I'm thinking about tackling Thomas Mayer's Octal Phono Preamp.
Circuit Here:
VinylSavor: The Octal Phono Preamplifier, Part 1 : Circuit
P/S Here:
VinylSavor: The Octal Phono Preamplifier, Part 3 : Power Supply
I can source parts and do a fair job assembling and soldering. I'm not an electronic/tube wizard though, so a quick question.
I want to incorporate a passive volume control, along with switching to other line sources without the addition of another line stage. This will be driving a ST-70 with a VTA driver board via a 3' (or shorter) cable. From his description, it sounds like I'll be OK, but I wanted to check before diving in too deep.
Circuit Here:
VinylSavor: The Octal Phono Preamplifier, Part 1 : Circuit
P/S Here:
VinylSavor: The Octal Phono Preamplifier, Part 3 : Power Supply
I can source parts and do a fair job assembling and soldering. I'm not an electronic/tube wizard though, so a quick question.
I want to incorporate a passive volume control, along with switching to other line sources without the addition of another line stage. This will be driving a ST-70 with a VTA driver board via a 3' (or shorter) cable. From his description, it sounds like I'll be OK, but I wanted to check before diving in too deep.
Osvaldo, there is no tone stage. This is a phono preamp.
Chad, just think of the phono output as another source. So phono out to selector to volume.
3 feet shouldn't be a problem provided the preamp can drive the volume control, and the amp's input impedance isn't stupidly low (which it isn't).
Chad, just think of the phono output as another source. So phono out to selector to volume.
3 feet shouldn't be a problem provided the preamp can drive the volume control, and the amp's input impedance isn't stupidly low (which it isn't).
Just as a hard won “PS:”
I've found good utility in having the 'volume controls' individual per source, and the selector after them. The input is thus “padded” (in the live-music mixing-board sense), and can be easily adjusted to near-equal levels before the master volume control. The pots don't even need to have front-panel knobs. For all intents, they can be panel-flush flat-tip screw adjusting pots. However, not having them can at times be a pain.
In that way, as you experiment with different sources, they at least can be tamed, level-wise.
I hope this little bit helps!
Just saying,
GoatGuy ✓
I've found good utility in having the 'volume controls' individual per source, and the selector after them. The input is thus “padded” (in the live-music mixing-board sense), and can be easily adjusted to near-equal levels before the master volume control. The pots don't even need to have front-panel knobs. For all intents, they can be panel-flush flat-tip screw adjusting pots. However, not having them can at times be a pain.
In that way, as you experiment with different sources, they at least can be tamed, level-wise.
I hope this little bit helps!
Just saying,
GoatGuy ✓
Thank you everyone for the confirmation. I know I looked at several other phono-sections, RCA manual for example, that require a line stage, but keeping things as simple as possible is what I'm after. It this gets too complex, the project won't happen.
I like the per/input pot idea, but again..trying to keep things simple. I'm in the habit of turning the volume all the way down before switching, and currently, my only secondary line source PC via a DAC, with volume available in software on that end.
I like the per/input pot idea, but again..trying to keep things simple. I'm in the habit of turning the volume all the way down before switching, and currently, my only secondary line source PC via a DAC, with volume available in software on that end.
Chad,
Do yourself a favor and use a buffered, not passive, volume control. You're going to need at least 100 Kohms in the control and that leads to trouble in combination with cable capacitance plus power amp I/P impedance.
On the source padding front, individual pots. are not necessary. Custom padding resistors in the lines between I/P jacks and source selector switch get things done.
Do yourself a favor and use a buffered, not passive, volume control. You're going to need at least 100 Kohms in the control and that leads to trouble in combination with cable capacitance plus power amp I/P impedance.
On the source padding front, individual pots. are not necessary. Custom padding resistors in the lines between I/P jacks and source selector switch get things done.
OK - so can I simply "LEGO" this after the Phono-stage? Is the Power supply linked with the phono-section above sufficient to run this also?
VinylSavor: The Octal Line Preamplifier, Part 1 : Signal Section
Sorry, I've never "piece-meal'd" several circuits together - only full schematics to date. I don't know why I'm fixated on big octals, other than they look "kewl.", and these NOS tube types appear in good supply and affordable.
VinylSavor: The Octal Line Preamplifier, Part 1 : Signal Section
Sorry, I've never "piece-meal'd" several circuits together - only full schematics to date. I don't know why I'm fixated on big octals, other than they look "kewl.", and these NOS tube types appear in good supply and affordable.
Each diode in the 6BY5 can handle 175 mA. of DC. Plenty of capability there. The UF4007 SS diodes are good for 1 A. each and, again, are not a problem. What is needed is a power trafo with some "stones" and another filament winding for the buffering cathode followers. AC heating should be fine, once "line level" is reached. 6J5s make good cathode followers.
Antek's AS-1T300 power trafo should be quite adequate, at modest cost. Just to be sure, increase the final reservoir cap. in the B+ PSU from 220 μF. to 470 μF. This part will get the job done.
Antek's AS-1T300 power trafo should be quite adequate, at modest cost. Just to be sure, increase the final reservoir cap. in the B+ PSU from 220 μF. to 470 μF. This part will get the job done.
This cap costs slightly more, but I have used them for years... They are excellent.
PEH534YDG3470M2 KEMET | Mouser
PEH534YDG3470M2 KEMET | Mouser
Wow - good stuff - Thank You! I'll need to let it sink in. It would probably do me good to re-draw the whole mess into a single schematic, to be sure I understand and don't accidentally make a dumb assumption. Also, CAD up the component layout, enclosure, etc. I think I can do this. The big problem-sigh- is time.
OK - so I'm still hot to build this. I'm going to start sourcing and ordering parts.
Another dumb question: I see the suggested Antek T-former has two 300v windings. Would I need to simply leave one of the windings unconnected, or should I parallel them (in phase of course.)
I started to redraw the circuits using TinyCAD (good 'nuf for what I need, I'm not going to try to sim anything.) When I'm done, can I post it up for some quick QA?
OK - so I'm still hot to build this. I'm going to start sourcing and ordering parts.
Another dumb question: I see the suggested Antek T-former has two 300v windings. Would I need to simply leave one of the windings unconnected, or should I parallel them (in phase of course.)
I started to redraw the circuits using TinyCAD (good 'nuf for what I need, I'm not going to try to sim anything.) When I'm done, can I post it up for some quick QA?
Thank you for holding my hand 
I made my best attempt at compiling the three circuits into a single schematic. If anyone is up for it, I would appreciate a quick once-over to be sure I didn't goof somewhere. The only changes from the original circuits are:
* Changed the last cap in the power supply from 220uf to 470uf as suggested.
* Only the filaments for the phono stage are on DC power (6SF5 and 6J5)
* Moved the rectifier filament, along with the line stage, onto the other filament winding.
I know this is too small to see. I also have a PDF attached.
I made my best attempt at compiling the three circuits into a single schematic. If anyone is up for it, I would appreciate a quick once-over to be sure I didn't goof somewhere. The only changes from the original circuits are:
* Changed the last cap in the power supply from 220uf to 470uf as suggested.
* Only the filaments for the phono stage are on DC power (6SF5 and 6J5)
* Moved the rectifier filament, along with the line stage, onto the other filament winding.
I know this is too small to see. I also have a PDF attached.
Just a couple of small things.
The filament annotation says '5SF5', but the front-end valves are labelled 6SF5. Typo!
From
I also looked at the DC heater bridge-and-filter. 6.3 VRMS has VPEAK = √(2) × 6.3 = 8.9 V; each rectifier diode drops “something” … conventionally 0.7 V for ordinary silicon PN junctions at modest current, but in practice more like 0.9 V it seems. In any case, you've always got 2 of them in series for each half-phase of the AC, so let's say 1.8 V drop. 8.9 - 1.8 = 7.1 V which the capacitors will capture and hold. I guess the 0.5 Ω resistor in series will drop the volts a bit more. Let's see. 2 ea. 6SF5, 2 ea. 6J5 each with 300 mA filaments. 1.2 A. E = IR, E = 1.2 A × 0.5 Ω = 0.6 V. 7.1 V - 0.6 V = 6.5 V DC at the heaters. OK, just fine, no objection, good design.
Lastly… none of the valves' grids are drawn with 220 Ω (good enough) so called “grid stopper” resistors to protect against RF oscillation. Let's not forget that the first stage is designed for some pretty steep gain, so RF oscillation is a real possibility. I'd highly recommend them. On every valve's grid. Soldered to socket, close.
Other than those points, looks like a nice design.
Just saying,
GoatGuy ✓
The filament annotation says '5SF5', but the front-end valves are labelled 6SF5. Typo!
From
E = I R
P = I E … substituting E gives
P = I² R and solving for I
I = √( P/R ) … and keeping in mind
P = E²/R as well
I look at the various resistors to see where they sit on the spec-vs-actual power dissipation 'curve'.P = I E … substituting E gives
P = I² R and solving for I
I = √( P/R ) … and keeping in mind
P = E²/R as well
I = √( P/R )
I = √( 5 W ÷ 4,700 Ω )
I = 0.033 A … 33 mA (aggregate load ain't going to be even ¼ this!)
I = √( 5 W ÷ 10,000 Ω )
I = 0.022 A = 22 mA (same comment as foregoing)
P = E²/R
P = 350² V² ÷ 100,000 Ω
P = 1.2 W (zero circuit load max
And I see that the 5 watters are pretty substantially over-spec'd for the application. Not that this is bad. Especially if you get a great deal on some nice 5 watters. But 2 W would do just fine, for this circuit. I = √( 5 W ÷ 4,700 Ω )
I = 0.033 A … 33 mA (aggregate load ain't going to be even ¼ this!)
I = √( 5 W ÷ 10,000 Ω )
I = 0.022 A = 22 mA (same comment as foregoing)
P = E²/R
P = 350² V² ÷ 100,000 Ω
P = 1.2 W (zero circuit load max
I also looked at the DC heater bridge-and-filter. 6.3 VRMS has VPEAK = √(2) × 6.3 = 8.9 V; each rectifier diode drops “something” … conventionally 0.7 V for ordinary silicon PN junctions at modest current, but in practice more like 0.9 V it seems. In any case, you've always got 2 of them in series for each half-phase of the AC, so let's say 1.8 V drop. 8.9 - 1.8 = 7.1 V which the capacitors will capture and hold. I guess the 0.5 Ω resistor in series will drop the volts a bit more. Let's see. 2 ea. 6SF5, 2 ea. 6J5 each with 300 mA filaments. 1.2 A. E = IR, E = 1.2 A × 0.5 Ω = 0.6 V. 7.1 V - 0.6 V = 6.5 V DC at the heaters. OK, just fine, no objection, good design.
Lastly… none of the valves' grids are drawn with 220 Ω (good enough) so called “grid stopper” resistors to protect against RF oscillation. Let's not forget that the first stage is designed for some pretty steep gain, so RF oscillation is a real possibility. I'd highly recommend them. On every valve's grid. Soldered to socket, close.
Other than those points, looks like a nice design.
Just saying,
GoatGuy ✓
Thank you again for the help. I definitely came to the right place and am feeling much more confident I can pull this off. I'll make the changes as suggested, and I'm sure I'll have more questions as I get deeper.
This weekend, hopefully, I'll have time to start on an enclosure and begin planning the component layout in earnest. I'll post progress pics
Planning out the actual component layout. Yah - I'm a ways out from knowing the size of the enclosure and building anything yet. Looking up component specs and drawing them is tedious.
I want the orientation of components to be pretty close to the schematic, since I think it'll help me from making mistakes. After I get the larger circuit assemblies figured out, I want to see if I can "smoosh" them closer together, and reduce the enclosure size. Right now, the big square is 17"x 16" (huge). I'm okay with a standard 17" width, but want to reduce the depth. Everything would be inside the chassis, except the tubes on top.
I'm thinking about planning the signal circuits around a turret board, then cutting/drilling my own. Not sure yet though.
This weekend, hopefully, I'll have time to start on an enclosure and begin planning the component layout in earnest. I'll post progress pics
Planning out the actual component layout. Yah - I'm a ways out from knowing the size of the enclosure and building anything yet. Looking up component specs and drawing them is tedious.
I want the orientation of components to be pretty close to the schematic, since I think it'll help me from making mistakes. After I get the larger circuit assemblies figured out, I want to see if I can "smoosh" them closer together, and reduce the enclosure size. Right now, the big square is 17"x 16" (huge). I'm okay with a standard 17" width, but want to reduce the depth. Everything would be inside the chassis, except the tubes on top.
I'm thinking about planning the signal circuits around a turret board, then cutting/drilling my own. Not sure yet though.
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