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24.27.41.140
In Reply to: RE: Eli liked this Version of the RCA phono circuit posted by Chip647 on April 21, 2025 at 12:43:15
I avoid semiconductors in the signal path whenever possible. My version of the RCA preamp adds a cathode follower rather than a MOSFET. This will drive 25V RMS across 10K. That's more than enough drive capability for the high level input of any preamp or integrated amp.
Follow Ups:
TK,Thanks for sharing the schematic. I like that you have the cathode resistors for both gain stages un-bypassed. Just to clarify, are both sections of the 6BQ7 CF buffer connected in parallel for each channel?
Edits: 04/22/25
The 6BQ7 is shared between channels, one section each. Here's a pic of the preamp I built that uses this circuit. The phono tubes are shielded, and the 6BQ7A is just to the right of those. It puts on a nice light show!
Is that all point to point wiring or printed circuit board?
Any pics under the hood?
Yes, all the audio is point-to-point with many parts held on turret/eyelet boards. The control circuitry and most of the power supply parts are on PCBs. Sorry, I'm not anxious to post the innards. It's extremely crowded and by no means an example of how to build a preamp.
I was curious how you packed all that wiring in that small chassis.
TK,
Thanks for the clarification. Congrats, that's one heck of a build! Very nicely done! It could certainly pass as a commercially bought preamp with all these nicely machined parts as part of the chassis.
for your schematics. Is that RIAA accurate?
Using standard off-the-shelf parts matched channel to channel, RIAA compliance in my unit measures within 0.1dB in one channel, and within 0.2dB in the other. These numbers are the result of a frequency sweep using the Lipshitz pre-emphasis network ("reverse RIAA"). This is right at the edge of my measurement capabilities.
Edits: 04/24/25 04/24/25
Isn't the filter we use in our playback preamp a reverse RIAA circuit and the one used to cut a record a RIAA filter?
The RIAA filter reduces the bass and boosts the highs and then we reverse that process when we play the record back.
I think it would be better to call the cutting RIAA filter a pre-emphasis filter and the playback filter in our preamps a de-emphasis filter.
That would make the "Lipshitz reverse RIAA circuit" a "Lipshitz pre-emphasis RIAA circuit".
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"I think it would be better to call the cutting RIAA filter a pre-emphasis filter and the playback filter in our preamps a de-emphasis filter."
Can't argue with that!
Your preamp looks nice but a little busy :)
Edits: 04/24/25
"Busy" - and you haven't even seen the inside! In addition to the buffered MM phono preamp and switchable unity-gain tone controls, under the skin there's also an OTL/OCL line stage. This uses large 90H chokes to load the cathode follower output stages and an auto-bias circuit that holds the cathodes at 0V DC. The cathodes are connected directly to the output through mercury-wetted relays. The relays are activated by a protection circuit that provides a startup delay and also quick disconnect if voltage rises above a few millivolts. B+ has a separate regulator for each stage to minimize crosstalk and other unwanted coupling, as well as a pre-regulator to accommodate wider line voltage variations. The filaments are powered with DC that's regulated by a buck converter. Despite this overall complexity, the line stage signal path is short and extremely transparent. It uses an EF86 in triode mode feeding a high-current pentode-as-triode follower. Nested feedback reduces maximum gain to about 6dB, and there is only one small polystyrene coupling cap in the entire chain. Frequency response is beyond audibility at both ends and distortion at 1V RMS output is triple digit. This project consumed several years of my spare time, and although I probably wouldn't build it as one unit again, the effort has proven to be worthwhile. This preamp took the sound quality of my system to a whole new level.
Edits: 04/24/25
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