Newcomb TR-91 Input Transformer

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CJ

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the Electric Funeral Fire continues long into the night, and this Newcomb?

it is about to hit the tarmac, Hard, know what i'm sayin? du hast eine leiter hosen?  ;D

ok, it is a plug in, so lets plug it in, for the last time, ol buddy, ol pal, the parole board says No!  :D!

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CJ

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this guy is in the dirt,

looks like a 1:13 tuns ratio, 1:170 Z ratio, so 170 times 200 ohms is 34 K sec imp.

smaller lam than the 31 UI, call it a 25 UI, the legs are .25 inches, weird color on I bar is oxidation,

wound 540 turns pri #38, 7020 turns sec #46, 180 T/layer,  lots of Henries, but a wimpy stack,

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there is your core tube, pri, sec, screen and lams,

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mich

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Israel
What a Mess...... ;D    do you count the secondry? or just calculating from primary's reading?

waiting for the assembly line to begin...
 

CJ

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print>

 

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PRR

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> a wimpy stack

Off the cuff: they wanted "small" winding for low C.

10H primary implies 1,690H secondary. Assuming (wrongly) that leakage L is 1/1000th of good L, so 1700mH; and 100pFd load on secondary; gives resonance at 13KHz and 132Kohm. Newcomb was desperate for free gain, unconcerned about 20 even 15KHz, so a teeny core and winding.
 

CJ

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makes sense as this is a vocal transformer,  good for most except maybe Minnie Riperton,

and it might keep the feedback down,

they do have screens in there, need to do a sweep with and without if we ever get another one,

 
Joined
Sep 24, 2017
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I just found in my e-mail (not on PC) a spreadsheet of data for two TR-91's, using an AP ATS-1.

I didn't really know what I was doing as far as what level to inject and if any other impedance termination than what I got with the ATS-1 was a good/better idea.

I concluded that they had decent enough BW and a slight response rise above 19 kHz before a rapid rolloff might be normal or might be my poor termination choice.

I tried measuring inductance once on an impedance analyzer but only remember bring unable to call my data credible...it's always level-dependent, and I don't know what level to measure that at (probably at a typical mic signal level...and I don't think I was able to get down to 2.5 mV on the Z-analyzer without building a pad...I was already in someone's way & in a hurry.

I concluded also they were worry more to me than selling them because one had a poor ground connection in the soldered 9-pin plug. If condition was iffy, not worth much to someone on eBay. If I have to solder to wires inside the 9-pin plug & it's ugly, only I will see it.

I swept from 9.5-40000 Hz.

I'd prefer to look at the graph from the collected data before posting, in case I remember what all my notes in the spreadsheet meant.

The only other mic transformers I had to compare to were some Altec's from a solid state preamp and some Peavey (Architectural series(???)...as in church PA system?)...those were smaller, had lower turns ratio, and I don't think came close to the Newcomb data...I might not have even saved it because the markings in them never lead me anywhere.

A 1960 catalog said the TR-91 had sextuple copper and magnetic shielding. Elsewhere I read triple mu-metal, so 3 copper and 3 mu sounds believable.

This just boiled to the surface as I have a need to add a mic preamp input to a small project, and the Newcombs were free.

Feeling reckless on my phone. I'll try uploading the spreadsheet...

Guess not....xlsx is not a valid image format.
 
Joined
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I made Snipping Tool .PNG images for two Newcomb TR-91's.
Apparently 100-630 mV input is NOT offensive... I did a few spot checks at the higher amplitudes and THD continued to drop.

The ATS-1 manual says it has a differential input with 100k resistance from each side to ground, so that's what the transformer secondary saw as a load. I don't remember (2013 experiments) how I connected the transformer to the ATS-1. Because of the 9-pin plugs, I think I used alligator clip coax cables to the banana jack inputs on the ATS-1...I am not sure how that looks relative to the differential input. I have forgotten what the ground connection was to.

One transformer had a poor shield connection. My tables show s/n ratio with and without a ground (to compare the bad ground and good ground specimens), so that suggests the banana jack inputs were differential and not SE to ground.

In 2013 I was using an assumption that secondary as 'grid' might be 50k ohms, but the 1:13 step-up is nominally 22.3 dB (20*log(13).
 

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No freq. sweep on the Altec & Peavey input transformers, but some other measurements
 

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Joined
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I saw the teardown with phantom images...at least the schematic/pinout was still visible...that's why I posted my info...now everything I have ever seen on those transformers happens to be on one website...sometimes I don't remember where I saw something so I send myself an e-mail again...with webmail I can find it across devices.
 

rackmonkey

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Something seems odd with your measurements. I never tested a TR-91, but I’ve got sweeps on its higher primary impedance twin, the TR-100. It was almost flat from 20-20k with 150 and 600 ohm sources and 40k - 80k loads. Actually a nice sounding transformer, which i used with a two channel MILA preamp build for a customer. He loves it.
 
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I found Chapter 11 of Handbook for Sound Engineers, 3rd Ed. online. Fig 21. shows a rising response like I measured, due to inadequate damping resistance (my ignorance of what the ATS-1 Z-in looked like and failure to put a 'nominal' load on the secondary).

So, terribly inaccurate, but perhaps not meaningless. Tells me they were neither open nor shorted!

Not sure how THD would be affected by my see-what-I-get experiment with improper load or termination, but I didn't think they looked too bad...this could the type of mis-measurement that gives lucky data.

I have a project for one...I won't commit real estate for it until I see what it sounds like...
 
Joined
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Using a digital caliper on my phone screen, I estimate there is only about 2 dB rise from the midband level, and it seems to make it to 15 kHz on the rolloff back to the midband level.

The project is a suitcase amp for my daughter's violin which has a lavaliere electret mic on it. That works well with everything commercial she's plugged into, so at least I have an idea what it sounds like plugged-in and unplugged now.

If it sounds bad, back to the lab with the rats it goes.

The mic took over for a homemade piezo ribbon pickup that was a nightmare to EQ for sound people, but it was reliable and she was never the person with feedback with that.

Hard to beat a decent mic.
 

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