Do you think telling us the specific equipment you're talking about will better the quality and usefulness of the replies? |
Scott, you will need less gain in the amplifier. Most preamps have gain after the volume control and if you have high efficiency speakers this is a common problem. An attenuator is one solution but I think you will find that it degrades the sound in other ways. There may well be other solutions... |
Agree with Atmasphere.... |
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 |
thanks for all the great insights, all. to clarify a few questions...
the speaker i am hearing this trough is a Wilson Cub 2 used as a center channel. as ralph surmised, it is relatively efficient at 95 db.
Al, great memory! yes, at one point i was uing the W4S DAC2 as a preamp (can't say enough good things about that piece) however, i have since reconfigured my system enabling both 2-channel and multi-channel operation. the preamp driving the center channel is a Meridian 568.2mm and the power amp is a McCormack DNA-HT1. the interconnects between the amp and preamp are single ended, as the HT1 only offers RCA inputs. |
Seems to me that the McCormick has about 30db of gain. This is going to be a bit of a trick.
Do you know how much output your source has? Is is enough to drive the amp directly? |
Just to be different, Prometheus TVC's offer up to 54dB attenuation and isolation as well for under $500. Haven't read of any shipping backlogs for years. You can take a look at the Bent Audio stuff as well. If you're up for some DIY, I believe both Goldpoint and Dact have "fine tuning" naked attenuators. The problem with the inline attenuators is that they're only 10K Zin, which is not a problem in itself except in more complex parallel arrangements or with some tube and passive preamps. |
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
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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. It's important to remember that the noise voltage from every noise mechanism from every part of every piece of electronics adds up, in the fashion of the square root of the sum of the squares. For the simplest analysis of a single opamp gain stage, there are seven: 1. Johnson noise from source impedance of non-inverting input 2. Non-inverting input's input noise voltage 3. Non-inverting input's input noise current, times its source impedance 4, 5, 6. Same as 1, 2, & 3 for the inverting input 7. Output "build-out" resistor. The noise voltages from 1 thru 6 are multiplied by the circuit's noise gain (usually simplified to be the circuit's closed-loop voltage gain), and then added together (square-root-of-the-sum-of-the-squares). Any one of these sources by themselves is a pretty small number, but every single one of them (from every stage) contributes to the final result. In the real world, optimising noise performance usually boils down to choosing the input device(s) and their operating parameters so that their voltage noise/current noise characteristics fit the input source impedance, then going through everything else and shaving it down a few dB at a time . . . and when it's done properly, it's the noise of the preceding device or transducer that dominates. So with my previous attenuator example, it's not the absolute number that matters . . . it's the fact that we've taken the previously insignificant noise mechanism of amplifier source impedance and turned it into a huge one. If we use the common expression of Noise Figure (NF) . . . (the difference between an ideal amplifier and the real one for a given source impedance), sticking the attenuator in the back can change a good amp's NF (for a 150-ohm source) from i.e. 6dB to 24dB! I think in the majority of cases this will be instantly noticable with 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. 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? In general, I would say no, and definately not from a noise perspective. The possible exception is if the volume control is operated in such a low range where channel-balance and wiper contact resistance is an issue. But in a well-designed conventional (input-pot followed by active stage) line preamp, the Johnson noise from the volume control is the dominant noise source. And it's output impedance increases as the volume is turned up until it reaches it's maximum value at -6dB (plus electronic gain). Also, keep in mind that all the noise we've discussed is "post-fader" . . . that is, it's unattenuated by the volume control. So from a noise standpoint, the best way to reduce a conventional preamp's gain by passive, resistive means (assuming 12dB, and keeping input/output impedances the same) is to reduce the value of the volume pot to one-quarter of what it was, and insert a series resistor with the input to bring the impedance back up. Now, all the resistor's noise is attenuated along with the signal, and the active gain stage sees a lower source impedance to boot. But if it's the active stage that's noisy . . . This of course won't help. |
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 |
Those attenuators you mention ruin the sound (at least they did for me when I tried to reduce the gain between either my source and preamp or between the preamp and amp). They sound like an extra preamp in the chain, taking the life out of things and making the bass muddy. |
If you want to buy my Rothwells or Godenjacks, SE or XLRs,
I can post them for sale on Audiogon. They're sitting in a drawer. They do reduce the gain, though. |
thanks, all, for the EXTREMELY detailed and technical feedback - this is a fantastic community.
Ralph, the performance of your gear is exceeded only by your commitment to this community and your customers.
Al and Kirkus, while much of your exchange was well beyond my comprehension, i do appreciate the education. thanks for taking the time to comment here - i found this to be a very enlightening read.
Rgs92, i'm not sure how to contact you. seems like the 'contact member' feature was not carried over to the new audiogon system.
Thanks again, all, i've now got a few new avenues to pursue.
Scott |
