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.

8 Tracker Meter Bridge

The 8 Tracker attempts to squeeze a lot of functionality into a small space. The larger part of the mixer is taken up with the 8 channel amps and their faders. All the other functions of the mixer need to be contained in the 3U meter bridge running along the top. Here is a suggested layout for the 3U top sub-rack:

There is no room for a master slider fader so a large rotary knob is used instead. As I am right handed I have placed this in the right most module of the meter bridge. This module also contains a Twin Line Amp (TLA) that provides the mix bus amplifiers for the master left and right outputs. Another TLA is used for the AUX mix bus amplifiers. This is shown at the far left of the meter bridge. Next to the AUX sends are the four direct inputs housed in two modules. Each module uses one TLA which can handle two direct inuts. On the right, next to the master fader, is another TLA which provides the PFL bus and the talkback amplifiers. This leaves just three slot widths in which to house the meters and monitor section. A Sifam AL29SQ meter will fit in one module width with enough room below for its switching so the two central module spaces are used for the meters. This leaves one slot to the right of the meters for the monitor section. As there is a lot of cabling behind the meter and monitor panels and some of it is interconnected, it probably makes sense to combine these into a single three module wide panel. The monitor panel will probably need its own small board to accommodate the 10K:600 monitor isolation transformers.

Frame, Module and the 8 Tracker

In the last post I promised an update of the monster frame once the last parts arrived. Here it is:

As it takes up rather a lot of room in my small workshop, it is currently being used as storage space while I work on other aspects of the Mark 3. However, I think you can see the how the 4U fader section works and at the rear there is a 9U section. The bottom 3U of this would house the rear mounted line amps and the 6U above would be used for connectors.

One of the other aspects I have been working on is the module electronics. I have now built the first prototype channel module PCB as shown below:

All the basic enhancements over the EZTubeMixer PCB are included:

• Provision for  Molex KK connectors for internal wiring
• Ground reference the gain resistors so each amplifier can be used as a virtual earth mixer
• Moved the mic line switch to be before the phase switch so the latter works on both line and mic
• Amplifier gain preest pots moved to be accessible from the front panel
• Tubes moved so as not to foul the enclosure
• Output transformer (direct out|) included
• Provision for several EQ types

The next step is to test the board and then finish one of the EQ boards and try that out.

In the meantime I have been giving further thought to the first mixer I will build with the new modules. I said a few posts back that I would use the 500 frame as a test bed for the new modules and I still intend to do this. However, it occurred to me that it might just be possible to make a compact 8 track all tube mixer based on this frame so I have been investigating that possibility further. Also, the 500 frame is pretty simple; there is no room for lots of modules so it might even make a fairly feasible DIY project. Here is by first stab at a block diagram of the 8 tracker:

At the top left are the 8 identical channel modules. These have similar facilities to the modules I am building for the EZTubeMixer demo in that the mic pre, EQ and routing are all in the one channel module. The main changes are:

• A permanently connected PAN control instead of the smart pan
• Addition of pre/post switching for the two AUX sends
• Addition of pre-fade listen (PFL) switch to allow level setting and quality checking
• Addition of a channel mute switch. This mutes the signal feeding the PAN and AUX sends but leaves the direct out unaffected.

At the top right are the audio bus amps for  the two main buses (L & R), the two AUX sends and the PFL. The AUX sends and PFL would normally have simple rotary controls but you would expect the main bus output to be on a slider fader. Unfortunately, as we only have 8 slots, and all 8 are used for channels, there is no sensible place to put a main bus slider fader. So I have decided to use a single large rotary knob in the meter bridge section for the master fader. It will be on the front panel of the Twin Line Amp used for summing the buses so it can easily be a couple of inches in diameter. As I am right handed, I think I will place it in the right most module in the meter bridge. It's a compromise but a small one I think.

At the bottom left is the talkback and direct inputs. The talkback uses half of a Twin Line Amplifier (the other half is used for the PFL bus amplifier). A simple pot sets the gain and I envisage a front panel mounted XLR into which the talkback mic can be plugged (much the same way as we used to do it at Neve). Two momentary action switches control the routing of the talkback. One switch routes it to the L & R buses for slating and the other routes it to AUX1 for artiste communications. When either switch is operated, a dc signal labeled 'DIM' is produced which is used to operate two relays in the monitor section to reduce the level fed to the control room monitors to prevent howl round.

The direct inputs are modelled on the AUX Returns of the EZTubeMixer demo and indeed they can be used for  FX returns. Each direct input has level and pan controls and uses half of a Twin Line Amp. I think there is enough room in the meter bridge section of the 500 frame to house four direct inputs which means there will be 12 inputs available at mixdown.

Lastly, at the bottom right are the monitoring and metering sections. I envisage just a pair of meters placed centrally in the meter bridge of the 500 frame. At present I am thinking of using a couple of Sifam AL20SQ meters for a nice retro feel.

Each meter has its own 5 way switch that selects between four of the direct outs and one AUX send thus covering all eight channels and both AUXes between them. The output from these two switches feeds a second switch that connects the meters either to the 5 way switches or straight to the main bus outputs. This allows the meters to monitor the main output and easily be switched to read other outputs when desired. The signal fed to each meter is also fed to the monitor section.

The monitor section is quite straight forward. It consists basically of a three way monitor select switch and a level control. The monitor select switch selects between the the main L/R buses, a two track playback input or the meters. Normally you would listen to the main buses and occasionally switch to the two track playback to check sound quality. Selecting the meter position allows you to listen to whatever the meters are connected to. The output of the monitor select switch goes via a couple of relays to a 10K:600 transformer and thence to the monitor level pot. The 10K:600 transformer is there to ensure the monitor section applies a minimal load to the signals it monitors. It also means a simple passive 1Kohm level control can be used on the secondary for the monitor level control (this is pure Neve).

The relays affect what reaches the monitors and its level. RY1 and RY2 are the PFL relays. When a PFL button on a channel is operated, the PFL dc operates the relay and automatically routes the output of the PFL bus amp direct to the monitors so it can be heard. RY3 and RY4 are the DIM relays which apply a 20dB attenuation to the monitor level. These relays are operated by the talkback switches and, as mentioned above, are designed to prevent howl round when talkback is used.

As it stands, this design uses 8 channel modules and 5 Twin Line Amps for a total of 39 tubes.

The next step in the design is to sketch out some front panels for the modules and the components of the meter bridge.