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I like my tone darker for jazz too but when playing rock and fusion, I like a little more highs. I'd prefer to be able to roll it off on the amp. That's the problem I have with the quilter is that cannot do that but with the capacitor trick, the problem may be solved!
Originally Posted by nevershouldhavesoldit
Incidentally, the tweeter on the henriksen is not efficient enough to really influence the sound a lot. I thought i could use it for acoustic guitar but because of the inefficiency of the tweeter, it doesn't do as good a job as I had hoped. I'd rather have an efficient tweeter with a vol control like many of the bass cabs have.
I'm not sure how they rate it and obviously each manufacturer has a different system of measurement but it appears to be 1/2 the volume of the tonker. I'm thinking about maybe getting another quilter TB and doing the cap trick with it...That’s a separate issue. The little SICA is less sensitive than the EMI, assuming the graph you linked shows the output at 1 meter from a 1 watt input. But it handles a lot of power and makes giggable SPLs even with amps like the Superblock. I’ve used mine with my DV Mark EG250, and it makes a lot of sound.
That EMI has a very audible peak from about 1.6kHz to 5kHz after an equally audible dip from 1 to 1.6kHz. It probably sounds quite bright in any enclosure.
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03-09-2023 07:08 PM
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Actually, there’s an industry standard. When determining sensitivity, SPL is supposed to be measured at 1 meter from the speaker driven with an audio signal of 1 watt. I think the original standard frequency was 1 kHz, but some use random noise in the band from 300 to 3kHz and others use white or pink noise. The graph I got from the SICA website states that it was measured with 1 watt at 1 meter. Originally Posted by jzucker
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On SICAs: The 6L 1.5SL version used on Metro 6.5GP+ and BG is a bass/midrange pro audio speaker. Some sources call it a woofer. Strangely, the response curves posted by the factory and other sources show quite some variation at the treble end. On some, there's a sharp drop around 5 kHz, whereafter the speaker comes alive again. The current factory curve posted by Never suggests otherwise.
About a year ago my stock of said SICAs was wearing thin, and I knew the next shipment wasn't due until September. I found a stash of 16 NOS SICA 6L1 1.5SL versions from Sweden and got through the summer with those. They sounded perfect. Most of them were installed in what I called a Metro 6.5BG JR which, despite being shorter than the "real" BG and lacking the internal felt lining, sounded almost identical on both guitar and bass.
The speaker on Never's Metro 6.5BG is actually one of these 6L1s. It appears there's been more than one variant even under this name, with different-looking treble ends in the response curves. What matters to me is that they all start from around 60 Hz and none comes with an annoying degree of top end sizzle.
The SICA 6D 1.5SL dual-cone version has the same 130 W Neo motor but nominally starts from around 100 Hz and extends to 15 kHz. In a Metro 6.5FR II enclosure, it handles even bass quite ok. Again, different curves and even sensitivity ratings (91 vs almost 94 dB - a significant difference) have been posted for this speaker.
What I think is worth pointing out is this: Stated frequency ranges are based on cutoff points (minus X dB under the average SPL) - arbitrary in the sense that there's life outside the range. Basses, mids and trebles can be EQ'd in or out. Plus, in e.g. guitar amplification, the player intuitively adjusts to what comes out from the speaker and just hits the notes harder or softer, whether inside or outside the "usable range."
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To return to the original experiment. I asked Quilter what is the impedance at the FX loop connection. The answer is like resistors, 1k going out and 47k return when the send side of the loop is in use. That makes the -3dB frequencies with the the three capacitor values to be 3k4Hz, 1k1Hz and 480Hz.
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where can I get the 1/4 phone plugs with the terminals and do they make a right angle ver?
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This capacitor trick doesn't seem to work on my Quilter MicroPro amp. I don't think I'm doing anything wrong. I have a small bag of 473 capacitors. I tried with a bunch of them in case some of them were faulty. When I insert the plug to the f/x receive, it cuts out the circuit and the amp goes quiet. It's possible that MicroPro's are wired differently.
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Regular phone plugs will work, if you use a small (physically) capacitor. If you want something other than that, Mouser has lots of plugs, of different characteristics. I buy most of my electronic stuff there. If they don't have it, I don't need it.
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If you search for "1/4 phone plug with screw terminal" several different ones pop up on Amazon. The terminals on all of them appear to be at a right angle to the 1/4" plug. For example, this one. Originally Posted by jzucker
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From your numbers, I suspect you used the standard formula for the "cutoff point" of a simple RC filter (1/2[pi]RC) under the assumption that the output impedance was the resistor in a simple RC filter with the capacitor. Unfortunately, it's not that simple. This is the filter to which the formula I think you used applies: Originally Posted by aquin43
But output impedance is not static resistance - it's frequency dependent resistance. You can't use it in the formula for a simple low pass filter, which is based on a single fixed resistance and a single capacitance as in the diagram above. The capacitor between the signal path and ground is functionally a variable resistor with infinite resistance to DC. As the frequency of the signal rises, the resistance of the capacitor to signal flow drops. The "cutoff point" is the frequency at which capacitive reactance (the term for the resistance of a capacitor at a given frequency) becomes low enough for signal to begin to flow through the capacitor. As the frequency goes above this, the resistance drops further and more current can flow. So that capacitor we stuck between the signal path and ground is behaving as a variable resistor whose value goes down as the frequency of the signal goes up.
The output impedance of the effects loop send is not a fixed resistance like the resistor in the RC filter diagrammed above, so the simple formula 1/2[pi]RC is not applicable. The circuit is much more complicated than this anyway. We placed another capacitor among multiple interacting sources of resistance, capacitance, inductance etc. Determining the mathematical effect can be very complex, depending on the circuitry. The easiest way to do this for our little experiment would be to plot the 4 curves with a consistent signal of fixed composition and intensity, overlay them, and determine the exact differences between each pair across the fequency spectrum.
This is all great fun, and knowing stuff is a reward in its own right. But I'd rather practice than go any further with this one.
Last edited by nevershouldhavesoldit; 03-10-2023 at 05:04 PM.
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Thanks now all I need is a toneblock 202... Originally Posted by John A.
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I think you can probably find one of those without me Originally Posted by jzucker
It'll be interesting to get your take on this when you do.
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I had one before but hated it with the toneblock 1x12 and BN300 speaker. That cab is too small IMO and the insert that holds the amp takes up a big amount of space inside so it doesn't have a very big sound. And the BN300 speaker was not my favorite. I replaced it with a jensen tornado and used that rig for several gigs and it was one of the worst sounds I've ever had! Originally Posted by John A.
I think you can probably find one of those without me It'll be interesting to get your take on this when you do.
But I think it was the speaker(s) and cab. The amp sounded good but had that "bright switch" effect that couldn't be turned off. But it sounds like the .05ufd cap trick may solve the problem...
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I tried the 101R with the 1x10" . I didn't spend a whole lot of time with it, but didn't particularly like it. I tried an Aviator Cub more recently and liked that much more, especially the tweed sound. But I'm guessing you would want more power than that. Originally Posted by jzucker
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The FX output is almost certainly a 1k resistor from an op amp output and the input will be either a 47k into a shunt feedback op amp or a 47k in parallel with a series feedback op amp. Why would Quilter make it more complicated? Is it more complicated? Do you have a schematic diagram? If you have, it is a trivial matter nowadays to determine the response. Originally Posted by nevershouldhavesoldit
From your numbers, I suspect you used the standard formula for the "cutoff point" of a simple RC filter (1/2[pi]RC) under the assumption that the output impedance was the resistor in a simple RC filter with the capacitor. Unfortunately, it's not that simple. This is the filter to which the formula I think you used applies:
But output impedance is not static resistance - it's frequency dependent resistance. You can't use it in the formula for a simple low pass filter, which is based on a single fixed resistance and a single capacitance as in the diagram above. The capacitor between the signal path and ground is functionally a variable resistor with infinite resistance to DC. As the frequency of the signal rises, the resistance of the capacitor to signal flow drops. The "cutoff point" is the frequency at which capacitive reactance (the term for the resistance of a capacitor at a given frequency) becomes low enough for signal to begin to flow through the capacitor. As the frequency goes above this, the resistance drops further and more current can flow. So that capacitor we stuck between the signal path and ground is behaving as a variable resistor whose value goes down as the frequency of the signal goes up.
The output impedance of the effects loop send is not a fixed resistance like the resistor in the RC filter diagrammed above, so the simple formula 1/2[pi]RC is not applicable. The circuit is much more complicated than this anyway. We placed another capacitor among multiple interacting sources of resistance, capacitance, inductance etc. Determining the mathematical effect can be very complex, depending on the circuitry. The easiest way to do this for our little experiment would be to plot the 4 curves with a consistent signal of fixed composition and intensity, overlay them, and determine the exact differences between each pair across the fequency spectrum.
This is all great fun, and knowing stuff is a reward in its own right. But I'd rather practice than go any further with this one.
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To the best of my knowledge, there isn't any publicly available schematic. So I don't know what's inside a Superblock. The output impedance of an ideal op amp is zero. But most real world op amps have an output impedance of at least 100 Ohms over much of their range. Some, especially CMOS/RRIO ones, are in the 500-800 Ohms region because they're voltage controlled current stages (and MOS conductance is poor). With typical loop gains, this still results in output impedances <<1 Ohm, at least for low frequencies. To get that 1 kOhm value that Quilter specs, I'm assuming that the SB has a simple resistor in line with the output of the preamp stage that drives the effects send. But as far as I know, the output impedance is not fixed across the entire frequency spectrum - that's why they call it impedance. Originally Posted by aquin43
I may be wrong, but I don't think the output impedance of the send line can be plugged into the equation for a simple RC filter to come up with the cutoff point for a cap that bridges it to ground. And I doubt that there's a simple 6 dB/octave slope on the resulting curve. The plots I posted support this belief. Hopefully, we have a forum participant with more knowledge of this than either of us who will provide a definitive answer.
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i'm still curious about which value cap works the best through a wider range speaker like an EV. I wish that terminal version of the 1/2 plug was available in a right angle form factor. Of course, I don't even have a toneblock at this point but i'm very interested... Originally Posted by nevershouldhavesoldit
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Here you go
Mouser Electronics - Be back soon...
Right angle plug with screw terminals, room for a small cap.
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That link appears broken. Here's a working link to the Switchcraft 225, which is a great choice. Originally Posted by sgosnell
The only way to know how a given cap will work with any amp and speaker combo is to try it, Jack. I know of no formula that can be used to calculate the specific response curve resulting from the addition of one grounding cap to a complex circuit and system like these. You can project the likely effect of a simple isolated filter on the signal passing through it, but once it's interacting with the circuitry and devices ahead of it plus those following it in a signal chain, all bets are off. A very long old fashioned guitar cable is probably the most predictable source of added capacitance to ground that you can find and try. The longer the cable, the darker the sound will be.
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I don't have a Superblock but I just measured the output impedance of a Toneblock 202, which has a similar specification. It is 1k resistive over the whole guitar audio range.
I determined this by setting up a frequency response measurement at the FX send. This was essentially flat. Loading with a 1k resistor caused the level to drop by 6dB, while the frequency response remained the same.
I would guess that the Superblock uses the same or similar circuitry.
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I’m not sure the Toneblock uses the same circuit design as the Superblock - they’re very different devices. Again, we don’t know the circuit design so we can’t know the proper method for measuring output impedance. You might be right, but I don’t think so. And I don’t think your method measures output impedance accurately. A broadband audio signal is neither an appropriate source for measuring impedance nor an indicator of it. Full range output impedance is a continuous function of resistance vs frequency, not a single number. When a single number is stated, it’s for a single frequency. Originally Posted by aquin43
The general approach to measuring output impedance is this:
- Put a sine wave generator on the input of the source to be measured, and set it to deliver a signal somewhere in the middle of the operating range of input voltage for the test device.
- Connect the unloaded output (in this case, the send jack) to a scope and adjust the scope’s input sensitivity to display the signal over a large part of its vertical space.
- With no load on the send jack other than the scope, measure the output level on the screen.
- Load the send jack with a pot (or a decade resistance box) and adjust until the level on the scope drops by half. [Half power is -3dB, not 6.]
- Measure the pot's resistance at half power (which is 3dB below the unloaded output level). This is the output impedance at the test frequency.
You can also do this with a VTVM or other high input impedance voltage measuring device. A VOM is not sufficiently high impedance to leave the source virtually unloaded. But measuring op amp output impedance is not always done this way, eg some circuits require separate measurement for sourcing and sinking. Just sticking a 1k resistor in series with the output of an op amp doesn’t make the source impedance 1k.
Last edited by nevershouldhavesoldit; 03-12-2023 at 11:40 AM.
- Put a sine wave generator on the input of the source to be measured, and set it to deliver a signal somewhere in the middle of the operating range of input voltage for the test device.
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This is somewhat removed from music, but:
You can't measure impedance with a resistive load unless the impedance is a pure resistance so your method won't work in the general case.
The logic of my measurement is this. If I load the output with a 1k resistor and the voltage level drops by a half (-6dB), I will know that the output impedance is 1k and that it must appear to be resistive. I measure this over a wide frequency range and. lo and behold, the level drops by half over the whole range, i.e. the frequency response stays the same but at half the level. I can safely say that the output impedance looks like a 1k resistor over the whole range.
One useful feature of my measurement set up is that it can normalise the response at any frequency so that the measurement can be made independent of level. In this mode, when I connected the 1k load, nothing appeared to change, showing that the effect of the load was the same at all frequencies in the range.
Only a 1k resistive impedance will give a 6dB loss with a 1k resistor load, Obviously the output is an op amp stage with an output impedance of a fraction of an Ohm, shielded from the outside world by a 1k resistor which is large enough to protect the op amp but small enough to be unaffected by the normal loading of an effect input.
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First, you're starting with an error. A doubling of output power is a 3 dB change, not a 6 dB change. Originally Posted by aquin43
Second, "my" method is actually a standard method endorsed by a huge cadre of engineers, designers, and manufacturers. Here are just a few references to reinforce this: homediyelectronics, sengpieaudio, and songbirdfx. The exact same method is described in textbooks used in engineering schools throughout the world. I didn't make it up. Even when doing it the cheap and dirty way by using music as the signal rather than a sine wave, the method is the same. But what you get is basically a composite of the resistance vs frequency curve that roughly approximates the weighted mathematical mean of the actual output impedance curve. It's not nearly precise enough to use as a factor in a calculation of any filtration effects at the output of the stage.
Third, measuring the output impedance of an op amp is not as simple as you seem to think. and it depends on the design of the circuit. For example, here's a simple method that addresses voltage follower and other circuit designs, and it requires changing the sink current. If you want to dive deeper into this, here's a fascinating discussion from Electronic Design of different models that can be used to determine output impedance. Their summary of the task is useful:
"Generally, the amplifier’s open-loop output impedance can be modeled as a resistor with some frequency dependency:Z(s) = R*(Z(s)/P(s))The precise characteristics of impedance across the desired range of frequency depend on the amplifier’s output stage under test."
The term "open loop" is critical, because without knowing how much feedback is being applied in the circuit, you simply can't assume it's linear. And you can't determine its output impedance with a simple resistive load unless you know that the circuit design permits this.
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I don't know what happened to that link to Mouser. I'll try again.
Access Denied
The link title is munged, but it goes to the plug.
NSHSI linked the same plug, I now see.
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link still broken. Did mouser to under? Originally Posted by sgosnell
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The convention with voltage measurements in dB is to use a notional power at the measurement point, ignoring the actual impedance. This is proportional to Vsquared and makes for the formula 20log10(V2/V1) which gives -6.02dB for a halving in voltage level. Originally Posted by nevershouldhavesoldit
The scheme for measuring impedance using the resistor that produces a voltage drop of one half works for impedances that are resistive or nearly so. This is true of most audio line level and Fx drives because the designers make it so, wishing to avoid frequency response problems when loaded. The method cannot be used generally, e.g. when there is a reactive component to the impedance.
You were suggesting that the impedance might not be purely resistive so that my calculation of the frequency response with capacitive loading would be unreliable.
I used the method of loading with a resistor to determine the output impedance of a similar amplifier but I took it a stage further by measuring a complete frequency response both loaded and unloaded. These were the same, showing that the effect of loading with a 1k resistor was the same over a wide frequency range. The only way that this could happen is for the output impedance to be independent of frequency, i.e. to be an output resistance, so the use of this method is justified in this case. This measurement confirms that when Pat Quilter says 1k he means a resistive 1k.
The output impedance of an op amp stage is easily determined using information from the data sheet. Since op amps have high gain and are used with a high degree of feedback, the output impedance in the audio frequency range is almost always so low compared with the other impedances in the circuit that it can be ignored completely. For example, a TL072 with its 190 ohm output resistance, when in a stage with a gain of 5 will have an inductive output impedance of less than 0.4 Ohms at 1kHz. This would rise to under 4 Ohms inductive at 10kHz, still negligible compared to 1k, even assuming that the buffer gain were so high.
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