DIY Audio Projects Forum

I have been asked to build a tube preamp for some powered speakers. The problem is long interconnects are required to connect from the system to to the powered speakers either side of a massive screen for movies. I have included three schematics based around a 6SN7 the tube I would like to use.

Is there a problem with any of these preamps driving very long interconnects? I realise capacitance from long leads may effect performance. I like the two stage schematic with the additional Rk (430ohms). Is this the most suitable and would make any changes to passive parts values?

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Projects: Oatley K272C - headphone amp in a tin can | "K272C" - Oatley's newest headphone amp | "VoXItBassO" - stereo 12" sub-woofers ]
Website: [ retro-thermionic ]

Well, first you need to decide if you want gain in the preamp or do you just want a line driver. This will dictate whether you want one or two stages.

Frankly I would never direct couple to a cathode follower, unless there were some bias issue I was trying to work around, because it messes with the bootstrapped input impedance of the CF. If you use impedance coupling then the CF doesn't load the driver except with many megohms and there is virtually no rotation of the AC load line in the driver stage. This means lower distortion.

Just my opinion.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com

Matt: A line driver would be perfect. Can you make a suggestion for a schematic based on a tube I may have: 6SN7, 12A??,?

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Projects: Oatley K272C - headphone amp in a tin can | "K272C" - Oatley's newest headphone amp | "VoXItBassO" - stereo 12" sub-woofers ]
Website: [ retro-thermionic ]

Mark;

Just use the first circuit that you posted. Here is the CF analysis for that circuit. It's a fine line driver as is. Overall distortion peaks at 0.4%, output impedance is just 828Ω plus the 1kΩ series resistance, and the total drive voltage loss is only 0.6dBv. This should drive any typical line, even one of extended length, without problems. And at typical line levels, the distortion should be almost nonexistent.

Let us know how it goes.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com

Matt: perfect, I wanted to build this schematic anyhow and use my favorite driver tube, the 6SN7.

_________________
Projects: Oatley K272C - headphone amp in a tin can | "K272C" - Oatley's newest headphone amp | "VoXItBassO" - stereo 12" sub-woofers ]
Website: [ retro-thermionic ]

Hi Mark, The simple one would be my choice just to drive long cables. My first rule is not to complicate things if you don't have to.

Matt, I differ a bit on the use of direct coupled stages. I do it in the phono preamp and it works great. There is transforms the relatively high Z of the SRPP output to a low Z cathode follower output. I like it there as it eliminates at least one capacitor in the signal chain. I grant that it does mean the tubes have high cathode voltages, but then because of the SRPP the heaters are elevated anyhow. The only real difficulty is getting the DC parameters on the cathode follower right. That's where PCs help a bit. Do the what ifs.....and adjust fire as needed.

Good listening
Bruce

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Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

The only power amp I have listen to with two of three stages direct coupled was the 6C33 of my friends Don. It direct coupled the first and second stage of the amp before the power stage. That amp is one of the best I have ever heard. Not saying it was good because of direct coupling but in this case it appeared to work extremely well. I guess one day I'll just have to give it a go!

_________________
Projects: Oatley K272C - headphone amp in a tin can | "K272C" - Oatley's newest headphone amp | "VoXItBassO" - stereo 12" sub-woofers ]
Website: [ retro-thermionic ]