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Old 10th November 2001, 12:48 AM   #1
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Location: Salt Lake City
I recently built a pair of Power Follower 99 MOSFET class A power stages. Could anyone reccommnd a vacuum tube input stage that would work well with this folllower. Needs to be 1K ohm output impedance or less and able to swing 10 to 15 V.
Any suggestions here would be greatly appreciated.
Mark A. Gulbrandsen
Salt Lake City, UT
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Old 10th November 2001, 09:17 AM   #2
Freddie is offline Freddie  Sweden
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Look at Headwize, there are some good tube circuits there: http://headwize.com/projects/index.htm .

I think Andrea's "No-Compromise OTL Tube Headphone Amplifier" is just what you are looking for.
Kurt Strain has designed a nice looking linestage/headphone amp too..

BTW: I run my Power Follower with a BoZ with higher PSU voltage.

/Freddie
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Old 10th November 2001, 10:27 AM   #3
stigla is offline stigla  Norway
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One good page to begin with would be:

http://www.users.globalnet.co.uk/~valveamp/

This is a simple anode-follower circuit thingy, usable with a great vareity of tubes. If you want low output Z, then the 417A/5842 would be a great tube. Pretty high gain, low Ri, and very good sound! Other tubes could be the new ECC99 or equ. (5687, E182CC).
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Old 11th November 2001, 01:49 AM   #4
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Hi Freddy, I may actually consider building the BoZ instead. What did you raise the PSU to and were there any other mosd done or did you build it as shown? My object is to keep it as simple as possible.
Thanks,
Mark Gulbrandsen
Salt Lake City, UT
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Old 11th November 2001, 09:55 AM   #5
Freddie is offline Freddie  Sweden
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Mark,

I increased the PSU voltage to around 95V, and I also increased the gain a bit by lowering the source resistor from 100Ohm to 80.
Otherwise it looks pretty much like the original BoZ.

/Freddie
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Old 11th November 2001, 07:43 PM   #6
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I use BoZ. It's nice.
Matheus
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Matheus
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Old 11th November 2001, 10:00 PM   #7
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Mark,
Any medium mu tube--12AT7, 12AU7, 6SN7, 6DJ8, etc.--will give you a 15V swing with a reasonable (1-2V) input. Hi mu tubes such as the 12AX7 will easily swing even more. If you're wanting Zout 1k or less, you'll need a cathode follower. No big deal there, either. So, if you're wanting a tube front end you could easily use the two halves of, say, a 6DJ8, the first as gain, the second as a follower. 150-200V rail, 10k plate resistor, 200 ohm cathode resistor. This will run out about 25-28 dB gain...more than enough. Burn any surplus gain as feedback, or use a smaller plate resistor to reduce the gain. For a cathode follower, use Rk 10-20k or so, same rail, and you're done.
If you want to go solid state, any of the Zen variants will do just fine.
If you want to get fancy, set the front end up as a differential and use the backside as the feeback input point.

Grey
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