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Do I need a preamp with gain?
Do I need a preamp with gain?
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Old 3rd March 2018, 03:04 PM   #1
stephenmarklay is offline stephenmarklay  United States
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Default Do I need a preamp with gain?

I am looking at a Primaluna Prologue Four amplifier. The specs state it has an Input Sensitivity of 775mV and Input Impedance: 100k Ohms.

I have a passive pre but I am not sure if this would be a wise choice?
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Old 3rd March 2018, 03:10 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by stephenmarklay View Post
It has an Input Sensitivity of 775mV and Input Impedance: 100k Ohms. I have a passive pre
but I am not sure if this would be a wise choice?
That should be good enough to use a passive preamp with most sources, even phono.
Give it a try. You'd have to run the volume control higher on phono than for CD.

Last edited by rayma; 3rd March 2018 at 03:14 PM.
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Old 3rd March 2018, 04:21 PM   #3
GoatGuy is offline GoatGuy  United States
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@stephenmarklay

I have to say… I agree with RayMa. However, (and I admit this is personal), I also like the subtle tone “sweetening” that a decent, minimalist vacuum valve preamp delivers so effortlessly. Yes - to DF96 who hasn't replied yet - I'm admitting I like a subtle but audible “effects box” between signal and amplifier.

Another of the things tho' that I like about such an arrangement is the ability to step up the phono raw cartridge output (using on the order of at most a handful of millivolts) to the 778 (or so - its not an accurate spec in the end) millivolts of standard amplifier input. Ideally, each input of a preamp should have its own pad (volume control or attenuator); the phono input, a front-end RIAA pre-preamp stage, the rest of the inputs padded, then switched onto the preamp “front end”.

And the preamp does NOT need to be terribly exotic to achieve decent results. I very much appreciate the 'phase splitter front end' idea, with its conversion of single-ended "unbalanced" sources to "balanced" complimentary signals. Perhaps your Primaluna Prologue doesn't have balanced input. But if you're going to build for the future, then why not at least explore it?

The topology is simplicity itself: If you want a lot of tube-effects, use a triode. If not, use a pentode (think of it as a cascode-in-a-single-glass-envelope). Using raised grid bias, and well matched cathode and anode resistors, you get +6 db in a cathode-follower configuration (for balanced output). 0 dB for single-ended output.

Run at about 5 ma, 150 or so volt anode, separate well buffered screen grid supply, and a 30 volt cathode (anode) resistor drop. (E = IR, R = E/I … = 30 V ÷ 5 ma = 6,000 Ω). If you are into E–96 (2%) scale, 6,040 Ω.

Myself? I'd buy a cheap 50–100 pack of 6.2 kΩ 5% resistors, and watch the TV, and use a 4 digit meter to test 'em all, and arrange in order. You'll get some pair which is almost identical (0.5%!) (tho' you need 2 pairs to make stereo). The individual stereo channels do NOT need to have their resistors cross-matched, just the anode-and-cathode resistors, internal to each channel.

(PS: when you're testing a bunch, as long as they've got wire leads, you can just tape 'em to a piece of paper or cardboard. Use painter's tape, for easy removal. Tape a bunch down, use the meter to measure each one, write down the measurement near one of the leads. Do the next one. Etc. Takes about 5 minutes for a hundred.) Leave them attached to the paper, but roll 'em up, for future use. Once measured, always treasured!

__________

The tubes I like? 6SN7 of course, from reputable source for triode(s). For pentodes, the 6267 (EF86) remains a fertile collection of fine tubes. Not too expensive ($10-$20) for decent units. Believe it or not, I prefer FETs for the RIAA phono front end, if you go that direction. I'd use a 100 kΩ 'top' resistor and a 25 kΩ 'bottom' resistor to make a 30 volt grid supply; a 2.2 μF 100 volt non-polarized paper cap (to ground) to filter the supply (2.9 Hz rolloff), and a 220 kΩ supply-to-grid resistor feed. You get your 5 ma; The circuit also can deliver ±10 volt tracking of input signals. Plenty enough for a 778 mV rated amp!

PS: I'd also have a 'universal output pad' too: kind of like a volume control, but adjustable with a back-of-chassis screw, not knob. Keep out of curious hands. Keeps those 10 volt spikes from killing your amp. Perhaps I'd also include 5 volt back-to-back zener clamps on the output too, for the same reason. You never hear them in regular service. You need them to squash the transients. Trust me. Safe way-beats simplicity-without-protection.

GoatGuy
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Old 3rd March 2018, 04:26 PM   #4
stephenmarklay is offline stephenmarklay  United States
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Thanks you guys. I have a nice passive pre so if it will work for now and then I can add a tube gain stage later that would be perfect.

I would not mind building that myself.
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Old 3rd March 2018, 07:15 PM   #5
Lingwendil is offline Lingwendil  United States
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Do I need a preamp with gain?
^ good advice and info, as usual, from GoatGuy.


I'll add to his suggestion of the 6SN7, of its nine pin counterpart- the 6CG7 and 6FQ7, and also what has very quickly become my absolute favorite tube, the Russian subminiature 6N16B. It has fantastic curves, and is compact. More gain though, although running no-feedback and no cathode bypass will reduce gain and further increase its wonderful linearity. They also make very, very nice cathode followers, which I'm taking advantage of in a handful of projects. Including a nice tube crossover, and a build of sy's heretical linestage.
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Old 3rd March 2018, 10:37 PM   #6
GoatGuy is offline GoatGuy  United States
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Default UNBAL -> BAL, 0 or 6 dB gain

In case all-y'all desire a circuit diagram, see attached. Pentode based. Documentation is in diagram. Works well.

GoatGuy
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File Type: jpg unbal-to-balanced-preamp-180303a.jpg (126.7 KB, 131 views)
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Last edited by GoatGuy; 3rd March 2018 at 10:48 PM. Reason: forgot the component IDs
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Old 4th March 2018, 01:03 AM   #7
stephenmarklay is offline stephenmarklay  United States
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Quote:
Originally Posted by GoatGuy View Post
In case all-y'all desire a circuit diagram, see attached. Pentode based. Documentation is in diagram. Works well.

GoatGuy
You are one step ahead of me
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