Changing Impedence like Audere does in their onboard preamp

[Owen]

I will freely admit to have no idea what is going on here.

 

A long time ago I had and Audere preamp in a bass. It had some jiggery-pokery going on where you could change the impedance the pickups were seeing (I think, but as I mentioned above.......). What this did was create some BONKERS LF boost. I miss it. I plugged it into a Berg IP 2x 1 x 12 stack in the first Bassbash I went to and eybrows went up when I flicked that switch.

 

I went to look at their website to buy one. They will not ship outside the US.

 

So. is this something which could be done using off the shelf parts by someone who knows what they are doing in a passive circuit? Or is this an active thang?

 

TIA

[Woodinblack]

Not in a passive thing.

Well, I guess you could have two volume pots and a switch between them, although it would still depend on what the impedance of your amp was, and you would have to have 2 redundant volumes, which doesn't sound like a great idea. I suppose it could be a custom dual gang hand made pot, with a swtich between them, and then being careful that you picked the right amp (and get rid of the tone).

Basically not practical.

[Owen]

Now I know But unless I ask, I just will not know.

 

Thanks.

[Downunderwonder]

It wasn't something as simple as changing the pickups from series to parallel?

[Bigguy2017]

You can use a tapped audio transformer like the Cassidy bass does.

[Aidan63]

30 minutes ago, Bigguy2017 said:

You can use a tapped audio transformer like the Cassidy bass does.

is this the same thing that Yamaha use on the new Revstar guitars as a boost ? 

[rmorris]

Is it simply a matter of simply reducing the impedance seen by the output ?

If so you could just use a pot' wired as a variable resistor in parallel with the amp input.

The transformer method is also an option - literally 'transforming' the impedance and can be wired to increase or decrease the impedance.

[itu]

Once more the bass signal chain:

 

pickups - blend - vol - tone stack - output

 

If all parts of the chain here are working without a power supply (usually a battery), this circuit is high impedance, a.k.a. passive.

 

If any part of the signal chain has a power supply, the system output impedance becomes low, a.k.a. active.

 

Low impedance output is easier than hi-Z to any amp input. If anyone has had issues with piezos (very high impedance), understands this. You know the thin sound that lacks bass. A quality preamp might help.

 

If your amp has issues with the poor bass response, a preamp (or pickups, or active blend or...) may help. The output impedance can be tweaked with the circuitry somewhat, but the main point is that low impedance lets the whole frequency band out.

 

Every component, and especially high impedance component in the signal chain affect the pickup response, usually by reducing it. This means that replacing any passive component with a lo-Z one will widen the response. Even the blend and volume pots, which are usually left alone while only the tone stack is replaced. If you still read this, take a look at the signal chain: both pots are right after the pickups - and reduce the freq response!

 

As all things have two sides, many effects behave differently if the signal is hi-Z, or lo-Z. I have tried far too many OD/fuzz/dist pedals, as well as compressors, to find out that some pedals just work and sound better with different signal types. Make lots of trials and find this out by yourself.

[Owen]

I am PAINFULLY aware of the piezo into a front end which is not high impedance. Nothing is quite as sad as an unbuffered Double Bass piezo.

 

What I was enquiring about was what Audere offer in some of their on board preamps. I had an Audere once. It had some kind of impedance switching option. It was utterly bonkers, in a good way. All I know is that when I flicked that switch back in the day all sorts of super gnarly stuff happened which was very attractive. Not just LF, but grunt and edge.  I should have left the bass as it was, but........ you know how it is.......

It could just have been a level rise which in turn drove the front end of the amp harder. It could have been how the preamp was interacting with that specific pickup (also long gone).

[rmorris]

Worth looking here (legacy site)

http://www.audereengineering.com/TechDetails.htm

[itu]

10 hours ago, rmorris said:

Worth looking here (legacy site)

http://www.audereengineering.com/TechDetails.htm

Yes. You can make trial of this impedance change by using let's say a megaohm vol pot and putting different resistors (1 M, 500k, 200k and so on) in parallel with it. This way the pickup sees the vol as a 1 Mohm pot or less.

 

You have here an LR-circuit. The pickup represents L (inductance) and the pot is a Resistor. Naturally the impedance will change, this is very bassic electronics.

[Woodinblack]

46 minutes ago, itu said:

Yes. You can make trial of this impedance change by using let's say a megaohm vol pot and putting different resistors (1 M, 500k, 200k and so on) in parallel with it. This way the pickup sees the vol as a 1 Mohm pot or less.

 

To be the pedant, the pickup sees the vol as max 500k or less, with the 1M resistor (assuming you have not plugged the other side of the pot into something) and you are just measuring with a theoretical infinite measuring tool.

[itu]

13 minutes ago, Woodinblack said:

 

To be the pedant, the pickup sees the vol as max 500k or less, with the 1M resistor...

It is good to be pedant. I would also start with no resistor parallel to the pot, so 1 Mohm is the first trial and option.

[rmorris]

Better (imho) Just take the signal from the top of the pot (rather than the wiper) and wire the wiperto the bottom of the pot to give a single variable resistor : Zero Ohm to 1M0 (well near Zero due to pot limits but that area not of interest anyway).

If we assume amplifier resistive input impedance of 1M0 then impedance range seen by pickup is variable between 500K and Zero. 

Lower impedances will, of course, give lower level into amplifier so you need to compensate for that for real comparison. 

 

If really into it you could 'LT Spice' simulate it with varying L / R / C values...

[Woodinblack]

2 hours ago, rmorris said:

If really into it you could 'LT Spice' simulate it with varying L / R / C values...

 

But pretty hard to work out as you would also need the r / c network of the lead and amplifier input for the given amplifier.

 

All in all I am not sure what you would be gaining for any of this compared to one pre-amp with a bass boost!

[rmorris]

4 minutes ago, Woodinblack said:

 

But pretty hard to work out as you would also need the r / c network of the lead and amplifier input for the given amplifier.

 

All in all I am not sure what you would be gaining for any of this compared to one pre-amp with a bass boost!

 

Well I did caveat with "If really into it" 😳

I tend to agree wrt bass boost preamp although the resonance / peak due to the L of a pickup and the C of any tone cap / Cable capacitance / and amplifier input C is interesting. If it interest you of course 🙂

[Passinwind]

On 02/04/2022 at 14:35, Owen said:

I am PAINFULLY aware of the piezo into a front end which is not high impedance. Nothing is quite as sad as an unbuffered Double Bass piezo.

 

What I was enquiring about was what Audere offer in some of their on board preamps. I had an Audere once. It had some kind of impedance switching option. It was utterly bonkers, in a good way. All I know is that when I flicked that switch back in the day all sorts of super gnarly stuff happened which was very attractive. Not just LF, but grunt and edge.  I should have left the bass as it was, but........ you know how it is.......

It could just have been a level rise which in turn drove the front end of the amp harder. It could have been how the preamp was interacting with that specific pickup (also long gone).

 I had one in a bass with Q-Tuner pickups briefly and the "Low-Z mode" barely did anything at all. When the owner of Audere, David Meadows, let me try his own bass at a GTG it was fairly dramatic though. He invited me over to his shop to investigate but that particular config had other issues and I decided to just start building my own preamps again, and have never looked back. It's a really cool feature when it all works though, for sure.