Another amplifying stage, inspired by Blowtorch

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dimitri

dimitri

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Joined 2002
Hello,

I opened this thread to answer questions on the amplifying stage, inspired by Blowtorch. The project was published in Audioxpress, July 2020, pp.44-49. Schematic and PCB files in ExpressPCB format, BOM, gerber files can be downloaded from Audioxpress web site.

I have a couple sets of fully stuffed and tested boards, which I can lend to interested parties for auditioning. You should have 30V bipolar dc supply to power it.

Thank you,
Dimitri
 

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dimitri

dimitri

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Joined 2002
When there is large difference (>0.5V) between input voltage and voltage across capacitor, then diode is conducting and capacitor (dis)charges through resistor.
When the voltage difference is small diode is not conducting and effectively disconnects resistor from capacitor. Capacitor holds the charge.
This is conventional technique for condenser/electret microphone input stage.
 
bogdan_borko

bogdan_borko

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Joined 2005
dimitri said:
When there is large difference (>0.5V) between input voltage and voltage across capacitor, then diode is conducting and capacitor (dis)charges through resistor.
When the voltage difference is small diode is not conducting and effectively disconnects resistor from capacitor. Capacitor holds the charge.
This is conventional technique for condenser/electret microphone input stage.
Click to expand...

Thanks, nice tip! I love these practical things I've never seen in common literature..
Btw about "cap multiplier" you're using mosfet/bjt, why not just irf610 type mosfet, or darlington connection instead? All the variations have similar performance, is it about the sound?
Cheers, Borko.
 
bogdan_borko

bogdan_borko

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Joined 2005
Bigun said:
mosfet has high input impedance, bjt provides best transconductance as a follower and if needed it can be used with lower voltage drop. Higher transconductance means better sound. Mosfet conductance is less of an issue at high current.
Click to expand...

Ok, I know that part, as i remember from the simulation, PSRR of cap mul. varieties is fairly similar, that's why I asked. Never tried to compare different cap. multipliers with low psrr gain stage, I guess there could be a difference..
 
dimitri

dimitri

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Joined 2002
bjt provides best transconductance as a follower and if needed it can be used with lower voltage drop
Click to expand...
That is correct. BJT also has a base current, which makes a voltage drop on RC filter resistor. Resistor value should be not too high for low voltage drop/power dissipation on transistor. For the desired time constant filter capacitor value should be high enough.
 
bogdan_borko

bogdan_borko

Member
Joined 2005
dimitri said:
That is correct. BJT also has a base current, which makes a voltage drop on RC filter resistor. Resistor value should be not too high for low voltage drop/power dissipation on transistor. For the desired time constant filter capacitor value should be high enough.
Click to expand...

That's all correct, but just checked in MicroCap, there is almost no difference between IRF610 or darlington BC546/BD140 as serial pass devices in the same type of cap.mult., darlington only needs 47K instead of 100K in the RC for the base current. In the simulation, you get (in both cases) about - 65dB PSRR and less than 0.8 ohms output impeance with alo similar bandwidth etc..
Do you think it's up to models?
 
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