Preamp for a single XLR measurement mic

[john61ct]

Have not tried it before, but looked at this previously when I was finding a solution

SWAMP Microphone 48V Phantom Power Supply

Get the best performance from your mic with SWAMP's 48V Phantom Power supply for XLR microphones. Includes 18V AC power supply.

www.swamp.net.au

All the preamps I'm looking at deliver PP built-in at the full 48V

 

[john61ct]

From Macki FAQ

Is an insert jack an input or an output?

It's both! The entire signal from the corresponding channel, subgroup, or mix bus is sent through this jack. The tip of this jack is an output, which is commonly used to send signal out to a serial processor (compressor, equalizer, sonic maximizer, etc.); the ring is an input which allows signal to return from the processor; and the sleeve is a ground connection. Because the insert jack is an input and an output, it creates a loop, which is formed when a signal processor is inserted between the output (tip) and the input (ring).

The insert jack can also be used as two different types of direct outs. If a tip-sleeve 1/4" cable is plugged all the way in to the second click, the signal is sent out from the tip (output) of the insert jack and does not return to the ring (input), which breaks the loop and does not allow signal to flow through the rest of the channel. This is referred to as a direct out with signal interruption. If a tip-sleeve 1/4" cable is plugged in only to the first click, the signal is sent out the cable and continues through the channel, since the insert loop has not been broken. This is referred to as a direct without signal interruption.

 

[john61ct]

> Earthworks M30

Simply out of the question

 

[Rja4000]

From Macki FAQ

Is an insert jack an input or an output?

It's both! The entire signal from the corresponding channel, subgroup, or mix bus is sent through this jack. The tip of this jack is an output, which is commonly used to send signal out to a serial processor (compressor, equalizer, sonic maximizer, etc.); the ring is an input which allows signal to return from the processor; and the sleeve is a ground connection. Because the insert jack is an input and an output, it creates a loop, which is formed when a signal processor is inserted between the output (tip) and the input (ring).

The insert jack can also be used as two different types of direct outs. If a tip-sleeve 1/4" cable is plugged all the way in to the second click, the signal is sent out from the tip (output) of the insert jack and does not return to the ring (input), which breaks the loop and does not allow signal to flow through the rest of the channel. This is referred to as a direct out with signal interruption. If a tip-sleeve 1/4" cable is plugged in only to the first click, the signal is sent out the cable and continues through the channel, since the insert loop has not been broken. This is referred to as a direct without signal interruption.

Extract of MacKie 802 VLZ4 User manual

 

[john61ct]

deleted, covered more precisely below

morevthan many care about I'm sure :-0

 

[Rja4000]

Interesting

For my research into the 1202-VLZ4 I am considering, I concluded a TRS plug that has its Tip and Ring soldered together and TS at the destination works like the "first click" trick, signal is sent into the mixer circuitry as well as externally.

While a regular TS or TRS jack inserted fully breaks the internal signal path, signal gets muted in the Main Mix because the return path is interrupted, sends the cleanest possible unbalanced signal.

I don't know if this works differently across the various VLZ4 models

I just checled both methods (fully inserted vs half inserted with a TRS balanced cable) and, as expected, that makes no difference.

 

[john61ct]

I just checled both methods (fully inserted vs half inserted with a TRS balanced cable) and, as expected, that makes no difference.

So with both, the direct out is Sent but with no signal to the mixer, correct?

 

[Rja4000]

So with both, the direct out is Sent but with no signal to the mixer, correct?

No, that's not what I meant.
Sorry for the confusion.

I measured noise in both cases and the level is the same - or close enough to be reproducibility error.

 

[john61ct]

Aha yes, there should be no **audible** difference in signal quality (fidelity, frequency response, or noise floor) between any of these two methods, provided the cabling is wired correctly for the destination device

The Onyx preamps in the VLZ4 series have sufficient driving capability to feed both the external processor and the internal channel strip simultaneously without signal degradation from loading effects.

 

[john61ct]

OK, I'm trying to compile the various ways the channel inserts feature can be used.

To start, Connector types by function.

Direct Out signal Sent regardless of connector type.

1. "Insert Send+Return" for mixing channels live, while also externally monitoring/processing the raw signal

2. "Insert Send-only pass-through" for mixing signal live while also playing back/monitoring or recording externally, but no Return

3. "Insert Send-only muted", signal on that channel is blocked from mixer circuitry

This last is for my current use case, isolates the preamp completely from the rest of the mixer signal chain, fader and EQ sections

effectively connecting my interface TS port directly to the Mackie preamp output alone. The low cut HP filter of course will be switched out for measurement purposes.

 

[john61ct]

In general, remember the Channel Insert port, although TRS is unbalanced.

Beware using a regular TRS cable fully pushed into the channel insert, going out to a balanced input port.

The Ring (cold) wire then connects to the mixer's Return, which is an input, not a ground. This leaves the cold line "floating," acting as an antenna that can introduce hum or other RF noise.

Also a regular TRS when inserted fully will break the return signal path, muting that channel from the mix. Hence the need to solder a special "sniffer" cable for the

2. "Insert Send-only pass-through" use case

(see below)

...

Channel Insert feature

all VLZ4 series models work the same

802VLZ4: 2 inserts (on channels 1–2)
1202VLZ4: 4 inserts (on channels 1–4)
1402VLZ4: 6 inserts (on channels 1–6)
1604VLZ4: 16 inserts (on channels 1–16)
1642VLZ4: 10 inserts (on channels 1–8 and 9/10, 11/12)
2404VLZ4: 20 inserts (on channels 1–20)
3204VLZ4: 28 inserts (on channels 1–28)


As stated, these channel insert I/O jacks are unbalanced, designed to Send out the post-preamp signal - after the gain/trim stage and switchable HPF / low cut control - for playback/monitoring or recording as "direct out" signals, excluded from the mixing, EQ and level control circuitry completely.

and/or for patching / looping in external processors, such as compressors, limiters, or equalizers, before returning it into the channel's mixer signal path via the same unbalanced TRS cable - see "Insert Send+Return" use case below

 

[john61ct]

Sorry for repetition, summary about Mackie VLZ Analog Mixers in general:

signal routers only, line-level in and output, no power amp - in fact actually outputs a lower signal level after the mixer circuitry

High-resolution meters, reliable longevity from solid build quality and sealed rotary controls.

Super value just for the Onyx mic preamps, a respectable substitute for much more expensive standalone "boutique" mic pre's, with a strong reputation as high "straight wire with gain" SQ - accurate, neutral and uncolored.

From the 802 model up, the 1/4" "channel insert" ports in parallel with the XLR inputs, are designed as pre-mixer external processing loops for the mic input. But this feature also enables the preamps to act as standalones using "direct out" cables, bypassing all the mixer circuitry.
See *Mixer Muted

Wide dynamic range, up to 60dB gain

Clean, quiet low distortion signal when gain structure is set properly - don't let the stereo bus meters peak much above +2dB.

The Insert preamped channels have a "virtual pad",
20 dB of attenuation fully down and 40 dB of gain fully up, with a “U” (unity gain) mark at 10:00.

The XLR input jack has 0 dB of gain with the knob fully down, ramping
up to 60 dB of gain fully up.

Full 48V phantom power for mic's that need it.

Standard screw holes underside to fit a mic stand

to and adjust
height and level.

 

[john61ct]

Connector types by function.

1. "Insert Send+Return" for Live + Processing, mixing channels live, while also externally monitoring/processing the raw signal, then finally inserting the Return into the mix.

This is the purpose-designed use case for the Channel Inserts feature, as an addition or alternative to using the AUX Sends to feed an external processor, returning it into the mix via the Stereo Aux Return or a standard line input channel.

...

HowTo: you need the special soldered breakout "Y" cable illustrated above, TRS wired as unbalanced send/return circuits.

Plugging this specially wired cable into the insert jack fully, intercepts the preamp output signal from internal channel signal path.

The Tip conductor is the signal Send output from the mixer, a low-impedance (120Ω) path capable of driving any line-level device.

The Ring conductor is the signal Return - input back into mixer - at high impedance (over 2.5kΩ, maybe 5kΩ) so can be driven by almost any processing device.

So, that mic signal goes out via the Tip, to the external monitoring / recorder - or through the processor(s) - and then returns via the Ring wire into the mixer EQ/fader circuits, all though the same unbalanced TRS cable.

DIY HowTo: a standard 1/4" TRS plug at the mixer end, then at the processor end, gets split out into two TS plugs, one for the Send and one for the Return - see the illustrations in the mixer user manual.

Sleeve to ground (earth) at both ends - ideally both the mixer and processor are powered of the same AC grid supply circuit.

 

[john61ct]

2. "Insert Send-only pass-through" cable for mixing the live signal, while also playing back/monitoring or recording externally, but no Return

To send the signal out to a recording or playback device WITHOUT interrupting the signal flow inside the mixer.

A TRS "sniffer" or "trick shorting cable" allows you to fully insert the plug into the channel insert port without blocking the signal path into the mixer.

HowTo: Connect the Sleeve to the shield (ground) at both ends.

Inside the mixer end, solder Tip and Ring together to maintain the connection between the Send and Return.

At the recording / playback end use a TS plug (not TRS), connect the Tip (from the T-R bridge on the other side) to the hot wire.

The signal on the Tip and Ring will be identical, so if your recording / playback device uses a balanced input port, then using a modified TRS jack on that end will cause the balanced input circuitry to subtract the signals from each other, resulting in silence or extremely faint audio.

see also https://www.audiosciencereview.com/...single-xlr-measurement-mic.70078/post-2549939

 

[john61ct]

To use a standard TS cable for this #2 "Insert Send-only pass-through" use case, with no Return from an external processor, there is also a workaround, the "First Click" method.

OK in a pinch, but IMO physically too unstable in a live situation, could be bumped or pushed in too far, blocking the signal from the mix.

HowTo: just insert a standard TS 1/4" plug, but only to the first click. This contacts the "send" part of the jack without breaking the internal connection that returns audio to the mixer.

With a TRS cable, Ring might touch the Return part of the port, which can cause grounding issues or phase cancellation depending on what's on the other end, so a TS cable is much better.

 

[john61ct]

3. "Insert Send-only muted", signal on that channel is blocked from mixer circuitry (signal interruption)

*Mixer Muted

This last is for my current use case, isolates the preamp completely from the rest of the mixer signal chain, fader and EQ sections

effectively connecting my interface TS port directly to the Mackie preamp output alone. The low cut HP filter of course will be switched out for measurement purposes.

HowTo: Use a standard TS cable, fully plugged in.

This mechanically grounds the return, muting the channel safely, and sends the cleanest possible unbalanced Direct Out signal for measurement purposes.

(whew!)

 

[john61ct]

All constructive feedback welcome

And of course keep suggesting good value smaller portable standalone alternatives (not interfaces), even if I end up getting the 1202VLZ4 for this use case

especially data points & comments on the three units I asked about bottom of my OP

Hopefully will be useful to other noobs investigating the topic ;-)

 

[john61ct]

example 1402VLZ4, 6 mic preamps

https://ebay.us/m/X4SdFO

$5 each x 14 channels

"compact ha!" faders not just knobs

just the rackmount ears sell for half what they're asking for the bundle

 

[john61ct]

E1DA also proposes a purely analog APU that may be used as pure fixed-gain mono preamp with 48V phantom power available. Quality is excellent

So I infer you mean Cosmos APU. Love the SFF and USB-C power input!

I have to say the primary usage as a test measurement device is way over my paygrade, is that to measure distortion including speakers?

But yah sure seems like healthy overkill at $140 per channel, in a different way from the VLZ4 units.

> you'll have to have enough with a fixed gain

Have to have enough what? Please ELI5, within the context of my first REW baby-steps.

Do you mean with a low-output mic I might need a boost like ?Cloudlifter

+60dB and 10Vrms max output seem plenty to noobie me??