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Old 29th June 2003, 04:49 PM   #1
Freddie is offline Freddie  Sweden
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Default Buffered Passive Preamp

Just read the thread regarding the Passive preamp by HarryHaller, and got inspired. Passive Preamp

So I quickly drew up this schematic, anything wrong with it? I found a cheap alps 10k motorized pot ($4), so this is going to be real cheap... I think it's worth a try.

What do you think?, suggestions?

/Freddie
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Old 29th June 2003, 05:03 PM   #2
jam is offline jam  United States
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Connect the sources of the fets with a small value trim-pot to adjust for dc offset. The wiper is now your output.

Regards,
Jam
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Old 29th June 2003, 08:09 PM   #3
Freddie is offline Freddie  Sweden
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Thanks for the suggestion Jam.

/Freddie
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Old 29th June 2003, 08:46 PM   #4
Freddie is offline Freddie  Sweden
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Suggestions for powersupply?

I'm thinking of using LM317/337 or perhaps 7824/7924.

/Freddie
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Old 29th June 2003, 10:43 PM   #5
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Default Re: Buffered Passive Preamp

Konnichiwa,

Quote:
Originally posted by Freddie
So I quickly drew up this schematic, anything wrong with it? I found a cheap alps 10k motorized pot ($4), so this is going to be real cheap...
Well, as drawn the worst case input impedance is around 1200 Ohm. I'd rather scale up the Calues by a factor 10 or so, meaning a 15K lawfaking resistor and a 100K Pot.

Sayonara
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Old 30th June 2003, 12:38 AM   #6
jleaman is offline jleaman  Belgium
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Could this schematic be used to drive a gain clone. ? I was thinking about maybe Building one for my small amps that i have completed..?
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Old 30th June 2003, 01:55 AM   #7
SY is offline SY  United States
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You'll definitely need some source degeneration; the 50 ohm pot will help, but you'd do better putting a hundred or so ohms in each source lead, then select devices (or, gasp, servo) to minimize output offset.

The input resistor, R4, is much too small. At higher volume control settings, you'll be putting quite a load on the source components. And unless you have TOTAL confidence in the source components, I'd capacitively couple the input. That will make output DC nulling with an integrator easier, too.
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Old 30th June 2003, 08:06 AM   #8
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Konnichiwa,

Quote:
Originally posted by SY
You'll definitely need some source degeneration; the 50 ohm pot will help, but you'd do better putting a hundred or so ohms in each source lead, then select devices (or, gasp, servo) to minimize output offset.
With the selected J-Fets and if they are selected for an Idss that makes sure that the maximum dissipation is not exceeded no further degeneration is neccesary. That is not only my own experiences, but also that of John Curl and Erno Borbely. Adding any more source resistors actually not only increases the output impedance, it also makes (surprisingly) the buffer less linear....

Sayonara
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Old 30th June 2003, 08:21 AM   #9
Freddie is offline Freddie  Sweden
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Quote:
Well, as drawn the worst case input impedance is around 1200 Ohm. I'd rather scale up the Calues by a factor 10 or so, meaning a 15K lawfaking resistor and a 100K Pot.

Sayonara
Yes, 1200 ohms is a little low. The only reason why I chose the 10k linear pot, is that I can get a motorized version for $4.. But I will choose a 100k log pot instead in shunt mode.
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Old 30th June 2003, 08:27 AM   #10
Freddie is offline Freddie  Sweden
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Quote:
You'll definitely need some source degeneration; the 50 ohm pot will help, but you'd do better putting a hundred or so ohms in each source lead, then select devices (or, gasp, servo) to minimize output offset.

The input resistor, R4, is much too small. At higher volume control settings, you'll be putting quite a load on the source components. And unless you have TOTAL confidence in the source components, I'd capacitively couple the input. That will make output DC nulling with an integrator easier, too.
Mr. Borbely himself doesn't use any source resistance at all. He said that a DC offset can be as low as 1mv with good matching. But I have included the 50 ohm put anyway. I will try the circuit without extra source degeneration resistors.
My sources have no DC offset and my power amplifier is capacitor coupled, so I will not place a cap at the input.

However thanks for the suggestions

/Freddie
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