Will An Attenuator Help Reduce This Hiss?

Hi Scott,

It might help, but I'm not certain.

And as Ralph points out some people have reported adverse sonic effects from in-line attenuators, although others have used them successfully. FWIW a while back I had used a pair of 10 db Rothwells at the inputs of the amp I used at the time, with no adverse effects that were apparent to me.

It seems clear that the noise is being introduced at a circuit point in the preamp that is "after" the volume control. So adding a 10 db attenuator, for example, would presumably reduce the level of both signal and noise by 10 db. You would then turn the volume control up by a corresponding amount, which would result in the original signal level being restored but would leave the noise at the 10 db lower level.

As a rough rule of thumb, btw, 10 db corresponds to what would be subjectively perceived as half the volume. 20 db corresponds to what would be subjectively perceived as 1/4 of the volume.

If you presently turn your volume control more than 1/2 of the way up, or perhaps even somewhat less, I would be hesitant to use the 20 db value, because you might find yourself running out of range at the upper end of the volume control's range of settings.

The main reason for the uncertainty I indicated is that the mute function does not reduce the noise. I would expect that in most cases the location of the mute circuitry in the signal path would be very close to the output of the preamp, and that it would kill any noise originating upstream of that point. Which suggests the possibility that noise generated by the preamp is either radiating through the air to the power amp, or is coupling to the power amp through the ground connections between the two components, or possibly through power wiring. An attenuator would probably not help in those cases.

IIRC you are using a W4S DAC2 as your preamp, with balanced connections to the power amp. The balanced connections presumably reduce the likelihood of coupling via the grounds, but the noise produced by the digital circuitry in the DAC2 could conceivably be coupling through the air or through power wiring.

Best regards,
-- Al

NGJockey raises a good point about impedance compatibility. According to my not particularly accurate analog multimeter, though, my 10 db Rothwells consist of a resistor of about 22K in series between input and output, and a resistor of about 10K or 11K shunted from the output to ground. That will result in an overall input impedance, as seen by the preamp, in the vicinity of 30K, the exact value depending on the input impedance of the amp. As NG indicated, that shouldn't be any problem for a solid state component such as the Meridian.

The output impedance of the attenuator, seen "looking back" from the amp, will be in the vicinity of about 7K. Given that the attenuator plugs directly into the input jacks of the amp, with no intervening cable, that would only be a problem if the input impedance of the amp is low AND varies significantly as a function of frequency. Perhaps that is why some people report adverse effects from these things, and others don't?

Best regards,
-- Al

Hi Kirk,

Thanks for the characteristically knowledgeable and thoughtful analysis.

However, while I recognize that the high source impedance presented by the resistive attenuators may significantly degrade amplifier noise performance in terms of numbers, assuming it is excellent to start with, is that degradation really going to be audible? For unbalanced inputs, amplifier sensitivity will typically be in the rough ballpark of 0 dbv (i.e., 1 volt or so). So -115 dbv into the amplifier would result in a noise level out of the amplifier that is 115 db below full power. Let's say that full power corresponds to an SPL at the listening position in the vicinity of 110 db. In that situation -115 dbv at the amplifier inputs would result in an SPL at the listening position of -5 db, surely not audible. And that is without A-weighting. And I would expect that overall upstream noise performance would be considerably worse than that as well.

Concerning the distortion effects that may result from the increases in voltage and current that have to be provided by the preamp if a resistive divider is used, yes that is certainly an effect that can occur. But isn't it generally considered to be sonically preferable for the preamp's volume control to be operated at higher points within its range, rather than at lower points, to minimize the sonic effects of the volume control mechanism itself? It seems to me that the overall effect on preamp sonics resulting from inserting a resistive attenuator would reflect a net balance of multiple effects, that in any given case may net out unpredictably for the better or for the worse or without significant difference.

In any event, thanks again for the good inputs, that wouldn't usually be thought of. Best regards,

-- Al

Thanks for the response, Kirk. I see what you are saying. It is not so much the Johnson noise produced by the resistors themselves that is particularly significant (about 1.8 microvolts for your 9700 ohm example, or 1.5 microvolts for the Rothwells I referred to), but the fact that the amplifier's own input noise currents will be increased in significance by the increased source impedance.

I think in the majority of cases this will be instantly noticeable in a quiet room with no source playing and one's ear near the speaker. But at the very least it seems awfully ham-handed to instantly nullify all the hard engineering work it takes to build a low-noise power amplifier.

That provides a good perspective on the magnitude of what we are discussing. Which, as I see it, does not necessarily exclude the possibility that resistive attenuators could still be a reasonable low cost solution in some and perhaps many cases. Especially given that those kinds of noise levels will often be present anyway, as a result of the noise produced by the upstream components, ground loop effects, RFI/EMI pickup, the source material, etc.

Best,
-- Al