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    VTA PH15 Build thread PH14

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    plexus

    Posts : 94
    Join date : 2012-02-18
    Location : Toronto, Canada

    VTA PH15 Build thread PH14

    Post by plexus on Sun Apr 01, 2012 6:50 pm

    I will post my phono stage build in this thread. For now this is just and intro. The kit and other parts have been ordered and I will get building underway as parts come in and time permits. So, some background: I started with audio in the early 80's. Then in the late 80's-90's I was able to spend a lot of time a high end shop and listen to and explore all kinds of gear and expertise. Through that time I went through a number of upgrades. By 2002 I had pretty much the system I have now: Oracle Delphi MKI, Linn Ittok LV2, Benz Micro Ruby 2, Parasound PH100 phono stage, Dynaco Mk3, Martin Logal SL3 electrostatics.

    The Dynaco's are originals that had some upgrades when I get them. I put in a Welborne cap board. Eventually I bought glass epoxy driver boards and used all high quality matched parts with Jensen Al PIO bybass caps. This made a big improvement. In the last few months I bought my first VTA product: the MK3 driver boards with 6SN7s. That has been a major jump in clarity and depth over the stock design.

    Onto the phono stage: at the time I got the parasound I didn't want to jump into anything too expensive; and it seemed like you could push $10k with a phono stage. over the years I have learned (for myself) that there is a lot of hokus pokus in high end audio. I subscribe to the "anything better than 18 awg lamp cord is a waste" as quoted by Stanley Lipschitz and John Vanderkooy, members of the AES and the profs that run the Audio Lab at the University of Waterloo, where I went. I spoke with them at length over the years and they shared their research into the "sound of cable" which concluded that beyond good quality copper and well shielded interconnects, and 18 awg hardware store lamp cord speaker wire, anything else was simply a waste - there was no recorded difference when measured in their labs, nor was there any statistical relevance when testing with a wide range of listeners in a true lab quality A/B/X test scenario.

    On the other hand, I do believe that capacitors make a sonic difference. I know, like me with cabling, there are people that feel caps don't make much a difference past a certain inexpensive threshold. In my own testing (not scientific) I have heard large differences between audio cap types and so this is going to be a big part of my PH15 build.

    So back to my phono stage. I didn't want to make a big commitment with this and its basically the weak link in my system. I did however upgrade the parasound as best i could: upgraded all the components to higher quality and closely matched, replaced the opamps with ultra low noise ones ($30 each), beefed up the power supply and filtering, other things. here was the page I made covering all the mods: http://www.theplexus.com/pph100/.

    After trying the VTA octal MK3 drivers I was confident in the VTA kit sound quality and decided to order a PH15 kit. I am going to build the PH15 with the highest quality parts I can afford. The topology uses 4 gain stages, the first which is a JFET. From what I can tell VTA bases their designs on Audio Research and the AR PH5/6 also use a JFET "front end". I'm not afraid of semi-conductors and I think in the case of the first gain stage of a phono stage, a JFET is a good choice. when you spread the gain over 4 stages, there is a strong benefit to using a JFET like that. although it makes the pre a hybrid and not a pure tube design. however I think, from the design, that the JFET "compromise" is only that in the sense of making the pre a hybrid - the sound quality should be better than a "pure" tube design (use google to search and you will see the reasons why).

    Although the kit is only $200, I project the final cost of my PH15 to be around $1200. most of the additional cost is going to go into the component choice I made: Caddock MK132 resistors in all the critical places, V-cap cuTF bypass caps between stages, Jensen CuPIO on the output and Multicaps for all other RIAA points. All caps and resistors matched in pairs. I am also going to build in a pair of Jensen 34K step-up transformers because I need the gain for the Ruby 2. I current am running the Jensen step-ups with the Parasound set to the same gain at the PH15 (55dB) and it working out great in terms of final output level.

    This is going to add an additional $550 for the caps, $125 for the resistors. I got a good deal on the Jensens which was $140 for the pair.

    I am taking an (expensive) stab at the V-caps. I do like the Jensens Al PIO's and I did listening between those and Multi-caps, in the MK3 and in a 1 stage 12AU7 preamp I have. The Jensens had a wider, warmer, deeper sound to them. Larger sound stage. And now when I researched for cap choices for the PH15 I see that people do not like the Jensen Al caps very much, but the Cu caps are considered "good". the V-caps appear to get across the board top grades for sonic quality. they are expensive but they are not the most expensive caps. those, in listening tests, didn't do as well as the V-caps. so this is part of what fed my decision to go with V-caps. It's a risk however because I haven't personally heard them.

    I decided to put V-caps in the 0.22uF bypass positions to, based on-spec, retain the sonic quality through the first 3 gain stages. I do know that the Jensen PIO caps have a different kind of colouration, more to a warmer fuller sound, from first hand experience. that and price drove the decision to go with Jensen Cu PIO on the final output, a 1.5uF cap. these are very expensive and the 2 Jensens will cost me $150 vs V-caps which would be $430. It's already a crazy amount to spend on 4 bypass caps, $400!

    The Audio Research PH5/6 use Rel-cap PPMF's which ended up in Tier F "Best to Avoid" in one comprehensive cap testing effort (http://www.laventure.net/tourist/caps.htm). So if these caps can justify a $3500 phono stage which i have a feeling is very close in design as the VTA PH14/PH15 then perhaps my cap and resistors selection will raise it to a higher level of sonic quality? The Audio Research PH7/PH8 appears to use proprietary caps (http://www.drhifi.net.au/Reviews/ARC%20PH7%20Review/1.htm). The PH8 is $7000 so who knows, with my component choices will we be able to approach a $7000 phono stage with a kit cost of $1200?

    I am thinking I will build the PSU in a separate case. I know I ordered the PH15 which has the PSU on the same board but it looks like I can slice the board in 1/2 because the PSU appears to be trace-discrete from the rest of it.

    Speaking on which, any body know of a place I can get custom project cases made with proper punch outs? I found one place online and its $300 for each case! I could use hammond cases too but they don't appear to have cooling vents. I'd like cases with wood sides and ideally stainless steel.

    PS. I also got a quad of V-Cap CuTF 0.22uF and a quad of Jensen CuPIO to try in my MK3's, replacing the Jensen AlPIO that are in there now. I want to "get to the bottom" of this capacitor issue (does it matter) and the best way is going to be listening to them myself. I'll pick out what I like and sell the rest. I don't go in with any preconceptions other than the AlPIO I have now sound pretty good and better than the Multicaps I was using.

    Tube Nube

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    Re: VTA PH15 Build thread PH14

    Post by Tube Nube on Mon Apr 02, 2012 9:36 am

    Very good start to an interesting build thread, Plex'. I look forward to reading of your project as it unfolds.

    plexus

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    Re: VTA PH15 Build thread PH14

    Post by plexus on Mon Apr 02, 2012 5:06 pm

    Update: the caps are going to cost me more. total cap costs are going to be $650. i have also re-planned the resistors and will put Vishay "nudes" in some critical places. i would have preferred to use those for the 210k and 38.3k however the available values would have meant 3-4 resistors for each of those values at about $50-60 each! so i opted to do those with Caddocks MK's.

    I am still planning on 2 cabinets, one for the PSU and the other for the phono stage. I still have to source cabinets. I do like the Hammond ones but there are no air vents. Again, any suggestions for cabinets would be a great help.

    The total cost will be around $1600. This should put it on par with the AR PH8 @ $7000 depending on what their "proprietary" caps are and if they are better than V-caps.

    Tube Nube

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    Re: VTA PH15 Build thread PH14

    Post by Tube Nube on Mon Apr 02, 2012 5:21 pm

    If you know anyone with a CNC machine, they could do a custom vent cutting job for you.

    plexus

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    Re: VTA PH15 Build thread PH14

    Post by plexus on Mon Apr 02, 2012 5:33 pm

    Tube Nube wrote:If you know anyone with a CNC machine, they could do a custom vent cutting job for you.

    Hi TubeNube. no I don't, unfortunately. There must be someone out there that does customs cabinet work for this kind of thing, or can do cut outs on an existing cabinet like the hammonds. I have been googling but I can't find a source. I guess I need to post in the forums. I have looked in a few DIY forums but found nothing.

    So for now the cases are up in the air.

    Tube Nube

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    Re: VTA PH15 Build thread PH14

    Post by Tube Nube on Mon Apr 02, 2012 7:14 pm

    I know a guy who knows a guy... Here in Calgary. Story is this guy just got this cnc machine in the last couple weeks, a real steal, and my friend (an I.T. Guy) is going to be in charge of the software end of the "make it work" project.

    I m headed over there now to hang out. I ll ask if they can do it, if you like?

    plexus

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    Re: VTA PH15 Build thread PH14

    Post by plexus on Tue Apr 03, 2012 12:45 pm

    Tube Nube wrote:I know a guy who knows a guy... Here in Calgary. Story is this guy just got this cnc machine in the last couple weeks, a real steal, and my friend (an I.T. Guy) is going to be in charge of the software end of the "make it work" project.

    I m headed over there now to hang out. I ll ask if they can do it, if you like?

    yes please. i also found a person on ebay who appears to make cases and emailed them but no reply back yet. his cases look great too. i just need to know if he can do the punchouts i need. most people make their amps with the tubes exposed but i want the tubes to be inside the cabinet for best hum/noise rejection.

    I am about to put my order in for the caps, resistors, etc. its kind of scary because of the cost of all these parts. i just have to keep telling myself "this will cost around $1500 and then you will have the equivalent of a $7000 phono stage". that is, if the PH14/15 are as good as assumed. i have yet to hear one or even read a comprehensive review. i did review the schematic in detail and the design looks very good. i am not afraid of the JFET input stage - i think it makes sense to do this. Audio Research does too and apparently so do their customers.

    Tube Nube

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    Re: VTA PH15 Build thread PH14

    Post by Tube Nube on Tue Apr 03, 2012 12:53 pm

    Hey Plex,

    If any inferences can be drawn from the SP14 pre amp, and extended to the PH15 phono, then I'm sure you can build in confidence. As I reported on another thread, I'm finding the SP14 stunningly better than the SP8 that I'd been happy with for over a year. Our man Roy has been designing up a storm of product updates lately!

    I cant' recall who, but another member here also built/bought PH 14 phono . . . I seem to recall he was happy with it. Shouldn't be hard to find.


    WntrMute2

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    Re: VTA PH15 Build thread PH14

    Post by WntrMute2 on Tue Apr 03, 2012 3:27 pm

    plexus wrote:[
    I am about to put my order in for the caps, resistors, etc. its kind of scary because of the cost of all these parts. i just have to keep telling myself "this will cost around $1500 and then you will have the equivalent of a $7000 phono stage". that is, if the PH14/15 are as good as assumed. i have yet to hear one or even read a comprehensive review. i did review the schematic in detail and the design looks very good. i am not afraid of the JFET input stage - i think it makes sense to do this. Audio Research does too and apparently so do their customers.

    I always shudder a bit when the kit designer makes claims that his or her "insert your price here" is equivilant to a 5X "insert your price here". I'm NOT saying that many of these kits are great value for the money but it seems disingenuous to me. No one can know what the 2,000, 3,000, or 10,000 amp, phonostage or preamp sounds like. Otherwise we could just buy our equipment on price alone. You are loading up what is probably a very nice design with the most expensive boutique caps and such you can find without knowing if the design can support them. Why not build in stock or close to stock form and then experiment?

    plexus

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    Re: VTA PH15 Build thread PH14

    Post by plexus on Tue Apr 03, 2012 5:57 pm

    There are photos on the net of the Audio Research PH5,6 and 8 and you can tell a lot from a board layout without seeing the schematic if you know a little something about tube amp design. from the photos it looks to me like the VTA PH14/15 are based on the AR phono stages from a circuit design perspective. After you get a good circuit board layout, decent tubes the rest are passive components, chassis design and wiring quality. The AR phono stages use pretty decide passive components adding to the cost of the final product (because of course they need to make very high mark ups too on their component costs). the PH8 uses what they say are their own proprietary bypass cap designs however I am sure they are just re-branding an off the shelf teflon cap or having them made for them. there is cost there too. also the AR phono stages have a fancy remote control for switching load, etc from your chair. big deal. pointless.

    So I have confidence even without hearing the PH15 that its going to be a good phono stage and if it is in fact based on the AR designs it will comparable. My role in this is going to be using the best passive components that it makes sense to use, including Vishay "nude" resistors in critical audio-path locations, Caddock MK132's in the next tier of critical locations, I'll try V-cap CuTF's in the bypass locations and Jensen CuPIO in the output. everything matched as pairs or quads where necessary. The PSU will be in a separate cabinet with a much greater than needed cabling to the phono stage. solid silver wire for hook-up. that's the best I can do. from a component perspective it will be at or greater than the AR offering. if the VTA circuit is sound (no pun intended) the phono stage should sound as good if not better than the AR PH8. but of course I'd need to find one in order the A/B them.

    Yes its a lot of $ to put into a "spec" project. but I don't see anything in the VTA design that makes me question the quality of it nor the rationale to use top quality matched passives.

    Oh and yes you are right, it might be prudent to build the stock kit and then expand on it from there. that's one approach. however there is no evidence I have been able to find that contradicts the additional quality of using Caddocks, Vishays and V-caps. so i feel i might as well get all these parts now because i know i am going to put them in anyway. there is also a pretty great discount on v-caps if you buy in quantity. i am going to try a quad in my MK3's so buying 8 I save $60 per cap!

    plexus

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    Re: VTA PH15 Build thread PH14

    Post by plexus on Thu Apr 05, 2012 1:10 pm

    All the upgrade parts came today as well as the chassis and PSU parts. I just need the actual VTA kit now to get started. A little scary sitting here looking at $1000 in parts but they indeed lovely. I am also going to do a little upgrading in the VTA 6SN7 MK3 drivers too, since I am putting V-Cap CuTF's in there I might as well upgrade the other coupling cap and some of the audio chain resistors. I won't start building/modding until I have everything though. Maybe this weekend for the amps because I dont think the PH15 kit will show up until next week.

    Jensen 2.2uF CuPIO caps are HUGE!

    Any idea what the spec on the mains fuse should be for the PH15? I figured the heater is 12V and 4 tubes at 0.2A per = basically 12V@1A=12W, and the B+ 300V@1A=300W so figure 400W total so 3.5A fuse??

    plexus

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    Re: VTA PH15 Build thread PH14

    Post by plexus on Thu Apr 05, 2012 3:45 pm

    Some link for the parts I a going use.

    Commentary on resistors for audio: http://www.ariaaudio.com/WVVishay.html

    Link to datasheet PDF for TX2575: http://www.texascomponents.com/pdf/tx2575.pdf

    I am going to be using these TX2575's in the PH15 in places where the audio path is through the resistor and to another part. audio to ground (etc) will go through Caddock MK132's

    Comprehensive "capacitor shoot out" including V-cap TNTF: http://www.laventure.net/tourist/caps.htm#AAA

    I can not find any commentary online from people who have used V-cap's that questions their high sonic quality, unlike other caps where there are differing opinions. the only way to really find out is to get some and try them - an expensive proposition.

    That's it for now. I'm basically "on hold" until the kit shows up.

    jjones3318

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    Re: VTA PH15 Build thread PH14

    Post by jjones3318 on Thu Apr 05, 2012 3:58 pm

    Surely the B+ will be no more than 50mA, most likely in the 30-40mA range? Certainly not 1A.

    On the boutique components, I feel compelled to offer my .02, having been there. I strongly advise you to take the time to start with standard fair and put in the boutique parts in stages. If you go all out at the start, you won't know which components made a difference and which ones did not. It's good to know you're putting your $$ in areas that actually make a difference - great knowledge to have when you plan your future projects.



    plexus

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    Re: VTA PH15 Build thread PH14

    Post by plexus on Thu Apr 05, 2012 4:27 pm

    jjones3318 wrote:Surely the B+ will be no more than 50mA, most likely in the 30-40mA range? Certainly not 1A.

    On the boutique components, I feel compelled to offer my .02, having been there. I strongly advise you to take the time to start with standard fair and put in the boutique parts in stages. If you go all out at the start, you won't know which components made a difference and which ones did not. It's good to know you're putting your $$ in areas that actually make a difference - great knowledge to have when you plan your future projects.


    Thanks jjones3318. I am just learning about tube amplifier design and I didn't know what the draw on B+ would be so i guessed. I was wrong. Very Happy So I guess a 0.5A fuse would be good then taking into account loss on the transformers and some over-head?

    As for the upgraded parts, I hear you. however I have already experimented with various caps and resistors, wiring, etc over the 25 years or so I've been into high end audio. I don't design my own projects but rather buy/build and look for ways to increase quality of off the shelf products. In this case I don't really need to experiment with "standard fair" parts and move up from there. i do know that to my ears anyway, better caps and better resistors and matching everything does make a difference as does careful wiring, grounding and shielding (including taking into account capacitance).

    Over the years I've upgraded many components (both products and parts), done extensive A/B testing (true A/B and A/B/X but also non-blind comparison) and so I am very comfortable with going straight to upgraded parts. The only unknown are the V-caps as I have never used them before. The only way to really know is to hear them and since I do not know anyone who can do a cap A/B test with me with their own V-caps, I am left to buy them and try them myself. Alas.

    I know enough about tube amp design and phono stage design to be able to read a schematic and determine weaknesses in the design (to a point). I think the VTA PH15 design is really nice and like I said I would bet $ that its based on the AR PH5-8. likely with some refinements. but even if not, its still a good design. it doesn't push the tubes too hard and from what I can tell it keeps the tubes in a nice linear area of operation, spreading the gain over 4 stages. the use of the JFET as a cathode driver is a nice touch and although not a "pure" tube design I think makes sense as a compromise in this kind of application (low initial signal level).

    What I am saying is, thanks for your advice, but I am comfortable jumping right to high end parts on this. I am not so interested in comparing the "standard fair" to "upgraded parts" but rather getting down to listening to music through a phono stage as good as I am able to make it.

    I do however plan to try out the V-caps in my MK3's and see what the difference is there between those and the Jensen AlPIO's I am using now. The Jensens were way better than the Multicaps the original driver boards came with when I bought the amps (the PO upgraded to those). If I don't hear a difference then fine, I will accept that the V-caps are going to be lost on me and likely sell them (and the quad for the phono stage) and get something more cost effective.

    I know there is a lot of controversy about upgrading parts in audio gear. Over the years of looking at the innards of gear ranging from crappy to ultra-high end, and examining schematics as much as I can, and this applies more to tube equipment, once the circuit design gets to a certain point of quality all you can do from there is upgrade parts. The expensive commercial equipment usually offers little change in terms of circuit but goes on to integrate higher quality parts. or rather, more expensive parts on the promise of higher quality. I think the AR PH5 and PH8 are an example of this where at first glance it appears the major change are the parts quality (and the resulting price increase).

    The question of "does it sound better" is as always a can of worms. for me, in my experience, caps make a huge difference. resistors less so however I have not done a lot of listening to different resistor types. and wiring makes no difference what so ever beyond a certain easy to attain and inexpensive point. others disagree. others agree. others partially agree. so lets not go there. Very Happy

    This is not an experiment for me. this is about making a very high quality phono stage that I can start listening too sooner than later (yes the 400 hour burn in for the V-caps is going to be a challenge).

    tubes4hifi
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    Re: VTA PH15 Build thread PH14

    Post by tubes4hifi on Sun Apr 08, 2012 8:32 pm

    for the PH15 phono preamp you shouldn't need anything bigger than a 1/2 amp slo-blo.
    That's 60watts power from the AC power socket. Next time I build one I'll measure the actual current used,
    the main criteria is the turn-on surge due to the filaments. I'm guessing 1/4 amp while running.

    heyraz

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    Re: VTA PH15 Build thread PH14

    Post by heyraz on Mon Apr 09, 2012 9:46 am

    Regarding OPA 637 oscillation, if you still have it, I tried 2 things that helped for me. First, in addition to the standard 0.1uf decoupling caps on each of the op-amp's +/- power rails, I added 0.01uf and 0.001uf (to remove the higher frequencies). I also bypassed the NFB resistor with a 10pf mica to limit high frequency RFI from entering the loop from the backside. I tried this after reading an article about taming noisy op-amps and it seemed to eliminate the "hashy" noise my phono pre would make during loud complex passages.
    The values I chose for the decoupling and bypassing capacitors were not calculated, I guessed and used what I had around.

    plexus

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    Re: VTA PH15 Build thread PH14

    Post by plexus on Mon Apr 09, 2012 2:58 pm

    @heyraz i was able to eliminate the oscillations in the Parasound, its covered somewhere on that web page. but thanks. what you suggested is pretty much what I did.

    I now have all the upgraded parts for the PH15 as well I am going to put "nude" bulk-film and caddock resistors in key places on my MK3's along with the V-Cap CuTF's in the phase splitter and a Jensen CuPIO to couple the gain stage to the phase splitter. Its all about a $700 upgrade to the MK3's and should bring them up to snuff.

    I did put the V-Cap CuTF's in the MK3's to burn in. My expectations were low in that I expected when I put them in and tried them out right away that I would be faced with a very subtle difference and the part of my brain that *wants* them to sound better would lose, as usual, against the part of the brain that has to admit there is not a sonic difference (I exercised the ability to be objective about audio when I did extensive A/B/X testing many years ago with speaker wire and interconnects). Well, the audible difference with the V-Caps was immediate and apparent - there was no subtly at all. I also experienced this going from Multi-caps to Jensen AlPIO however that "upgrade" was a little more subtle than going from the Jensen's to the V-Caps.

    The V-Caps sounded very clear and detailed. I was hearing way more nuances than I ever had. I could definitely also hear a "hardness" or "strident" quality which I think is from the caps being fresh and no broken in. For the first 10 hours or so, they were definitely a major step up from the Jensen Al PIO's I had in there for 10 years. Things like more detail in the recordings of room sound (reverb), the nuances of the bow and fingers on violin and acoustic guitar strings, etc. just more sound and not colouration or "phantom" sound - more actual real qualities in the recordings. of course the quality of the recording has to be high too. so definitely so far sold on the V-Caps. Before listening to them I felt a little like a fool for shelling out $550 for these caps but now, so far at least, I think its an easy and well justified upgrade for the price.

    After about 20 hours the sound went down hill. it was like the sound was trying hard to "get through" and losing. like if you break your way through a brick wall, by the time you get through you are still you but you are worn, haggard and bleeding. however, i have heard that this is not uncommon with caps that there are phases during burn in where they can sound bad. these caps apparently need 400 hours (17 days @ 24 hours a day) to burn in. so i have my MK3's running 24/7 now with a set of "junk" tubes and Caddock 8 ohm 100W film resistors on the speaker terminals. I am not running them full power, only enough to make the resistors "very warm" to the touch. I am also burning in the other set of V-Caps going into the PH15 so they are ready or close to ready when the kit arrives and I am ready to solder them in. Those caps are in a 12AU7 pre-amp I have with outputs running to ground. I am using the same burn in track I created that I mentioned before: equal mix of 1-22kH 10s sweep, 1-22kH 0.2s sweep and white noise all repeated.

    So, totally surprised by the V-Caps, and like I said, I don't just say something sounds better unless it actually really sounds better to me - if its a subtle thing then I err on the side of it just being psychological and if I can't do a real blind A/B test then I will even more err on the side of non-objectivity. That was not the case with the V-Caps - very apparent and different sound. I am not going to say "better" yet because they need to burn in.

    For the MK3's, the V-Caps are in and burning in. Once they are burned in I plan to spend a few weeks listening to it extensively. then i will do the resistor upgrades and see if there is a change there. I am skeptical about it but we'll see. I can understand how caps can make an audible difference but resistors, not sure.

    heyraz

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    Re: VTA PH15 Build thread PH14

    Post by heyraz on Mon Apr 09, 2012 4:53 pm

    Resistors probably contribute the most noise to a circuit. There's a couple of articles out there describing the different types and what situations to use them in. I spent a lot of time researching that when I built my ST70. In another amp I have, metal oxide was too harsh, carbon film was just plain noisy, and metal film was just right. Each situation is different (plate-grid-cathode) so research the article. When I replaced carbon resistors with metal films in the signal path of my phono preamp, it was very noticeable. Much quieter, more air and depth to the music.

    plexus

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    Re: VTA PH15 Build thread PH14

    Post by plexus on Wed Apr 11, 2012 1:24 pm

    The PH15 kit arrived today. It looks really good. There are no step by step instructions, only schematic, board layout and parts list. That's fine for me but for others who need step by step, sorry. Here are some pictures of the contents. The parts quality is decent, it will at the very least get you started. There are no cheap resistors and caps here - only quality 1% resistors, many are Dale (one or two steps above run of the mill 1%'s), the film caps are Wima's which are "ok" and the electrolytics appear to be good quality too. Everything is sorted out very nicely so that will help with picking the right parts when populating the board, however you should still double check before you plop a part on the board. check and double check and re-check.

    I am going to cut the board to separate the PSU from the phono stage because I want to put them in separate cabinets. as you know I am going "full bore" with this build - most of the parts Roy supplied I am going to be using top quality parts for. I know its a bit of a waste but I wasn't sure about what I was going to replace while the board was in transit so I decided to just get the full kit and whatever is left over I can put in my parts drawer. As mentioned I will be using V-Cap CuTF's, Jensen CuPIO, Auricaps, Caddock MK132 and TX2575 bulk film "nude" resistors. Around $1000 in parts upgrades.




    plexus

    Posts : 94
    Join date : 2012-02-18
    Location : Toronto, Canada

    Re: VTA PH15 Build thread PH14

    Post by plexus on Wed Apr 11, 2012 10:38 pm

    The build is done, at least as far as the PCB's are concerned. I can't do much more until I get the cases, so likely 2-3 weeks. Alas. I might hook up some RCA's and test it out at some point but for now, enjoy...



























































    plexus

    Posts : 94
    Join date : 2012-02-18
    Location : Toronto, Canada

    Re: VTA PH15 Build thread PH14

    Post by plexus on Thu Apr 12, 2012 1:51 am

    Had to try it out. So its just hanging off the wires. Works well and less hum than I expected considering its just sitting in open air. The sound is definitely a big step up from my modded Parasound PH100. Wider soundstage, more depth of image, more detail gets through giving instruments and voice more "layers" in the textures of the sounds. I am hearing more things, nuances and in some cases instruments in recordings I am well acquainted with. I am also hearing more tape hiss from the master being pressed into the vinyl, where I haven't in the past. Just more detail is getting through, and deeper quieter sonics are being resolved where before they were not.

    The whole thing needs burn in time. The V-caps have 100 hours on them and they need 400 to be officially burned in.








    TMadden

    Posts : 99
    Join date : 2011-12-22

    Re: VTA PH15 Build thread PH14

    Post by TMadden on Thu Apr 12, 2012 2:35 am

    You are hearing exactly what I was hearing when I upgraded to the PH14! I have bypassed the electrolytic on the grid of the first tube with a 2 uf film cap, seemed to me it really opened up the sound. Next I tried eliminating the voltage divider and electrolytic completely and biased the tube with a 9 volt battery, sounds fantastic, haven't been inclined to mod it any more since! I left the grid reistor in place.

    plexus

    Posts : 94
    Join date : 2012-02-18
    Location : Toronto, Canada

    Re: VTA PH15 Build thread PH14

    Post by plexus on Thu Apr 12, 2012 3:45 am

    Warmed up, the PH15 sits at 115.5VAC @ 0.25A or 29W. However a 0.5A regular blow fuse will pop on start up, some of the time. So I switched to a 1A fast blow and that seems to hold ok.

    @TMadden Can you explain in more details your mods and also explain what each does and why it improves performance? I just want to learn as much as I can.

    TMadden

    Posts : 99
    Join date : 2011-12-22

    Re: VTA PH15 Build thread PH14

    Post by TMadden on Thu Apr 12, 2012 4:48 am

    OK, well I'm no engineer but here's my understanding of it. The first stage of the ph14/15 is a j-fet cascoded into a grounded grid triode. This gives us the low noise of a j-fet combined with the sonic signature of a triode. I don't have the schematic in front of me, but you have the grid of that triode being held at about 9 volts by using a voltage divider composed of 2 resistors and an electrolytic capacitor. Electrolyitc capacitors are handy for lots of capacity in a small package but they are not ideal sonically. Hence the film bypass. To take it one step further as done in the Loesch style preamps, you can eliminate the voltage divider and cap entirely and just use a 9 volt battery to bias the triode. If you google "Loesch preamp" or "Oz Loesch" you will see several variations on this, all in very high end, highly regarded designs. I just borrowed the idea!

    plexus

    Posts : 94
    Join date : 2012-02-18
    Location : Toronto, Canada

    Re: VTA PH15 Build thread PH14

    Post by plexus on Thu Apr 12, 2012 11:13 am

    Build time for me on the PH15 was 5.5 hours to get all the boards together and do a smoke test on the PSU. I took my time and ensured each solder connection was sound, top and bottom. Also measuring series and parallel resistors to get the odd values to ensure the channels were balanced within 0.5%. Bending all the resistors pins to fit also took a lot of time. Another 1.5 hours was spent hooking it up, powering it up and getting it connected in the system for a preview.

    Special thanks to Parts Connexion for all the help and stock of the upgraded parts I used, high quality wire, connectors, etc. A great resource and a pleasure to deal with.

    @TMadden - great thanks for that. I am actually learning about tube amp design right now and although tube circuits are simple when you look at the schematic, they are very complex devices - even more tied their own "physics" than transistors, it seems. So any little details like this are helpful in further understanding. thanks!


    Last edited by plexus on Thu Apr 12, 2012 12:08 pm; edited 1 time in total

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