Help Understanding Audio Output Transformer Specifications. - Page 1
Forum section: electronics / beginners

author: Rene
author_rank: Regular Contributor
timestamp: September 18, 2017, 12:54:06 am
content: Hello,The attached picture shows the specification of an "Audio Output Transformer", since the picture is not very clear, here are the specifications:Impedance: 1KHz, 1V, 0mADC.Input: 1 Kohms +- 10%DC resistance (input): 70 ohms +- 20%DC resistance (output): 0.62 ohms +- 20%Frequency respoinse: 300-10000Hz +- 3dBInsulation resistance: > 100Mohm @ 100VDC.I was wondering if someone could help me understand what some of the specifications mean, specifically:Impedance: 1KHz, 1V, 0mADC:What does this means? Why do they talk about impedance and don't mention ohms?Input: 1 Kohms +- 10%:What is this input? Is this what the source impedance is supposed to be? Why 1 Kohms specifically?Insulation resistance: > 100Mohm @ 100VDC:Why do they mention ohms and volts? Shouldn't a max volt spec be enough so specify insulation resistance? And is this primary, secondary insulation?Ok, so at this point you can clearly see that I have no idea what I am talking about so some direction would be really appreciated. Also, could someone tell me when would a transformer like this become useful? What would be a specific application?Thanks.

author: Audioguru
author_rank: Super Contributor
timestamp: September 18, 2017, 01:38:02 am
content: It was used a long time ago (60 years) to match the high impedance of vacuum tubes to an 8 ohm speaker.Radio Shack knew nothing about electronics which is why they are gone now and why this low quality old transformer has no maximum power rating.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: September 18, 2017, 01:43:37 am
content: "I have no idea what I am talking about..." Not so. You are just labouring under the handicap of a badly written spec sheet.From the top:- Impedance: This impedance lists some conditions under which the impedance is measured. It is measured with a 1KHz signal, 1V amplitude (maybe RMS, maybe not) with a primary DC current of 0mA. This is actually significant as a large DC current through the primary will tend to saturate the core of the transformer and reduce its effectiveness as a transformer as far as ac currents go.- Input: This is the AC impedance seen into the primary winding under the above conditions with a typical secondary load attached. Although not specified as it should be, I expect the secondary load would be about 8 Ohms. It is telling of Radio Shack that it is not specified.- DC resistance (input): This is the DC resistance of the primary winding.- DC resistance (output): The DC resistance of the secondary.- Frequency response: The 3dB frequency response of the transformer- Insulation resistance: when 100 V is applied between the primary and secondary windings then the minimum insulation resistance is this.These specs are typical of a small audio output transformer for something such as a transistor radio or some such. Another very important spec that is missing is its power handling capability, but I expect it is not more than 1W.If you are using this for some other purpose than an audio transformer, it is sometime convenient to know its turns ratio. In this case it can be guessed if we assume the secondary impedance is 8 Ohms. So it would be TR = SQRT(Zpri/Zsec) = SQRT(1000/8) = 11:1 (approx)Hope this helps

author: Ian.M
author_rank: Super Contributor
timestamp: September 18, 2017, 01:49:14 am
content: +1.Remove 'guessed' as the pinout has the secondary marked as '8 Ohms'While we are guessing, I believe it isn't good for more than 200mW into 8 ohmsThat's 1.265V RMS at the secondary, which would require 20V pk-pk on the primary - that's possible from a 12V supply if one feeds the center tap from the supply and drives it push-pull with two transistors of the same type, one on each end of the secondary, with antiphase base drive. It probably would be more typical to use it with a 9V supply, and get about 130mW out.Here's how center tapped transistor output transformers were typically used with Germanium transistors 'back in the day':http://www.diale.org/deacy.htmlYou'd need to recalculate the bias if you wanted to use those circuits with silicon transistors, and also adjust the drive level and quiescent current to account for the different transformer ratio and primary resistance.

author: drussell
author_rank: Super Contributor
timestamp: September 18, 2017, 04:37:21 am
content: Yeah, that's not a vacuum tube type output transformer, it is a 1K -> 8 ohm like was often used in the early transistor days before higher output currents directly into typical speaker level impedances became common.They are very handy for other uses, though, like a step-up transformer from battery voltage to 200-500 volts or so. I've used them to make things like strobe lights, etc.Remember also that impedances on transformers are ratios so it will look like a 2k load into 16 ohms, etc.