Who here is going straight from receiver pre-outs to a pro-amp for their subwoofer?? - Page 3

On pro amps the input level control is only for attenuation, this means it does not boost like a powered preamp, is can only reduce the input signal with resistance. It goes from 0db (no attenuation) to -∞ or full attenuation which is just about equal to a shorted out input signal..
Edited by joehonest - 10/15/12 at 7:52am

So you can set the net gain for the amp with the knob, but it's not a gain control? It doesn't control the effective gain of the amplifier? The amp provides some gain of the input signal on it's output, it doesn't attenuate the input signal to the amp so the output from the amp is smaller than the input. Do you also refer to the volume control on your receiver / pre-pro as an attenuator knob also?

Not necessarily. That will depend entirely upon the gain structure in the amp and DSP. If your signal was at 0dBFS wideband and then you decided to boost +10dB at 20Hz, the DSP would need to attenuate everything else by 10dB so that 20Hz could be at at 0dBFS. Signal cannot go above FS or else it is clipped.

Agreed, but the only reason you would use gain is if the pre-amp output was exceptionally weak - clipping is not the concern. If one has a pro amp with DSP, one really truly doesn't need some box between their receiver's pre-amp out and the pro amp.

I am making a simple point: don't boost then attenuate, it's redundant and can only add noise/distortion and take away from dynamic range.

And, they're not referred to as attenuation knobs in the manuals of pro amps. QSC calls it a "gain control", Yamaha calls it a "volume control knob", Crown calls it a "level control", & Behringer calls them "gain controls". No amp maker calls them attenuation knobs.

Yes that is true, but for your info and how to best use this hardware you should read what we wrote to make the best of it...

OK here is a hyper-clear explanation of what to do when setting up a pro amp, courtesy of Crown. It's clear 'volume knobs' are strictly for attenuation. You can attenuate the signal, but if the signal is weak, you don't necessarily want to. Pay special attention to the part about noise and hiss. It does not come from the amp, it comes from the source components. I can see how people are being confused by this but the key is that the amp is not generating the noise, and if your source is sufficiently noise-free you can take advantage of that by having extra headroom. That's why I have a s/n ratio of 122 db at output on my audio interface.

Also, since we are talking subwoofers here you'll never hear the background hiss from running your amp full power. If you do hear something, it's much more likely you are hearing ground-loop hum from your cable line.


Edited by imagic - 10/15/12 at 8:52am

They should. But for that matter most volume controls/attenuators that are referred to as gain controls aren't. True gain controls alter the resistance in the amplifier gain stage feedback loop, a technique that's seldom used.

I know what they are and how they work electrically, but they still change the effective gain of the amplifier input to output.

That link says that "If power amps were always meant to be turned up full, they wouldn't have level-control knobs. There is an optimum setting for those knobs in any sound system".

When people mix and match various pro-level equipment with consumer level equipment as well as balanced and unbalanced connections, it is easy to see how people can get themseves into trouble with gain structure.

My external power amplifiers do not have any user adjustable level control / gain control features at all. However, the gain structure is adjustable via mixer unit (NHT X-1) that is placed in front of the power amplifier. So the gain structure is adjustable, and nothing is set to full on / all tthe way up.

Does any of that really matter? Gain is just a ratio of voltage in to voltage out of the power ampliifier. For the end user, it does not matter how you get there.

True

Both of my Onkyo's an 886P and an 806 will dump over 10 volts out of the SW output with less than 1% thd. Actually if I remember right they start clipping at around 12 volts. Anyway it is enough to clip the input of a DCX2496 which is a "pro" piece of gear while still being clean. The output stage on the DCX clips at about the same time with no gain applied inside of the DCX at all. My bottom of the line, many years old Pioneer 517k will even put out over 5 volts before clipping. There are a couple of audio mags and sites that test receiver or pre pro output stages. You don't need your particular model to be tested just something of the same brand and somewhat close to get a good idea. Manufacturer's aren't going to radically change that part of their design between units in a family. I'd be very surprised to see any modern reciever or pre-pro incapable of a clean 4 to 5 volts out the SW jack. That is enough to work with most any amp.


The main problem is that most people do not understand gain structure or spend any time at all optimizing it.

This Rane article is a good read.

The more and more powerful your system gets, with higher sensitivity and output headroom the more important it is to start paying attention to gain structure. In general you want to use as much gain as early as possible in the chain to minimize noise and hum. Basically you should take the time to measure what type of signal level your front end unit and any EQ units behind it are capable of producing or accepting cleanly, in addition to what your amplifier input sensitivity control should be set to to match up with them. Otherwise you may end up with a much higher amount of noise from your speakers and mismatched gains that produce clipping in the electronics well before the amplifier produces full output. All it takes is: A couple of hours of time, hacking up a cheap cable or two, a signal generator, a spectrum analyzer and a dmm really. Oh and unplug all the speakers. Basically you want to give the amplifier the hottest signal that is still clean that you can. The amplifier should clip first but it should only be just prior to the electronics before it in the chain. Otherwise you are either limiting your output headroom or you are increasing the noise in the system more than needed.

For example my system is easy.

886P SW channel = >12volts clean and clips the DCX input before it distorts itself.
DCX can accept right around 12 volts which clips the input
DCX output can produce right about 12volts prior to clipping. Both the input and output at 0 gain clip at just about the same time.
The K10 takes 6.34v input to produce clipping at it's lowest sensitivity setting. I have much more clean voltage than that available so the amplifier clips first which is what we want so I use this setting.
Using a signal that produces clean output from the 886P into the DCX, but that is just below clipping the DCX input stage and output, but results in heavy clipping of the K10 amplifier input.... I then lower the DCX output section level until it is just below clipping the amp input and then raise it back up until it produces solid clipping, but just into it instead of 6dB into it. At this point the 886P is producing a clean signal that is very hot. The DCX input is accepting as much as it can cleanly and the output is then attenuated some which means it is also clean at this level, which then runs into the amplifier input, which is set at its least sensitive setting and produces moderate clipping of the amp. The amp clips first before anything else in the chain and the signal is as hot as possible throughout the chain to the amplifier. After this point you should not touch any gain other than that in the front end receiver in the chain otherwise you will change the relationship between units.

If EQ is used in the receiver it does not matter and won't affect anything. However if you use EQ like I do, in a unit after the receiver, you may want to account for that by looking at the max amount of cut used which in my case is around 12dB at 45Hz. What that means is that with a 45Hz signal the input to the DCX will get clipped before the output into the amplifier produces clipping so that you will not be able to reach full power from the amp at that frequency range without readjusting the gain in the unit supplying the EQ, upwards to account for the amount of cut. However many times such as in my case a cut is used to pull down a large peak in the response. Full output at amp clipping at 45Hz in my room would produce something near 145dB at my seat which is more than I ever intend to use or need. So the loss of 12dB of headroom there is no big deal to me and I just leave it alone.

My main channels are a similar deal. I have an Emo XPA-5 for them which only takes about 1.5volts or something like that to produce full power. Meanwhile the 886P produces less clean voltage from the main channels than the SW channel but it is still way more than the EMO needs. The DCX can still spit out 12 volts give or take. Since the Emo is a fixed sensitivity and the 886 and DCX both have many times more voltage potential than needed I simply run a 60Hz sine wave into the 886P and run it into the analyzer and push it to just below 1% THD. Then noting the voltage with my dmm and the SPL level recorded inside of the spectrum analyzer I then switch to the internal pink noise used for calibration and without touching any of the volume levels or input trims I note what SPL the pink noise is with the spectrum analyzer and take note of the output level of the channel being used for the pink noise. I know that this signal is a -30dB level from what would produce REF level. I compare the SPL from the 60Hz sine that produces just under 1% thd and that recorded by the -30 pink noise signal. The setting of the channel trim and the master volume setting for the 60Hz max clean sine wave, along with the spl recorded by the analyzer can be compared against the SPL recorded by the analyzer for the pink noise at the same channel trim. This will tell you what the maximum output the main channels can be set at to avoid clipping at REF level. Allowing you to use maximum signal strength from the receiver or pre-pro. In my case the 886 will be just below clipping at +8.5dB with a signal 30dB over the pink noise and the master volume at 0dB. From there the XPA-5 gain is fixed and the 886 is capable of putting out much more than it takes to clip the XPA-5. Also the XPA-5 has no clipping indicators and I'm not set-up to measure voltages as large as the amp will put out so I use the manufacturer's input sensitivity rating and measure the output from the DCX while putting the max clean 60Hz signal from the 886 through it. Then the DCX output stage is adjusted to produce the manufacturer's specified input voltage needed plus some just to make sure. From there plug the speakers back in and adjust the speaker levels to 75dB using the pink noise and adjust using only the channel levels in the receiver. If using EQ account for it the same way as outlined with the subs above.

That's just how I do it. YMMV. Anyway if you really want to get this stuff right you need to dig out a volt meter and a spectrum analyzer.
Edited by Ricci - 10/15/12 at 12:24pm

You amps already are turned 'all the way up' that's the whole point. A Pro amp that has volume attenuation knobs - in order to get it to act like your amps, you need to turn those knobs all the way up. That's the whole point of this exercise, to let people know that those knobs do not add any gain to the input signal, and when set to maximum (attenuation = 0) they act like an amp that has to volume/gain/attenuation function. Simple as that.

^^^^^

The main problem I believe is level matching, if there really is any problem, yes the AVR will put out full voltage drive the pro amp to full rated output, but will the AVRs SW output be able to match what is needed to balance with the mains though the full volume range or will it fall short..

If the main speakers are the subwoofer are in the same ballpark efficiency-wise then there should be no problem - your typical 90db 1w/meter speaker will match up just fine with most subwoofer designs. If you start mixing high-efficiency speakers (horn-based) with inefficient subwoofer drivers the disparity could become too large. I was at the outer limits with some inefficient, sealed 12" subwoofers being paired to dual 15" P.A. cabinets that were rated at 104db 1w/meter but my (former) Sony AVR still pulled it off.

Thats what those clean boxs are for, to make up for what is needed to match the mains, if needed. So there really is no problem at all, just easy fixes, if needed. So whats the big deal about ?
Edited by joehonest - 10/15/12 at 11:53am

The main reason I got the pro amp was to drive a $$$ car sub. I didn't want to invest in a high priced home HT sub plate amp which may become useless at some point. The crown has many uses and will never get dusty..

I agree with you 100%


I like to tell that the crown XLS 1000 to 2500 have DSP, but no gain boost, only filtering

Yes you are right, no guarantee that gain is a DSP feature.

Now you are saying that a volume control does not increase the gain of an amplifier. That's pretty obvious. The amplifier volume control does reduce the effective gain of the amplifier if it is used.

Some amplifiers even have an adjustable input sensitivity.

I have an NHT X-1 crossover in line between my AVR and my power amplifiers. I don't need a volume control on the amplifier unit because I can adjust the volume level (gain structure) after the the AVR but before the power amplifier. I run unbalanced, but I can run balanced at any time if I wanted to with no level matching problems.

http://www.nhthifi.com/ServiceCenter/X1-Electronic-Crossover_2
Edited by J_Palmer_Cass - 10/15/12 at 4:48pm

Seems like you are good to go with your nice, simple crossover unit.

You young whippersnappers don't remember it but once upon a time power amps had no volume controls, only the pre-amp or mixing console did.

In the old days, power amplifiers were available both ways (with or without a volume control).

I don't recall seeing volume controls on power amps until the 80s, though I guess some could have had them. It was the 90s before I owned any with them, when I replaced my 60 pound BGWs with newfangled lightweight Carvers that were only 30 pounds.

A lot of them had a screwdriver type volume adjustment located on the rear of the unit. It was easy to miss.

fuuuuuu maybe the drivecore I was sent was faulty? a faulty crown power amp though... now ive heard it all.

This may sound like a stupid question, but most of this talk is about using a pro-amp to drive a subwoofer, but, does driving other speakers, such as the mains in a home theater setup, pose any additional problems for pro-amps when connected to an AVR's pre-outs for those channels?

I did for a little while as well. I had them on a crown xls2000 which was WAY more than they needed, but I actually did prefer it this way over the emotiva as I could adjust the gain on the pro amp where you cant on the emo. With that said, it was super overkill for my room and I barely ever even saw the lights flicker on the crown.