NTP cards - Is it a FET compressor?

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rafafredd

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Jun 3, 2004
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I´ve got two cards sometime ago with four of those circuits. I was ready to take it apart for the parts (it has Lundahl output transformers, a nice output stage and an interesting NTP balanced line receiver) when I had a second look at the schem. Now I think I´m going to keep it and trying to use it as it is, because now I see it as a FET compressor. So, Is it a compressor?

http://hps.infolink.com.br/rafafredd/ntp_card_schem.gif

There´s also another part of the board that is a logic/control, but I haven´t got the schem for this part, and I don´t intend to use it.

So, I think I just need to add a side-chain and I´m in???

Let me know what do you think, and sorry if it´s not a FET compressor at all. I know nearly nothing about FET compressors. If this is the case I´ll just take it apart for parts.

Thanks for any inputs!
 
I don't know if that would be a compressor, but I offer another suggestion.. A fet in the audio path can also be used as a mute device. my console has automated muting, and it uses FETs to do the job.

hope this helps. :guinness:
 
C106/R104 and the strange A103 feedback suggests that there has been done something to reduce distortion outside of the on/off states. Which means that it is probably some simple (non-chritical) remote volume control?

For precision-predictable work, you would use fet to gnd (like 1176), not in series. But that won't give as large an operating range as the series coupling like this one. The fet seems to act like an adjustable series resistor into the inverting opamp input - giving you close to zero voltage across it when it's open. I like.

Try it with a variable DC control voltage. If it sounds anywhere near-decent between on and off, my guess is that it's a remote level control. To make it a compressor, throw together a simple sidechain.. :grin:
 
this is the only info i could find on a fet as a mute. kinda similar to what we see on your schematic..

http://www.ham-kits.com/ProData/PDschematicPg1.PDF

EDIT: just got done looking at the NTP schematics at Jakob's site.. they all show the FET to ground like he mentioned.. this one is different. I think we all agree that it's some kind of volume control, but I still don't think it's a compressor. :sad:

are there any numbers on the boards? like part numbers maybe?
 
Well, if it´s a gain control, I think I can try it as a compressor... I hope it´s fast enought. If I like it, I´ll use it as a compressor.

Can I just feed some DC at A102 pin 2? Anyone have a guess on the DC range for 0 to infinite attenuation?
 
I´m no designer, so make sure you understand it before clicking on the link below:

http://hps.infolink.com.br/rafafredd/ntp_card_schem_test_mod.gif

This is a possible mod for a simple test if this thing can work as a compressor. Do you think the mod is ok? I hope you can read my mouse-drawn-letter and symbols.

Maybe I should add a dual opamp for buffering the signals before the bridge rectfier...
 
[quote author="rafafredd"]Well, if it´s a gain control, I think I can try it as a compressor... I hope it´s fast enought. If I like it, I´ll use it as a compressor.

Can I just feed some DC at A102 pin 2? Anyone have a guess on the DC range for 0 to infinite attenuation?[/quote]
Those two stages look like a drive level limiter followed by a 1/2 wave rectifier stage to me... and so you'd feed some sort of AC into them, I'd think. It's not too easy to estimate exactly what the function of the FET is, but since one side of it is AC coupled into the summing point of A103 which is a virtual ground point, and the DC biassing looks like it's come from R103 and R109 and is slightly unusual... I'd say that this was likely to be a mute circuit, simply because any signal modulation of that FET is going to introduce rather a lot of modulation distortion in the output.

I'd try feeding some signal into A102 via a series resistor (pin 2 is also a virtual ground) - I suspect that when the threshold level's been reached it will switch. But without staring at this for ages, I'm not entirely sure whether you are driving this thing on or off - if you see what I mean.
 

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