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Old 14th December 2015, 09:50 AM   #1
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Default Classi RIAA preamp questions

Dear folks,

I know, there are a lot of RIAA preamps arround and most of them are quite "better" than the solution I would like to build. But start with" simple" things is better (and cheeper) than build a high-end which cost horrobil mony and dont work Click the image to open in full size.

So I decided to build a preamp which Eli show here:
12ax7 Phono Preamplifier

I routed a small PCB with two channels and it works so far. A little humm for which I have work on ( other routing ?? )

Questions:
1) I do not understand the 20M resistor at grid of second triode. Spice simulations shows no difference if I use a standart 1M grid leak resistor instead.
2) Measure the output with open input ( no cable attached to PCB ) I measure some hum but also a random DC level which move +/- 200mV. At one channel I measured +/-400mv so I changed the tube. Its better now but I wonder what is the reason for this fluctuation??
3) Routing the PCB to get as best as possible hum free output is the major nightmare on all my projects. So I decide to use 12V DC for heaters and big ground areas. Does a star grounding make sens on PCB if I already use ground areas ( possible double sided PCB )?

Thanks a lot for any hint.

Best regards
Karsten
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File Type: pdf RIAA-7025.pdf (37.9 KB, 79 views)
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Old 14th December 2015, 10:41 AM   #2
DF96 is offline DF96  England
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Quote:
Originally Posted by karsten21
I do not understand the 20M resistor at grid of second triode. Spice simulations shows no difference if I use a standart 1M grid leak resistor instead.
If your simulation shows no difference between 1M and 20M grid leak resistor then this means that the triode model does not correctly do grid current.
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Old 14th December 2015, 11:35 AM   #3
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Hello DF96,

Yes might be that the used spice model does not implement complete grid current. But in fact this does not enlight me concerning how a 20Meg resistor has to be set as grid leak resistor.
Do you have any explanation for this..please ?

EDIT:

Aiieee: Did the schematic use a grid bias using the 20Meg and the few micro Amp from Grid current to bias the tube?
It seemed to me correct but... I read that this way of biasing a tube is not preditcable and therefor should not to be used.

The issue now is: I can change this to a "normal" kathode biasing but how to measure the used bias voltage? A normal multimeter has 10Meg as input resistance... measureing on a 20Mge weill return... soemthing...

Sorry for editing...

Karsten


Thanks a lot
Karsten
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Last edited by karsten21; 14th December 2015 at 11:46 AM.
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Old 14th December 2015, 12:44 PM   #4
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by karsten21 View Post
I can change this to a "normal" kathode biasing but how to measure the used bias voltage?
Measure the anode voltage. Then use 'normal' bias and adjust until you get the same anode voltage. I would just use a red LED though.
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Old 14th December 2015, 01:08 PM   #5
RajkoM is offline RajkoM  Bosnia and Herzegovina
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DF96 is right.

Karsten21, try with this Nakabayashi model for 12AX7, works good in LTspice.


*
* Generic triode model: 12AX7
* Copyright 2003--2008 by Ayumi Nakabayashi, All rights reserved.
* Version 3.10, Generated on Sat Mar 8 22:41:09 2008
* Plate
* | Grid
* | | Cathode
* | | |
.SUBCKT 12AX7_AN A G K
BGG GG 0 V=V(G,K)+0.59836683
BM1 M1 0 V=(0.0017172334*(URAMP(V(A,K))+1e-10))**-0.2685074
BM2 M2 0 V=(0.84817287*(URAMP(V(GG)+URAMP(V(A,K))/88.413802)+1e-10))**1.7685074
BP P 0 V=0.001130216*(URAMP(V(GG)+URAMP(V(A,K))/104.24031)+1e-10)**1.5
BIK IK 0 V=U(V(GG))*V(P)+(1-U(V(GG)))*0.00071211506*V(M1)*V(M2)
BIG IG 0 V=0.000565108*URAMP(V(G,K))**1.5*(URAMP(V(G,K))/(URAMP(V(A,K))+URAMP(V(G,K)))*1.2+0.4)
BIAK A K I=URAMP(V(IK,IG)-URAMP(V(IK,IG)-(0.00058141055*URAMP(V(A,K))**1.5)))+1e-10*V(A,K)
BIGK G K I=V(IG)
* CAPS
CGA G A 1.7p
CGK G K 1.6p
CAK A K 0.5p
.ENDS
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Old 14th December 2015, 06:13 PM   #6
rongon is offline rongon  United States
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Quote:
1) I do not understand the 20M resistor at grid of second triode. Spice simulations shows no difference if I use a standart 1M grid leak resistor instead.
I've modeled both Eli's RIAA and Thorsten Loesch's 'El Cheapo' RIAA (ECC83 to ECC88). Both use grid leak bias (aka contact potential bias) on the second stage.

The Ayumi 12AX7 model that RajkoM posted is the one I use. I find it still doesn't model grid current correctly, so grid leak bias doesn't work well with it.

The idea behind using grid leak bias is that you can put a mammoth resistance loading the first stage and EQ (20M ohms). The bass EQ will work much better that way.

If you're worried about it, maybe you could make your PCB with pads for a cathode resistor and cathode bypass capacitor for the second stage, and simply short them between the tube's cathode and ground to try out grid leak bias with the 20M resistor. If that doesn't work for you, you can change the value of the grid leak to something like 510k or 1M, take out your jumper wires, and install your cathode resistor and bypass cap (or an Infrared LED, or whatever). You'll also have to change some values in the EQ if you put in a cathode resistor.
--

What happens if you put a 1 ohm cathode resistor in your simulation? You should see maybe 0.001V across that resistor, which would indicate 1mA being drawn through the second stage 12AX7.
--

Last edited by rongon; 14th December 2015 at 06:18 PM.
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Old 14th December 2015, 06:20 PM   #7
DF96 is offline DF96  England
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The original reason for grid leak bias is that it allows the cathode to be directly grounded. This means that noise from AC heater-cathode leakage is grounded too; a direct connection will always have lower impedance than a bypass capacitor. Useful for low noise input stages.
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Old 14th December 2015, 06:33 PM   #8
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Grid leak, AKA contact, bias works well only in low signal level situations. A phono preamp is a low signal level situation. As previously stated, the huge grid leak resistor lightly loads the EQ network, which improves bass extension. Thorsten Loesch originated the concept.

FWIW, something on the order of 2 V. of bias should be found, when the 20 Mohm resistor is employed.

While computer simulations can be beneficial, only cut and try yields fact(s).
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Old 14th December 2015, 06:37 PM   #9
rongon is offline rongon  United States
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Quote:
While computer simulations can be beneficial, only cut and try yields fact(s).
Amen to that!
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Old 14th December 2015, 07:25 PM   #10
rayma is online now rayma  United States
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Quote:
Originally Posted by Eli Duttman View Post
the huge grid leak resistor lightly loads the EQ network, which improves bass extension.
The RIAA bass roll off is set by the coupling capacitor and the DC load after it.
A 1M grid resistor and 0.1uF cap (1.6Hz) would be fine. No need for 20M in this respect.

Last edited by rayma; 14th December 2015 at 07:27 PM.
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