Amplifier Voltage Gain matching your Preamp and Amplifier

[gene]

A point, which I concede was a surprise to me, was that there are apparently so many preamp outputs in the world that clip at 1Vrms or perhaps a few dB above that, which is just bad design. But if that's the case (which I'm sorry I can't confirm reading the receiver tests posted here), it's worth knowing. You could have stopped right there.

The noise point was an exaggeration, the cable impedance point was inaccurate, the amplifier gain/bandwidth and DC offset points were irrelevant, the preamp load point was inaccurate and cited an unrealistic load condition...all of this and more has been clearly spelled out.

Examples...make them realistic, not extreme. Scale the example to probability...what real world chances are there that my Onkyo AVR can't drive my Emotiva amp? IF an extreme example is necessary, how about a specific to watch for, like "the BIGGIE3 AVR preamp out and the XTC amp don't work well together because the AVR output clips at 1Vrms and the XTC amp needs 5Vrms for maximum output." But, if most amps are fine with 2Vrms, and most preamp outputs clip at 2.2Vrms, say so. You gave many specific examples of preamp outs with plenty of headroom, no examples of those with clip levels so low as to be a problem.

And then, what would be REALLY nice is, measure and publish the preamp clip point and output impedance in all reviews. While you're at it, how about along with input sensitivity, measure and publish the input impedance on power amps? The reviews that Gene writes are quite complete (though still missing amp input Z tests), the other reviewers, not so much. Since I was accused of not being familiar with the technology, and told I should read the reviews, I did. Dozens of them, right here. Those figures are missing from all of them, except the ones Gene does, where he states the pre-out clip point. If preamp/amp match is such a huge deal, we need that info tested and published, both out clip and in Z. But what I suspect is, since input impedance of amps is usually found in the spec sheet, and is 50K or higher most of the time, it's viewed as unnecessary to publish in a review. But then, don't cite the one-off 600 ohm input as an example without naming it! (BTW, didn't that device have an input termination switch that would change that from 600 to something much higher?)

You're more than welcome to start your own website. Invest in test equipment and solicit manufacturers to send you products so you can measure and report on them. I'm sure you can make this all a reality in no time. Then you can create a forum so others (myself included) can come on and start critiquing your life work. Of course, we would do this all anonymously.

 

[Steve81]

The noise point was an exaggeration

Not necessarily. Emotiva for example doesn't recommend running their higher gain models with higher sensitivity speakers (I discussed my options with them, at the time having speakers with a rated sensitivity of 99dB w/ 2.83V). Having once been an active member at the Klipsch forums, I also noted several cases where noise was an issue and people turned to things like inline attenuators because it was a real problem. Is it something everyone should be concerned with? Of course not. But if you are running high sensitivity speakers and in the market for a separate amplifier, it's a potential problem to be aware of, and one that is easily avoided.

Examples...make them realistic, not extreme

"Extreme" showcases the problems you might come across, which is important in the context of such an article (at least IMO). Of course, it should be noted that just because an example is extreme, it can still be quite realistic. The XPR-1 has been recently reviewed here and requires considerable output from a pre/pro to drive it to full tilt. It's obviously an extreme example, but it's quite real.

IF an extreme example is necessary, how about a specific to watch for...

I suppose I don't really see a great benefit to it. The whole point of an article like this is to give people the tools/knowledge to make an informed decision regardless of the specific circumstances, not to point out one or two real world examples.

But, if most amps are fine with 2Vrms, and most preamp outputs clip at 2.2Vrms, say so.

Because
A. Unless I've actually benched most preamp outputs I can't state that authoritatively.
B. Generalizing problems away doesn't help those people who do end up having issues (i.e. people with less capable equipment).

 

[Saril]

Application Question

Again, have to disagree, especially when real world examples are later given. The example is (as should be fairly obvious given the meager capability 1Vrms represents) meant to demonstrate the issue, nothing more, nothing less. Of course that Gene has measured receivers whose preouts aren't capable of much more is just icing on the cake, regardless of whether you trust his statement or not.


There lays the problem: sans somebody actually bench testing your receiver, how do you know its preouts qualify as "reasonable"?


That's rather the point of reviews here, no?

How would I determine if the following is a good match using the math provided? Thanks.
Marantz AV8801 preamp with balanced pre out = 2.4V
Vienna Acoustics - Beethoven Baby Grands & Maestro (mains & center) - 4 Ohms and 91dB sensitivity
Emotiva XPA 100 amp - 32 dB gain - 400W @ 4 Ohms

For surrounds - Noble Fidelity L-65 - 8 ohms & 91 dB - driven by Emotiva XPA-200 - 32 dB gain - 150W @ 8 Ohms

 

[Saril]

The XPA-200 has a gain of 29 dB, not the 32 dB I originally posted.

 

[Steve81]

How would I determine if the following is a good match using the math provided?

You'll convert the power output of the amplifier for a given load to voltage (for example 400W = 40V into a 4 ohm load). Then use the formulas provided to see what you need from the pre/pro to drive your amplifiers to full power.

 

[TLS Guy]

The same could be said about jumping to conclusions and making unsubstantiated claims in a forum post to discredit this article when you have no data to back it up.
1. I have measured MANY receiver preouts that clip at 1Vrms either b/c of the limitations of the Volume controls or b/c the preamp was using a single +5V rail for the opamps.
2. There are esoteric amps that have 600 ohm input impedances. I don't agree with making a design like this, but they exist.
3. Cable impedance DOES matter depending on the input impedance of the amp and how esoteric the actual cable is (capacitance being the most concerning parameter). A preamp driving an amp with a 50k input impedance connected to an interconnect cable of 1000pF would yield a -3dB loss at 3.1kHz. This is certainly NOT acceptable. So we prefer a lower capacitance cable of course!

That is really important information, and I have often wondered about the head room of receiver preouts. A preout that clips at 1 volt is virtually useless.

OP amp chips these days are almost always able to be driven by +5 volts or +5/-5 volts so 10 volts for the latter. I have also suspected that there is corner cutting with just 5 volts to the preamp in a lot of designs.

The very first thing I did after unpacking my Marantz AV 8003 was to put it on the bench and test its headroom. Clipping occurred just under 14 volts peak to peak, which I regarded as adequate.

We have had posts here about receiver preouts not being able to drive some pro amps, so it is an issue that has surfaced on these forums, and your can't get more real life than that.

One of the things Peter Walker impressed on me, is that you need to design in lots of headroom for audio circuits. He always did, and all his Quad products had superior headroom.

Concerning the issue here, his Quad 44 preamp, has 500 mv and 1.6 volt outputs, and both have lots of headroom, but it allows the volume control to get in the right range, to cope with amplifiers of different sensitivity.

Nowhere is this issue more problematic than microphone mixer input stages, where enormous headroom is required and few deliver.

When I was doing a lot of live recording, I picked my mixers with enormous care.

Unfortunately headroom is a way under reported specification, but perhaps it betters sorts out the chaff from the classy.

 

[Nestor]

That is really important information, and I have often wondered about the head room of receiver preouts. A preout that clips at 1 volt is virtually useless.

OP amp chips these days are almost always able to be driven by +5 volts or +5/-5 volts so 10 volts for the latter. I have also suspected that there is corner cutting with just 5 volts to the preamp in a lot of designs.

The very first thing I did after unpacking my Marantz AV 8003 was to put it on the bench and test its headroom. Clipping occurred just under 14 volts peak to peak, which I regarded as adequate.

We have had posts here about receiver preouts not being able to drive some pro amps, so it is an issue that has surfaced on these forums, and your can't get more real life than that.

One of the things Peter Walker impressed on me, is that you need to design in lots of headroom for audio circuits. He always did, and all his Quad products had superior headroom.

Concerning the issue here, his Quad 44 preamp, has 500 mv and 1.6 volt outputs, and both have lots of headroom, but it allows the volume control to get in the right range, to cope with amplifiers of different sensitivity.

Nowhere is this issue more problematic than microphone mixer input stages, where enormous headroom is required and few deliver.

When I was doing a lot of live recording, I picked my mixers with enormous care.

Unfortunately headroom is a way under reported specification, but perhaps it betters sorts out the chaff from the classy.

What was your headroom test?

 

[TLS Guy]

What was your headroom test?

Well since the peak to peak voltage was almost 14 volts, it gave me at least 8db of head room over my required line voltage of 1 volt RMS.

 

[Nestor]

Well since the peak to peak voltage was almost 14 volts, it gave me at least 8db of head room over my required line voltage of 1 volt RMS.

I realize those are your results. More specifically, what kind of signal did you feed it?

 

[Saril]

Help Please

You'll convert the power output of the amplifier for a given load to voltage (for example 400W = 40V into a 4 ohm load). Then use the formulas provided to see what you need from the pre/pro to drive your amplifiers to full power.

Thanks for the reply - can you confirm my calcs please:

Emotiva XPA-100
Vin = 1.2V
Load Impeadance = 4 ohms
RMS @ 4 ohms = 400W

my calculated Vout = 40
my calulated gain = 30.45 (Emotiva states 32 dB)

using 10^(gain/Vout) I get 5.77
preamplifier needs to deliver no less than: 10^(40/5.77) = 5.77V RMS to drive amplifier to full power
The Marantz balanced out is 2.4V

Is this a bad match, i.e. the 8801 will never be able to fully drive the XPA-100.
I'm hoping that I made a calc error. Any help greatly appreciated. Thanks.

 

[TLS Guy]

I realize those are your results. More specifically, what kind of signal did you feed it?

I fed it from my bench Eico 378 oscillator.

 

[Steve81]

Thanks for the reply - can you confirm my calcs please:

Emotiva XPA-100
Vin = 1.2V
Load Impeadance = 4 ohms
RMS @ 4 ohms = 400W

my calculated Vout = 40
my calulated gain = 30.45 (Emotiva states 32 dB)

You can stop at this point as you've found that 1.2Vrms (the XPA-100's input sensitivity) from the pre/pro is more than enough to drive the XPA-100 to full rated output into a 4 ohm load, assuming 32dB of voltage gain. Presuming the gain/input sensitivity are consistent for the Emo's balanced inputs, there shouldn't be any problems.

 

[Saril]

You can stop at this point as you've found that 1.2Vrms (the XPA-100's input sensitivity) from the pre/pro is more than enough to drive the XPA-100 to full rated output into a 4 ohm load, assuming 32dB of voltage gain. Presuming the gain/input sensitivity are consistent for the Emo's balanced inputs, there shouldn't be any problems.

Thanks again for your help. Using Excel I plug in the items I know, amp Av, speaker impeadance, and amp power and solve for the minimum required Vin as shown:

Your 50 Watts example:

Gain of Amplifier in dB 28 Gain (Av)
Speaker impeadance in ohms 8 Impeadance
Amp power in RMS Watts 50 Watts
need Vout = SQRT(Imp*RMS) 20.00 Vout
Vin Vin=Vout/10^(Av/20)
Minimum Required Vin in volts 0.796


using Vin = Vout/10^(Av/20) and plugging in the known values I get:

Emotiva XPA-100 at 4 ohms, 400W, gain @ 28dB:
Minimum Required Vin in volts 1.592

Emotiva XPA-100 at 4 ohms, 400W, gain @ 32dB:
Minimum Required Vin in volts 1.005

The Marantz 8801 has balanced pre-outs of 2.4V so I'm good.

I was just wondering if you agreed with the required Vin numbers I came up with.

 

[Steve81]

Emotiva XPA-100 at 4 ohms, 400W, gain @ 28dB:
Minimum Required Vin in volts 1.592

Emotiva XPA-100 at 4 ohms, 400W, gain @ 32dB:
Minimum Required Vin in volts 1.005

The Marantz 8801 has balanced pre-outs of 2.4V so I'm good.

I was just wondering if you agreed with the required Vin numbers I came up with.

Those numbers appear accurate to me.

 

[cpp]

Just wondering, are there any NON-PRO amps that allow for gain adjustments ?

 

[jomark911]

Yes there are, either pro, or hi fi amps. But they do not alter their gain , but their input sensitivity.
That's what the pots do.

 

[cpp]

Yes there are, either pro, or hi fi amps. But they do not alter their gain , but their input sensitivity.
That's what the pots do.

Interesting can't find a thing on that with my ATI amps

 

[RichB]

Yes there are, either pro, or hi fi amps. But they do not alter their gain , but their input sensitivity.
That's what the pots do.

I am not sure.
But the Parasound A31 allows you to adjust the gain upto +6 above the THX level.
The A21, allows you to adjust the gain below THX level.

- Rich

 

[Irvrobinson]

Just wondering, are there any NON-PRO amps that allow for gain adjustments ?

The ATI AT602 and AT1202 both have gain controls.

 

[cpp]

The ATI AT602 and AT1202 both have gain controls.

Irv what about the 2002, and where would it be ?