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Old 1st March 2004, 07:56 AM   #1
hacknet is offline hacknet  Singapore
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i`m planning my TDA1543`s analogue stage and i`ve stumbled upon a couple of designs involving tubes.

i`ve seached around and can`t seem to find the reason why everyone`s output stage that uses tubes will be a 6922 in SRPP.

could a do a simple cathode follower?
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Old 1st March 2004, 09:36 PM   #2
fdegrove is offline fdegrove  Europe
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i`ve seached around and can`t seem to find the reason why everyone`s output stage that uses tubes will be a 6922 in SRPP.
First of all you want to do I/V conversion.
This requires a low value gridleak resistor so this demands a tube that can handle this without excessive distortion.

A quick glance at the curves of the ECC88 family will show you this is a good candidate for this kind of service.
Add to that it's medium gain and high transconductance and you see why this is indeed the prime candidate for this kind of application.

Now, why the SRPP?
Well, the ordinary CF having a bad rep for playing see-saw with the B+ and currents you'd want to look at an alternative.
Enter the SRPP with its improved linearity and lowish Zo and by the look of things it seems as if we've killed two birds with one stone....
Alas, looks can be deceiving: the SRPP is far from perfect, it requires a very good PS since it's PSRR is very low. It also works best into a predefined load, no problem for a DIYer who only has to cater for his own gear.
For a manufacturer however this solution is far from ideal.

Can it work O.K.?
Certainly can and it's often the better of a set of compromises, just not always.

Hope this helps,
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