2. amps-preamps
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Matching Preamp Output & Power Amp Input Voltage

I am setting up my first home theater system and have run into an issue that I could sure use some guidance on.

For a bit of background, I purchased an Onkyo TX-NR807 7.2 receiver that is rated at 135 watts/channel (but, with only 2 channels driven, which is a typical spec for many brands of AVR's). For a more realistic 7 channels driven spec, I am probably looking at something closer to 70 watts/channel. Based on this, I wanted more available power for my ProAc Super Tower speakers that I am bringing over from my existing stereo system to serve as the right front & left front speakers in my home theater set-up. Since I already have a nice Nakamichi Stasis PA-7AII 2-channel stereo amp (225 watts/channel), I planned to use the RF/LF pre-outs on the Onkyo receiver, so the stereo amp could power my ProAc Super Towers and then relieve the Onkyo to directly power the remaining center channel speaker and the surround speakers.

But here's the rub. The voltage output spec for the Onkyo pre-outs is 1 volt, however, the input sensitivity for my 2-channel power amp is 2 volts. (And I am not worried about impedance matching in this instance, as I am well beyond the 10:1 rule, since the 2-channel amp is at 50,000 ohms and the Onkyo pre-outs are at 470 ohms).

I have not had a chance to hook things up yet to do a bit of experimenting, but I am nevertheless concerned that with the 1 volt to 2 volt mismatch, that my 2-channel amp will never be able to develop its rated power and that higher gain settings will be needed to get the same relative volume level out of the speakers.

I have also contemplated getting a input matching transformer to tackle the problem, however, that puts yet another device into the path, and it will also require XLR to RCA adapters since I will be going from RCA pre-outs on the Onkyo to RCA inputs on the 2-channel amp. Then there is the further consideration of the impedance value (and potential noise) associated with the in-line input matching transformer device.

Anyway, I would sure appreciate it if someone can weigh in on all of this as to whether the 1 volt to 2 volt mismatch is that big of a deal (and what should ideally be done about it), or whether I should just stop worrying about it.

Thanks,
Don
decj
5 responses Add your response
almarg's avatar
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almarg
9,670 posts
 
Hi Don,

The manual for the Onkyo, which can be found here, does not appear to specify the levels of the pre-outs. So I'm not sure where you found the 1 volt number, and I'm also not sure that it can be interpreted properly without specifications that allow it to be related to the signal level coming in from the source. For instance, a specification on the gain of the preamp section, or else a specification that the 1 volt output corresponds to some number of millivolts in.

If indeed the maximum output of the preamp section is only 1 volt, that would reduce your 225W amplifier (with 2 volt input sensitivity) to a (225/2squared) = 56W amplifier.

However, I suspect that is not the case, and perhaps the 1 volt pre-out spec you saw somewhere corresponds to a 200mv input, that being the input sensitivity spec that is indicated in the manual for the Onkyo (meaning the input voltage to the Onkyo's line-level inputs that will result in its own power amplifier being driven to full power). If so, and if your source is a cdp having the 2 volt full scale output that is common, you would be able to put 10 volts into the Nak with the volume control at maximum!

Regards,
-- Al
almarg's avatar
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almarg
9,670 posts
 
Correction: At the very end of my preceding post, I should have added the words "... assuming the clipping point of the pre-outs is not exceeded."

Regards,
-- Al
decj OP
2 posts
 
Al,

Thanks much for your response.

The story behind my getting the Onkyo pre-out specs is long and convoluted. I too noticed that this spec was not in the manual (I have the hard copy version), nor was it published on Onkyo's website page for the TX-NR807 receiver. When I called Onkyo tech support, the first rep didn't know what it was, but he talked to someone else who supposedly did. Here's what this effort yielded ... they stated it was "less than 1 volt but greater than 300 mV", and surprisingly that this "was not exact" ... and there was no impedance info to be had. I then followed up with a couple of emails to Onkyo customer support and they came back with the 1 volt and 470 ohm pre-out information. Now whether that latter into is actually correct or not, I cannot vouch for, but it's all I have to go on.

And I understand what you're saying about the lack of addition info to make the Onkyo pre-out output spec more meaningful. For example, the second input sensitivity spec for my Nak amp is 140 mV at 50k ohms (1 watt output). Anyway, the Nak CA-7A control amplifier that is currently hooked up to my Nak amp has an output level/impedance of 2.0 volts at 1,000 ohms, so as one wouldn't be surprised coming from the same manufacturer/model line, they are both matched to each other from an output/input voltage perspective.

In any regard, I also heard from someone that since the Onkyo TX-NR807 is a THX Select 2 Plus certified receiver, that THX certification specs demand that pre-amp pre-outs need to meet 2 volts. However, I have not been able to confirm this elsewhere, so unless you know that off-hand, I tend to not trust that info.

So, what do you suggest? Just hook my Nak up to the Onkyo pre-outs and see if it sounds OK and I'm hopefully not running the volume up inordinately high? Or if needed, get one of the input matching transformer units and see if that corrects the voltage mismatch issue (should it turn out to be a problem)?

Please let me know.

Thanks again,
Don
almarg's avatar
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almarg
9,670 posts
 
Thanks for the additional background. I suspect that if you just go ahead and hook it all up as you are envisioning it will work fine.

It sounds like the people who provided you with the numbers are not knowledgeable about what they mean. And I suspect that the 2.0 volt number for the Nak preamp matches the number for the Nak power amp not because the components are better matched, but because the specmanship is better matched.

Most likely the 1.0 volt output of the Onkyo is relative to some input voltage that is significantly lower than 1.0 volts, such as the 200mv number I mentioned (i.e., the preamp section provides significant gain). And I would expect the preamplifier section to maintain that same amount of gain as the input level rises above 200mv until the pre-outs reach the clipping point, which will almost certainly not occur below an output level of at least several volts or more.

So if your source puts out say a volt or two you'll have no trouble getting 2 volts into the Nak. And the +/- 12db range of the independent volume adjustments that the Onkyo provides for its individual output channels (what it calls "level calibration") seems reasonable in terms of allowing you to match the volumes of those channels to the ones driven by the Nak.

Regards,
-- Al
decj OP
2 posts
 
OK Al ... I am feeling a lot better about all of this. I of course realize that there are no guarantees that all will come together perfectly as we really don't have all of the desired upfront information, but I will give it a shot and see what happens.

In looking over a number of threads here, you have obviously helped out a lot of people and I am glad to count myself in that group.

Thanks again for your help and encouragement!

Don
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