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Old 14th June 2009, 03:54 PM   #1
hug96 is offline hug96  France
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Default valve preamp for quad 303

HI,

I was thinking yesterday about simplifying my quad 33, and then realised that the output level was only 0.5V for full amplifier output.

I then thought it could be a good idea only to have a minimalist preamp, with unity gain, only to adapt the impedance ( quad 303 input impedance is quoted as 22K and 33 output impedance is 1K).

so I was thinking, would a simple circuit with a potentiometer and a simple classic cathode follower be a good idea of realisation? would it work well ?

I'd be happy to have some advices.

Hugues.
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Old 14th June 2009, 06:21 PM   #2
SY is offline SY  United States
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It will work very well as long as the cathode follower is designed competently. I published a circuit like this here and on my website- you don't have to do the full-blown circuit, but there are some basic design hints in the article which will (I hope) help you avoid the most common mistakes.
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Old 15th June 2009, 02:44 AM   #3
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Hugues,

You want to use a high gm type, for a low O/P impedance. The current production JJ ECC99 is very well suited to this job.

IB should be substantial, as charging and discharging the interconnect cable capacitance is not problematic.

A bipolar PSU allows you to avoid a cap. in the I/P circuitry.

The 10M45S "chip" will allow you to CCS load the CF, with very little muss or fuss. CCS loaded CFs yield superior performance.
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Old 15th June 2009, 02:52 PM   #4
hug96 is offline hug96  France
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thanks for the answers.

so, I'd like to start on a simple cathode follower with single psu.

I've seen some vintge circuits with ecc82 cathode follower but don't know if this could be useful as tube amps have usually higher Z imput thant solid state...

so I'm looking for a simple circuit if anybody can help me ( the 2 capacitors, 5 resistors kind) but well calculated, as I've read many times that poorly conceived cathode followers gave very poor results...

Click the image to open in full size.

thanks for your help !

Hugues.
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Old 15th June 2009, 09:02 PM   #5
pete.a is offline pete.a  United Kingdom
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Default Quad 33 Improvements

Hello there, My first system was a 33/303/Fm3 Circa 1997

Have you surfed through the net and discovered the Quad improvent site ?
I used to own a set of 303 amps , that i ran as Bi amped , one amp per speaker, just using a tweaked 33 pre ( i added a seperate on off button and fitted a 50k noble vol pot ) Big difference..didn't need to change much more after that.
I then changed the main power supply caps in the 303 . I had a valve FM3 tuner. I think that was the best sounding radio for a long time..until all the stations here went compressed ..sorry to ramble... Have you tried any other amp combination yet , as the old 303 can sound a bit soft . There's no doubt about it's build quality..bomb proof...

I have since that first system had the upgrade bug ... sold the quad stuff , tried a couple of amps and have now
built my own system using kits...and im now very satisfied..all transistor..as house isn't suited to valve ..small children !
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Old 16th June 2009, 04:31 PM   #6
hug96 is offline hug96  France
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Default Re: Quad 33 Improvements

Quote:
Originally posted by pete.a
Hello there, My first system was a 33/303/Fm3 Circa 1997

Have you surfed through the net and discovered the Quad improvent site ?
I used to own a set of 303 amps , that i ran as Bi amped , one amp per speaker, just using a tweaked 33 pre ( i added a seperate on off button and fitted a 50k noble vol pot ) Big difference..didn't need to change much more after that.
I then changed the main power supply caps in the 303 . I had a valve FM3 tuner. I think that was the best sounding radio for a long time..until all the stations here went compressed ..sorry to ramble... Have you tried any other amp combination yet , as the old 303 can sound a bit soft . There's no doubt about it's build quality..bomb proof...

I have since that first system had the upgrade bug ... sold the quad stuff , tried a couple of amps and have now
built my own system using kits...and im now very satisfied..all transistor..as house isn't suited to valve ..small children !

actually, I'm very satisfied with my system. I just thought that the signal goes through a lot of useless components through the quad 33 ( too much gain, eq control etc...)

so I was thinking of a minimalist preamp, more to be seen as an experiment.
and I'm on holiday now, so I thought this would be nice to do it now.

so I found this circuit in a morgan jones book with an e88cc, and I happen to have a spare 6922 jan philips.



Click the image to open in full size.


what do you think of that schematic? should it be working alright?

any comments welcomed !

Hugues.
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Old 16th June 2009, 07:48 PM   #7
SY is offline SY  United States
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MJ's circuits are well-engineered and an excellent model to follow.
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Old 23rd June 2009, 03:47 PM   #8
hug96 is offline hug96  France
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hi,

here is the circuit i'm gonna try.

I'm reusing a transformer I already have...

I'm not sure about the value of the volume pot, 47K or 100K?

Thanks for your help, Hugues.
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