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Old 28th December 2002, 01:52 PM   #1
diyAudio Member
Join Date: Dec 2002
Location: Houston
Default Help with Penultimate Zen amp.

I am starting the construction of a Penultimate Zen amp this week. My speakers are causing a few changes to be made to the biasing and output. The speakers are 15 ohm Lowthers in a very large folded TQWT enclosure. The bias is lowered to one amp, and the output coupling cap lowered to a 6800 ufd, instead of a 10,000.
My question is on the front end. Does the input 47.5K resistor need to be loaded down with a low impedance? Most sources would put a very low output impedance there. But I would really like to hang a passive front end on with a couple of mono pots and a switch to make a integrated Penultimate Zen.
My concern is that since the 47.5K resistor sets the input impdance putting a pot on the front end would add to this and screw up the feedback loop. Could something like a 100K pot be used instead of the resistor. Or is it okay to just add a pot, would prefer a 250K, to the circuit as designed.
I have a couple of tube output sources that like a least a 100K load due to the size of the coupling caps used. But for now I use 250K and it does seem to help.
My Zen amp is being put together to mimic a SET in several areas. The high impedances are a start. The lower power is another.

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Old 29th December 2002, 09:15 PM   #2
diyAudio Member
Join Date: Feb 2001
Location: Columbia, SC
While you can lower the bias, you don't have to. Actually, the amp should sound better with the bias left as it is. The power won't be reduced by lowering the bias (assuming that you don't take it so low that you current limit). If you're seeking to reduce the power, you will need to look at reducing the rail voltages.
The output coupling cap can be reduced without fear of penalty. Cheaper, too.
If the value of the pot is high enough, it would change the feedback ratio. This might or might not suit you. (Of course, you could always increase R3 in proportion.) The circuit would work, however. It seems to me that it would be easier simply to increase the value of the DC blocking cap in your sources.
I'm short on sleep. Maybe something else will occur to me after I get some rest.

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Old 29th December 2002, 10:30 PM   #3
diyAudio Member
Join Date: Dec 2002
Location: Houston
Default Changes to Penultimate Amp

Thanks for the input. I am fumbling along with the details, trying to change the amp to be better matched with a higher impedance and efficient load.
I am going to try the 1 amp current for two reasons, less heat and getting by with just one power transformer. I ordered the 38 volt Plitron instead of the 18 volt to alow using the outputs seperately instead of series connecting.
By reducing the value of the output coupling cap I hope to be able to use a higher quality cap. I have ordered both 4700 35v and 6800 25v Nichicon Muse caps. These should be better as a coupling cap compared to the Panasonics TSHA used for power supply filtering.
The input may be better left alone for now. I would like to get it working first and be able to judge the sonics before screwing around with the feedback.
In fact now I am leaning toward using a BUF634 between the volume pots and the input of the amp. Used one on the output of my old Pioneer PD-91 player and it was eye opening. I usually play with tube gear and this is my first project in solid state since the late 80's. And I still have some BUF634 left over.
My parts likes are flavored from using tubes so long, but have gotten some 200 v 20A MUR2020 rectifiers, the 63 volt 10,000 ufd Panasonics TSHA caps, big mutha heat sinks from Tanner, lots of old Holco and Allen Bradley resistors. and plan on using back to back 47 ufd Silmic caps instead of the 10 ufd poly caps. The small value caps (220 ufd) required are either KZ Muse or the ES that is used for coupling.
I figure this is what DIY is all about. The amp could be built just as Nelson put the parts list together. But I would not have left it alone long enough to hear it broken in.
Chasing the additional parts to build the input has set me back about about a week. Hopefully next weekend I can stuff the boards and start and the power supplies and casework.
Looking at the pictures in the PassDIY gallery has me now going with a wood box to house it. That should be much easier than aluminum.
I am excited!
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Old 30th December 2002, 03:26 AM   #4
diyAudio Member
Join Date: Feb 2001
Location: Columbia, SC
Tubes are cool by me. I was going to do my active crossover with 6922s until I started counting how many I'd need. That little reality check stopped me cold. I went solid state. However, I still have pair of monoblock tube amps (currently down for reworking--possibly back to class A) and a CJ Premier 3 in the system. Fortunately, the DC blocking caps in the preamp are big enough that they leave me latitude for a wide range of Zin. In addition, I built the crossover to have a fairly high (for solid state, anyway) Zin.

<i>Ha!</i> I have it!
That's what you need to build...a Zin amp...

Uh, sorry. That just kinda popped out. I promise not to do it again until the next time it happens.

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