1800VA Torroid to get 40A???

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
So, I have learned that my Martin Logan speakers like alot of current, so I bought a (what I thought) was large 420 VA 22V transformer which delivers 9.5 Amps pre rail.

I just found out that the ML's like amps that run around 40 Amps per rail. Always, with the revisions!

So now I am thinking that to get this, I would need an 1800VA torroid @ 22V or a 1600VA @ 20V.

But, I also know that the primary Ampere input can affect the overall output...this is my inquiry...

1. How do I figure the primary Ampere input and its effect on Ampere output?

2. If I were to find an 1800VA @ 22V Torroid, would I really be capable of pushing 40A with the LM4780? What is the limit? 35V is the only restriction I have ever seen quoted on here.

3. Would the 1600VA @ 20V or 1800VA @ 22V be the better choice?

Thanks,
Dominick
 
In an ideal world it would be:

Vpri x Ipri = Vsec x Isec

However all transformers have a level of effiency usually stated on the datasheet. The closer the output voltage to the input usually the higher the effiency. on 230V->22V efficiency is around 90%, i'm not sure about 110->22V. Have a look on the datasheet.


So the Equation becomes:

Vpri x Ipri = (Vsec x Isec) / Efficiency. Well the exact formula is a bit different but this is the simplest way to do it.

So the primary current would be Ipri = (Vsec x Isec) / (Efficiency x Vsec).

If for example you have a 1800VA transformer, primary at 110v and eff around 91%:

Ipri = 1800/ (110 x 0.91) =~18A.

I'm not sure about the US standards but in europe usually sockets are connected to 16A fuses... Anyway you would need quite a beefy cable for it (at least 4mm) on the primary (normal power cord wouldn't be enough).... and double (not exactly) on the sec.

What Amp are you using ?

BTW the cables to your ML speaker should be quite beefy too!
 
I just finished an LM4780 design that you can peek at in this section. I guess the sunfire grand cinema pushes 40A per channel and current is the hot commodity for ML's so I thought I would make that rating my goal.

The sunfire is also a favorite among ML owners.

What does the Ipri, Isec stand for? I guess just the I is unknown to me!

Thanks for the help,
Dominick
 
Hi Dom,
I think Martin Logans need amp performance that chips will never be able to deliver.

But back to your query.

The transformer charges the smoothing caps. The designer selects a transformer Vac and VA to suit the maximum average power the rails will need to supply for normal operating conditions.
For a normal ClassAB amplifier it is usual to specify a VA rating equal to 1.5 times the maximum power that you can push into your normal load.

The capacitors supply the transient current to the output stage. This transient current can be many times the average current that you charge the caps with.

Some well designed small amplifiers can achieve large peak current ability. I do not know the US market, but here in the UK the audio press and the better manufacturers specify the peak current ability of the amplifer.

An example would be a 50W into 8r amp that can do 25Apk.
This peak current is about 10 times the rms current into an 8r load.
Even the peak current into an 8r load is only 3.5Apk
But the specification confirms that the output stage and Power Supply can achieve short term transients that very demanding speakers can require.

I am confirming that it is possible to design an amplifier that can successfully drive the Martin Logans, but you do need to know what you are trying to achieve and how to get there.
 
There is not such a thing as 40A instantaneous currents being sourced by amplifiers or flowing through a loudspeakers while listening to music.

The only recommendation that I can make is to place a precision non-inductive current shunt in series with the loudspeaker and to monitor the current (voltage drop) waveform across it with a suitable oscilloscope (be careful with earthed equipment and ground loops).

Then try to reach just 10A transients with music, without causing the amplifier to clip, and without blowing the loudspeakers, your ears or both...
 
Hi Eva,
I realise you are trying to be helpful by putting some pretty obvious reality into the discussion.
not such a thing as 40A instantaneous currents being sourced by amplifiers or flowing through a loudspeakers while listening to music
but the thread starter has come across a recommendation for 40A peak current capability.

If this information is from the manufacturer, would you accept it?

Taking the discussion down a slightly different route.

Some very difficult to drive speakers have an impedance vs frequency plot that shows a wide range of impedance variation, that falls well outside the recommended norm of +-20% of nominal.
I have seen some plots for a nominal 8ohm speaker that averaged just under 8ohm (from memory about 7.5ohm) but fell to under 1.5ohm at high frequency and judging by the shape of the curve appeared to keep falling beyond the 20kHz limit of the test/display set-up.

Traditionally, big beefy amplifiers are reputed to also have a high peak current ability. I have found, from many sources, that this correlation rarely follows. For example some low power amps can and do source enormously high peak currents into test loads and MANY high power amplifiers can only just manage to drive resistive loads and little else.

The Martin Logan range of speakers is reputed to be difficult to drive.
Although I have no data to hand I would not be surprised if users/dealers/manufactuer(Martin Logan) have found that they are best driven by amplifiers that have a very good to excellent peak current ability.

Let the original query follow it's course and let's see if any conclusions that can be drawn by the poster are sensible/achievable.
Trying to put a stop, for whatever your own reasons are, would, in my opinion, not be helpful at this time.
 
AndrewT said:
Hi Eva,
If this information is from the manufacturer, would you accept it?

When it comes to electronics, the only recommendations that I accept are the ones coming from my oscilloscope and from my multimeters.

You know, we as humans are prone to take advantage of hype when we know that other people does not have the knowledge or the resources to check our claims.
 
Funny, i've been waiting to hear which Martin Logan system is featured in this thread.

The ML's that were difficult and current suckers were the CLS and "The Statement". I think i'm one of few on diyA who have actually heard The Statement (1st ed. that is), and it needs a truckload of amplifiers. The CLS had a 81 dB/W.m sensitivity but a 200 watt HK Citation was enough.
All hybrid ML models from the Sequel 1 and onwards were pussy loudspeakers compared to the CLS.

An amplifier circuit capable of 40 amps peak output current doesn't need a 1800VA transformer to do that trick.
 
Yah gotta remember that the human ear works on simple level and timing placement of transients. A diode-like device, in terms of what it recognizes as 'signal'. The other 90% of the signal? It don't mean all that much. Which is why we can stand listening to and sometimes do ascribe 'clarity' to a horn speaker, which suffers tremendous amounts of distortion.

If you look at this situation with only a multimeter and scope, etc, you'll think that the last few x.xx% of current and drive capacity are not worth bothering with, but when you look at the place which those last little bits will have their effect in terms of signal reproduction, -and compare that to how the human ear works- they end up being the most important bits of signal reproduction and distortion that one can possibly pursue.

So yeah, it's ALWAYS worth one's while to go after those last little bits. As they are the critical ones, and you WILL be rewarded for your efforts. That is, if you are looking at things properly. :)

eg: Lets say that one particualr amplifier 'drive' situation made it so that the transients under heavy load were 'blurred' or distorted to a level that equated to ~1% more "Overall" measured distortion, compared to the circuit with the use of the beefy transformer.

Human perception (if one was simly listening to the two different amplifiers on the same system) would say that there was a Tremendous difference between the sound of the two given amplifiers, but it would barely register as a measurement.
 
Hi,
These Martin Lorgans are probably electrostats , which means they look close to capacitive load for an amplifier. The 40A peak current recommendation might be argued that the amp should drive the speakers up to 20kHz at full output voltage .The current requirement is dependant on amp's maximum output voltage.
So stating 40A without voltage does not provide enought information.
By the way , Dominick , the output current of LM chips are limited internally(at around 10A or so) , so adding a large trannie has no effect anyway.Neither too much filter capacitance does.

Regards,
Lukas.
 

AKN

Member
Joined 2005
Paid Member
I did lookup Martin Logan and find one model Summit witch i think is a very expensive model. According to specs max 300W 4 ohm impedance and one interesting statment 0.7Ohm at 20kHz.

300W into 4Ohm is ~35V and ~9A

Assuming amplifier contant voltage drive we take that 35V into 0.7Ohm 20KHz and we get 50A and 1750W!

For simplicy no phase angles included but they could only make things worse.

So to keep straight undistorted frequency response at 300W one would need a powerstation in class of a beefy Crell or simular capable of 0.5Ohm drive.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.