Amp comparison based on Specs?

[nikos]

I'm trying to learn what each measurement in an amps spec sheet mean..even though I understand that sometimes the paper can have a different opinion than the ear and is not always the absolute measure. I having trouble understanding some of this And I need some help...

I've picked the Rotel 1075 rated at 120x5 and the Adcom GFA-7605 125x5

Most of the specs are equal yet I'm listing a few below for your opinion on how much they matter. I guess another issue would be that it wasn't a 3rd party listing these..but each manufacturer so in theory they could have made these numbers up...

Here are some of the specs:

-----------------------------Rotel 1075------------Adcom 7605

Frequency Response------- 15Hz to 20KHz-------- 10Hz to 20KHz
Toroidal capacity ----------- 80000 ----------------100000
Signal to Noise "A Weighted" ---- 115 ------------------ 117
Input Impedance ----------------27k ohms -------------48k ohms
Power Consumption --------------800W ----------------1350VA ???
*Rise Time found value only for Adcom
5kHz, 120V peak-to-peak square wave, 20% to 80% ------ 6.0µS

Damping Factor ------------------180-------------------- => 498

And this is what really stood out to me....the 1095 has a 400 damping factor.

I started reading a def from http://www.classic-audio.com/marantz...ingfactor.html
and quote
"If you don't understand basic electric current and voltage, let's just say that damping factor is a figure of merit that tells you how good an amplifier is at controlling a speaker system. The larger the number, the better it is. At 100 or above, it's pretty darned good. Below 20 or so, it's pretty poor.

When one amplifier's damping factor is higher than another's, that tells you that the amplifier with the higher damping factor can better control the speaker systems, and all other things being equal (frequency response, phase shift, distortion, etc.) that it is a better amplifier" end quote

I don't have a great knowledge of electricity and currents to fully understand what I'm typing...and was hoping for some input on the above.

What do the above numbers tell you? What does he mean by "control" a speaker system? How much do these numbers really matter? what are the key points you guys look at?

Thanks for the lesson ;x(

*Also distortion was listen differently on each spec sheet and made hard to understand Rotel just gave one number @0.03%max for diff power levels and Adcom listed results from 0.012% to 0.07 for different Hz.

Nick

Full specs on each company's website:

http://www.adcom.com/pdfs/gfa760x_manual_v1.pdf

http://www.rotel.com/support/pdfs/manuals/RMB1075.pdf

[Elvis]

It would be nice if we could look at numbers and say "wow thats a great sounding amp".Great numbers could suck and worse numbers sound great,numbers are just a small part.I've had light amps that rocked and back breakers that sounded pretty bad.I've put the 1075 up against several 125 watt amps,some 15 lbs heavier with better numbers and could tell no difference to speak of.Any difference was so slight you'd be reaching to describe it,as it should be.However,I would'nt want you to think there is no difference in 125 watt amps,there is.If you step up to the next level and spend the big bucks then you'll hear an improvement even at the same 125w per ch.All imo.

[BWzes03]

One thing about the 1075, it has a Toroid Power Supply capacity of 1.5KVA, not 800VA, the average consumption at non-idle is 800W.
Also, the Rotel is able to output 120W on all channels at the same time with a THD of less than 0.5%. (with an 8 Ohm load) The Numbers with a 4 Ohm load is 200 W on all channels with a THD of less than 0.5%
The maximum ouput power in dynamic (music) use is more like 150W on all channels.
(the ouput transistors are all speced at 150W, 4 devices for each channel, 20 devices in total.)
No doubt the Adcom can hold his own, but my guess is, the Rotel has a bit more potential.

[nikos]

Thanks for the replies...

I do understand that the numbers can be very misleading on a amp and to be taken with a grain of salt...yet thats what I'm questing to learn and understand the numbers better. Specs are there for a reason...and amps to be within spec or over spec means something... On the high end amps at the 125W x channel point...apart from better parts and circuitry....do they reflect this in numbers and show it on paper as well?



Straight from the Rotel website quote
"1,500 VA toroidal transformer and eight 10,000 µf British-made Slit Foil capacitors."
and thats how I came up with the 80000 toroidal capacity..8x10,000 mf.... the Adcom just plainly lists 100,000 mf Is this the correct assumption?

Also straight from the Adcom manual
"
Power Consumption (Continuous, All Channels Driven)
Quiescent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .116VA
Maximum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1500VA
100 watts into 8 ohms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1380VA" end quote

Again I understand that the conventional wisdom of "if its using more power it must be outputing more" is less obvious as in a case that a low end amp consumes lets say 100W fully driven and claims to output 800W....
And could plainly be that its not efficiently using the power and wasting it. What are the numbers if any that would support the idea that its outputing what its consuming? Does the above tell us that the ADCOM amp needs to consume more power to output the same power the Rotel achieves by using less??

Nobody touched the damping factor.... what are your thoughts on that if any?

Nick

[mjb]

Damping factor is the ability of the amp to not be effected by impedance changes and back emf produced by the load. Its like a backward input impedance. If a speaker cone is wobbling away due to inertia (ie, not being driven), it will be inducing current in the amps output circuit, and this current will effect things. Damping factor minimises the effect. The higher the number the better, but anything over 100 is very good.

[jim777]

Comparing amps by spec is almost impossible, especially to compare two different companies or two different technologies (digital vs analog).

You would have better luck comparing them by price although that isn't always true either!!

And the difference between two great amps is more subjective than objective.

So have fun with a real listening test!!

[grit]

:lol:

Damping factor is the ability of the amp to not be effected by impedance changes and back emf produced by the load. Its like a backward input impedance. If a speaker cone is wobbling away due to inertia (ie, not being driven), it will be inducing current in the amps output circuit, and this current will effect things. Damping factor minimises the effect. The higher the number the better, but anything over 100 is very good.

And that, IMO, is almost the SINGLE most important stat an amplifier has, but almost never discussed in stats. Then again, it's an odd stat to measure, given the non-linear nature of inpedance loads of speakers.

As Jim said, you really need to test them. Take Krell and Classe. Both make excellent amps. I would gladly take EITHER, but depending on WHICH speakers I was driving and for WHAT application (movies, types of music, etc.), I might pick one over the other. Which is not to say that one is better, just that one might fit *my* preferences more than another (for a given application).