Newcomb TR-91 Input Transformer

[CJ]

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!  !

 

[CJ]

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,

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

 

[mich]

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

waiting for the assembly line to begin...

 

[CJ]

print>

 

[PRR]

> 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]

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,

 

[murrayatuptown]

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.

 

[murrayatuptown]

But there's this...

 

[murrayatuptown]

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).

 

[murrayatuptown]

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

 

[rackmonkey]

CJ tore one of these down ages ago. Carnage photos long gone but diagram still there:

Newcomb TR-91 Input Transformer

 

[mjrippe]

CJ tore one of these down ages ago. Carnage photos long gone but diagram still there:

Newcomb TR-91 Input Transformer

Careful there, you'll start a feedback loop!

 

[rackmonkey]

Careful there, you'll start a feedback loop!

Oscillation at all frequencies

something about the new look on mobile doesn’t mix well with being in a hurry for me. I wasn’t even drinking this time.

 

[murrayatuptown]

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]

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.

 

[murrayatuptown]

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...

 

[murrayatuptown]

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.

 

[C12VR]

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.

The TR-100 is said to be for 150 ohm or 500 ohm mikes; do you have to connect the two primaries accordingly for each impedance?

 

[CJ]

Usually two primaries are parallel wired if using the 150 mic and series wired for 500/600 mic.

Exception would on something like a UTC ouncer which might have only one primary.
In that case, you would connect to taps on that single coil primary to get everybody happy. Then you would puke your guts out when you listen to that small core POS. At that time, you would go shopping for the best input XFMR there is, that being the peerless k 241 d.

Or the WE single coil job which is flat from 20 to 60 k unloaded.

 

[C12VR]

Usually two primaries are parallel wired if using the 150 mic and series wired for 500/600 mic.

Exception would on something like a UTC ouncer which might have only one primary.
In that case, you would connect to taps on that single coil primary to get everybody happy. Then you would puke your guts out when you listen to that small core POS. At that time, you would go shopping for the best input XFMR there is, that being the peerless k 241 d.

Or the WE single coil job which is flat from 20 to 60 k unloaded.

what do you consider the best mic input transformer available that won't cost me a kidney?