16 - 18V tap on Stereomour Transformer

[drewh1]

is there anywhere to tap 16 - 18 v DC on the Stereomour - I know this is blasphemy but I am curious about adding a stepper motor remote volume control.

Anyone else looked into this?

[rlyach]

Drew,

I don't want to speak for those who have been working with this amp for years but I believe that the answer is no. At least not easily. You could tap into the 12.3 AC filament and build a rectifier, but the transformer may not be able to supply the current to the stepper motor and the 12AT7 at the same time. The 12AT7 takes ~300mA.  Stepper motors can take a lot of current, depending on the amount of torque they supply. Also, you may not have enough voltage. the 12.3AC has a 17V p-p that you can rectify but as soon as you pull some current from the regulator you just built that 17V will probably drop below 16V. You could also use a voltage doubler from the 12.3VAC output and drop the voltage down with a regulator or resistor, but again you may not have the power available. You could also draw current from the 400VDC supply using a large high power resistor, but this may affect the power to your output tubes. I don't know the current rating of the HV supply. I think your best bet is to add a small switching power supply inside the box specifically for the stepper motor. You can just tap into the 120VAC main. This is just food for thought and I am sure PJ may have a more complete answer.

[Paul Birkeland]

You could add a small 12V transformer and power supply to the Steremour, just screw it to the wood on the back side of the chassis. 

I think there's a little headroom on the 12V winding, but I agree with Randy, those motors can draw surprising amounts of current.

[drewh1]

I can use an external power supply for the remote system, it just seems not very elegant.  I suppose I could take 120 V off after the switch so at least it would only be powered when the amp was powered.  The motor only uses current when turning (400 ma) and 4ma standby current. So maybe that is a better solution.

[Paul Birkeland]

Hmm, 400mA might be doable, considering the motorized pot is not always running, hopefully PJ can chime in!

If this is the case, you'll have to do some head scratching about whether pin 9 on the 9 pin socket should be lifted.

[drewh1]

In fact, the motor will not be used very much, occasional adjustment. I am finding I only move the volume control 2 notches or so to adjust for the various source levels. Since I have my music collection on a server, I tend to skip around a lot and don't want to get up every time! OK, so I am a little lazy 

I like the idea of tapping the 12 volt since it simplifies the circuit and I don't really want a switching power supply lurking underneath polluting my everything with EMI!

[Paul Joppa]

The 12.6v winding can handle 600mA rms easily, and close to 1 amp for short periods (say, less than 10 seconds?). You could rectify it and get 14-15 volts unregulated. I suggest the best would be to leave the 12AT7 with AC heater supply, and I'll strongly encourage you to retain the grounded heater tap (pin 9 on the socket) - this tap serves other purposes as well.

[drewh1]

Great! thanks for help - I am wondering if anyone else has put the BentAudio stepper motor solution.  I was going to build my own but this looks like a very expedient and elegant solution.

[drewh1]

So I want to make sure I have this right:

1. I tap off of the 12.6 v AC on the transformer (where the red and black go to 9 pin socket, pins 4 and 5).  i assume it is better to tap off the transformer than the socket?

2. Use a bridge rectifier to convert 12.6 v AC to  variable 12 to16 v DC (unregulated, low current so voltage will drift up?)

can I use any reasonably rated bridge rectifier (100v 4amp?) ? or should I use a low vF Shottky 2A 40V? 

[Paul Birkeland]

Your presumptions are all sound, I'd recommend 1N5820 diodes for your bridge. 

[rlyach]

Drew,

I put the circuit in circuit lab. I was not exactly sure what the RS values for the filament winding was so I made an educated guess. I have also included two resistors to model the filament of the 12AT7. There are two problems you will face. First, because the the filaments have their center point connected to ground (pin 9) your stepper motor will not have the same ground reference. If the motor has its chassis shorted to ground this may be a problem. Second, you will not reach 16VDC, especially when the stepper motor is running. The specs for the motor you gave say that it should work down to 12VDC. If we can get someone to give us the detailed specifications for the 12.6 Vac winding we can get a better idea of the performance of the rectifier under load. Also, I used 1N4007 diodes but you can update the parameters for any diode. Here is the link to the circuit. You can open it, make changes, simulate it, and save it to your own workbench.


https://www.circuitlab.com/circuit/92f74h/stepper-supply/

[Paul Birkeland]

The UF4007 will drop a little more voltage than a 1N5820, and the 10uF cap will be more responsive to the load at 10,000uF.

Let's hop that the motor supply is allowed to float with respect to the metal on the motor itself!

[rlyach]

I have updated the circuit to use the 1N5820 instead of the 4007. This reduces the forward voltage by about 200mV but increases the Rs by about 25 miliohms. I also increased the cap to 10000uf. I did a quick check and found several suppliers for a 12,000uF cap. The only question is what is the output series resistance of the winding? I gave a guess but it will make a difference. Keep in mind that the 400mA load is constant in this simulation, making this very much a worse case scenario. I would have liked to filter the output a little more but we don't have the headroom. All this is still dependent on being able to float the stepper motor below chassis.

[drewh1]

You guys are awesome!  I am going to learn a lot from this project.

[Paul Joppa]

The 12-v winding is about 0.65 ohms (including the reflected primary) with an unloaded voltage of 12.94 volts (assuming the power line is providing 120.0 volts, of course).

Use Schottky diodes! Silicon diodes will have a reverse recovery spike, which radiates energy from the wiring and couples to the other windings inside the transformer capacitively. If the voltage is marginal, consider 80SQ045N 8-amp diodes; they have an even lower voltage drop.