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Pre amp advice
Pre amp advice
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Old 30th January 2016, 06:16 PM   #1
kawihornet is offline kawihornet  Canada
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Join Date: Mar 2015
Location: Cambridge, Ontario, Canada
Default Pre amp advice

Hey guys
Here is what I want to do
I wanna build a tube pre amp with HT Bypass
all I need is 3 inputs for sources
and 1 input for my pre/pro to bypass to a 5 Channel amp.
I don't care if I copy a manufacturers design or have to buy a kit or
if someone has already designed one on another thread.
I just want the thing to sound amazing for 2 - channel
audio listening with that beautiful warm sound you get from tubes.

I know you can buy a high end preamp that will do all that for around 3k but I don't have 3k so... and besides even if I did my wife would never understand the expense lol.

any help would be very appreciated.

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Old 31st January 2016, 10:57 PM   #2
indianajo is offline indianajo  United States
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Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
The main difference between my "legendary" dynakit PAS2 tube preamp, and my $15 op amp disco mixer that I upgraded massively for $20, is the disco mixer hums a little. I think the hum has to do with the packaging or the wall-transformer power supply, but I've stopped upgrading for how.
Oh, yeah, the PAS2 sound a bit trebly now that one of the paper caps failed, and I replaced 9 of them with polyester film caps. **** the ninth one I changed was the one with the wax case burned through by the installer back in 1961. I turn the tone control down on it now, which I hever used to have to do.
"Warm tube sound" has to do with what happens in clipping. Tubes sound okay clipping, solid state sounds awful. The best thing to do about that is not clip. My disco mixer has +-8v power supplies, which provides plenty of headroom for 50x LP input, 2 v CD player input, or even for 4 v headphone out signal of an FM radio. My "legendary" dynaco FM3 tube tuner has no sensitivity anymore, I can't pick up the 200 W high school radio station 8 miles away which plays more than the 20 song rotation the commercial stations all play to their narrow niche market. The re-e-capped Reader's Digest 6 band radio is now the FM fidelity queen.
What is cool about a mixer, you don't have to walk around your table to change the input from FM to CD to radio or back again, when you put a new disk/record or station one. All inputs are on at the same time and none hisses too bad.
If you're not inputting RIAA LP's, you don't need a 50x RIAA input. In that case a good mixer would be a good place to start. I recommend 1975-1988 Peavey, the pots are good and you don't need to put up with the surface mount parts and ribbon cables you get in numark and behringer. However 4580 op amps in peaveys' are not quite ideal in the hiss arena. I like ST33078 right now but people recommend a whole menu of op amps that are more expensive than $.38 each. If driving 100' cables to the stage or to the effects unit outside in the truck you need the drive of a 4580 on the output, otherwise a 600 ohm drive op amp is fine. 4558 is hissy, also 741.
I'm fighting headroom problems this week while I modify my ST120 to cope with transistors that are not 120-150 beta like they could buy in 1966. More like 400 these days. So, Im using a Schober organ mono amp, that is okay with a 1.5 v signal but sounds awful at 3 v input. The 7 transistor/cy dynakit ST120 uses a 80 v power supply and has huge headroom for 70 db s/n cd's, although it is not a PA amp and will overheat if used continuously at more than ~10 W/ ch. I love having a lot of headroom. The CS800s sounds just as good in my living room as the ST120 at 1.5 vpp base volume, although it does have the heat sinks and fans to run 260 W/ch all day and night. Pity the CS800s blows the breaker cold start, I will get into the power supply after the ST120 & tube ST70. ST70 is a bit fuzzy compared to the ST120 and CS800s even with new e-caps and output tubes, as 1% distortion spec at 35 w/ch would indicate. Think maybe one of the new tubes is shorting out at 20 minutes - another problem you get into with tubes, quality is not the same as it was in 1966.
For a blow by blow account of the upgrade of the RA88a disco mixer, if you're interested, see this thread: Improving a "Disco mixer" to mid-fi performance
Have fun.
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300
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Old 31st January 2016, 11:31 PM   #3
rayma is online now rayma  United States
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Join Date: Apr 2011
Originally Posted by kawihornet View Post
I wanna build a tube pre amp with HT Bypass all I need is 3 inputs for sources
Do you need any gain? If not, a passive preamp with a selector switch and a 10k volume control may be just the thing.

Last edited by rayma; 31st January 2016 at 11:35 PM.
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