Thank you Doug, no argument from me however the Coil website specs call out 150 mic. Perhaps other brand preamp specs are also publishing partial data.
Specifications
CA-70 Amplifier:
Two-stage 6J7/6C5 vacuum tube circuit
Negative Feedback tone adjustment with HIGH/LOW filters
42db-54db of available gain
Flexible input pad section for mic or line level operation
150 Ohm Mic, 20k Line input, 600 Ohm output
Interchangeable/Optional input and output transformers
NOS tubes, signal caps & resistors, PEC 2W carbon pots
Up to Six CA Series Amplifiers mount into our PS6 Rack
Hand Built and Tested in the USA
Nevertheless, the theory was in response to my own listening dozens of times for each error I made in leaving the P4 in the circuit but not turned on.
I am hearing an excellent difference in the sound with P4 circuit in and interested in the why. When I heard this with the 160, I chalked it up to being something specific to the mic. (and ordered a second 160 as now, with the P4 in circuit, it is transformed to an excellent vocal mic for me). When I made the same error with the TG Mic (left the P4 circuit in with phantom off) and noticed the same excellent difference (when I βcorrectedβ the error, the excellent TG Mic sound changed very noticeably) , I was intrigued enough to post my experience. OT: I am really enjoying the TG mic with Coil preamps whether P4 is in, or out, of circuit, merely noting the difference.
When I have vocalists in next Saturday I will ask if they will humor me and record examples of each I can post. If we are able to do this, they will be separate takes. Not ideal for comparison but I am a strong believer that mic voicing affects performance.
This GS thread and post aligns with Scott's best guess when I requested an explanation based on his Pueblo P4 design:
Mic/preamp impedance matching
"And all this is before we even consider the effects of the mic pre input. Most modern mic pres are electronically balanced so as long as the phantom power blocking caps are large enough in value, the input impedance across the audio spectrum will be almost a flat line. In other words they are source agnostic. Transformer input mic pres on the other hand have the flattest response when fed from a specific source impedance (Typically 150 ohms these days). If fed from a lower source impedance they also exhibit a rising response at the top end of the audio spectrum. So combine any low impedance source like an electronically balanced output condenser with a transformer input mic pre and you either get air or shrillness depending on if you like the result or not."
Quote:
Originally Posted by
emrr
β‘οΈ
It isn't 150. It's a bridging input Z, transformer secondary to grid with no secondary load resistance. It's DESIGNED for a 150 ohm source. Very common interpretive mistake to make.