Anybody Familiar With A Bogen T725 Speaker Transformer? - Page 1
Forum section: electronics / beginners

author: MikeTannel
author_rank: Contributor
timestamp: September 28, 2025, 09:21:34 pm
content: Hi All, I have the above transformer and I'm trying to figure out what the Watt label assigned to each tap means.I've attached a spread sheet I made trying to make sense of it. The 'Watt' & 'Range' columns are taken right from the transformer's label. The rest of the values are measurements. The turns ratios are my estimates based on the measurements.None of it helped me figure out what the given Watt values are trying to tell me. Can anybody explain?btw - the only thing I could find by way of a datasheet was a marketing blurb (also attached below).Thanks!Mike

author: MikeTannel
author_rank: Contributor
timestamp: September 28, 2025, 09:30:53 pm
content: Here's the spreadsheet. I thought I attached it with the original...

author: Analog Kid
author_rank: Super Contributor
timestamp: September 28, 2025, 09:56:46 pm
content: Isn't it what you would think it is, the maximum power available at each tap?My guess®™ would be that because the windings are current-limited due to wire size, the available power goes down with decreased voltage, since the current can't be increased without damage.

author: themadhippy
author_rank: Super Contributor
timestamp: September 28, 2025, 10:04:31 pm
content: the 70v colums makes me think its for pa speakers.whilst it mentions 100v, 70v is just the samehttps://www.monacor.com/magazine/100-v-speakersEdit to add.Yep its a 70v line tranny . Datasheethttps://www.farnell.com/datasheets/1578920.pdf

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2025, 10:31:08 pm
content: Distribution from public-address amplifiers to multiple loudspeakers typically use lines specified for voltage (e.g., 70.7 V) at maximum amplifier power.Individual transformers for each loudspeaker have primary taps for a discrete set of powers drawn from that line voltage transformed into 8 ohms (or whatever) at the speaker.The power settings should add up to the total power specified into the 70.7 V line.This allows flexibility, where some loudspeakers get more power than others.

author: MikeTannel
author_rank: Contributor
timestamp: September 28, 2025, 11:06:53 pm
content: Ok, so if I got this, the idea is that the transformer labeling is intended to support its use as a component in a larger sound system. For example, I might have a main amp rated at 70V, 100W driving a distribution line. I then hang speakers off this line, using one of these transformers at each speaker. At each speaker, I choose how much power I want to devote to it by connecting to one of the taps as labeled. (I might want 4W at the back further away and 1/2W up at the front closer by.) Then, as long as the sum of the Watts across all the speakers doesn't exceed the 100W rating, I should be good to go. Yes/No?

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2025, 11:13:45 pm
content: That's my understanding, although I haven't done PA stuff since 1970 in college.If the impedance match to the amplifier is important, the settings should add up to approximately the amplifier rating into 70.7 V.(If I remember correctly, 70.7 is the correct value, but obviously 70 V is close enough for any purpose.)

author: themadhippy
author_rank: Super Contributor
timestamp: September 28, 2025, 11:25:03 pm
content: Quoteyes

author: MikeTannel
author_rank: Contributor
timestamp: September 29, 2025, 12:41:27 am
content: Awesome. As always, your guys help is much appreciated.Cheers!Mike