Dual MOSFET in a package.

[JohnRoberts]

I have killed a lot of brain cells messing with VCAs even back before they didn't suck (THAT VCAs at least).

One topology that promises low noise and low distortion for unity gain is to use the VCA subtractively. For unity gain the VCA is completely out of the circuit so zero noise and zero distortion from a simple op amp. To command attenuation you just subtract more of the input signal. -20dB of path gain results from subtracting 0.9x of the original signal. Of course there is no free lunch so this is higher noise for deep attenuation than conventional topology. This is actually quite nice for a limiter comfortably running in the 0db to -12dB range. A little higher noise/distortion when commanding -12dB attenuation seems acceptable.

JR

 

[abbey road d enfer]

Another noise contributing factor is the OSI (Optimum Source Impedance) of the input circuitry. Increasing the input resistor (i.e. decreasing the AC current) allows increasing the input voltage without distortion penalty. However, when the input resistor exceeds the OSI, the Input Noise Current becomes dominant.
The 2164 datasheet is silent about that.
I had been made aware of that a long time ago, when THAT introduced the 202X.
A defence contractor had to produce a new batch of tank simulation consoles and the noise figure was not adequate because they used 200k resistors, which was good with the original 202, since the discrete opamp in the original 202 used a FET, but incorrect with the 202X.
Noise analysis of the 202X suggest it used a bipolar input, with an OSI of about 10k.
Edit: since the 202X uses 8x monolithic VCA's in parallels, the OSI is also divide by 8.

 

[Analog_Fan]

When in doubt, follow the datasheet recommendation. The "Pin Configuration" diagram shows clearly pin 4 labeled as V-, pin 11 labeled as V+, meaning the substrate pins can connect to power supply voltages.

Many MOSFET devices connect substrate to source internally, but for that device with shared substrate you would only do that if using a circuit configuration which connects the source pins of both devices together (see the examples in the "Typical Applications" section of the datasheet showing current mirrors and differential pairs with substrate pin connected to source connection).

true, mosfet are actually 4 pin devices.
Saw the video of Sam Zeloof making a DIY mosfet.

I have added Solder Jumpers, will see how it turns out.

 

[Analog_Fan]

I would just like to add that the N and P MOSFET pairs in the ALD1105 are not paired with each other (N with P) and that the difference is considerable. In addition, the P JFET pair LSJ689 is available, which can be paired with the N JFET pair LSK489 with some effort. The price is high (Part Search Results For "linear systems"), but the days when good JFET transistors were cheap are long gone.

Yeah, check the company InterFet, like 20us$/piece or more for a dual N channel.
But couldn't find the dual P channel. JFET Master Table - InterFET

i completed the pcb routing earlier today, I'm gonna stick to it.

 

[Analog_Fan]

I would just like to add that the N and P MOSFET pairs in the ALD1105 are not paired with each other (N with P) and that the difference is considerable. In addition, the P JFET pair LSJ689 is available, which can be paired with the N JFET pair LSK489 with some effort. The price is high (Part Search Results For "linear systems"), but the days when good JFET transistors were cheap are long gone.

what do you think of these 2 devices?
Fairchild - FDC6306P
Fairchild - FDC6561AN

Just in case, this works out satisfying, using a smd assembly service.
Other people have used parts in the past that weren't meant for audio application, but it turned out popular, like a CD4000 logic chip.

 

[JohnRoberts]

I looked at the first part and it is a self described switching device. I see no mention of Vgs threshold matching within the pair, perhaps important for use in a long tail pair.

JR

[edit- I don't understand the CD4000 reference, while CD4007 allowed addressing the individual devices. Back in the 70/80s it was not uncommon to repurpose medium power transistors for low noise use due to their inherent low Rbb. /edit]

 

[Analog_Fan]

[edit- I don't understand the CD4000 reference, while CD4007 allowed addressing the individual devices. Back in the 70/80s it was not uncommon to repurpose medium power transistors for low noise use due to their inherent low Rbb. /edit]

they used the CD4069UB in a VCF.

this is a clone:


It's sounds awesome.

you can barely see it.

Doepfer A-124 Wasp Filter​

 

[JohnRoberts]

they used the CD4069UB in a VCF.

this is a clone:


It's sounds awesome.

you can barely see it.

Doepfer A-124 Wasp Filter​

It was not unusual to use CMOS logic as low open loop gain inverting linear amplifiers.... Using a multiple section gate with feedback could make one shots or astable oscillators. In fact for an add on to my flanger kit back in the mid 70s I made a variable clock circuit using a CD4001 (quad nor gate).

For high precision voltage to period conversion (my preference for use in professional studio delay efx) I used actual high speed comparators like LM311. Keep in mind that was 40+ years ago.

JR

 

[Analog_Fan]

FDC6306P & FDC6305N are probably corresponding parts.
both have VDSS of 20V.
V GSS +/-8 V (don't know what that means)
but what about the "2.5 V" reference in their datasheet?

Supersot6 means TSOT23 6-Lead
i knew already about the FDC6306P and found the FDC6305N by accident while searching what supersot6 means.

 

[moamps]

Yeah, check the company InterFet, like 20us$/piece or more for a dual N channel.
But couldn't find the dual P channel. JFET Master Table - InterFET

https://store.nacsemi.com/products/search?part=LSK489USD7
https://store.nacsemi.com/products/search?part=LSJ689USD10.5

i completed the pcb routing earlier today, I'm gonna stick to it.

Good luck.

Fairchild - FDC6306P
Fairchild - FDC6561AN

Switching Mosfets, nothing special, a lot of similar faulty I've seen in SMPSs and servos. It may be possible to use them in audio designs, but I will stick to JFETs and old lateral types in my modest designs.

 

[ccaudle]

FDC6306P & FDC6305N are probably corresponding parts

The threshold voltage of the two devices is significantly different. If by "corresponding" you mean suitable for complementary pairs, that would probably be an important parameter.

both have VDSS of 20V.

Both have an absolute maximum Vdss of 20V. That just means they were made on the same or similar semiconductor process, so they have the same rated breakdown voltage.

V GSS +/-8 V (don't know what that means)

Maximum voltage from gate to source. More than 8V difference between gate and source voltages may damage the gate.

 

[moamps]

but what about the "2.5 V" reference in their datasheet?

It's optimized for (battery) power management applications.

 

[Analog_Fan]

https://store.nacsemi.com/products/search?part=LSK489USD7
https://store.nacsemi.com/products/search?part=LSJ689USD10.5

Bummer
Mouser has the FDC6305N ordered for Feb 2023, but the FDC6306P you need to request.
so they fall off.

Thank you for showing me the alternative parts (LSK489 & LSJ689)

TO-71 package looks interesting and making the footprint took me some time.

eevblog.com TO-71 pcb footprint

the leads are approx 0.5 mm, i had to set the outer diameter to 0.75 mm, auto was no option as they would overlap.

for the SOIC8 parts?
pin 3 and 7 are substrate?
negative for the P and positive for the N channel?

The down part is Mouser doesn't sell them, that will be an experiment for a later date, but the ALD1105 version will be build.

Using Mosfets the resistors over the differential amplifier are 22K, using jFET it will be 220K shows the simulator.
is that a good thing or not?

 

[abbey road d enfer]

One bandaid to this is sliding bias: at low signal levels (noise dominates) run it in class AB, then as the input amplitude rises (distortion dominates) slide the bias over to class A.

Your mention of sliding bias took me back 60+ years, when one of my first electronics book was a compilation of audio amplifier circuits.
Transistors were expensive at the time, many of these amps were single ended, and there was a whole section devoted to sliding class A single-ended amps.
Googling sliding class A returned a number of audiophools links where it was assimilated with the quite common arrangement of pre-drivers in class A, drivers in class AB and output devices in class B.
Only a few described proper sliding class A as an arrangement that increases the transistor bias in accordance with teh signal amplitude.

 

[abbey road d enfer]

TO-71 package looks interesting and making the footprint took me some time.
View attachment 93224
eevblog.com TO-71 pcb footprint

the leads are approx 0.5 mm, i had to set the outer diameter to 0.75 mm, auto was no option as they would overlap.

It is fairly common for this type of package to create a footprint with a larger diameter, requiring to fan-out the leads for insertion.

 

[Analog_Fan]

It is fairly common for this type of package to create a footprint with a larger diameter, requiring to fan-out the leads for insertion.

Thanks a lot for that suggestion, didn't think of that, normally i follow the datasheet's mechanical data strict.

theirs lead should be 1.27 cm, should work.

 

[Monte McGuire]

Using Mosfets the resistors over the differential amplifier are 22K, using jFET it will be 220K shows the simulator.
is that a good thing or not?

22kΩ will be a lot quieter than 220kΩ, all else equal.

 

[Analog_Fan]

22kΩ will be a lot quieter than 220kΩ, all else equal.

but, the amplitude is barely visible in the falstad sim.
used the mouse wheel to gradually increase resistor value to get output of a 10 V.p.p., where it needs to be.

Downloaded LTSpice to repeat the job again.
for now it seams the mosfet return a good result, compared to jFet, i can use 22K.

i tried introducing the the "differential" amp from the Moog Filter, but got no result, also no space on the pcb for through hole ...
unless i get it right and could use smd parts. the TL071 is used and fits and fully routed.

 

[abbey road d enfer]

but, the amplitude is barely visible in the falstad sim.

The amplitude of what?

Downloaded LTSpice to repeat the job again.
for now it seams the mosfet return a good result, compared to jFet, i can use 22K.

Since we don't know what is a "good result", what can we say?

i tried introducing the the "differential" amp from the Moog Filter, but got no result, also no space on the pcb for through hole ...

Have you noticed that the Moog filter circuit has almost nothing in common with the actual discussion?
You are losing us. Not the best to get help.

 

[Analog_Fan]

i know but that's where i choose the la

The amplitude of what?

Since we don't know what is a "good result", what can we say?

Have you noticed that the Moog filter circuit has almost nothing in common with the actual discussion?
You are losing us. Not teh best to get help.

sorry, not my intention, but that's was the inspiration to the choice of the (big) 100uF came from.