Will An Attenuator Help Reduce This Hiss?

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 Al, one of the things that causes confusion is the difference between PVCs and TVCs, and the various configurations of each and their interactions with the preamp, cables and amplifier.

The result is there is no single exact solution for every situation, often thus resulting in loss of bass, perhaps brightness, loss of dynamics, etc. One has to be careful!

If it were me I'd shy away from buying an attenuator until going over the entire system architecture from top to bottom and, as thoroughly as possible, give it the full measure of attention paid to impedence and gain matching that (from what you've described) was evidently never successfully done to start with. Review and replace any and all components that can be said to be violating Ohm's Law so that you'll be able to get back to square one and you won't be eternally confronted with trying to buy expensive bandaids to put on something that needs surgery, knowwhaddamean, Verne?? I really think this is actually the best way forward for anyone in this situation and it will give you better performance for it, to boot!

A two-resistor attenuator like the one Al describes seems simple and elegant, but there is another serious side-effect besides the issue of excessively loading the source -- it's the fact that raising the source impedance driving the amplifier will introduce much more thermal noise (a.k.a. "Johnson noise") at the same time you're trying to attenuate the noise coming from the preamp.

Consider typical values of a preamp with a 150-ohm output impedance, an amp with a 50K input impedance, and a 12-dB attenuator. Even with a perfect, noiseless source and preamp, the absolutely lowest noise level that can occur at the amplifier input is about -133dBv (noise from 150-ohm source at 20KHz bandwidth). This sounds like an incredibly low number, but the noise performance of a well-designed conventional solid-state amp (and even a few exceptional tube units!) can get within a handful of dB to this level (equivalent input noise or EIN).

So let's add the attenuator, and keep the 50k loading on the preamp the same . . . That would be about a 36k series resistor and a 16k shunt, making the amplifier's source impedance about 9.7k. The resulting input noise is now about -115dBv . . . a whopping 18dB worse. And even though it's still a 50k input impedance, the preamp must still put out four times the output current, because it has to put out four times the voltage as it did without the attenuator.

Scaling down the impedances seems like a good idea at first, but it doesn't really help as much as one might think. If we decide that the preamp's okay with a 12k load, we can change the values to 9.1k and 3.3k, and the amp then sees about 2.5k . . . input noise is now about -121dBv. 6dB better is certainly nothing to sneeze at, but don't forget the preamp must now deliver sixteen times the current (4x from lower loading, 4x from increased output voltage) . . . which should NOT be taken lightly from a distortion standpoint. And the Johnson noise is still 12dB higher than with no attenuator.

All of these figures assume the electronics are completely noiseless . . . in the real world, noise performance will (definitely, NOT probably) be worse. So the only time you get a net improvement in noise performance is if the driving source is itself both pretty noisy (completely overwhelming the Johnson noise), and at the same time doesn't mind being heavily loaded . . . . an example would be a typical 1970s pro broadcast console. But in consumer/high-end . . . It'll probably make things worse all the way around.

Atmasphere does make a good point regarding a transformer volume control . . . In fact, a Jensen JT-10KB-D 4:1 input transformer will solve the above example nicely. 12dB attenuation, 47k-ish input impedance, 250-ohm-ish secondary source impedance, excellent noise figure. They also sound fantastic . . . I've used them in several designs.

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