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Old 24th May 2006, 02:35 PM   #1
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Default preamp /amp combination question

How can a noobie (me) determine if he has a setup that is reasonable sensible? I have a Ned/triodeel ST70 that is nice and a 6SN7 cathode follower preamp I built from a VTV issue several years ago. It all seems OK, but I heard a ridiculously simple preamp using a single twin-triode (is this a single-ended circuit?) with a no frills PS and it sounds way better than my more elaborate 6SN7 preamp. It's open, airy, detailed, blah, blah.

So my question is: do I have a bad match with a CF preamp and a ST70? I was thinking of reworking the preamp to a SRPP as it's really simple, but would this be any better? Would this SRPP sound more like the simple single-ended preamp I heard?

Thank you very much for any input.
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Old 25th May 2006, 12:07 AM   #2
arnoldc is offline arnoldc  Philippines
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That's what you call Synergy. All your system components must interact well together- from your source to the speakers including all cabling in between.

I currently use a ridiculously simple preamp as well- the Western Electric 407A/B and its 6.3V cousin- 2C51, 5670,396A. How simple? One plate resistor, one LED cathode bias, one coupling cap and one load resistor. In *my system* I prefer it over the VTV 6SN7 and variants.

Another ridiculously simple is the 12B4 preamp which has tons of posts in this forum.

Since I have high end dealer friends, I borrow their gears, try them in my room, in my system, and see what I'm missing. Sometimes a get a wide grin when the borrowed equipment or accessory sounds inferior to what I have.
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Old 25th May 2006, 02:32 PM   #3
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Thanks so much for your reply.

Yes, synergy is the perfect word. The simple preamp I tried was basically as you described yours to be, except mine has a resister/cap bias, not led.

I think I'll try the SRPP circuit as it's easy to rewire my current preamp to this for a listen and check if I get better synergy!
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Old 25th May 2006, 08:46 PM   #4
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I converted one channel from CF to SRPP according to the attached schematic because no PS changes appeared to be necessary. Unfortunately, the gain is extremely low. With the volume pot near maximum it is barely at a "background music" level. Needless to say the CF channel would be blasting at this setting but I left that channel turned off. Any idea what may be the problem?

TIA
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Old 25th May 2006, 11:43 PM   #5
arnoldc is offline arnoldc  Philippines
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Is the cathode resistor bypassed? According to TubeCAD, the 6SN7 with 8mA, 300V B+, Ra=Rk=800R gives around 23dB gain. Without bypass, gain is 19dB.
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Old 26th May 2006, 12:04 AM   #6
lndm is offline lndm  Australia
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The CF stage has no gain (a gain of almost 1). The SRPP stage should be providing gain.

I assume(?) your preamp was a gain stage followed by the CF. A CF will provide the ability to drive a range of power amps with good distortion figures. If you simply removed it, your earlier gain stage would be under greater loading, but, if you were careful to match it to your power amp, you may like what you hear.

I think the SRPP stage may be unnecessary for this location, but I understand it has a cult following and you might enjoy the sound of it. Ordinarily, I would start with the single triode gain stage (unless there was a reason to do otherwise), and only change it if I didn't like the results.
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Old 26th May 2006, 03:23 PM   #7
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Thanks so much for your replies. Perhaps my Ra/Rk resistors are off. I used 1K5 as the schematic called for. I'll try something in the 800R range and listen for an improvement.

Indm, the original circuit I have is the VTV 6SN7 linestage. One triode is used as gain, followed by the CF. My amp is a Dynaco ST70 and I have not figured out if SRPP, CF, or a simple single stage is best sounding. I generally listen to pop and rock and low to moderate volumes (I have a baby daughter!). As aronldc said earlier, I'm trying to find some "synergy"!!!

Thanks again, I'll let you know what I find once I make some adjustments.

(note: I did attempt to attach the schematic but it failed, perhaps because I'm "under probation"?
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File Type: jpg pre_6sn7.jpg (17.6 KB, 181 views)
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