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Old 8th February 2005, 06:36 PM   #1
beppe61 is offline beppe61  Italy
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Default Preamp schematic.

Dear DIYers,


I have found the attached schema in an old audio magazine.
I was wondering if this schema is suitable for a simple line preamp.
The voltage gain should be about 3 (if I am not wrong).
The supply voltage is 60 V (single supply).
Unfortunately the two BJTs are not named.

Does anyone have any suggestion for a couple of suitable BJTs?

Thank you very much to anyone.
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Old 8th February 2005, 09:48 PM   #2
Netlist is offline Netlist  Belgium
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Something like BC639/BC640 should do the trick IMO.

/Hugo
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Old 9th February 2005, 08:24 AM   #3
beppe61 is offline beppe61  Italy
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Dear Sir,

Thank you very much indeed for your kind and extremely valuable reply.
What do you think about the schema ?
I intend to replace the linear attenuator at the input with a log potentiometer and take the signal off the output coupling cap (this will be a film cap of course, like tho other one at the input).
I think that this kind of circuit is called " Sziklai pair " but I am not sure.
In general I am looking for a minimalist line preamp.
I would like to stay with BJTs even if I understand that the Nelson Pass Bride of Zen is very very good, if well built.
I don't know if this preamp is able to drive my power amp that has an input impedance of about 8 kohm.

Any your further suggestion would be very much appreciated and valuable for me.

Thank you very much again.

Kind regards,

Beppe61
Torino__ITALY
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Old 9th February 2005, 09:13 AM   #4
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I am willing to bet that even an opamp circuit will sound better.
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Old 9th February 2005, 09:21 AM   #5
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Hi.

BC550C and BC557C work well in this circuit. Use polyester btpassed with polysteyrene for good cheap coupling caps.

Super hint make the 120k with 20k from the supply decouple with a good cap and then a 100k to the base of TS7, this removes supply interaction and tightens the sound.

If you are an experimenter then replace TS8 with a p chanenel fet like a BS250 and reduce the values of R33 and R34 for a lower output impedance.

Chris
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Old 9th February 2005, 09:48 AM   #6
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Replacing the lin pot with a log pot is not a good idea, as the parallel combination of the 27k and 120k resistor bend the curve of the lin pot to an approximate log curve. I think you will be just about OK to drive an 8k load with this circuit.
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Old 9th February 2005, 10:07 AM   #7
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Quote:
Originally posted by analog_sa
I am willing to bet that even an opamp circuit will sound better.
And I'm willing to bet that even this simple circuit will sound
better than any opamp....
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Old 9th February 2005, 10:18 AM   #8
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Quote:
Originally posted by Netlist
Something like BC639/BC640 should do the trick IMO.

Quote:
Originally posted by Chris Jennings
BC550C and BC557C work well in this circuit.
The be honest, almost any small signal transistor will work.

fT > 100MHz
Hfe > 100, >300 even better
Min 30 V

BC55x
BC56x
BC54x

Even old BC108 will work
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Old 9th February 2005, 10:31 AM   #9
beppe61 is offline beppe61  Italy
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Ok dear friends,

I have to reveal the little dirty secret.
The schema is "stolen" from that one of the line input stage of and old but very estimable Radford ZD22 preamp.
The full line stage is much more complex including also tone control section (I personally hate it).
Unfortunately, but as usual, the schema doesn't report the BJTs' codes.
So the search is open for suggestion.
I think that TS8 should be cooled with a dissipator.
So a TO92 case BJT wouldn't be the best option, I think.
I was thinking about the 2SA968. An handy TO220 device.
Is it too much ?
Thank you in primis to the Radford designer (who is he/she I really don't know) and then to everyone who takes part in this discussion.
I am very curious in how this extremely simple stage can sound.
Really an op-amp based preamp would be better ?
Don't know.

Kind regards,

Beppe61
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Old 9th February 2005, 11:20 AM   #10
beppe61 is offline beppe61  Italy
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Dear Moderator,

If what I am writing is not allowed please forgive me and delete the following message.

Dear friends,

The original BJTs are:

TS7 = BC107
TS8 = BC303

The BC303 must be cooled.
So I would like to replace the BC303 with a TO220 case Toshiba 2SA968.
Is it possible without other modifications to the circuit ?

Kind regards,

Beppe61
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