audio transformer, about 30W size, easily available?

No I'm talking about the secondary ! The primary is unknown until the OP gives some data.

No you twit ! That's the *SECONDARY* load impedance / voltage ! 160 odd ohms isn't 8 ohms. Do you know nothing ?

Are you the original OP btw ?

Graham

Well, are we talking about 70.7 volt PA transformers? The output of the PA (Public Address) amp puts out a nominally 70.7 volts. At each place where there needs to be a speaker, (like, in a classroom), there's a transformer that kinda comes with the speaker. Its primary goes right across the 70.7 volt PA line, and it has taps for the appropriate speaker. A quick web search turned up a couple that can handle 30 watts, but they're already attached to loudspeakers, for like three hundred bucks.

Apparently the PA folks aren't used to selling stuff to hobbyists. )-;

Thanks, Rich

Now, wait a minute. I thought I was the one who's been drinking here.

What you just said - a 70.7 V line doesn't look much like 8 ohms.

That's what the transformer is for, duh!

If he's talking about a 70.7 volt line, then he needs a speaker transformer.

If he's talking about driving that 70.7 volt line with transformers from his output stage, then all bets are off - that'd be kinda an exercise in anachronism, given the transistors they have these days.

;-) Cheers! Rich

Errr - no !

Graham

Thanks for the additional info. I ordered the one from Edcor, it will be here in about 2 weeks.

mw

Uh ?

The BFoelsch guy incorrectly thought I was talking about the 70V transformers used to connect the speakers to the 70V line.

I mentioned them in passing.

My main point was that a 70V line providing 30 Wattts doesn't look much ( or even at all ) like 8 ohms ( which is what the OP wants to connect to ) hence '

70V transformers ' are no use to him whatever.

Graham

[snip]

Mains toroids can have quite a reasonably low leakage inductance, but the associated downside is the high capacitance and early LC resonance.

You really need to state the maximum primary voltage and source impedance of your transformer driver.

--
Tony Williams.

[...]

RS Components had one in their 2001 catalogue (actually a 100v-line version for PA systems in the UK) Part Nr. 210-6532 It was priced at

11.98 UKP.

I seem to remeber that this type didn't have particularly well coupled windings and the output fell off towards 20 Kc/s. Best to ask them for the spec first if you intend to buy.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

There is so much confusion about audio distribution transformers in this thread that I just had to put in my two cents. First, I never did figure out just what turns ratio or (alternatively) impedance ratio the OP wants, but a great variety of ratios are available from audio distribution transformer vendors. Unfortunately, their datasheets don't always make it obvious....you'll have to do some interpretation. Second, there is no necessary distinction between transformers used to boost voltage up from low impedance amplifier outputs to the higher

70.7V and 100V nominal lines levels common for audio distribution and those used to transformer back down to low impedance drivers at the loudspeaker end. You simply connect the transformer appropriately at either end. However, it is true that many audio distribution loudspeakers come with transformer included, often with a series capacitor and/or other components wired in. In any case, transformers suitable for milliwatts to hundreds of watts are readily available in any qty from pro audio distributors and catalog houses. To make sense of audio distribution transformer voltage, impedance, and power specs, simply use Ohms Law. For example, a power amplifier rated 100 watts 8 ohms has a 28.3 volt rms output capability, so a distribution transformer designed to couple the amplifier output to a 70.7V line has a turns ratio of 28.3:70.7 = 1:2.5. When the 70.7V line is fully loaded with loudspeakers (each with their own step down transformer), their impedance is Z = V^2/P = 70.7^2/100 = 5000/100 = 50 ohms. The same transformer can be turned around, so that a driving source will 'see' a 50 ohm load with 8 ohms on the 'secondary'. Audio distribution transformers are readily availalble for 25, 70.7, 100, and 140 volt systems, and power ratings range from milliwatts to hundreds of watts. So, a great many turns ratio and core size combinations are out there. Note, however, that a good many of them are actually AUTOFORMER connected, meaning that primary and secondary share turns and provide no galvanic isolation. Autoformers can be smaller and less-expensive than true transformers, and they are almost guaranteed to have lower leakage inductance.

Transformers designed for audio distribution should differ from power transformers in these respects: a) less insulation and more intimate coupling between primary and secondary (by means of multifilar winding and/or interleaving) to reduce leakage inductance to improve high frequency response b) thinner laminations and/or better steels to reduce high frequency core losses c) power ratings much higher than the corresponding mains transformer, because audio transformers are not used for continuous power output (audio average power is much lower than audio peak power for voice and music) d) in consideration of c, a well-designed audio transformer will give much less consideration to temperature rise

For those transformers designed to couple significant power at frequencies below mains frequencies, the transformer design will also use greater core cross-sectional areas and/or higher turns counts than a similar mains transformer. However, this is rarely the case, since audio distribution systems usually are not called upon to deliver much power in the lower octaves.

Also, some audio distribution transformers may be designed to tolerate significant DC offsets present at the outputs of some high power amplifiers. Deliberate increases in winding resistance and/or the inclusion of small air gaps in the core accomplish this.

Some toroidal mains transformers will provide good fidelity up to 10kHz and above, provided that thin insulation was used in their construction and the windings are close together and well distributed around the core. However, most will probably saturate badly at frequencies just below their mains rating and/or voltages slightly higher than rated.

Speaking of low frequency saturation, it's not widely understood that transformer behavior in saturation, as measured by distortion on the secondary, depends strongly on power amplifier characteristics. When the core saturates, coupling between primary and secondary remains good, but the amplifier sees an incredibly low inductance in PARALLEL with the reflected secondary load impedance. In effect, the power amp suddenly has to supply current into the winding resistance. Saturation is a voltage-times-time phenomenon (that has nothing to do with secondary loading), with core magnetization flux building continually during each half cycle of VOLTAGE input. Consequently, the onset of saturation as input amplitude increases, normally occurs near a voltage zero-crossing, just prior to the end of a half-cycle. This is the point of greatest vulnerability for a Class AB power amplifier, where the voltage across its output transistors is greatest. The sudden current spike associated with transformer saturation generally causes some combination of these symptoms: voltage distortion due to finite output impedance, voltage distortion due to deliberate SOA protection, output transistor damage. You don't need much equipment to test a transformer for low frequency saturation: a signal generator, and oscilloscope, and a low ohms series resistor. You'll also need a power amplifier if your signal generator doesn't have enough voltage output. Connect the resistor in series with a low impedance transformer winding and drive the series combination with the signal generator, with common connections, including scope ground connected to the open end of the resistor. Connect one scope channel across the resistor to monitor current and connect another channel across the signal generator (or power amp) output. Increase driving voltage and/or lower driving frequency until the voltage across the resistor exhibits spikes near voltage zero crossings. You have discovered the saturation limit of your transformer at the test frequency. Normally, you should expect the saturation voltage limit to halve for each octave decrease in frequency, since it is determined by the integral of voltage over time with units of volt-seconds. Consequently, power handling without saturation declines by a factor of 4 for each octave. Again, what happens when an audio signal reaches the volt-seconds limit for a transformer depends most strongly on amplifier output characteristics, since the transformer still happily delivers a turns-ratio-modified replica of driven winding voltage to the other windings. For good reliability and fidelity, "low-cut" audio signals so that you just don't go there.

Paul Mathews

Paul Mathews

Thanks for your contribution.

That's because in the "big picture" it doesn't matter. While it would make sense to expert designers that these parameters would be neatly defined and calculated, I am instead working empirically and changing things as I go along. That's why I said in my original post "I am experimenting with a custom audio amplifier design"... that means I have plenty of flexibility with the design. I can work backwards in a way, first I'll find a good transformer and then adjust my design to match it.

It's a fun way to do things.

I'll let you know in a couple of weeks how it turns out with the transformer. The other aspects of the design are working very well, and even with the 60Hz power transformer I am getting a lot of my hoped-for results.

mw