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View Full Version : Distortion during loud tracks - my amp is too sensitive - what to do?



rh2600
2008-11-12, 22:53
I've turned off volume control, and RG on my SB3.

My marantz SR-45 had a tendancy to distort audio coming from its paired CD player - which I used to address via it's own volume control.

Anyway, its evident that my amp is too sensitive for the Sb3 outputs too.

The specs for my amp say: Input sensitivity - 185mV/47Ω.
I assume I need to adjust the Preamp Volume Control, but does anyone know how to calculate exactly how much?

Cheers

Ross

Mnyb
2008-11-13, 00:03
? are you sure your not describing your phono input ?

The phono one is not suitable for line level sources and also has an equalizing curve (for the pickups used with your gramophone for "vinyl")

47Ohm or 47KOhm looks like phono input to me

cliveb
2008-11-13, 06:30
? are you sure your not describing your phono input ?

The phono one is not suitable for line level sources and also has an equalizing curve (for the pickups used with your gramophone for "vinyl")

47Ohm or 47KOhm looks like phono input to me
185mV sensitivity can't be a phono input - thay would be more like 5mV. And although phono inputs are 47kOhm, that's also a perfectly reasonable input impedance for line level inputs.

But having said that, any decent line level input should be able to handle up to about 10V without overloading. I'm surprised that a Marantz would have this problem. Ross: have you tried switching the SB3's volume control back on to see if backing off its output level solves the problem? If it does, and you'd prefer to leave its output fixed at maximum, then you could add some passive attenuators between the SB3 and SR45.

lanierb
2008-11-13, 10:23
The first thing to try would be to back off the SB3 volume control. It won't affect the audio quality down to at least 50% and probably a lot lower. If that doesn't work (and even if it does) then I'd suspect there's something wrong with the Marantz.

chinablues
2008-11-13, 10:44
ha ha, distortion during loud tracks! Rephrase it to "distortion playing tracks loud" and you have the expensive path to a brand of audiophilia. How many have played tracks loudly late in the evening & have seen those ripples of discontent on the surface of the Lagavulin when a distorted chord crosses the room and hits the crystal. Gotta upgrade those speakers, amps or whatever... Solution is easy. $$$ A whole industry out there waiting.

dan

Mnyb
2008-11-13, 11:13
185mV ok missed that, I also assumed that the OP mised something with 47ohm ? instead 47k which is normal.

but why 185mV ? strange, even old stuff with din connectors have 0.5V sensitivity.
I think never stuff hovers about 2V.
Must google this amp ?

Solder some attenuators i think there is instructions in the wiki.
http://wiki.slimdevices.com/index.php/DIYPassiveAttenuation

or be lazy like me and get 10dB attenuators from rothwell

http://www.rothwellaudioproducts.co.uk/html/attenuators.html

Other sugestions in the wiki, including links to other brands of attenuators

http://wiki.slimdevices.com/index.php/Connect_To_PowerAmp

I'm not up to speed on logarithms just now om not sure of 10dB cuts enough voltage but i combination with preamp volume in the settings for the player you can get a decent match.

Phil Leigh
2008-11-13, 11:46
185mV sensitivity is crazy! - even old Quads from the 60's had 250mV!

Surely this can't be right - you need at least 2V for modern gear and 10V would be a lot of headroom.
Where's google...

cliveb
2008-11-14, 02:22
185mV sensitivity is crazy! - even old Quads from the 60's had 250mV!
My recollection is that line level input sensitivities around the 100mV mark was quite common back in the 70s. Most devices such as FM tuners and tape decks output fairly modest voltage levels. It was only when CD players arrived on the scene in the 80s that outputs of 1V and 2V became commonplace.


Surely this can't be right - you need at least 2V for modern gear and 10V would be a lot of headroom.
10V was the minimum overload capability you'd expect from a decent input with 100mV sensitivity. For a 2V input, 10V is only a little over 12dB of headroom, which is hardly adequate.

Phil Leigh
2008-11-14, 08:25
My recollection is that line level input sensitivities around the 100mV mark was quite common back in the 70s. Most devices such as FM tuners and tape decks output fairly modest voltage levels. It was only when CD players arrived on the scene in the 80s that outputs of 1V and 2V became commonplace.


10V was the minimum overload capability you'd expect from a decent input with 100mV sensitivity. For a 2V input, 10V is only a little over 12dB of headroom, which is hardly adequate.

hmmm - yes I meant 150 not 250.
Many preamps with split-rail supplies use +/-12v so max swing is (theoretically) 24 v pk-pk...

I think that these days most line inputs are set to 500mV sensitivity, with a proper CD input being at 2V.

Anyway, 187mV is too low for a CD or squeezebox. It will need an attenuator in-line to avoid overloading the input stage IMHO, depending on its gain staging and overload margin (headroom).

rh2600
2008-11-16, 19:42
I've attached some photos of the amp, and the page from the manual so you can see I ain't making this up or getting confused about the phono input.

So the Preamp Volume Control isn't going to do what I need?

This amp is only about 10 years old - so I'm a little suprised it can't handle a standard input from a CD player...

Mnyb
2008-11-16, 23:35
http://www.sengpielaudio.com/calculator-volt.htm

some more calculators

Preamp volume can do what you want, we are only suggesting inline attenuators because you need to do a very large adjustment.
Meaning you are one of the few that actually can impair sound quality with to low volume on SB.

Attenuators have the added benefit of protection, if your SB goes nuts and suddenly outputs full volume (a possibility in some rare cases ), then you won't blow your speakers.

Must rush to work, sorry havent made any calculation for you

cliveb
2008-11-17, 03:49
Anyway, 187mV is too low for a CD or squeezebox. It will need an attenuator in-line to avoid overloading the input stage IMHO, depending on its gain staging and overload margin (headroom).
An attenuator possibly makes sense in order to free up a decent amount of preamp volume control range, BUT...

The point I was originally trying to make is that even an amp with an "old-fashioned" input sensitivity around the 150mV mark should not be overloaded by the 2V signal that typically comes out of a CD player.

rh2600 needs to perform a very simple experiment:

1. Set the Squeezebox volume to maximum (100), play a loud piece of music, and turn the Marantz's volume control up so you can hear the distortion.

2. Now turn the Marantz's volume control down. If the distortion disappears, it was either due to clipping in the Marantz's power amp section (nothing you can do - you need a more powerful amp), or it might even be due to overloading of the speakers (you need speakers with higher power handling).

On the other hand if the distortion remains even at low listening levels, it's due to overload of the Marantz's input ciruitry. Try turning the Squeezebox's volume down - the distortion should go away. In this case, attenuators will fix the problem.

Mnyb
2008-11-17, 09:51
Clive is probably right, no one knows where or when it clips.

But any how, now my amateurish calculations SBR has an output speced as 4.8Vpp
which is circa 1.7Vrms the Marantz wants 0.185Vrms (I assume rms ).

Via the calculator i found on the web i get this:
1,7 volt is 6.83dBu or 4.61dBV
0,185 volt is -12.44dBu or -14.66dBV

In any case the needed damping is 19.27dB say 20dB

Please correct me if i'm wrong it was way back i calculated such thing's

So 20dB damping in preamp volume control will do the trick.

Now the excell spread sheet for calculating damping resistors is gone ? any one got a copy ? well a part of the jobb is done.

Never mind Lpad calculator here: http://www.sengpielaudio.com/calculator-voltagedivider.htm

cliveb
2008-11-18, 01:51
Now the excell spread sheet for calculating damping resistors is gone ? any one got a copy ?
You're right - it's disappeared. Must have got lost when the wiki was restructured. You can find the spreadsheet here: http://www.delback.co.uk/general/atten_calc.xls

Mnyb
2008-11-18, 10:39
Nice round values for an Lpad R1=10kOhm R2=1kOhm.

There is many possible results for this, this is just one suggestion.

I use a combination of attenuators and power amps with adjustable gain, in my hifi and powered speakers with gain controls on the back for the kitchen.

There is a general need for attenuators as there's to much gain everywhere.
The loudness race is on in amps and CD players too !
Imagine the hifi show room when the sales person just switches between two CD players without adjusting volume, which one will be "better".
I remember some old hifi of mine where i never used more than half of the volume knob often less.
I seen people actually complaining on amps calling them "weak" when they have turn up the volume knob more !

Generally i like it when I can use most of the range of my volume controls.


Btw I just loaded SC in the classic skin where the volume goes from 1 to 11 yeah ! much better than 1-10