with the wiring in the turntable leads than tonearm and cartridge. Typically they use a coax-like cable with a ground/shield/return and a single hot to an RCA plug, not a twisted pair under a shield to an XLR. So indeed, going single-ended for the cable may matter more than making cart wiring being balanced differential, unless I want to make my own cables and connectors.
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