Double Bass Headway EDB-2 w/ Gage Copperhead input distortion

sumeursault

sumeursault

Jun 21, 2006
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#1
I recently switched to the Gage Copperhead from the Fishman Full Circle, and immediately realized that it was distorting the input of my old Headway EDB-1 preamp, regardless of the input gain setting. I contacted John Littler and he suggested I upgrade to either of the two preamps they currently offer. I chose the EDB-2 since it seemed to provide everything that I needed at a lower price point. However, testing it out I found that it exhibits the same ugly input overload distortion when I dig into the strings. Frustrating. Anyone else have this problem?
 
#2
For attenuating a passive piezo pickup you can put a capacitor in the low nF range between hot and shield.
That‘s what is inside the Full Circle Jack and you can easily make or modify a plug/jack adapter/extension cable by soldering a capacitor between hot and shield. It doesn‘t matter on which end.
For testing you can use crocodile clamp cables to connect the capacitor to the opened jack/plug to find the right value.
Be prepared to get some noise with the unshielded croc cable on the hot wire.
 
#3
I have a similar problem with one of my Realist Lifeline pickups and the HE circuit on my EDB-2 HE. The distortion goes away when I switch off the harmonic enhancer. I also tried swapping my two Lifelines between basses, and it only happens with the one pickup. Modifying a cable as double midi described fixed the problem, but it's a pain to always need a special cable. I also found that I needed to cut the bass knob quite a bit with the modified cable to get the tone back to normal.

I think that it is unfortunate that Headway put the HE circuit before the input gain.
 
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#4
If you can open the output jack of the pickup, you can put the capacitor in there instead.
The adapter cable is easily reversible by not using it and nice to try out the value you need.

It is debatable where the problem is located, pickup or preamp. As there is typically data for the preamp under which conditions it works, I would say it is a pickup problem or a wrong preamp that does not fit the pickup.
So the capacitor in the output jack or part of the pickup seems to be the right solution. That’s also the way Fishman uses for the Full Circle.
 
#5
DoubleMIDI said:
For attenuating a passive piezo pickup you can put a capacitor in the low nF range between hot and shield.
That‘s what is inside the Full Circle Jack and you can easily make or modify a plug/jack adapter/extension cable by soldering a capacitor between hot and shield. It doesn‘t matter on which end.
For testing you can use crocodile clamp cables to connect the capacitor to the opened jack/plug to find the right value.
Be prepared to get some noise with the unshielded croc cable on the hot wire.
Click to expand...

Thanks, I'll try this. Wouldn't introducing a capacitor to the signal chain act as a kind of filter as well, thus changing the sound?
 
#11
My issue isn't a problem with either unit per se, but simply a problem with the pairing of the two. It was pretty obvious that the issue was the signal level of the pickup going into the HE circuit on my Headway--only the very front of the note distorts and only when the HE circuit is enabled for that channel. I absolutely love my Headway preamp and highly, highly recommend it, BUT I don't understand why the HE effect is before the input gain or why it distorts at a lower level than the rest of the circuit.

meursault42 said:
Wouldn't introducing a capacitor to the signal chain act as a kind of filter as well, thus changing the sound?
Click to expand...
Yes, I think it has to, and that's certainly what my ears told me. I assumed at first that it was behaving as a low pass filter since it is essentially a tone knob without the knob. So I tried to adjust by boosting the highs, but that didn't help much. Then I read that the capacitor extends the low end when connected to a piezo this way. I don't know why that is, but attenuating the bass frequencies cleaned up the sound very nicely. I have the pickup on a backup bass now that hasn't seen a gig in ~9 months, so I don't really have to deal with it now.
 
#12
meursault42 said:
Thanks, I'll try this. Wouldn't introducing a capacitor to the signal chain act as a kind of filter as well, thus changing the sound?
Click to expand...
Not in the case of a passive piezo which is a capacitor itself and a second capacitor from hot to ground.
In this case the flow of the electrons are shared, part of it stay in the piezo capacitor and part of it go to the hot to ground one. A capacitive divider. It does not change the sound, because it doesn’t interrupt the hot wire. Less electrons flowing means less voltage and therefor the signal volume goes down. The value of the capacitor influences how much the signal gets attenuated.
The values depend on the capacitance of the piezo. We have no experience values for the Realist yet, only for the Yamahiko and the Full Circle.

The additional capacitor might have an influence on the loadability (impedance), so if the input impedance of the first amp input stage following the piezo is too low, it might have an influence on the sound, but that should not be a problem with an input impedance in the MegOhms range.
 
#13
DoubleMIDI said:
Not in the case of a passive piezo which is a capacitor itself and a second capacitor from hot to ground.
In this case the flow of the electrons are shared, part of it stay in the piezo capacitor and part of it go to the hot to ground one. A capacitive divider. It does not change the sound, because it doesn’t interrupt the hot wire. Less electrons flowing means less voltage and therefor the signal volume goes down. The value of the capacitor influences how much the signal gets attenuated.
The values depend on the capacitance of the piezo. We have no experience values for the Realist yet, only for the Yamahiko and the Full Circle.

The additional capacitor might have an influence on the loadability (impedance), so if the input impedance of the first amp input stage following the piezo is too low, it might have an influence on the sound, but that should not be a problem with an input impedance in the MegOhms range.
Click to expand...

Interesting, thanks! When I put my little external volume pot in line it does clear up the distortion, but seems to attenuate the low frequencies. Why would that be?
 
#14
You drop the input impedance by using that volume pot. I haven‘t seen a volume pot above 5 MegOhms and even that would act in parallel with the input impedance of the amp, so dropping the total input impedance below the smaller impedance value of pot and amp.
That‘s the reason why a capacitive divider is used to drop the volume for a piezo. The pot is a resistive divider and won‘t act well with a piezo because of the high impedance needed for a piezo.

A resistive divider can work if you split the hot wire with the resistor. But then it depends on the input impedance of the preamp/amp (first stage) how much the signal gets attenuated.

Since you want more flexibility for the amp, it makes more sense to use the capacitive divider in the pickup jack, as it is then coupled with the pickup and does not depend (as much) on the input impedance of the preamp/amp.
 
#15
DoubleMIDI said:
For attenuating a passive piezo pickup you can put a capacitor in the low nF range between hot and shield.
That‘s what is inside the Full Circle Jack and you can easily make or modify a plug/jack adapter/extension cable by soldering a capacitor between hot and shield. It doesn‘t matter on which end.
For testing you can use crocodile clamp cables to connect the capacitor to the opened jack/plug to find the right value.
Be prepared to get some noise with the unshielded croc cable on the hot wire.
Click to expand...

So like a 10nF ceramic capacitor maybe? I don't even know where to begin...
 
#16
If you or a friend has a meter that can sense capacitance, you can measure the capacitance of the piezo pickup and try a capacitor in the same range as a first try. On the other hand the capacitance of the Yamahiko is rather small (below 1 nF if I remember correctly) and the recommended additional capacitor was about ten times larger. But you shouldn‘t take that as granted, my memory is not very good.

I would get a 10 nF and a 4.7 nF and compare them. Then you can decide if one works or if you need a larger or smaller value than one of them.
A plastic foil axial capacitor with a low voltage 10 to 50 volts) might fit into the jack and sounds best.
 
#17
DoubleMIDI said:
If you or a friend has a meter that can sense capacitance, you can measure the capacitance of the piezo pickup and try a capacitor in the same range as a first try. On the other hand the capacitance of the Yamahiko is rather small (below 1 nF if I remember correctly) and the recommended additional capacitor was about ten times larger. But you shouldn‘t take that as granted, my memory is not very good.

I would get a 10 nF and a 4.7 nF and compare them. Then you can decide if one works or if you need a larger or smaller value than one of them.
A plastic foil axial capacitor with a low voltage 10 to 50 volts) might fit into the jack and sounds best.
Click to expand...

I can't seem to find an online vendor that stocks both 10nF and 4.7nF axial film caps in low voltage. Any suggestions?
 
#18
meursault42 said:
I can't seem to find an online vendor that stocks both 10nF and 4.7nF axial film caps in low voltage. Any suggestions?
Click to expand...
Not really.

Typically these kind of capacitors are not axial, but Styroflex are (not sure about the value range) and I got some others in a bunch of parts decades ago, but cannot remember the values I got.
I cannot look for them, I‘m not at home and cannot remember where I had put them.

The smaller the voltage the smaller the dimensions. Maybe even a rectangular might fit. But for testing the size does not matter. If you tested the value you need you can look for one that fits in the jack. And better open the jack first with a plug inside to see how much room is left.

Since I live in Germany what I maybe able to get here may not help you.
 
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