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Old 25th July 2008, 10:15 AM   #1
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Join Date: May 2008
Default High pitched whine when using active guitar preamp with phantom power

Hiya,

I've just put together a little onboard guitar preamp which uses dual rail opamps (yes I know disceet is normally the way to go, but the same circuit needs to do a few other things & therefore for simplicity I've used TL074s)

I did all my testing using two PP9 batteries - the preamp output is actually quiet as can be.

However, there's also a need to power this same onboard guitar preamp by phantom power sourced from specialised large manafacturer music 'widget' (I use the term widget' as the actual device name isn't that important here - & it's a fairly specialised item).

Well, when I connect my onboard preamp to the +v & -ve sourced from this widget - I get a higher noise floor but that aside, the main problem is a high pictched whine.

Since the same preamp is very quiet when on batteries, this 'whine' must be coming in from the widget supply rails (or bad ground). The dual rails this widget provides would normally be to drive a small preamp for which the audio output you'd never get to hear (in that circumstance, the actual audio signal is just used to derive guitar pitch information)...therefore I guess the manafacturer hasn't paid too much attention to the fidelity & therefore quality of the two rails (or even the ground).

My questions....

1. What's the best way to trouble shoot this?

2. Is there a 'standard' issue manner in which to tidy/clean up supply rails (I'm sure it makes no difference but the rails are +7V & -7V).

I'm figuring I have two options...

1. Some form of cap between the rails & earth to shunt the whine to ground.

2. Some form of inductance wired into each rail.

My problem is I don't know which is the better route nor what values to use. I've not been able to establish the frequency of the whine but I'd say it's in the 2-4khz range.

On a side tack, I'm also a little uncertain the best way to handle the grounds when it comes to two separate external boxes using the same circuity.

1. My widget supplies +v, -ve & 'ground'.

2. The guitar will be driving a soundcard (but could be an amp) which has it's own 'ground'.

I've simple soldered the two separate 'device' grounds onto the rear of a volume pot...is this correct? Does it contravene good working practise?
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Old 25th July 2008, 11:23 AM   #2
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
are there any unused channels in the tl074?
is the PSU regulated and/or smoothed?
is the opamp decoupled at the supply pins.
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regards Andrew T.
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Old 25th July 2008, 11:39 AM   #3
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Join Date: May 2008
Hi Andrew,

>> are there any unused channels in the tl074?

No...they're all in use.

>> is the PSU regulated and/or smoothed?

I have no idea - the 'widget' I mention is an industry standard device made by a large German manafacturer. I'd have to summise it would be as the spec here is for the device to provide +7V & -7V to the guitar. I've got a couple of small smoothing electrolytics added to my circuit.

>>is the opamp decoupled at the supply pins.

No...I'm aware it's good working practice to do this, but since my preamp is totally silent when on battery power, I eliminated this from my enquiries (perhaps I shouldn't have?)

A bit more info...even when I'm not using the preamp at all (ie circuit switched 'out of operation'...just on passive guitar pickups), I'm still getting the whine from my guitar...*especially* when the magnetic pickup is turned down to zero - however if I disconnect the 'widget' the high pitched whine disappears. The only thing still connected through when my guitar is in passive mode is the widget's ground cable.

This leads me to think the whine is possibly being derived from the external widget's ground (which happens to be soldered to the case of the guitar's passive volume pot). I can't account for any other why when turning my guitar's volume pot to zero (when it's just passive ...ie pickup-> vol pot -> guitar out) ramps the whine up.
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