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In Reply to: RE: Well heck... posted by Lew on February 12, 2008 at 14:01:21
I'm wondering how old your original schematic is! The 'new' CCS was not created around noise issues, although it did help. The primary goal was to increase the differential effect of the voltage amplifier. It was an improvement in that regard over the older circuit (which also had zener diodes in it). Because it increased the differential effect, it also increased gain, decreased distortion, reduced noise and to a minor degree increased bandwidth.
Follow Ups:
Please clarify. Which of many possible CCSs in both the amp and preamp are you referring to?
The amps there is one for the voltage amplifier.
In the MP-1 there is one for each stage of gain in the phono.
But I've been referring to the one in the voltage amplifier of the power amps.
Notwithstanding the question about zeners, the CCS at the input of my amps is composed of both sections of an ECC99 in cascode configuration, so the net impedance is very high and it works very well. (I do understand the role of a CCS in the context of a dual-differential amplifier. I thought it was you who brought up the issue of noise, not me.) The ECC99 (not a 6SN7) was used as CCS because of its lower Rp and superior transconductance compared to a 6SN7. These qualities of the CCS tube were needed, because the 6900 needs a lot more current than does a 6SN7 to operate in a linear fashion. Obviously if you're using the 6SN7 in the dd-cascode, you don't need as much current, so another 6SN7 as CCS works fine there.
My major reason for using the 6900? I had a bunch of them, had heard them compared to a 6SN7 in my former MFA Luminescence, and wanted to try them in the Atma context.
if it were in production I'd be likely to use it. It would be more effective with the zener circuit I mentioned.
We tried something in the neighborhood of 20 different CCS configurations. Most of them represented some form of compromise- the one we settled on gave the lowest noise (it is noisier if you *bypass* the zeners, BTW) while at the same time the best differential effect- the best of both worlds. Plus it has a low parts count :)
Holy cow, I'd better hunt down a few more. All my CCSs are done with ECC99. I've got about a dozen spares.
So that opens up some possibilities!
Ralph,
Would a change to ECC99 apply to MA-1 amps?
Do let us know if you decide to offer such a CCS change.
I have a bunch of unused Soviet military version of these tubes now going to waste...
jb
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