Tube Guitar Preamp with a Chip Amp

[dbishopbliss]

A little while ago I put a bluetooth enabled chip amp from PE into a box to use in my workshop. Its very small and has more than enough power for my speakers - 50W/Channel into 8 Ohms.


I just came across a demo of a Victory Duchess V4 amp. The description says it has 4 preamp tubes going into a chip amp power section. Milkman The Amp has a similar description.


That got me thinking? Could I just build the preamp-section of an amplifier and send its signal to this power amp (or any mono power amp)? For example, If I used only the Normal Channel of a 6G3 Deluxe amplifier and omitted the tremolo (keeping things simple for now), could I just use a single 12AX7 and the circuit up to and including the .01 coupling cap before the phase inverter? That is... use that signal to feed the chip amp either through a hard-wire in the same box, or using a jack from the preamp to a jack in the power amp.

[sluckey]

You can do that. May need to use a bigger output cap depending on which power amp you chose. Experiment.

If you expect long cables then you'll likely need a cathode follower output.

[dbishopbliss]

The amp I want to use is the same as the one used in the Victory, Milkman, Seymour Duncan PowerStage and Fender Rumble. According to the documentation it requires a balanced input.

I found a video using a hammond transformer that converts a signal to balanced but it is expensive - more than the amp. I found a cheap DI box ($12) that I could gut and use in my project but my guess is the components cannot be very expensive given it has an enclosure, two switches, two 1/4" jacks and an XLR jack.

Here's my questions:

1. Is is possible to connect an unbalanced signal to a balanced input?
2. Is there a downside connecting an unbalanced signal to a balanced input given the distance is so short (same enclosure not long runs of cable where noise can be injected)?
3. Is there a simple/inexpensive way to convert an unbalanced signal to balanced without a transformer?


https://www.parts-express.com/ICEpower-50ASX2BTL-Class-D-Audio-Amplifier-with-Power-Supply-Module-1-x-170W-326-214

[sluckey]

https://www.google.com/search?q=connect+an+unbalanced+signal+to+a+balanced+input&oq=connect+an+unbalanced+signal+to+a+balanced+input&aqs=edge..69i57j0i546l2.1498j0j4&sourceid=chrome&ie=UTF-8

[dbishopbliss]

https://www.google.com/search?q=connect+an+unbalanced+signal+to+a+balanced+input&oq=connect+an+unbalanced+signal+to+a+balanced+input&aqs=edge..69i57j0i546l2.1498j0j4&sourceid=chrome&ie=UTF-8

I did a similar search, I just don't always understand the information and how to discriminate between which answer is best.


If I am understanding correctly, the simplest answer seems to be connect the signal from the preamp to Pin 2 and connect Pins 1 and 3 to ground (see diagram below). Of course there won't really be any pins, there will be wires on the amp board. Is this correct?

[PRR]

According to the documentation it requires a balanced input.
https://www.parts-express.com/ICEpower-50ASX2BTL-Class-D-Audio-Amplifier-with-Power-Supply-Module-1-x-170W-326-214

Where does it say that??

https://www.parts-express.com/pedocs/manuals/326-114--icepower-50asx2btl-class-d-amp-with-power-supply-data-sheet.pdf

Fig 16 page 26 clearly shows single-ended inputS at 40k impedance.

Ummmm, fig 17.... OH!! You want to drive the two channels bridge-mode for the 170Watt rating. Good luck. The most rational plan would be a 19 cent dual op-amp, one buffer (because you are coming from tubes), one inverter (as shown). With TL072 the input impedance can be high enough for any tube preamp. The AUX supply on the board is probably not a good power source for the op-amp (way too high V, and finicky).

[sluckey]

If I am understanding correctly, the simplest answer seems to be connect the signal from the preamp to Pin 2 and connect Pins 1 and 3 to ground (see diagram below). Of course there won't really be any pins, there will be wires on the amp board. Is this correct?

I think that's right.

[dbishopbliss]

According to the documentation it requires a balanced input.
https://www.parts-express.com/ICEpower-50ASX2BTL-Class-D-Audio-Amplifier-with-Power-Supply-Module-1-x-170W-326-214

Where does it say that??

https://www.parts-express.com/pedocs/manuals/326-114--icepower-50asx2btl-class-d-amp-with-power-supply-data-sheet.pdf

Fig 16 page 26 clearly shows single-ended inputS at 40k impedance.

Ummmm, fig 17.... OH!! You want to drive the two channels bridge-mode for the 170Watt rating. Good luck. The most rational plan would be a 19 cent dual op-amp, one buffer (because you are coming from tubes), one inverter (as shown). With TL072 the input impedance can be high enough for any tube preamp. The AUX supply on the board is probably not a good power source for the op-amp (way too high V, and finicky).

I'm resurrecting this thread. Finally getting the time to start building stuff again (gotta finish a guitar first).


I was looking around at different schematics for creating a balanced signal using a TL072. While the chip itself is inexpensive and it only requires a handful of resistors, the power source seems like that could be the more tricky part (as you point out). Would something like the circuit in the attached diagram work? Assuming that does, I have a number of unused Line Transformers (Speco T7010) that were used as Output Transformers on some old Bottlehead amplifiers. Could I repurpose those?