Posh DI Box- is it worth it?

[Owen]

Could you expand on passive v active DI use? Is it a buffering thing? Thanks.

[jensenmann]

Wow, that´s along story. Here we go:

Passive pickups can be seen as a mixed circuit which consists of resistance, inductance and capacity, hence creating a resonant peak at a certain frequency above which the frequency response is going downwards. This resonant peak is highly important for the sound of the instrument because it sits in the presence area of our hearing range.  Now if you plug such a PU into an amp then the amp´s input impedance will interfere with this resonant point, usually lowering and eventually broadening the peak as well as lowering the frequency. We´re used to that and we want to hear our passive instrument like that to a certain degree. To make things worse the cable has it´s impact, too, as it sits between amp and PU. That´s why some people like cables of a certain length.

Now there comes the DI with it´s own input impedance, typically 50kOhm for a passive DI and 1MOhm for an active DI. This again will interfere with the PU resonance and alter the sound. And things get more difficult if you go through the DI unbuffered into an amp because that will increase the load on the pickup and the resonance point will go lower again. Technically spoken both input impedances are parallel to the pickup giving an even lower impedance than each single one of DI and amp.

For historical reason most bassamps have an input impedance of 220kOhm (many exceptions, though) while guitar amps typically have 1MOhm. To have a proper matching between output impedance of a PU and input impedance of an amp/DI the later one should be x10 of the source impedance. Pickups of passive basses have worst-case output impedances of 17kOhm, hence there should be a load of at least 170kOhm. Passive DIs cannot deliver such high impedances. They are the wrong choice. You´re better off with an active DI. Active basses have worst case output impedances of a few hundred Ohms. Here both passive and active DIs are fine regarding impedance matching. The PU resonances are already buffered by the active electronic inside the bass in a way that both DI and amp cannot have an influence on that.

For the studio nerd there´s more to check out: if we have micpres with variable input impedance then this will be the next parameter to look for in a chain of passive bass, passive DI + amp. A lower input impedance setting of the micpre (e.g. certain vintage Neve preamps have this possibility) will be reflected through the transformer of the DI. That means the DI seems to have a lower input impedance than it´s nominal value (wrt 1,2-1,5kOhm input impedance of the micpre) and will load the pickup even more down, eating the treble range even more. Historically this problem has been recognized in times when there were no active basses around. The solution were active DIs, back then made with tubes.

All that is theory. You will not destroy either bass, amp or DI with the technically wrong choice. It does influence sound, but if you like it then all is good. If not, then see above.

Edited June 9, 2018 by jensenmann

[Happy Jack]

Excellent stuff Jensen ... I just wish I understood it all better.

I assumed that I couldn't go far wrong by plugging my passive basses through good-quality DI boxes by Radial. It seems I misunderstood the situation.

 

[Owen]

A comprehensive reply. Thanks

[Chienmortbb]

On 09/06/2018 at 17:20, jensenmann said:

Wow, that´s along story. Here we go:

Passive pickups can be seen as a mixed circuit which consists of resistance, inductance and capacity, hence creating a resonant peak at a certain frequency above which the frequency response is going downwards. This resonant peak is highly important for the sound of the instrument because it sits in the presence area of our hearing range.  Now if you plug such a PU into an amp then the amp´s input impedance will interfere with this resonant point, usually lowering and eventually broadening the peak as well as lowering the frequency. We´re used to that and we want to hear our passive instrument like that to a certain degree. To make things worse the cable has it´s impact, too, as it sits between amp and PU. That´s why some people like cables of a certain length.

Now there comes the DI with it´s own input impedance, typically 50kOhm for a passive DI and 1MOhm for an active DI. This again will interfere with the PU resonance and alter the sound. And things get more difficult if you go through the DI unbuffered into an amp because that will increase the load on the pickup and the resonance point will go lower again. Technically spoken both input impedances are parallel to the pickup giving an even lower impedance than each single one of DI and amp.

For historical reason most bassamps have an input impedance of 220kOhm (many exceptions, though) while guitar amps typically have 1MOhm. To have a proper matching between output impedance of a PU and input impedance of an amp/DI the later one should be x10 of the source impedance. Pickups of passive basses have worst-case output impedances of 17kOhm, hence there should be a load of at least 170kOhm. Passive DIs cannot deliver such high impedances. They are the wrong choice. You´re better off with an active DI. Active basses have worst case output impedances of a few hundred Ohms. Here both passive and active DIs are fine regarding impedance matching. The PU resonances are already buffered by the active electronic inside the bass in a way that both DI and amp cannot have an influence on that.

For the studio nerd there´s more to check out: if we have micpres with variable input impedance then this will be the next parameter to look for in a chain of passive bass, passive DI + amp. A lower input impedance setting of the micpre (e.g. certain vintage Neve preamps have this possibility) will be reflected through the transformer of the DI. That means the DI seems to have a lower input impedance than it´s nominal value (wrt 1,2-1,5kOhm input impedance of the micpre) and will load the pickup even more down, eating the treble range even more. Historically this problem has been recognized in times when there were no active basses around. The solution were active DIs, back then made with tubes.

All that is theory. You will not destroy either bass, amp or DI with the technically wrong choice. It does influence sound, but if you like it then all is good. If not, then see above.

I agree almost 100%. Most of the bass amps I have worked on use 1MR as the input impedance although that is probably higher than neccesary. 

[Dan Dare]

Orchid make a nice, modestly priced active DI box. I have a few and they've always worked well for me.

[itu]

On 27/04/2021 at 19:31, Chienmortbb said:

Most of the bass amps I have worked on use 1MR as the input impedance although that is probably higher than neccesary. 

It helps a lot with piezo pickups. The higher, the more universal. A lo-Z input needs an extra buffer if a piezo is used.