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#392514 - 04/30/13 10:18 AM Denon 3808 Preamp input sensitivity
osborne Offline
frequent flier

Registered: 08/20/09
Posts: 16
Loc: Labrador
Last week I ordered a Bryston 4BSST2 as the first step in upgrading my home theatre system. My long term plan was within two years to buy a quality pre-amp to match the amplifier.

My question is about the Denon’s preamp input sensitivity and max output I can expect without clipping. From the spec sheet I believe the input sensitivity is 1.2V for teh Denon 3808.

Using the equation Gain = 20 Log (Vout / V in) I have played with some numbers and come up with a max Vout of 33.72v or 142 watts based on setting the amplifier gain to 29 dB (assuming 8 ohm load). Now I just paid nearly 6 grand for the amp, new cables and shipping so needless to say the fact that I will be clipping the output of my 3808 at 142W from the Bryston is not all that exciting news, and things get worse if I assume the 6 ohm load of the M80’s.

So looking at it the other way to cleanly drive the amp to its max power without clipping will require an input sensitivity of 1.74V at 8 ohms and 1.94v at 6 ohms.

In terms of real world performance this may not even be an issue, but I feel like I jumped the gun by buying the amp and not looking at it and assuming the denon would be sufficient for the next year or two. The other reason this may be an issue for me is I may watch movies loud but my wife blasts music. She has managed to shut my Denon down a quite a few times now by putting it into thermal overload protection mode over the last few years. Hence the new amp.

So do I have the input sensitivity of the denon correct at 1.2v?

It looks as if a new pre-amp may be on the way sooner then expected. Any decent pre-amps you can recommend 3k and below with at least 2v of input sensitivity?

Osborne
_________________________
Epic 80 - 500 system in 7.1
Denon 3808CI



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#392516 - 04/30/13 12:06 PM Re: Denon 3808 Preamp input sensitivity [Re: osborne]
Andrew Offline
Axiom Engineer
veteran

Registered: 12/18/01
Posts: 172
Hi Osborne,

I would hold tight until you receive the amp and have tried it with the 3808. The 1.2V figure given likely represents the output voltage at unity gain, or 0dB on the Denon volume control. It is VERY unlikely that the Denon will not be able to output the 1.8V required to drive the 4B to full output. The only thing you will find is that you have to have the Denon volume control at a higher setting that you are used to. If you are still using the Denon to power the other speakers in your system you will have to reduce the level settings for those channels to balance the level of your M80s.

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#392534 - 04/30/13 10:55 PM Re: Denon 3808 Preamp input sensitivity [Re: osborne]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10383
Steven, the bottom line is what Andrew said above: it's "VERY unlikely" that the Denon pre-amplifier section can't output the 1.8V needed for the maximum power number of the Bryston.

I looked at the units and did some quick calculations, so I'll go into some detail. The first point is that the terminology is a bit tangled-up. You use "input sensitivity", but the relevant spec is the pre-amp output level. The 3808's input sensitivity is 200mV for a 1.2V output.

Your 20log voltage ratio calculation for gain is correct, but it's incorrect that it represents a "max". The 1.2V output isn't max but is the level needed for the 130 watt 3808 output into 8 ohms. Using Ohm's Law(won't bother with all details unless someone wants them), 130 watts into 8 ohms requires 32.25V. The 1.2V to 32.25V gain is 28.59dB in the 3808 amplifier, a typical number for audio amplifiers. Quality pre-amplifiers, such as the one in the 3808, have the ability to output considerably more than just what's needed for the rated maximum power output of the receiver. Didn't see a full lab test on the 3808, but the 4306 with similar specs(including 200mV input sensitivity, 1.2V output and 130 watts into 8 ohms)showed over 6 volts of pre-amplifier output in the test reported here . You already have a "quality pre-amp" and don't need to spend more money for one.

The Bryston(29dB) and Denon(28.59dB)amps are similar in gain and any slight imbalance is easily corrected by the auto-calibration. They'll work well together.

Now as to the horrendously loud listening levels that created this problem, I'd be inclined to suggest yelling to your wife "turn that damn thing down!"(easy for me to say). Maybe less dangerously, spending $20 or so on a little cooling fan to prevent the overheating shut down might have been more cost-effective than a few thousand on a new amplifier.
_________________________
-----------------------------------

Enjoy the music, not the equipment.



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#392539 - 05/01/13 12:52 AM Re: Denon 3808 Preamp input sensitivity [Re: osborne]
brwsaw Offline
connoisseur

Registered: 10/12/12
Posts: 1018
Or...you can enjoy the music AND the equipment.
_________________________
Not sure which is better,a little extra money in the account or days off.

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#392540 - 05/01/13 04:15 AM Re: Denon 3808 Preamp input sensitivity [Re: osborne]
CatBrat Offline
axiomite

Registered: 08/05/09
Posts: 5838
Loc: Some random location
Or..... Just watch some movies and forget about the music.

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#392541 - 05/01/13 07:11 AM Re: Denon 3808 Preamp input sensitivity [Re: osborne]
osborne Offline
frequent flier

Registered: 08/20/09
Posts: 16
Loc: Labrador
Thakns for the quick advice guys. I did look on the net for a lab report on the 3808 but could not find one.

As for the overheating issue i solved that a while ago. I bought an AV cabinet from Standout Designs that has fans installed in the cabinets controlled by a temperature sensor. Things have been cooled down since then.

I should ahve my amp on Friday so I'll let you know how things turn out.

I am going to mention to my wife that when the light on the front of the new amp flashes red it means it is clipping and needs to be turned down!
_________________________
Epic 80 - 500 system in 7.1
Denon 3808CI



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#392557 - 05/01/13 09:42 PM Re: Denon 3808 Preamp input sensitivity [Re: osborne]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10383
Actually, there is a lab test on the 3808 here , but it isn't "full" in that the pre-amp output which was your interest wasn't tested. It rarely is tested, possibly because a very low proportion of receiver buyers use the pre-outs, and those that do should almost never have good reason to be concerned about the output level.

Nevertheless, the test did show uniformly excellent results, including power(186 watts into 8 ohms and 238 into 4 ohms)which indicates audibly transparent amplification at all safe listening levels with the M80s.

It's good to hear, although hardly surprising, that providing adequate ventilation solved the problem with the 3808.
_________________________
-----------------------------------

Enjoy the music, not the equipment.



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#392654 - 05/03/13 05:28 PM Re: Denon 3808 Preamp input sensitivity [Re: osborne]
Amie Offline
connoisseur

Registered: 02/06/02
Posts: 1620
I've seen ads for those AV Cabinets. How do you find them? Are the fans super-quiet? They look beautiful online.
_________________________
Amie Colquhoun
Axiom Audio

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