High Current Amplifier

[Vishal]

Is a NAD 356 considered a "high current amplifier"?

The PSB Imagine B has an impedance of 4 Ohm and is suited for a high-current amplifier. Is the NAD 356 high-current enough to drive the Imagine B well?

PSB Imagine B specs:
http://www.psbspeakers.com/content/121003081630-PSB-Imagine-Specs_English_Sep17.pdf

NAD 356 specs:
C 356BEE Stereo Integrated Amplifier - NAD Electronics



Thanks

 

[joyous]

Nad perceived as strong, it can tame hungry speakers.

Perhaps most powerful in amp categories of same price. (this is the perception i got from reading. But haven't read about its technical side, about 'why nad?')

 

[BLASTO]

^^^ True.
When NAD says 50w, It is actually 100w.
The 356 is at least 150w in normal scale used by other manufacturers.

It can drive the Imagine.

 

[sonosphere]

Hi,
I don't know if NAD 356 is a High-current amp, but I've seen it driving low sensitivity speakers like Usher S-520 and Usher V-601 (both have sensitivity around 85 dB approx) with hefty, tight bass (Marantz 6003 /7003 series amps haven't done as well in bass, with same speakers).

BTW if you are using a NAD 326 (or higher one like 356) try using it as a power amp with a good external pre-amp (NAD's own internal pre is weak in low-level details). You'll be surprised at the all around improvement in sound.

Regds,

 

[Thtpro]

Sonos,
Which external Preamp do you suggest?

 

[denom]

This one, right Sonos???

Transformer Volume Control

 

[kanwar]

But how much HIGH CURRENT.............IN AMPERES, lets talk some science. If any......

 

[alankar1977]

very true.....

 

[rishiguru]

But how much HIGH CURRENT.............IN AMPERES, lets talk some science. If any......

kanwar it will be great if you do some science talk on this high current capability of amps. With your knowledge regarding amps it will be of great help to us.

Many amplifier companies boast unrealistic claims about the current capabilities of their amps. They do this by making an instantaneous measurement lasting only microseconds to boast outrageously and unrealistic amperage ratings.

I believe if one needs to know the true current capability of an amp, checking the value of their fuses will dig out the truth.

Thanks in advance!!!

 

[Vishal]

Is there a commonly agreed rating for a "high current" amplifier?

 

[odyssey]

One needs to be a bit careful here. Both the ushers are easy to drive - which has little to do with sensitivity, it has to do more with the crossover, phase, impedance. The Ushers in that sense are easy. The NAD in general are the better mass market branded amps - compared to the usual suspects like the Marantz, onkyo, etc

Hi,
I don't know if NAD 356 is a High-current amp, but I've seen it driving low sensitivity speakers like Usher S-520 and Usher V-601 (both have sensitivity around 85 dB approx) with hefty, tight bass (Marantz 6003 /7003 series amps haven't done as well in bass, with same speakers).

BTW if you are using a NAD 326 (or higher one like 356) try using it as a power amp with a good external pre-amp (NAD's own internal pre is weak in low-level details). You'll be surprised at the all around improvement in sound.

Regds,

 

[kanwar]

kanwar it will be great if you do some science talk on this high current capability of amps. With your knowledge regarding amps it will be of great help to us.

Many amplifier companies boast unrealistic claims about the current capabilities of their amps. They do this by making an instantaneous measurement lasting only microseconds to boast outrageously and unrealistic amperage ratings.

I believe if one needs to know the true current capability of an amp, checking the value of their fuses will dig out the truth.

Thanks in advance!!!

First of all we need to look at the factors which demand the output stage of an amplifier[typical class-AB] should be of high current capacity.

When you have passive XO between amplifier and driver. Following condition occurs.

The net impedance comprises of Resistance[voice coil DCR, XO inductor DCR] + Inductance[voice coil inductance, XO inductor's inductance] + Capacitance [XO capacitors].

Resistance part of impedance is linear hence the current draw is proportional to the voltage applied to driver which depends on instantaneous music signal, hence the power dissipation is also dependent on the resistive current draw which is always a finite positive value.

Inductance is reactive in nature which poses a resistance like load which is variable with frequency, output stage can drive any inductance value but the problem occurs with zero crossing of current and the back EMF applied in reverse to output stage, this condition needs the amplifier must have a low output impedance in order to absorb[sink/source] the back EMF current which is fed back by inductance to the amplifier output stage. Lower the output impedance, higher the damping factor.

Capacitance is also reactive in nature which again poses frequency based resistive behavior. Since the current leads the voltage in capacitive load, which makes the amplifier to deliver the current to the load even when there is no voltage across the load, which means you have full rail voltage across the output transistors but still need to conduct the current to load which causes extremely high heat dissipation in the output stage of amplifier. This is the toughest load for any amplifier output stage. For example an amplifier having its rail voltage +/-45VDC, needs to deliver say 1 ampere of zero crossing current into capacitive load resulting in dissipation of 45 to 70W[depending on conduction angle] of heat thru output transistors alone, calling for nice headroom in output stage dissipation capability.

Now to sum up, in order to satisfy the R+L+C parts of an impedance the amplifier needs :

1 High current sink/source capacity[capacitive + resistive part]
2 High heat dissipation capacity [capacitive part]
3 High current gain stage.[Inductive+ capacitive part]

So what makes the amplifier's output stage a high current capacity one.

1. Number of output devices in output stage to provide adequate current sink/source for capacitive + resistive load impedance.

2. Number of current gain stages in order to give enough low impedance drive to absorb back EMF of inductive impedance of load.

3. Power supply size, capacitance bank reservoir, simply to provide the energy.

In layman's format,
A proper 150W@4ohms class-AB amplifier must have atleast 2 pairs of parallel metal transistors along with triple deep darlington current gain stage and 22000uF of storage reservoir provided the rails are +/-45VDC and adequate heatsinking.

I hope this helps.

 

[asliarun]

But how much HIGH CURRENT.............IN AMPERES, lets talk some science. If any......

There is a very nice article I found on this subject. I haven't understood it fully (need to re-read it a few times) but it explains the fundamentals of amperage, power supplies, and capacitance.

Here's a relevant piece from the article:

"Power-wise, what this means in practice in case of a true voltage source amplifier down to 0.5 Ohm loads, is outlined in the list below:

50W/8 Ohms = 28.3 V peak / 3.53 Amperes peak

100W/4 Ohms = 28.3 V peak / 7.07 Amperes peak

200W/2 Ohms = 28.3 V peak / 14.14 Amperes peak

400W/1 Ohm = 28.3 V peak / 28.28 Amperes peak

800W/0.5 Ohm = 28.3 V peak / 56.57 Amperes peak

So, 50W/8 Ohms equals 28.3V peak. Good. It follows that with 8/4/2 Ohms loads, our required currents will be (20:8/4/2) 3.54/7.07/14.14 A. Small wonder, since we want our amp to provide 50/100/200W into 8/4/2 Ohms. Remember, this is in terms of the power supply only, and it completely disregards the capabilities of the output stage and heat sinking, which, for 200W/2 Ohms, will need to be massive."

-----------------

There is also this piece about capacitors:
"First and foremost, their capacities. If you're willing to overlook their cost, a simple rule of thumb is to say that you need 1,000uF per every RMS Ampere of current. In practice, this will keep your amp working well even into lower loads. However, just to be on the safe side, it's better yet to say you need 1,000uF per every PEAK Ampere of current - this effectively increases your filter capacity by a factor of 1.41.

Using our example amplifier, delivering 50/100/200W into 8/4/2 Ohms, obviously we need bother only with the largest figure, that of 200W/2 Ohms. So, our RMS current will be as follows:

Sq. Root of (power:load impedance) = Sq.Root (200:2) = 10A.

Therefore, we would require 10,000uF per supply line at the very least. If we want to be really safe, we'd cater for our PEAK current, which is 1.41 times greater:

10A x 1.41 = 14.1 Amperes.

The nearest standard value is 15,000uF, so we'd use that for every supply line. If using just one transformer, you'd need four such capacitors; if using separate supplies for each channel, you'd put 2x15,000uF for each channel.

There is another approach. It is accepted that one needs 1-2 joules of energy per every 10W of output power. For a 50W/8 Ohms amplifier, we need 10-20 joules of energy to be stored. We can use a formula, 1/2CVsquared (where C equals capacitance and V equals voltage, the voltage being squared) to calculate that 15,000uF, fed by say 33V (worst case, full load on) allows for 8.16 joules per capacitor, or 16.3 joules per channel - enough to fry quite a few unsuspecting speakers in the low end class, and even some in the midrange class. If 33V is our worst case, we can assume 36-38V supplies with the load off (say 37V), which means that just before a transient, we will in fact have some 20.5 joules stored in each channel's capacitors."

-------------------

I would assume that a high current amplifier (which is what I was told to get for my speakers as well, by the way) is an amplifier that can reliably supply these amperages and accounting for phase swings, I would assume it should be able to drive your speakers one impedance level below your speaker's impedance rating. That is, if you have 8ohm speakers, it should be rated to drive 4ohm loads reliably.

For the record, my amp is rated to deliver 35 amps, and a good power amp like the Odyssey Stratos can deliver 45 amps. That might be overkill for 8ohm speakers, but my guess would be that for 4ohm speakers that also have big phase swings, the amp can cater to the "worst case scenario" - at least for short bursts of time.

Edit: I'm quite sure both the amp numbers I mentioned above are probably theoretical peak output amperage the amplifier can supply for short bursts of time. Otherwise, the numbers are too high.

 

[kanwar]

Asliarun,

Nice article but it only speaks about resistive nature of load, which is not the reality.

For example a 100W @ 4ohms needs :-

20Vrms , 5Arms or 28.28Vpk, 7.07Apk current.
Now this value is what a resistive load will require to dissipate 100Watts of power. So the amplifier must be capable of delivering the same. If the amplifier is connected directly to driver such as woofer, it will only see DCR of voice coil, 4ohm driver has 2.5 ohm typical as its DCR and around 1Millihenry as its voice coil inductance but no capacitive reactance. Here the resistive calculation will suffice even if you have phase shift with inductance of voice coil, since the current lags the voltage in inductors, its not a problem. So the accepted capability of amplifier to drive a woofer of 4ohms [2.5 ohms DCR]directly must be atleast 12Ampere peak. The impedance cannot DIP beyond this point. To give a headroom one can have 20Apk capability in 100W 4ohm rated amplifier, its more than sufficient.

The scenario changes when you have passive XO in between a woofer & an amplifier, a normal 2nd order passive low pass filter consists of inductor in series and a capacitor in shunt. The problem arises when the a music signal which has full range frequency content goes to this section and the XO shunts the HF portion of current towards the GND[-ve terminal] and remaining LF goes to woofer in theory. Now this capacitive element as in my previous post elaborates that the current leads the voltage by 90degree worst case phase shift. This phase shift causes huge dissipation of heat in output transistors, here you don't need a peak current capability, here what you need is extension in SAFE OPERATING AREA of the output transistor in the amplifier, which can only be gained from high SOA rated device or paralleled pairs. This 90deg phase shift causes the output device to deliver current even when there is no voltage across the load which makes the output stage to enter in a region known as SECOND BREAKDOWN where the stress is both high voltage across the device + high current demand to satisfy capacitively reactive load due to phase shift. If this SOA is exceeded then the device gets damaged.

Lets do some math:

L=1.45mH, C=12uF typical passive XO value. Voice coil DCR =2.5ohm

Now lets calculate the current needed at 1khz frequency to drive the driver + passive XO.

Capacitive reactance XC=1/2*PI*F*C ,gives 13.2 ohms
Inductive reactance XL= 2*PI*F*L, gives 9 ohms

Total reflected impedance @ 1khz will be XL 9ohms in series with net resulted shunt impedance of XC 13.2ohms parallel with DCR 2.5ohms gives 11.1ohms as impedance of network at 1khz.

With this value we need about 2.54Amperes peak at worst case 90deg phase shift with full rail voltage across the transistor resulting in very high power dissipation in form of heat. This value holds true when we talk about capacitive reactive playing major role in phase shift, there are many cases where poorly designed passive XO exhibit phase shift caused by overly damped inductors and in such cases the current peak demands are very high and amps have difficulty to drive them.

Now the claims of having 45Ampere pk in an amplifier all go in vain when you don't have the headroom for reactive current at zero crossings of music wave.

In other words, apart from resistive current capability, the amplifier must have both capacitive and inductive current capability also. This statement goes for all linear class-A, AB, G,H amps.

 

[kanwar]

Class-D amplifiers in general don't have such limitations of class-AB amps, the reactive energy is recycled and fed back to power supply, in this way class-D amplifier works like a power converter and very efficient.

When we say some XYZ is tough speaker, its not the driver, its the passive XO in it which demands currents at phase shifts.

Please note, Active speakers and Active systems are not prone to such crappy situation either because of absence of passive XO in them.

 

[kanwar]

For the record, my amp is rated to deliver 35 amps, and a good power amp like the Odyssey Stratos can deliver 45 amps.

Lets do case study on Odyssey Stratos:

As per the link given by Asliarun,

It says its uses 2SA1216 & 2SC2922 2 pairs of transistors in parallel.

Datasheet link from manufacturer:
http://www.sanken-ele.co.jp/en/prod/semicon/pdf/2sc2922e.pdf

Amplifier Power = 150W @ 8ohms.

49V peak, 6.1A peak voltage and current requirement for resistive 150W load.

Transistor rated at 17A each @ 25C absolute maximum ratings. 2 parallel pair gives 34A in theory.

I really don't know from where the claimed 45A pk comes from in amplifier's specs. If manufacturer is stating such specs for some "milliseconds" he/she should give the time duration as well, but no mentioning of it.

Lets do some SOA calculation from datasheet of transistor itself.
Considering 90deg worst case phase shift, 49V peak across the device during zero crossing gives you max current of almost 5A, with 2 pairs doubles to 10A, which is very good value for the stability.

This amplifier is no doubt designed to handle good phase shifts across passive XO + driver .

 

[asliarun]

Thanks, Kanwar. Your posts are very informative, although I will admit some of the technical details are a bit hard to fully understand.

What should one look for, in terms of specs, for hard to drive speakers?
Edit: Please ignore the question - you already answered it in your previous post.

You also mention Class D which is interesting. I had a Tripath based Topping amp which I had to replace because my speaker upgrade was simply too demanding for the Tripath. Hence, I played it safe and got a "high current amp" - or at least what people consider to be a high current amp.

However, I am discovering the whole world of high-end Class D amps that seem to have quite amazing specs. One other forum member (I think GeorgeO) mentioned Hypex NCore. Are there any Class D amps that you recommend or that you have had good experience with?

 

[moktan]

Thanks, Kanwar. Your posts are very informative, although I will admit some of the technical details are a bit hard to fully understand.

What should one look for, in terms of specs, for hard to drive speakers?
Edit: Please ignore the question - you already answered it in your previous post.

You also mention Class D which is interesting. I had a Tripath based Topping amp which I had to replace because my speaker upgrade was simply too demanding for the Tripath. Hence, I played it safe and got a "high current amp" - or at least what people consider to be a high current amp.

However, I am discovering the whole world of high-end Class D amps that seem to have quite amazing specs. One other forum member (I think GeorgeO) mentioned Hypex NCore. Are there any Class D amps that you recommend or that you have had good experience with?

i am pasting a google translation of a review of a good class D amp (Red Wine Audio Signature 30.2)...
Generally reviews of this amp is usually of a non-technical nature. but given the tenor of the discussions in this thread i guess it will be of some help to folks who understand the significance of such things....
the original link is here Group Test: 11 Integrated Amplifiers [avmentor.gr]
(for graphs etc please refer to the original site)
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There is no doubt that the Signature 30.2 is the most exotic amplifier of this test. This is a handmade in America, which is based on the Tripath modules which has improved in places (especially the filters output) by the company. Particularly interesting point is that the power is supplied by two lead-acid closed type which are loaded by a smart circuit, only when needed without the intervention of the user. The Red Wine Audio has not hesitated to use top materials at certain points, such as motorized stepper attenuator 24 positions Goldpoint as both capacitors oil / paper used at the entrance.
terms of connection possibilities, now, should settle - literally-at the absolute minimum. O Signature 30.2 has a single input single ended, for driving an output terminal (signal simply passes through the attenuator) and a pair of terminals for connecting speaker per channel.
Build quality is very good with top materials where necessary, good assembly careful wiring and simple aesthetic which has, however, view. We would like a more elegant remote control, however ...

Measurements
In load of 8W and limit deformation of 0.3%, the Signature 30.2 yielded 22.2W, while in the load 4 Ohm with distortion 0.3%, the maximum power was reduced to 10.3W. Here we have the second amplifier test which has a higher rate of lower power unit, a performance that justified, however, by the output stage of Tripath and logic low power device. The extinction coefficient was calculated 101.
frequency response of the amplifier is very smooth with slight deviation at low frequencies, about 0.5dB at 10Hz and a trend for high-rate, which starts at about 5kHz and reaches-3dB just above the 20kHz . The similarity of the two channels is excellent.
During driving the composite load not low response shows some variation but the synergy of the output filter / load seems to lead to a trend of increasing profits which exceed 2dB at 40kHz (we saw similar behavior to also switchgear, Onkyo). Growth this behavior combined with a dip which is centered around 6.5kHz and reaches 1.5dB.




Frequency response for both channels. Reference level: 1kHz, 0.3% thd + N/8Ohm, two channels Frequency response for both channels. Reference level: 1kHz, 0.3% thd + N / Composite load, both channels




Square wave 1kHz, maximum output at 8 ohm load thd + N depending on the output power. 8 ohm load (red), 4 Ohms (green)
The first signs of overload the amplifier to the load of 8W 17W manifested in distortion with 0.04%, while the output stage "resistant" to the 0.25% and thus yields about 27W. At 4 Ohms, amplifier 20W exceeded without visible signs of overload behavior, however, then made ??the measurement impossible.
30.2 The proven amplifier with a slow rise time 14mS rate and lift just 1.3V/mS. These values ??should not cause very bad impression (as in the case of Onkyo) because the "speed" the switching output stage has the same meaning and the same value as that of conventional and additionally depends primarily on the load output, the filter used and their relationship. The shape of the square wave is characterized by a smooth slope, and high-pitched noise that we have learned to expect a switch stage.
harmonic spectrum of the amplifier with a signal of 1kHz, including declining smoothly with redundant components are generally the even stronger but with all their fall relatively quickly below-100dBr.
spectrum shows noise quiet construction, this apparent absence of hum and noise from the lift and any harmonic, a fact quite reasonable when you consider that these circuits simply do not exist in the case of Signature! The amplifier also remains silent at high frequencies. The static noise measurement is-60dBr (A).




Harmonics spectrum signal 1kHz, 10Wrms/8O. Noise spectrum. Reference level: 1kHz, 0.1Wrms/8Ohm, shorted input.



Accuracy level regulator
The regulatory level in the middle of his journey, made two observations: First, the attenuator Goldpoint you used is indeed exceptional, the Signature 30.2 to show the smallest difference between the channels we saw in testing and, secondly, that the conduct in it is better to buffer "inside" than when it is in place minimum attenuation. This, of course, requires an explanation which does not currently have: Is it possible to insert a resistor in the signal path to balance a disparity between channels? Unknown.
Hearings
Given its low power, we decided to assign the Signature 30.2 to lead a more sensitive speaker, replacing the ATC SCM-50 with the second loudspeaker (Audio Spectrum Eros with a passive sub). However, this was ultimately not necessary, as the amplifier drove 50aria, although marginally, but no serious problems at levels which exceeded 92dB at the listening position with occasional clipping phenomena. In any case, you need to combine it with a sensitive speaker in a small room to the fun.
Amplifier attributed the bass range with a distinctive combination of nerve tumor which balance, in our opinion, successful, impeccable between description of minutiae and creating a sense of the big bands or projects that use low frequencies to create atmosphere. The listener hears and feels simultaneously very low frequencies, a feature which easily leads to excess and unavoidable due to the low shear strength. The middle area was characterized by neutrality and important features are described, with the human voice has a joint choir and formed distinctive comfort space. High, the amplifier appeared quick and pleasant with a mild trend-by-case sounds more slender than we would like, but do not be annoying or never "little".
The stereo image is focused in the middle distance and is distinguished for its excellent detail both in the amplitude axis and in that the depth.
Finally ...
... here must have already become clear that we are not dealing with an integrated amplifier generic but with a special construction with unique design options and solutions, and-of course-the expected whims, which appeals to listeners who are looking for different and are ready work in this direction (looking a suitable, good and easy speaker). We recommend that you listen definitely worth it!

Price: 2,400 euro
info: Sound Elevator, tel: 210-282.8060, web: News | Sound Elevator - High End Audio Distribution , Red Wine Audio – battery powered purist hi-fi
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[kanwar]

Thanks, Kanwar. Your posts are very informative, although I will admit some of the technical details are a bit hard to fully understand.

You also mention Class D which is interesting. I had a Tripath based Topping amp which I had to replace because my speaker upgrade was simply too demanding for the Tripath. Hence, I played it safe and got a "high current amp" - or at least what people consider to be a high current amp.

However, I am discovering the whole world of high-end Class D amps that seem to have quite amazing specs. One other forum member (I think GeorgeO) mentioned Hypex NCore. Are there any Class D amps that you recommend or that you have had good experience with?

Class-D amplifiers are efficient power converters. They recycle reactive energy. They give you much more current then class-AB of same power rating operating from same power supply. Tripath can't be considered as power converter because of its limitations in current conversion. Hypex class-D modules are good because of load independent behavior due to post filter feedback and you can give a try but they are very expensive also.

Similar type of discussion was held in another thread also

http://www.hifivision.com/amplifiers/8711-class-d-amplifiers-20.html#post172188

 

[kanwar]

i am pasting a google translation of a review of a good class D amp (Red Wine Audio Signature 30.2)...

This amp is nothing but a chip based from Tripath which went bankrupt. T-amps are low on power, cheap to manufacture, easy to reproduce for mass market and have an aura of distinct sound characteristics because of its poor frequency and phase response due to pre-filter feedback[though people like it sometimes], you cannot correct for the non-linearity introduced by output filter unlike post filter feedback amps.

Chip class-D amps come from many different manufacturers such as NXP[philips] , ST, TI, IRF . All of these use pre-filter feedback and non are great in anything but targetting mass market. So much hype has been created and marketing is done in such a way that the end user is made to believe that such chip would do a miracle, but the moment you test it in lab and later with ears, things become clear.