How important is transformer output?

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I'm needing a ss amp with a great deal of dynamic power for some real power hungry CS3.5 speakers. These are notoriously low impedance. When I'm assessing amplifiers from the power side, how critical is the transformer size or is capacitance the key? If for example, I am looking at units with 600VA vs 2000VA and both have 120,000uF capacitance, is it a given that the 2000VA will be far superior, given more or less equivalent wattage rating and technology? Is it that simple or are there other things to consider?
 
My question is poorly posed, I think. The speakers in question are 88dB/W/m, nominal impedance of 4 ohms with minimum recommended power of 40W (though I think 400W @ 4 ohm is more realistic for normal use), but my desire is to understand the relative importance of capacitance and transformer output in the ability of an amp to drive speakers. Using a water analogy, the capacitance is a storage tank, the speaker load someone opening and closing a valve, the transformer some kind of pressure throttling device. If I'm only lightly opening and closing the outlet valve, the amount stored in the tank is more than sufficient to keep up with my demand and the throttling valve throughput adequate to replenish the tank. Once I impose more serious demand, at some point, the amount stored becomes taxed and the ability of the throttling device's throughput begins to have an impact on meeting the demand.

I'm using an Aragon 8008BB with 2200VA transformers and 120,000uF capacitance to adequately drive these speakers. If I were to use an 800VA transformer in the same amp instead, with same capacitance, and given a very demainding speaker load (there must be some bottom limit to what is practial or in use generally), would the system be taxed or not? Is the 2200VA severe overkill when 120,000uF is present, even, for let's say, the most demanding speaker loads, a Magnepan, perhaps?
 

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I would take an amp module designed for say 300watt/8ohm, with say 4 output pairs each channel
Lower the supply voltage to +/- 50-55Vdc
750VA rating Andrew suggest should be fine for each mono
2 pairs of 33.000uf each channel should be ok
Use heavy heatsinks

With good amp modules and done properly I expect it will be better than Aragon

btw, isnt Aragon true balanced amps? They may not like low impedance

I believe that amps should be designet for at least half the impedance load of the actual speaker impedance
 
Originally posted by DreadPirate
real power hungry CS3.5 speakers. These are notoriously low impedance

The CS3.5 is actually not that difficult a load. It is still above 4 ohms. A notoriously low impedance speaker would be the Watt-Puppy 8 where impedance hovers around 2 ohms.

For a normal amplifier, a 800VA transformer is more than enough for 400W @ 4 ohms. 2200VA is an overkill. For high performance amps, 2200VA with 120,000uF per channel is not unrealistic.

Regards
Mike
 
You need both a large transformer and lots of capacitance to keep the rail voltage from having excessive ripple (dropping below the no load voltage). The capacitance is the current source when the transformer is off (not sourcing current) which is typicaly 80% of the time at high power output. When the transformer is on it has to be large enough to "fill" the capacitors in the short time its on. At full power into a low impedance there is a max ripple voltage before it effects the sound. There is a certain combination of transformer/ capacitance size that will meet this criteria and if either one is to small you will have excesive ripple. After this point increasing one without the other will not be benificial. (No matter how much larger the tranny is, the cap voltage will still drop x amount when it is the current source).

So going from 2200VA tranny to 800VA, you willl no longer need 120,000uf. Its a waste of caps.
 
keep the rail voltage from having excessive ripple (dropping below the no load voltage)

Using a cap-only psu you will always get ripple, no matter how much caps you add. If you're really that concerned, go for an actively regulated psu. Sure, not easy to do, but very effective. Feel free to mail me if you're interested in pcbs :D

Anyway, if you want a high-current capable amp, I would think more in terms of an amp that is designed for that (large number of output transistors for example, high power drivers if BJT...). No use for a large cap bank and toroid if the output stage is not up for it.

Have fun, Hannes

PS: power needs are usually also largely overestimated...just use your multimeter to measure voltage across speaker terminals when playing music. You could be surprised ;)
 
h_a said:
power needs are usually also largely overestimated...just use your multimeter to measure voltage across speaker terminals when playing music. You could be surprised ;)
Yes and no.

Average power needs are often overestimated.
I posted data on speaker terminal voltages when testing a new satellites + bass speaker system. The voltages were tiny and surprisingly the bass speaker voltages were much less than the satellites drive signal.

Peak transient power/voltage/current needs are often underestimated.

Allow at least +20dB above your average level and some say that +30dB is required for very dynamic sound sources.

If one is ticking along @ 200mW into 92dB/W/m 8ohm speakers (about 80dB at the listening position) and you need +20dB that is a peak transient of 100dB then the power requirement jumps to 20W and that is far below what real sounds in real life sound like.

Using the +30dB for transient overhead requires 200W into 92dB speakers to generate that 110dB.

Is it Gedlee that suggests we should be aiming for 130dB SPL for transient peaks?

Heatsinks do not need to be designed for transients!!!!
 
Interesting, Andrew.

It would be, however, interesting to know where these figures come from. Especially +30dB headroom will be completely useless if one likes to listen to contemporary rock or pop, where the loudness war kills the dynamic range.

Also not everyone has neighbours that tolerate 110dB nor a listening room with a noise floor below 30dB...

Last I'm wondering what all these 5W amp owners think about that headroom ;) Or the Aleph-owners. Anyway, the headroom can be easily examined by shoving your music onto a PC and having a look at the waveform.

Have fun, Hannes
 
Like always energy is needed if you want to do some work.
Never save on the transformer power.
I always use like 2x20V 1000VA for 40W/8Ohm output one channel.
Calculate with factor 4 to have a nice capacity.
Almost always the transformer is to small in commercial power amplifiers.
They always calculate in the momentary capacity and then the sound is like sh-it.

regard

Progg70
 
Yes Andrew, 20W enough but always keep in mind speaker control, impedance dips, so it's safer to go to 50W/100W. Also distortion rise as the amp is approaching the power limit; it's harder on your speakers, more heat dissipation.

Very good advices for the Pirate !!

If you want same sound than the Aragon go for no less than 75 watt SS. If you tell us what is your new amp maybe we could advice more
 
The reason I came up with this question is that I was seeing a great deal of variation among commercial amps I was considering for purchase in the 150-200W @ 8 ohm range as to VA ratings for the transformer and total capacitance. For example:


Adcom 700VA 60,000uF 200W
Aragon 8008ST 2000VA 60,000uF 200W
Odyssey Stratus 400VA 60,000uF 150W

Other amps of similar capacity, Classe 201 and Threshold S300 I was unable to find this information for.
 
It would be nice to know how large of space you are using the speakers in. In a smaller room you can use a smaller amp where as in a large room you need a larger more powerful amp that can put out some current to control the drivers. The 3.5 as stated previously is not a difficult load but does need an amp that can control the drivers. A friend had a pair of 3.5s in a room that was 18ft by 26ft driven by a pair of highly upgraded Dyna MK 3s. He was very pleased with the sound and the bass was very good also. He tried many different solid state amps but always went back to the MK 3s. Room size is more important than size of the power transformer in the amp. Most decent amp manufacturers will design an amp with proper power transformer for the rated power and current output.
 
Power at 8 Ohms is a pretty, but not so interesting number as - as you say yourself - most speakers dip far below that.

Large transformer, large cap bank and powerfull output stage becomes important for lower impedances, like 4 or 2 Ohm.

You can check your list of amps how power increases with lower impedances. Check also the prices of the amps, in the end you get what you pay for - and the big transformers cost big money.

By the way, the transformer rating is also not always honest, depends on manufacturer. Better compare size and weight.

Speaking of ratings, also power ratings of amp manufacturers are not always honest and often stretched for/by marketing.

Have fun, Hannes

PS: that still does not mean that any of these monsters is necessary in real life...
 
I have found that the way an amplifier delivers power into lower impedances has a big influence on the way the bass sounds.

Cordell seems to agree by stating that an Xohm capable amplifier should be able to deliver 180% of it's maximum rated power into X/2 resistor load.

Your 100W into 8ohm, must be able to deliver 180W into 4r0.
Recently I have come to modify my opinion slightly and have started using X/3 as the test resistive load.

100W into 8ohm is 40Vpk and 5Apk into 8r0.
180W into 4r0 is 38Vpk and 9.5Apk into 4r0 (Cordell's rule)
160W into 4r0 is 35.7Vk and 8.9Apk (AT's old rule)
240W into 2r67 is 35.7Vpk and 13.4Apk (AT's possible new rule).

If you decide to select a 4ohm capable amplifier you just half all the impedances.

eg.
200W into 4ohm requires an amplifier that puts out at least 360W into 2r0 by Cordell's rule or at least 480W into 1r33.
This ensures that the current output meets most of the demands set by difficult 4ohm speakers.
 
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