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PostPosted: 17 Jan 2012, 01:06 
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This is all new to me (tube audio) and what great fun. While I am awaiting parts I did up the circuit with what I have on hand (all recycled) with 1/8 watt carbon film resistors and Vishay caps and it sounds really good. I wondered about the high voltage at 215VDC and all I have that will go that high is a Variac and powered up the filament with a lithium battery. I noticed the stray noise goes away at 180 or so volts but still works down to a few volts but the hum is really bad if you pick up the battery pack or touch anything grounded at those lower voltages. I have nothing going to earth ground or a line filter in line yet but like the results so far. The higest temperature I get in the circuit (bottom side) is 120F and of course the tube (topside) is around 160F.

I am running it through a Quad 33 for testing. With the 33 wide open the preamp is quite impressive and all I had on hand were 1.0uF caps for the preamp and 470uf caps for the power supply. I am sure when I build it to specs it will be even better but so far I am having a blast. Can't wait to run it with some serious mosfet power as I am using a 7868 based amp out of an old organ and am quite pleased. Couldn't find any 1uF K40Y9's but did find some .47's and will parallel them hoping to get the same or close to it results. The first prototype had .22 Wima's but the 1uf sure brought out the lows and no need for an EQ now.

I will have to say those big caps gave quite a spark when I plugged in the Variac and will sure get the load resistor and X2 type cap in the finished preamp but in the meantime I am getting great results. Can't wait to get the PIO's, Film's and transformer in and think a remote volume control is also in order for the $$$. If I had it all over to do again and if I build another it will sure be the kit at < $500.00 (hopefully after shipping) is really quite a bargain if you have to find and purchase all the parts as simple as it looks the price of components have really skyrocketed in the last 20 or so years.

I will also have to say I have been using Morcheeba and Crystal Method tonight for testing and it sure is cool not having tone controls or EQ as they don't need any tailoring with this dude so far. Tomorrow I will try Dark Side of the Moon and Time Warp after I build the other channel.

In the meantime I am still having fun building it from scratch with old junque.

EC


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PostPosted: 17 Jan 2012, 08:23 
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Found the sites

http://www.riccibitti.com/pcb/pcb.htm

http://www.youtube.com/watch?v=D8M5BIo0 ... r_embedded

Had good luck with these. Your mileage may vary.
EC


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PostPosted: 17 Jan 2012, 15:09 
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Hi, I believe when you get it all together you will be quite happy with it. There are many out there now, both diy and kit and they perform well. There are some new updates that I have not added to the original project yet. The basic changes are to increase the B+ on the anode of the SRPP to 280-305 VDC (by reducing the resistor values in the power supply filter string) and change the cathode resistors in both the upper and lower triode sections to 820 ohms. The changes shift the operating point slightly to a lower distortion point. The changes were not intuitive and didn't show up in CAD simulations. They were found when I made up test beds for the design and varied the values of resistors and voltages while measuring the distortion on my distortion analyzer. The new values reduce the already low distortion by nearly 50%. It is now pretty much at the limits of my signal sources and ability to measure.

Good listening
Bruce

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PostPosted: 26 Jan 2012, 15:20 
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I am reprinting a question I PM'd Bruce here so if answered it can benefit all.

I am interested in scratch building this preamp and have a couple of newbie questions. I am looking to use this design as a bare-bones line stage preamp and connect to a solid-state amp with an input impedance of 15K 2v sensitivity (Proton D1200). You spec this design to have an output impedance of 3K and I can't tell if the 2K2 resistors determine this or if they are only to buffer the sub out. I am not going to add the sub out so are they necessary at all as in other desings the coupling cap directly to the output? You also discuss these resistors can change in value from 220 to 2700 as appropriate - what determines this considering my or any other application?

You do mention in discussion that this design is appropriate for higher impedance (input) amplifiers, yet you also seperately mention you do not design for solid-state. I read this as you will not design a solid-state amp/preamp not that this would not work with a SS amp.

One other design question not detailed on the print. If the heaters are running DC is there a polarity WRT pin 4 and 5 on the ECC802S?

Thanks for the great thread on this design.


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PostPosted: 26 Jan 2012, 21:42 
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Hi Everyone, Here is the answer I sent earlier in response to the PM (I deleted some extra information that is not germane to this thread)

The 2.2K resistors do two things. First they provide isolation between the two outputs. If you don't have the need for two you can eliminate one of the resistors. The second function is to help mitigate the effect of cable capacitance on the preamp. You can lower the resistance if the cable runs are short and / or low capacitance. With cables under 6 feet and of reasonable capacitance feeding power amplifier impedance over 10K you can go to a value in the 1K range.

The output impedance of the stage itself is in the 3K range. This is a fairly typical value and will work with nearly any power amplifiers. The best matches are when the ratio of impedance (preamp output to amp input) are more than 1 to 3 (approximately). That is why I said it was really best with tube amps. They are nearly always above 50K and more usually in the 100 to 500K range. However, most newer solid state amps now have impedances over 10K and for them it seems normal to be in the 50K to 100K range.

Yes the preamp uses DC heaters. I have not been able to determine which polarity through the heaters is better than any other one. Generally if I use a dual triode (like the ECC803S) with six volts on the heaters I will generally connect the center tap (pin 9 in this case) to the ground side of the heater supply. A consideration sometimes occurs. That is in the SRPP design when you are running in the 300B+ range or higher. The middle cathode will be at about half the B+ applied to the upper anode. To prevent heater to cathode failure you need to apply a dc reference voltage to the heater supply (like in the preamp). At the higher B+ levels it does make a slight difference. Then you need to figure out which half of the tube's heaters is heating which triode. Then you want to make sure (on 12 volt heater systems) which heater is going to be +12 and which will be 0. The six volt difference can make a difference in the dc reference needed. All this is to say that unless you use really high B+, the actual place you apply the dc reference is not important.

None of the components in the Forewatt are difficult to get. Most can come from Parts Express, but Mouser is probably a better source. Oddwatt Audio is not presently providing parts or partial kits.

The SS regulator is quite helpful in making the preamp quiet and a consistent performer.

Added: I want to thank all the folks that have interest in my projects. I try to design things that excel in performance and are fairly easy to duplicate. I appreciate the questions such as above as it is a trigger to either learn something new or pass on knowledge to others so that they may learn. That is a key aspect with my conception of diy. :)

Good listening
Bruce

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PostPosted: 27 Jan 2012, 13:07 
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Bruce,
Thank you so much for the clairification. So I am to understand your original Forewatt PS print, since both left and right B+ is depicted, that the filiment represented is the parallel of both filiment sections for each tube. That is to say you would connect pin 9 to 6v negative and tie pin 4-5 together to +6v regulated, referenced to B+ with one 0.1uf 50v film cap across 9 and 4-5?

Sorry if this should be obvious to me.


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PostPosted: 27 Jan 2012, 14:23 
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Hi You are correct. :)

Good listening
Bruce

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PostPosted: 29 Jan 2012, 09:10 
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gofar99 wrote:
The basic changes are to increase the B+ on the anode of the SRPP to 280-305 VDC (by reducing the resistor values in the power supply filter string) and change the cathode resistors in both the upper and lower triode sections to 820 ohms.


Hi Bruce,

You are reducing both 2k2 1W resistors on the power supply schematic? Do you have recommended values?

And to be clear, the 820 Ohm resistors are replacing the 470 Ohm resistors on the preamp schematic?

Thanks!


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PostPosted: 29 Jan 2012, 12:12 
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Hi, Sorry for the confusion. There are two sets of 2.2K resistors. One in the output of the preamp (like a post just a few days ago) and the others in the power supply filters.

Reducing the 2.2Ks in the filters to 1.5K or even 1.0K will boost the B+ a little. Try to get 260-265 at the input to the LR8Ns. Then set them for 250 going to the tubes.

Yes replace the 470R resistors with 820R resistors.

The changes shift the operating point slightly and improve the sound noticeably.

Good listening
Bruce

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PostPosted: 30 Jan 2012, 00:25 
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gofar99 wrote:
Try to get 260-265 at the input to the LR8Ns. Then set them for 250 going to the tubes.

Thanks again for your reply, but now I'm really confused. Where is the input to the LR8Ns?

John


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