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Old 15th July 2012, 08:14 AM   #1
andyr is offline andyr  Australia
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Default Effect of different values for R1 in a passive composite RIAA network?

Please see the passive composite RIAA network shown in Figure 1 of this paper:
Phono Stage Design

In the schematics for several phono stages in this section, I have seen a similar RIAA network used - but with both 20K & 47K for R1. In another circuit (not from here), they used 240K.

Is anyone able to tell me what effect the value of R1 has on the RIAA circuit? In experiments I've done, it seemed that using 200K instead of 47K resulted in a signal loss - so the signal out of the RIAA when R1 was 200K was less than when R1 was 47K. Does this seem reasonable?

If so, then surely a low value for R1 is preferable, to minimise the (huge! ) signal loss produced by the passive composite RIAA network?

Secondly, I have seen values for R0 from 510K to 2M2 - is there any benefit in using 2M2 (compared to 510K)?

Thanking you in advance,

Andy
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Old 15th July 2012, 10:35 AM   #2
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Filter theory always assumes zero impedance drive and infinite impedance load. The variations you are seeing are due to the intrusion of reality on this scenario.

xkcd: Experiment
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Old 15th July 2012, 11:59 AM   #3
andyr is offline andyr  Australia
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Quote:
Originally Posted by Mark Kelly View Post

Filter theory always assumes zero impedance drive and infinite impedance load. The variations you are seeing are due to the intrusion of reality on this scenario.
C'mon, Mark ... reality is what we have to deal with.

Sure "Filter theory always assumes zero impedance drive and infinite impedance load". Given this, I would like to know what the effect is of having:
* a lower R1 compared to a higher R1, and
* a higher R0 compared to a lower R0.

Vintage is over ... shirley you've got the spare time to answer this (simple to you, with your knowledge) question.

Regards,

Andy
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Old 15th July 2012, 12:38 PM   #4
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That's what I'm trying to say. It depends on which particular reality is intruding.

If you are using a V/T stage based on high mu, high impedance V/Ts such as the 12AX7 the optima will be very different from those that will apply if you are using a bipolar input op amp such as the AD797.

There are no generalisations
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Old 15th July 2012, 01:23 PM   #5
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The series resistor plus the following cap and R values determine the filter time constants, and that is fixed for RIAA.
But it also means that you can play with the values for R and C as long as you keep the time constant right.
This is all very well described in the page you linked to.


In a tube preamp you usually deal with higer impedances and lower currents, so you need to scale impedances up. That means higher R, lower C for the same TC. In ss opamp circuits you deal with lower impedances and higher current so you can scale the impedances down, lower R, higher C.

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Old 15th July 2012, 09:05 PM   #6
andyr is offline andyr  Australia
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Quote:
Originally Posted by Mark Kelly View Post
That's what I'm trying to say. It depends on which particular reality is intruding.

If you are using a V/T stage based on high mu, high impedance V/Ts such as the 12AX7 the optima will be very different from those that will apply if you are using a bipolar input op amp such as the AD797.

There are no generalisations
I'm using JFET gain stages either side of the RIAA network.

I'm not asking you for the "optimum" value of R0 & R1 ... simply whether there's any advantage in having 200K for R1 rather than, say, 47K - given the higher value seems to increase the loss in the RIAA network.

And is there an advantage in having 2M2 for R0, instead of 510K?

Thanks,

Andy
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Old 15th July 2012, 09:34 PM   #7
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Quote:
Originally Posted by andyr
I'm not asking you for the "optimum" value of R0 & R1 ... simply whether there's any advantage in having 200K for R1 rather than, say, 47K - given the higher value seems to increase the loss in the RIAA network.
Assuming the correct values are used so the network has the right time constants when loaded: a high impedance network will be easy to drive but needs a higher impedance load and will have more thermal noise, a low impedance network is harder to drive but will have less noise and can cope with a lower impedance load.

The network loss will be exactly the same in any case, if correctly driven and loaded. A quick scan through that article shows no mention of thermal noise, which may be why it is not clear on the actual range of values needed.
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Old 15th July 2012, 09:41 PM   #8
andyr is offline andyr  Australia
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Quote:
Originally Posted by janneman View Post
The series resistor plus the following cap and R values determine the filter time constants, and that is fixed for RIAA.
But it also means that you can play with the values for R and C as long as you keep the time constant right.
This is all very well described in the page you linked to.


In a tube preamp you usually deal with higer impedances and lower currents, so you need to scale impedances up. That means higher R, lower C for the same TC. In ss opamp circuits you deal with lower impedances and higher current so you can scale the impedances down, lower R, higher C.

jan didden
Thanks, Jan.

Yes, I know R1 (and the Zout of the 1st stage) needs to be taken into account when calculating the rest of the RIAA component values, and 200K will produce different values than 47K.

I'm just wondering - given I have a choice - which would be better in my situation: R1 = 200K or R1 = 47K?

Regards,

Andy
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Old 15th July 2012, 09:43 PM   #9
DF96 is offline DF96  England
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If you can drive 47K then use 47K as you will get lower noise. Or even lower if you can drive it.
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Old 16th July 2012, 02:40 AM   #10
andyr is offline andyr  Australia
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Quote:
Originally Posted by DF96 View Post
If you can drive 47K then use 47K as you will get lower noise. Or even lower if you can drive it.
Thank you, DF96 - that's exactly the kind of answer I was looking for . I will go for 47K, then, as I know I can drive this. (I may well be able to drive 22K but I would have to experiment first.)

Can you provide any advice on my 2nd Qu ... given that Zin of the 2nd stage JFET is around 15 Meg, is there any advantage (or disadvantage) in using 2M2 for R0, rather than 510K?

Thanks,

Andy
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