Deep bass (<40Hz) and the Quickie

[Dr. Toobz]

Now that I've upgraded my S.E.X. amp to the "enhanced" version, I've begun to notice something interesting with the Quickie that I'd like to fix. The S.E.X. amp (sans Quickie, straight from the DAC) now has really deep bass, such that I can actually feel the lowest notes in some recordings through my headphones. That's really impressive! The problem is that I like how the amp sounds with the Quickie between it and my sources - recordings are more natural, and midrange more vivid and "real" sounding through the Quickie. The soundstage comes forward a bit as well, and is less flat. I'm assuming this is the storied "DHT" sound I hear so much about? The cost, though, is low bass. The Quickie seems to begin its "roll off" sooner than the S.E.X. amp, so although I'm gaining warmth and sweet mids, I'm losing the impact of low notes (and thus, one of the biggest the benefits of my new MQ irons) when inserting it in my chain. In fact, I've noticed that bass notes are less clean with it in the signal path, which I'm attributing to the loss of the lowest frequencies.

The question is, are the coupling caps at fault here, or is it the chokes? With 3.7k/150H plate chokes, I'd assume the parafeed caps would have to be slightly bigger than the stock 2uF in there now. Larger caps would give a better damping factor and lower -3dB point. But, I also recall that plate chokes tend to be better at loading the tubes at higher frequencies, and are worse at low ones than something like a CCS. So, I'm wondering if I should bother a) raising the value of the coupling caps to 3 or 4uF, or b) just get rid of the chokes altogether in favor of the PJCSS? For the latter, I'm assuming a 2uF parafeed cap would still be flat down to sub-sonic frequencies?

[Paul Birkeland]

I would give the CCS a try.  Chokes will certainly impact the frequency response of the circuit.

[Dr. Toobz]

Considering that the dynamic impedance of a plate choke rises with frequency, is a CCS mainly constant across a wider range of frequencies? If so, I would assume there would be less interaction with the circuit (and therefore, a more consistent sound)?

I should receive a PJCSS within a week or two, as I emailed Eileen about one but was told the PC boards weren't in yet.

[Paul Joppa]

The 150 henries should give extremely deep bass, down to 2-3Hz. The parafeed cap is loaded with the 100K input of the SEX, also plenty deep. I don't remember, do you have the Speco in place for headphone listening to the Quickie? That would kill the deep bass.

[Dr. Toobz]

I excised the Specos from the Quickie a while ago, once I discovered that I would get a lot more use from it in my main rig than as a standalone headphone amp. So, there shouldn't be any reason why the bass would seem different, a least not technically speaking. However, I can reproduce it with almost any recording - switching from the DAC -> Quickie ->S.E.X. to the DAC -> S.E.X. arrangement sounds much cleaner, with less midbass bloat and low bass notes that actually "move" the headphones. It seems to be the last 10 or 15Hz of "hearable" bass that is effected on the low end. For example, I was listening to an electric Herbie Hancock recording from the early 70's that has some strange sort of synth in it towards the end of one of the tracks. There's a sustained bass synth note that just sounds like a very deep rumbling and is pitched at a G0 or A0 - so probably about 25-27Hz. Without the Quickie, the S.E.X. amp puts out enough deep "rumble" to physically move the Beyers on my head. Inserting the Quickie gets rid of that effect, and seems to round/bloat other bass notes.

I could just get rid of the Quickie from the chain, but I do like having the extra gain, and as I mentioned before, the Quickie warms up the mids considerably and puts things like pianos and saxophones closer towards the listener while leaving high frequencies like percussion instruments further back. The net result is a more natural, realistic sound with a more immersive soundstage than the S.E.X. amp alone. In fact, going back to the S.E.X. amp by itself almost sounds like going from the stereotypical "tube sound" to solid-state - the S.E.X. amp alone seems to have a blacker background, faster treble, and a pushed-back soundstage without the Quickie in tow.

I'll see what the PJCSS does once it arrives, but my hunch is that it may be an improvement over the plate chokes despite similar benefits.

[Keim]

Have you tried different tubes in the Quickie?  I don't have a QUickie, but tuberolling made big differences in bass with my Foreplay 2.

[Paul Joppa]

Long shot here - when the Quickie is in the circuit, is the SEX gain turned all the way up? I'm just wondering if the Quickie is working near its maximum output or not.

[Dr. Toobz]

Have you tried different tubes in the Quickie?  I don't have a QUickie, but tuberolling made big differences in bass with my Foreplay 2.

Not yet - but I will soon.

[Dr. Toobz]

Long shot here - when the Quickie is in the circuit, is the SEX gain turned all the way up? I'm just wondering if the Quickie is working near its maximum output or not.

Interestingly, it's more the other way around, i.e., the Quickie is turned up pretty high (2 o'clock position) with the S.E.X. amp turned down pretty low ( 9 or 10 o'clock). And I think that's the source of the problem, per my fiddling with it this evening. When the Quickie is turned up 50%, the S.E.X. amp sounds nearly the same as when it's not in the path in terms of how far "back" the soundstage is and how clear bass notes are. When the Quickie gain is raised, the soundstage starts to compress and come forward, the bass becomes "bloatier," and the overall sound is warmer but less clear. So, perhaps this is an example of triode compression - i.e., the little driver triodes on the 6DN7's start to become swamped and add harmonics/distortion to the signal, causing a change in focus and overall sound? I hear no obvious distortion at that level, but my understanding of tubes is that they start to have a different spectral signature prior to clipping. Is there any validity to this theory?

Either way, getting the chokes out of the picture will be interesting....

[Paul Joppa]

I think it's most likely that the Quickie is overloading rather than the SEX driver stage, but the effect is the same - more second harmonic from the overloaded stage makes a warmer but bloated sound and a more forward sound stage. I would guess the deep bass is getting soft clipped which is losing its impact. Tube soft clipping is much softer and more acceptable than the hard clipping of high-feedback amps, so it's harder to hear it as "distortion".

Because both the Quickie and the SEX amp have very low noise/hum, this gives you the interesting opportunity to tune this effect to your liking. I'd go for the lowest distortion with spectrally rich music such as symphonic or big band, and bring in the richness for sparse music like solo guitar or string quartets, where you want the sound stage to move more into the room. But experiment and see what floats your own boat. We always say it's about the music, but really good music is about emotional communication and it's really about what works best to grab hold of your emotions.

[Dr. Toobz]

I think it's most likely that the Quickie is overloading rather than the SEX driver stage, but the effect is the same - more second harmonic from the overloaded stage makes a warmer but bloated sound and a more forward sound stage. I would guess the deep bass is getting soft clipped which is losing its impact. Tube soft clipping is much softer and more acceptable than the hard clipping of high-feedback amps, so it's harder to hear it as "distortion".

Because both the Quickie and the SEX amp have very low noise/hum, this gives you the interesting opportunity to tune this effect to your liking. I'd go for the lowest distortion with spectrally rich music such as symphonic or big band, and bring in the richness for sparse music like solo guitar or string quartets, where you want the sound stage to move more into the room. But experiment and see what floats your own boat. We always say it's about the music, but really good music is about emotional communication and it's really about what works best to grab hold of your emotions.

I've actually viewed the arrangement this way already - i.e., I can play around with the "type" of sound I get by manipulating the gain on the different amps, esp. the Quickie.

For example, adding a lot of "tube sound" to things like Bill Evans piano recordings and 50's/60's jazz in general gives a warm, emotional sound that really draws the listener in. Saxophones and pianos really stand out with the extra harmonics, and electric guitars do to some extent as well. The effect is not as impressive with things like Pink Floyd or The Who, where I've noticed the complicated arrangements and addition of keyboards leads to things getting obscured by the extra "warmth." Believe it or not, I actually prefer the less accurate, warm and fuzzy sound on most recordings, which seems to be the antithesis of high-fidelity reproduction! Oddly enough, things just sound more natural that way, especially digital recordings.

If the Quickie is most likely the stage that is adding a lot of 2nd harmonic distortion, that leads me to wonder how much the 6DN7's can take before they overload. It seems like I can get the sound to start compressing in a similar way, but it's not until the S.E.X. pot is turned up almost all of the way. I'd assume this means that the S.E.X. has a decent amount of headroom? I've also noticed that even with the Quickie turned up all of the way, the S.E.X. doesn't distort.

[Dr. Toobz]

I think it's most likely that the Quickie is overloading rather than the SEX driver stage, but the effect is the same - more second harmonic from the overloaded stage makes a warmer but bloated sound and a more forward sound stage. I would guess the deep bass is getting soft clipped which is losing its impact. Tube soft clipping is much softer and more acceptable than the hard clipping of high-feedback amps, so it's harder to hear it as "distortion".

Because both the Quickie and the SEX amp have very low noise/hum, this gives you the interesting opportunity to tune this effect to your liking. I'd go for the lowest distortion with spectrally rich music such as symphonic or big band, and bring in the richness for sparse music like solo guitar or string quartets, where you want the sound stage to move more into the room. But experiment and see what floats your own boat. We always say it's about the music, but really good music is about emotional communication and it's really about what works best to grab hold of your emotions.

I wired up a sort of pass-through/tape monitor switch on the Quickie, going from the selector switch to a DPDT switch with the other side  going to the parafeed cap/resistor (which I disconnected from the RCA jacks), and the center going to the output RCA jacks. This has allowed me to compare what things sound like with the Quickie in the circuit vs. passed through. Interestingly, with the gain on the Quickie set to a point where the volume is the same on both sides of the switch (i.e., like not having the Quickie at all), the sound is very nearly the same. The only difference I can detect (and a slight one at that) is that the S.E.X. amp sounds a bit "cleaner" and "blacker" on its own, and that maybe the very lowest notes roll off sooner with the Quickie. Otherwise, they're almost identical, esp. with good Amprex tubes. I think the extra circuitry in the signal path is what leads to a tiny loss of clarity (my mantra is to keep it simple - there's no point in having more electronics in the signal path if you don't need extra gain, which is why I switch the Quickie off for loud recordings). Things only get really different when the Quickie gain is turned up - basically, what I reported before (decreasing sound stage depth, warmer sound, rolled-off bass). It's interesting that tubes give you this option of "fattening" up the sound, even prior to getting nasty and distorted like a guitar amp. Some recordings do benefit from the extra warmth, and some don't - that's why a pass-through switch seems to be a good idea.