Thank you, this is currently a bit over my head, but I'm going to read it through it and see if I can understand it enough to incorporate it into the current schematic.
This comes up over and over again.
CD players and DACs have 2V rms output at 0dBFS (max output).
RIAA phono preamps typically have 1V to 2V rms output on program peaks.
Power amps typically reach full power with 0.5V to 1V rms signal input.
There is absolutely zero need for a line level preamp with gain.
If you really, really, really want to add a tube line stage to your signal chain, consider a cathode follower with an input selector switch and volume control at its input. Salas' 6V6-triode line preamp in its cathode follower version is well liked:
https://www.diyaudio.com/community/threads/6v6-line-preamp.102352/page-8#post-1995090
It's very simple.
CD players and DACs have 2V rms output at 0dBFS (max output).
RIAA phono preamps typically have 1V to 2V rms output on program peaks.
Power amps typically reach full power with 0.5V to 1V rms signal input.
There is absolutely zero need for a line level preamp with gain.
If you really, really, really want to add a tube line stage to your signal chain, consider a cathode follower with an input selector switch and volume control at its input. Salas' 6V6-triode line preamp in its cathode follower version is well liked:
https://www.diyaudio.com/community/threads/6v6-line-preamp.102352/page-8#post-1995090
It's very simple.
The input 12AX7 stage is a mu-follower or SRPP or whatever it's called. Since there's no loop feedback, some gain can be lost by removing 3C7 and 3C8 cathode bypasses. Maybe about 5 or 6 dB but with better linearity. It's perfectly safe (here) to replace the 12AX7s with 12AU7s, worth another 10+ dB but with (a little) lesser linearity.
All good fortune,
Chris
All good fortune,
Chris
Thank you Chris!
Elsewhere, 5751 tubes, to replace the 12ax7 tubes, was also mentioned to me. Is this also a reasonable tube swap to reduce the output? Would they be preferable, or more effective than the 12au7 tubes as replacements for the 12ax7 tubes?
Ha! Chris is right! I had to re-draw the schematic to see it, but it's an SRPP DC coupled to a cathode follower. (It's not a mu-follower, though.)
5751 tubes are very similar to 12AX7, just a bit less gain (5751 mu = 70, 12AX7 mu = 100). The gain will be about the same (i.e., too much gain). If you're trying to reduce the gain then 5751 is not a better choice than 12AU7.
I'd try 12AU7 for V203 and V302 (the SRPP stages). 12AU7 mu = 20, which is 1/5th that of 12AX7. and less than 1/3rd that of 5751.
SRPP gain is about half of mu, so figure the gain using a 12AU7 will be about 8X.
Just swapping the 12AU7 in place of the 12AX7 should tell you if you're close enough to what you want.
As Chris mentions, you could also try removing 3C7 and 3C8 cathode bypass capacitors to reduce the gain a bit more, perhaps down to 6X or thereabouts. That would be a very simple modification.
I can't believe I couldn't figure out that drawing. It seems so obvious now. For what it's worth, I would draw that SRPP with cathode follower output stage like this:
Note that 12AU7WA is just a different ('ruggedized') version of 12AU7.
Another problem to be solved is that you will want to elevate the heater supply by about 80V, so that the cathode of U3 (in the schematic above) is not 160V away from its heater voltage, Most tubes of this type have a maximum heater to cathode voltage of 100V.
I hope that helps.
5751 tubes are very similar to 12AX7, just a bit less gain (5751 mu = 70, 12AX7 mu = 100). The gain will be about the same (i.e., too much gain). If you're trying to reduce the gain then 5751 is not a better choice than 12AU7.
I'd try 12AU7 for V203 and V302 (the SRPP stages). 12AU7 mu = 20, which is 1/5th that of 12AX7. and less than 1/3rd that of 5751.
SRPP gain is about half of mu, so figure the gain using a 12AU7 will be about 8X.
Just swapping the 12AU7 in place of the 12AX7 should tell you if you're close enough to what you want.
As Chris mentions, you could also try removing 3C7 and 3C8 cathode bypass capacitors to reduce the gain a bit more, perhaps down to 6X or thereabouts. That would be a very simple modification.
I can't believe I couldn't figure out that drawing. It seems so obvious now. For what it's worth, I would draw that SRPP with cathode follower output stage like this:
Note that 12AU7WA is just a different ('ruggedized') version of 12AU7.
Another problem to be solved is that you will want to elevate the heater supply by about 80V, so that the cathode of U3 (in the schematic above) is not 160V away from its heater voltage, Most tubes of this type have a maximum heater to cathode voltage of 100V.
I hope that helps.
Just my experience with a high gain valve preamplifier [26 dB] and a 2v DAC, and a 2V for full output valve power amplifier:
I tried the L pad volume reduction between pre and power- it lost a lot of dynamics and sounded flatter (you get into challenges with impedance matching too)
I tried a volume pot on the power amplifier (it picked up a little hum, and sounded cleaner but lost energy and transient dynamics)
I tried the Rothwell attenuators (bloody awful sound)
I have a preamp with negative feedback, and there is an option to change the feedback resistors to change the gain (26 goes to 16 dB) this solved the problem but again lost some of the immediacy and magic.
In the end I went to about 21dB of gain by changing the value (and quality) of the feedback resistor to my taste, and put in a higher quality pot on the pre amplifier.
I tried some more standard gain valve preamplifiers (sone very expensive ones too) and they also sounded flatter, and less like live music.
I now have very high level listening volumes at around 9 o clock on the volume wheel, and listen mostly at 7.30 t 8.30 o clock, and yeah I have to make small adjustments not large ones,
BUT it sounds fantastic to me and anyone who has shared the experience has agreed. So to buck the obvious logic I think GAIN is GOOD!!
It's an SRPP 12AX7 direct coupled to a 12AU7 cathode follower. Like Chris said removing first tube cathode bypass cap will reduce gain and changing it to a lower gain tube like 12AU7 or 12AY7 tube will reduce gain further so your volume control can have wider range. I personally rather like having more gain than not enough and the sound, as tonescout pointed out earlier, has more immediacy and "jump factor." I'm a fan of having line stage because it can influence and flavor the sound. (Please don't lecture me about neutrality. If I can I would add tone controls to knock down the gain.) Yes, you end up boosting noise too. No free lunch.... so is life.
I was wrong.
I forgot to put on my Chinese thinking cap to see that it was an SRPP drawn in Chinese configuration.
Even though it is on a PCB, it would be easy to convert the input stage to a 12AU7 gain stage with a resistor plate load (not the other 1/2 of the 12AU7).
Then DC couple that to the 12AU7 cathode follower output.
The new input 12AU7 cathode self bias resistor could either have a parallel bypass cap, or not. That gives a gain selection, to get how much gain you want.
Excess gain solved.
The nice thing about changing to a triode gain stage from an SRPP is that there would never be hum from the top SRPP tube filament to cathode insulation leakage resistance; the top tube is not there in a non-SRPP) There is a preventative for the problem, but only if you employ it.
I know and have heard and measured the problem.
If you do not take care of that problem (and some brand new and/or NOS tubes already have the problem, with or without installing the extra circuit that is required).
The bad thing for some, is they can not sleep at night, they worry about the slight extra distortion of a non SRPP triode stage.
And, the single triode mod almost certainly could be done with only 1 or 2 traces cut on the PCB, and a couple of parts "bridge wired" point to point over the PCB.
With careful cutting of the traces, it could be easily restored to original. Then you could restore it to original, and sell it to someone who would probably modify it to my lower gain proposed circuit.
Just saying
I forgot to put on my Chinese thinking cap to see that it was an SRPP drawn in Chinese configuration.
Even though it is on a PCB, it would be easy to convert the input stage to a 12AU7 gain stage with a resistor plate load (not the other 1/2 of the 12AU7).
Then DC couple that to the 12AU7 cathode follower output.
The new input 12AU7 cathode self bias resistor could either have a parallel bypass cap, or not. That gives a gain selection, to get how much gain you want.
Excess gain solved.
The nice thing about changing to a triode gain stage from an SRPP is that there would never be hum from the top SRPP tube filament to cathode insulation leakage resistance; the top tube is not there in a non-SRPP) There is a preventative for the problem, but only if you employ it.
I know and have heard and measured the problem.
If you do not take care of that problem (and some brand new and/or NOS tubes already have the problem, with or without installing the extra circuit that is required).
The bad thing for some, is they can not sleep at night, they worry about the slight extra distortion of a non SRPP triode stage.
And, the single triode mod almost certainly could be done with only 1 or 2 traces cut on the PCB, and a couple of parts "bridge wired" point to point over the PCB.
With careful cutting of the traces, it could be easily restored to original. Then you could restore it to original, and sell it to someone who would probably modify it to my lower gain proposed circuit.
Just saying
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From what I immediately saw, R11 is -- NOT -- connected to C4 as R5 is connected to C3. In other words, there is -- NO CONNECTION DOT -- at the junction of R11, C4 & C6 as there -- IS -- at the junction of R5, C3 & C5!!! By the way, which schematic program was used for this schematic? What's with everything being preceded by the numeral "3"? I have never seen that being done before!!! WEIRD!!! IMHO.....this schematic needs to be redrawn BIG TIME!!!Can this be done without a major overhaul and just some parts swapping?
This is an existing circuit on a PCB, and I don't want to totally reengineer it, hopefully just swap some part values to reduce the overall gain.
yaquin 845 pre schem by Barry, on Flickr
Thank you
/
😉 If I ever read a review of HiFi with an apparent compliment of neutrality... I think bland, vanilla, grey and am not inspired to have a future listen.
on a similar trajectory of changing the feedback circuit responsible for RIAA, I am actually thinking about doing the opposite (increasing gain)
Can I alter any of these feedback resistors here and just change the gain, not spoiling the RIAA equalisation?
Can I alter any of these feedback resistors here and just change the gain, not spoiling the RIAA equalisation?
Looks like an Audio Research. You'd run out of open loop gain at low frequencies, but might could squeeze a couple dB by bypassing R6 and shunting R4 with a cap plus 2K Ohm.
A classic approach is to add some positive feedback cathode to cathode, sometimes with the R4 shunt. Still only a couple dB.
All good fortune,
Chris
A classic approach is to add some positive feedback cathode to cathode, sometimes with the R4 shunt. Still only a couple dB.
All good fortune,
Chris
5751 stil have to much amplification factor.
The problem is in the very wrong and dangerous schematic.
Advice is to just leave this device and get something more into the technical specification?
Isn't that the driver stage for an 845 amp? (Going by the '845' on the schematic). 845s, who need massive voltage swing.
Using that as a preamp is like using a sledgehammr to crack an egg shell.
Using that as a preamp is like using a sledgehammr to crack an egg shell.
If you look at the full schematic, on the pdf I linked in a post after my original, you'll see that there are multiple boards/diagrams and the number refers to the particular board in the diagram.
Yes, this pre was designed to drive an 845 power amp, I'm learning...