School me on power amp/pre-amp setups

[DiabolusInMusic]

I want to get into a power amp/pre amp setup but I have zero clue on how this all works, so I am going to ask some experts.

First and foremost, does power amp matter? I already ordered one, and it has arrived so I am not changing my mind now, but does it make a huge difference? I ordered a Carvin HD2000 from a fellow off tb, he had it modified to 1 V input sensitivity. Is that good? On tb mods = good, so I went for it. I liked it's weight most of all, I am coming (happily) from an 8lb head I carry in a backpack.

I am thinking power amp doesn't matter, they are all the same and I can collect pre-amps like trading cards. Is this incorrect thinking? What should I be looking for when buying pres? Are there some pairings that just don't work?

I am not too sure on hooking it all up either. Can somebody explain this whole bridged setup thing? I know this amp can run 2 ohms, I think it can run two 4 ohm cabinets individually, am I wrong? How could I do this? I have one 4 ohm cab and one 5.3 ohm cab, I want them to get individual power so the 5.3 ohm one doesn't see more juice.

Thanks in advance!

 

[will33]

The 1V input sensitivity is good. Means it will run with a wider variety of preamps....pretty much all of them. A bad pairing would be a preamp that doesn't have as strong an output signal, like .775 volts and a poweramp with a higher input sensitivity, like 1.4 volts. Then the preamp wouldn't be able to "drive" the poweramp properly, resulting in a weak/anemic sound. So, the 1 volt input avoids this scenario.


Your cabs will be just fine. The poweramp is really 2 amps in one case. Just hook the 4ohm cab up to one channel and the 5.33 to the other. Use the preamp as your tone control and use the poweramp attenuators (volume knobs) to balance the 2 cabs.

Your poweramp choice is fine. Theoretically, all poweramps should sound the same, simply taking what is fed in the front and putting it out the back louder, but otherwise unchanged. The difference between cheap ones and expensive ones should come down to parts/build quality, reliability/longevity, how they behave when pushed really hard or when dealing with funky electricity, etc. All that said, you can still hear slight differences in the sound between various poweramps, but it is neglegable, especially in all the commotion of a live gig. I'd rather focus on durability, dependability, other features, etc, and of course, bang for the buck.


Your preamp should have a 1/4" output you simply patch to the poweramp input with a shielded small signal cable (guitar cord/patch cable, etc.). The poweramp should have some configuration or switch or something to recieve a mono input and put it in both channels, use that, then connect one speaker to one channel and the other speaker to the other channel and play.

With a poweramp as strong as yours, I would not bridge it. That just runs the risk of blowing speakers. Yours has plenty of power to push any speaker without bridging.

Bridging is basically tying the two separate amps together to make a single, more powerful mono amp out of it.

 

[will33]

FWIW, a "bass amp" is just a preamp and a poweramp all built together in one case. You're doing basically the same thing, just with 2 separate pieces of equipment.

 

[BassmanPaul]

I have no experience with the HD series but the DCMs have a button on the back marked "Parallel". This feeds the Channel One signal to both power amps. This is how I run all my amplifiers with a cabinet on each channel. The power amps faders can be used to "balance" the system to your liking.

 

[Major Softie]

And, to address some of your other questions:

Yes, the power amp does matter. There are even Solid State power amps (very few) that are optimized for bass (SWR made one). Also, Tube power amps, especially when driven hard, can give you a very different sound than Solid State power amps (which generally add no, or very little, "color" to the sound).

So, power amps can contribute more than just amplification, they can modify your tone, but not NEARLY as much as the preamp.

I am not too sure on hooking it all up either. Can somebody explain this whole bridged setup thing? I know this amp can run 2 ohms, I think it can run two 4 ohm cabinets individually, am I wrong? How could I do this? I have one 4 ohm cab and one 5.3 ohm cab, I want them to get individual power so the 5.3 ohm one doesn't see more juice.

Some (not all) power amps will have a "stereo/mono" switch (or a "parallel switch" as mentioned above). In stereo, each 1/2 of the amp will use a different input jack, and in mono they can both use the same input jack. In mono, you could adjust the amount going to each cab as you desire. Not all amps have such a switch, and not all power amps even have a volume control (although most do).

You can still do this with a stereo only amp if your preamp has more than one output and volume controls for each or a balance control. Some do, some don't. So, there's a variety of solutions depending on the features of both the preamp and power amp you choose. Some preamps even give the option to send the lows to one amp and the highs to another (Bi-Amping). Your cabs are probably not appropriate for that.

As far as plugging it all together, your pre may have 1/4" or may have XLR outputs. Either way they will most likely be "balanced" outputs (3 conductor), so even if they are 1/4" you want 3 conductor plugs and cable (TRS or "Stereo" 1/4" connectors). If they're XLR, they only come in 3 conductor configuration. When it gets to the amp, who knows how it will connect up. There could be screw terminals, 1/4" balanced, or XLR.

I agree with Will33's suggestion to avoid bridging. Not only for the reason he gave, but because bridging would make it impossible for you to send each cab a different amount of amplified signal. Also, I've never seen an amp that can handle 2 ohms when bridged (doesn't mean they don't exist, but I haven't seen one). Generally, if they will handle 2 ohms, it is only when the two 1/2's are being used independently (not bridged).

 

[bucephylus]

So, this may be a foolish question; but, its been a while since I ran a rack system so....

I recently put together a Demeter VTBP201/Power amp rack. My understanding of power amps is that they like to have their gains set at maximum and then control the volume with the volume knob on the preamp. However, the Demeter only has a single volume knob; and I suspect the tone may vary slightly depending on the volume setting, due to the preamp tube characteristic. Is it acceptable to control volume by setting the preamp volume for optimal tone and then adjust the power amp volumes? Or is that just totally wrong. I just don't remember. Thanks.

 

[Floyd Eye]

I don't know about the Demeter, but I can tell you that I don't dime the power amp in my bass rack.

 

[WretchedExcess]

To me, the whole idea of buying a high power SS amp and a separate preamp is to isolate tonal control and clipping to the preamp stage, and to have a power amp that's always clean and never clips, because SS power amp clipping is abrupt, harsh and ugly. to me the whole idea behind separate components is to isolate tone control / preamp line level function from the voltage amplification function.

> I don't dime the power amp in my bass rack.

I do just the opposite. I dime the power amp, so that it is always configured to produce maximum output. Giving it maximum output means maximum headroom. This prevents the amp from clipping when it doesn't have to. Then I use the preamp output to control total voltage output to the speakers.

Doing it this way maximizes headroom and minimizes the likelihood that your power amp will clip. IMO if you don't dime the power amp, then you're not using all of the power that's available in your amp, and you've bought a large amp for no good reason. As an example, if you keep the volume down on a 500W amp, all that does is to limit it's maximum voltage output to a lower wattage level. If you're going to do that you might as well just spend less money and buy a smaller amp.

See? There are two schools of thought on this.

 

[Floyd Eye]

I don't dime the power amp, nor do I dime the pre. In fact they are both probably about halfway. 1000 watts into a SansAmp through an 810. ( This is my B rig by the way. My A rig is a CL)

 

[Downunderwonder]

You can dime the power amp if you can get your desired tone from your preamp without blowing cabs or eardrums.

The other prickle is you may get excess noise if you are running minimal gain in the preamp with the amp wide open.

The "correct" way is to turn up the pre to get tone and bring up the amp to get volume, subject to what the cabs can handle.

 

[Downunderwonder]

To me, the whole idea of buying a high power SS amp and a separate preamp is to isolate tonal control and clipping to the preamp stage, and to have a power amp that's always clean and never clips, because SS power amp clipping is abrupt, harsh and ugly. to me the whole idea behind separate components is to isolate tone control / preamp line level function from the voltage amplification function.

> I don't dime the power amp in my bass rack.

I do just the opposite. I dime the power amp, so that it is always configured to produce maximum output. Giving it maximum output means maximum headroom. This prevents the amp from clipping when it doesn't have to. Then I use the preamp output to control total voltage output to the speakers.

Doing it this way maximizes headroom and minimizes the likelihood that your power amp will clip. IMO if you don't dime the power amp, then you're not using all of the power that's available in your amp, and you've bought a large amp for no good reason. As an example, if you keep the volume down on a 500W amp, all that does is to limit it's maximum voltage output to a lower wattage level. If you're going to do that you might as well just spend less money and buy a smaller amp.

See? There are two schools of thought on this.

Diming the amp can actually reduce clean headroom if your cabs are the limiting factor, you'll be having a noisy quiet dynamic.

Dialling down the amp actually doesn't limit the amp itself. It only raises the input needed to push it to full output.

 

[pnchad]

I don't know if I would 'peg' it but I come from the pro-audio world and I can tell you most engineers do leave the power stage at a high-gain level and adjust by input, whether it's a board or pre

however, in most playing situations (unless you're doing really big rooms or theaters) you do have to consider the match of the pre out and the power amp in because you don't need the headroom

 

[will33]

Oddly, the "volume controls" on poweramps do not affect the amount of power the poweramp makes. The power stage multiplies the input voltage by whatever amount it's designed to all the time.

They are really attenuators. When you want to play quieter, or have more range of travel to adjust the knobs on your preamp, they reduce the level of the signal coming in that the amplifier is amplifying. They don't actually adjust the amount of amplifying it's doing. They reduce the level of the signal that is being amplified.

This is why you can make a tube amp that has a master volume control act a lot more like a non-master volume tube amp by cranking the master volume wide open, or at least setting it quite high. Same holds true for many "all in one bass amps" with a master volume control.

There really isn't one right or wrong way. It's whatever gives you the best sound, at the desired level, with enough range of travel in all the various gain and volume knobs to be able to dial in what you want.

 

[bucephylus]

The "correct" way is to turn up the pre to get tone and bring up the amp to get volume, subject to what the cabs can handle.

And that's what I remember. But, I've also seen the other opinions above; so, was curious. Thanks for the responses. Seems this is not necessarily so obvious.

 

[WretchedExcess]

> Dialling down the amp actually doesn't limit the amp itself. It only raises the input needed to push it to full output.

Looking at this non-musically, from a tech perspective It's important not to confuse the function of an input sensitivity control with that of a volume control.

It's also important not to confuse the gain of the amplifier with it's power output.

Will's point is true -- that the gain of a closed NFB loop won't be changing. OTOH the function of a "volume" control on an amp depends entirely upon where it's been placed in the circuit. They typically work by shunting a variable portion of the input signal to ground. Shunt enough signal to ground and then you aren't ever going to drive the amp to full output, even though the closed loop gain may not be changing. Throw away enough signal and you're never going to be able to drive the amp to it's full power output, regardless of how high an input signal you might supply:

Throw away enough input signal by shunting it to ground will result in the need for an an infinite level of input signal to push the amp to full output. Obviously, that can never happen, because there is a limit to how large an input signal can be before it will overdrive / clip the input stage of the power amp. Clipping the input stage will happen long before the amp ever gets driven to full power by your high level input signal. If you turn down the power amp to the point that you have to overdrive the input stage then the gain of your 1kW power amp is accurately reproducing a clipped input signal. What's the point in that?

If you have a 1kW amp that never gets the throttle opened beyond 12:00, then it's never going to deliver 1kW. You might as well have bought a smaller amp.

If your amp has a noise problem that's a separate issue. A properly functioning power amp should have a low noise floor and can be run full-open without producing a disproportionate amount noise. (As an example of this, many HiFi amps don't even have volume controls, they run flat-out at all times and they rely entirely upon the level of input signal coming into the amp to determine their power output. That, in turn, is defined by the preamp's volume control.) Similarly, a properly functioning and properly configured set of separate components should maintain a high SNR so that volume adjustments at the last amplification stage in the signal chain won't need to be relied upon as a noise gate.

 

[WretchedExcess]

The "correct" way is to turn up the pre to get tone and bring up the amp to get volume, subject to what the cabs can handle.

You know what, I'm going to refine my statement in what might amount to what looks like a change in my position, but it really isn't. There's a difference between doing what's right for HiFi & SRO vs. what's right for musical use.

In a guitar amp you're using the preamp circuit as a tone coloration device. In a HiFi or SRO environment that never happens. Similarly, in the MI application you're using a variable output control on an amp to limit the total volume output, or "size" of your amp, which isn't really something that gets done in HiFi / SRO applications.

In my experience, every good preamp has a "send" volume control on it's output, which functions as a master volume for the entire circuit. That makes the volume control on the separate power amlifier unnecessary. In fact, many power amps are designed without pots (where you're expected to limit the line signal elsewhere, like in the preamp out) or they're designed with really small diameter crappy pots, which are inaccessible, hard to fine tune, and wear out quickly. They're typically designed for level matching on an installation, being set and then forgotten. IMO then, the master volume that controls the amp's total output should be on your preamp.

Just my $0.02.

 

[Nephilymbass]

Pre amp + power amp is my choice. It's nice to be able to use the same power amp and cab for everything and just swap the pres for different vibes. Sometimes i use my swr pre but for rock i prefer my sans amp RBI. The deal with power amps is just the ratings are all over the place. For example my swr 750 power amp is louder than my QSC which is rated double the power.

 

[WretchedExcess]

They are really attenuators. When you want to play quieter, or have more range of travel to adjust the knobs on your preamp, they reduce the level of the signal coming in that the amplifier is amplifying.

This may well be one of the most important, but most easily overlooked points in the thread. As a practical application, it's worth revisiting this point -- by turning down the input attenuator on the power amp, you'll end up increasing the amount of rotation of the preamp's MV control that corresponds to the desired change in volume. In other words, decreasing the amplifier's "sensitivity" or "volume" allows you to have a finer degree of granularity in the preamp's MV control. For some people this might be important.

Changing the controls on the amp is a great way to accomplish this if you have a single amp running to your cabs. But balancing output can be a nightmare if you've got 2 slaved amps running to different cabs, or if you're biamping. In that scenario it's easiest to just set and forget the level matching on the amps and stay away from the controls on the amp and stick with the controls on the preamp.

Basically, you can design the system to work any way you want.

 

[SemiDriven]

Appreciating the dialogue!

Longer-term goal for me is to also go the pre amp/power amp route.

 

[Downunderwonder]

Backpedalling much.

Noise in the preamp stages is exacerbated by having a wide open poweramp, any decent poweramp is indeed virtually noise free, I didn't rate that worthy of mention.

An attenuator does nothing to the available voltage out. Sensitivity controls are rare.