Alternative pre-amp

[Linnovice]

Hi all. Over recent years I’ve collected/acquired a quite diverse array of front end equipment. Reel to reel decks, cassette decks, tuners, minidisc, dat, etc, etc. All these get an airing at one time or another so are not just for show or bragging rights.
My main system has a Linn Av5103 system controller being fed by most of the above but it only has three record out connections (analogue that is) which is a bit restricting. I’ve had a small light bulb moment and am thinking about introducing a medium sized mixing desk into the mix (excuse the pun). Something like a 20/4/4 unit would be ideal for flexibility. Can anyone think of a reason not to do this? They do not appear to demand extravagant prices and presumably the SQ must of a quite high standard.

[paulsherwin]

You could just buy or build a big passive switch box.

[jamesperrett]

If you are looking at mixing desks then go for an established UK manufacturer rather than a cheap brand. I rather like the Allen and Heath range but there are alternatives too.

Another cheaper alternative would be to use a patch bay instead of a mixer or switch box.

[Radio Wrangler]

Mixers normally operate as concentrating devices, bringing many inputs into fewer outputs. This seems to be the wrong way to go if your problem is not enough outputs. But won't you need extra inputs to go with all these implied tape machines?

The heart of a mixer is a summing amplifier where all the inputs are combined. The channels have gain control faders and pan pots. The problem with the summing amplifiers is that with a channel faded right down there is still a noise contribution from that channel and the overall noise floor is worse than you would get from a switch arrangement. So a lot of care goes into being able to switch unused input channels off to kill their noise. What you gain with a mixer is the ability to fade smoothly from one inout to another. You haven't mentioned wanting this, and it is an unusual facility for a domestic hifi system.

So with a mixing desk, you'd have to switch channels on or fully off anyway, so it looks like all you need is the switching function and the output buffers for driving the recorders.

It looks to me like a job for a switch box to handle the inputs, and either another switch box to handle the recording outputs. A buffer amplifier could be used to drive multiple outputs at once, if you have a need for multiple recordings to be made at once.

One disadvantage to a simple switch would be if you wanted to have individual sensitivity adjustments so that the loudness of all your different sources could be set equal, allowing you to switch around the different sources without jumps in volume.

Signals from turntables would need RIAA equalisation for magnetic cartridges, and maybe other equalisations if you play older records. Outboard RIAA stages might be the best approach. Mixing desk manufacturers put their best efforts into getting low noise in their microphone inputs. A friend was brought in to manage one such firm, and what he had to say about Rohm obsoleting their favourite low noise transistor was, um, memorable.

Mixing desk design is a battle against noise while trying to engineer-in versatility. If you look at the schematics of these things, you'll find circuitry and devices which are definitely not audiophile-style. It works, though.

Another thought is that switch boxes and mixers switch signal connections, but not earth connections which for unbalanced signals just get lumped together. With a large assemblage of lots of sources, the opportunities for earth loops and hum or RF pick up increase geometrically. This would be a vote for connector fields and patch cords.

I'm assuming that many/most/all of your sources are unbalanced. Balanced systems keep earths separate from signals and fix the problems of the inevitable loops.

They say that you should never see how sausages or laws are made. Maybe recording studio gear belongs in that list

David

[Craig Sawyers]

Mixers usually have a listen switch that removes that channel from the summing bus in order to minimize noise. Likewise the record outputs.

That switch can either be physical, a relay, or a FET.

So only active channels are mixed, and contribute to the noise.

Analogue mixing desks tended to use massive number of 5532's. Although some earlier desks used discrete opamps, such as the Quad8 OM10.

Craig

[Ted Kendall]

A little Mackie mixer - a 1202 of some stripe - would do most of what you want. The output is stiff enough to drive several domestic machines in parallel.

[Welsh Anorak]

Presumably you won't want microphone inputs, effects or multi-band EQ so go for one with fewer bells and whistles. Several stereo inputs make life easier in that you only have one fader per input, and the L/R EQ won't be accidentally knocked out of balance.

Cheap makes and models are good for small halls and so on, but you'll be more critical so I'd echo the suggestion of Mackie, mid-range Allen and Heath or Soundcraft. Worth looking at little used gear as well - you might strike lucky.

A custom-made patch bay in conjunction with your mixer would be the ultimate in flexibility.

[williamsunique]

I use a Behringer passive patch bay. Super easy, plenty of ins and outs. All ins and outs I have labled. I don't hear any degradation of sound.
Most of my equipment uses phono sockets and the Behringer sockets are stereo jack 6.3mm ie: 1/4" in old money. Rather than purchase all new leads as mine are phono to phono, I just purchased adapters (Y shaped) two phono sockets to 6.3mm stereo plug.
Patch bays were normal in recording studios, I see no reason why they can't be used in domestic situations.
All the feeds to and from this patch bay are at line voltage. I wouldn't use it for switching between phono cartridges output though.
The fact the patch bay is passive as opposed to active means there is no extra noise introduced.

Paul

[Linnovice]

Hi again. Thank you all for your advice and suggestions. This week I have obtained from a well known auction site a Soundcraft Spirit Folio SX 20 channel mixing desk. It was a bit dirty and dusty with a few knobs missing. After stripping it down the the basic board, giving it a thorough clean and vac out with a few new knobs it now looks very good. What's more, it even sounds good.
I am very surprised to be honest. I didn't know what to expect so had no pre conceived ideas but hey, it works.

My AV5103 has 10 analogue input pairs and 3 analogue outs. So I have connected all my tape outs to the preamp, this includes a DAT unit, two minidisk units and two tuners (Troughline and Kudos). I also output the desk to the preamp. The record ins I have going through the mixer along with another two tuners, vinyl, DAC, cassette deck, etc.
It's taking me a little while to get the hang of how the desk works but so far I'm impressed with the flexibity it offers. I've made up a range of leads XLR to 6.25mm jacks and likewise rca to 6.25mm jacks and it makes for easy recording and/or listening. All in all for £80.00 an easy solution.

Sound wise, I am very satisfied. I can't honestly hear any difference in quality when putting an FM signal through the preamp direct or via the desk. The vinyl is good through the desk as is the DAC which carries the CD/Streamer/Computer signal.

The only caveat I can make is that at 75 my hearing is not what it was so maybe it is down a little but, in all honesty, it sounds very good to my ears. May be the new suit of clothes syndrome but I don't think so.

Thanks again for all the advice.

Mike

[Radio Wrangler]

I understand that Douglas Self used to work for Soundcraft.

David

[Hartley118]

Soundcraft always had a strong reputation for audio quality. Just make sure that everything is working as it should and that the switches and pots are clean.

Back in the day, we lost one or two Neve engineers to Soundcraft.

Martin

[arjoll]

I got a wee Mackie ProFX8v2 for "switching inputs" in the office, had the benefit of a built-in USB interface. Probably didn't really need 8 channels but it was the cheapest with slide faders! My issue was more multiple inputs, not multiple outputs, but the idea of introducing a mixer certainly has merit. I could have built a switch box or used some kind of patch bay, but this was quick, easy and not overly expensive.

[mhennessy]

The trouble with most sound desks is that they primarily deal with mono sources. There will be a number of stereo line inputs (4 in the case of the Folio), but using stereo sources with the other channels involves moving two faders together and hard-panning each to left and right as appropriate. Plastic strips can be used to join faders together for this use.

With a digital sound desk, once you have run out of dedicated stereo inputs, the normal workflow is to join an adjacent pair of channels. This means the faders move together when you move either fader (if you try to move both together, the servos sometimes fight you!). Having done that, you can move one of the faders to another fader layer and pretend it doesn't exist.

Once you've got a sound desk working the way you want for source selection, you can think about using the aux outputs to drive record inputs. Set them to pre-fade, and you can make recordings independently of what you choose to listen to.

Here, there is more variation between desks. Most auxes are mono, so you need a pair. Smaller desks might only have 2, 3 or 4. The pre or post fade selection might be fixed. In the case of the Folio, one is fixed to pre-fade, the second is switchable, and the third is fixed to post-fade. That's a pain, but a reasonable compromise. In live sound, for example, you need pre-fade for monitor "wedges", and post-fade for FX units. Two wedges and an FX send is Ok for simple acoustic shows, but limiting for larger acts.

Again, this is where digital scores. I have an Allen & Heath Qu16, which is their most basic digital desk, yet it has 10 aux outputs. The first 4 are mono; the remaining are arranged as 3 stereo pairs. More expensive desks have more flexibility here, naturally. With modern digital sound desks, you do "sends on faders", where you hit the appropriate aux (or mix) button and the faders move to show you the current mix and allow you to adjust it as required. When finished, the faders go back to controlling the main mix as normal. So much easier than twiddling tiny little knobs on each channel strip.

Another advantage of digital is remote control. You can control almost every feature from a free app running on a tablet. I've done complete shows that way, with the desk hidden on the stage to make the cabling easy.

Obviously, these are much more expensive than analogue desks. But worth it - I'd never go back. You just get so much more flexibility - much more than can be mentioned here.

All that said, for this application, an analogue matrix (or router) is a much better bet. There are many variations, but they are available with almost any number of inputs and outputs (usually powers of 2, so 8, 16, 32, 64, etc). They are usually "square", in that they have the same number of inputs and outputs - or "sources" and "destinations". Some have a simple front panel interfaces; others require serial instructions to be sent to them from a separate control system.

Here's an example: https://www.ebay.co.uk/itm/166048487547

Funnily enough, I have one of these in the office at work, pending a suitable project. I can't imagine that one would be hard to fix - probably just a PSU fault. But collection-only probably restricts this particular one to just an example.

It switches composite video as well, but you can just ignore that. It works in stereo, and has balanced inputs and outputs which can easily be wired to unbalanced equipment. The front panel interface is all you need to use it - just select the destination - which mostly will be your stereo, but might be record inputs - and press the source buttons. Very easy.

A matrix does nothing to the audio, so won't change the gain or EQ. The audio path is a lot cleaner than it would be in a small analogue sound desk. Of course, while that is easy to measure, it might not be audible. But peace of mind, etc...

You can achieve similar things with a patch bay, but a matrix is much easier, especially if you wish to send the same source to many destinations (you end up needing "parallel strips" and lots of patchcords). It's the way we used to do it, but as the cost of buying and installing jackfields has risen, the cost of matrices has fallen. Most routing is done on touchscreens these days - somehow, it's not the same!

Oh - if feeding your signals into the mic XLRs, make sure you don't accidentality hit the 48V button! It's safest to go in on the 1/4" inputs, though be aware that most desks have a 10 or 20dB pad between them and the mic pre, so that raises the noise floor. Rarely a problem for live sound, and probably OK for analogue sources at home, but worth keeping mind (use the dedicated line input channels for the digital gear). Some larger analogue desks had dedicated pre-amps for mic and line respectively on every channel, but that's never seen with these compact types.