Preamp Help

[shutterdial]

I Am wanting to build a Tube Phono Preamp for a friend, and have been looking at the STA1 because He wants bass and treble controls. He also Wants 2 od3 Tubes because mostly of looks but can i use a power transformer with the tubes and make it work also will this amp put out enough power for the power amp. Thanks for all the help.

[Alain Poitras]

Originally Posted by shutterdial

I Am wanting to build a Tube Phono Preamp for a friend, and have been looking at the STA1 because He wants bass and treble controls. He also Wants 2 od3 Tubes because mostly of looks but can i use a power transformer with the tubes and make it work also will this amp put out enough power for the power amp. Thanks for all the help.

The STA1 with tone controls is not a magnetic phono preamp, you will have to add 2 or 3 more double triodes for the phono for a total of 5 or 6 tubes ... The preamp transformer can be very small and cheap since the current is very low but a transformer for a power amp can be very big and expensive depending on the voltage and current needs.

If the power amp is on a separated frame, you are better to use distinct transformers for the power amp and the preamp but you can also use the high voltage and heater supply of the power amp for the preamp with a cable between them or if they are on the same frame. In that case, you need a voltage dropping resistor to get 220VDC for the preamp supply because the power amp supply voltage is ususally higher.

Alain.

[shutterdial]

It is in a different frame, do you know were i can find a schematic for a more powerful preamp with tone controls? i'm also going to use a power transformer with od3 for the b+

[aardvarkash10]

if all he wants is visual aesthetics, build a SS phono preamp into a box with a couple of OD3s sitting on top feeding a dummy resistor load.

You'll save a lot of heartache.

Alternatively, get a technical rather than aesthetic understanding of what is needed (what is the required cartridge load, what the power amp load is presenting, an idea on the required gain etc) and then ask your first question again.

There are a couple of reasonably current "what phono pre should i build?" threads on the forum at the moment - they give plenty of background too

[shutterdial]

Its a pioneer turntable some thing like a pl-150 running to a 6550 stereo amplifier.
Amp has 4 6sn7 tubes and 4 6550 tubes

[Alain Poitras]

Originally Posted by shutterdial

Its a pioneer turntable some thing like a pl-150 running to a 6550 stereo amplifier.
Amp has 4 6sn7 tubes and 4 6550 tubes

Is turntable must be old, mine is a Pionner Pl-200 with a LM10 ortofon cartrige and I think I buy it more than 25 years ago and I hope it will work at least another 25 more years ...

This tubes phono preamp look nice :



If you plug it to the STA-1 input, change the volume control for a 50K and add a source selector, you will have a complete preamp for any power amplifier because is output is a cathode follower stage.



You really don't have to use a OD3 regulator tube for a preamp, just a simple supply with a good CRC or CLC filter will do and a DC supply for the heaters ... You also don't have to use the zeners and R12, C10, C11 of the phono preamp if you have a well design supply. The supply of a tubes preamp don't need to have the exact voltage to work very well, even if there is some line voltage variations, you will not notice any change and since the current of the preamp is very low and stable, there is no need for a load regulation, just a good ripple filtering is OK.

A total of 6 double triodes for magnetic phono and also any number of auxiliary for CD, radio, etc ... The best is using shielded tube sockets to avoid hum because phono input is very sensitive. You can also use small DIP relays to select sources near the RCA plugs at the back, with just a cheap rotary selector to select the DC coil relays, the unselected relays put the corresponding sources outputs to ground to avoid unwanted interferences like in any good sound system ...

Another possibility is to look at the commercial preamp section of a well know complete amplifier and reproduce only this part, there is hundreds of them on the WEB.

Alain.