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Any 4S Universal Preamp 12a*7 builds here
Any 4S Universal Preamp 12a*7 builds here
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Old 18th January 2019, 02:56 PM   #21
DF96 is offline DF96  England
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Quote:
Originally Posted by eldarvanyar
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.

Quote:
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.
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Old 18th January 2019, 09:40 PM   #22
Diyengineer is offline Diyengineer  United States
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Originally Posted by Shawnstium View Post
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.

Last edited by Diyengineer; 18th January 2019 at 09:43 PM.
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Old 18th January 2019, 10:56 PM   #23
Shawnstium is offline Shawnstium  United States
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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.
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Old 18th January 2019, 11:24 PM   #24
eldarvanyar is offline eldarvanyar  United Kingdom
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Quote:
Originally Posted by DF96 View Post
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
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Old 19th January 2019, 01:04 AM   #25
GoatGuy is offline GoatGuy  United States
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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
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Old 19th January 2019, 01:11 AM   #26
GoatGuy is offline GoatGuy  United States
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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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Last edited by GoatGuy; 19th January 2019 at 01:30 AM.
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Old 19th January 2019, 02:22 AM   #27
Duke58 is offline Duke58  United States
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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.
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Old 19th January 2019, 07:05 PM   #28
eldarvanyar is offline eldarvanyar  United Kingdom
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Any 4S Universal Preamp 12a*7 builds here 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?
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Old 19th January 2019, 07:32 PM   #29
GoatGuy is offline GoatGuy  United States
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Default Options, designs, overall...

Quote:
Originally Posted by eldarvanyar View Post
[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
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Old 19th January 2019, 08:08 PM   #30
Suncalc is offline Suncalc
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Hmmm... Seems you guys all kind of missed the whole point behind the 4S.

“Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less.”
― Marie Curie
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