Audio Transformer Questions - Page 1
Forum section: electronics / beginners

author: dorrisant
author_rank: Newbie
timestamp: September 15, 2022, 02:47:58 am
content: I am looking to replace an audio TX in a clone of a circuit. I measured the voltage ratio... the primary is the output side, so this is a step up TX.I injected 10V AC into the primary and got 277mV AC on the secondary side. So the turns ratio is 36:1. It looks like the impedance ratio would be 1296:1.The device I am cloning is a audio line out derived from the speaker jack of an audio amp. It is, in essence, a microphone simulator. It will be connected to the input of a microphone preamp. I know that the DCR is not really relevant except to give a rough idea of maybe what the wire gauge may be. That Primary measures 1000Ω DCR and the secondary measure 77Ω DCR. I also measure the impedance of the two coils, I will add these measurements here, although I don't know that they are relevant. The primary measured 13H and the secondary measured 291mH.The size and form factor closely resemble what is found here: chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://datasheet.octopart.com/42TM017-RC-Arcol-datasheet-8742.pdfMy questions are... Where can I find a TX that is at least PCB mounted if not close to the depiction in the link above with specs that will work as given in the text at the beginning of this post? Is it something that I can buy off the shelf somewhere? Or is it something that I can wind with parts... DIY? If so, is there any way to determine the wire gauge and approximate number of turns? I have a coil winder, so this may be rather easy. I thought I stumbled across a site that sold parts for winding your own but can't remember where that may be.Thanks for your reading and thought.Tony

author: SpecialK
author_rank: Regular Contributor
timestamp: September 15, 2022, 03:32:53 am
content: I'll admit I don't know really what you are trying to accomplish. I don't think you want a transformer here. I will try to explain:if you take a signal that is sufficient in amplitude to drive an 8 ohm speaker and run it into a step up transformer it will overdrive with several volts the preamp that's designed to see a microvolt signal.I think you need a resistive network to condition the signal. Like a 10 ohm pot across the speaker out terminal with something like a 25 kohm resistor off the wiper and into the preamp. Maybe the transformer is needed for balanced audio?

author: dorrisant
author_rank: Newbie
timestamp: September 15, 2022, 03:48:30 am
content: Just to be clear... I am cloning a device. The input of the device will take signal from the speaker jack. At the end of the circuit within this device there is a step up transformer. The circuit prior to the TX in question will attenuate the signal down to a reasonable level. That said the TX looks to be 200mW. The output of the TX is connected to an XLR for a balanced, line out. I repair and build tube amps as a full time job. I understand what you are saying, and agree, but would never do it that way. Sorry for any confusion and thanks for replying.Also, this is an existing device and proven to work. There is no question about that. I am just trying to source the small, microphone level output TX for this line out device.

author: jonpaul
author_rank: Super Contributor
timestamp: September 15, 2022, 06:01:28 am
content: please post a schematic or blocks diagrams as your system description is very vague.A photo of the transformers tested, mfg and model?The URrL Produits link is dead, a Chrome standby temp fieMicrophone transformers typically are NOT rated for speaker leveleg 600 Ohm....50k ohmFinally your test of 10 vac doesn't specify the frequency appliedBack to work!Jon

author: dietert1
author_rank: Super Contributor
timestamp: September 15, 2022, 06:25:16 am
content: Maybe a transformer is needed for isolation. How about this one:https://eu.mouser.com/ProductDetail/Hammond-Manufacturing/140NEX?qs=XJfXErqHgA6XqvWkAYNzIg%3D%3DRegards, Dieter

author: dorrisant
author_rank: Newbie
timestamp: September 15, 2022, 01:01:16 pm
content: I had previously done the test from a small wall-wart TX that put out approximately 14V AC. I had run this down to 10V AC via my Variac. This was obviously @ 60Hz.This time I tested with my signal generator Set to 3.845V AC @ 1kHz applied to the primary. The secondary measured 590mV. This, according to my calculation, results in a turns ratio of 42.47:1 which would be an impedance ratio of 1803.754:1. Please correct me if I'm wrong.The data sheet I had tried to attach above is now available here as a pdf, referring to the Xicon 42TM series. The attached jpg is the schematic as far as I have gotten with it. I understand that the value for the TX on the right is not correct. It shows what I found a 10V AC @ 60Hz. This is the output TX I am trying to copy. At the left side of the schematic, the amplifier speaker output (which would be connected to the speaker) is connected to J? + and J? GND.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 15, 2022, 02:15:39 pm
content: Like DI box? Just buy one! A pasive transformer DI can be had for less than $30 egwww.amazon.com/Passive-direct-instrument-balanced-unbalanced/dp/B009AARY2I/ref=sr_1_4?crid=1KTZIUP1WPUD3&keywords=DI+box&qid=1663251196&sprefix=di+bo%2Caps%2C313&sr=8-4More findable a10k to 600R transformer. 4:1 eg Hamond 107N. Still very expensive.You can make an electonic transformer.It will outperform any iron unit at a fraction of the cost.eg Add a THS4531 to a transformer you can get..This gives you the option of diff or single ended output. That Corp and TI make nice low cost audio line drivers.Input iron and active output electonics give the best of both worlds. ie safety isolation and hum loop taming.

author: CaptDon
author_rank: Super Contributor
timestamp: September 15, 2022, 02:37:46 pm
content: That mic simulator circuit is too complicated!!!! I have for years used 600 ohm to 10k ohm microphone impedance matching transformers to do exactly what you are trying to do. I robbed the transformers out of RadioShack microphone matching units. Either version is usable, one version is female XLR to male mono 1/4 inch, the other version is female 1/4 inch to male XLR. I mounted a male chassis mount XLR connector on the amplifier chassis and inside the chassis (Away from the power transformer!!!!) I mounted the impedance matching transformer. Low impedance side connected to the XLR and high impedance side driven from the speaker output but I included a 5K ohm pot to vary the input drive to the transformer. You don't want to overdrive the little transformer. You turn the pot up just enough to get mic level at the XLR output! It works great and I have two gig amplifiers in daily use using this homebrew D.I. to send a signal to our F.O.H./Monitor sound engineer. Two things, I replaced the 5K pot with a 1K and added a series resistor of 3.9K going to the speaker so it would limit the maximum voltage being applied to the transformer. Also with that arrangement I can still easily turn up the pot and XLR output to slightly less than 'line' level for mixers that expect line level from a keyboard synth and so forth. The transformers that I extracted are probably only .5 inch square. Their frequency response is fine for guitar work, after all, they were intended to be used with a high impedance microphone driving a 600 ohm snake cable to a 600 ohm input mixer and sound good doing it!!

author: The Electrician
author_rank: Frequent Contributor
timestamp: September 17, 2022, 02:37:53 pm
content: Quote from: dorrisant on September 15, 2022, 01:01:16 pmThis is a turns ratio of 3.845/.59 = 6.517For a primary DCR of 1000 ohms and secondary DCR of 77 ohms, it looks like a Xicon 42TM017-RC would be a good match.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 17, 2022, 06:20:44 pm
content: It is. Horses for courses though. OTT but utterly controllable.I the real world a DI box is just the thing, even a cheap one.