ToolShedAmps Posted November 10, 2015 Share Posted November 10, 2015 Ok then, here we are. For those of you that haven't been following along I have been asked, nay, invited to build Matt Renaud's "Open Source" 4S Universal Preamp for the benefit of all forum members (both DIY'ers as well as Klipsch owners wanting to "dip their toes in the water (with Tubes)" and simply purchase one of mine.) I have agreed to create this "build thread" with the express purpose of informing through pictures and text, MY process of constructing a "variant" of this pre-amplifier. DIY'ers are more than welcome to build their own and leverage this thread for help from myself as well as other knowledgeable forum members to achieve a successful outcome. I will detail my thought process (as best as possible) to provide some insight into the whys and hows of the variant I will be building. DIY'ers are welcome to build the project "as written" (the link to the original project published on DIYAUDIOPROJECTS is listed below) OR, they can "feel free" to build my variation (in fact, I hope they do, I can always use more POSITIVE feedback). Without further ado, lets take a look at the original project...... http://diyaudioprojects.com/Tubes/Universal-Tube-Preamplifier/ After you have familiarized yourself with the circuit and design elements, we will look at what I propose to build instead... Stay tuned.... Matt. 2 Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted November 10, 2015 Share Posted November 10, 2015 Generous endeavor, thanks. Quote Link to comment Share on other sites More sharing options...
Schu Posted November 10, 2015 Share Posted November 10, 2015 In... Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 10, 2015 Author Share Posted November 10, 2015 Y'all get on in here and we'll talk about this a bit. The things that interest me about this design include; the use of a volume control "outboard" of the gain-stage. I will freely admit that I have NEVER done this before. However, from a purely pragmatic perspective, it is an interesting idea and it will be retained (unless of course there is some absolutely compelling reason it should not.) What is interesting is that it is not only attenuating "volume" but, also shifting the output impedance. AND, any "noise" being generated or induced by the control itself is not being "amplified", and only becomes a minor contributor to the sonic puzzle. I VERY much like the simplicity of the design, in my experience "less stuff" in the signal path the better. However, I will side with a few individuals who concur with the idea that "universality precludes optimization". SO, this build will NOT be universal. If anyone out there thinks they need a pre-amp with nearly 35db of gain, please chime in and state your case. (You "Low Output Moving Coil" guys/gals need a phono-stage any way you dice it. ) Anyway, this ought to get the party started. Cheers! Matt. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 10, 2015 Share Posted November 10, 2015 I can't comment too much before I see the schematic. I presume we are still dealing with a common cathode configuration with the only difference being that the load resistor and or cathode resistor's values will be optimised for the specific valve chosen. This is essential to the design for if the volume control is staying where it is the stage needs to be very linear for the valve will be seeing up to 5.6V peak to peak at it's grid with some sources. Same thing with the bias, stock it sits at 2.3v, this will clip with some sources. For whomever decides to build or buy this I strongly recommend using as short as possible interconnects with this preamp. William has built this and has a thread on it somewhere in the forums. I am sure he will chime in with the changes he has made. Quote Link to comment Share on other sites More sharing options...
MyOwn Posted November 10, 2015 Share Posted November 10, 2015 This is a qoute from Bruce at Transcendent Sound High current output stage drives very long interconnect cables. This feature greatly improves the sound of any system as cable capacitance degrades sonic purity. Quote Link to comment Share on other sites More sharing options...
MyOwn Posted November 11, 2015 Share Posted November 11, 2015 As I have already stated I have built this preamp and I am well satisfied with it. The distortion figure of 1% is at 2V PP if I am reading it correctly and I do not believe I am using near that much so it stays below this figure. Music without some 2nd harmonic is very sterile and not very romantic. Not a very good choice of words but that is what comes to mind. Most say it has no soul. I have had amps in the past with basement distortion measurements and the sound is not nearly as pleasing as one with some 2nd and a touch of 3rd. Jean Hiraga is noted for one of the first to make this statement years ago and it is still the standard of higher end amplifiers. This is probably what makes a tube pre sound so much better to me with a SS amp. I did make a few changes but nothing significant. JP and Maynard both contributed and the biggest change was the plate resistor from 100K to around half 47K the specific figure if my memory is correct specific for the 12AU7 tube. I have not buttoned mine up yet and am still thinking of making some minor changes such as capacitor upgrades. It has a great sound stage and will make a SS amp with a decent sound stage a great sound stage. The choices of tube preamplifiers start with the cheap Chinese offerings and then jump up into the thousands. I believe this will have a good market base and with quality parts and point to point build it should last many years unlike the Chinese counterparts. I'm thinking about doing this.... http://www.transcendentsound.com/Masterpiece.html with this.... http://www.firstwatt.com/sit2.html Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 11, 2015 Share Posted November 11, 2015 The distortion figure of 1% is at 2V PP if I am reading it correctly and I do not believe I am using near that much so it stays below this figure. G'day William! Where did you place your volume control? Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted November 11, 2015 Share Posted November 11, 2015 For those interested, attached is the schematic of the preamp variant (optimized for William's use) proposed by myself and former member "thesloth." As Matt pointed out regarding the unit he's initially going to build, the gain and output impedance will vary. I'm not a big fan of "universal" designs as I've found the compromises to outweigh any advantages. I'm curious to read about the outcome. It might make more sense to make the output impedance adjustable to compensate for the preamp's possible use with SS equipment having only a 10-20k input impedance. Maynard wdecho preamp.pdf Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 11, 2015 Author Share Posted November 11, 2015 Good Morning William, Good Morning All, My last post started to describe what I thought was interesting about the design. This post I will continue with another item that I find interesting and beneficial about the original design. The "Ground Lift Switch", after reading through much of the associative feedback from the original build, many who had built this pre-amp had a bit of difficulty "wrapping their head" around this in terms of how it was actually done. In this build thread I will describe in great detail how to do this. Additionally, many DIY'ers can benefit from some of the grounding methodology that I employ as a "rule", many of my amplifiers are used on speakers that exceed 102db efficiency and are "dead" quiet. We will talk about "Earth" ground, "Chassis" ground, and, "Signal" ground. This will be a great primer for those of you who wish to minimize noise in your own builds. I will not talk about grounding "theory" but rather, I will simply describe and execute the methodology that I have found to be successful regardless of application. Secondly, the original project uses a power supply design that I find to be an excellent example of a "Tube Rectified" design in keeping with my own power supply design ideals. Those of you who want to design your own SS bridge or voltage doubler to employ a less expensive solution or perhaps use a power transformer you already have, please feel free. However, if you do this, please keep in mind that a SS rectifier imparts its own signature to the sonic "footprint" and therefore will no longer represent what the original designer intended. My next post will provide a revised schematic (based on what the designer intended) of the circuit. Additionally, I will follow that up with changes to the power supply to support the changes to the basic circuit. The last thing of note; if DIY'ers intend to employ (at some point) a "motorized" volume control, the value of the Stereo pot MUST be changed from 250K (per the original schematic) to 100K as there are no motorized 250K Stereo pots available. In an effort to maintain the "sweep" of the original designed in volume pot, we can detail how to "strap" a fixed resistor to the Stereo 100K pot for those of you who wish to build the pre-amp "straight". Until the next post, Cheers! Matt. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 11, 2015 Share Posted November 11, 2015 I decided to at least try this preamplifier as the original designer made it with the volume control at the output. I guess I am the last one to the party this morning. I figured someone else would have picked up on this and pointed it out but I guess I will do it. William if you want to confirm by doing some measuring that would be cool. Since you have placed the volume control at the output the distorion (1%) won't change where the volume control is no matter how low you go it will always be 1% or higher. I do not believe I am using near that much so it stays below this figure. Like I said it might be beneficial to actually measure distortion at different output levels, but in theory the distiortion will be 1% even at 20mV output. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 11, 2015 Share Posted November 11, 2015 (edited) Hey Matt, where's the dang schematic already However, if you do this, please keep in mind that a SS rectifier imparts its own signature to the sonic "footprint" and therefore will no longer represent what the original designer intended. Please explain to the class and myself how in this design (Class A) a SS rectifier imparts it's own sonic "footprint". Edited November 11, 2015 by xxJPMxx Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted November 11, 2015 Share Posted November 11, 2015 Hey Matt, where's the dang schematic already However, if you do this, please keep in mind that a SS rectifier imparts its own signature to the sonic "footprint" and therefore will no longer represent what the original designer intended. Please explain to the class any myself how in this design (Class A) a SS rectifier imparts it's own sonic "footprint". In laymans terms for the kids at the back of the class if possible. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 11, 2015 Share Posted November 11, 2015 Hey William, you don't have to change the pot if you like it the way it is. I was just hoping you could throw the distortion analyzer on it and measure distortion at different output levels. Also let us know what load it's being put onto to. If not I am certain the distortion on the output doesn't change. I call them effect boxes and people get offended sometimes. If you are intentionally adding sound (2H is an octave above fundamentals) to your recordings it's not far from the truth. I have no problems with people doing this, some hi fi guys do though. I try to design stuff with low distortion myself, but there will always be some......second harmonic is the most harmonious. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 11, 2015 Share Posted November 11, 2015 (edited) FFT of original circuit w/ 12AU7 and 100k plate load. 1kHz @ 1Vrms input. 250mV output 1.9% THD FFT with 12AU7 and William's 47k plate load. 1kHz @ 1Vrms input. 250mV output .98% THD (which is what William measured in real life also) As you can see it's not all 2H. The 47k looks better! Edited November 11, 2015 by xxJPMxx Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 11, 2015 Author Share Posted November 11, 2015 Alright, here we go again, I guess I just like a certain amount of "SAG" in my PS. The 1st thing you will notice about my "revision" is that there isn't a 12AU7 in sight. In fact, this tube isn't a "Dual-Triode" at all. It IS however approximately the same as 1/2 of a 6SN7 in it's own envelope. I've done this for a couple of reasons which I'll get into after you all take a look at this. Ok, lets talk about this then. Matt. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 11, 2015 Share Posted November 11, 2015 Alright, here we go again, I guess I just like a certain amount of "SAG" in my PS. What are you measuring for a current change in the PS during signal? Class A operation generally doesn't sag since the average current draw doesn't change. And thanks for the schematic!! Any valve choice is better than the 12AU7 IMHO Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 11, 2015 Author Share Posted November 11, 2015 Alright, here we go again, I guess I just like a certain amount of "SAG" in my PS. What are you measuring for a current change in the PS during signal? Class A operation generally doesn't sag since the average current draw doesn't change. And thanks for the schematic!! Any valve choice is better than the 12AU7 IMHO You wanted an answer for why I don't like SS rectifiers, so, I provided one, actually, I believe there to be a difference in sound beyond the "scientific" breakdown of any power supply, if you don't believe me, build the circuit with both then do a "listening" test and judge for yourself. And, you are absolutely correct. ALL Class-A, Single-Ended designs draw "constant current" regardless of load, that's why forum members should be informed to NEVER turn on a Single-Ended Class A amplifier without speakers attached as you may damage your output transformers. Matt. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 11, 2015 Share Posted November 11, 2015 Looks like up to 22db gain. I ran some quick simulations using the same parameters. Average source input of 1Vrms, 1kHz sine wave. The distortion spectrum looks better, mainly 2H, but still rather high at 1.8%. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 11, 2015 Share Posted November 11, 2015 You wanted an answer for why I don't like SS rectifiers, so, I provided one, actually, I believe there to be a difference in sound beyond the "scientific" breakdown of any power supply, if you don't believe me, build the circuit with both then do a "listening" test and judge for yourself. And, you are absolutely correct. ALL Class-A, Single-Ended designs draw "constant current" regardless of load Your initial response for why you like tube rectification was "sag", which this circuit doesn't exhibit by nature, just pointing that out so don't kill the messenger here. Quote Link to comment Share on other sites More sharing options...
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