• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Any 4S Universal Preamp 12a*7 builds here

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eldarvanyar said:
Would I be better off with a passive pre amp on the 606 then?
Possibly. A 10k pot should do it, provided that your source can drive it OK.

Where would I seek to build a valve preamp? Is it really only for a valve power amp?
Sorry, I don't understand your questions. A valve preamp provides gain, but you don't need gain. A valve power amp is something different; it does not contain a preamp, but it does contain a voltage amplifier. Some people confuse 'preamp' with 'voltage amplifier in power amp'.

If you want to include a valve in your system then a cathode follower buffer may be your best bet. A well-designed buffer will be audibly transparent i.e. you won't know it is there.
 
I plan to build the ECC802S SRPP Tube Preamplifier (DIY ECC802S (12AU7 / ECC82) Vacuum Tube SRPP Preamplifier) next to drive the Pass F4 power amp. Has anyone built this preamp with success?

I have built that pre amp and am very happy with it. Simple to build and great results for me. I’ve been using it pretty much every day for a few months now.

Not though, that page does not have the latest schematic. The latest schematic can be found in the build thread in their forum. It has a regulated supply for each channel. Along with some other changes.
 
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Thank you DIYeng for the input. I’m using my own regulated power supply design, but I did not know they had changed the 470R to 820R. Unfortunately I already purchased all my components... whomp whomp. Maybe there will be some old stuff around home, but they won’t be similar to the ones I procured. The plan was to use resistors that all had the same coefficient for temperature drift.
 
Possibly. A 10k pot should do it, provided that your source can drive it OK.


Sorry, I don't understand your questions. A valve preamp provides gain, but you don't need gain. A valve power amp is something different; it does not contain a preamp, but it does contain a voltage amplifier. Some people confuse 'preamp' with 'voltage amplifier in power amp'.

If you want to include a valve in your system then a cathode follower buffer may be your best bet. A well-designed buffer will be audibly transparent i.e. you won't know it is there.

Thanks, can you recommend a cathode follower buffer? I am wondering about the MUCHEDUMBRE BUFFER PREAMP
Muchedumbre Buffer Preamp – wauwatosa tube factory

I have already worked out the majority of the BOM with Mouser UK

Thanks again
 
I admit I'm “late to the party”, but in my last 50 years, I've come to prefer “buffers when buffers are not going to harm, and possibly help things” and “gain preamps when gain is going to help and not harm things”. Dunno. Simple thinking comes from having the decades go by, and the IQ drop in inverse proportion to one's waist size. … :)

Anyway.

What all the posters are saying ultimately is, “you have a very fine setup of rigs; you have a fine preamp for your magnetic cartridge turntable; your other sources (likely) aren't substantially 'special' (i.e. very low output, or very high for that matter), and you almost certainly (with your equipment) not 'starving the amp's input' by the overly high impedance of any particular source”. Got that?

In other words, apart (possibly) from simply needing to PAD your various sources to be outputtting in their “sweet spot”, viz a vis the 606 (or 909, or 303) amplifier input, apart from that, you're probably good to go.

B • U • T …

What DF96 saith, well back, about post 5 or so, about wanting to create a tube-driven effects-box with musically (and audibly) said-to-be-beneficial-and-at-times-just-so characteristics, well … that's fun too.

Eli Duttman early on railed about too much gain. It is definitely true. The 4-S configuration with 1.2 kΩ cathode resistor, 100 kΩ anode resistor, is quite easily going to produce a gain of 20× to 35×. DF96 has the math, but that's what I remember.

Having all that gain, and having a potentiometer on the output (100 kΩ resistor side) is kind of silly. "Taming the shrew" sort of (i.e. it would be wicked-loud without the pot). Yet, since the thing would have almost too much gain for any but your most “weird” sources, why not "tame that" as part of the simple design?

Apart from choosing a different tube (than the 12AX7, with its 100 mu), there are severalp pieces of advice from the foregoing posters that make sense.

First, if it really is a "preamp", then by rights, it should have 3 things that really make a lot of pleasurable sense

(1) An input selector.

A decent preamp is the “switching center” to choose your sources with. Yes, yes, very probably you already have one. But the thing is, if you're going to invest in making a darn-good-quality preamp that does something useful, you might as well invest in making it a front-end-source selector.

(2) PADs for each input

So, it selects. But your sources are all variously "able". One might nominally do best (have the largest headroom with minimized floor noise) at 300 mVRMS output. Another might be happy outputting 1500 mVRMS. Another odd bird (like a naked dynamic microphone, no accounting for Karaoke situations!!! LOL!), might have 20 mVRMS output for a closely held mike. The magnetic pickup turntable … is very probably "normal", with a 0.773 VRMS nominal output; however, it it has a very fine cartridge and excellent high dynamics vinyl, then peak excursions upwards of 3.0 VPP are quite possible. Even higher, I've personally measured.

THE POINT is that you will want not just to switch between sources, but to “adjust the input level” of each of them … utilizing a fairly high impedance ("low load") level control for each. Note … you can certainly entertain the consumer-unfriendly idea of having dâhmned-good independent pots for L and R channels, for each input. While it is very convenient to have a single dual-section stereo pot, the actual tracking of resistance between the pair of sections is very often not great. The expedient of having not-ganged pots gives you the freedom to adjust both quite exquisitely. You're on DIYAudio. Take the plunge.

(3) Amplification, high-impedance input, buffering

Finally, let's say that you're going all idealistic. Doesn't matter what the 606 ÷ 909 ÷ 303 wants, you want to give each of your sources a featherweight high impedance "output load". Thus, the input to the preamp, on all switched source channels, needs to be reasonably high impedance.

How high? At times, I've designed for 50 kΩ "input load impedance". At other times, 100 kΩ. 250 kΩ. Unless you've got the most ridiculous source demanding 1,000 kΩ or higher, there is almost no reason to go above 250 kΩ.

And lower usually results in better control over cable capacitance effects. So, say "100 kΩ" (or 50).

________________________________________

Now you have your design criteria for a simple preamp.
100 kΩ pots.
Independent for L and R channels
Modest internal voltage gain.
Robust output buffering.
non-inverting on the polarity of the signal
4 (6? 8?) input switching
Lowest noise
Modest "tube sound" co-linear distortion.
No super fancy tubes, transformers, power supplies, switches.
Once you work with that, you have the basis for several pretty-straight-forward designs.

Just saying,
GoatGuy
 
PS — I'm hoping that DF96, Eli Duttman, others, … will put forth links to schematics and preamps that satisfy the above criteria list. I could, I might, but frankly, I know when I'm in the presence of “my betters”. GoatGuy

PPS — the suggestion by Dinolobe for an amplifier is marred by the second stage having a completely out-of-band 100 kohm resistor in the cathode side. Kind of "ruins" it. Just saying...
 
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GoatGuy is giving some good info to the OP. Instead of whining, he's making some fine wine...

I built the 4S. My compliments to the person who took the time to post the schematic and the write up. I built it because of the completeness of the info.

It isn't a universal preamp for every amp. When I connect it to my McIntosh MC2205- it's too much gain. But when I connect it to my homemade piece of junk looking 5881 amp- SWEET! That is using a 12AU7 tube.

Frankly Mr Frankley- my solid state Krell PAM-7 preamp is sometimes too much. You want a buffer with some amps. If you've got the sickness like me and have about 10 different amps, get yourself a buffer. That's a volume control for most modern devices like a iPod, smartphone, computer, music server, whatever you want to call it.

Just remember- there may not be a "universal" preamp. I built the 4S because I have a bunch of 12A*7 tubes. All my crappy amps don't sound good with every 12A*7 tube. But for me and tinnitus, voices in my head, I have found audio nirvana (not the website) as I'm listening to some of my junk at rock concert level in my man cave.
 
[emoji238] Goatguy that’s really helpful thanks!
I suppose in order to help me and more importantly others to build a preamp that ticks all the box’s you mention then the right circuits to choose from would be really helpful and welcome.
Probably just as important is the help that’s needed to build it in choosing the right components and layout.
It’s certainly a journey isn’t it?
 
Options, designs, overall...

[emoji₂₃₈] Goatguy that’s really helpful thanks!
I suppose in order to help me and more importantly others to build a preamp that ticks all the box’s you mention then the right circuits to choose from would be really helpful and welcome. Probably just as important is the help that’s needed to build it in choosing the right components and layout. It’s certainly a journey isn’t it?

It is true, “that it is quite the journey”. Yet — this is where I guess I differ from many on DIYAudio — I don't think it is an over "hard" proposition. The design goals at the outset were simple enough:

• № 1 Inexpensive
• № 2 Tube
• № 3 Valve "sound"
• № 4 Fairly easy to build
• № 5 Useful for learning

along with

• № 6 'Nice' Input impedance
• № 7 'Sturdy' output inpedance
• № 8 No need for circuit heroics
• № 9 Fairly easy to understand
• № 10 Useful as a system-enhancing preamp

I added № 6 thru № 10.

№ 3 actually makes it reasonable to specify rather "older fashioned and less linear" circuit topologies. Simpler days, simpler ideals. That in turn partially delivers № 4 and № 1. № 9, too for that matter, along with № 8.

The fairly optimal design would simply be this:

Input selector and pads
→ VAS (Voltage amplification stage)
→ → Cathode-follower or Aikido low-impedance output stage
→ → → another "overall volume control" pad
The input selector should switch independent source-side attenuators. This allows each of them to be preset to "what's right" for the whole setup. And to allow source swapping and switching, effortlessly.

The VAS might be a simple "long tailed pair" of triodes, all in one bottle, with a nice simple IXYS or FET constant current cathode bias leg. Having a LTP is also non-inverting (one of my criteria for a preamp).

The IRS (impedance reduction stage) could be a simple cathode follower; it could be more significant, like the Aikido topology. Or as i've recently seen a shunt-cascode configuration. All would do the trick of delivering less than 10 kΩ output impedance.

Having an output pad (volume control, one per channel) is helpful for feeding amplifiers that don't have such niceties. Or for subtly balancing the left and right channel power to accommodate the idiosyncrasies of the listening area. On my system for instance, I added 3 output pads per channel, since each channel is driving 3 different amps: headphones, kitchen/house and main rumpus/listening room. It was nice to be able to do it this way.

In the end, broken down thus, designing each section isn't all that hard. There are plenty of resources here at DIYAudio outlining all nature of opportunities.

Just saying,
GoatGuy
 
Oh, to be sure we “missed the point” of the 4-S, in that it is a

• preamplifier
• with way too much gain (nominally)
• 1950s era single-triode simplicity
• way high output impedance
• easily ridiculed potentiometer placement
not universal (in any sense) for substitution of other tubes\
• and the very subject of much conjecture of how it fits between OP's equipment.

Or, as the really-not-that-funny saying goes, “But apart from that Mrs. Lincoln, how was the show?”

No. My point(s) are straight: if you are going to build a competent preamp for a system already outfitted with awfully nice stuff, and you want to have that preamp serve a combined-useful-purpose, well … then the foregoing mostly-monologue-but-was-intended-to-be-a-discussion framework holds.

If, on the other hand, the OP (or you, or anyone reading this in the future) just wants a simple diagram, something to breadboard, something to hook up and "see what it sounds like", fuzz, fur bâhlls, dandruff, and frayed wires notwithstanding, well … by all means.

4-S without a single modification.

Just saying,
GoatGuy
 
Part of the issue is that the hobby needs more technically well-designed circuits that are also documented and explained in a way that is approachable for beginners. The 4S gets the latter correct, but not the former.

A build recommendation must be the right design for the right level of knowledge. Sometimes we forget that expertise in a hobby is gained through gradual experimentation and learning. From the perspective of a progressive hobby, an inferior or imperfect design still holds some value for the builder.

In the spirit of well-designed and documented, maybe a TubeCAD CCDA would be a good place to look?

CCDA: constant-current-draw amplifier
 
Been there, done that, wore out the t-shirt to prove it.

The guy who designed and wrote up the 4S build, created the schematic gets my sincere THANK YOU! I learned and built the preamp. I still use it with some of my amplifiers. No, it's not universal, but I built it because I have a ton of 12A*7 tubes and I've learned why they sound different, which ones sound better with different amplifiers. Most importantly, I learned the difference between buffers, line stages and phono stages. The bad news is there probably isn't one preamp that will work for every single amplifier. That's part of the journey, the fun part about this hobby.

So- to all you who went to trouble of offering an alternative preamp- Thank you!
 
This is an interesting, and certainly polarized, discussion.

I built the 4S pre for a specific reason, because it was about the simplest preamp design that I had seen, and because I wanted to drive my amp a bit harder.

For the background:
1. I'm not an electrical engineer or a tube guru. I'm a mechanical engineer who is interested in electronics and I like playing around at the bench. It does seem to keep me out of the bars in the evenings :)
2. My first tube build ever was a Fender Champ 5F1 clone since I'm a (albeit bad) hobby guitar player, and since that went well, I wanted to try and build a tube audio amp.
3. My second build was the Claus Byrith 8W SE amp using Edcor instead of Lundhal iron. This was my most complicated foray into tube electronics to date and I was quite satisfied by the results.

Now the issue I had was that my sources really didn't drive the SE amp hard enough to get a good output level with the speakers I had on hand, B&W CM1s, which have a sensitivity of only 84dB. So I built the 4S pre using a solid state power supply as a fun little project using junk bin parts to add a bit of gain into the front end. The SE amp took weeks to design and build and cost north of $500 in components. I wasn't ready to commit to a project of that scope again for a simple preamp.

And that's where it ends. It works for me, it was fun, it gave me the increased output i was looking for, it gave me more practice, some knowledge (whether good or bad) and to my untrained ears, it sounds just fine.

Is it 'universal'? Probably not. I'm not tube rolling. I run a 12AU7 without cathode bypass and that's what works for me with my single tube amp. But I do think it's was a useful exercise and I'm proud that I was able to build it and make it work. We're not all greybeards who dream of SRPP stages and chase the glass white whale. Some of us are just having fun. And for $0 out of pocket using maybe $50 worth of components I already had laying around, I was able to solve a problem over a weekend and gain a little knowledge.

So to the OP, I say go for it if you want a nice inexpensive design to play with. If you want a pro-grade preamp, listen to the other posters. Though that will cost significantly more and require a lot more time and complexity to build. Listen to the criticism, and learn from those who actually provide constructive expainations of the shortcomings of the design. But at the end of the day, it's your time, effort and money.
 
The 4S is not by any means perfect, but if you look at the limitations of the idea as an opportunity to add your own well thought out design improvements then it's very versatile. I can think of several ideas for a good improvement. Just a few off the top of my head-

Output buffer- add a simple mosfet or a low impedance tube (6N1P, 6DJ8, 12AU7, etc) as a follower to decrease output impedance and increase the drive capability. A simple dirt cheap IRF820 or similar mosfet, a small heatsink, and a couple more resistors should work very, very well, and with the right considerations could even drive headphones or small speakers. The high gain tubes such as 12AX7/6N2P would really shine if given an output buffer. Time and dollar considering, this would be the one modification I would most recommend that be considered.

Choose only linear tubes- the 6CG7/6FQ7 will work in place of a 12AU7 and offer lower distortion. Affordable and cheap, same socket, different filament wiring. 6N1P gives higher gain but would also work well, and have lower output impedance.

Be realistic about the gain you need- how many of your devices actually need a 50x voltage increase at the input? Realistically you may find you only need 6-20x depending on your source material and amplifier. A 6CG7 will give a gain of ~10x or so without bypass capacitor, ~15x or so with. This would likely be plenty for many purposes. Running a cellphone as a source into an older less-sensitive amplifier can even get by with that with room to spare in all instances I've come across.

Don't be afraid to use it as a front end for something cool! Built one with higher gain already and can't bear to rebuild it because it sounds just too darn good? Awesome. Now you have an excuse to build some of Nelson Pass's mosfet power amplifier designs that require a preamp with high gain to drive. See? Now you have justification to build even more cool stuff.

I ran a 4S with a 6FQ7 and IRF820 (running hot on big heatsinks)source follower as a preamp for about eight years, and even used it to drive Grado SR80 headphones directly. I loved it, and gave it to a buddy at work who now uses it to directly drive a pair of small desktop speakers.
 
I'm like you, cbattles- just went for it and built the 4S because it is well documented and I'm not a golden-ear EE or audiophile. Just a ME who's not part of the DIY "church," not afraid to make the mechanical parts of this homemade gear. Yes, it is my time, effort and money. I'm the proud owner of what might be to some people, a big pile of junk that is the best sounding pile of junk in the world to ME. No one is going to tell me what sounds good to me, I'm going to just do it and see for myself.
 
12AU7s can be linear enough to not be too objectionable. I built a 12AU7 preamp (gain stage and a cathode follower) and got THD+N numbers around .05% as measured. I consider that to be fairly reasonable for a tube preamp.

That said, the 4S has a major design issue- its output impedance. You might get away with it with short cable runs (low capacitance) and a power amp with a very high input impedance, but it would struggle hopelessly to drive my MC^2 MC650, which has a 10k input impedance when used as a differential input. Since the 4S doesn't have a differential output it would see more like a 5K input impedance. This is not all that uncommon.
 
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