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Common amp myths

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#421
rag said:
When you are driving a speaker all Watts are not created equal. An eight Ohm speaker is not the same as a real eight ohm load. The eight Ohm RMS power rating of an amplifier is measured into a real eight Ohm load. A real load means that it is eight Ohms at all frequencies. An eight Ohm speaker on the other hand is only eight Ohms at a few specific frequencies. Over 99% of the audio band the speaker is no where near eight Ohms. A typical eight Ohm speaker will start out at 5 Hz as 6 Ohms. If mounted in a bass reflex cabinet (ported cabinet) it will shoot up to over 50 Ohms at about 30 Hz, then drop down to 8 Ohms at 50 Hz, then shoot back up to 50 Ohms at 80 Hz then drop back down to 8 Ohms at 200 Hz. From 200 Hz through the rest of the audio band (20 kHz) its impedance will increase 6dB per octave (double every time the frequency doubles).
Tube amps have a soft supply that sags with low impedances and returns with high impedances. This means the amp will put out more power into a high impedance than an amp with a hard supply that did not sag. The 8 Ohm real load that the tube amp is rated into causes the supply to sag a lot compared to the supply in a typical solid state amp. So when the tube amp is driving a speaker it can deliver a lot more power to the high impedances it sees than a comparable solid state amp. Of course that assumes the solid state amp has a hard supply as most of them do. But guess what? Some solid state amps have soft supplies.
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This is probably the best tube vs. solid state explanation that I've ever seen on the net.

Thank You!
 
#422
roflol said:
amp x is going to be reallllllll loud if it can put out 300watts of volume at 20hz
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let me introduce you to my power amp:

Link Removed

It's a heavy weight, around 50lbs, but it's relatively cheap on the used market, lighter than the SVT, a bit more sterile, and of course a good preamp is necessary.

You should hear it bridged (600W @ 20Hz) with a good tube pre.

Edit: It's not so much loud as it is, hmmm, powerful, for lack of a better word . . . I'll get back to you on this!
 
#424
pedro said:
Mine too. I'm curious how Jimmy's head is.
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Very painful. Been moving since Monday. Just finished today. 3 trips in a 24 ft truck. I want to die!

Anyway, sorry I haven't been on. We get internet tomorrow afternoon. I'm sponging from a hotel right now. Will yack at youse guys tomorrow...great thread! I've learned a lot. And isn't that why we're all here???

Isn't it?

Anyone?

:confused:
 
#425
adbass said:
Right, I understand that and it's an important point. How would you predict the two amps I provided in my example would sound as compared to each other?

just as a reminder:
Amp X (300W @ 20Hz)

vs.

Amp Y (300W @ 1kHz)
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While there is not enough information, some general comments are possible assumng an "ideal" amp. Mainstream solid state power amps come pretty close to ideal.

An "ideal" amp puts out the same output power into a resistive load (which is what most ratings are based on) over the entire audio frequency range. So I would say that if Amp X and Y are ideal amplifiers, they could be the same amp. They would drive a given speaker to produce the same volume.

I will overlook the detail that the gain of most bass amps begins to drop off above 20 Hz, because the amp could deliver just as much power but would require a higher input signal amplitude at 20 Hz than at 1 kHz to reach its rated power.

How one amp could be louder than another, yet have the same power rating, would have to reflect departures from ideal performance, which is why there isn't enough information to answer this question in general terms.
 
#426
fdeck said:
While there is not enough information, some general comments are possible assumng an "ideal" amp. Mainstream solid state power amps come pretty close to ideal.

An "ideal" amp puts out the same output power into a resistive load (which is what most ratings are based on) over the entire audio frequency range. So I would say that if Amp X and Y are ideal amplifiers, they could be the same amp. They would drive a given speaker to produce the same volume.

I will overlook the detail that the gain of most bass amps begins to drop off above 20 Hz, because the amp could deliver just as much power but would require a higher input signal amplitude at 20 Hz than at 1 kHz to reach its rated power.

How one amp could be louder than another, yet have the same power rating, would have to reflect departures from ideal performance, which is why there isn't enough information to answer this question in general terms.
Click to expand...

That's going to keep me busy for a while! I'm still learning, thanks the reply Fdeck. (Still enjoying that preamp of yours btw.)

Just to be clear, I am asking these questions with reference to the solid state power amplifier I linked above, which is not bass specific.
 
#427
The best explanation I can come up with in the tube vs solid state is that a clean SS amp you will hear distinct spaces in the sound where you hear no notes. But with tube amps due to compression & harmonics will fill in these spaces making it sound fuller & perceived to be louder when it is really not.

Same thing that was brought up that commercials sounding louder then the programs. They are more or less the same volume, just compressed so much you are constantly hearing sound that fill in the gaps.
 
#429
billfitzmaurice said:
More to the point, the output of a driver is directly proportional to the excursion of the cone, and excursion is directly proportional to voltage applied, not power.
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More specifically, the output is proportional to the velocity of the cone (at frequencies above resonance), and the velocity is proportional to the voltage applied.
 
#430
fdeck said:
I think Bill nailed it. The way I think of it is that flat on-axis response from an ideal piston translates into constant acceleration, not constant excursion, at all frequencies. In frequency space, acceleration is the excursion times the square of the angular frequency, so excursion goes down as the frequency goes up. As it turns out, an ideal cone speaker has the same frequency relationship above resonance when driven by an ideal voltage source.

In terms of measuring and adjusting the voltage at each frequency, a modern amp does that with negative feedback. Both tube and solid state power amps do this, but in different ways.

Overdriving the amp effectively turns off the feedback mechanism, so that the amp becomes current limited by the internal impedance of the circuit.
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+1
 
#433
derrenleepoole said:
That is so cool :)
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(Reference to the Yamaha kick-drum mic made out of a speaker, in a casing that looks like a little drum, the assembly mounted on a little tripod stand)

You can make one of these out of a small woofer and an old mic cable. I forget exactly how, but it's not a big deal. I understand it's a nice way to get very low bass from a kick drum, and you can combine it with another mic at the beater head inside the drum: 'Thwack' and 'Thump' mixed together. The Reese's Peanut Butter Cup of drum mic techniques!

Fun, right? :hyper:
 
#434
Count Bassie said:
(Reference to the Yamaha kick-drum mic made out of a speaker, in a casing that looks like a little drum, the assembly mounted on a little tripod stand)

You can make one of these out of a small woofer and an old mic cable.
Click to expand...

The mic cable doesn't have to be old.

I think there was a construction article in Recording magazine a few years back.
 
#435
Count Bassie said:
(Reference to the Yamaha kick-drum mic made out of a speaker, in a casing that looks like a little drum, the assembly mounted on a little tripod stand)

You can make one of these out of a small woofer and an old mic cable. I forget exactly how, but it's not a big deal. I understand it's a nice way to get very low bass from a kick drum, and you can combine it with another mic at the beater head inside the drum: 'Thwack' and 'Thump' mixed together. The Reese's Peanut Butter Cup of drum mic techniques!

Fun, right? :hyper:
Click to expand...

You don't really have to do anything. Just plug a speaker cabinet in to a mic pre. It will work.
 
#438
shakerattleroll said:
The Abbey Road engineer(s) (Ken Townsend maybe?) used this technique on Pauls bass around the time of Revolver. I will check my Recording the Beatles book tonight and post the researched info.
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I'd heard of this. From the sound of the bass on that record, I'd say it was pretty successful. If you like that sound, and I do...

When I get my housework finished I'm going to build some cool recording stuff.
 
#439
Count Bassie said:
(Reference to the Yamaha kick-drum mic made out of a speaker, in a casing that looks like a little drum, the assembly mounted on a little tripod stand)

You can make one of these out of a small woofer and an old mic cable. I forget exactly how, but it's not a big deal. I understand it's a nice way to get very low bass from a kick drum, and you can combine it with another mic at the beater head inside the drum: 'Thwack' and 'Thump' mixed together. The Reese's Peanut Butter Cup of drum mic techniques!

Fun, right? :hyper:
Click to expand...

This actually takes me right back to being 15 (1987/8) when I plugged an old set of headphones into the mic input of our really old stacking stereo system (the type in a wooden case with turntable and tinted glass doors) and realising I could get a sound through the speakers by tapping the earpieces. I was trying to impress a girl at the time, and no she wasn't :rollno:

I just thought the Yamaha thing looked cute :hiding:

I'm so making one of these :bassist:
 
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