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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Which pre-amp for my tube amplifier?
Which pre-amp for my tube amplifier?
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Old 24th February 2020, 03:53 PM   #1
KolfMAKER is offline KolfMAKER  Netherlands
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Default Which pre-amp for my tube amplifier?

I have a tube amplifier in a Philips tube radio (model B2X 80U) built in 1958. I have completely restored this unit, and currently it is functioning perfectly.

For my next step I would like to add some modern technology to this radio. As it has a (switchable) input for a turntable, I would like to use it for attaching a bluetooth receiver (for streaming music to this radio). I have already made a test setup with a bluetooth receiver board (KRC-86B), and it works.

There's only one problem. The audio signal output from the bluetooth receiver board is not strong enough. It means I have to turn the volume quite high to get a reasonable volume. Which also means that when I switch to tuner, the volume blasts my ears off!

Question:
So my question is: what can I best do to get the right audio signal input?

a) I guess I could put a pre-amp board in between the bluetooth receiver output and the tube radio input. What pre-amp board would you suggest? (I am looking at PCB's not pre amp's in casings, as I wil have to built everything inside the radio)
b) Is it possible to exchange the pre-amp tube in the radio for one that has a stronger pre-amplification?
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Old 24th February 2020, 05:48 PM   #2
Mark Tillotson is offline Mark Tillotson
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Do you have +/-15V supplies in the unit already?
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Old 24th February 2020, 06:49 PM   #3
KolfMAKER is offline KolfMAKER  Netherlands
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Thnx Mark.

The Bluetooth receiver I will probably use is powered by 5 volts. I have an AC/DC converter already incorporated in the radio unit. It switches on/off together with the original power switch of the radio.
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Old 24th February 2020, 07:18 PM   #4
stocktrader200 is offline stocktrader200  Canada
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A 5x gain op amp (Se5532) or OPA2134 will work. You will need at least +- 9 volt supplies to prevent clipping.
The op amp can be internal or external whichever is easier.
Rectification and some filtering will provide this power from the 6.3v heater tap that should be referenced to chassis ground.

Last edited by stocktrader200; 24th February 2020 at 07:21 PM.
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Old 25th February 2020, 02:36 AM   #5
PRR is online now PRR  United States
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Which pre-amp for my tube amplifier?
It's a transformerless radio. Touching the circuitry can be fatal. They got a Phono input by using small capacitors which pass audio better than shock; also because cheap phonos do not have exposed wiring.
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