I have found a valve preamp kit in an online store, to go between an amplifier based on an LM380 chip, and a crystal or regen radio, but it uses a microphone where I want an audio input. If anyone can tell me how to connect an audio in one this or give me a schematic for a low voltage preamp using a single 12AU7 tube, I'd be grateful
Thanks Dave
Oops, forgot the link to the preamp kit,
They do have (had) another similar preamp with an audio input, but none are in stock now (discontinued)
Forget. It's designed for nuts who like to add a special type of distortion to their hi-fis'
For God's Sake buy the National Semiconductor Audio handbook (I think it's back in print now) and/or the TI Bifet manual. Or download them.
And /or also 'op-amps for everyone' (IIRC) from TI.
And stop looking at crackpot sites using 30 year old chips ffs.
Graham
Fascinating in its own way but utterly irrelevant to what you require.
Stop looking for 'kits' and do some *learning*. Then you can design your own kits. Less than one day's appropriate study should do it.
Graham
Can you upload the regen schematic to a picture host and post the link?
Replace the headphones with a logarithmic pot - the kit instructions should indicate the resistance value to use, you need to make sure the pre-amp's input capacitor's working voltage is rated to withstand the full HT on the regen, or you could add a coupling capacitor to the pot's wiper.
I just built a two transistor (BC549,BC558) preamp kit that works well, will use that. I have a question regarding another regen I have. The 45 volt HT is placed across the headphone terminals, how would I connect an IC based amp to the radio, without frying the chip?
Schematic of the radio is at
Dave Australia
Put a resistor of the same resistance as specified for the phones in place of the phones, then couple the plate/anode to the top of a volume pot via a non-polar capacitor (0.1 - 0.47uF) rated to withstand the HT and take the O/P to the IC from the wiper - the bottom of the pot is obviously taken to ground.
You may find the IC input impedance is a little on the low side (often less than 47k) resulting in not much volume, an easy quick fix is to add a simple JFET source follower between the wiper and the IC amp chip - take the Vdd from the IC supply rail.
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