Mic Preamp Power Supply Questions

[Jonas]

Hello, I'm interested in some general feedback and have a couple of questions regarding the attached power supply (2 separate attachments). This is a mic preamp showing only 1 of 2 identical channels (4 tubes). I believe it's called the Slowblown pre?

The HT power supply (CRCLC) is extraordinarily robust!! There are 3 x 1000uF caps in the HT power supply; the last stage 1000uF cap (320V B+) then connects to each preamp plate (attachment 2 "preamp") with each preamp plate further individually filtered with a 47uF cap, but I'm noticing these stages are not separted so all the 47uF caps are essentially paralleled?

My questions are:

1) Does this power supply look to be correct?
2) do I need to separate the preamp plates with additional resistor stages to separate the additional 47uF caps? 
2) what are the purposes of all the 1uF parallel caps at the filter caps?

[shooter]

second 2;
usually used as "spike suppressors" or transient suppression.

[HotBluePlates]

2) what are the purposes of all the 1uF parallel caps at the filter caps?

100-1000µF caps imply they will be aluminum electrolytic caps, due to the cost & size of using anything else.

We expect a capacitor to be a "short-circuit" for high frequencies, but the physical realities of constructing capacitors means that reactance falls as frequency rises (as we expect) up to a point, then it begins rising again.  That latter bit indicates some amount of inductance, and means the cap is not a "good capacitor" at some high range of frequencies.

The 1µF caps bypass the large 100-1000µF caps to overcome the inductance of the big caps.  It helps make sure the big caps don't oscillate in the radio-frequency range.

Hello, I'm interested in some general feedback and have a couple of questions regarding the attached power supply (2 separate attachments). This is a mic preamp ...

Almost 30 years ago, I bought some custom-built mic preamps that were a pair of UA 610 preamps packaged in a single rack mount unit.  That was before Universal Audio began making those again.

There was a separate power supply chassis that connected to the preamp chassis via a 4-pin XLR cable.  In the power supply chassis were 2 linear regulated power supplies:  a 250v 100mA supply for B+, and a 12-15v 0.8-1A supply for the heaters of 4x 12A_7 tubes.

If I were to build my own mic preamp today, I would copy that approach:  buy off-the-shelf linear regulated power supplies & deal only with how to package them within a finished product.

[Jonas]

If I were to build my own mic preamp today, I would copy that approach:  buy off-the-shelf linear regulated power supplies & deal only with how to package them within a finished product.

This is a very good suggestion, and I think I will give this approach a try soon. There is also a 48V supply (same manufacturer) for phantom power builds.

For now, I have an old chassis and most of the parts on hand to build per the schematics. Thanks for the help and tips

[Merlin]

1) Does this power supply look to be correct?

Yes, it's a bit overkill, but not wrong.

2) do I need to separate the preamp plates with additional resistor stages to separate the additional 47uF caps? 

Not that's not really necessary.

2) what are the purposes of all the 1uF parallel caps at the filter caps?

Some people believe the smaller caps will make the bigger caps "work better" at high frequencies. In reality the smaller caps do practically nothing until well above the audio range, but you know what audiophiles are like.

[HotBluePlates]

If I were to build my own mic preamp today, I would copy that approach:  buy off-the-shelf linear regulated power supplies & deal only with how to package them within a finished product.

...  There is also a 48V supply (same manufacturer) for phantom power builds.

The builder of the mic press I mentioned simply made a voltage-divider across the 250v supply output to obtain 48v for phantom power.

There's not much current-demand for phantom power (typically well below 10mA).

[Jonas]

I recently completed this build and happy with the results. I followed the schematic using mostly parts on hand; I used a 6H choke on hand and the PT is new. At the moment, it is not equipped with any input or output transformers, so it is basically a DI box until I figure out what to use for I/O transformers. Not sure what to expect once it it equipped with input transformers and start plugging mics in. No 48V phantom power so it will have limited use.

All the parts are inside one chassis; I anticipated some noise from the AC components but it is dead quiet. I want to try the linear power supply approach using 2 chassis per HotBluePlates recommendation soon.

Anyone else build this? Any suggestions or recommendations for a different tube mic pre?