Juno / pre-amps / impedance issues?

[jimbobbley]

Hi all

Excuse the first world problems here as I know I'm very lucky to have this equipment but I can't work out what's going on here... and also it might read like a 'all the gear, no idea' post too but honestly I do know what I'm doing (except here!)...

I'm recording some Juno 106 into a UAD Apollo soundcard - it sounds fine, level is very healthy etc. For a bit of colour / an experiment I thought I might run it through some pre-amps rather than just going straight into the UAD.

I plugged it to two Chandler TG series 500 preamps and the sound instantly went thin and weedy. There is only one input to these units, and switching between mic/line level or toggling the impedance had no effect whatsoever. Hmm, weird.

I then tried the same thing into the line input of a Neve 1073. Same thing - weird, thin sound with no low end at all. Tried pressing Lo Z, all the EQ and shelving was off, even tried going into the mic input but that just went the other way and hideously distorted.

As a last resort I thought I'd try two Shadow Hills Gama units I have and - hey presto - up popped the Juno and sounded great.

Then I re-tested the Chandler with the outputs from a Nord Stage 3 - it sounded fine. Same with the Neve.

So.. what's causing the difference? It's not purely a 'level' thing as if both are plugged in to the UAD directly, if anything the Juno has significantly more level. Is there some kind of buffer issue on the outputs, a la guitar pedals? Impedance switching didn't seem to have any effect.. and why does the Nord Stage work fine into the Chandler and Neve but the Juno doesn't.. and why does the Juno sound great into the Shadow Hills?!

I have no idea and would greatly appreciate any info which could lead to me becoming less ignorant and confused!

Thanks

John

[Sam Spoons]

Sounds like a classic case of a stereo output into a balance input*...

Tell us exactly which outputs and inputs and the cable connectors you are using please.

* TRS headphone output to TRS or AXLR line input would exhibit these symptoms, splitting the stereo into two unbalanced cables solves it.

[jimbobbley]

Ha oh my god, I didn't even think of that.. but it still doesn't quite answer why one pre-amp works, and the input to the UAD Apollo on its own works...?

The signal path is:

Juno - 1 x TRS output from 'Mono' out, 1 x TRS output from 'Stereo' out, running into a DSub into a Bantam patchbay.

This is normalled to a UAD Apollo 16 normally but for this I obviously patched to various other pres patched in via bantam / d-sub / XLR connector looms.

So maybe the Shadow Hills and the UAD both have differently-wired inputs to the Chandler and the Neve? Still a bit odd to me...!

[Sam Spoons]

It looks like the 'mono' labeled output is also one side of the stereo pair so not the stereo>balanced issue unless you are using the headphone output (which would definitely cause the issue). Is it possible you have a mis-wiring in the patchbay?

[jimbobbley]

I don't think so, no, because the following all apply:

Juno -> direct in to UAD = works
Nord Stage -> direct in to UAD = works
Juno -> Neve 1073 - doesn't work
Nord Stage -> Neve 1073 - works
Juno -> Chandler = doesn't work
Nord Stage -> Chandler = does work
Juno -> Shadow Hills = works
Nord Stage -> Shadow Hills = works

so we know the output from the Juno is working, the inputs to all 3 pres are working, and the Nord Stage as a test device is also working... I'm baffled!

[Hugh Robjohns]

Extremely unlikely to be an impedance issue, especially if feeding a line input, and even most mic inputs present a high-enough input impedance to keep a modern electronic output happy.

So the most likely reasons, in order of likeliness, for a 'thin and weedy' sound are:

1. Feeding a stereo unbalanced source into a mono balanced input. Any elements that are the same (ie central in the stereo image) in both channels will cancel out, leaving only elements that are widely spaced. If the source is heavily stereo chorused that's likely to mean the low end all disappears and you're left with a rather flat phasey high end.

2. Phantom power present and reverse-biasing the source's output capacitors or otherwise upsetting the output circuits biasing arrangements.

3. Input transformers being heavily saturated due to far too high a signal level.

[jimbobbley]

Thanks Hugh, and Sam too. I think it must be the 'stereo' unbalanced -> mono balanced issue then, especially as the Juno Chorus is - quite rightly - always on. But I'm still baffled as to why the problem doesn't present itself on the UAD inputs (which are mono, balanced) and the Shadow Hills pre (which presumably is the same).

[Hugh Robjohns]

Ah... reading your list of 'wot works and wot doesn't ... '

Option 0. Balanced TRS cables feeding balanced TRS preamp transformer inputs causing no end of confusion when used with keyboard unbalanced TS output sockets.

If the ring contact is not grounded at the keyboard end -- which is very likely in a keyboard of the Juno's age -- then preamps with floating transformer inputs won't have a full signal path circuit and will end up effectively being capacitively coupled, resulting in a very thin and weedy sound indeed.

The simple and instant solution would be to use standard instrument TS cables.

[jimbobbley]

Aha! Now that makes sense. In that case we could deduce (well, idiots like me anyway) that the Mono Gama and the UAD line inputs are not transformer pres / lines in...?!

Thanks a lot, this was making me very confused!

[Sam Spoons]

According to the blurb the Shadow Hill Gama's microphone input is transformer balanced but it's 'DI' input may not be...

[James Perrett]

It is often far safer to use TS jacks with unbalanced inputs/outputs - as you've found out. To make matters worse, there are some TS connectors that have their ground contact right where the insulation between the ring and the sleeve goes in a TRS connector. If you are unlucky enough to use a TRS connector with one of these sockets you'll end up with a big hum.

[Hugh Robjohns]

According to the blurb the Shadow Hill Gama's microphone input is transformer balanced but it's 'DI' input may not be...

Quite so...

Although a transformer input with tie-down resistors of some form would still work too, albeit with a loss of level. It's only floating transformer inputs that would suffer in this way.

Rule of thumb: if something has an unbalanced output, use a TS plug!

[jimbobbley]

I suppose I should've known without checking that Roland gear from that era would be unbalanced, much like the Space Echoes etc.. lucky for me that I've managed to get away with it for so long I suppose! Thanks for the education folks

[jimbobbley]

According to the blurb the Shadow Hill Gama's microphone input is transformer balanced but it's 'DI' input may not be...

I've got the 'discrete' input selected - I wonder if that's making the difference here, rather than using the nickel or steel transformer colouring options...

[Martin Walker]

According to the blurb the Shadow Hill Gama's microphone input is transformer balanced but it's 'DI' input may not be...

I've got the 'discrete' input selected - I wonder if that's making the difference here, rather than using the nickel or steel transformer colouring options...

I've been reading quite a lot just lately about musicians who want to pass their synths through boutique preamps to add some colour/drive/mojo, and it seems quite common for the line/DI inputs of said preamps to bypass the input transformer, which somewhat defeats the object of the exercise e.g.



Any synthesist fancying this approach should read the preamp specs very carefully before purchasing.

If on the other hand you've already purchased the preamp, adding a DI box to drop your synth signal low enough to be put through its mic input will also restore the input transformer to the signal chain

Final thought - good synths tend to sound good already, but cheaper synths and various drum machines can often benefit from being passed through a preamp to add more character.

Martin

[Sam Spoons]

The manual for the Shadow Hill Gama doesn't say if the 'DI' input is balanced or not, maybe it isn't which is why the Juno works with it but not the others? The Nickel. Descrete, Steel switch selects the characteristics of the output transformer.

[Hugh Robjohns]

... it seems quite common for the line/DI inputs of said preamps to bypass the input transformer...

Yes, and for good reason. Instruments usually have unbalanced outputs at a moderately high level, and geetars in particular need to see a very high input impedance. A mic input transformer doesn't like high levels, isn't needed for bal>unbal conversion, and struggles to present a high input impedance.

Instead, it's a lot easier to use a single-sided FET buffer stage for the unbalanced instrument input, and drop its output signal into the main preamp gain stage just after the input transformer. It works nicely from a gain structure point of view, presents a suitably high input impedance, and if designed sensibly the FET can be encouraged to introduce some nice musical colouration.

And if the preamp has an output transformer and the ability to push it a bit, you can have some tranny saturation as well...

[Martin Walker]

100% agreed Hugh, especially for guitar use, but nevertheless unfortunate for this situation of wanting to add input transformer colour to a synth (that doesn't require a high input impedance)

Martin