Amp for 4 Ohm Speakers

[3db]

Autotransformer in electrical term has nothing to do with being "automatic". An autotransformer is simply a one winding transformer without multiple taps to change the output voltage.

Of course it will try to double down, that's simply Ohm's law but the fact that it use an autotransformer to change the voltage, it will not have the same efficiency as OTL designs, hence more heat and not good to do so as per Mc's email to Zumbo. It also has lower damping factor.

So the whole argeument is kinda mute since we're playing with an autotransformer and a not regular transformer. I have to refresh myself on the theorey of those again. If memory serves me (which is seldom these days) these autotransformers are more hazourdous than a normal transformer since there is no isolation whatsoever from the line voltage coming in. Why would one use such a beast?

 

[PENG]

I wouldn't call that a good thing...

Exactly, OTL amps that double down, e.g. Krell amps, will not tell you not to do it. Even a Denon AVR-5308 will double down when run in two channels, but the distortion will likely be deemed too high by them. This is also a moot point but I am trying to make a point.

 

[3db]

Double down? Qu'est ce que c'est?

 

[PENG]

So the whole argeument is kinda mute since we're playing with an autotransformer and a not regular transformer. I have to refresh myself on the theorey of those again. If memory serves me (which is seldom these days) these autotransformers are more hazourdous than a normal transformer since there is no isolation whatsoever from the line voltage coming in. Why would one use such a beast?

Most likely for impedance matching to achieve "maximum power transfer". Doesn't maximum power transfer theory ring the bell? If you were like me, the txt books are in the basement all boxed up. You can still Google as an alternative, for a brush up. The theory is sound (meaning good) but I am not sure if it affects sound quality, obviously not if you ask Mc.

 

[TLS Guy]

I wouldn't call that a good thing...

Well here's why.

No transformer can create energy, in fact there is always waste (loss).

Now in a conventional transformer all of the power passes through the transformer primary and secondary.

In an auto transformer, only the power of the voltage change is passed though the transformer, so losses are reduced.

However the power from the output devices must equal the power in the load minus transformer loss.

Now lets take a good direct coupled amp that can deliver 105 watts into 8 ohms. That requires a 29 volts and a current of 3.625 amps.

Now it is a good amplifier and lets lower the load to 4 ohms. A good amplifier will maintain voltage and so the current will increase to 7.25 amps and the power delivered will be 210 watts.

Now lets look at the auto transformer situation.

To deliver 105 watts into four ohms requires a voltage of 20.5 volts and the current is 5.1 amps.

Now lets assume the ratio is 1:1 at four ohms, so all the voltage is delivered by the load.

Now to deliver 105 watts into 8 ohms as stated above requires 29 volts and 3.625 amps.

So the voltage from the power transistors is 20.5 volts, so we need 8.5 volts from the auto transformer. This is a ratio of 1:1.4 rounded off.

So the power transistors are still delivering 105 watts + a small loss from the transformer, which for this illustration we will ignore.

The voltage at the power transistors will still be 20.5 volts and the current 5.1 amps.

Now lets use the 8 ohm tap, but make the load 4 ohms.

The current into the load will double to 7.25 amps for a power of 210 watts.

Now if the power at the load is 210 watts, then the power delivered from the power transistors must be 210 watts.

The voltage at the power transistors is 20.5 volts which requires a current of 10.24 amps.

Now the direct coupled amp only needed to provide 7.25 amps.

Now heat generated id the square of the current X the resistance. The heating effect in a power transistor is the square of the current X its internal resistance.

Now Mac can't deny this when you push them and they don't.

Basically by use of the auto transformer you get the same power into all loads, which for this amp is 105 watts.

 

[3db]

Well here's why.

No transformer can create energy, in fact there is always waste (loss).

Now in a conventional transformer all of the power passes through the transformer primary and secondary.

In an auto transformer, only the power of the voltage change is passed though the transformer, so losses are reduced.

However the power from the output devices must equal the power in the load minus transformer loss.

Now lets take a good direct coupled amp that can deliver 105 watts into 8 ohms. That requires a 29 volts and a current of 3.625 amps.

Now it is a good amplifier and lets lower the load to 4 ohms. A good amplifier will maintain voltage and so the current will increase to 7.25 amps and the power delivered will be 210 watts.

Now lets look at the auto transformer situation.

To deliver 105 watts into four ohms requires a voltage of 20.5 volts and the current is 5.1 amps.

Now lets assume the ratio is 1:1 at four ohms, so all the voltage is delivered by the load.

Now to deliver 105 watts into 8 ohms as stated above requires 29 volts and 3.625 amps.

So the voltage from the power transistors is 20.5 volts, so we need 8.5 volts from the auto transformer. This is a ratio of 1:1.4 rounded off.

So the power transistors are still delivering 105 watts + a small loss from the transformer, which for this illustration we will ignore.

The voltage at the power transistors will still be 20.5 volts and the current 5.1 amps.

Now lets use the 8 ohm tap, but make the load 4 ohms.

The current into the load will double to 7.25 amps for a power of 210 watts..


Now if the power at the load is 210 watts, then the power delivered from the power transistors must be 210 watts.

Assuming the transformer can deliver it. From what I've just read, the autotransformer has better regualtion characteristics than of a conventional transformer which took me my surprise. I guess its only vice but its a biggy is that it cannot providing electrical isolation. Is trhis vice the only reasoon why they are not being used as it is a considerable saftey hazourd?

 

[TLS Guy]

Assuming the transformer can deliver it. From what I've just read, the autotransformer has better regualtion characteristics than of a conventional transformer which took me my surprise. I guess its only vice but its a biggy is that it cannot providing electrical isolation. Is trhis vice the only reasoon why they are not being used as it is a considerable saftey hazourd?

The advantage is size and efficiency. In a conventional transformer all off the power is passing through the both primary and secondary.

However in the auto transformer only the power supplied by the portion if the coil supplying the difference in voltage from source to load carries any power, the rest of the power being carried by the source directly.

So that means that the transformer can be made much smaller.

 

[ashergill]

That was a heck of a discussion! Though I didn't mean to spark any controversy, I actually learned quite a bit about amps from these posts. Thank you all, especially Zumbo.

I've bought an ATI 1802, but still plan to keep an eye out for a mint-condition McIntosh down the road. On the bad-news side of things, I did not get the VA concert grands after all that; they sold to someone else So, it's back to square one, but perhaps that is a good thing. I've setup a couple more auditions; planning to listen to B&Ws and Paradigms.

 

[AcuDefTechGuy]

That was a heck of a discussion! Though I didn't mean to spark any controversy, I actually learned quite a bit about amps from these posts. Thank you all, especially Zumbo.

I've bought an ATI 1802, but still plan to keep an eye out for a mint-condition McIntosh down the road. On the bad-news side of things, I did not get the VA concert grands after all that; they sold to someone else So, it's back to square one, but perhaps that is a good thing. I've setup a couple more auditions; planning to listen to B&Ws and Paradigms.

Cool, the ATI 1802 is an excellent amp.

Now let's have a heated discussion on speakers.

Let's talk about why the MB Quart speakers are just awesome.

 

[cpp]

Now let's have a heated discussion on speakers.

Let's talk about why the MB Quart speakers are just awesome

so what speaker price range ?


quarts work nice in my car

 

[zumbo]

Good choice on the amp.

Now, MB Quart QLS 830?

Just missed a set for $510.

A good while back, I won my third set for $469.

They are more than worth the asking price at Yawa. $999pr.

Or, the waiting game on ebay.

 

[zumbo]

The 830's are better than the 1030's. Not open for debate.

http://www.yawaonline.com/speakers.html

 

[zumbo]

This seller will sometimes start an auction at $9.99. Yes, nine dollars, and ninety-nine cents.

As mentioned, I won my last set for $469.

They also accept reasonable offers. Who knows what they will accept.

http://cgi.ebay.com/MB-Quart-QLS-830-6-5-3-Way-Tower-Speakers-Pair-New-/150423012801?pt=Speakers_Subwoofers&hash=item2305e905c1