Transformer advice needed.

[Count Yorga]

I need some help understanding transformers. I need a transformer for an audio project, preferably a toroidal transformer. I want to out 240v UK mains into it and get 18 volts out if it. All the various ones I see to buy online have multiple secondary outputs. I wish to understand if I can buy a transformer with say 2 x 9v secondary and combine them to achieve 18v. Basically I want to get 18v AC from a transformer. Can anyone advise me about this?

Thanks.

 

[Nigel Goodwin]

I need some help understanding transformers. I need a transformer for an audio project, preferably a toroidal transformer. I want to out 240v UK mains into it and get 18 volts out if it. All the various ones I see to buy online have multiple secondary outputs. I wish to understand if I can buy a transformer with say 2 x 9v secondary and combine them to achieve 18v. Basically I want to get 18v AC from a transformer. Can anyone advise me about this?

Thanks.

Yes, 2x9V is 18V - just join them in series - having the taps makes the transformers more useful.

I presume you're aware that the output is AC?.

 

[Count Yorga]

Thank you vwry much for this and, and yes I am aware a transformer puts out AC. The amplifier board I'm using has an integrated rectifier.

I don't need to for this project, but for future reference, if I should run this into a rectifier with say six 10,000uf capacitors, this will result in a higher voltage? Thus if I should need say 50v DC, I should use lower rated transformer? About 30v? Have no experience with power supplies, but wish to gain a better understanding.

Thanks.

 

[Visitor]

Capacitors will charge to a peak voltage of 1.414 × the transformer voltage minus 0.7V per diode voltage drop (probably 1.4v if a bridge rectifier is used).

 

[Visitor]

Also, the transformer windings must be connected in phase; if they are out of phase, the output will be close to zero volts instead of 2×.

 

[Count Yorga]

Thank you Visitor this is very helpful. I am considering a move ambitious project which will probably involve a rectifier. There is some leeway with the two amplifier boards I'm thinking of using as they are rated at >= + -30V, <= + -50V so I am thinking 30v (2 x 15v secondary from transformer) into the rectifier should be towards the top end of this rating without exceeding the 50 volt upper limit. Thank you for your assistance with this.

 

[Visitor]

2 × 15 volt windings would provide about 41 volts DC when rectified and filtered.

 

[Count Yorga]

That's great, thanks very much for your help.

 

[audioguru]

Why do you talk about 18VAC, 2 x 15VAC, +/-30VDC, +/-50VDC and 60,000uF of capacitors?
18VAC rectified and filtered produces 18V x root of 2= 25.5VDC - 2V (bridge rectifier and filter)= +23.5VDC.
Also, 18VAC will not produce +/-50VDC.
Also, 2 x 15VAC will produce +40.4VDC or +/-20.2VDC, not +/-50VDC.
60,000uF is too much capacitance for an amplifier.

 

[Count Yorga]

That's a bit too much Greek salad for me If I wanted to drive two 50 volt dc amplifier boards, what would you recommend? The rectifier in question is an off the shelf one on ebay I'm considering. Go easy I've never dealt with this before.

 

[audioguru]

You do not know what you are doing and I also do not know what you are doing.
You did not post a link to the high power +/-50V amplifier circuit for is to see if it is a You Tube or ebay fake or if it can produce a true output of 127W into an 8 ohm speaker.

 

[Dick Cappels]

Can you post a link to the amplifier in eBay? That might help you get the correct transformer.

 

[rjenkinsgb]

the two amplifier boards I'm thinking of using as they are rated at >= + -30V, <= + -50V

If they need dual positive AND negative supplies - which is common for many analog and audio circuits - you need a different rectifier configuration and transformer setup.

eg. This would work for something like you describe, using a dual 30V secondary or a centre tapped 30-0-30 secondary transformer:

The diodes could be separate or a bridge rectifier module.

It's like two, two-diode centre tap full wave rectifiers back to back, giving opposite polarities from the same transformer windings:

Center Tapped Full Wave Rectifier Operation - Inst Tools

A center-tapped rectifier is a type of full-wave rectifier that uses two diodes connected to the secondary of a center-tapped transformer,

instrumentationtools.com

Another important point with transformers and rectifiers:
Transformers are not perfect; the voltage reduces as the load increases and the rated secondary voltage is (or should be) what is given under full load, and assuming the primary voltage matches.

They have a regulation factor which defines how much the voltage changes from no load to full load.

Larger transformers generally have better regulation than small ones; eg. a 200VA rated one may have 8% regulation, while a 1VA one could have 20% regulation.

That is how much the "off load" voltage is likely to increase, compared to the rated voltage.

That's important for capacitor voltage ratings and maximum supply voltage of such as the amp you mention.
A 30-0-30 transformer should give around positive and negative 41V using the rec circuit above.
Under full load.

If it's regulation was poor, 20% or more, the off-load voltage could be around 50V

And, if you use a "230V" primary transformer on UK mains which is still actually 240V, the secondary voltages will be increased that few percent further as well.

A lot of the time it's not critical - but if the output voltage from a transformer/rectifier combination could be anywhere near the voltage limits for any component, it's worth a bit of extra thought and care.

 

[Count Yorga]

These are the power amplifiers. Cheap ones until I have a better idea about this.

1pair QUAD405 HiFi Stereo Power Amplifier Board w Aluminum Angle Dual Finished 7544462265438 | eBay

Find many great new & used options and get the best deals for 1pair QUAD405 HiFi Stereo Power Amplifier Board w Aluminum Angle Dual Finished at the best online prices at eBay! Free delivery for many products.

www.ebay.co.uk

This is typical if the various power supply boards available.

4 * 10000uF / 100V High-grade Power Supply Bpard for AMP Amplifier Power Board | eBay

âRectifier: 4 Japanese FCU20A40 high speed rectifier diodes, and each rectifier is equipped with a heat sink. Capacitor capacity: 4 10000uF/100V high speed low resistance audio capacitors, 2 0.33uF/160V thin film audio capacitors.

www.ebay.com

I have a preamp with tone controls which runs on 12v AC and has integrated rectifier.

 

[rjenkinsgb]

Interesting, but I suggest you read this, especially the second half; those are clones of clones by the look of it.

Quad 405 / 405-2 Power Amplifier Review

This is the second article in my QUAD 405 / 405-2 series, this time a review of the legendary Quad 405-2 power amplifier.

liquidaudio.com.au

 

[Count Yorga]

Yeah they are cheap. Once I get the hang of this I'll progress to better quality kit. Have rebuilt many amplifiers but never built one before and am unsure of transformers and rectifiers. I could buy boards with onboard ones but that wouldn't teach me much really.

 

[MaxHeadRoom78]

What I generally do is purchase Toroidal transformers, they are easy to modify or add windings to.
If removing double secondary windings they are generally wound bi-filar (wound together) so remove any turns with both at the same time.
To find out the turns/volt, wind on 10 turns and power up and take a reading.

 

[audioguru]

The amplifier modules are very expensive for a Chinese copy of a 48 to 50 years old design.
The modules will severely over-heat since they are designed to be mounted on the huge heatsink in the original Quad 405 amplifier.
The design is class-A that makes a lot of heat.

Reviews of this old amplifier are mixed, some like it and others don't. They say the updated Quad 405-2 is much better.

 

[Count Yorga]

I'm going to use a toroidal transformer. These amps are rated at between 30 and 50 volts so a transformer with 2 x 15 volt secondaries should give about 45v after the rectifier. I have no experience with rectifiers as I've always used boards with them built in before. This is the part I am most in the dark regarding.

The heat won't be an issue as I plan to attach two stonking great big heat sinks. I've use cooling fans with enclosed valve amps quite successfully so this is always an option.

 

[audioguru]

A transformer with two 15VAC secondaries can produce rectified and filtered plus and minus 20.2VDC. Then the "100W" amplifier that needs a minimum of plus and minus 30VDC will not work.

You were wrongly connecting the secondaries in series to make 30VAC which produces a rectified and filtered +40.2VDC with no negative voltage.