DIY Audio Projects Forum
http://diyaudioprojects.com/Forum/

ForeWatt Tube SRPP Preamplifier - Support / Update Thread
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2448
Page 48 of 68

Author:  gofar99 [ 14 Apr 2016, 22:01 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi, I not so sure I would call some of the things improvements...rather alternate ways of building it. If you use between 180 and 220 volts on the anodes then 470 ohms is the cathode resistor of choice. If you go up to the 250-270 then 820 ohms is the value of choice. These values were chosen for two reasons. First the make the current in the tube fall in a good range for linearity. Second was for low distortion. I must have done 100 test runs on various combinations of voltages and resistors on my distortion analyzer and those values were best.

The preamp in active mode (not the passive bypass if you built it with one) inverts the signal. So reasonabley you should swap the leads on the speakers (plus then goes to negative and negative to plus) to "re-invert" the signal. So much for theory. The problem is that not all source material was recorded with correct phase. It varies quite a bit and is absolutely unpredictable in advance and often in listening as well. Not only that but not all companion gear (like your amps, CD player, Ipod etc) have correct phase output either. There really is not good answer to this problem. To even complicate more relatively few individuals can tell if something is phased correctly and depending on speaker placement in your listening room the reflected sound may mask an incorrect phase.

Some clues...If you listen to certain kinds of music, often that with very clear central female vocalists and some instruments. If the artist or instrument seems to be in front of a line drawn between the speakers then the phase is likely to be correct for that particular piece of music. If the artist or instrument seems to be behind the line then it is likely to be reversed, again for that piece of music. In my system I have verified that all the phase relations are correct (not inverting) except the preamp (yes I use a modified Forewatt) so I swapped the speaker terminals. Is it always right, no. But it is what the source material was made with.

Good listening
Bruce

Author:  wdecho [ 15 Apr 2016, 02:53 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Thanks Bruce, you have cleared up the questions I had. I do understand the concept of phase. My speaker manufacturer, Klipsch, once sold a speaker with a certain crossover network and then had a notice sent out years later to reverse the mid driver connections at the crossover to be more in phase. This is common in crossovers. The fact that the sound was better in phase with the mid driver was missed when the crossover was designed. I have heard of the better hearing bunch that can tell if a recording is recorded with positive phase 2nd harmonic or negative phase 2nd harmonic. I guess what is best sounding with the Forewatt is determined by everyone individually by trying it both ways. I was waiting to hear from you about the cathode resistors and higher plate voltage changes and maybe do the reverse change at the output while in the preamp. With my old ears it sound great the way it is. I doubt I can discern the difference changing the output connections.

Author:  wdecho [ 15 Apr 2016, 10:27 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

I changed the cathode resistors to 820R this morning and bumped my B+ to 250VDC. I also reversed the wires on the output and no sound. I am thinking I need to put the 2.2K resistor (1.2K in my circumstance) and the output capacitor on the negative wire that is now going to the positive output. After changing the output as before it is playing again.

Author:  wdecho [ 15 Apr 2016, 19:14 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

I reversed the wires to the output again leaving the cap and resistor to the center of the output RCA Connector. Still did not work. My guess is that the input wires need to be inverted but I am not sure. Maybe Bruce will tell me how it is done. It works fine as the schematic shows the output.

Author:  gofar99 [ 16 Apr 2016, 11:36 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi, You don't change the preamp....just the leads at the speakers. If you swap them in the preamp it will effectively short out the signal and you will get nothing. Put the conections back in the preamp.

Good listening
Bruce

Author:  wdecho [ 16 Apr 2016, 12:03 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

gofar99 wrote:
Hi, You don't change the preamp....just the leads at the speakers. If you swap them in the preamp it will effectively short out the signal and you will get nothing. Put the conections back in the preamp.

Good listening
Bruce


I found out the hard way but have changed them back a couple of times experimenting. I am more familiar with SS amps and preamps and all that is involved inverting phase is switching the output connections. Live and learn.

Author:  wdecho [ 18 Apr 2016, 06:47 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Bruce the phase swapping is just as you said. I have a dozen or so amps I and have my system where I can change and compare amps in 2 or 3 minutes. Changing the phase at the speakers changes the sound differently from one recording to the next. On some it is better with normal hookup and others reversed. For amps that I use the forwatt with I will just reverse the connection to the speakers more for what I know in my head (knowing the pre inverts) than my ears. With the changes made (more B+ and 820R cathode resistors) I am liking the sound of this pre more and more with my SS amps. Very clean and quick.

Author:  a-soares [ 15 Jun 2016, 04:54 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi Bruce (and everyone else in the forum of course).
I was planning to build the Forewatt using the 6N1P some time ago (page 30-31), but for a number of reasons the construction stopped at the casing. Meanwile i've built a 6N2P/6P3S power amplifier so i returned to the idea of building the 6N1P version of the ForeWatt to use it as a preamplifier (i will eventualy submit the 6P3S built to DIY Audio Projects so that others can try it out).
My question is: with a transformer of 200v like the one i have for the pre-amp i think i will get around 240v DC under load (correct me if i'm wrong pls), and with the CRCRC filter i plan on getting around 200v DC rectified. With that voltage as B+ are the 390R cathode resistors the correct ones for the 6N1P?
My transformer has 2x200v Sec so i plan on using each separate secondary winding for each different channel. It also has 2x 6.3v so the build will be in fact a double mono design.
Hope to put some pictures soon

Author:  gofar99 [ 25 Jun 2016, 21:22 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi, That value may work, but figuring on a loaded down supply of about 220V and an idle current of 10ma (per channel) I would think a 270R or 250R would be be better. I seem to recall cooking one of these up some time ago. If I can find the schematic I'll post it. The problem with using those tubes is too much gain for a line stage. Most line stages run in the 3-7X range. The 12AU7/ECC802/ECC82 version is at 7 and really is quite sufficient.

Good listening
Bruce

Author:  a-soares [ 27 Jun 2016, 07:57 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

I already had some experience with th ECC802 and it is a really good tube. I've used them to replace the original tubes on a AR amp. But I don't have them on hand right now. On the other hand I have a bunch of 6N1P's collecting dust-also a great tube- and that is why I plan on using then. That without the cathode cap so that the gain is lower of course.
I will play around a bit with the values after I build the power supply section and see exactly what's the B+ value.

Page 48 of 68 All times are UTC - 6 hours [ DST ]
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/