Why do inverse-RIAA filters reduce sound level?

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jeffthompson
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Why do inverse-RIAA filters reduce sound level?

Post: # 36842Unread post jeffthompson
Thu Aug 20, 2015 12:30 pm

I'm curious why the schematics for inverse-RIAA filters all reduce the sound level, often by -60db or even more? On my lathe (which is new to me, so I'm still getting used to it), the sound level from my filter is too low to cut.

The other question would be: where does the filter go in my signal chain? If it's reducing volume, I assume it's the last step before the cutter head?

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markrob
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Re: Why do inverse-RIAA filters reduce sound level?

Post: # 36847Unread post markrob
Thu Aug 20, 2015 12:51 pm

Hi,

The reason is that with a passive network of this type (e.g. no transformers) the highest voltage gain you can achieve is unity (a straight wire). If you look at the RIAA curve for cutting, you can see that the curve provides for boost starting from the lowest frequencies with the maximum boost at 20 Khz. The only way to achieve the desired curve this is to start out by have a very high insertion loss (large gain cut). As you approach 20 Khz the gain approaches, but never achieves unity gain. You need to apply makeup gain in front or after (or some combination of the two) the network. Hope that makes sense.

Mark

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jeffthompson
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Re: Why do inverse-RIAA filters reduce sound level?

Post: # 36848Unread post jeffthompson
Thu Aug 20, 2015 12:56 pm

That does make sense, thanks Mark!

To clarify: you're saying that I can/should put an amplifier either before (to beef up the signal before being cut) or after (to bring it back up)?

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markrob
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Re: Why do inverse-RIAA filters reduce sound level?

Post: # 36851Unread post markrob
Thu Aug 20, 2015 4:06 pm

Hi,

You'll need to add gain someplace ahead of the power amp input to bring the level up to that needed by your amp. You and do this in one place (on either side of the network), or allocate some gain blocks on each side of the network. Keep in mind that the passive network wants to see a low impedance drive source (so it is not loading down the driver stage) and a high impedance input (so it is not loaded down by the following stage). You also have to be very careful that you don't run out of headroom in the stage that drives the network.

Mark


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studiorp
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Re: Why do inverse-RIAA filters reduce sound level?

Post: # 36855Unread post studiorp
Thu Aug 20, 2015 5:19 pm

Otherwise an impedance transformer ?

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markrob
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Re: Why do inverse-RIAA filters reduce sound level?

Post: # 36856Unread post markrob
Thu Aug 20, 2015 5:37 pm

Looks like it would work. They don't provide a noise specification, so that may be an issue. This would be best placed after the network (to provide 40db of makeup gain). You need to need to check if 50K ohms is high enough of an input impedance to avoid loading the network down.

Mark


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jeffthompson
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Re: Why do inverse-RIAA filters reduce sound level?

Post: # 36865Unread post jeffthompson
Fri Aug 21, 2015 8:23 am

This is helpful, thanks. My knowledge in this area is pretty limited (I have a good understanding of mic preamps and guitar equipment but this is new territory for me).

I did a quick search, but are there any good threads on here about people's suggested signal chains? I think that would be a really helpful conversation, but probably separate from this thread.

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dimi751
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Re: Why do inverse-RIAA filters reduce sound level?

Post: # 37133Unread post dimi751
Sun Sep 06, 2015 8:44 pm

Funny that you raised this question I had the same problem a few days ago I did get enough excursion to the cutted head.. As soon as I placed an dj mixer to before the amplifier I was then about to get the proper levels to the cutted head, great advices from Jesus thanks!

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