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Old 11th May 2002, 02:20 PM   #11
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Default LCR RIAA EQ

Jocko,

several people expressed concerns about hum or noise pick up in LCR EQ coils. However I've never seen any issues with this in practice. I've build LCR RIAA phonostages with the potted RIAA from Tango and also 'discrete' networks with seperate (unshielded) coils. No hum pick up, whatsoever. Also other friends of mine who built such phonostages never had problems. One of them uses homebrew coils.

Dicinger,

LCR RIAA EQ are not 'compound' they consist of two independent networks in series, one for the 318/3180 uS time constant and one for the 75 uS timeconstants. They can be tweaked separately with very little interaction. The only interaction between the two networks is due to the DC resistance of the coils. Theoretically the two timeconstants can be placed between different stages. But then you need two gain stages which are capable of driving 600 Ohms. It's also no problem to introduce the 'secret' 3.18 uS timeconstant into the 75uS part.

Regards

Thomas
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Old 11th May 2002, 05:48 PM   #12
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Glad the LCR network is ok. I still wouldn't do it that way. You can make a passive EQ with 2 RC pairs. Easier, cheaper, and probably a lot less work to get it accurate.

Jocko
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Old 11th May 2002, 06:10 PM   #13
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Default Passive RIAA RC EQ

Like this:

Jocko
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Old 11th May 2002, 06:14 PM   #14
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Default Re: LCR RIAA EQ

Quote:
Originally posted by Vinylsavor
concerns about hum or noise pick up in LCR EQ coils. However I've never seen any issues with this in practice.
This is good to hear... i have been tossing the idea around in my head of using LCR filters to build an active XO.

dave
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Old 11th May 2002, 07:01 PM   #15
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Jocko,

before trying LCR EQ, I used RC networks. Split networks, as well as compound ones, similar to the one you posted. After I tried the LCR EQ the first time, I never looked back.

One of the key advantages of the LCR variant is it's constant input impedance with respect to frequency. The RC network you showed, has an input impedance which drops as the frequency increases. It's accuracy will depend on the driver stage. For example in a tube circuit, the response will change with different tubes, or with ageing tubes.
Dave,

I've done exactly that in my system, which is two way active, with a passive crossover after the preamp. I've built a 2nd order LCR type XO. Works nicely.

Thomas
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Old 11th May 2002, 11:09 PM   #16
dice45 is offline dice45  Germany
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Thomas,

Quote:
LCR RIAA EQ are not 'compound' they consist of two independent networks in series, one for the 318/3180 uS time constant and one for the 75 uS timeconstants. They can be tweaked separately with very little interaction. The only interaction between the two networks is due to the DC resistance of the coils. Theoretically the two timeconstants can be placed between different stages. But then you need two gain stages which are capable of driving 600 Ohms. It's also no problem to introduce the 'secret' 3.18 uS timeconstant into the 75uS par
thank you, stand corrected, good news
You have probably forgotten more about forgotten more about inductors and trannies than i ever will know. However, whether LCR RIAA is compound or split is not such a vital detail for me. What its vital to me: the 4th TC, 3.18 s. Did you try it out already? Or does your LCR network happen to have an approx. 50kHz rolloff by occasion?
Me just knows rumours, but terrific ones. As you know, i am a bit slllooowww in building stuff, too much musing and planning. But it is so evident to me, it must be terrific.

Allen raves about it, Morgan Jones explicitely seconds Allen's recommendation and also the explanation for it in his book and two other guys tried it out already whom i had correspondence with about preamps and whom i told i plan to use split RIAA / 4 TCs in my preamp design. One tried it i a discrete SS preamp , the other one in his SRPP tube preamp. Both enthusing about it, never looking back to 3 TCs.
My preamp won't make it till Aarhus this year, but it will have this feature for sure. Until then i am happy to hear any other comment about the topic.
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Old 11th May 2002, 11:56 PM   #17
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Default Passive RC EQ

Quote:
It's accuracy will depend on the driver stage.
Yes. That is why I showed the exact value of R2 as 31912 ohms. You need to take into acccount the drive impedance, which we will assume is low and constant over the audio band.

Jocko
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Old 12th May 2002, 01:24 AM   #18
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Quote:
Originally posted by Vinylsavor
I've done exactly that in my system, which is two way active, with a passive crossover after the preamp. I've built a 2nd order LCR type XO. Works nicely.
Thomas,

Any tips. What kind of stages did you sandwich the filter between?

dave
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Old 12th May 2002, 05:07 PM   #19
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Dave,

I'm using an all-tube based system. The Xo is placed between the output of the linestage and the power amps. The linestage has a transformeroutput with a step down ratio of 4.5:1 yielding about 200 Ohms Zout. The XO has an Zin of about 700 Ohms.

Thomas
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Old 13th May 2002, 07:15 PM   #20
BrianL is offline BrianL  United States
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Default Calculating RIAA Eq.

"Transducer: wanted to know about how to design
RIAA playback EQ circuits:

Before designing an RIAA playback EQ circuit,
RUN, don't walk, and get a copy of
"On RIAA Equalization" by Stanley Lipshitz,
from the JAES in the now-forgotten pre-CD
days. Be sure to read also the follow-up
letter from Peter Baxandall. I'm sure this
can be purchased from the AES. (an earlier
version appeared as an AES convention preprint)

However, several people make a valid argument
for another zero in the playback eq curve at
something like 50k Hz. to compensate for the
fact that the EQ circuits in the record amps
don't continue to infinite gain at high
frequencies. This was not considered by
Lipshitz. See comments, for example in:

http://www.users.nac.net/markowitzgd/phonopre.html

As to using an inductor, if one were to do
so, why not a largish pot core of appropriate
ferrite material? (not that I'd necessarily
recommend the inductor approach)
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