Double Bass iAmp200 and infrasonics-- FIXED! Kudos to EA

I decided to post this as a new thread because I didn't want it buried at the end of the old one:

(http://www.talkbass.com/forum/showthread.php?t=210002)

A quick recap: I use a RS Solo pickup and noticed wild speaker-cone excursions with my new iAmp200. This resulted from the amp's response to infrasonic frequenncies. Mike Dimin put me in touch with Gary Gibilisco, the amp's designer.

The bottom line: I couldn't be happier! First of all, Gary was enormously appreciative of my detailed analysis of the problem, exhibited a great sense of trust, and an absolute devotion to solving the problem. He was very candid in telling me that EA had heard mention of the problem but had never before had a customer with sufficient expertise to provide them with detailed feedback.

The iAmp200 does have a "roll-off" (high-pass) circuit to attenuate extremely low frequencies. In the case of a double bass with a piezo pickup, it just was not severe enough. Gary immediately issued a call-tag for my iAmp200 and it was shipped back at EA's expense. Within about ONE DAY, Gary sent me a loaner amp which he had modified such that the roll-off began at a slightly higher frequency and using a capacitor with smaller tolerance around its nominal value. It worked! The wild cone excursions were tamed and the desired low-end response (40 Hz and above) was not reduced to any substantial extent.

Given the success, Gary modified my amp in the same way and I received it yesterday. I'm all set. Gary also encouraged me to keep his loaner for a few days so that I could fully compare the two. Well, it only took a short time last night to discover that I could hear no difference.

Kudos to the guys at EA. They stood FIRMLY behind their product and went beyond the call as far as I'm concerned. Customer service just doesn't get better than what I experienced. We in the bass community are, indeed, fortunate to have companies like EA serving us.

Techy note: How does a double-bass with a piezo element produce extremely low frequencies given that the fundamental of the vibrating E-string is around 40 Hz? When you tap a string (as when a finger lands on the string during normal playing), you create an impulse that contains a broad range of frequencies. Normally, the very low frequencies contained in the impulse are never transmitted-- certainly not acoustically through the bass. The piezo element, on the other hand, is a broadband device and it will pass these frequencies. One does not expect them to be passed by the amplifier and speaker either. In my case, the iAmp200 and Wizzy cabinet were quite willing to pass them! Providing the iAmp with a bit more roll-off below 30 Hz cured the problem.

Freddels said:

Is this going to be a fix on all iAmps going forward or only if you ask for it?

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I knew someone was going to ask about that. At the moment, I have no idea. I cannot imagine that making the change going forward would prove unsatisfactory to any user unless that user just likes to see the cone jump around. On the other hand, the benefits of making this a design-change seem self-evident. In any case, that's not my call to make. I leave that to the folks at EA.

Aleph5 said:

I don't remember whether this was discussed before, but what if you just shunted the amp input with a resistor, say, 1/2 to 1/10 the value of the input impedance of the amp? Would there by a deleterious effect besides shifting the piezo's LF cutoff frequency upwards by a factor inversely to the reduction of the input Z (as I'm sure many are aware)?

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I think a switchable capacitor would be more in order.

Mike Dimin said:

The "fix" that works so well for uprights is not so desirable for electric players, especially those that slap. So the question, now, is how can we provide the modified amps to upright players and the originals to electric players?

Mike

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Well, there really seems no good reason to have the amp respond to inaudible frequencies. If they're inaudible, they rob power from the amp whether you play double bass or slap a BG. Now, if it is the case that BG slappers desire the distortion that is brought about by bottoming out the speaker cone, then they will want the infrasonic response of the original design.

Now, the modification Gary made does cause the response below 30 Hz to be attenuated more than in the original design. To the extent that BG players want THOSE audible frequencies to be reproduced, the modification will decrease their intensity by a very small amount. The parametric equalizer would remedy that. They could always be boosted given the Q and center-frequency adjustment on the lowest band of the iAmp.

Frankly, I do not believe there is any good reason to believe that BG players would find Gary's modification undesirable (unless, as I stated above, the desire is to bottom out the cone). It seems only conjecture that it would alter the sound in a way that would be undesirable. After all, the roll-off BEGINS at about 30 Hz. If a BG player has tested an iAmp with the original and modified roll-off and found the latter to be less desirable (in a blind test), then I stand corrected.

Finally, if for whatever reason, even if just superstition, EA wants to supply the iAmp to BG players with the original design, it seems a simple matter to offer the option of the modified roll-off when ordering.

Mike Dimin said:

Gary is sending me an amp with the mod so I can A/B them especially with a low B five string

Mike

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Excellent! Here's what I would be interested to know. If you do hear a difference in the low end, can you remedy it easily with the parametric EQ? It should only be about a 2-3 dB effect. If it can be satisfactorily taken care of with the EQ, then it would seem to make sense to build the amps with the mod. That way, just about everyone would be happy with the sonic character of the amp and would, at the same time, benefit from the amp not having to waste power reproducing the undesirable low frequencies.

tombowlus said:

Dear lord, please don't use the "switch" word!

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Okay, sorry. I as merely pointing out that if one WERE to switch something, it would be the capacitive element. I agree with you. I suspect (and I await Mike's evaluation), that the mod will produce a very desirable overall result.

Aleph5 said:

No, the equivalent circuit to a piezo is a voltage source in series w/ a capacitor (hopefully a biggun'). So the load resistance also needs to be big (usually 1-10 Mohm) to set the LF pole as low as possible (though in your case, only to just around 40Hz or so). Most people don't notice problems if it's too low, but if it is, lowering the resistive load will raise the cutoff frequency. The only reason I can see why a fancier solution is needed is that the rolloff is only 6dB/octave and a steeper slope is probably needed.

A capacitor across the input lowers the HF rolloff point. (I get paid to be an EE for SOMETHING, anyway. )

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That's why you don't put the capacitor ACROSS the input! I was talking about changing the capacitive and not the resistive element in the RC circuit. I should have been clearer in that I was not referring to a capacitive shunt across the input. It seems the last thing you want to do is lower the input Z as suggested in your post. The input Z of the iAmp is 1M-ohm.

Edit-- Upon re-reading your post, I see what you mean. My preference, however, is to tweak the filter stage withiin the pre-amp and to not mess with the loading at the input. After all, the amp may not always be used with a piezo element and it is far more desirable to have the frequency response remain constant and independent of the source impedance. This, apparently, is Gary Gibilisco's preference as well. Yep, there's a bunch of us trained in EE here.