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Author Topic: 60 HZ hum, only in Backline Amps - on same power as Sound System  (Read 8304 times)

John Roberts {JR}

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #20 on: July 22, 2015, 10:59:14 am »

A active DI is actually a preamp.  It dosn't have gain but it is a amp before your board.  That brings up questions
Transformer DI raise similar questions
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How does this amp sound?
How does the transformer sound?
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What is it's noise floor.
How effective is magnetic shielding?
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How flat is it?
How flat is the transformer?
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What does it do when over-driven?
How is saturation characteristic?
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What does it do when it's powersupply changes?
OK no PS in passive DI.
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None of the above are show stoppers. Just things to find out.

With a good transformer, you look up the numbers, say "that will work"  Now as long as you know that transformer is in your DI and all the switches that change the sound are off you know what will happen when you use it.
Active and passive DI both have their places.

JR

PS: Frank have you ever made an active DI? (I have).
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Steve M Smith

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #21 on: July 22, 2015, 11:33:27 am »

Me too.  But you know that!


Steve.
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Frank DeWitt

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #22 on: July 23, 2015, 09:58:59 pm »

Transformer DI raise similar questionsHow does the transformer sound? How effective is magnetic shielding? How flat is the transformer? How is saturation characteristic?OK no PS in passive DI. Active and passive DI both have their places.

JR

PS: Frank have you ever made an active DI? (I have).

Good questions.  I like Jensen equipped DI boxes because the above questions are easy to answer.
http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-db-epc.pdf
The sound and shielding is predictable and they are near flat over a wide range.

I built passive DI boxes because I needed them and I didn't want a bunch of switches in them that could be messed with while they were in use on the platform and I was at FOH a long distance away.

So far I haven't needed a active DI so I haven't built one. 
Yes, they have there place.
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Steve M Smith

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #23 on: July 24, 2015, 01:50:53 am »

I have mentioned before that I do a lot of folk/acoustic acts.  Often musicians turn up with a piezo equipped instrument and a lead and ask "where do I plug in?".  I found it easier if I had my own high impedance input DIs to preserve the full tone of the pickup.

They also work fine on everything else (so far!). They have a -20dB pad switch on them but I can't recall ever using it.

It's a simple three transistor design and is easy to make.


Steve.
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Frank DeWitt

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #24 on: July 24, 2015, 03:04:39 pm »

I have mentioned before that I do a lot of folk/acoustic acts.  Often musicians turn up with a piezo equipped instrument and a lead and ask "where do I plug in?".  I found it easier if I had my own high impedance input DIs to preserve the full tone of the pickup.

They also work fine on everything else (so far!). They have a -20dB pad switch on them but I can't recall ever using it.

It's a simple three transistor design and is easy to make.


Steve.

Sounds like the perfect place to use them and not a good place for the a passive.  Is the input and output isolated as far as grounding?  (I realize that is not a issue for a pickup.)  Would you point me to a circuit?  I wouldn't mind having one in the bag. 



 
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Steve M Smith

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #25 on: July 24, 2015, 09:21:57 pm »

The ground is only semi-lifted through a 10 ohm resistor and parallel 0.1uF capacitor.  A full lift wouldn't provide a return path for the phantom power.



It's just a FET buffer with complimentary outputs driving a Schoepes circuit (as used in some phantom powered microphones).  Each half uses the 6K8 resistor in the mixer as the load resistor in an emitter follower circuit whilst the collectors are held at 12v by the zener which provides the voltage for the FET.

I can send you a blank PCB if you would like one.



It's not actually a 'printed' circuit.  the missing copper was routed out with a CNC router.


Steve.
« Last Edit: July 24, 2015, 09:25:27 pm by Steve M Smith »
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Mike Sokol

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #26 on: July 24, 2015, 10:06:45 pm »

It's not actually a 'printed' circuit.  the missing copper was routed out with a CNC router.
Steve.

How come I don't have my own CNC router? Gotta work on that... ;D
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Mark Cadwallader

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #27 on: July 24, 2015, 10:55:37 pm »

How come I don't have my own CNC router? Gotta work on that... ;D

Because a full-on CNC milling machine is way more versitile...?  (As visions of Bridgeports danced in their heads.)
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Mac Kerr

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #28 on: July 24, 2015, 11:06:32 pm »

How come I don't have my own CNC router? Gotta work on that... ;D

Here ya go!

Mac
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Steve M Smith

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #29 on: July 25, 2015, 03:41:18 am »

It's an Excellon drill/router which we have at work.  It was bought 30 years ago to make PCBs (the traditional way).  We don't make our own PCBs any more but we kept the machine and I use it to make test fixtures and assembly jigs (and other things!).

A CNC Bridgeport would be nice but it's a bit overkill for holding a 1mm diameter cutter.

I have the stepper motors and linear slides to make my own machine. It will be similar to the X-Carve.

It's also useful for making perfect fit woodworking joints. This is a current project nearing completion: http://stevesmithphoto.webs.com/5x4-3.JPG

I seem to have uncharacteristically veered this thread off topic!


Steve.
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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #29 on: July 25, 2015, 03:41:18 am »


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