Phono pre-amp discussion

Sounds like the power supply needs some work.

 

Why the hell would they use 12ax7's for a buffer? Surely there are tubes with lower gain and less noise they could have used...

 

Currently using a K&K Maxxed Out Phono.. love it to death. Got it used and haven't had the urge to upgrade any further. Did a tube swap and it sounds even better than before. I highly recommend it if its in your price range.

 

12ax7 is very popular in phono applications. Not sure why, I'm sure Marv or someone else can explain. You need about 40dB of gain for a MM stage.

 

Lotsa gain. Thing is, high Rp makes it less than ideal if it's directly going to drive an EQ network. Also not sure how it stacks up to the best low-noise triodes in terms of equiv. input noise resistance.

 

12AX7 isn't a good choice to use as a buffer. 12AU7 or AT7 are better suited for this role.

 

@Donald North - You're a DHT fan, what tubes would you use for a DHT phono?

 

Didn't @baldr recently disparage that whole line of tubes for phono pre amps on head-fi?

 

Yes, I love DHTs and enjoy them in my speaker setup in addition to powering headphones. Using DHTs for phono is not a simple task, to put it mildly. First, you need to use dead quiet filament supplies. DHTs are microphonic and using them in a phono stage is especially sensitive. You would need to mechanically and probably acoustically isolate it from speakers. DHTs are as lower gain than many of the high performance input tubes like the 417A, so you will probably need to add an extra gain stage. For tubes you can consider the EML type 20 and 30 DHTs, which are higher gain than a type 10, for example.

 

The last time I saw a proposal for a DHT phono preamp it suggested a split riaa network and 3 gain stages with EML type 30s. Of course, the power supply schematic was more complicated than the gain and equalization...

 

Allnic makes a DHT phono amp:
http://allnicaudio.com/H5000DHT

You can read the implementation details in the manual on the site. Cool stuff but it's crazy expensive... ($33K)

 

300b's as voltage regulators...that's a little bit insane. At least the main gain stage tubes are still in production by KR.

 

Glad to see another 834p owner/fan on here, seems like you enjoy it well enough at least! I very fortunately was able to hear one before I purchasing at a local shop. During my ownership I built a 'Thorston modified' version based off the circuit with a tube rectified PSU mostly out of curiosity. My friend thought it may sound better, so it turned into a fun project. We collectively liked the Thorston build best, so I've since sold my actual 834p. I still feel it's still a really good tube phonostage for what it cost. I do recommend rolling in some new tubes, specifically something NOS if you can. I had favorable results with some Telefunken, Amperex and Mullard ECC83 specifically, preferring Teles myself. I never replaced any of the caps in the original, but I've seen that done a few times where people say it makes a big difference as well. I recently built it into a chassis along with my linestage, before it was a point-to-point situation in a random box while we were testing...so I'm quite satisfied with how the final build turned out.



Regarding the talk on DHT phonostages, the only ones I had seen or really read about were a few made by Thomas Mayer, off his blog a few years back. He seems to work with a ton of DHT in general though, so it was bound to happen I guess

 

Wow, that's a beauty! I heard you can upgrade the components, you mentioned you didn't change the caps, but anything else? How much of an improvement do you hear if any?

 

I had just tried out various tubes myself, though it seems alot of people do swap the coupling caps out with different types (from pictures, alot have the fancier stuff like MusiCaps, ClarityCaps or Mundorfs) and I recall reading it's generally recommended to gain some overall refinement and various sonic improvements, added dynamics and greater detail retrieval from some reports. What's in there stock are some pretty standard Polypropylenes, and you can get better quality stuff without breaking the bank. I'd have to recommend something like Obbligato Golds, I used those in my build...but there are dozens of options from 'nicer' poly caps, film and foil, even inexpensive paper in oil (you just have to make sure they measure/match since they tend to have lower tolerances).

Thorston talks about the 'what and why' on his site: http://www.goodsoundclub.com/GetPost.aspx?PostID=2052

I tried new production 12AX7, NOS US-made 12AX7 types, various euro ECC83 but also got to try some specialty versions of those tubes. The 5751 / CV4017 tubes basically has the same characteristics of a 12AX7 so they're swappable with no mods, but a Mu rating of 70 instead of 100 so lower gain. Depending on where you put those, would yield different presentation/tonal results for example. There are a few specialty versions out there, usually industrial or military types, designed to have low hum / microphonics that work great in phono stages (12AX7A / 7025A for example) but I have not tried any of those yet. I preferred the ECC83 variants I tried, specifically the Telefunkens as they seemed to give a slightly more linear response, a bit better dynamics as I recall. Unfortunately, they tend to come with a cost...many of the NOS options are just becoming so rare since alot of things use 12AX7. Every once in a while a good deal pops up though, especially for re-branded Telefunkens and Mullards.

You can find some descriptions of the various sound characteristics reported here: http://www.audiotubes.com/12ax7.htm It's a rabbit hole of sorts

It's good to experiment a bit with other tubes and see if you can achieve more ideal results that way, I went on to read some forum posts about other's results before I started collecting tubes though...so for example: https://forum.audiogon.com/discussions/which-12ax7-tubes-for-ear-834p-phono-stage

If you're feeling adventurous the more advanced mods swapping other parts will do the same though likely to a greater degree. I believe there are a few guys out there that will do such mods for a fee.

Do you use yours with a MM or MC cart? I saw you have an external step-up transformer so I wasn't sure if you were using the MM output, bypassing with the TF-1 to use a MC, and whether it was better than the standard MC output. I've only had experience with MM output/carts but I remember reading that the MC output left some things to be desired.

 

great reading list! Yes, I'm using an MC cart into the TF-1 into the MM setting. I found the MC setting on the 834p to be slightly harsh and the TF-1 helped with that.

 

Phono stages i have had:

Musical fidelity v-lps (with homemade lps and Musical Fidelity v-psu) GOOD
Cambridge AZUR651 Phono GOOD
Yoshino EAR 834P FAIR
Musical FIdelity M1 Vinl VERY GOOD
NVO SPA - II EXCELLENT
NVO SPA ONE VERY GOOD
ASR Mini Basis VERY GOOD
ASR Basis EXCELLENT

My current (and only) phono is the ASR Basis and i am looking for a good deal on a ASR Basis Exclusive (similar and newer design but two phonos in one box for two tonearms).

If you are looking for absolute Fidelity these two are major contenders.

The close second is the NVO SP- II but pricey (even for me beeing a friend of the manufacturer) and also too many tubes.

The biggest let down is the EAR 834p, i could only live with it for a few months. Too much noise and roll off, very pleasing mids but that's all there is to it unfortunately.

 

Once upon a time, shortly after the earth cooled, cheap self-contained phonographs were manufactured which were single box, mono, ceramic cartridge types which were used in children's classrooms to provide support for lessons. They were zillions of them made, virtually all with 12AX7 tubes for gain. Ceramic cartridges had higher output, and being amplitude output devices, required less EQ than magnetic types. The stuck urinal noise of the 12AX7 was irrelevant in this application, which was driven by cost, for affordability for .edu and non-profit entities. It was cheap, no CHEAP. It was also well suited for applications of audio stages for ham and rf gear, and quite popular for musical instruments, which take advantage and view as desirable the lack of linearity in the whole 12A_7 line.

Audio engineers, being the red-headed stepchildren of the electronic engineering community have been treated as dolts by the providers of components who have provided cook-book style docs as well as example circuits supplied as literature. These documents are proper for the understanding of bright simians. Such docs were provided in the early days of stereo proffering the 12A_7 line. This practice continues to this day with ds dacs. This probably explains why MacIntosh, Marantz, and virtually everyone else were full of 12A_7s in their glory days. This encouraged the design of ubiquitous early stereo two tube (or three tube with the cheesy mis-applied 12A_7 cathode follower) featuring feedback RIAA (usually at least 2-3 db off and worsening at the bottom end as the tube wears and gain falls). You treat people as dolts, it encourages truly awful designs. Real designs require the ability to read and understand a data sheet.

Finally, no, I am not a 12A_7 family hater as long as they are kept within the boundary of cheap stuff, such as the phonograph described above, or an application where distortion is a virtue, such as a guitar amp.

 

Thoughts on this? http://www.decware.com/newsite/ZP3.htm