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Double Bass Amps with 10 MΩ input impedance
- Thread starter Ethan Eriksen
- Start date
"Only" an preamp, but the Felix offers a 20 MOhm-input and works really fine!
If you can find one, try an Eden Time Traveler WT330. Unfortunately it is no longer built, but it has also a 10 MΩ and is a really fine amp for piezo-PU's.
In Germany you can find Eich Amplification, who offers an 5 MΩ-option for all amps:
Bass Heads | EICH Amplification
If you can find one, try an Eden Time Traveler WT330. Unfortunately it is no longer built, but it has also a 10 MΩ and is a really fine amp for piezo-PU's.
In Germany you can find Eich Amplification, who offers an 5 MΩ-option for all amps:
Bass Heads | EICH Amplification
Actually,
From my experience, of the pickups I have used only the Fishman Full Circle and BP 100 need that 10 Meg Ohm load. Recently, there are three standalone boxes
that deal with impedance loading differently. The Red-Eye, Headway EDB-2 H.E. and the Sarno Blackbox. Rather than me try and explain it with my layman’s knowledge, I’ll just say that these new approaches yield a better tone quality than just setting the load at 1 or 10 Meg Ohms. There are exhaustive threads on both the Sarno and Red-Eye.
From my experience, of the pickups I have used only the Fishman Full Circle and BP 100 need that 10 Meg Ohm load. Recently, there are three standalone boxes
that deal with impedance loading differently. The Red-Eye, Headway EDB-2 H.E. and the Sarno Blackbox. Rather than me try and explain it with my layman’s knowledge, I’ll just say that these new approaches yield a better tone quality than just setting the load at 1 or 10 Meg Ohms. There are exhaustive threads on both the Sarno and Red-Eye.
Still few & far between it seems.
The Noble Amp fits the bill but apparently he only made 5 - and I'm not crazy about the form factor.
I'm still waiting and looking for that amp that will pass my real-world A/B testing and let me leave my HPF-pre at home.
But until it finally arrives, life's not that bad. (I can patch in one extra unit and replace a battery every 6 months or so.)
The Noble Amp fits the bill but apparently he only made 5 - and I'm not crazy about the form factor.
I'm still waiting and looking for that amp that will pass my real-world A/B testing and let me leave my HPF-pre at home.
But until it finally arrives, life's not that bad. (I can patch in one extra unit and replace a battery every 6 months or so.)
Same here. The pickups I've used seem just fine with something like 1-2 Meg Ohms. That would be: the original copper Realist, the Realist Lifeline, and Vic's single and dual bridge wing elements.
Regarding the EDB-2 H.E., I have not heard that it does anything unusual with respect to impedance buffering. The manual just says that it has a 6.8 Meg Ohm impedance. Some of the earlier versions had a three way switch to select different impedance levels, but EDB-2 H.E. does not have that feature.
There is a case for a standard op-amp based mod to do this. R.M. Marston wrote a book on op-amps, and the Newnes pocket book on active designs (which he also wrote) probably shows it too.
It's basically an AC-coupled voltage follower with a guard ring round the non-inverting input to raise the input resistance to 500 Mohms, and the same circuit, without the guard ring on the PCB will do 50 Mohm input resistance.
It is cheap to build, and could be used to raise the input resistance spec of any preamp input.
It's basically an AC-coupled voltage follower with a guard ring round the non-inverting input to raise the input resistance to 500 Mohms, and the same circuit, without the guard ring on the PCB will do 50 Mohm input resistance.
It is cheap to build, and could be used to raise the input resistance spec of any preamp input.
What is a guard ring (in this context)?
As a non-native speaker I need some help with that.
A schematic might help a bit too.
I don't know if R.M. Marston's books are online, I can't find the exact circuit on Google. You may need to look for his books to find it, but my description below might help to find other circuits online that will do the same thing. Google's images might help, it's a small circuit so a small image will cover it.
A guard ring is also called a 'driven guard'. It's not needed unless you need to go beyond 50 Mohms. What it does, is use the output or other low resistance signal line that has the same potential (voltage) as the input, and from there, a copper trace on the PCB goes to the input pin's pad on the circuit board, and surrounds it, not contacting it. By doing this, it raises the input resistance. Sometimes the ring is open, overlapping at its open ends, to avoid inductive coupling.
EDIT:
I think this is Marston's original:
http://t0.gstatic.com/images?q=tbn:...ZhlTwlpe7x2EklUT1q5hFfolm9wMuPXi4srdzAKdnQw&s
If you don't need more than 50 Mohm, ignore the guard ring. Also, it's worth using a better op-amp, but it's interesting that LM741 was apparently good enough. It probably is, because the circuit works at unity gain.
Last edited:
Thanks a lot.
Now I understood what it is, but I might need some more time to understand why it works that way. Maybe the coin will drop sometime later …
You're welcome. When I look into small circuits for making critical parts or modules for other contexts I take plenty of time. It never comes all at once.
The nice thing about this circuit is that once you have it, you might be able to adapt it to any preamp input. Once it's laid out on a small board, wiring it to an existing op-amp power supply would be easy. All-valve amps would need the supply brought into them though, probably no 15V rails in those unless they already have some solid state circuits too.
