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Old 2nd May 2003, 08:29 AM   #71
miguel2 is offline miguel2  Portugal
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In this schematic by Joe Rasmusssen there are a couple of changes in the chip circuit relative to Kuei's version. I believe, for quick test purposes, that the tube can be implemented in this inverted version proposed by Kuei. In this case we just have to add the tube. But the gain of the tube may overload the chip so some resistor in between may be necessary. Also, I have +/-28V rails, maybe I can get away with it. The only problem is that I have a problem with catode followers

Miguel
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Old 2nd May 2003, 08:32 AM   #72
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Quote:
Originally posted by miguel2
But the gain of the tube may overload the chip so some resistor in between may be necessary.
The gain in Joe's tube stage would be just under 1 so no need to pad it down.

dave
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Old 2nd May 2003, 08:41 AM   #73
miguel2 is offline miguel2  Portugal
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Thanks Dave.

I just noticed a 1n3 cap in the chip input. This is something to worry about? Does the tube input can cause some oscillations in the chip?

Miguel
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Old 2nd May 2003, 08:48 AM   #74
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Quote:
Originally posted by JAZZ2250
What do you think about this schematic? I have no idea how low the output impedance will be. But, according to the schematic, it provides "low impedance". Sorry for my ignorance. I'm still reading a tube related book. Very slowly.
Doesn't seem anyone has responded to this, so maybe I can be of some help.

It's a mu-follower, a variation of SRPP (Single Rail Push-Pull) and as such is a PP circuit (some will disagree with that).

Download this PDF file from TubeCad (allow it a little time if you have a modem connection), it explains both of them in depth, first SRPP, then goes onto the mu-follower variation:

http://www.tubecad.com/articles_2002...onstructed.pdf

The output Z is not that low, we did a 6922 SRPP and got, from memory, 1600 Ohm.

To be honest, this is probably my least favourite tube circuit!!! It can sound nice but rarely excites, it is prone to vagueness. BUT many love it, maybe you will, after all this is only an opinion. Too me it is almost too tubey, if you know what I mean. I like precision too.

This leaves me with an opportunity to air this hooby-horse of mine. Why is it that I've come across so many 2A3 & 300B SE amps, and yet they use SRPP input/gain stages. Not really Single-Ended IF you use a PP front-end.

Joe
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Old 2nd May 2003, 09:52 AM   #75
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Quote:
Originally posted by miguel2
Thanks Dave.

I just noticed a 1n3 cap in the chip input. This is something to worry about? Does the tube input can cause some oscillations in the chip?

Miguel
In short... no... and no.

The end product of this is to shape the HF bandwidth and its value is tunable by ear, so once built do experiment.

It is not unusual for SS designers to put a passive bandpass filter on the input stage, to block possible DC and, to me more importantly, reduce potential slew induced distortion. In this case, I believe you will find the end result a warmer sound.

Joe
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