High output, low volume? Input impedence mismatch?

[bongomania]

A meter won't do it. You can get a close approximation using Thevenin's Theorem http://en.wikipedia.org/wiki/Th%C3%A9venin's_theorem and few small components and a meter, but it's not super convenient. When manufacturers provide the in/output impedance spec, it's not because they measured it at the jack, but because they calculated it since they know all the internal component values.

[SGD Lutherie]

Originally Posted by bdplaid

how does one measure impedance? I don't think my meter will do it. ohms on it seems to be for resistance only.

No need to do that. Here's the specs on that amp:

PIEZO INPUT: -13 dBu / 4.7 M ohms
NORMAL INPUT: -10 dBu / 1 M ohms (PICKUP TYPE: PASSIVE)
0 dBu / 50 k ohms (PICKUP TYPE: ACTIVE)

So, plugging into the "Normal" jack, and then toggling between active and passive changing the input impedance from 1M to 50K. I would not use the 50K with EMG pickups, except the X series, because their output impedance is 10k.

You can also see that the passive input is padded (-10 dBu), so the active should give you more volume, except for the impedance change.

Have you tired the Piezo input?

[bdplaid]

I saw those specs but kind of took them with a grain of salt since I think there's some thing wrong in there.

Let me reason this though... if I read that correctly, it's kind of backwards? I mean, I am accustomed to inputs for *active* being padded something like -10 dB, rather than the active. and the active input would be padded -10dB. So if this is correct and working properly, I take these specs as a problem - as in, no wonder the active input pegs and clips the volume, especially with a hot output like the EMG provides.

the piezo input? actually, no, obedient lad that I am. Hmmm....

In the meantime, I have re-coverted the bass back to active. and it sounds so much better, it'll stay that way. I am convinced that the basses aren't the issue, but the amp is. and maybe it's a design issue rather than something broken. which I think i said a million posts back. maybe.

[wcriley]

Those specs are in dBu. They likely show how much voltage the input is designed to (nominally) handle.
0 dBu = .775 volts.

[khutch]

If the dBu numbers on your amp are the sensitivity figures (the input required to generate full output at max gain for example) then it is the active input that is padded as you would expect. It takes less signal to generate full power on the other inputs.

Yes, you do have an impedance mismatch and a pretty severe one at that. However a 2k Ohm or so preamp will have no trouble driving 50k, 1M, or 4.7M. Impedance mismatches are an issue primarily when a high impedance tries to drive a low impedance and in RF circuits which are often designed for maximum power transfer and that means the impedances must match and in high speed digital circuits where energy reflections from mismatched impedances disrupt the signal waveforms. Driving a high impedance with a low impedance will not cause the issues you are having.

Ken

[SGD Lutherie]

Originally Posted by khutch

Yes, you do have an impedance mismatch and a pretty severe one at that.

How do you figure? It's a 2k output impedance driving a 50k input impedance. That works fine. It's a low Z feeding a high Z. That's what you want.

However a 2k Ohm or so preamp will have no trouble driving 50k, 1M, or 4.7M.

Exactly. Roland knows what they are doing.

Impedance mismatches are an issue primarily when a high impedance tries to drive a low impedance...

That's what I said above. Plug your passive bass into a low Z input on a mixer, and you will hear an impedance mismatch.

[BogeyBass]

its either the amp or the active preamp or pretty much both

2k seems really high for a active device, and 50k seems like its a little low.

depends on the active preamp design if its direct from a active component, or if its a active circuit still driving passive tone/volume controls.

lots of poopy designs out their so most likely both.

Now I get to sound like a real stick in the mud and say this is why active preamps never have done anything for me. Old fashion passive bass and good old 1meg input.

I dont see a need for a bunch of circuits to be squeezed into a bass running on a low voltage single ended power supply. When the same circuits and function can just be included in the amp on a much better power supply.

[bdplaid]

This bass is passive Bart pickups, CTS 250K passive blend, CTS 250K volume into EMG BTC active tone circuit. I think EMG knows what they're doing. But me, on the other hand...

Generally I prefer passive, but it depends on the bass. Modulus wouldn't be Modulus without active electronics. This bass just sounds better with active than it does passive (as we all know, it's been both several times now).

Every active bass I've pumped in this amp has had the same issue - clipped the input, or just didn't have the right amount of output.

[dmusic148]

Just for giggles, once you've restored the active electronics plug a cable into the bass and put a meter across the other end set to DC volts. What do you see?

[bdplaid]

I get nuthin. 000. i think I have a crappy meter.

[dmusic148]

Originally Posted by bdplaid

I get nuthin. 000. i think I have a crappy meter.

Nah, you SHOULD get nothing. That's what I was looking for.
Anyway I see you have this issue with this amp with any active bass, so, not your bass.

[bdplaid]

probably the only time in history when zero was a positive.

[khutch]

Originally Posted by SGD Lutherie

How do you figure? It's a 2k output impedance driving a 50k input impedance. That works fine. It's a low Z feeding a high Z. That's what you want.

Yes, I said that works fine because in the audio world low impedance sources are compatible with high impedance loads but it is an impedance mismatch nonetheless. Impedances do not match unless they are equal. In this case the mismatch is 25:1 and that would be disastrous in many applications whether the source impedance was higher or lower than the load impedance.

Ken

[bongomania]

Hm. Can you give a specific instance where that is both (a) true, and (b) applicable in this context?

[Smilodon]

In which order do you plug in your cable, and is it a mono cable?

Sometimes if you plug in at the amp before you plug in at the bass you could send DC to the amp as you plug inn the bass. (as the contacts will touch the pins used for grounding the preamp in the bass.

Sending DC to a input could cause it to "peak" and stay peaked for a while before it "recovers".

[bumperbass]

Originally Posted by khutch

In this case the mismatch is 25:1 and that would be disastrous in many applications whether the source impedance was higher or lower than the load impedance.

Ken

Seriously, please explain this. We're speaking of voltage, right? Using your theory, I can't use an active bass with a passive input because it would be "disastrous"?

[bdplaid]

Smilodon: I think amp first with volume down, then bass. But I don't think I have strict routine for that. In other words, I don't know.

[Smilodon]

Originally Posted by bdplaid

Smilodon: I think amp first with volume down, then bass. But I don't think I have strict routine for that. In other words, I don't know.

Try bass first, then amp. Just to be sure.

[khutch]

Originally Posted by bongomania

Hm. Can you give a specific instance where that is both (a) true, and (b) applicable in this context?

(a) Already done in my first post.

(b) I've already said that it does not matter in this case. This is the third time now.

Originally Posted by bumperbass

Seriously, please explain this. We're speaking of voltage, right? Using your theory, I can't use an active bass with a passive input because it would be "disastrous"?

No, I have already said, and this is now the fourth time, that it will not be disastrous in this case.

So why bring it up?

Impedance matching is a very specific engineering term that most of you are misusing: matched impedances are EQUAL. Actually that is not quite right, to be precise they are complex conjugates but those who have not understood what I have been saying up to this point certainly will not understand that! I bring it up because it seems like the OP does understand that, at least to an extent, and therefore is concerned that the MISMATCH in his impedances could be the source of his problem. So I have been trying to reassure him that this is not an issue in this case. Meanwhile I seem to have totally confused the rest of you because you do not understand that in audio applications (but not all others) MISMATCHED impedances can be COMPATIBLE. At audio frequencies and for reasonable sized equipment layouts when you drive a high impedance amplifier input from a low impedance preamp output you have COMPATIBLE set of impedances, NOT a MATCHED set of impedances.

In old analog telephone systems reasonable care was taken to maintain a 600 Ohm impedance throughout the system, in other words they were matched. True impedance matching does not become an issue until the system becomes comparable in size to the wavelength of the frequencies you are using. Audio frequencies have wavelengths that are many miles long so even HUGE outdoor concert venues do not run into impedance matching issues as long as you are careful to ensure that low impedances drive high and not the other way around. Miles long analog telephone connections, on the other hand, needed to be matched because they were as big as a wavelength and bigger, as do RF and high speed digital systems were wavelengths can be as small as a few millimeters.

Ken

[bdplaid]

Ken: Being the OP I might as well chime in. Having friends that are , like yourself, bona fide engineers, I have learned that there are different ways of describing things - some are precise and others are not. Engineers are precise, and, not being an engineer, I had to learn that language. so now I see what you are saying. you are making a precise distinction between "matched" and "compatible." However, I did misspeak in my OP - I really meant "compatible" rather than matched.

I'm remarkably untalented when it comes to electronics. Over the years I have devised ways of understanding things in layman's terms (at least in my own personal head). Because on the surface I could see no incompatibility between the Roland amp's input gain and the bass's output, I looked to areas where my understanding is weak: impedance. About the only thing i get about it is that Low z = low noise and spurious signal rejection. I've never understood WHY a Low Z signal needs to drive a Higher Z signal. I just viewed it as a push-pull kind of thing - High Z needed to "pull" low Z. yes, wrong and stupid, and I already said I didn't get it.

In the case of this amp, because the active input lowered itself to 50K Z, and it just didn't behave like virtually every other amp on Earth, I was wondering if the difference in behavior wasn't caused by impedance issues. apparently not. but i can't help but think it has something to do with it. My suspicion is that my bass with active will do better into the *passive* side of things, if I pad the amp input down. remember that the specs for this amp actually *raise* the input gain for active electronics, as well as lower the Z, so I see that gain change as a problem right there.

anyway, I hope this explains a little better where I was coming from.

Greatly appreciate your (and others) comments, technical and otherwise.