Possible to add a Pre out to a tube amp?

[CraigRobbo]

Hello guys.

I am not well skilled in DIY audio and hope this isn't an obvious question, i did some google without an obvious answer.,

I am thinking of buying a stereo tube amp but would love to Bi amp so would like to have a pre out controlled via the volume knob. Is this possible to add if the amp doesn't have one.

The amp in question is an ebay " Music Angel" amplifier which I have the opportunity to get very very cheap.

I would prefer not to get into the debate of " get a different amp" as at the price I can get it for, if it can be done would be worth while

Thanks

Craig

 

[rayma]

I am thinking of buying a stereo tube amp but would love to Bi amp so would
like to have a pre out controlled via the volume knob.

It appears there are 3 inputs on the rear, so trace the cables back from one pair of the inputs,
and disconnect the far ends of the hot wires. Connect those ends to the volume control's wipers instead.
The ground leads in the cables can remain as they are. Those inputs are now volume-controlled outputs.

 

[Mooly]

Ideally you should take the signal from the wiper of the volume control and feed it into a buffer amplifier. The output of the buffer would be the 'line out' adjustable via the volume control.

Omitting the buffer adds to many variables and is not likely to be satisfactory in the long run.

Depending on the existing circuit within the amp, there may be the possibility of taking a buffered feed already... we would have to see the circuit though.

Adding an AC coupled opamp buffer to a valve amp would be straightforward and use a minimum of parts but you would need to be able to figure the internals out to do it.

 

[rayma]

Ideally you should take the signal from the wiper of the volume control and feed it into a buffer amplifier.

Yes, it's a 100k pot, so it's certainly not ideal to run unbuffered. For a woofer amplifier,
it might do ok through an isolation resistor.

 

[CraigRobbo]

Thanks guys, here I have attached a photo of the underside.

Hopefully this may help, but certainly sounds do-able.

 

[rayma]

certainly sounds do-able.

There's an internal low voltage housekeeping supply, and a filament supply.
You could run a transistor or single supply op amp buffer off one of those, with
appropriate inut/output coupling capacitors. Or, use an external buffer box
connected to the unbuffered outputs, wired as above.

 

[Mooly]

^ As rayma say really. If the low voltage supply is clean and rated at 12 volts upward then a simple opamp or transistor emitter follower would suffice. To do it neatly would need a small PCB securing to the chassis though.

 

[rayma]

^ As rayma say really. If the low voltage supply is clean and rated at 12 volts upward
then a simple opamp or transistor emitter follower would suffice. To do it neatly would
need a small PCB securing to the chassis though.

I would probably use an external buffer box, ebay or self-built, it's easier.
A foot of cable off the wipers isn't going to have much HF effect.
Some reverse engineering would be necessary in the amp's grounding system
to add an active circuit inside, but it would be a useful learning project.

 

[CraigRobbo]

Thanks for all the help guys. Much appreciated