Need help with a Toroidal Power Transformer

Thread Starter

ProWannaBe

Joined Feb 13, 2017
21
I do not have any experience with toroidal power transformers at all. I am building a 300watt power amp and need some information. I bought a used toroidal transformer that is 32v-28v-0-28v-32v secondary. It is "300VA" 120v primary and is only 120v. I will be using the 28v-0-28v part of this transformer. Am i right in assuming that the 0 "CT" works as the ground on the secondary winding side of the coil? Also my power amp schematic calls for a 30v-0-30v 10amp transformer.

The person who sold me this power amp kit told me to buy a "300VA" 28v-0-28v transformer but i am also wondering if the amperage is sufficient for the 10amp transformer that the kit calls for?

One other thing is that when I read the secondary voltage on the 2 x 28v lines, it is correct reading from each line individual and grounding to the 0 "CT" line, but when i read the lines that are supposed to be 2 x 32v each, each one reads over 60 plus odd volts also grounding to the 0 "CT" line. Am i doing this correctly?

I would appreciate your help in figuring this out and i also want to thank you ahead of time for any help given!
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
A simple measurement will show you, a 30-0-30 means there should be 60v across each end with a CT creating a zero point. Of which you should read 32 from each end to the CT (ov) point
The term used is closer to Common than Ground.
Max.
 
I understand your frustration. been there. Done that. Many years ago.

A 300 VA transformer is NOT suitable for a 300 W amplifier in the strict sense. Maybe. maybe not.
The DC rail has to be derated about =1/.62 so about 1.6 the RMS current you need.

Before we go there, you have to know what your limits are and what your load is.
So, it's a speaker. Probably an 8 ohm speaker. So,lets specify 6 ohms because it isn't 8 at all frequencies.
Plan to parallel speakers?
This gives you an RMS current that you need on the secondary per amplifier.

So, lets say 150 VA per amp. Derated it's about 93 W.

The secondary has to be able to deliver 1.6 * the RMS current that you want for the speaker.

So, if your only limited to 3A, then (3*3)*8 is 72 W. I used 3 as an example. Being able to deliver higher currents is a good thing. I have 30 Amp output devices in my Amp, but not a 30 A power supply.

Since this is an audio application, it's likely that only one rail is active at at a time, so you PROBABLY can have heaver wire, but less VA.

You have a 28 V secondary, so that's 1.414, so that's a power supply about +-40 V - (1.2 or about two diode drops)
You can't deliver +-40 V p-p. Estimate the losses in the emitter resistors etc. Let's just say it's 4 Volts. So, 72 V p-p.
Which is 72/1.414/2 is aprox 28 V RMS, so that limit is 200 W into 8 ohms.

So, you already have a 300 W amp, that can only deliver 200 W into 8 ohms BECAUSE of the 8 volt possible drop.
Three to 4 drops of 0.6 is definate (2.4) on each rail.

Nor, it MIGHT be able t deliver higher into a lower impeadance.

it's this weird puzzle with limits and ranges all over the place. The transformer may have 10% regulation. It has a VA rating.
The DC currrent you can get out of a rail is about 0.62 the AC winding rated current.
The speakers are all over the place and spec'ed for some DC load.
Each limit has to be figured separately and for each channel. An RMS voltage with an RMS current and a DC load (speaker).
Check each available.

the key being the DC rails can only supply 0.62 of the AC rated current of the secondary.

This https://www.google.com/url?sa=t&rct...k5oUMa3KwjgqL3-YA&sig2=CZFTg1_wya5qpkNIYtVxZg will certainly help.

The weird part: you don't need the current if driving electrostatic speakers. If your driving horns, you need lots of voltage, so tube amps sound better or very high power solid state amps.

So, the key to understanding is a collection of limits. One will define your power output. You can have a 1000 VA transformer, but if it supplies 0.1 A and you have an 8 ohm speaker?
 

Thread Starter

ProWannaBe

Joined Feb 13, 2017
21
A simple measurement will show you, a 30-0-30 means there should be 60v across each end with a CT creating a zero point. Of which you should read 32 from each end to the CT (ov) point
The term used is closer to Common than Ground.
Max.
I agree and knew that technically, common was the correct term to be used. I guess i slip back to a bad habit of mine every so often and call it the maintenance man type of lingo!
Anyway I appreciate your time in helping me and i had assumed that the "CT" was common. I also knew you can add the lines together to make higher voltages. But i wanted to make sure that i was using the "CT" line correctly as i had said, (i had assumed)! I have found that it is better to make sure than to assume anything! Thank you!!!
 

Thread Starter

ProWannaBe

Joined Feb 13, 2017
21
I understand your frustration. been there. Done that. Many years ago.

A 300 VA transformer is NOT suitable for a 300 W amplifier in the strict sense. Maybe. maybe not.
The DC rail has to be derated about =1/.62 so about 1.6 the RMS current you need.

Before we go there, you have to know what your limits are and what your load is.
So, it's a speaker. Probably an 8 ohm speaker. So,lets specify 6 ohms because it isn't 8 at all frequencies.
Plan to parallel speakers?
This gives you an RMS current that you need on the secondary per amplifier.

So, lets say 150 VA per amp. Derated it's about 93 W.

The secondary has to be able to deliver 1.6 * the RMS current that you want for the speaker.

So, if your only limited to 3A, then (3*3)*8 is 72 W. I used 3 as an example. Being able to deliver higher currents is a good thing. I have 30 Amp output devices in my Amp, but not a 30 A power supply.

Since this is an audio application, it's likely that only one rail is active at at a time, so you PROBABLY can have heaver wire, but less VA.

You have a 28 V secondary, so that's 1.414, so that's a power supply about +-40 V - (1.2 or about two diode drops)
You can't deliver +-40 V p-p. Estimate the losses in the emitter resistors etc. Let's just say it's 4 Volts. So, 72 V p-p.
Which is 72/1.414/2 is aprox 28 V RMS, so that limit is 200 W into 8 ohms.

So, you already have a 300 W amp, that can only deliver 200 W into 8 ohms BECAUSE of the 8 volt possible drop.
Three to 4 drops of 0.6 is definate (2.4) on each rail.

Nor, it MIGHT be able t deliver higher into a lower impeadance.

it's this weird puzzle with limits and ranges all over the place. The transformer may have 10% regulation. It has a VA rating.
The DC currrent you can get out of a rail is about 0.62 the AC winding rated current.
The speakers are all over the place and spec'ed for some DC load.
Each limit has to be figured separately and for each channel. An RMS voltage with an RMS current and a DC load (speaker).
Check each available.

the key being the DC rails can only supply 0.62 of the AC rated current of the secondary.

This https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjVsfmf_Y7SAhVH7SYKHeNXDdsQFggeMAA&url=http://www.hammondmfg.com/pdf/5c007.pdf&usg=AFQjCNFPNgWFEfkADk5oUMa3KwjgqL3-YA&sig2=CZFTg1_wya5qpkNIYtVxZg will certainly help.

The weird part: you don't need the current if driving electrostatic speakers. If your driving horns, you need lots of voltage, so tube amps sound better or very high power solid state amps.

So, the key to understanding is a collection of limits. One will define your power output. You can have a 1000 VA transformer, but if it supplies 0.1 A and you have an 8 ohm speaker?
KeepItSimpleStupid,
T
hank you very much for the extensive education in working with amplifiers and power transformers! I really appreciate the amount of information you have given me to chew on.

The guy that i have bought this kit from says that this transformer is enough? I have my doubts!

This is his response to me about the amp:
The VA rating is basically Volt x Ampere. So if a 28V output has a 300VA rating it should have a 300/28 = 10.7A current output. So 28V - 0 - 28V 300VA CT transformer is ok. The amplifier can handle up to about 35V with proper heat sinks. However the recommended operating range is about 26V - 30V. So anything between 26 - 30 would be perfect. you can even use 32V. but you should keep the power transistor temperature lower than that of operating with 28V. otherwise transistor may damage slowly and eventually give away. If there are any problems feel free to contact me.
Back to me:
So I am going to have to chew on all this information you've given me and make sure for myself! I also appreciate the link to the pdf document which i definitely have saved for a reference!!! The amp i am building uses a full wave circuit which i see in the pdf information that it seems to be better for power output.


Again thank you very much and now i am going to have to do my homework!!!
 
What i can't tell is are you talking stereo, mono and what speaker load.

I built a version of this amp http://leachlegacy.ece.gatech.edu/lowtim/ in the early 80's. The article in Audio Magazine specified a 3 A, 35 V secondary for 2 channels. The article mentioned it's use with electrostatic speakers.

The first transformer I used, used a 20 A constant voltage transformer, but the hum was objectionable. I had a custom transformer built with 4x35Vac windings which still isn't enough. They had 9600 uF capacitances on each of the 4 rails. Each amp had two.

later, I was able to procure a 500 W, 120 VAC regulator (List price was about $1500) and the sound got noticeably better.

In school, I missed the de-rating stuff, but it makes sense. So, I have an effective 100 V supply assuming no losses. It can swing +-50 V, no losses and a 3A RMS secondary. I measured about 125 W into 8 ohms resistive, 1 channel driven and no AC pre-regulator.

When you look a the power supply as a bunch of limits, it makes sense.

I made changes to the component selection of the original design. You don't run a 50 V rail with 50 VDC capacitors. Learned the hard way. That was what was specified. Nearly all resistors went metal film. Bias pot went 10 turn. Power supply got upgraded and went independent. Ground went true star. Anti-thump and audio ramp at power up was added. Custom case from scratch.

What's unique about the Leach Amp is the ground plane construction and the rise time.

In my professional life I did occasional power amp repairs for a shop at home. At work, I worked on some high power stuff, but not full time. 100 kV 0.1 A DC supplies for X-ray generators, 30 kW, 15 kV at 1.5A ebeam supplies and 1000 W RF transmitters. then there was the arc lamp supply with 22 VDC at 50 A with a 40 kV kick to start. I also did work at the low end of the spectrum. Like 100 V and 2E-12 Amps (Picoamps). And where voltages were around 1 V and 25 mA. The 3000 A at 6.3 Vac supply never had any issues. The scanning electron microscope with EDAX was fun to work on too.
 
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