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PostPosted: 09 Mar 2011, 12:41 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, I would try a 100k in series with the input and see if it will do what you need. I agree on the padding of the output :( . Generally not a good idea. A cathode follower can be used. If you carefully select the parts, you can make it lose a lot of the signal, maybe as much as 50%. usually this is not what you want, but it can work. A good tube for the cathode follower is the 6N1P dual triode. Cheap and has good noise performance as well as low impedance output as a cathode follower. If you juggle the values carefully you can direct connect it to the SRPP (no coupling cap). You still will need a cap on the output. It will need to handle about 1/2 the B+ and be fairly large. I used WIMA 4.7uf polys in this location. 10uf would be better. You can use a quality electrolytic there, but be sure to bypass it with a poly.

Good listening
Bruce

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PostPosted: 10 Mar 2011, 16:43 
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Joined: 31 Dec 2010, 16:53
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Location: Florida, US
Thank you,

It is not finish yet, but following your advice, hopefully I will make sound good. :D
Attachment:
preamp1.jpg

Attachment:
preamp2.jpg

Laz.


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PostPosted: 10 Mar 2011, 20:27 
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Joined: 14 Feb 2010, 13:13
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Location: Lisbon, Portugal
Congratulations Laz! It looks very nice! :)

Enjoy the tone! :P

Miguel


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PostPosted: 10 Mar 2011, 21:10 
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Location: Arizona, USA
I like the red paint, it matches the wood very well. All my reference amps, preamps and such have wood cases. I think it adds a nice touch to a diy project. Good show :king:

Good listening
Bruce

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PostPosted: 27 Mar 2011, 09:57 
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Joined: 26 May 2009, 18:51
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Location: Lilburn, Georgia USA
Hello, I have finally gotten around to building my ForeWatt preamp and have a question before I proceed. After finishing the power supply section I plugged it in and checked the voltages. It has 311vdc B+ no load no tubes in it. Would it be ok to plug in some old 12AT7 tubes I have to check the loaded B+ voltage? Thanks, rparsh


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PostPosted: 27 Mar 2011, 11:25 
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B+ will decrease when You put tubes....It's OK.

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Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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PostPosted: 28 Mar 2011, 21:27 
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Joined: 26 May 2009, 18:51
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Hi Azazello, My voltages turned out perfect values. I just wanted to be sure before I put in the $20 each.ECC802S JJ tubes. It works fine and really wakes up my KT88 OddBlocks. Sounds great too. I used 1uf Solen for output caps. Used 1N5422 Schottky bridge for heaters into a 7806 regulator. Comes out 6.1vdc. There was more to building the Forewatt than it looks like on paper. I haven`t seen many pictures of Forewatts in here. Thanks, Robert Parsh


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PostPosted: 29 Mar 2011, 15:51 
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Hi Yes the voltages are about as expected without tubes in place. The preamp is deceptively simple, but is a great match for the Oddblocks. I suppose it should be as I planed it that way. One thing to consider if I haven't already mentioned it is that the output is inverting. With some systems this can cause sonic issues. It will usually show up as having soloists seem to be behind the speakers instead of in front of them. The simplest way around this is to reverse the polarity of connections ate either the power amps or at the speakers (not both). Depending on your system the change can be rather dramatic. BTW the Oddblocks do not invert.

I'm doing a long term listening test of a 6N1P variation of the Forewatt. It has more gain (not really needed in a line stage) and a lower output impedance that may be of benefit to some systems. the original Forewatt works best into 5K ohms or more, the 6N1P version can go as low as 1K. As soon as I am happy with it, I'll post that change here.

Good listening
Bruce

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PostPosted: 30 Mar 2011, 18:35 
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One thing to consider if I haven't already mentioned it is that the output is inverting. With some systems this can cause sonic issues.

Is it possible to invert the signal again (un-invert)?

Have you heard of someone using an opamp to invert the signal?


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PostPosted: 31 Mar 2011, 15:51 
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Hi, A good question and yes it can be inverted again. However to do so will add compenents to the signal path. Nearly always, the fewer parts in the signal chain the better. In a related design that I am working on I do that. It requires a second gain stage to do it though. That is more a consequence that I needed more gain (it is a phono preamp) than the desire to flip the signal. The second part of your question is also good. An op amp can be used as a unity buffer/inverter. Sure. Same thoughts though. Why complicate something if all you need to do is swap the speaker leads. This is simple, as the preamp will most likely be in line for all input sources. There is the arguement about mixing SS and valve components as well. It makes no difference to me, both work, I prefer the sound of all tube amplification, but have been known to use SS stuff when it does what I want best. So yes you can do it, my choice in this case would be a good audio IC in the OPA family. IHMO less coloration than trying to get a tube stage to do the same thing.

Good listening
Bruce

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