Hi,
I am considering a new RIAA based on the Pearl 1. It's for MM only, so I want to use only two parallel 2SK170 in the input stage.
As I want to keep the same gain level as the original, I will increase R19 from 499 to 1000...
But I think I then also will have to change the value of the RIAA resistor R23 (6k8), as the output impedance from the first stage will be increased...
Is that correct?
Thinking Ro=R19, then RIAA resistance is 499+6800 = 7299 in the original.
In the modded: 7299 - 1000 = 6299, this must be the new R19 value?
BR🙂
I am considering a new RIAA based on the Pearl 1. It's for MM only, so I want to use only two parallel 2SK170 in the input stage.
As I want to keep the same gain level as the original, I will increase R19 from 499 to 1000...
But I think I then also will have to change the value of the RIAA resistor R23 (6k8), as the output impedance from the first stage will be increased...
Is that correct?
Thinking Ro=R19, then RIAA resistance is 499+6800 = 7299 in the original.
In the modded: 7299 - 1000 = 6299, this must be the new R19 value?
BR🙂
Tough choice -- the paralleled JFET's reduce the noise in the front end and increase the gain, but this stage will overload with a MM cartridge at frequencies over 10K with peak in put.
Bob Cordell showed the way in which passively loading the JFET input (from 47k to, I believe 18k) rolls off the high end, but you need to know the inductance of the cartridge, and make some other adjustments down the line.
Bob Cordell showed the way in which passively loading the JFET input (from 47k to, I believe 18k) rolls off the high end, but you need to know the inductance of the cartridge, and make some other adjustments down the line.
Well, I have found some related topics about the overload issue, and it seems to be basically unclear (information wise).
So far I have concluded:
1. The overload issue has nothing to do with parallel FETs (2 or 4), if gain is equal. The problem is the high gain linear first stage at high frequency combined with high-output MM...
2. A solution would be to reduce first stage gain, and by keeping R19 at 499R resistance will half the gain....(with 2 FETs)
3. As many users are reporting no problems with MM on Pearl (and a few with some problems), it is likely not all, but high output MM above a certain level...
It could be nice if someone could state a guideline regarding this, like: "With MM putting out above 3mV/1kHz, the first stage gain can be maximum 23dB"
Anybody? 😛
So far I have concluded:
1. The overload issue has nothing to do with parallel FETs (2 or 4), if gain is equal. The problem is the high gain linear first stage at high frequency combined with high-output MM...
2. A solution would be to reduce first stage gain, and by keeping R19 at 499R resistance will half the gain....(with 2 FETs)
3. As many users are reporting no problems with MM on Pearl (and a few with some problems), it is likely not all, but high output MM above a certain level...
It could be nice if someone could state a guideline regarding this, like: "With MM putting out above 3mV/1kHz, the first stage gain can be maximum 23dB"
Anybody? 😛
Hi,
the pearl 2 has 53dB of gain which is a lot for an MM cartridge.
Normally a gain of around 40dB is used. So you must reduce the resistor value even further (by aprox. 10 dB.)
William
the pearl 2 has 53dB of gain which is a lot for an MM cartridge.
Normally a gain of around 40dB is used. So you must reduce the resistor value even further (by aprox. 10 dB.)
William
Hi,
sorry, I overlooked the pearl 1 in the first post. Pearl one has almost 60dB of gain which is even more than the pearl 2 so also a bit much for a normal MM cartridge.
Reducing R to 120R would give around 10dB of gain for the first stage and around 40 in total.
William
sorry, I overlooked the pearl 1 in the first post. Pearl one has almost 60dB of gain which is even more than the pearl 2 so also a bit much for a normal MM cartridge.
Reducing R to 120R would give around 10dB of gain for the first stage and around 40 in total.
William
You are right about the 60dB gain, but you forget the -20dB loss in the passive RIAA...
The Pearl 2 has +10dB more gain than Pearl 1, made in the second stage...
The Pearl 2 has +10dB more gain than Pearl 1, made in the second stage...
The first stage gain will be reduced 3dB by halving the number of JFETs, but the problem isn't in the first stage. (On its own, with no feedback the first stage measures under 0.02% and gain is ~27dB.) So you don't want to cripple the first stage.
Take a look at the "Building a Pearl 2" thread for some suggestions. http://www.diyaudio.com/forums/pass-labs/204336-building-pearl-2-a-49.html
Since Pearl 1 and 2 have similar input and passive RIAA stage, could the problem be in the +10dB additional gain in the Pearl 2 second gain stage?
Your measurements in post #485 in the Building a Pearl 2 thread does seems alarming....
Have you observed the same thing with a Pearl 1?
Your measurements in post #485 in the Building a Pearl 2 thread does seems alarming....
Have you observed the same thing with a Pearl 1?
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