Insert Options

All modern mixers provide some means of inserting external devices into the signal path.  These can often also be used as direct outputs and sometimes these are provided separately. When I was at Neve back in the 70s, the provision of inserts was straightforward; wherever there was a balanced output you could add an insert. Direct outs were not common, but if you wanted one you could connect it to the same place. All Neve consoles had channel amplifiers containing mic preamps followed by EQ. These were balanced in and out so the first available option for an insert  or direct out was at the channel output. So this was a pre-fader, post EQ insert. After the mix bus, the group amplifiers were balanced out so there was another opportunity to add inserts. No special arrangements were necessary. All you had to do was bring out the necessary signals to the patch bay

With the advent of project mixers, inserts and direct outs began to appear at other points in the signal chain. The result is that inserts and/or direct outs may nowadays be required or preferred at almost any point in the signal chain.The diagram below illustrates the places where an insert and/or direct out could be included in the Mark 3 tube mixer.

The top most option shows an unbalanced insert directly after the mic preamp and before the fader and EQ. This type of insert was first popularised by the project mixer manufacturers. It can also be used as a direct out by inserting a mono jack so that the internal signal flow is not interrupted. An alternative direct out can be taken after the EQ gain make up amplifier. You could even switch the direct out between the insert point and the output of the EQ gain make up amplifier. The only disadvantage of this approach is that the insert is unbalanced.

The second diagram shows a balanced version of the first example. An output transformer has been added to the output of the mic pre and an input transformer has been added before the fader. This type of insert is often used as a direct out during tracking so the signal can be recorded dry. The fader and buses are used for creating a monitor mix during tracking. At mixdown, the inserts are used to add effects.

The third diagram moves the balanced insert to post the EQ gain make up amp so it is now post fader, post EQ. In its unbalanced form, this is the typical insert arrangement for project mixers like the Mackie CR1604 for example. Again the insert can be used as a direct out for tracking but this time the fader controls the level recorded to the track and the level sent to the mix. This allows EQ to be applied to tracks as they are recorded but prevents the channels being used as a monitor mix.

The last diagram is the same as the third but moves the fader to after the insert. This means the fader can now be used for monitor mixing but the recorded signal can also include EQ. This the classic Neve configuration.


The Mark 3 mixer channel amp can handle the first two configurations unaided using its  two built in amplifiers .

The third diagram requires an additional input transformer, space for which is provided on the new channel amp PCB. There are then two options for the source of post fader signal to be sent to the pan and AUX send controls. One is from the output of the second amplifier which would be post fader but pre-insert. This amplifier is designed to drive buses so this does not present any problem. The second choice is from the secondary of the input transformer. This means any device plugged into the insert would be expected to be able to drive the bus. In most cases this should not be a problem but, of course, the drive capability cannot be guaranteed. Also, if passive mixing is used, the crosstalk via the pan control depends in part on the driving source impedance. As long as any inserted device has a low output impedance this should not be a problem. Provided these limitations are realised, this scheme is fine for simpler mixers like the 8 Tracker which have a limited number of amplifiers.

The fourth option also requires an input transformer but has one added complication. The post fader drive to the buses now comes directly from the fader itself. However, the fader does not have the drive capability needed to drive the buses so an extra post fade amplifier is needed for this scheme to work. In the one metre monster this is not a problem as there is plenty of space for additional Twin Line Amplifiers (TLAs) in the rear facing 3U sub-rack. Only one amplifier is required for each channel so one TLA will do two channels. In the 8 tracker, amplifiers are at a premium so this is perhaps not a viable option. The pre-fade sends can come pre or post the insert. The post insert, pre-fader send only needs to drive AUX send buses so pan pot crosstalk is not  an issue.

Clearly, the one metre monster can be configured to provide any of the insert/direct out options described above. In the 8 tracker block diagram shown in a previous post, there is no provision for inserts although each channel does have a post fader, post EQ direct out. The 8 tracker could be configured to provide the pre fader, pre EQ unbalanced or balanced insert point shown in the first two options above. It could also be configured to provide the post fader, post EQ insert shown in the third option, using the extra input transformer on the new channel amplifier board, provided the limitations noted above are acceptable. It is the last option that it has problems with as this requires additional TLAs. These would have to be housed in the 3U meter bridge. However, all the available space in the meter bridge is currently accounted for, so to include the TLAs needed to do this type of insert, something will have to be deleted. Here's a reminder of the current layout of the 8 Tracker meter bridge:

On the far right is the TLA used for the master buses and on the far left is the one used for the AUX sends. These we have to retain. We could drop the four direct inputs to provide four  of the extra amplifiers we need but then there is no way to bring in FX returns to the mix bus except via a channel line input and we need all of those for 8 track mixdown. Similarly, unless we are talking about a one person project studio where only one track at a time is recorded, then we cannot really afford to lose the PFL or talkback and anyway these provide us only with two amplifiers. For such a project studio we could perhaps compromise and lose two direct inputs as well as the PFL and talkback amplifiers to get four amplifiers and this would still leave two direct inputs for FX returns. However, we still only have half the amplifiers we need. Although the meters and monitor is only a passive panel, experience in building the EZTubeMixer prototype has demonstrated that this cabling takes up a lot of space so we cannot really put any TLAs in this section. However, as this section is passive, it does not have to be in the meter bridge; it could be anywhere, So one possibility is to house the meters and the monitor circuits in a kind of power bump on top of the meter bridge. That would free up two slots in the meter bridge (one would still be needed for all the cables to pass through) which, if fitted with a couple of TLAs would give us the final four channels we need.

At this point in time, I have not decided which type of insert to include in the first 8 tracker I build. However, it will be a fully balanced one so the additional input transformer will be needed on the channel amps.