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Richard Lee's Ultra low Noise MC Head Amp
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Old 20th July 2019, 11:53 AM   #731
Hans Polak is offline Hans Polak  Netherlands
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Digging a bit deeper into the LF bump problem, I tried to find the pro's and con's of various measures.

Starting with the damping as suggested by Bonsai:
It has no impact on noise and some minor effect on THD and Gain, nothing to worry about,
Damping is indeed applied, but for a Duraglit amp that has just enough dynamic range without damping, it seems rather unacceptable that DR may be reduced up to 10dB depending on the setting.
So when using this damping by placing resistors in the output line, this has to be done with great care.

Another way of getting rid of the LF bump is to reduce the output cap from 220uF to a much lower value like somewhere between 10uF and 22uF.
No reduction in DR will happen and no tuning will have to be done when using large caps for the base voltages of both transistors.

In the Image below, the left column applies when using a collector current of 2.8 mA, the right column for 6mA to reduce noise if needed.
Simulation is done for a 5R (in green) and a 10R (in bue) Cart, having an output of 0.5mV@5cm/sec@1kHz, thus needing a Headamp with a gain of 10.

The three rows are resp. for 1) "no damping+220uF", 2) "220R damping+220uF" and 3) "no damping and 10uF".
For each individual situation, the effect of using a 0.5mF or a 5mF Cap is shown.
The effect of a larger Cap is mainly shifting the Bump to the "left", as can be well seen in the various plots.

I did the same simulations for a 2R and a 5R 0.25mV Cart, needing a gain of 20, and for a 1R and 2R 0.1mV Cart needing a gain of 50.
Results were quite comparable with the above.
So to conclude it can be destilated from the findings, that the larger Rs, the lesser the Bump problem, but I would strongly advise to always substantially reduce the two 220uF output Caps.


Hans
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Last edited by Hans Polak; 20th July 2019 at 11:55 AM.
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Old 20th July 2019, 06:26 PM   #732
Bonsai is offline Bonsai  Europe
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What load resistor have you used on the output Hans? 100 Ohms?
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Old 20th July 2019, 09:27 PM   #733
Hans Polak is offline Hans Polak  Netherlands
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Quote:
Originally Posted by Bonsai View Post
What load resistor have you used on the output Hans? 100 Ohms?
I have set the gain to exactly 20dB as needed for a 0.5 mV Cart, see text and images.
So for the four variations of Rs and collector current I have used 4 different load resistors to get exactly this 20dB gain, resp. 99, 162, 73 and 127 Ohm.

Hans
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Old 21st July 2019, 01:21 AM   #734
gerhard is offline gerhard  Germany
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Just stumbled across some Zetex/Diode, Inc transistors with healthy hfe.
BF=1100. There are more.

.MODEL ZXTN07012EFF NPN IS=1.7p NF=1 BF=1100 IKF=5 VAF=25 ISE=3E-13 NE=1.43 NR=1 BR=470 IKR=1 VAR=6.5 ISC=1.2p NC=1.5 RB=0.1 RE=0.03 RC=0.0097 RCO=0.2 GAMMA=10E-10 CJC=106p MJC=0.33 VJC=0.55 CJE=285p MJE=0.41 VJE=0.80 TF=0.4n TR=1.3n XTB=1.4 TRC1=0.003 TRB1=0.003 TRE1=0.003 QUASIMOD=1 Vceo=12 Icrating=4.5 mfg=Zetex

I was really looking for some DPAK to relief op amps of unnecessary heat (high VCC)
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Old 21st July 2019, 07:25 AM   #735
Hans Polak is offline Hans Polak  Netherlands
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Quote:
Originally Posted by gerhard View Post
Just stumbled across some Zetex/Diode, Inc transistors with healthy hfe.
BF=1100. There are more.

.MODEL ZXTN07012EFF NPN IS=1.7p NF=1 BF=1100 IKF=5 VAF=25 ISE=3E-13 NE=1.43 NR=1 BR=470 IKR=1 VAR=6.5 ISC=1.2p NC=1.5 RB=0.1 RE=0.03 RC=0.0097 RCO=0.2 GAMMA=10E-10 CJC=106p MJC=0.33 VJC=0.55 CJE=285p MJE=0.41 VJE=0.80 TF=0.4n TR=1.3n XTB=1.4 TRC1=0.003 TRB1=0.003 TRE1=0.003 QUASIMOD=1 Vceo=12 Icrating=4.5 mfg=Zetex

I was really looking for some DPAK to relief op amps of unnecessary heat (high VCC)
And the only supplier is Aliexpress ??
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Old 21st July 2019, 08:41 AM   #736
gerhard is offline gerhard  Germany
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< ZXTN07012EFFTA Diodes Incorporated | Diskrete Halbleiterprodukte | DigiKey >

Probably Mouser, too.

:-) Gerhard
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Last edited by gerhard; 21st July 2019 at 08:44 AM.
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Old 21st July 2019, 11:39 AM   #737
Hans Polak is offline Hans Polak  Netherlands
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Quote:
Originally Posted by gerhard View Post
Interesting transistor with a PNP complement although again with an unrealistic Rb = 0R1.
Once Bill will be ready to send me his “Wayne” design, I could compare these trannies with the ZTX pair.

Hans

Last edited by Hans Polak; 21st July 2019 at 11:41 AM.
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Old 21st July 2019, 01:07 PM   #738
Hans Polak is offline Hans Polak  Netherlands
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This is a similar simulation as #731, but now for 0.1mV output 1R and 2R Caps, the Headamp thus needing a gain of 50.
Because these Carts demand a very low amp noise, I have simulated this time for collector currents of 6mA and 12mA.
The image below is just for the version with 10uF output caps and no damping, just like the lower version of the three in #731.
Again, using 5mF instead of 470uF base Caps in combination with 10uF output Caps, gives a perfect FR.


Hans


P.S. Correction: In the right plot is written Ic 6mA, this should be 12mA.
The left one is correct with 6mA.
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Last edited by Hans Polak; 21st July 2019 at 01:11 PM.
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Old 21st July 2019, 02:23 PM   #739
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by gerhard View Post
Just stumbled across some Zetex/Diode, Inc transistors with healthy hfe.
BF=1100. There are more.
Interesting high beta is usually a counter indicator for low rbb but this device also has very low Vcesat.

The 60C/W for SOT23 is interesting also since the pads look like standard size, seems to almost defy physics. Notes say big 2oz copper land needed but I'm used to big metal area on back of package for that.
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Old 21st July 2019, 03:55 PM   #740
Bonsai is offline Bonsai  Europe
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Hans, on the Hawking, if you run your sims but look at the collector voltage of the transistors before the DC blocking caps (with or without dampers) what are you getting? This is with the 10uF capacitors
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