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Tube Buffer Amp
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4144
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Author:  philip davies [ 13 Nov 2012, 15:08 ]
Post subject:  Tube Buffer Amp

Hi,

I am currently using a digital 31 band graphic eq betwean my cd player and SE valve ampliier. I am using my eq to remove room resonance with excellent succes. The graphic eq uses XLR's for input and output and because of this I have to use a sound craft mixing desk as I can connect it to the graphic using the XLR's. I would like to remove the mixing desk from the audio path and use something else such as a tube buffer amp or some kind of passive preamp using transformers.

I had thought about connecting the graphic eq directly betwean the cd player and amp but doing this will have terrible impedance miss match as the cd player has a high impedance output and the graphiq has a low impedance input.

Any suggestions on what to do would be very gratefull.

Thanks

Phil

Author:  Les [ 13 Nov 2012, 15:56 ]
Post subject:  Re: Tube Buffer Amp

How about the suggestions made here: viewtopic.php?f=5&t=4078

You basically just need impedance matching transformers. It would be the simplest approach. The more complex the route, the lesser the return in "investment". While a tube buffer would inject certain "tube ambiance". You will need to know the impedances of the equipment being matched, regardless of approach.

Author:  philip davies [ 13 Nov 2012, 17:02 ]
Post subject:  Re: Tube Buffer Amp

Hi Les,

I was thinking of using impedance matching transformers as suggested in that post. I was going to put them in a solid state preamp that I was going to build for my solid state amp but after getting my valve amp to work and replacing all the cheap parts with good quality items i've ended up with an amp that sounds superb compared to my solid state power amp therefore I'm going to concentrate on my valve set up.

The impedances of the equipment are :-
CD Player - 47Kohms @ 1Khz
Valve Amp - 47Kohms
Graphic EQ Input - 22Kohms @ 1Khz
Graphic EQ Output - 100ohms @ 1Khz

Thanks again

Phil

Author:  Geek [ 13 Nov 2012, 23:58 ]
Post subject:  Re: Tube Buffer Amp

A 6SN7 or 6FQ/CG7 cathode follower will do you nicely :D

Author:  philip davies [ 14 Nov 2012, 02:50 ]
Post subject:  Re: Tube Buffer Amp

Hi Gregg,

Thanks for yor suggestion, do you have a circuit diagram that I can look at?

Phil

Author:  Geek [ 14 Nov 2012, 10:06 ]
Post subject:  Re: Tube Buffer Amp

Here's the one I use. Unconditionally stable and one tube for stereo! Good for miniature 6FQ7/6CG7 too.
Attachment:
6SN7_CF.png

For 6SL7 or miniature 5751, use 820 ohms instead of 4.3K on the cathode.

There's lots more like it on Google Images, if you want to use another type.

Cheers!

Author:  philip davies [ 14 Nov 2012, 13:28 ]
Post subject:  Re: Tube Buffer Amp

Hi Gregg,

Thanks for posting that schematic. I have two questions:-

1. Does this circuit add any gain?
2. Should this buffer amp go betwean the cd player and graphic eq or betwean the graphic eq and amplifier?

Sorry for all the questions.

Thanks for your help :up:

Phil

Author:  Geek [ 14 Nov 2012, 18:20 ]
Post subject:  Re: Tube Buffer Amp

No problem!

1) Nope. Gain is about 0.95 actually. And given the very high dynamic range, a volume control isn't needed.

2) Anywhere you need a buffer. I find it handy for testing crappy big-box SS gear as my tube preamps have trouble driving 2K input impedance ;)

I forgot to mention the heaters of the buffer.... don't ground them. Either let them float with a noise bleed capacitor, or lift them 40V or so above the cathode (exact amount depends on the HV you use).

Cheers!

Author:  philip davies [ 15 Nov 2012, 07:57 ]
Post subject:  Re: Tube Buffer Amp

Hi Gregg

Thank you very much for your help. Is there an equation that I can use to work out the heater bias?

Thanks again

Phil

Author:  HT Performance [ 15 Nov 2012, 08:40 ]
Post subject:  Re: Tube Buffer Amp

The heater bias voltage is generally about 1/3 of your HV.


Cheers,

Miguel

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