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Monitoring the signal going into a non-grounded speaker?

Without differential scope amplifier/probe, audio out is missing something in the bass range about 70-80 Hz. Line-out signal faithfully follows the bass guitar input signal . Shure microphone, capable of those frequencies, monitoring the sound from the speaker, does not match the input at those sort of frequencies. But is it a failing of suspension or something of the speaker or some inductive/capacitive interaction with the amplifier when loaded with the speaker? Not yet monitored the main rails to see if dipping on this bass amp. I will then be subbing the speaker, as a test, but it still would not determine whether an amp or a speaker problem , speaker is original to the combo. Any simple component to put in series or parallel with the speaker to check if an output matching problem ?

Reply to
N_Cook
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Does this amp have a graphic equaliser? The line out may be pre graphic, and one graphic fader may be faulty.

Gareth.

Reply to
Gareth Magennis

something

frequencies,

the

the

speaker

Volume , tone controls etc are in the DSP sextion and as line out is fine , I assume not a problem in that piece of magic.

Reply to
N_Cook

Is this a Line 6?

Gareth.

Reply to
Gareth Magennis

Then get a differential scope. You have one in your computah in the form of the stereo input of your sound card. Just install an oscilloscope emulator program. Put some large value resistors in series with the inputs so you don't blow anything up. Favorite software: You'll also find it handy when troubleshooting constant voltage speaker system.

Incidentally, looking at my pile of about 15 assorted scopes (dead and alive), the only one that doesn't have a differential input is an ancient Heath Vectorscope. Finding two scope probes that are matched and/or haven't been broken by my mishandling is a bit more difficult.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

Battery operated (AC) DVM? Or the green/yellow wire in the scope plug...

Reply to
Geo

NEVER do that.

Reply to
tm

I guess I should offer a few warnings to prevent the usual disasters. I sometimes use a sound card scope when sniffing around switching power supplies, where the input section is usually ungrounded. It's kinda difficult to see the waveforms due to the limited bandwidth of the sound card, but I have a good imagination. However, there are a few precautions.

  1. Use a USB sound card dongle and not the device on the computah motherboard. If you do something wrong, and burn up the motherboard sound chip, you will not be very happy. A dongle is essentially a throw away device. It's also nice to have a long extension on the USB cable, instead of a long cable on the probe.
  2. Use a protective resistor in series with the input cable. It protects against overcurrent, but also protects against idiot errors, such as unplugging the 3.5mm jack while the probes are connected to the AC mains. The jack will be shorted on its way out, and will make a nifty spark.
  3. Build an attenuator if you're playing with high voltages. My guess(tm) is that the sound card input can handle about 5V peak to peak. 10:1 for audio is fine. 100:1 for high voltage. Also, don't count on the input capacitor, which is probably only good for about
15VDC. 
--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

Can you be more descriptive about what you're doing? If you're connecting a sound card to the line side of a switcher, I can't imagine anything more dangerous...as in smoke/fire/death.

Reply to
mike

Left -------------------------/\/\/\/------> hot

Right ------------------------/\/\/\/------> common

Shield ------------------------------------> ground

(For a switcher, pretend the resistor is a 100:1 attentuator network as I'm too lazy to draw it). As long as the sound card can handle the voltage difference between hot and ground, and common and ground, nothing will blow up. With the 100:1 attenuator, it should be no more than about 5V on the inputs. Incidentally, using a series capacitor for isolation doesn't work as the initial charging current will blow up the sound dongle.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

something

frequencies,

the

the

speaker

bog

I suspect a current transformer would affect any PA/speaker/Zobell resonance/interaction effect. I do have a scope with differential inputs but as its not been used for 15 years I would bet good money on it having at least corroded switch contacts. I may sort out replacement rechargeable batteries for an ancient Tektronix

212 hand held scope This is a Roland Cube Bass 30. There may be a faulty , going open, "shape" switch and may sometime settle into the default at the bottom of the resistor chain which is the Octave Bass setting which , may explain the odd effect
Reply to
N_Cook

A properly loaded currrent transformer can be ten millliohms, or a few hundred microohms. There needn't be any significant effect on the system being measured.

Reply to
whit3rd

/

system

with

it

formatting link

So you would pass a speaker lead through the hole, ignoring the primary sense turns. In the meantime I may as well try a solenoid without pole piece

Reply to
N_Cook

few

a

the

from

designed

formatting link

power

you

nasty

the

I tried a solenoid but not enough output and poor at sub 100Hz. Do you know what the response is like below 100Hz on your pass-thru Tx ? Dug out a couple of microphone Tx and checked they worked down to 40Hz , taking readings, less response but not too much. Then some more important jobs came in. One for the phones out line ( so same f response) and one with series 1K or so over the speaker, should work fed to a normal dual channel scope

Reply to
N_Cook

I tried a couple of physically larger microphone Tx and the response is essentially flat from 2Hz (the bottom limit of my sine gen ) and about 20KHz , rapid tail-off after 30KHz. So will try one or both of them when I get back to this speaker/PA problem. RTFM time : the "shape" switch on a Roland cube 30 bass does not switch in and out the COSM selection. I think there was a problem in this sw and when the wiper disconnects then the function stays with whatever was selected before, the control lines stay high. Incidently if o/c at switch on then disables the amp totally. The speaker/PA is perhaps fine after all , but will try comparing at some point , to at least see if microphone Txs are useful for this

Reply to
N_Cook

Mic Tx and dropper worked well, no difference other than amplitude between line out and speaker input in normal low colouration amp types (perhaps

10Kdropper for a more powerful amp). I will not be subbing the speaker as the owner went through the same thinking process as me, thinking the "shape" switch disabled any type selection. Not only that but he usually uses this amp flat with no reverb or effects, with an acoustic bass. To switch out reverb and flanger etc is a matter of turning the pots fully ACW in this DSP amp. He did the same with amp type following the same reasoning (plus shape off) but ACW selects the octave bass , an effect rather than colouration and the source of the non existent problem.
Reply to
N_Cook

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