Level matching mixer insert sends with a high-end compressor

[chanceoperation]

Hi Musicians.

I have a slight level mismatch that I don't know how best to resolve.

The units involved are the Allen & Heath GL2400 mixer and the Dangerous Music Compressor. I want to patch the compressor through the inserts on two of the mixer's channels.

The problem is that the signal out of the mixer's send is a tad weak for hitting the compressor where it works best.

The GL2400 manual says it sends at 0dBu. It does not mention impedance.

The Dangerous compressor manual says it's input impedance is 20K Ohms. It does not mention level.

I am looking for a solution that:

• gives the compressor a slightly stronger signal,
• is adjustable/trimmable,
• is transparent,
• is either not overkill, or,
• offers my project studio some other useful features (for example, a good stereo pre-amp I can use elsewhere, if that applies to this use case too)

Many thanks in advance,

Andrew Culver

[Hugh Robjohns]

The problem is that the signal out of the mixer's send is a tad weak for hitting the compressor where it works best.

This is quite a common issue as desks of this type tend to feature unbalanced inserts at a depressed operating level (usually somewhere between -6 and 0dBu) -- the lower insert level being necessary for valid internal technical reasons.

And this is compounded by some outboard which only starts to sound 'interesting' when pushed quite hard -- and the DMC is designed to handle signals up to a whopping +27dBu.

There are several possible options, although some may not be deemed practical:

The most obvious option is simply to run the source channel 10 or 12dB hotter than you are currently doing. The downside is reduced headroom in the input (and potentially EQ) stages of the desk, and you can probably get away with that, although it may be an issue depending on how and what you're recording.

Another relatively simple option, albeit expensive and requiring some DIY, would be to introduce a step-up transformer connected to the DMC input. That would balance the signal into the DMC and give the signal a useful uplift... But you'd need a couple of high quality transformers designed for high line levels and they won't be cheap, and you'd have to put them in a box and wire them up.

Next, you could put a line-level preamp in front of the DMC to boost the level by 10-12dB, but the downside is a more complex signal path and potentially more noise/distortion (or beneficial character, depending on your viewpoint).

And lastly, but probably not practical in your situation, a better technical solution, would be to modify the input stages of the DMC to make it more sensitive....

The GL2400 manual says it sends at 0dBu. It does not mention impedance.

Impedance really isn't an issue with line level signals. We are dealing with voltage transfer, so as long as the input impedance is more than 5-10 times higher than the source impedance, the precise value doesn't matter. Output impedances are almost always less than 150 Ohms, and input impedances always more than 10k Ohms, so there's never anything to worry about in that respect.

I am looking for a solution that:

• gives the compressor a slightly stronger signal,
• is adjustable/trimmable,
• is transparent,
• is either not overkill, or,
• offers my project studio some other useful features (for example, a good stereo pre-amp I can use elsewhere, if that applies to this use case too)

Personally, I think I'd run the channel hotter... Or change the desk for something more capable, or choose a more versatile compressor.

H

[chanceoperation]

Another relatively simple option, albeit expensive and requiring some DIY, would be to introduce a step-up transformer connected to the DMC input. That would balance the signal into the DMC and give the signal a useful uplift...

Not a DIY guy when things are small and electricity is involved. But would this fit the bill? :

http://www.radialeng.com/j4.php

J+4™ Balanced -10dB to +4dB Signal Driver

The Radial J+4 is a stereo interface designed to accept unbalanced low output level devices and boost the signal to a balanced +4dB level.

The design begins with 100% discrete components for optimal signal transfer. Careful attention has been paid to eliminate noise by combining the advantages of active signal buffering with transformer isolation. The J+4 delivers better than 95dB signal to noise while ensuring hum and buzz caused by ground loops along with spurious noise from clocking does not pollute the signal path. For ease of use, controls have been kept to a minimum. This includes a stereo level control, a high pass (low cut) filter and ground lift. The choice of input connectors include a stacked set of ¼" jacks for DJ mixers or keyboards, RCAs for CD players and a mini 3.5mm for iPods®.

[Hugh Robjohns]

Yes, that will boost the level by about 12dB, and can cope with inputs up to +17dBu which should be enough.

H

[chanceoperation]

Sound advice indeed.

Thanks Hugh.

[ef37a]

Sound advice indeed.

Thanks Hugh.

As ever of course! But, Ooof! That box will be the thick end of £200 by the time it hits Blighty, no sure how Canada fairs?

All you really need is an NE5532 OP AMP and a supply I bet Behringer could bring that in well under $50.

Maybe Orchid Electronics have something?

Dave.

[Hugh Robjohns]

About ca$240 I think. There is a cheaper alternative in the form of the ART CleanboxPro which would be about ca$75, and it provides an additional -12dB conversion path to reduce the elevated level from the DMC back to unbalanced insert level.

http://artproaudio.com/isolators/product/cleanbox_pro/

H

[chanceoperation]

There is a cheaper alternative in the form of the ART CleanboxPro which would be about ca$75, and it provides an additional -12dB conversion path to reduce the elevated level from the DMC back to unbalanced insert level.

http://artproaudio.com/isolators/produc ... anbox_pro/

I will give this one a try, and report back here.

Many thanks all.

[Wonks]

If you simply run the mixer input gain 10dB hotter than normal, then surely as the compressor you have can reduce its make-up (output) gain by up to 10dB i.e. to -10dB, you should be able to bring the level back down through the mixer as if the compressor wasn't there and the input gain was set as normal?

[Hugh Robjohns]

... surely as the compressor you have can reduce its make-up (output) gain by up to 10dB i.e. to -10dB, you should be able to bring the level back down through the mixer as if the compressor wasn't there and the input gain was set as normal?

Yes, the DMC is very unusual in having a Make-Up Gain control that offers attenuation, so that could be used to reduce the compressor's output level (if the top-down compression doesn't already provide enough) and restore normal headroom margins through the channel path downstream of the insert point.

But, as I said earlier, adopting the 'run the channel hotter' solution does mean that all circuit points upstream of the insert point would have 10dB less headroom. Whether that's an issue or not depends on the console design, the dynamics of the source material, and how hot it is normally being driven anyway.

The -10 > +4 / +4 > -10 boxes have the benefit of allowing standard levels and headroom margins through the channel path, while providing a hotter and balanced signal for the outboard compressor. The downside is an extended signal path potentially with more noise and distortion (although that's very unlikely to be an issue in practice).

H

[resistorman]

Ebtech makes some solutions:

http://www.ebtechaudio.com/llsheinf.pdf

[Hugh Robjohns]

Yes, a lot of companies provide something along the lines of a domestic/pro interface that provides the unbalanced/balanced conversion while shifting nominal I/O levels between -10dBV and +4dBu.

The thing one has to be a little careful about when using such a device in this mixer insert application is to ensure that the unbalanced input/output has sufficient headroom. This is because these boxes are expecting the unbalanced input signal at a nominal -10dBV (which is about -8dBu), but the nominal insert level is going to be 0dBu (or perhaps a little lower in some cases). So the nominal level is already 8dB hotter than expected, with uncontrolled transients way above that (which is why the user want's to apply a compressor). So headroom can be an issue in some cheaper designs.

The ART model mentioned earlier can cope with at +21dBu according to the specs, so I doubt headroom will be an issue in practice... but if an alternative unit crapped out at, say, +12dBu you might have a problem with it in this application (although it would probably be perfectly fine when interfacing a domestic source).

H

[chanceoperation]

I just realized I never reported back on the ART Cleanbox Pro. I did buy it and it's been sitting between the GL and the Dangerous compressor ever since, doing a standout job in both directions. I can't hear any distortion at all, and the compressor is happy working within its ideal range. I think the boost is only 6db. I set it once and forgot all about it. Highly recommended for this application.

[Hugh Robjohns]

Grand. Thanks for the update.