DIY Preamp for use with Dynaco ST70

[liggs17]

Recently came into possession of a Dynaco ST70 with upgraded driver board. I would like to build a preamp for this, but have three questions:

1. Is a preamp necessary?
2. Would an S5 Electronics stereo kit work as a suitable preamp?
3. If not, what is a good DIY preamp kit?

I am able to solder competently, but get lost with regards to the mathematics. I have put an S5 stereo amp together, and I liken this to a "paint by the numbers". What I'm looking for is either an all-included kit or a board with a list of parts and the suppliers from whom I can order them.

 

[Eli Duttman]

Recently came into possession of a Dynaco ST70 with upgraded driver board. I would like to build a preamp for this, but have three questions:

1. Is a preamp necessary?
2. Would an S5 Electronics stereo kit work as a suitable preamp?
3. If not, what is a good DIY preamp kit?

I am able to solder competently, but get lost with regards to the mathematics. I have put an S5 stereo amp together, and I liken this to a "paint by the numbers". What I'm looking for is either an all-included kit or a board with a list of parts and the suppliers from whom I can order them.

1. You will need some sort of control center. The question is how complex the control center will be. What will be used as the source(s)? If CD is all you use, try driving the ST70 with a portable CDP. If you comfortably achieve satisfactory listening levels, you are a candidate for a "unity" gain control center, either passive or buffered.

2. I don't think so.

3. The Bottlehead Foreplay is a possibility. However, the Foreplay has a fair amount of gain. What you wind up with will depend on the portable CDP test.

 

[liggs17]

Eli,

Thanks for the response. What is the benefit of using a portable cd player for testing? I want to know just for knowledge's sake.

What is a gain control center? Is there a link or book that explains these concepts?

Thanks.

Liggs17

 

[Sherman]

Basically, if the signal from the portable CD player is sufficient to drive the ST70 to full power or thereabout then you don't need a preamp. What you need in that case is simply a way of controlling the volume and switching sources (CD, DVD, tuner, etc.) if so desired.

This setup is often called a "passive preamp" (not really a preamp since there is no gain, nor even any active components). You put a volume control (potentiometer, stepped attenuator, remote control, really your choice) in a box. If you have multiple sources then install RCA input jacks in the box and wire them to a rotary switch to select the proper pair. Wire the rotary switch to the volume control and then wire the wiper of the volume control to a pair of RCA jacks for the output.

Now just connect the output of your new passive pre to the input of the ST70. Connect your sources to the input jacks of your passive pre. That's it. You can select your source and control the volume from the passive pre.