Keen-eyed readers and visitors to my YouTube channel may have noticed the two octal phono preamps I've been using in my recent uploads. I built them for the following reasons:
- Although the Loctal EAR 834P had no problems driving the Intact Audio nickel autoformers, the 5k Ohm output Z of the Shure M65 phono preamp sounded very anemic.
- In the spirit of not leaving stones unturned and my curiosity for other sonic flavors notwithstanding, I looked back to passive RIAA EQ phono circuits I've been wanting to build, which have low enough output impedance to drive an Autoformer Volume Control.
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| Please read my thoughts on cathode followers below |
I initially wanted to revisit the above circuit, which I built in the early 90s. However, it's basically the same RCA phono circuit I've been enjoying for decades direct coupled to one section of a 6SN7 configured as a cathode follower. I explored what others have done with the high mu 6SL7 octal triodes.
6SL7- split passive RIAA EQ - 6SN7
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| Gain = 45 dB |
I was enticed by this split RIAA EQ circuit published by an audio hobbyist* in the UK. I simplified it slightly by removing the ferrite bead at the input, the input capacitance load, Zener regulation and then I lowered the value of the output coupling capacitor. This topology evolved from the phono schematic found in the back pages of the RCA tube manual (ex: page 594 of RC23) and popularized around the turn of the millenium by Sound Practices aficionados, Bob Danielak and Jeremy Epstein.
This circuit sounds very similar to my long-time musical companion, the RCA derived octal preamp sans cathode follower, which is rich, dark and robust! Come to think of it, that's just the way I like my coffee.😉
Through the years I've learned that the power supply topology has a sonic signature that's especially audible on phono preamps. I combined the more front row oriented capacitor-choke-capacitor power supply to balance with the mid to back row presentation of the 6SL7-split RIAA EQ-6SN7 circuit.
Cathode Follower
A cathode follower (CF) is a necessary evil at the output of phono stages which employ high mu/low gm triodes like my favorite 6SL7 if one requires a low output impedance. I started experimenting with cathode followers during my Dynaco hacking days in the 80s. Whether AC or DC coupled to a medium mu/med gm triode like a 6SN7 configured as a CF, I always perceived a compression of dynamics when this type of phono stage was ganged with a linestage that also had a CF output. Could it be due to two consecutive 100% negative feedback circuits working back to back?
Hence, I've adhered to the recipe of using a cathode follower only at the output of the linestage like I wrote in my homebrewer article for the final Sound Practices issue. To a certain extent, it still holds true today because I can't recommend these CF equipped phono circuits to be combined with the Bruce Berman-style 76 linestage preamp or my plate choke coupled 76 update. However, the active RIAA EQ'd EAR834P is a notable exception. I've successfully paired this phono stage with those aforementioned linestage preamp circuits.
6SJ7- passive RIAA EQ - 6SL7 - 6SN7
I've been intrigued by this RCA pentode based phono preamp for many years and always wanted to build an octal version.
Since I'm very familiar with its sound quality, I chose the 6SJ7/5693 for the front end followed by a 6SL7. Instead of AC coupling in the original, I direct coupled to a low Z 6SN7 cathode follower.
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| Gain = 44 dB |
This is a lively and airy sounding phono preamp. It has remarkable midrange presence and its rendition of complex tone colors is reminiscent of a fine Pinot Noir. The reproduction of dynamics from the most subtle pianissimo to hair raising fortissimos is compelling.
An authoritative choke input power supply complements its sonic virtues in depicting a three dimensional mid hall perspective.
Words, schematics, pictures and even videos are limited in describing sound. Since these are not commercially available designs, you'd have to build them to hear them. As for me, I've added two low output Z phono preamp flavors to my tools for enjoying music. Mission accomplished!
look at QUAD 22 preamp !
ReplyDeleteToo complicated and I don't need tone controls. The RCA with a pentode front end is similar but a lot simpler.
DeleteNice work on the two new phono stages. If I can trust what I hear on my computer speakers, I like the Pentode version a bit more. Regarding cathode followers, have you ever tried replacing the cathode resistor with a solid state current source? I've found it to be a nice upgrade in some cases. A simple bipolar transistor current source with its base powered by the DC filament supply is easy to implement.
ReplyDeleteIn the past I've built a kit with CCS loaded 6DJ8s and it wasn't the sound I was looking for. Since then I've avoided mixing solid state with tubes. Besides, I admit not having enough experience working with solid state.
DeleteThanks for the suggestion I'll keep it in mind!
Looks like my next project. :-)
ReplyDeleteIf you've only tasted all hi-mu triode phono stages, I'd recommend trying the pentode front end for a different flavor. 😊
Delete[Scrolling on my phone so the schematics aren't viewed yet - downloaded and soon-to-be-printed. Cathode-following with great interest.]
ReplyDeleteYou've reminded me of one of my very early projects before I knew my ass from my elbow: a 6SL7 phono/line stage that bypassed the RIAA and reduced the gain when in the other source positions.
What could possibly go wrong? (Everything.)
I ripped out the switching feature years later, and reduced the signal path to component leads only. Strangely enough, the hum was much less of a problem after that!
I found my octal experiments, which I liked very well, sounded "slow" or "lush" as compared the the hi-gm projects. Gonna have to mess with the pentode idea, I like a metal 6SJ7 every now and then. One of my stalled projects is a guitar amp.switchable from 6SL7 to 6SJ7 front ends.
I need to rebuild my DIY system. Thanks for the shout-out!
- another JE
Hey JE, have fun solder slinging, again! ;)
DeleteHi! Really interesting. Since you are battling with a low input autoformer, did you consider placing the autoformer at the output of your line stage (after the low output cathode follower) and placing a load of say 100 k at the input of your line stage? Then you could use any phono, low or high output. Correct me if I’m missing something here.
ReplyDeleteWhen I assembled the Finemet AVC in 2018 for a friend of a friend, I had to install it at the output of my line stage for testing. However, there are instances wherein an active line stage becomes redundant e.g. power amp input sensitivity is ~ 1 Vrms or below. Then, a low output Z phono preamp has to drive the AVC directly.
DeleteHi, thank you for you reply. Yes, with a passive line stage I agree that you are forced to have the AVC at the output of the phono stage. However, if we assume an active line stage is required, is there any drawback to having the AVC at the line stage output and therefore allowing successful pairing with any phono stage, regardless of high or low output impedance.
DeleteAlso, could you notice any difference in performance when having the AVC at the active line stage input, compared to having the AVC at the line stage output?
DeleteGood questions and I'll get to that later. I got sidetracked when I had a discussion with J-Rob about transformer coupled line stages, which I tried and wrote about in my SP homebrewer article moons ago. Right now I have three pairs of preamp OPTs ( ~ 5k:600 ohm) which I've built into line preamps and have been trying different tubes to drive them. I've also been experimenting where to position the AVC. Stay tuned! ;)
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