Removing limitations
Join Our Community Subscribe to Paul's PostsI had written in yesterday’s post the limitations of 30 or so steps of mechanical stepped attenuators concerns the switch itself. It’s near impossible to add too many more switch contacts in the small space available.
The limited number of steps isn’t the only drawback. Adding remote control requires a motor to do the turning, and we’re still stuck with large level jumps between steps.
But the idea of using individual resistors to control level in steps is still a great sounding option. How to get more steps when mechanical means don’t permit it?
Eliminate the restriction of the mechanical switch.
If you’ll recall our first posts on input selection in a preamp you’ll remember we had this very same problem. Mechanical input selectors are difficult to remote control. We use an electronic switch instead—a relay, or its FET equivalent.
Once you figure this out—using an electronic switch instead of a mechanical one—a new world of possibilities come into play. You’re no longer restricted on the number of steps and remote control is a cinch. Two birds with one stone and you’ve not sacrificed anything.
This is what we do in the BHK preamplifier and how we achieve 100 steps in 1/2dB increments. A combination of relays and FET switches manage expensive, great sounding resistors control ing level. The results are near perfect.
Still expensive and challenging for designers to build and implement, but problems solved.
I was going to move on and write how similar results can be achieved without such expense—the innovations of the new Stellar Gain Cell DAC—but many have asked for one more post about outlier technologies for level changing: light resistors and transformer level shifters.
Tomorrow.
Paul, if your “Since pot’s legal” headline was a grabber – surprised you overlooked Buddy Holly’s “Not Fade Away” opportunity.
Many high-end companies begin their preamplifier marketing story with ‘machined out of a single billet of aluminum’ which indeed is important and expensive, however the primary function of a stereo preamplifier beyond their gain stages is source selection and level control.
A preamplifier is essentially an adjustable line amplifier, I think we can all agree that it isn’t always or necessarily the part or device, but the circuitry that surrounds the part that makes it good, bad or indifferent.
Interesting to note how few high-end manufacturers actually disclose the technology and design criteria of level controls on their websites. We’ve recently discussed Spectral’s essay for uncompromising attenuation, so here’s a brief list in no particular order.
My compliments on your company’s full disclosure technology policy.
Audionet Stern
The volume control is realized using enhanced, discrete Audionet operational amplifier modules with state-of-the-art mica capacitors. Volume controller on a double ball-bearing axis with magnetic ratchet and optical sensing controlled by electronic switches and real-time linearized precision resistor network.
Soulution 725
The volume control uses high-precision, low-noise metal foil resistors, and controls both volume and balance. To block switching peaks during volume changes which may be hazardous to power amplifiers a second volume control with PGA (Programmable Gain Amplifier) is used. Active only when the volume is changed, it is similar to a potentiometer and enables the volume to be adjusted in smooth steps without clicks. As soon as the new desired level has been selected, the unit returns control to the precision resistors.
Audio Research Reference 10
A new volume control with finer step gradation encoder-controlled rotary knob. Volume adjusts the loudness in 103 steps.
VTL TL7.5 Series III Reference
Single fully balanced differential 95-step 70dB discrete resistor relay attenuator for all attenuation functions, using low signal instrumentation ruthenium relays for greatest resolution.
Pass Labs Xs Preamp
99 volume steps, fabulous relays and switches, there are a couple of tricks I can’t talk about, such as how we got this great performance out of the volume control.
Constellation Altair II
Benefits from what may be the most precise, sonically benign volume control ever created. Instead of the usual mechanical potentiometer, we use optically controlled resistors that can be adjusted to an accuracy of 0.1 dB. Because the connection between the control circuit is optical, not electronic, there is no possibility for interference between the control circuit’s microprocessor and the audio signal path.
D’Agostino Momentum Preamplifier
Volume adjustment is achieved via an optical controller and a relay-switched resistor ladder.
Nagra Classic Preamp
Does not release information on their motorized volume control pot, probably either a Noble or Alps. But you have to admit that the jewelry grade chassis and front panel Modulometer are pure sex.
Lamm LL1.1 Signature Tube Preamplifier
TKD 41-step volume control potentiometer of the highest quality available on today’s market.
Ayre KX-R Twenty
Following in the legendary footsteps of the KX-R preamplifier, the Ayre KX-R Twenty lives up to the challenge with dynamics powered by our Diamond output stage and a finesse achieved by eliminating the traditional attenuating volume control with our 60 step volume control, each of 1.0 dB Variable Gain Transimpedance (VGT) amplification circuit.
Rowland Criterion battery-powered Preamplifier
Since 1993, Jeff Rowland has adopted highly accurate optically encoded volume control subsystems, which exceed the performance and stability of more traditional carbon tracks and step attenuators based mechanisms.
Unlike many traditional volume controls, Criterion’s optically encoded volume control maintains the full frequency bandwidth intact across its entire gain range with perfect right/left balance. It never acts as a progressive band-pass filter at lower volume levels.
Dual rate volume control allows precision level adjustments over nearly a 100 dB range. The volume control incorporates a permanently noiseless optical encoder which exceed the performance and stability of more traditional carbon tracks and step attenuators based mechanisms that always maintain its tactile feel, accuracy, and channel balance at all settings.
Unlike many traditional volume controls, Criterion’s optically encoded volume control maintains the full frequency bandwidth intact across its entire gain range with perfect right/left balance. It never acts as a progressive band-pass filter at lower volume levels.
The circuit is velocity sensitive. Single taps on the remote transmitter’s volume buttons–or slow rotation of the main volume control–yield an impressive +/-0.5dB gain resolution, from 0dB to 99dB in 199 identical gain steps. Rapid gain changes in 1.5dB increments are achieved by pressing and holding a volume button on the remote transmitter, or by rotating rapidly Criterion’s main volume knob.
Bryston BP-17
The BP17 utilizes a brand new balanced action software controlled motorized analog volume dial. This approach enables the preamp to be operated remotely while still adhering to fully discrete analog circuits eliminating another source of distortion while maintaining channel-to-channel tracking within 1/10dB over the full range of control.
Boulder 2110
Balanced version of Boulder’s CMOS-controlled resistor ladder analog discrete step volume control is programmable for 0.1, 0.5, and 1.0 dB steps.
Most interesting was the volume control by Ed Meitner in his DAC BIDAT simply controlling the reference voltages. Thus channel imbalances were no issue. Nor did this concept reduce the resolution.
@ dr.goodears
Very illustrative post
Thank you for your time and patience.
Thanks AM for your acknowledgement. One of the more interesting V/C disclosure’s imho was Pass Labs refusal to discuss their secret sauce, “there are a couple of tricks I can’t talk about”. 🙂
Obviously what matters most is actually listening to music played back through the units in various music system environments and configurations.
What do i know? my personal favorite preamplifier in terms of transparency, musicality and actually revealing what the microphone receives, getting closest to the music, still utilizes a variable potentiometer.
Other than the Bryston BP-17 (i think it’s safe to say that Bryston’s best reputation are their power amplifier’s) these preamplifier models are all ‘significantly’ more expensive when compared to Paul’s BHK Signature Preamplifier value.
Is there a recommendation how to set the volume on the DS DAC when the BHK preamp is in the chain, Paul? 100?
It doesn’t really matter since the DS doesn’t lose resolution at any setting. For peace of mind I always set mine at 100. I just sleep better at night.
Paul…try the DS 99 setting. If you listen very, very closely,the 100 setting has a very slight layer of hash/distortion overlaid within the music. I hear this every time I toggle between 99-100. The music set to 99 just sounds so right. Crazy I know,but it is what it is.
I was wondering whether you were leading up to a next gen Gain Cell
I am!
While in a “purest phase”, I ventured down a long, expensive jounrney of line stage preamps. I’m going to date myself, but I started with a Mod Squad Line Preamp and went through several flavors. The one I thought was as closest to a straight wire, source to amp, was a Placette Audio Remote Volume Control. At first I liked it….a lot. After a while though, I lost interest. It was ruthlessly agnostic. I loaned it out to several others and they had the same reaction. Loved it at first, then ultimately grew tired of it. The group had varying sources from tubed phono preamps to solid state DACs.
I have found a great active preamp actually adds something positive (personal preference) to the overall sound that I could not get from a source. It’s counter intuitive for sure.
I have found exactly the same thing and it has always driven me nuts to have found that. But there it is. A great active preamp in the chain beats a DAC direct. Go figure. Usually less is more but in this case…..
No question. Used the Placette passive controller for many years, loving its absolute clarity. But it ran out of gain on a number of discs and gave up some dynamics and punch. No substitute for a good active preamp which actually defines the system.
Hey Paul. What ever happened to the Gain Cell idea? I thought it was supposed to be the cat’s meow of volume controls? It still seems like a great idea.
“Because of the PS Audio Gain Cell™, the GCC Series of products do not attenuate the input signal
and do not “throw away” the unwanted signal. Instead, the GCC Control Amplifiers vary their gain in
response to the user’s front panel or remote commands.”
Tuesday’s post should answer your question. Gain Cells are back!
I said it a few posts ago…
I’ve tried to connect my DSJ (PSAudio dac) directly to the power amp.
I hear a loss of resolution when I turn the volume down, although not as much as with the older DAC (PWD).
So far I did’nt hear a single dac without this problem, despite what the manufacturers and experts say.
For me the preamp is the most important part of the electronic chain.
And my DSJ volume ? A little below 100. Sounds better to me than 100. I agree with Mark-d.
And here I was sure you were going to tell us: “and for our next trick we’ve invented a clever new way of adjusting gain by changing the bias voltage in a way that doesn’t alter the sound.” Some sort of PS breakthrough that would blow the competition away. No resistors at all in the signal path. How disappointing.
I believe the way the Gain Cell works is different levels of amplification are turned on as needed Soundmind.
I have an old SAE preamp, needs new caps “when I get to it,” that I haven’t been able to part with because it can drive anything, and has a nice EQ, though the phono stages are kinda “meh.” Anyway, its step attenuator has a workaround: a second, smaller one switches between -24, -12, 0, +12, and +24 dB. Sounds a bit obtuse, but in practice worked pretty well. Didn’t really need to switch ranges often; just chose from “don’t wake up the wife” all the way to “neighbors call police” and adjusted the main knob to taste. If I ever get rid of it I’ll pull the attenuators for my DJ console.