Feeler: ezAM16-500 - Christmas ideas for your loved one ?

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Slenderchap

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Santa and his little elves are busy getting these new toys ready for Christmas ....

                                                   
ezam16-500_600.jpg


It is the ezAM16-500  ..... 500 series version of a fully balanced "capacitor-less" mic-pre..... there's a clue is in the name....

It is a significantly simpler project than the other kits we do so it can be completed by less experienced project builders.

Build instructions are here:
http://www.audiomaintenance.com/downloads/ezam16-500_colourbook.pdf

........... "any interest" ?

Colin
www.audiomaintenance.com
 

gregnody

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Very cooooool. ;D
Indeed, Santa has been  generous this year.
Just ordered one
 

emrr

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dmp said:
Looks very nice.
Gain control is a balanced input pad?

Yeah, looks like a bridged H pad.  Very odd choice for a preamp input, likely with highly variable results with different mics. 
 

Slenderchap

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emrr said:
Yeah, looks like a bridged H pad.  Very odd choice for a preamp input, likely with highly variable results with different mics.
It's a pseudo balanced bridged T attenuator ....... We didn't get any complaints about "variable results"  from the people who tested it...  they were happy with the way it sounded/performed.

[... impedance/gain error only comes into play at 50dB gain (zero attenuation) of about +1.5dBu but is up at 30kHz].

Colin
www.audiomaintenance.com
 

dmp

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How does  it compare to an attenuator on the output? I assume you are terminating the OT with a 600 ohm anyway - so a 600 ohm attenuator would work easily.
I would guess the output attenuator would be a better control of level without impacting the tone / loading of the mic.
 

Slenderchap

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It's a constant impedance input pad so should load the mic equally at all settings.... the reason for doing it this way is that you can feed the input with very high signal levels.... as they are allegedly very good for recording drums.... cannot do that with an output pad.

Colin Adshead
www.audiomaintenance.com

 

dmp

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Cool. Thanks for the reply. 
Are the Carnhills reproductions of the original iron? I don't see any specs on them in the design guide. Are they custom?
 

mitsos

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Looks like a nice easy build.  Is the impedance switch for input impedance: 150/600?
 

emrr

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This IS an interesting study in how to change a +24VDC device to work with +/-16VDC, and cool to see a version of this circuit again. 

Happy to see polarity on the output side. 

The Z switch is for input, changes series/parallel connections on input transformer after the H pad.  This is also a +/- gain switch, and changes the steps of the H pad a bit too. 

Slenderchap said:
It's a pseudo balanced bridged T attenuator

It's a bridged H pad.  Textbook definition of one. 

I'll speak without benefit of any measurement analysis you may have taken in approving this approach.  Simple signal generator source would not be an adequate test, bearing little resemblance to a wide array of potential mic sources. 

No one uses an H or T on a mic input for very good reason, pretty much any designer would advise against, it violates bridging principle.  Great volumes of commentary have been written over decades against using a constant impedance pad with microphones.  With an H pad, this is set up as a line amp that happens to amplify mic signals. You have the same load/source Z presented to the input transformer and the mic.  Heavier mic loading than standard, light transformer loading (which tends to come with diminished bandwidth).  Neither mic nor transformer want to have this constant impedance between them.  Textbook commentary, loading any dynamic/ribbon like this introduces a LPF, and tends to increase distortion in condenser mics along with reduced headroom. 

Take it to -5dB (45dB) and it's 1Kish R load to each. 
Take it to -10dB (40dB) and it's 645ish R load to each. 
Take it to -15dB (35dB) to -50dB (0dB) and it's 600ish R load to each.
At no place other than full gain, no pad, is the mic subjected to a typical fully bridging input. 

This all works out sorta OK for the input transformer when set to 600 input, less so for 150.  The mic load from -10 on is ideal for the Shure SM57, but not a lot else.  The source Z from a mic is always increased by the series resistance, also true with a U pad yet the U fixes the source Z to the input transformer unlike the H.

The U pad has been the universally recognized method for padding mic inputs for decades, and works equally well with the vast majority of line sources.  I would have built out a U pad with multiple steps,  starting at -15 or -20.

But hey, if no one complains, does it make any difference?  Load Z closer to matching is a ‘feature’ of at least one other product, yet it still uses the U pad. 

I went down the road of hacking this layout to a U pad, it could be done but it's fairly complex with at least 20 trace cuts and probably a wired path restoration or two.  Given the 150/600 Z switching, I'd make the U shunt for the 150 since errors should be lower to the 600 than if you shunted for 600 and switched to 150. 
 

mitsos

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No one uses an H or T on a mic input for very good reason,

So, U-pad on the front and T-pad on the back would be the recommended method? 
 

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