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Post by mcirish on Nov 16, 2022 9:36:49 GMT -6
A while back, I picked up a Stam SA-1073MPA. I just started working with it and played around with the impedance switch. Apparently, it changes the impedance from 300 to 1200 when in the HiZ (in) position. I think I have those numbers right but feel free to correct me.
Anyway, in my mind, I figured a higher input impedance for a mic would be a good thing but I found this to be exactly opposite in my test. I used tube mics in my test, so that probably has a big effect on it. Two mics, Wunder CM7GS/M7 and a Miktek CV4 (BN-K7 capsule). I started off with the switch in the HiZ (in) position.
Results:
I notice reduced hi end and some saturation when in HiZ. When pushed, this becomes very noticeable.
In the standard low impedance position, the mics sound better to my ears. Clear and undistorted. It doesn't start distorting when pushed.
I don't know why I thought that should be the opposite. Maybe I'm just thinking about DI bass with passive pickups. The higher the input impedance, the clearer the sound of the bass. I thought that would be the same with the mic input impedance. I guess I learned something today. With these particular mics, I will definitely be keeping it in low impedance.
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olionajudah
Full Member
just learning .. one mistake at a time
Posts: 48 
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Post by olionajudah on Nov 16, 2022 11:48:32 GMT -6
How a given mic interacts with an amp's input impedance depends significantly on the microphone.
In my admittedly limited experience, passives are the most dramatically affected. Increasing input impedance seems to improve transient response (less sluggish) and mid-range articulation and extend top end frequency response. This phenomenon has been most obvious to me using passive ribbons, the overall effect being less 'darkness' (less top end roll off, more energetic transient response). It seems to have a similar effect on passive dynamics as well, though less pronounced to my ears.
For tube condensers and fet condensers with transformers, the effect of switching input impedance is often audible but less predictable, and sometimes fits your description. In other cases, the effect of switching input impedance is much more subtle, and in some cases, not audible at all to me.
My general preference is to use higher input impedance pres for passive ribbons and dynamics. Alternatively, I tend to prefer very low (300 ohms) input impedance when patching km84s into my BAE 1073. The general guideline is to match a mics output impedance with a mic pre with an input impedance approximately 10x the mics output impedance.
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Post by EmRR on Nov 16, 2022 12:03:43 GMT -6
That is counterintuitive on the surface. It's more complicated than it seems, as this is still early SS stone age technology tricked out to the max. The switch Is series/parallel for the transformer, so gain moves 6ish dB, and the various gain steps can sometimes throw away gain through voltage dividers, or add additional active gain. You change Z and add/subtract gain, you move the gain switch to compensate, probably can't even match it exactly, and you then might be adding more active gain, you might be tossing more transformer gain with resistive dividers (pads), headroom relationships of various stages change, clipping points may change. The transformer loading relationship also changes, as does the inductive relationship between mic and input transformer. Lots going on. Same 'concept' on a different preamp is easily a totally different experience. The mic type affects it greatly too.
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Post by theshea on Nov 17, 2022 12:02:51 GMT -6
on my AML1073ez i exclusively use the 1200 ohm setting. it sounds better and has less hiss.
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Post by robo on Nov 17, 2022 16:56:12 GMT -6
That is counterintuitive on the surface. It's more complicated than it seems, as this is still early SS stone age technology tricked out to the max. The switch Is series/parallel for the transformer, so gain moves 6ish dB, and the various gain steps can sometimes throw away gain through voltage dividers, or add additional active gain. You change Z and add/subtract gain, you move the gain switch to compensate, probably can't even match it exactly, and you then might be adding more active gain, you might be tossing more transformer gain with resistive dividers (pads), headroom relationships of various stages change, clipping points may change. The transformer loading relationship also changes, as does the inductive relationship between mic and input transformer. Lots going on. Same 'concept' on a different preamp is easily a totally different experience. The mic type affects it greatly too. Thank you for this. I’ve always wondered why the variable impedance on my APA Juggernaut and ISA have predictable effect and the switch on any Neve or Neve clone seems like a crapshoot. |
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