DIY EAR 834p phono stage - a short evaluation/ test with those chinese PCBs

[niclaspa]

Frippel, regarding using a separate box for the PSU.  This is what I wrote in David's build thread:

Putting the power supply in a separate box has the advantage that you don't need to worry about it introducing hum into the phono stage.  But, there is extra cost and work involved.  You need a connector with at least five poles: 2 for high tension, 2 for heater supply and one for earth.  From a safety aspect, it important that the power supply chassis and the amplifier chassis are connected with an earth cord.  The connector and the cable needs to be rated for the high tension voltage.  Make sure that you choose a combination of male/female connector that don't leave high tension on any pin that can be touched.  Amphenol make good connectors, which I have used for this purpose:





Unfortunately, not very cheap.

I think that the easiest and cheapest option is to get a enclosure that is big enough to keep the transformer at a good distance from the amplifier.

The umbilical connectors are also quite fiddly to solder.

[Fripppel]

Thanks, I think I will go with one proper box and work with shielding the transformer and PSU. ZZmoko I made it really simple with the resistors like this. 

[ZZMoko]

That's perfect I couldn't imagine how you would have the space, but that makes perfect sense

[niclaspa]

It is better if you add one resistor to each heater cable, i.e., one to the green cable and one to the white cable.  Usually heater supplies have a center tapped transformer (or two resistors to create a center tap).  The center tap is connected to ground to minimise hum.  If you add both resistors to one side, you will create an imbalance and could get more hum as a result.

[Fripppel]

Hmm, that makes sence Niclas, thanks. I will get rigth to it. By the way, I saw your single ended amp build. Impressed to say the least. Go sweden!!!
Regards Fredrik

[nostromo]

Hello. Can I suggest a change?

[ZZMoko]

Would the choke lower the voltage to the heaters at all, thereby making the extra resistors unnecessary?

Also would 3 watt resistors be adequate for lowering the voltage to the heaters (5 watt resistors are much harder to get in the auks) and if so what values would you suggest (now we know to add them on both leads )?

[niclaspa]

The values of the resistors are the same regardless if you put the two of them on a single lead or one on each lead.  3W is sufficient.

The choke is for the high tension supply and will not change the voltage to the heaters.  It will make the high tension DC cleaner and could improve the sound.

[Chris65]

The choke has nothing to do with the heater supply, it is part of the B+ (high voltage) supply.

In this circuit with a non-centre tapped DC heater supply, a single resistor is ok to drop the heater voltage if needed.
The value & wattage required for the resistor depends on how much the voltage needs to be reduced. (Ohm's Law)
                    
                

The values of the resistors are the same regardless if you put the two of them on a single lead or one on each lead.  3W is sufficient.

Guess we posted at the same time Niclas.
I'd be a little cautious saying a 3W resistor is sufficient. It may not be as you know, depending on how much voltage drop is needed.
It's likely to be sufficient in this case though.

Shouldn't a resistor in each leg be half of the total resistance required?
                  

[Chris65]

I made it really simple with the resistors like this.........

Just a suggestion - I would put those resistors at the power supply PCB connection. There is the possibility of generating a little bit of noise with the exposed leads close to the valve.

[Tone_deaf]

If it were me I'd increase the value of the resistor in the PSU CRC filter by 1 ohm rather than have unsupported components at the amplifier end.  They tend to get bent into places they shouldn't go when poking about with meter probes or just swapping a tube. Although heater voltage is low it is high current, and doesn't half make some sparks when it shorts to ground!
Glyn

[niclaspa]

Three 12ax4 pulls 0.9A. The required voltage drop is around 1V, which means 0.5v per resistor. The power in one resistor is 0.9 * 0.5 = 0.45W. The resistors should have at least three times that rating. 3W is fine.

If the heater supply is DC then my comment about two resistors and center tap does not apply. One resistor will do. Or two in series in one leg like Frippel had done.

[needlekiller]

1. the 6,3 volt dc can checked on the heater dc output terminals.
2. the correct value can only checked with the valves!
3. there are 2 resistores on the board, in post 51 the res have 0,22 ohm
4. so if the heater-voltage with tubes on board is highre then 6,3v dc, change the 2 res to 0,33 or 0,47 ohm
cheers uli

[needlekiller]

there seams to be some different psu boards. but the heaterpart looks allways the same! bridge rectivier, load elko, first 0,xx ohm
resistor, second elko, second 0,xx ohm resistor,third elko to + heater.
so in this case an aditional resistor in the +heater works, but one more thing that could fail! remember "KISS"

[Fripppel]

Resistors are now moved to PSU and values changed. Thanks again. Hmm, "KISS" meaning - keep if sounding super!?! I will!
/Fredrik