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VSSA Lateral MosFet Amplifier

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Lazy Cat

Lazy Cat

Member
Joined 2007
sonnya said:
How much current flows in the VAS transistors??
Click to expand...

Yesterday I tested only outputs with resistor divider providing Vgg and current was stable at 440 mA, so outputs with fixed Vgg (voltage drop on resistor) have stable bias completely.

VAS current set to 15 mA, very stable (14,5-15,5 mA) before employing bias to outputs.

When both running on the same heatsink, PTC is obvious, so output bias is going up because 2SA/SC gain is rising with temperature.

I will check with reference zener (1-100 mA shunt regulator) as bias spreader so the outputs will have constant Vgg, meaning constant bias, no matter of VAS current fluctuations. In that way cumulative process will be avoided, VAS bias will stop rising at certain temperature.
 
Lazy Cat

Lazy Cat

Member
Joined 2007
Lazy Cat said:
I will check with reference zener (1-100 mA shunt regulator) as bias spreader so the outputs will have constant Vgg, meaning constant bias, no matter of VAS current fluctuations. In that way cumulative process will be avoided, VAS bias will stop rising at certain temperature.
Click to expand...

TLVH431BQ adjustable precision shunt regulator from 1,24 to 18V, 100uA to 70mA, -40 to +125°C temperature range, most suitable to replace bias trimmer for ALF mosfet. In this way Vgg and therefore output bias current will stay constant as once set, regardless of VAS bias current variation. In this case 2SA/SC could stay on the main heatsink to ensure smallest PCB possible. :cool:
 
G

gaborbela

Member
Joined 2005
Hello
I used to have similar problem with these topology. Especially with higher bias.
All do I never mount the VAS to the main heatsink.
wahab made some change in the circuit, I want to see if that will improve the stability.
Also I'm interested how you guys succeed to resolve these issue.
The frustrating think to me the Darlington I used the best sounding amplifier I built over 22 years or so.
Soon I will test the Lat FET version..
I want to hear if will be difference sound wise between the darlington and the FET.

Greetings Gabor
 
sonnya

sonnya

Member
Joined 2001
mirand-audio.dk/?post_type=product
Lazy Cat said:
TLVH431BQ adjustable precision shunt regulator from 1,24 to 18V, 100uA to 70mA, -40 to +125°C temperature range, most suitable to replace bias trimmer for ALF mosfet. In this way Vgg and therefore output bias current will stay constant as once set, regardless of VAS bias current variation. In this case 2SA/SC could stay on the main heatsink to ensure smallest PCB possible. :cool:
Click to expand...

Yes this part fixes the bias problem, but still the bias of the VAS transistors Will be highly depending of the heatsink temperature.
 
Lazy Cat

Lazy Cat

Member
Joined 2007
gaborbela said:
The frustrating think to me the Darlington I used the best sounding amplifier I built over 22 years or so.
Soon I will test the Lat FET version..
I want to hear if will be difference sound wise between the darlington and the FET.

Greetings Gabor
Click to expand...

Hi Gabor

I suggest you to use ALF mosfets, they have low Cgs and high transconductance. I couldn't find better ones for the job in this simple topology. Also they're stocked in Farnell or Newark (Element 14).

I'm sure you'll be pleased with the sound with these new parts incorporated. ;)
 
A

AndrewT

R.I.P.
Joined 2004
VSSA has 6 transistors.
That qualifies in my book as very simple.

Do I have to read past post1 to discover that it becomes very complicated SA later?

I started reading the SSA and TSSA and had to give up with both because both completely forgot the topic of the Threads ! SIMPLE
 
lineup

lineup

Member
Joined 2005
sonnya

sonnya

Member
Joined 2001
mirand-audio.dk/?post_type=product
AndrewT said:
VSSA has 6 transistors.
That qualifies in my book as very simple.

Do I have to read past post1 to discover that it becomes very complicated SA later?

I started reading the SSA and TSSA and had to give up with both because both completely forgot the topic of the Threads ! SIMPLE
Click to expand...

Well.. The TSSA first couple of schematics are very simple and they works.
 
AKN

AKN

Member
Joined 2005
Paid Member
Hi,

Nice little amp.
AHsJFJhC1JVdTiwNFHjIzQRYghQooATggIBTTTRlyNVL0Z4COQLoGuCnQgAduFxBEWgoDaMibGzFWSPmU4csj+wM1PKj1okIIUjp6WEESB+BGGKhQHUrj5U6CaosGTVmjgxZfDhtogIpxgAJAgBgmZTpC5kNAxkcmaIq1B1PgSotWCgQzyIagHx4afCnFZqFdF5IKWCKgGENQxAxKdGi1yBCpWrMCCDCAC0TAdrsuGRDyAcYFIJ0QSDAAIIEiQBEYpWEh5JXTySZAQBmxYADD8KQaOQCTi8cXFZ5cGRBDYRUZd6AuiFnIIhOW1QQucCBFxJMLM7QFTgiioNZheYuAgwIADs=
:)

My :2c:
Importance of feedback resistor tolerance. How about positive and negative transfer functions, perfectly equal? Why not trim one feedback resistor in relation to the other? Great opportunity to 'zero' distortion.
 
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