usability of an old transformer

[zvir]

Hi guys.
I have an old 85 VAC center tapped transformer. I have no idea about it's power handling capabilities, but maybe it will help if I tell you that it used to power a mixer with two 200w class A amplifiers...

Now, what I wish to know is:
Can this be used to power up a Gainclone? Or perhaps two of them?
Please help me about this one, I'm a bit short on money to buy a new transformer...

 

[AndrewT]

85Vac centre tapped will give ~+-60Vdc.
Enough to blow up as many chipamps you can throw at it.

 

[RocketScientist]

Hi guys.
I have an old 85 VAC center tapped transformer. I have no idea about it's power handling capabilities, but maybe it will help if I tell you that it used to power a mixer with two 200w class A amplifiers...

Now, what I wish to know is:
Can this be used to power up a Gainclone? Or perhaps two of them?
Please help me about this one, I'm a bit short on money to buy a new transformer...

Do you mean it measures at 85 VCT (42.5 + 42.5) AC with no load? Or is it rated for 85 V? If it measures 85 V with no load you will get about +/- 60 volt DC rails. If it's rated at 85 V at full load, you'll probably have 64 volt or even higher no-load rails and 60 volt rails under load.

If it's 85 V no-load you're not likely to get a real 200 watts. If you have 60 volt rails and figure about 3 volts drop (which might be optimistic) to the load, that gives you 57 volts peak or 40 volts RMS which is exactly 200 watts RMS into 8 ohms. But the rails will droop under load--probably by 5% - 10% depending on beefy the transformer is. So 180 watts is more likely.

I don't think you really meant "Class A"? A true 200 watt two channel Class A amp would be a huge beast pumping out lots of heat. It would also need a really huge transformer and wheels to move it around

If you want to know the approximate VA (volts*amps power) capability of your transformer the best estimate is to simply put it on a scale. Transformers are about 100 VA per kilogram. Or for pounds, divide the weight by 0.022 to get VA.

It depends on who you ask, but a 400 watt (200 w/ch) Class A/B amp needs about a 600 VA transformer to deliver full power continuously into 8 ohms without the transformer getting too hot. You could get by with 350VA - 500VA for playing music into 8 ohms but if it's going to be used for demanding use you probably want 600 VA or more. If you want the amp to comfortably drive 4 ohm loads you want closer to 800 - 1200 VA. So it's hard to say how your transformer was sized for the old mixer it was in.

Ideally the transformer should weigh at least 6 KG or or 13 pounds to be large enough for a 2 channel 180 - 200 watt amp. If you want less power than that, and don't want to use at least 4 (ideally 6 or more) output devices per channel than you probably want a lower voltage transformer like 30 + 30 or 35 + 35 VAC. So you can use your transformer and spend money on big heatsinks and output devices, or you can build a smaller amp and have to buy a new transformer

 

[zvir]

Re: Re: usability of an old transformer

Well, now that you put it that way, it probably wasn't class A... But on the back of that mixer was written that it was 2 x 200w. And the transformer measures 85V under no load. About the weight, I'm not sure exactly how much it weights, but it's not very light... I'll put it on a scale tomorrow and post the results.

Anyway, what you are saying is that I am not able to use this one to power up, let's say, LM3886? So I have to buy a new transformer and/or some other devices?

 

[RocketScientist]

Re: Re: Re: usability of an old transformer

Anyway, what you are saying is that I am not able to use this one to power up, let's say, LM3886? So I have to buy a new transformer and/or some other devices?

Yes, or move up to an LM4702, LME49810 or LME49830 to build a chip amp with discrete output transistors. You can buy finished modules, kits and empty PCBs using these chips from from Asian suppliers. But you need one with 4 output transistors per channel to handle +/- 60 volt rails unless you want to live dangerously. For example, Panson_hk is here on diyAudio sells these boards which have 4 transistors per channel and would work well:

LME49810/LME49830 Power Amplifiers

These days, if low cost is your goal, it's hard to beat commercial made in China products. That's true for DIY PCB amp modules and also true for finished commercial amps. For example: The Behringer A500 sells for under $200 US and it would be impossible to build a similar complete DIY amp in a rack chassis with similar performance for less money unless you already had a lot of the parts. Some people say bad things about the A500 but here's an objective review showing it performs fairly well:

A500 Review

But, yeah, if you're set on using the LM3886 you need a different power transformer. Another source might be a dead A/V receiver rated at 100watts/ch or less.

 

[star882]

You can use voltage regulators to reduce the voltage.

 

[megajocke]

Or build a Leach amp:
http://users.ece.gatech.edu/~mleach/lowtim/

 

[RocketScientist]

You can use voltage regulators to reduce the voltage.

Except the regulators would probably cost more than the correct transformer or a LM4702 module. Regulated power supplies for power amps present all sorts of challenges and issues. And, in this case, the regulators would need a big expensive heat sink and pass transistors and the finished amp would be very inefficient. You're way better off using an unregulated supply.

 

[zvir]

I thought about voltage regulators, but there is a problem about them. I haven't seen simple regulators that can put out +-45V at high currents. I guess that the voltage regulator would be more expensive than the new transformer...

And the leach amp seems interesting... I'll look at it sometime soon...

 

[star882]

Except the regulators would probably cost more than the correct transformer or a LM4702 module. Regulated power supplies for power amps present all sorts of challenges and issues. And, in this case, the regulators would need a big expensive heat sink and pass transistors and the finished amp would be very inefficient. You're way better off using an unregulated supply.

I thought about voltage regulators, but there is a problem about them. I haven't seen simple regulators that can put out +-45V at high currents. I guess that the voltage regulator would be more expensive than the new transformer...

A well designed regulator is neither expensive nor inefficient. A simple buck converter will work fine. It will allow the beginner to learn about power supply design without having to deal with a direct connection to the mains.

 

[RocketScientist]

A well designed regulator is neither expensive nor inefficient. A simple buck converter will work fine. It will allow the beginner to learn about power supply design without having to deal with a direct connection to the mains.

You might want to read what some of the audio amp "gurus" like Douglas Self, Randy Slone, etc. have to say about regulated power supplies and audio amps that put out any real power (i.e. over 15 watts or so). Switching regulators, like a buck regulator, present their own problems--not the least of which is lots of high frequency "hash" from their high current switching pulses. And if the audio amp tries to draw more peak power than the buck regulator allows, most buck regulator chips will shut down or otherwise seriously glitch the supply voltage causing nasty noises in your speakers. So the regulator has to be sized for seriously high currents to prevent it from going into protection. As does the switching inductor to prevent it from saturating. It starts to get expensive even if you can solve the noise problems.

In Class-D amps (not what we're talking about here) switching power supplies are more popular because the Class-D amp already needs filtering to get rid of the high frequency garbage from the power supply. But in a Class-B amp, like a gainclone, the noise on the power supply from a buck regulator would likely be a problem.

It's also tricky to design high current, high voltage buck regulators. Most regulator chips don't meet the requirements. And the extremely wide range of load currents presents challenges with respect to sizing the inductor, minimum duty cycle, etc.

 

[star882]

You might want to read what some of the audio amp "gurus" like Douglas Self, Randy Slone, etc. have to say about regulated power supplies and audio amps that put out any real power (i.e. over 15 watts or so). Switching regulators, like a buck regulator, present their own problems--not the least of which is lots of high frequency "hash" from their high current switching pulses. And if the audio amp tries to draw more peak power than the buck regulator allows, most buck regulator chips will shut down or otherwise seriously glitch the supply voltage causing nasty noises in your speakers. So the regulator has to be sized for seriously high currents to prevent it from going into protection. As does the switching inductor to prevent it from saturating. It starts to get expensive even if you can solve the noise problems.

In Class-D amps (not what we're talking about here) switching power supplies are more popular because the Class-D amp already needs filtering to get rid of the high frequency garbage from the power supply. But in a Class-B amp, like a gainclone, the noise on the power supply from a buck regulator would likely be a problem.

It's also tricky to design high current, high voltage buck regulators. Most regulator chips don't meet the requirements. And the extremely wide range of load currents presents challenges with respect to sizing the inductor, minimum duty cycle, etc.

Put the bulk capacitors on the output of the regulator and only use small bypass capacitors on the input. Essentially an active PFC but with a buck instead of a boost converter. The operating frequency is generally well above 20kHz, nicely out of band. There should not be any in band noise unless the feedback loop is unstable. (If you really want engineering challenges, design a power supply for DSL equipment. Most switchers have the operating frequency and their harmonics right inside the bandwidth you're trying to work with.)

 

[RocketScientist]

Put the bulk capacitors on the output of the regulator and only use small bypass capacitors on the input. Essentially an active PFC but with a buck instead of a boost converter. The operating frequency is generally well above 20kHz, nicely out of band. There should not be any in band noise unless the feedback loop is unstable. (If you really want engineering challenges, design a power supply for DSL equipment. Most switchers have the operating frequency and their harmonics right inside the bandwidth you're trying to work with.)

That only addresses one of many problems. You also need two regulators (one each for the positive and negative rails), and unless they're running from a common clock, you're likely to get beat (difference) frequencies that will be audible. Even running from a common clock, the current demands for each rail will be constantly different, hence the duty cycles will vary with respect to each other and likely create audible artifacts. High current fast rise time PWM waveforms create all sorts of nasty broadband energy that likes to find its way into the audible band--especially when you have two such generators running in close proximity to high gain and relatively high impedance audio circuitry.

And many have demonstrated that even out of band HF energy can degrade the sound in various ways. And that still leaves cost, protection circuitry interfering, inductor saturation, lack of chips that can handle the voltages, etc.

Switching regulators are the way to go for digital power supplies of 5 volts or less. But they really are *NOT* the way to go for a class-B audio power amp. There's fairly clear consensus on that. Do what you like with your own designs, but I'm trying to help zvir avoid a potential nightmare.

 

[star882]

The easiest way is to center tap full wave rectify the transformer to get a single 60v output, then run a BTL from it.