AML ezAM16-500 output question

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Potato Cakes

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Jul 1, 2014
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Hello, everyone!

I have an AML ezAM16-500 kit that I recently built, and it does seem to work, but I noticed that there were two missing parts from the kit that are also not shown in the Colourbook instructions but listed in the schematic and the BOM. It is R64 (270R) and C8 (10nF) which are in series and tie the +/- connections from the output transformer secondaries. I've seen something like this on other designs, but I just don't know enough about electrical designs to know what those components' purpose is. I have a recording session coming up that will require all of my preamps, and before I put this one to use, I'd like to know if there is any real reason or benefit to having these components in or out of the circuit. I tried emailing Colin, but I think those guys have been super busy to respond to messages.

Thanks!

Paul
 

Attachments

  • AML ezAM16 output.pdf
    59.1 KB · Views: 12
Thanks for the info!

I was concerned that without it I would be missing out on some sort of subjective sound that Colin had intended to bestow upon this design. I'll just use it without it and see what happens. It is quiet and all the controls do what they say they should do.

Thanks again for looking at this for me!

Paul
 
Hi Paul,
The zobel network is designed to stabilise the output impedence  seen by the transformer ,
Lets say your transformer is designed to be loaded by 600 ohms ,and you load it with 10k there will be a subtle change in the frequency responce of the preamp.The zobel makes sure that what ever  the transformer is connected to  its presented with the same load , so stabilising the response of the preamp .

To illustrate ,
Here's two curves from the same transformer loaded by different values .

first the 600 ohm load,

5kohm to follow ......
 

Attachments

  • TUAC_TP125_Tx_Tst_600R.pdf
    7.8 KB · Views: 24
And the 5kohm  load,
You can see the effect ,
The capacitor and resistor in the zobel forms a hi pass filter across the transformer to damp out the high frequency bump you see in the curves .
As you can see the this all happens at a very high frequency ,so the difference wont really be knoticed by the ear .
 

Attachments

  • TUAC_TP125_Tx_Tst_5K.pdf
    7.8 KB · Views: 15
One other point worth mentioning ,
The amount of transformer core distortion  will also vary a bit depending on the load ,
Sometimes with transformer coupled preamps the 'core' effect is just what you want ,
Its worth checking the  impedence and maximum levels  on the inputs of your a/d convertor  ,just so your aware of the loading .
A transformer working at very high levels can benefit from an increase in load ,it will give a bit more headroom and a little less distortion ,
So basically,  for a slight  gain in  headroom and distortion ,you loose out a little on frequency responce.
Hopefully this gives you an idea of whats going on .
All the best,
 
Tubetec said:
One other point worth mentioning ,
The amount of transformer core distortion  will also vary a bit depending on the load ,
Sometimes with transformer coupled preamps the 'core' effect is just what you want ,
Its worth checking the  impedence and maximum levels  on the inputs of your a/d convertor  ,just so your aware of the loading .
A transformer working at very high levels can benefit from an increase in load ,it will give a bit more headroom and a little less distortion ,
So basically,  for a slight  gain in  headroom and distortion ,you loose out a little on frequency responce.
Hopefully this gives you an idea of whats going on .
All the best,

Tubetec,

I am just now seeing this post. Thanks for extra info.  Makes thing a little bit more clearer what is going on in there.

I did add the zobel components in there. Without them, it seemed that something funny was going on with the higher frequencies listening to my voice in a condenser mic. Your graphs may explain precisely what I was hearing, or at least perceiving.

Thanks!

Paul
 
Obviously different transformers, but a real AM-16 has a very large and audible treble boost if loaded with 5K+ versus 600R-2K. 
 
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